Wireless Networking Technologies

WLAN, WiFi Mesh and WiMAX

Dr. S S Mohan Reddy

Professor
Department of ECE
Course Outline
 Wireless Networks  Wireless LANs (WiFi)
– Difference from wired – 802.11 standards
– Mobility – Mobility support
– Voice and QoS support
 RF Basics
– Frequency, modulation  Mesh and Adhoc Networks
– Medium access control – Routing and Transport

 WiFi Overview  Wireless MANs (WiMaX)

– Basic elements – 802.16 standard
– Standards and variants – Voice and QoS support

 WiMaX Overview  Trends

– Basic elements – Overlay networks

2
Wireless Networks
Wireless networks
 Access computing/communication services, on the move

 Wireless WANs
– Cellular Networks: GSM, GPRS, CDMA
– Satellite Networks: Iridium

 Wireless LANs
– WiFi Networks: 802.11
– Personal Area Networks: Bluetooth

 Wireless MANs
– WiMaX Networks: 802.16
– Mesh Networks: Multi-hop WiFi
– Adhoc Networks: useful when infrastructure not available
4
Limitations of the mobile environment
 Limitations of the Wireless Network
 limited communication bandwidth
 frequent disconnections
 heterogeneity of fragmented networks

 Limitations Imposed by Mobility

 route breakages
 lack of mobility awareness by system/applications

 Limitations of the Mobile Device

 short battery lifetime
 limited capacities
5
Mobile communication
 Wireless vs. mobile Examples

  stationary computer
  laptop in a hotel (portable)
  wireless LAN in historic buildings
  Personal Digital Assistant (PDA)

 Integration of wireless into existing fixed networks:

– Local area networks: IEEE 802.11, ETSI (HIPERLAN)
– Wide area networks: Cellular 3G, IEEE 802.16
– Internet: Mobile IP extension

6
Wireless v/s Wired networks
 Regulations of frequencies
– Limited availability, coordination is required
– useful frequencies are almost all occupied
 Bandwidth and delays
– Low transmission rates
• few Kbits/s to some Mbit/s.
– Higher delays
• several hundred milliseconds
– Higher loss rates
• susceptible to interference, e.g., engines, lightning
 Always shared medium
– Lower security, simpler active attacking
– radio interface accessible for everyone
– secure access mechanisms important 7
Wireless Technology Landscape

72 Mbps
72 Mbps Turbo .11a
54 Mbps 802.11{a,b}
54 Mbps
5-11 Mbps 802.11b .11 p-to-p link
5-11 Mbps
1-2 Mbps 802.11 µwave p-to-p links
1-2 Mbps Bluetooth

3G
384 Kbps WCDMA, CDMA2000
384 Kbps
2G
56 Kbps IS-95, GSM, CDMA
56 Kbps

Indoor Outdoor Mid range Long range Long distance

outdoor outdoor com.

10 – 30m 50 – 200m 200m – 4Km 5Km – 20Km 20m – 50Km

8
Reference model

Application Application

Transport Transport

Network Network Network Network

Data Link Data Link Data Link Data Link

Physical Physical Physical Physical

Radio Medium

9
Effect of mobility on protocol stack

 Application
– new applications and adaptations
– service location, multimedia
 Transport
– congestion and flow control
– quality of service
 Network
– addressing and routing
– device location, hand-over
 Link
– media access and security
 Physical
– transmission errors and interference
10
Perspectives
 Network designers: Concerned with cost-effective
design
– Need to ensure that network resources are efficiently utilized
and fairly allocated to different users.

 Network users: Concerned with application services

– Need guarantees that each message sent will be delivered
without error within a certain amount of time.

 Network providers: Concerned with system

administration
– Need mechanisms for security, management, fault-tolerance
and accounting.
11
RF Basics
Factors affecting wireless system design
 Frequency allocations
– What range to operate? May need licenses.
 Multiple access mechanism
– How do users share the medium without interfering?
 Antennas and propagation
– What distances? Possible channel errors introduced.
 Signals encoding
– How to improve the data rate?
 Error correction
– How to ensure that bandwidth is not wasted?

13
 Frequencies for communication
twisted coax cable optical transmission
pair

1 Mm 10 km 100 m 1m 10 mm 100 m 1 m
300 Hz 30 kHz 3 MHz 300 MHz 30 GHz 3 THz 300 THz

VLF LF MF HF VHF UHF SHF EHF infrared visible light UV

 VLF = Very Low Frequency UHF = Ultra High Frequency

 LF = Low Frequency SHF = Super High Frequency
 MF = Medium Frequency EHF = Extra High Frequency
 HF = High Frequency UV = Ultraviolet Light
 VHF = Very High Frequency

 Frequency and wave length: = c/f

 wave length , speed of light c  3x108m/s, frequency f
14
Wireless frequency allocation
 Radio frequencies range from 9KHz to 400GHZ (ITU)

 Microwave frequency range

– 1 GHz to 40 GHz
– Directional beams possible
– Suitable for point-to-point transmission
– Used for satellite communications
 Radio frequency range
– 30 MHz to 1 GHz
– Suitable for omnidirectional applications
 Infrared frequency range
– Roughly, 3x1011 to 2x1014 Hz
– Useful in local point-to-point multipoint applications within confined
areas
15
Frequencies for mobile communication
 VHF-/UHF-ranges for mobile radio
– simple, small antenna for cars
– deterministic propagation characteristics, reliable connections
 SHF and higher for directed radio links, satellite
communication
– small antenna, focusing
– large bandwidth available
 Wireless LANs use frequencies in UHF to SHF spectrum
– some systems planned up to EHF
– limitations due to absorption by water and oxygen molecules
(resonance frequencies)
• weather dependent fading, signal loss caused by heavy
rainfall etc.

16
 Frequency regulations
 Frequencies from 9KHz to 300 MHZ in high demand
(especially VHF: 30-300MHZ)
 Two unlicensed bands
– Industrial, Science, and Medicine (ISM): 2.4 GHz
– Unlicensed National Information Infrastructure (UNII): 5.2 GHz
 Different agencies license and regulate
– www.fcc.gov - US
– www.etsi.org - Europe
– www.wpc.dot.gov.in - India
– www.itu.org - International co-ordination
 Regional, national, and international issues
 Procedures for military, emergency, air traffic control, etc

17
Wireless transmission

Antenna Antenna
Transmitter Receiver

 Wireless communication systems consist of:

– Transmitters
– Antennas: radiates electromagnetic energy into air
– Receivers
 In some cases, transmitters and receivers are
on same device, called transceivers.
18
 Transmitters
Antenna
Amplifier Mixer Filter Amplifier
Source
Oscillator Transmitter

Suppose you want to generate a signal that is sent at 900 MHz and
the original source generates a signal at 300 MHz.
•Amplifier - strengthens the initial signal
•Oscillator - creates a carrier wave of 600 MHz
•Mixer - combines signal with oscillator and produces 900 MHz
(also does modulation, etc)
•Filter - selects correct frequency
•Amplifier - Strengthens the signal before sending it
19
Antennas
Antennas
 An antenna is an electrical conductor or system of
conductors to send/receive RF signals
– Transmission - radiates electromagnetic energy into space
– Reception - collects electromagnetic energy from space
 In two-way communication, the same antenna can be
used for transmission and reception

Omnidirectional Antenna Directional Antenna

(lower frequency) (higher frequency)

21
Antennas: isotropic radiator
 Radiation and reception of electromagnetic waves,
coupling of wires to space for radio transmission
 Isotropic radiator: equal radiation in all directions
(three dimensional) - only a theoretical reference
antenna
 Real antennas always have directive effects
(vertically and/or horizontally)
 Radiation pattern: measurement of radiation around
an antenna
z
y z

y x ideal
x isotropic
radiator

22
Antennas: simple dipoles
 Real antennas are not isotropic radiators
– dipoles with lengths /4 on car roofs or /2 (Hertzian dipole)
 shape of antenna proportional to wavelength
 Gain: maximum power in the direction of the main lobe
compared to the power of an isotropic radiator (with the
same average power)

/4 /2

y y z

simple
x z x dipole
side view (xy-plane) side view (yz-plane) top view (xz-plane)
23
Antennas: directed and sectorized
 Often used for microwave connections or base stations
for mobile phones (e.g., radio coverage of a valley)
y y z

directed
x z x antenna

side view (xy-plane) side view (yz-plane) top view (xz-plane)

z
z

x
sectorized
x antenna

top view, 3 sector top view, 6 sector

24
Antenna models

In Omni Mode:
 Nodes receive signals with gain Go

In Directional Mode:
 Capable of beamforming in specified direction
 Directional Gain Gd (Gd > Go)

25
Directional communication

Received Power  (Transmit power)

*(Tx Gain) * (Rx Gain)
Directional gain is higher

Directional antennas useful for:

 Increase “range”, keeping transmit power constant
 Reduce transmit power, keeping range comparable
with omni mode
26
Comparison of omni and directional

Issues Omni Directional

Spatial Reuse Low High

Connectivity Low High

Interference Omni Directional

Cost & Complexity Low High

27
Antennas: diversity
 Grouping of 2 or more antennas
– multi-element antenna arrays
 Antenna diversity
– switched diversity, selection diversity
• receiver chooses antenna with largest output
– diversity combining
• combine output power to produce gain
• cophasing needed to avoid cancellation

/2 /2
/4 /2 /4 /2

+ +

ground plane
28
Signal Propagation and Modulation
Signals
 physical representation of data
 function of time and location
 signal parameters: parameters representing the value of
data
 classification
– continuous time/discrete time
– continuous values/discrete values
– analog signal = continuous time and continuous values
– digital signal = discrete time and discrete values
 signal parameters of periodic signals:
period T, frequency f=1/T, amplitude A, phase shift 
– sine wave as special periodic signal for a carrier:

s(t) = At sin(2  ft t + t)

30
Signal propagation ranges

 Transmission range
– communication possible
– low error rate
sender
 Detection range
– detection of the signal transmission
possible
distance
– no communication detection
possible
 Interference range interference

– signal may not be

detected
– signal adds to the
background noise
31
Attenuation: Propagation & Range

32
Attenuation

 Strength of signal falls off with distance over

transmission medium
 Attenuation factors for unguided media:
– Received signal must have sufficient strength so that
circuitry in the receiver can interpret the signal
– Signal must maintain a level sufficiently higher than
noise to be received without error
– Attenuation is greater at higher frequencies, causing
distortion
 Approach: amplifiers that strengthen higher
frequencies
33
Signal propagation
 Propagation in free space always like light (straight line)
 Receiving power proportional to 1/d²
(d = distance between sender and receiver)
 Receiving power additionally influenced by
– fading (frequency dependent)
– shadowing
– reflection at large obstacles
– refraction depending on the density of a medium
– scattering at small obstacles
– diffraction at edges

shadowing reflection refraction scattering diffraction

34
Multipath propagation
 Signal can take many different paths between sender and receiver due
to reflection, scattering, diffraction

multipath
LOS pulses pulses

signal at sender
 Time dispersion: signal is dispersed over time signal at receiver
  interference with “neighbor” symbols, Inter Symbol
Interference (ISI)
 The signal reaches a receiver directly and phase shifted
  distorted signal depending on the phases of the different parts

35
Effects of mobility
 Channel characteristics change over time and location
– signal paths change
– different delay variations of different signal parts
– different phases of signal parts
  quick changes in the power received
power long term
(short term fading) fading

 Additional changes in
– distance to sender
– obstacles further away t
short term fading
  slow changes in the average power
received (long term fading)
36
Propagation modes Signal

Transmission Receiving
Antenna Antenna
Earth
a) Ground Wave Propagation
Ionosphere

Signal
b) Sky Wave Propagation
Earth

Signal
c) Line-of-Sight Propagation
Earth

37
Modulation
 Digital modulation
– digital data is translated into an analog signal (baseband)
– ASK, FSK, PSK
– differences in spectral efficiency, power efficiency, robustness
 Analog modulation
– shifts center frequency of baseband signal up to the radio
carrier
 Motivation
– smaller antennas (e.g., /4)
– Frequency Division Multiplexing
– medium characteristics
 Basic schemes
– Amplitude Modulation (AM)
– Frequency Modulation (FM)
– Phase Modulation (PM)
38
Modulation and demodulation
analog
baseband
digital
signal
data digital analog
101101001 modulation modulation radio transmitter

radio
carrier

analog
baseband
digital
signal
analog synchronization data
demodulation decision 101101001 radio receiver

radio
carrier

39
Digital modulation
 Modulation of digital signals known as Shift Keying
 Amplitude Shift Keying (ASK): 1 0 1

– very simple
– low bandwidth requirements
– very susceptible to interference t

 Frequency Shift Keying (FSK): 1 0 1

– needs larger bandwidth

t
 Phase Shift Keying (PSK):
– more complex
1 0 1
– robust against interference
 Many advanced variants
t

40
Multiplexing Mechanisms
Multiplexing
channels ki

k1 k2 k3 k4 k5 k6

c
 Multiplexing in 4 dimensions
t c
– space (si)
t
– time (t) s1
f
– frequency (f) s2
– code (c) f
c
t
 Goal: multiple use
of a shared medium s3
f

 Important: guard spaces needed!

42
Frequency multiplex
 Separation of the whole spectrum into smaller frequency
bands
 A channel gets a certain band of the spectrum for the whole
time
 Advantages:
k1 k2 k3 k4 k5 k6
 no dynamic coordination
necessary c

 works also for analog signals f

 Disadvantages:
 waste of bandwidth
if the traffic is
distributed unevenly
 inflexible t

 guard spaces
43
Time multiplex
 A channel gets the whole spectrum for a certain
amount of time

 Advantages:
 only one carrier in the
medium at any time k1 k2 k3 k4 k5 k6

 throughput high even

for many users c
f

 Disadvantages:
 precise
synchronization
necessary t

44
Time and frequency multiplex

 Combination of both methods

 A channel gets a certain frequency band for a certain
amount of time
 Example: GSM k1 k2 k3 k4 k5 k6
 Advantages:
c
– better protection against
tapping f
– protection against frequency
selective interference
– higher data rates compared to
code multiplex
 but: precise coordination
t
required
45
Code multiplex
 Each channel has a unique code
 All channels use the same k 1 k2 k3 k4 k5 k6
spectrum at the same time
 Advantages: c
– bandwidth efficient
– no coordination and synchronization
necessary
– good protection against interference
and tapping f
 Disadvantages:
– lower user data rates
– more complex signal regeneration
 Implemented using spread
t
spectrum technology

46
CDMA Example
– D = rate of data signal
– Break each bit into k chips
• Chips are a user-specific fixed pattern
– Chip data rate of new channel = kD

 If k=6 and code is a sequence of 1s and -1s

– For a ‘1’ bit, A sends code as chip pattern
• <c1, c2, c3, c4, c5, c6>
– For a ‘0’ bit, A sends complement of code
• <-c1, -c2, -c3, -c4, -c5, -c6>
 Receiver knows sender’s code and performs electronic
decode function
Su d  d1c1  d 2 c 2  d 3 c3  d 4 c 4  d 5 c5  d 6 c6
• <d1, d2, d3, d4, d5, d6> = received chip pattern
• <c1, c2, c3, c4, c5, c6> = sender’s code
47
CDMA Example

 User A code = <1, –1, –1, 1, –1, 1>

– To send a 1 bit = <1, –1, –1, 1, –1, 1>
– To send a 0 bit = <–1, 1, 1, –1, 1, –1>
 User B code = <1, 1, –1, – 1, 1, 1>
– To send a 1 bit = <1, 1, –1, –1, 1, 1>
 Receiver receiving with A’s code
– (A’s code) x (received chip pattern)
• User A ‘1’ bit: 6 -> 1
• User A ‘0’ bit: -6 -> 0
• User B ‘1’ bit: 0 -> unwanted signal ignored

48
Spread spectrum technology
 Problem of radio transmission: frequency dependent
fading can wipe out narrow band signals for duration of
the interference
 Solution: spread the narrow band signal into a broad
band signal using a special code - protection against
narrow band interference

power interference spread power signal

signal
spread
detection at interference
receiver

f f

 Side effects:
– coexistence of several signals without dynamic coordination
– tap-proof
 Alternatives: Direct Sequence, Frequency Hopping
49
Spread-spectrum communications

50
Source: Intersil
 Effects of spreading and interference
dP/df dP/df

user signal
i) ii) broadband interference
narrowband interference
f f
sender
dP/df dP/df dP/df

iii) iv) v)
f f f
receiver

51
DSSS properties

52
Source: Intersil
DSSS (Direct Sequence)
 XOR of the signal with pseudo-random number
(chipping sequence)
– many chips per bit (e.g., 128) result in higher bandwidth of the
signal tb
 Advantages user data
– reduces frequency selective
0 1 XOR
fading
tc
– in cellular networks
chipping
• base stations can use the sequence
same frequency range 01101010110101 =
• several base stations can resulting
detect and recover the signal signal
• soft handover 01101011001010
 Disadvantages tb: bit period
– precise power control necessary tc: chip period

53
DSSS Transmit/Receive
spread
spectrum transmit
user data signal signal
X modulator

chipping radio
sequence carrier

transmitter

correlator
lowpass sampled
received filtered products sums
signal signal data
demodulator X integrator decision

radio chipping
carrier sequence

receiver

54
Frequency Hopping Spread
Spectrum (FHSS)
 Signal is broadcast over seemingly random series of radio
frequencies
 Signal hops from frequency to frequency at fixed intervals
 Channel sequence dictated by spreading code
 Receiver, hopping between frequencies in synchronization
with transmitter, picks up message
 Advantages
– Eavesdroppers hear only unintelligible blips
– Attempts to jam signal on one frequency succeed only at knocking
out a few bits

55
FHSS (Frequency Hopping)
 Discrete changes of carrier frequency
– sequence of frequency changes determined via pseudo random
number sequence
 Two versions
– Fast Hopping: several frequencies per user bit
– Slow Hopping: several user bits per frequency
 Advantages
– frequency selective fading and interference limited to short
period
– simple implementation
– uses only small portion of spectrum at any time
 Disadvantages
– not as robust as DSSS
– simpler to detect
56
Slow and Fast FHSS
tb

user data

0 1 0 1 1 t
f
td
f3 slow
f2 hopping
(3 bits/hop)
f1

td t
f

f3 fast
f2 hopping
(3 hops/bit)
f1

tb: bit period td: dwell time

57
FHSS Transmit/Receive
narrowband spread
signal transmit
user data signal
modulator modulator

frequency hopping
synthesizer sequenc
transmitter e

narrowband
received signal
signal data
demodulator demodulator

hopping frequency
sequenc synthesizer
e receiver

58
 OFDM (Orthogonal Frequency Division)
 Parallel data transmission on several
orthogonal subcarriers with lower rate
c
f
k3

Maximum of one subcarrier frequency appears exactly at a frequency

where all other subcarriers equal zero
 superposition of frequencies in the same frequency range
Amplitude subcarrier: sin(x)
SI function= x

59
OFDM
 Properties
– Lower data rate on each subcarrier  less ISI
– interference on one frequency results in interference of one
subcarrier only
– no guard space necessary
– orthogonality allows for signal separation via inverse FFT on
receiver side
– precise synchronization necessary (sender/receiver)

 Advantages
– no equalizer necessary
– no expensive filters with sharp edges necessary
– better spectral efficiency (compared to CDM)
 Application
– 802.11a, HiperLAN2, ADSL

60
ALOHA

Stations transmit whenever they have data to send

 Detect collision or wait for acknowledgment
 If no acknowledgment (or collision), try again after a
random waiting time

Collision: If more than one node transmits at the

same time
If there is a collision, all nodes have to re-transmit
packets

61
Aloha/slotted Aloha
 Mechanism
– random, distributed (no central arbiter), time-multiplex
– Slotted Aloha additionally uses time-slots, sending must
always start at slot boundaries
collision
 Aloha
sender A
sender B
sender C
t

 Slotted Aloha collision

sender A
sender B
sender C
t

62
Slotted Aloha
 Time is divided into slots
– slot = one packet transmission time at least
 Master station generates synchronization
pulses for time-slots
 Station waits till beginning of slot to transmit

 Vulnerability Window reduced from 2T to T;

goodput doubles

63
Error control
Bit level error detection/correction
Single-bit, multi-bit or burst errors introduced
due to channel noise
– Detected using redundant information sent along
with data
 Full Redundancy:
– Send everything twice
– Simple but inefficient
 Common Schemes:
– Parity
– Cyclic Redundancy Check (CRC)
– Checksum

65
Error detection process

 Transmitter
– For a given frame, an error-detecting code (check bits)
is calculated from data bits
– Check bits are appended to data bits
 Receiver
– Separates incoming frame into data bits and check
bits
– Calculates check bits from received data bits
– Compares calculated check bits against received
check bits
– Detected error occurs if mismatch

66
Frame level error correction
 Problems in transmitting a sequence of frames
over a lossy link
– frame damage, loss, reordering, duplication, insertion
 Solutions:
– Forward Error Correction (FEC)
• Use of redundancy for packet level error correction
• Block Codes, Turbo Codes
– Automatic Repeat Request (ARQ)
• Use of acknowledgements and retransmission
• Stop and Wait; Sliding Window

67
Block Code (Error Correction)
 Hamming distance – for 2 n-bit binary sequences, the number of
different bits
– E.g., v1=011011; v2=110001; d(v1, v2)=3
 For each data block, create a codeword
 Send the codeword
 If the code is invalid, look for data with shortest hamming distance
(possibly correct code)
Datablock (k=2) Codeword (n=5)
00 00000
01 00111
10 11001
11 11110
Suppose you receive codeword 00100 (error)
Closest is 00000 (only one bit different)

 Efficient version: Turbo Codes

68
Stop and Wait ARQ
 Sender waits for ACK
(acknowledgement) after
transmitting each frame; keeps
copy of last frame.
 Receiver sends ACK if received
frame is error free.
 Sender retransmits frame if
ACKnot received before timer
expires.
 Simple to implement but may
waste bandwidth.

 Efficient Version: Sliding Window

69
Bandwidth and Delay
Bandwidth

 Amount of data that can be transmitted per unit time

– expressed in cycles per second, or Hertz (Hz) for analog
devices
– expressed in bits per second (bps) for digital devices
– KB = 2^10 bytes; Mbps = 10^6 bps

 Link v/s End-to-End

71
Bandwidth v/s bit width

72
Latency (delay)
 Time it takes to send message from point A to point B
– Latency = Propagation + Transmit
+ Queue
– Propagation = Distance /
SpeedOfLight
– Transmit = Size / Bandwidth

 Queueing not relevant for direct links

 Bandwidth not relevant if Size = 1 bit
 Software overhead can dominate when Distance is small

 RTT: round-trip time

73
Delay X Bandwidth product

 Relative importance of bandwidth and delay

 Small message: 1ms vs 100ms dominates

1Mbps vs 100Mbps

 Large message: 1Mbps vs 100Mbps dominates

1ms vs 100ms

74
Delay X Bandwidth product

100ms RTT and 45Mbps Bandwidth = 560 KB of data

75
TCP/IP Basics
Interconnection devices
Basic Idea: Transfer data from input to output
 Repeater
– Amplifies the signal received on input and transmits it on
output

 Switch
– Reads destination address of each packet and forwards
appropriately to specific port
– Layer 3 switches (IP switches) also perform routing functions

 Router
– decides routes for packets, based on destination address and
network topology
– Exchanges information with other routers to learn network
topology

77
Switched networks

78
TCP/IP layers
 Physical Layer:
– Transmitting bits over a channel.
– Deals with electrical and procedural interface to the
transmission medium.

 Data Link Layer:

– Transform the raw physical layer into a `link' for the
higher layer.
– Deals with framing, error detection, correction and
multiple access.

79
TCP/IP layers (contd.)
 Network Layer:
– Addressing and routing of packets.
– Deals with subnetting, route determination.

 Transport Layer:
– end-to-end connection characteristics.
– Deals with retransmissions, sequencing and
congestion control.

80
TCP/IP layers (contd.)

 Application Layer:
– ``application'' protocols.
– Deals with providing services to users and
application developers.

 Protocols are the building blocks of a network

architecture.

81
Lower layer services
 Unacknowledged connectionless service
– No acknowledgements, no connection
– Error recovery up to higher layers
– For low error-rate links or voice traffic

 Acknowledged connectionless service

– Acknowledgements improve reliability
– For unreliable channels. e.g.: wireless systems

 Acknowledged connection-oriented service

– Equivalent of reliable bit-stream; in-order delivery
– Connection establishment and release
– Inter-router traffic

82
 Generic router architecture
Data Hdr Header Processing 1 1 Queue
Lookup Update
IP Address Header Packet

Address Buffer
Address Buffer
Table Memory
Table Memory

Data Hdr Header Processing 2 2 NQueue

times line
Lookup
IP Address
Update
Header
rate
Packet

Address Buffer
Address Buffer
Table Memory
Table Memory

N times line rate

Header Processing
Data Hdr N N Queue
Lookup Update
IP Address Header Packet

Address Buffer
Address Buffer
Table Memory
Table Memory
83
Typical TCP behaviour
14
Congestion
12
Congestion Window size

avoidance
10
(segments)

8 Slow start
6 threshold
Slow start
4
2
0
0 1 2 3 4 5 6 7 8
Time (round trips)
84
 Slow start phase
Host A Host B
• initialize: one segm
ent

RTT
• Cwnd = 1
two segm
• for (each ACK) ents

• Cwnd++
• until four segm
ents

• loss detection OR
•Cwnd > ssthresh
time

85
Congestion avoidance phase
Host A Host B
/* Cwnd > threshold */ four segm
ents
• Until (loss detection) {

RTT
every w ACKs:
Cwnd++ five segm
ents
}
• ssthresh = Cwnd/2
• Cwnd = 1
• perform slow start
1
time

86
TCP: Fast retransmit and Fast recovery
10
advertised window
8
Window size (segments)

6
4 After fast recovery
2
0

Time (round trips)

87
802.11 (WiFi) Overview
Wireless LANs

 Infrared (IrDA) or radio links (Wavelan)

 Advantages
– very flexible within the reception area
– Ad-hoc networks possible
– (almost) no wiring difficulties
 Disadvantages
– low bandwidth compared to wired networks
– many proprietary solutions

 Infrastructure v/s ad-hoc networks (802.11)

89
 Infrastructure vs. Ad hoc networks
infrastructure
network
AP: Access Point
AP

AP wired network
AP

ad-hoc network

90
Source: Schiller
 Difference between wired and wireless

Ethernet LAN Wireless LAN

B
A B C
A C

 If both A and C sense the channel to be idle at the

same time, they send at the same time.
 Collision can be detected at sender in Ethernet.
 Half-duplex radios in wireless cannot detect collision at
sender.
91
Carrier Sense Multiple Access (CSMA)
 Listen before you speak
 Check whether the medium is active before sending a
packet (i.e carrier sensing)
 If medium idle, then transmit
 If collision happens, then detect and resolve

 If medium is found busy, transmission follows:

– 1- persistent
– P- persistent
– Non-persistent

92
Collision detection (CSMA/CD)
 All aforementioned scheme can suffer from collision
 Device can detect collision
– Listen while transmitting
– Wait for 2 * propagation delay
 On collision detection wait for random time before
retrying
 Binary Exponential Backoff Algorithm
– Reduces the chances of two waiting stations picking the
same random time

93
Binary Exponential Backoff
1.On detecting 1st collision for packet x
station A chooses a number r between 0 and 1.
wait for r * slot time and transmit.
Slot time is taken as 2 * propagation delay
k. On detecting kth collision for packet x
choose r between 0,1,..,(2k –1)

 When value of k becomes high (10), give up.

 Randomization increase with larger window, but delay
increases.

94
Hidden Terminal Problem

A B C
– A and C cannot hear each other.
– A sends to B, C cannot receive A.
– C wants to send to B, C senses a “free” medium
(CS fails)
– Collision occurs at B.
– A cannot receive the collision (CD fails).
– A is “hidden” for C.

95
Effect of interference range

Transmission from 1  2 will fail

96
Solution for Hidden Terminals
 A first sends a Request-to-Send (RTS) to B
 On receiving RTS, B responds Clear-to-Send (CTS)
 Hidden node C overhears CTS and keeps quiet
– Transfer duration is included in both RTS and CTS
 Exposed node overhears a RTS but not the CTS
– D’s transmission cannot interfere at B

RTS RTS
D A B C
CTS CTS
DATA
97
 Components of IEEE 802.11
architecture
 The basic service set (BSS) is the basic building
block of an IEEE 802.11 LAN
 The ovals can be thought of as the coverage area
within which member stations can directly
communicate
 The Independent BSS (IBSS) is the simplest LAN. It
may consist of as few as two stations
ad-hoc network BSS1 BSS2

98
 802.11 - ad-hoc network (DCF)
802.11 LAN

STA1  Direct communication

BSS1 STA3
within a limited range
– Station (STA):
terminal with access
STA2
mechanisms to the
wireless medium
– Basic Service Set (BSS):
BSS2 group of stations using the
same radio frequency
STA5

STA4 802.11 LAN

99
Source: Schiller
 802.11 - infrastructure network (PCF)
Station (STA)
802.11 LAN – terminal with access
802.x LAN
mechanisms to the wireless
medium and radio contact to the
access point
STA1
BSS1 Basic Service Set (BSS)
Access Portal – group of stations using the same
Point radio frequency
Access Point
Distribution System
– station integrated into the
Access wireless LAN and the distribution
ESS Point system
Portal
BSS2
– bridge to other (wired) networks
Distribution System
– interconnection network to form
one logical network (EES:
STA2 802.11 LAN STA3 Extended Service Set) based
on several BSS

100
Source: Schiller
802.11- in the TCP/IP stack
fixed terminal
mobile terminal

server

infrastructure network

access point

application application
TCP TCP
IP IP
LLC LLC LLC
802.11 MAC 802.11 MAC 802.3 MAC 802.3 MAC
802.11 PHY 802.11 PHY 802.3 PHY 802.3 PHY

101
802.11 - MAC layer

 Traffic services
– Asynchronous Data Service (mandatory) – DCF
– Time-Bounded Service (optional) - PCF

 Access methods
– DCF CSMA/CA (mandatory)
• collision avoidance via randomized back-off mechanism
• ACK packet for acknowledgements (not for broadcasts)
– DCF w/ RTS/CTS (optional)
• avoids hidden terminal problem
– PCF (optional)
• access point polls terminals according to a list

102
802.11 - CSMA/CA
contention window
DIFS DIFS (randomized back-off
mechanism)

medium busy next frame

direct access if t
medium is free  DIFS slot time

– station ready to send starts sensing the medium (Carrier Sense

based on CCA, Clear Channel Assessment)
– if the medium is free for the duration of an Inter-Frame Space
(IFS), the station can start sending (IFS depends on service type)
– if the medium is busy, the station has to wait for a free IFS, then
the station must additionally wait a random back-off time
(collision avoidance, multiple of slot-time)
– if another station occupies the medium during the back-off time
of the station, the back-off timer stops (fairness)

103
 802.11 –CSMA/CA example
DIFS DIFS DIFS DIFS
boe bor boe bor boe busy
station1

boe busy
station2

busy
station3

boe busy boe bor

station4

boe bor boe busy boe bor

station5
t

busy medium not idle (frame, ack etc.) boe elapsed backoff time

packet arrival at MAC bor residual backoff time

104
 802.11 –RTS/CTS
 station can send RTS with reservation parameter after waiting for DIFS
(reservation determines amount of time the data packet needs the
medium)
 acknowledgement via CTS after SIFS by receiver (if ready to receive)
 sender can now send data at once, acknowledgement via ACK
 other stations store medium reservations distributed via RTS and CTS
DIFS
RTS data
sender
SIFS SIFS
CTS SIFS ACK
receiver

NAV (RTS) DIFS

other NAV (CTS) data
stations t
defer access contention

105
802.11 - PCF I

t0 t1
SuperFrame

medium busy PIFS SIFS SIFS

D1 D2
point
coordinator SIFS SIFS
U1 U2
wireless
stations
stations‘ NAV
NAV

106
802.11 - PCF II

t2 t3 t4

PIFS SIFS
D3 D4 CFend
point
coordinator SIFS
U4
wireless
stations
stations‘ NAV
NAV contention free period contention t
period

107
CFP structure and Timing

108
802.11 - MAC management
 Synchronization
– try to find a LAN, try to stay within a LAN
– timer etc.
 Power management
– sleep-mode without missing a message
– periodic sleep, frame buffering, traffic measurements
 Association/Reassociation
– integration into a LAN
– roaming, i.e. change networks by changing access points
– scanning, i.e. active search for a network
 MIB - Management Information Base
– managing, read, write

109
 802.11 - Channels, association
 802.11b: 2.4GHz-2.485GHz spectrum divided into 11
channels at different frequencies
– AP admin chooses frequency for AP
– interference possible: channel can be same as that
chosen by neighboring AP!
 host: must associate with an AP
– scans channels, listening for beacon frames containing
AP’s name (SSID) and MAC address
– selects AP to associate with
– may perform authentication
– will typically run DHCP to get IP address in AP’s subnet

110
 802.11 variants

802.11i LLC
security
WEP MAC
802.11f MAC Mgmt
Inter Access Point Protocol

802.11e MIB
QoS enhancements
PHY
DSSS FH IR

OFDM
802.11b
5,11 Mbps
802.11a
6,9,12,18,24
802.11g 36,48,54 Mbps
20+ Mbps

111
 802.11 Market Evolution
802.11

Industry Campus Public hotspots Broadband access

Enterprise
Verticals Networking Mobile Operators to home

Warehouses

Factory floors

Medical

Remote data Freedom from

entry; Mobile user wires for laptop Revenue generation Untested
business population users; opportunity; proposition;
process without any productivity low cost alternative attempts are on-
efficiency office space enhancement to GPRS going
improvement
112
Public WLANs

 Provide significantly higher data rates than wide-

area wireless networks
 Could take advantages of both WLAN and wide-
area radio technologies to create new services
and reduce networking costs
 Public WLANs are the first wave of all-IP radio
access networks
 New and innovative business models for
providing public mobile services

113
Worldwide WLAN sales

114
802.16 (WiMaX) Overview
Motivation for 802.16

 Broadband:
– A transmission facility having a bandwidth sufficient to
carry multiple voice, video or data, simultaneously.
– High-capacity fiber to every user is expensive.

 Broadband Wireless Access:

– provides “First-mile” network access to buildings.
– Cost effective and easy deployment.

116
IEEE 802.16
 WirelessMAN air interface
– for fixed point to multi-point BWA

 Broad bandwidth: 10-66 GHz

– Channel as wide as 28 MHz and
– Data rate upto 134 Mbps

 MAC designed for efficient use of spectrum

– Bandwidth on demand
– QoS Support

117
802.16 Architecture

118
Channel model
 Two Channels: Downlink and Uplink
 Supports both Time Division Duplexing and
Frequency Division Duplexing

 Base station maps downstream traffic onto time

slots with individual subscriber stations allocated
time slot serially
 Uplink is shared between a number of subscriber
stations by Time Division Multiple Access

119
Network initialization of SS

 Acquires downlink and uplink channel.

 Perform initial ranging, negotiate basic capabilities.
 Perform registration and authorization.
 Establish IP connectivity and time of day.
 Transfer operational parameters.
 Set up connections.

120
Bandwidth requests and grants
 Ways
– Bandwidth request packet.
– Piggybacking bandwidth request with normal data
packet.
 Request can be made during time slot assigned by base
station for sending request or data.
 Grant modes
– Grant per connection.
– Grant per subscriber station.
 Grant per subscriber station is more efficient and scalable
but complex than Grant per connection.

121
Uplink scheduling services
 Unsolicited grant service
– Support applications generating constant bit rate traffic
periodically.
– Provides fixed bandwidth at periodic intervals.

 Real-time polling service

– Supports real-time applications generating variable bit rate
traffic periodically.
– Offers periodic opportunities to request bandwidth.

 Non-real-time polling service

– Supports non-real-time applications generating variable bit rate
traffic regularly.
– Offers opportunities to request bandwidth regularly.

 Best effort
– Offers no guarantee. 122
802.16: Summary
 Higher throughput at longer ranges (up to 50 km)
– Better bits/second/Hz at longer ranges

 Scalable system capacity

– Easy addition of channels maximizes cell capacity
– Flexible channel bandwidths accommodate allocations for both
licensed and license-exempt spectrums

 Coverage
– Standards-based mesh and smart antenna support
– Adaptive modulation enables tradeoff of bandwidth for range

 Quality of Service
– Grant / request MAC supports voice and video
– Differentiated service levels: E1/T1 for business, best effort for
residential
123
 IEEE 802.16 Standard
802.16 802.16a/REVd 802.16e

Completed Dec 2001 802.16a: Jan 2003 Estimate 2006

802.16REVd: Q3’04

Spectrum 10 - 66 GHz < 11 GHz < 6 GHz

Channel Conditions Line of sight only Non line of sight Non line of sight

Bit Rate 32 – 134 Mbps at 28MHz Up to 75 Mbps at 20MHz Up to 15 Mbps at 5MHz

channelization channelization channelization

Modulation QPSK, 16QAM and OFDM 256 sub-carriersQPSK, Same as 802.16a

64QAM 16QAM, 64QAM

Mobility Fixed Fixed Pedestrian mobility –

regional roaming
Channel 20, 25 and 28 MHz Selectable channel bandwidths Same as 802.16a with
Bandwidths between 1.25 and 20 MHz uplink sub-channels to
conserve power
Typical Cell Radius 1-3 miles 3 to 5 miles; max range 30 miles 1-3 miles
based on tower height, antenna
gain and power transmit

124
802.11 Internals
Wireless LANs vs. Wired LANs

 Destination address does not equal destination

location
 The media impact the design
– wireless LANs intended to cover reasonable
geographic distances must be built from basic
coverage blocks
 Impact of handling mobile (and portable)
stations
– Propagation effects
– Mobility management
– Power management

126
Wireless Media
 Physical layers used in wireless networks
– have neither absolute nor readily observable boundaries
outside which stations are unable to receive frames
– are unprotected from outside signals
– communicate over a medium significantly less reliable
than the cable of a wired network
– have dynamic topologies
– lack full connectivity and therefore the assumption
normally made that every station can hear every other
station in a LAN is invalid (i.e., STAs may be “hidden”
from each other)
– have time varying and asymmetric propagation properties

127
Infrastructure vs. Ad hoc WLANs
infrastructure
network
AP: Access Point
AP

AP wired network
AP

ad-hoc network

128
Source: Schiller
IEEE 802.11
 Wireless LAN standard defined in the unlicensed
spectrum (2.4 GHz and 5 GHz U-NII bands)

 33cm 12cm 5cm

26 MHz 83.5 MHz 200 MHz

902 MHz 2.4 GHz 5.15 GHz

928 MHz 2.4835 GHz 5.35 GHz

 Standards covers the MAC sublayer and PHY layers

 Three different physical layers in the 2.4 GHz band
– FHSS, DSSS and IR
 OFDM based Phys layer in the 5 GHz band (802.11a)
129
802.11- in the TCP/IP stack
fixed terminal
mobile terminal

server

infrastructure network

access point

application application
TCP TCP
IP IP
LLC LLC LLC
802.11 MAC 802.11 MAC 802.3 MAC 802.3 MAC
802.11 PHY 802.11 PHY 802.3 PHY 802.3 PHY

130
Functional Diagram

131
 802.11 - Layers and functions

 MAC  PLCP Physical Layer

– access mechanisms, Convergence Protocol
fragmentation, encryption – clear channel assessment
 MAC Management signal (carrier sense)
 PMD Physical Medium
– synchronization, roaming, Dependent
MIB, power management – modulation, coding
 PHY Management

Station Management
LLC – channel selection, MIB
DLC

 Station Management
MAC MAC Management
– coordination of all
PLCP management functions
PHY

PHY Management
PMD

132
 802.11 - infrastructure network
Station (STA)
802.11 LAN – terminal with access
802.x LAN
mechanisms to the wireless
medium and radio contact to the
access point
STA1
BSS1 Basic Service Set (BSS)
Access Portal – group of stations using the same
Point radio frequency
Access Point
Distribution System
– station integrated into the
Access wireless LAN and the distribution
ESS Point system
Portal
BSS2
– bridge to other (wired) networks
Distribution System
– interconnection network to form
one logical network (EES:
STA2 802.11 LAN STA3 Extended Service Set) based
on several BSS

133
Source: Schiller
Distribution System (DS) concepts
 The Distribution system interconnects multiple BSSs
 802.11 standard logically separates the wireless
medium from the distribution system – it does not
preclude, nor demand, that the multiple media be
same or different
 An Access Point (AP) is a STA that provides access
to the DS by providing DS services in addition to
acting as a STA.
 Data moves between BSS and the DS via an AP
 The DS and BSSs allow 802.11 to create a wireless
network of arbitrary size and complexity called the
Extended Service Set network (ESS)

134
Extended Service Set network

135
Source: Intersil
802.11 - Physical layer
 3 versions of spread spectrum: 2 radio (typ. 2.4 GHz), 1 IR
– data rates 1 or 2 Mbps
 FHSS (Frequency Hopping Spread Spectrum)
– spreading, despreading, signal strength, typically 1 Mbps
– min. 2.5 frequency hops/s (USA), two-level GFSK modulation
 DSSS (Direct Sequence Spread Spectrum)
– DBPSK modulation for 1 Mbps (Differential Binary Phase Shift
Keying), DQPSK for 2 Mbps (Differential Quadrature PSK)
– preamble and header of a frame is always transmitted with 1
Mbps, rest of transmission 1 or 2 Mbps
– chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1
(Barker code)
– max. radiated power 1 W (USA), 100 mW (EU), min. 1mW
 Infrared
– 850-950 nm, diffuse light, typ. 10 m range
– carrier detection, energy detection, synchronization
136
Spread-spectrum communications

137
Source: Intersil
DSSS Barker Code modulation

138
Source: Intersil
DSSS properties

139
Source: Intersil
802.11 - MAC layer

 Traffic services
– Asynchronous Data Service (mandatory) – DCF
– Time-Bounded Service (optional) - PCF

 Access methods
– DCF CSMA/CA (mandatory)
• collision avoidance via randomized back-off mechanism
• ACK packet for acknowledgements (not for broadcasts)
– DCF w/ RTS/CTS (optional)
• avoids hidden/exposed terminal problem, provides
reliability
– PCF (optional)
• access point polls terminals according to a list

140
802.11 - CSMA/CA
contention window
DIFS DIFS (randomized back-off
mechanism)

medium busy next frame

direct access if t
medium is free  DIFS slot time

– station which has data to send starts sensing the medium

(Carrier Sense based on CCA, Clear Channel Assessment)
– if the medium is free for the duration of an Inter-Frame Space
(IFS), the station can start sending (IFS depends on service
type)
– if the medium is busy, the station has to wait for a free IFS plus
an additional random back-off time (multiple of slot-time)
– if another station occupies the medium during the back-off time
of the station, the back-off timer stops (fairness)

141
802.11 DCF – basic access
 If medium is free for DIFS time, station sends data
 receivers acknowledge at once (after waiting for SIFS) if the
packet was received correctly (CRC)
 automatic retransmission of data packets in case of
transmission errors

DIFS
data
sender
SIFS
ACK
receiver
DIFS
other data
stations t
waiting time contention

142
 802.11 –RTS/CTS
 If medium is free for DIFS, station can send RTS with reservation
parameter (reservation determines amount of time the data packet
needs the medium)
 acknowledgement via CTS after SIFS by receiver (if ready to receive)
 sender can now send data at once, acknowledgement via ACK
 other stations store medium reservations distributed via RTS and CTS
DIFS
RTS data
sender
SIFS SIFS
CTS SIFS ACK
receiver

NAV (RTS) DIFS

other NAV (CTS) data
stations t
defer access contention

143
802.11 - Carrier Sensing
 In IEEE 802.11, carrier sensing is performed
– at the air interface (physical carrier sensing), and
– at the MAC layer (virtual carrier sensing)
 Physical carrier sensing
– detects presence of other users by analyzing all detected
packets
– Detects activity in the channel via relative signal strength
from other sources
 Virtual carrier sensing is done by sending MPDU duration
information in the header of RTS/CTS and data frames
 Channel is busy if either mechanisms indicate it to be
 Duration field indicates the amount of time (in microseconds)
required to complete frame transmission
 Stations in the BSS use the information in the duration field to
adjust their network allocation vector (NAV)
144
802.11 - Collision Avoidance
 If medium is not free during DIFS time..
 Go into Collision Avoidance: Once channel becomes
idle, wait for DIFS time plus a randomly chosen
backoff time before attempting to transmit
 For DCF the backoff is chosen as follows:
– When first transmitting a packet, choose a backoff interval
in the range [0,cw]; cw is contention window, nominally 31
– Count down the backoff interval when medium is idle
– Count-down is suspended if medium becomes busy
– When backoff interval reaches 0, transmit RTS
– If collision, then double the cw up to a maximum of 1024
 Time spent counting down backoff intervals is part of
MAC overhead
145
Example - backoff

B1 = 25 B1 = 5
wait data

data wait
B2 = 20 B2 = 15 B2 = 10

B1 and B2 are backoff intervals

cw = 31 at nodes 1 and 2

146
Backoff - more complex example
DIFS DIFS DIFS DIFS
boe bor boe bor boe busy
station1

boe busy
station2

busy
station3

boe busy boe bor

station4

boe bor boe busy boe bor

station5
t

busy medium not idle (frame, ack etc.) boe elapsed backoff time

packet arrival at MAC bor residual backoff time

147
Source: Schiller
802.11 - Priorities
 defined through different inter frame spaces – mandatory idle
time intervals between the transmission of frames
 SIFS (Short Inter Frame Spacing)
– highest priority, for ACK, CTS, polling response
– SIFSTime and SlotTime are fixed per PHY layer (10 s and 20
s respectively in DSSS)
 PIFS (PCF IFS)
– medium priority, for time-bounded service using PCF
– PIFSTime = SIFSTime + SlotTime
 DIFS (DCF IFS)
– lowest priority, for asynchronous data service
– DCF-IFS: DIFSTime = SIFSTime + 2xSlotTime

148
Solution to Hidden Terminals

 A first sends a Request-to-Send (RTS) to B

 On receiving RTS, B responds Clear-to-Send (CTS)
 Hidden node C overhears CTS and keeps quiet
– Transfer duration is included in both RTS and CTS
 Exposed node overhears a RTS but not the CTS
– D’s transmission cannot interfere at B

RTS RTS
D A B C
CTS CTS
DATA
149
802.11 - Reliability
 Use acknowledgements
– When B receives DATA from A, B sends an ACK
– If A fails to receive an ACK, A retransmits the DATA
– Both C and D remain quiet until ACK (to prevent collision of
ACK)
– Expected duration of transmission+ACK is included in
RTS/CTS packets

RTS RTS
D A B C
CTS CTS
DATA

ACK

150
802.11 - Congestion Control

 Contention window (cw) in DCF: Congestion

control achieved by dynamically choosing cw
 large cw leads to larger backoff intervals
 small cw leads to larger number of collisions

 Binary Exponential Backoff in DCF:

– When a node fails to receive CTS in response to
its RTS, it increases the contention window
• cw is doubled (up to a bound cwmax =1023)
– Upon successful completion data transfer, restore
cw to cwmin=31
151
 Fragmentation

DIFS
RTS frag1 frag2
sender
SIFS SIFS SIFS
CTS SIFS ACK1 SIFS ACK2
receiver

NAV (RTS)
NAV (CTS)
NAV (frag1) DIFS
other NAV (ACK1) data
stations t
contention

152
802.11 - MAC management
 Synchronization
– try to find a LAN, try to stay within a LAN
– timer etc.
 Power management
– sleep-mode without missing a message
– periodic sleep, frame buffering, traffic measurements
 Association/Reassociation
– integration into a LAN
– roaming, i.e. change networks by changing access points
– scanning, i.e. active search for a network
 MIB - Management Information Base
– managing, read, write

153
802.11 - Synchronization

 All STAs within a BSS are synchronized to a common

clock
– Infrastructure mode: AP is the timing master
• periodically transmits Beacon frames containing Timing
Synchronization function (TSF)
• Receiving stations accepts the timestamp value in TSF
– Ad hoc mode: TSF implements a distributed algorithm
• Each station adopts the timing received from any beacon
that has TSF value later than its own TSF timer
 This mechanism keeps the synchronization of the TSF
timers in a BSS to within 4 s plus the maximum
propagation delay of the PHY layer

154
Synchronization using a Beacon
(infrastructure mode)

beacon interval

B B B B
access
point
busy busy busy busy
medium
t
value of the timestamp B beacon frame

155
Source: Schiller
Synchronization using a Beacon
(ad-hoc mode)

beacon interval

B1 B1
station1

B2 B2
station2

busy busy busy busy

medium
t
value of the timestamp B beacon frame random delay

156
802.11 - Power management
 Idea: switch the transceiver off if not needed
 States of a station: sleep and awake
 Timing Synchronization Function (TSF)
– stations wake up at the same time
 Infrastructure
– Traffic Indication Map (TIM)
• list of unicast receivers transmitted by AP
– Delivery Traffic Indication Map (DTIM)
• list of broadcast/multicast receivers transmitted by AP
 Ad-hoc
– Ad-hoc Traffic Indication Map (ATIM)
• announcement of receivers by stations buffering frames
• more complicated - no central AP
• collision of ATIMs possible (scalability?)

157
802.11 - Energy Conservation
 Power Saving in infrastructure mode
– Nodes can go into sleep or standby mode
– An Access Point periodically transmits a beacon
indicating which nodes have packets waiting for them
– Each power saving (PS) node wakes up periodically
to receive the beacon
– If a node has a packet waiting, then it sends a PS-
Poll
• After waiting for a backoff interval in [0,CWmin]
– Access Point sends the data in response to PS-poll

158
Power saving with wake-up patterns
(infrastructure)
TIM interval DTIM interval

D B T T d D B
access
point
busy busy busy busy
medium

p d
station
t
T TIM D DTIM awake

B broadcast/multicast p PS poll d data transmission

to/from the station

159
Source: Schiller
Power saving with wake-up patterns
(ad-hoc)
ATIM
window beacon interval

B1 A D B1
station1

B2 B2 a d
station2

t
B beacon frame random delay A transmit ATIM D transmit data

awake a acknowledge ATIM d acknowledge data

160
 802.11 - Frame format
 Types
– control frames, management frames, data frames
 Sequence numbers
– important against duplicated frames due to lost ACKs
 Addresses
– receiver, transmitter (physical), BSS identifier, sender (logical)
 Miscellaneous
– sending time, checksum, frame control, data
bytes 2 2 6 6 6 2 6 0-2312 4
Frame Duration Address Address Address Sequence Address
Data CRC
Control ID 1 2 3 Control 4

version, type, fragmentation, security, ...

161
 802.11 frame: addressing

2 2 6 6 6 2 6 0 - 2312 4
frame address address address seq address
duration payload CRC
control 1 2 3 control 4

Address 3: used only

in ad hoc mode
Address 1: MAC address
of wireless host or AP Address 3: MAC address
to receive this frame of router interface to
which AP is attached
Address 2: MAC address
of wireless host or AP
transmitting this frame

162
802.11 frame: addressing

Internet
H1 R1 router
AP

R1 MAC addr AP MAC addr

dest. address source address

802.3 frame

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

802.11 frame
163
802.11 frame: more
frame seq #
duration of reserved
(for reliable ARQ)
transmission time (RTS/CTS)

2 2 6 6 6 2 6 0 - 2312 4
frame address address address seq address
duration payload CRC
control 1 2 3 control 4

2 2 4 1 1 1 1 1 1 1 1
Protocol To From More Power More
Type Subtype Retry WEP Rsvd
version AP AP frag mgt data

frame type
(RTS, CTS, ACK, data)

164
Types of Frames

 Control Frames
– RTS/CTS/ACK
– CF-Poll/CF-End
 Management Frames
– Beacons
– Probe Request/Response
– Association Request/Response
– Dissociation/Reassociation
– Authentication/Deauthentication
– ATIM
 Data Frames

165
802.11 - Roaming
 Bad connection in Infrastructure mode? Perform:
 scanning of environment
– listen into the medium for beacon signals or send probes into
the medium and wait for an answer
 send Reassociation Request
– station sends a request to a new AP(s)
 receive Reassociation Response
– success: AP has answered, station can now participate
– failure: continue scanning
 AP accepts Reassociation Request and
– signals the new station to the distribution system
– the distribution system updates its data base (i.e., location
information)
– typically, the distribution system now informs the old AP so it
can release resources

166
802.11 - Roaming within same subnet

 H1 remains in same IP
subnet: IP address can router
remain same
hub or
 switch: which AP is switch
associated with H1?
BBS 1
– self-learning
– switch will see frame from H1 AP 1
and “remember” which switch
AP 2
port can be used to reach H1
H1 BBS 2

167
802.11 - Point Coordination Function

168
Coexistence of PCF and DCF
 A Point Coordinator (PC) resides in the Access Point and
controls frame transfers during a Contention Free Period
(CFP)
 A CF-Poll frame is used by the PC to invite a station to
send data. Stations are polled from a list maintained by
the PC
 The CFP alternates with a Contention Period (CP) in
which data transfers happen as per the rules of DCF
 This CP must be large enough to send at least one
maximum-sized packet including RTS/CTS/ACK
 CFPs are generated at the CFP repetition rate
 The PC sends Beacons at regular intervals and at the
start of each CFP
 The CF-End frame signals the end of the CFP

169
CFP structure and Timing

170
802.11 - PCF I

t0 t1
SuperFrame

medium busy PIFS SIFS SIFS

D1 D2
point
coordinator SIFS SIFS
U1 U2
wireless
stations
stations‘ NAV
NAV

171
Source: Schiller
802.11 - PCF II

t2 t3 t4

PIFS SIFS
D3 D4 CFend
point
coordinator SIFS
U4
wireless
stations
stations‘ NAV
NAV contention free period contention t
period

172
Throughput – DCF vs. PCF

 Overheads to throughput and delay in DCF mode come from

losses due to collisions and backoff
 These increase when number of nodes in the network
increases
 RTS/CTS frames cost bandwidth but large data packets
(>RTS threshold) suffer fewer collisions
 RTC/CTS threshold must depend on number of nodes
 Overhead in PCF modes comes from wasted polls
 Polling mechanisms have large influence on throughput
 Throughput in PCF mode shows up to 20% variation with
other configuration parameters – CFP repetition rate
 Saturation throughput of DCF less than PCF in all studies
presented here (‘heavy load’ conditions)

173
 174
ICCC 2002
WLAN: IEEE 802.11b
Connection set-up time
 Data rate – Connectionless/always on
– 1, 2, 5.5, 11 Mbit/s, Quality of Service
depending on SNR
– Typ. Best effort, no
– User data rate max. approx. 6
guarantees (unless polling is
Mbit/s used, limited support in
 Transmission range products)
– 300m outdoor, 30m indoor Manageability
– Max. data rate ~10m indoor – Limited (no automated key
 Frequency distribution, sym. Encryption)
– Free 2.4 GHz ISM-band Special
 Security – Advantage: many installed
– Limited, WEP insecure, SSID systems, lot of experience,
available worldwide, free ISM-
 Cost band, many vendors,
– 100$ adapter, 250$ base integrated in laptops, simple
station, dropping system
 Availability – Disadvantage: heavy
– Many products, many vendors interference on ISM-band, no
service guarantees, slow
relative speed only 175
 IEEE 802.11b – PHY frame formats
Long PLCP PPDU format
128 16 8 8 16 16 variable bits
synchronization SFD signal service length HEC payload

PLCP preamble PLCP header

192 µs at 1 Mbit/s DBPSK 1, 2, 5.5 or 11 Mbit/s

Short PLCP PPDU format (optional)

56 16 8 8 16 16 variable bits
short synch. SFD signal service length HEC payload

PLCP preamble PLCP header

(1 Mbit/s, DBPSK) (2 Mbit/s, DQPSK)

96 µs 2, 5.5 or 11 Mbit/s

176
Channel selection (non-overlapping)
Europe (ETSI)

channel 1 channel 7 channel 13

2400 2412 2442 2472 2483.5

22 MHz [MHz]
US (FCC)/Canada (IC)

channel 1 channel 6 channel 11

2400 2412 2437 2462 2483.5

22 MHz [MHz]

177
WLAN: IEEE 802.11a
 Data rate
– 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending
 Connection set-up time
on SNR – Connectionless/always on
– User throughput (1500 byte packets): 5.3 (6),
18 (24), 24 (36), 32 (54)  Quality of Service
– 6, 12, 24 Mbit/s mandatory – Typ. best effort, no guarantees
 Transmission range (same as all 802.11 products)
– 100m outdoor, 10m indoor
• E.g., 54 Mbit/s up to 5 m, 48 up to 12 m, 36  Manageability
up to 25 m, 24 up to 30m, 18 up to 40 m, 12
up to 60 m – Limited (no automated key
 Frequency distribution, sym. Encryption)
– Free 5.15-5.25, 5.25-5.35, 5.725-5.825 GHz
ISM-band
 Special
 Security Advantages/Disadvantages
– Limited, WEP insecure, SSID – Advantage: fits into 802.x
 Cost standards, free ISM-band,
– 280$ adapter, 500$ base station
available, simple system, uses
 Availability
less crowded 5 GHz band
– Some products, some vendors
– Disadvantage: stronger shading
due to higher frequency, no QoS

178
IEEE 802.11a – PHY frame format

4 1 12 1 6 16 variable 6 variable bits

rate reserved length parity tail service payload tail pad

PLCP header

PLCP preamble signal data

12 1 variable symbols

6 Mbit/s 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s

179
OFDM in IEEE 802.11a
 OFDM with 52 used subcarriers (64 in total)
 48 data + 4 pilot
 312.5 kHz spacing
pilot 312.5 kHz

-26 -21 -7 -1 1 7 21 26 subcarrier

channel center frequency number

180
 Operating channels for 802.11a
36 40 44 48 52 56 60 64 channel

5150 5180 5200 5220 5240 5260 5280 5300 5320 5350 [MHz]
16.6 MHz

center frequency =
5000 + 5*channel number [MHz]
149 153 157 161 channel

5725 5745 5765 5785 5805 5825 [MHz]

16.6 MHz

181
WLAN: IEEE 802.11e
 802.11e: MAC Enhancements – QoS
– Enhance the current 802.11 MAC to expand support
for applications with Quality of Service requirements,
and in the capabilities and efficiency of the protocol.

 EDCF
– Contention Window based prioritization
• Real-time
• Best effort
– Virtual collision resolved in favor of higher priority

182
 Extending DCF: EDCF
EDCF improves upon DCF by
FTP flows + unprioritised prioritising traffic
VoIP: larger contention  Each traffic class can have
window
a different contention
Access window
to  Different traffic classes to
channe use different interframe
l spaces, called Arbitration
Prioritised VoIP calls : smaller Interframe Space (AIFS)
contention window

EDCF contention window

parameters
 VoIP (priority): 7-31
 FTP w/o priority: 32-1023
 VoIP w/o priority:32-1023
183
IEEE 802.11 Summary
 Infrastructure and ad hoc modes using DCF
 Carrier Sense Multiple Access
 Binary exponential backoff for collision avoidance and
congestion control
 Acknowledgements for reliability
 Power save mode for energy conservation
 Time-bound service using PCF
 Signaling packets for avoiding Exposed/Hidden terminal
problems, and for reservation
– Medium is reserved for the duration of the transmission
– RTS-CTS in DCF
– Polls in PCF

184
Mobile IP

185
Traditional Routing
 A routing protocol sets up a routing table in routers

 Routing protocol is typically based on Distance-Vector or

Link-State algorithms
186
Routing and Mobility
 Finding a path from a source to a destination

 Issues
– Frequent route changes
• amount of data transferred between route changes may
be much smaller than traditional networks
– Route changes may be related to host movement
– Low bandwidth links

 Goal of routing protocols

– decrease routing-related overhead
– find short routes
– find “stable” routes (despite mobility)
187
Mobile IP (RFC 3344): Motivation
 Traditional routing
– based on IP address; network prefix determines the subnet
– change of physical subnet implies
• change of IP address (conform to new subnet), or
• special routing table entries to forward packets to new subnet
 Changing of IP address
– DNS updates take to long time
– TCP connections break
– security problems
 Changing entries in routing tables
– does not scale with the number of mobile hosts and frequent
changes in the location
– security problems
 Solution requirements
– retain same IP address, use same layer 2 protocols
– authentication of registration messages, …
188
Mobile IP: Basic Idea

MN Router
S
3

Home
agent

Router Router
1 2

189
Mobile IP: Basic Idea

move

Router
S MN
3

Foreign agent

Home agent

Router Router Packets are tunneled

using IP in IP
1 2

190
Mobile IP: Terminology
 Mobile Node (MN)
– node that moves across networks without changing its IP address
 Home Agent (HA)
– host in the home network of the MN, typically a router
– registers the location of the MN, tunnels IP packets to the COA
 Foreign Agent (FA)
– host in the current foreign network of the MN, typically a router
– forwards tunneled packets to the MN, typically the default router
for MN
 Care-of Address (COA)
– address of the current tunnel end-point for the MN (at FA or MN)
– actual location of the MN from an IP point of view
 Correspondent Node (CN)
– host with which MN is “corresponding” (TCP connection)

191
Data transfer to the mobile system
HA
2
MN

home network receiver

3
Internet

FA foreign
network

1 1. Sender sends to the IP address of MN,

CN HA intercepts packet (proxy ARP)
2. HA tunnels packet to COA, here FA,
by encapsulation
sender
3. FA forwards the packet to the MN

192
Source: Schiller
Data transfer from the mobile system
HA
1 MN

home network sender

Internet

FA foreign
network

1. Sender sends to the IP address

CN of the receiver as usual,
FA works as default router
receiver

193
Source: Schiller
Mobile IP: Basic Operation
 Agent Advertisement
– HA/FA periodically send advertisement messages into their
physical subnets
– MN listens to these messages and detects, if it is in
home/foreign network
– MN reads a COA from the FA advertisement messages
 MN Registration
– MN signals COA to the HA via the FA
– HA acknowledges via FA to MN
– limited lifetime, need to be secured by authentication
 HA Proxy
– HA advertises the IP address of the MN (as for fixed systems)
– packets to the MN are sent to the HA
– independent of changes in COA/FA
 Packet Tunneling
– HA to MN via FA 194
Mobile IP: Other Issues
 Reverse Tunneling
– firewalls permit only “topological correct“ addresses
– a packet from the MN encapsulated by the FA is now
topological correct

 Optimizations
– Triangular Routing
• HA informs sender the current location of MN
– Change of FA
• new FA informs old FA to avoid packet loss, old FA now
forwards remaining packets to new FA

195
Mesh and Adhoc Networks

196
Multi-Hop Wireless

 May need to traverse multiple links to reach destination

 Mobility causes route changes

197
Mobile Ad Hoc Networks (MANET)
 Host movement frequent
 Topology change frequent

A B
B A

 No cellular infrastructure. Multi-hop wireless links.

 Data must be routed via intermediate nodes.

198
MAC in Ad hoc networks

 IEEE 802.11 DCF is most popular

– Easy availability
 802.11 DCF:
– Uses RTS-CTS to avoid hidden terminal problem
– Uses ACK to achieve reliability

 802.11 was designed for single-hop wireless

– Does not do well for multi-hop ad hoc scenarios
– Reduced throughput
– Exposed terminal problem

199
Exposed Terminal Problem

A B
D
C

– A starts sending to B.
– C senses carrier, finds medium in use and has to
wait for A->B to end.
– D is outside the range of A, therefore waiting is not
necessary.
– A and C are “exposed” terminals
200
Distance-vector & Link-state Routing

 Both assume router knows

– address of each neighbor
– cost of reaching each neighbor
 Both allow a router to determine global routing
information by talking to its neighbors

 Distance vector - router knows cost to each destination

 Link state - router knows entire network topology and

computes shortest path

201
Distance Vector Routing: Example

202
Link State Routing: Example

203
MANET Routing Protocols
 Proactive protocols
– Traditional distributed shortest-path protocols
– Maintain routes between every host pair at all times
– Based on periodic updates; High routing overhead
– Example: DSDV (destination sequenced distance vector)

 Reactive protocols
– Determine route if and when needed
– Source initiates route discovery
– Example: DSR (dynamic source routing)

 Hybrid protocols
– Adaptive; Combination of proactive and reactive
– Example : ZRP (zone routing protocol)
204
Dynamic Source Routing (DSR)
 Route Discovery Phase:
– Initiated by source node S that wants to send packet to
destination node D
– Route Request (RREQ) floods through the network
– Each node appends own identifier when forwarding RREQ
 Route Reply Phase:
– D on receiving the first RREQ, sends a Route Reply (RREP)
– RREP is sent on a route obtained by reversing the route
appended to received RREQ
– RREP includes the route from S to D on which RREQ was
received by node D
 Data Forwarding Phase:
– S sends data to D by source routing through intermediate nodes

205
Route discovery in DSR
Y

Z
S E
F
B
C M L
J
A G
H D
K
I N

Represents a node that has received RREQ for D from S

206
 Route discovery in DSR
Y
Broadcast transmission

[S] Z
S E
F
B
C M L
J
A G
H D
K
I N

Represents transmission of RREQ

[X,Y] Represents list of identifiers appended to RREQ 207

Route discovery in DSR
Y

Z
S [S,E]
E
F
B
C M L
J
A [S,C] G
H D
K
I N

• Node H receives packet RREQ from two neighbors:

potential for collision
208
Route discovery in DSR
Y

Z
S E
[S,E,F,J,M]
F
B
C M L
J
A G
H D
K
I N

• Node D does not forward RREQ, because node D

is the intended target of the route discovery
209
Route reply in DSR
Y

Z
S RREP [S,E,F,J,D]
E
F
B
C M L
J
A G
H D
K
I N

Represents RREP control message

210
Data delivery in DSR
Y

DATA [S,E,F,J,D] Z
S E
F
B
C M L
J
A G
H D
K
I N

Packet header size grows with route length

211
Destination-Sequenced Distance-
Vector (DSDV)
 Each node maintains a routing table which stores
– next hop, cost metric towards each destination
– a sequence number that is created by the destination itself
 Each node periodically forwards routing table to
neighbors
– Each node increments and appends its sequence number when
sending its local routing table
 Each route is tagged with a sequence number; routes
with greater sequence numbers are preferred

212
DSDV

 Each node advertises a monotonically

increasing even sequence number for itself
 When a node decides that a route is broken, it
increments the sequence number of the route
and advertises it with infinite metric
 Destination advertises new sequence number

213
DSDV example
 When X receives information from Y about a route to Z
– Let destination sequence number for Z at X be S(X), S(Y) is
sent from Y

X Y Z

– If S(X) > S(Y), then X ignores the routing information received

from Y
– If S(X) = S(Y), and cost of going through Y is smaller than the
route known to X, then X sets Y as the next hop to Z
– If S(X) < S(Y), then X sets Y as the next hop to Z, and S(X) is
updated to equal S(Y)

214
Protocol Trade-offs
 Proactive protocols
– Always maintain routes
– Little or no delay for route determination
– Consume bandwidth to keep routes up-to-date
– Maintain routes which may never be used

 Reactive protocols
– Lower overhead since routes are determined on demand
– Significant delay in route determination
– Employ flooding (global search)
– Control traffic may be bursty

 Which approach achieves a better trade-off depends on the traffic

and mobility patterns

215
TCP over wireless

216
Impact of transmission errors

 Wireless channel may have bursty random errors

 Burst errors may cause timeout

 Random errors may cause fast retransmit
 TCP cannot distinguish between packet losses
due to congestion and transmission errors

 Unnecessarily reduces congestion window

 Throughput suffers

217
Split connection approach

 End-to-end TCP connection is broken into one

connection on the wired part of route and one
over wireless part of the route

 Connection between wireless host MH and fixed

host FH goes through base station BS
 FH-MH = FH-BS + BS-MH

FH BS MH

Fixed Host Base Station Mobile Host

218
I-TCP: Split connection approach

Per-TCP connection state

TCP connection TCP connection

application application application

rxmt
transport transport transport
network network network
link link link
physical physical physical

wireless 219
Snoop protocol
 Buffers data packets at the base station BS
– to allow link layer retransmission
 When dupacks received by BS from MH
– retransmit on wireless link, if packet present in buffer
– drop dupack

 Prevents fast retransmit at TCP sender FH

FH BS MH

220
Snoop protocol

Per TCP-connection state

TCP connection

application application application

transport transport transport
network network network
rxmt
link link link
physical physical physical

FH BS MH
wireless
221
Impact of handoffs

 Split connection approach

– hard state at base station must be moved to new base station
 Snoop protocol
– soft state need not be moved
– while the new base station builds new state, packet losses may
not be recovered locally

 Frequent handoffs a problem for schemes that rely on

significant amount of hard/soft state at base stations
– hard state should not be lost
– soft state needs to be recreated to benefit performance

222
M-TCP
 Similar to the split connection approach, M-TCP
splits one TCP connection into two logical parts
– the two parts have independent flow control as in I-
TCP
 The BS does not send an ack to MH, unless BS
has received an ack from MH
– maintains end-to-end semantics
 BS withholds ack for the last byte ack’d by MH
Ack 999 Ack 1000

FH BS MH

223
M-TCP

 When a new ack is received with receiver’s

advertised window = 0, the sender enters
persist mode
 Sender does not send any data in persist mode
– except when persist timer goes off

 When a positive window advertisement is

received, sender exits persist mode
 On exiting persist mode, RTO and cwnd are
same as before the persist mode

224
TCP in MANET
Several factors affect TCP performance in MANET:

 Wireless transmission errors

– may cause fast retransmit, which results in
• retransmission of lost packet
• reduction in congestion window
– reducing congestion window in response to errors is
unnecessary

 Multi-hop routes on shared wireless medium

– Longer connections are at a disadvantage compared to
shorter connections, because they have to contend for
wireless access at each hop
 Route failures due to mobility
225
 Impact of Multi-hop Wireless Paths
TCP throughput degrades with increase in number of hops
 Packet transmission can occur on at most one hop
among three consecutive hops
– Increasing the number of hops from 1 to 2, 3 results in increased
delay, and decreased throughput

 Increasing number of hops beyond 3 allows simultaneous

transmissions on more than one link, however,
degradation continues due to contention between TCP
Data and Acks traveling in opposite directions

 When number of hops is large enough (>6), throughput

stabilizes
226
Impact of Node Mobility
TCP throughput degrades with increase in mobility but not always
mobility causes
link breakage,
resulting in route Route is TCP sender times out.
failure repaired Starts sending packets again

No
throughput
No throughput
despite route repair

Larger route repair

delays are especially
TCP data and acks harmful
en route discarded 227
WiFi: Management and Security
Network management
 Five key areas (FCAPS):
– Fault management
– Capacity management
– Accounting(access) management
– Performance management
– Security management
 FCAPS at all layers of a stack (network,
middleware, apps)

 Security is the main area of concern

229
Wireless Network Management

 In addition to the wired network issues,

wireless network management needs to
address some specific issues:

– Roaming.
– Persistence of Mobile Units.
– Lack of SNMP Agents in Mobile Units.
– Mobile Adhoc Networks.

230
Wireless security

231
Threats

 Disclosure of sensitive/confidential data

 Denial of service (DoS)
 Unauthorized access to wireless-enabled
resources
 Potential weakening of existing security
measures on connected wired networks and
systems

232
Vulnerabilities
 Wired Equivalent Privacy (WEP) encryption
standard is weak
 Radio signals susceptible to jamming and
interference
 Protocol vulnerabilities allow
– Network sessions to be taken over by an intruder
– Injection of invalid data into network traffic
– Network reconnaissance
 Default configurations create “open” network

233
Vulnerabilities - 1

Example: The radio

signal from a
wireless network can
spill over from the
building where
access points are
located to
neighboring
buildings, parking
lots and public roads.

234
 Vulnerabilities - 2

Example: Many wireless

networks do not use WEP
or other encryption to
protect network traffic.

▲ = Access points using

encryption
▲ = Access points without
encryption

235
Vulnerabilities - 3

Example: These
packet traces show
highly confidential
data that can be
captured from a
wireless network

236
 Wireless security technologies
•SET for transaction security
•S/MIME and PGP for secure email
Applications •Java security (sandboxes)
Can use •Database security
higher level
services to
compensate •SSL and TLS
for lower layers Middleware •Web security (HTTPS, PICS, HTTP Headers)
•Proxy server security
Tradeoffs in
performance
and security TCP/IP
•IPSEC and wirless VPN
•Mobile IP

•802.11 security (WEP)

Wireless •WLL link security
Link
237
Security and availability
 The security S is provided at the following levels:
– Level 0: no security specified
– Level 1: Authorization and authentication of principals
– Level 2: Auditing and encryption (Privacy)
– Level 3: Non-repudiation and delegation

 Availability A can be represented in terms of replications (more

replications increase system availability):
– Level 0: No replication (i.e., only one copy of the resource is used)
– Level 1: Replication is used to increase availability. The resource is
replicated for a fail-safe operation
– Level 2: FRS (Fragmentation, Redundancy, Scattering) is used. FRS
schemes split a resource, replicate it, and scatter it around the
network to achieve high availability and intrusion tolerance
238
Being secure
 Develop wireless network policies
 Conduct risk assessments to determine required
level of security
 Limit access to wireless networks through the
use of wireless security measures (i.e. 802.11i
or WPA)
 Maintain logical separation between wireless
and wired networks
 Perform wireless scans to identify wireless
networks and applications (on a regular basis)
 Enforce wireless network policies
239
802.16 internals
IEEE 802 family
802.2 Logical Link

Data
802.1 Bridging
Link
Layer

802.3 802.4 802.5 802.6 802.11 802.12 802.16

Medium Medium Medium Medium Medium Medium Medium
Access Access Access Access Access Access Access

802.3 802.4 802.5 802.6 802.11 802.12 802.16

Physical Physical Physical Physical Physical Physical Physical
Physical
Layer

241
IEEE 802.16

 Purpose:
– to enable rapid worldwide deployment of cost-effective
broadband wireless access products
 802.16:
– consists of the BS (Base Station) and SSs(Subscriber Stations)
– All data traffic goes through the BS, and the BS can control the
allocation of bandwidth on the radio channel.
– 802.16 is a Bandwidth on Demand system.
 Standard specifies:
– The air interface, MAC (Medium Access Control), PHY(Physical
layer)

242
IEEE 802.16
 The spectrum to be used
– 10 - 66 GHz licensed band
• Due to the short wavelength
– Line of sight is required
– Multipath is negligible
• Channels 25 or 28 MHz wide are typical
• Raw data rates in excess of 120 Mbps
– 2 -11 GHz
• IEEE Standards Association Project P802.16a
• Approved as an IEEE standard on Jan 29, 2003

243
IEEE 802.16 MAC layer function
 Transmission scheduling :
– Controls up and downlink transmissions so that
different QoS can be provided to each user
 Admission control :
– Ensures that resources to support QoS requirements of
a new flow are available
 Link initialization:
– Scans for a channel, synchronizes the SS with the BS,
performs registration, and various security issues.
 Support for integrated voice/data connections:
– Provide various levels of bandwidth allocation, error
rates, delay and jitter

244
Basic services
 UGS(Unsolicited Grant Service)
– Supports real-time service flows that generate fixed size data
packets on a periodic basis, such as T1/E1 and Voice over IP
– The BS shall provide fixed size slot at periodic intervals.
 rtPS(Real-Time Polling Service)
– Supports real-time service flows that generate variable size data
packets on a periodic basis, such as MPEG video

 nrtPS(Non-Real-Time Polling Service)

– Supports non real-time service flows that generate variable size
data packets on a regular basis, such as high bandwidth FTP.
 BE(Best Effort service)
– Provides efficient service to best effort traffic

245
FDD based MAC protocol
 Downlink
– Broadcast phase : The information about uplink and
downlink structure is announced.
– DL-MAP(Downlink Map)
• DL-MAP defines the access to the downlink information
– UL-MAP(Uplink Map)
• UL-MAP message allocates access to the uplink channel
 Uplink
– Random access area is primarily used for the initial
access but also for the signalling when the terminal
has no resources allocated within the uplink phase.

246
FDD based 802.16 MAC Protocol
MAC Frame MAC Frame MAC Frame

Movable boundary
Downlink
Carrier
Broadcast Phase Downlink Phase
Broadcast Reserved

Movable boundary
Uplink
Carrier Uplink Phase Random Access Phase
Reserved Contention

247
Time relevance of PHY and MAC control information
Frame n-1 Frame n
DL-MAP n-1
UL-MAP n-1
Downlink
Subframe

Uplink
Subframe

Round trip delay + T_proc

Bandwidth request slots

248
Downlink Scheduling

 Radio resources have to be scheduled

according to the QoS(Quality of Service)
parameters
 Downlink scheduling:
– the flows are simply multiplexed
– the standard scheduling algorithms can be used
• WRR(Weighted Round Robin)
• VT(Virtual Time)
• WFQ(Weighted Fair Queueing)
• WFFQ(Worst-case Fair weighted Fair Queueing)
• DRR(Deficit Round Robin)
• DDRR(Distributed Deficit Round Robin)
249
 WRR

• It is an extention of round robin scheduling

based on the static weight.
Counter
VCC 1 (Source 1) 1 1 1 Reset
2 Cycle

VCC 2 (Source 2) 2 2 1 3 3 1 3 2 1 3 3 1 3 2 1

3
WRR
VCC 3 (Source 3) 3 3 3 3 3
scheduler

250
VT
 VT : aims to emulate the TDM(Time Division Multiplexing) system
– connection 1 : reserves 50% of the link bandwidth
– connection 2, 3 : reserves 20% of the link bandwidth

Connection 1
Average inter-arrival : 2 units
Connection 2
Average inter-arrival : 5 units

Connection 3
Average inter-arrival : 5 units
First-Come-First-Served
service order

Virtual times

Virtual Clock service order

251
Uplink Scheduling

Uplink scheduling:
– Responsible for the efficient and fair allocation of the
resources(time slots) in the uplink direction
– Uplink carrier :
• Reserved slots
• contention slots(random access slots)
– The standard scheduling algorithms can be used

252
Bandwidth allocation and request mechanisms

 The method by which the SS(Subscriber Station) can

get the bandwidth request message to the BS(Base
Station)
– Unicast
• When an SS is polled individually, no explicit message is
transmitted to poll the SS.
• The SS is allocated, in the UP-MAP(Uplink Map),
bandwidth sufficient for a bandwidth request.
– Multicast
• Certain CID(Connection Identifier) are reserved for
multicast groups and for broadcast messages.
• An SS belonging to the polled group may request
bandwidth during any request interval allocated to that CID
in the UP-MAP
– Broadcast
253
Bandwidth allocation and request mechanisms
 UGS :
– The BS provides fixed size bandwidth at periodic intervals to UGS.
– The SS is prohibited from using any contention opportunities.
– The BS shall not provide any unicast request opportunities.
 rtPS
– The BS provides periodic unicast request opportunities.
– The SS is prohibited from using any contention opportunities.

 nrtPS
– The BS provides timely unicast request opportunities.
– The SS is allowed to use contention request opportunities.
 BE
– The SS is allowed to use contention request opportunities.

254
Bandwidth Request-Grant Protocol

SS1 4.
1. BS
BS allocates
allocates bandwidth
bandwidth to
to SSs
SSs
2.1
5.1 for
for transmitting
transmitting data based on
bandwidth
their bandwidth requests.
request.
1
4
BS 2.1Bandwidth is also
SS1 transmits allocated for
bandwidth
requesting
requests. more bandwidth.
5.1
2.2 SS
SS12 transmits
transmits data and
bandwidth
2.2
5.2 SS2 bandwidth
requests. requests.
5.2 SS2 transmits data and
bandwidth requests.

255
Example
Total Uplink Bytes =
100
2 SS and 1 BS
Flows: UGS rtPS nrtPS BE
SS1 SS2 1st Round 40 30 20 10
Demands: Demands: 30 22 20 10
Excess Bytes = 18
UGS = 20 UGS = 10 2nd Round 30 22 20+12 10+6
rtPS = 12 rtPS = 10 30 22 32 16
Excess Bytes = 2
nrtPS = 15 nrtPS = 15 3rd Round 30 22 30 16+2
BE = 30 BE = 20 30 22 30 18

Total Demand Per Flow:

UGS = 30
rtPS = 22 SS1 Allocation = 20 +12 + 15 + 9 = 56
nrtPS = 30 SS2 Allocation = 10 +10 + 15 + 9 = 44
BE = 50
256
257
QoS and Voice/Video Applications

258
 Bandwidth and applications
UMTS
EDGE
GPRS, CDMA 2000
CDMA 2.5G
2G
Speed, kbps 9.6 14.4 28 64 144 384 2000

Transaction Processing
Messaging/Text Apps
Voice/SMS
Location Services
Still Image Transfers
Internet/VPN Access
Database Access
Document Transfer
Low Quality Video
High Quality Video

259
Applications: network requirements

Hig
h Streamin
g Video Video
Conferencing
E-mail with
Requirements

Attachment
Bandwidth

s
Voice
Internet/
intranet
E-commerce
Text ERP
e-
mail
Terminal
Lo Mode
w
Low Latency Sensitivity High
260
Quality of Service
 Network-level QoS
– Metrics include available b/w, packet loss rates, etc
– Elements of a Network QoS Architecture
• QoS specification (traffic classes)
• Resource management and admission control
• Service verification and traffic policing
• Packet forwarding mechanisms (filters, shapers, schedulers)
• QoS routing
 Application-level QoS
– How well user expectations are qualitatively satisfied
– Clear voice, jitter-free video, etc
– Implemented at application-level:
• end-to-end protocols (RTP/RTCP)
• application-specific encodings (FEC)

261
QoS building blocks
 What kind of premium services?
– Service/SLA design
 How much resources?
– admission control/provisioning
 How to ensure network utilization, load balancing?
– QoS routing, traffic engineering
 How to set aside resources in a distributed manner?
– signaling, provisioning, policy
 How to deliver services when the traffic actually comes in?
– traffic shaping, classification, scheduling
 How to monitor quality, account and price these services?
– network management, accounting, billing, pricing

262
QoS big picture: Control/Data planes

Control Plane: Signaling + Admission Control or

SLA (Contracting) + Provisioning/Traffic Engineering

Router

Workstation Router
Router Internetwork or WAN Workstation

Data Plane: Traffic conditioning (shaping, policing, marking etc) at the edge +
Traffic Classification + Claiming Reserved Resources (Per-hop Behavior- PHB),
scheduling, buffer management

263
Services: Queuing/Scheduling
Traffic Traffic
Sources Classes
$$$$$$
Class A
$$$ Class B
$ Class C

 Extra bits indicate the queue (class) for a packet

 High $$ users get into high priority queues,
which are in turn less populated => lower delay
and near-zero likelihood of packet drop

264
QoS and pricing
 QoS Pricing
– Multi-class network requires differential pricing
– Otherwise all users select best service class
 Service provider’s perspective
– Low cost (implementn, metering, accounting, billing)
– Encourage efficient resource usage
– Competitiveness and cost recovery
 User’s perspective
– Fairness and Stability
– Transparency and Predictability
– Controllability
265
Multimedia applications

 Audio
– Speech (CELP – type codecs)
– Music (MP3, WAV, WMA, Real)

 Video (MPEG –1, 2, 4)

 Streaming
– using HTTP/TCP (MP3)
– using RTP/UDP (Video)

266
 Multimedia protocol stack

Signaling Quality of Service Media Transport

MGCP/Megaco

Application
daemon
Reservation Measurement
H.261, MPEG
H.323 SDP RTSP RSVP RTCP
SIP RTP

TCP UDP
rk
netwo

IPv4, IPv6

el
kern
PPP AAL3/4 AAL5 PPP
li
physic

Sonet ATM Ethernet V.34

267
Session Initiation Protocol (SIP)

 Invite users to sessions

– Find the user’s current location
– match with their capabilities and preferences in order
to deliver invitation
 Modify/Terminate sessions

 Session Description Protocol (SDP)

– Used to specify client capabilities
– Example (client can support MPEG-1 video codec,
and MP3 codecs)

268
SIP components
 User Agent Client (UAC)
– End systems; Send SIP requests
 User Agent Server (UAS)
– Listens for call requests
– Prompts user or executes program to determine response
 User Agent: UAC plus UAS
 Registrar
– Receives registrations regarding current user locations
 Redirect Server
– Redirects users to try other server
 Proxy Server

269
SIP architecture

Request
Response SIP Redirect
Server Location Service
Media
2
3

5
4
6
1
7
11
12 10
SIP Proxy
13 SIP Proxy
8
SIP Client
14
9

SIP Client
(User Agent Server)

270
SIP call flow example
USER A PROXY PROXY USER B

INVITE
407 Proxy Authenticate
ACK

INVITE
INVITE
100 Trying INVITE
100 Trying
180 Ringing
180 Ringing
180 Ringing 200 OK
200 OK
200 OK

ACK
ACK
ACK
BOTH WAY RTP
BYE
BYE
BYE
200 OK 200 OK
200 OK

271
H.323
 H.323 is an ITU standard for multimedia
communications over best-effort LANs.
 Part of larger set of standards (H.32X) for
videoconferencing over data networks.

 H.323 addresses call control, multimedia

management, and bandwidth management as
well as interfaces between LANs and other
networks.

272
H.323 architecture

273
H.323 components
 Terminals:
– All terminals must support voice; video and data are optional
 Gatekeeper:
– most important component which provides call control services
 Gateway:
– an optional element which provides translation functions
between H.323 conferencing endpoints (esp for ISDN, PSTN)
 Multipoint Control Unit (MCU):
– supports conferences between three or more endpoints.
Consists of a Multipoint Controller (MC) and Multipoint
Processors (MP)

274
H.323 Gatekeeper
 Address translation
– H.323 Alias to transport (IP) address
 Admission control
– Permission to complete call
– Can apply bandwidth limits
– Method to control LAN traffic
 Call signaling/management/reporting/logging
 Management of Gateway
– H.320, H.324, POTS, etc.

275
H.323 example

1. A sends request to GateKeeper: Can I call B?

2. GK resolves “Bob” to IP address through H.323
registration or external name service
3. GK applies Admission Policy
4. GK replies to A with B’s IP address
5. A sends Setup message to B
6. B checks with GK for authorizing the connection
7. GK acknowledges B to accept call
8. B replies to A and alerts User
9. H.245 connection established

276
Media transport: RTP
 Transport of real-time data, audio and video
 RTP follows the application level framing (ALF)
– RTP specifies common application functions
– Tailored through modifications and/or additions to the
headers
 RTP consists of a data and a control part
– The data part of RTP is a thin protocol
– The control part of RTP is called RTCP
• quality-of-service feedback from receivers
• snchronization support for media streams

277
RTP (contd)
 RTP services
– payload type identification
– sequence numbering, timestamping
– delivery monitoring, optional mixing/translation.

 UDP for multiplexing and checksum services

 RTP does not provide
– mechanisms to ensure quality-of-service, guarantee
delivery or prevent out-of-order delivery or loss

278
Trends

279
 3G Network Architecture
Core Network
Wireless
Telephone
Access Network
Programmable Network
Gateway
Mobile Access Softswitch
Router
Application
IP Intranet Server

Acces (HLR)
IP Intranet
IP s Point User Profiles &
Base Stations Authentication
802.11

802.11
3G Air Wired Access
Internet
Interface
Acces
s Point
280
Overlay Networks - the global goal
integration of heterogeneous fixed and
mobile networks with varying
transmission characteristics

regional

vertical
handover
metropolitan area

campus-based horizontal
handover

in-house

281
 Future mobile and wireless networks
 Improved radio technology and antennas
– smart antennas, beam forming, multiple-input multiple-output
(MIMO)
• space division multiplex to increase capacity, benefit from
multipath
– software defined radios (SDR)
• use of different air interfaces, download new modulation/coding
• requires a lot of processing power
– dynamic spectrum allocation
• spectrum on demand results in higher overall capacity
 Core network convergence
– IP-based, quality of service, mobile IP
 Ad-hoc technologies
– spontaneous communication, power saving, redundancy

282
 References
 A.S. Tanenbaum. Computer Networks. Pearson Education, 2003.
 J. Schiller, Mobile Communications, Addison Wesley, 2002.
 Y-B. Lin and I Chlamtac, Wireless and Mobile Network Architectures,
Wiley, 2001.

 802.11 Wireless LAN, IEEE standards, www.ieee.org

 Various RFCs: RFC 2002, 2501, 3150, 3449, www.ietf.org

 Others websites:
– www.palowireless.com

283
Thank You

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