Communication links
Two types of “links”:
point-to-point
PPP for dial-up access
point-to-point link between Ethernet switch and host
broadcast (shared wire or wireless medium)
old-fashioned Ethernet
upstream in Hybrid Fiber Coaxial (HFC) network
802.11 wireless LAN
shared wire (e.g., shared RF shared RF humans at a
cabled Ethernet) (e.g., 802.11 WiFi) (satellite) cocktail party
(shared air, acoustical)
Multiple access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes on the
shared channel interference
collision if a node receives two or more signals that overlap
fully or partially in time domain packet loss
Multiple (or medium) Access Control (MAC) protocol
Algorithm that determines how nodes share channel, i.e.,
determines when a node can transmit
Communication about channel sharing must use channel itself!
no out-of-band channel for coordination
E.g., 802.3 and 802.11MAC
An ideal multiple access protocol
given: broadcast channel of data rate R bps
desiderable:
1. when one node wants to transmit, it can send at rate R.
2. when M nodes want to transmit, each can send at average
rate R/M
3. fully decentralized:
• no special node to coordinate transmissions
• no synchronization of clocks, slots
4. simple
MAC protocols: taxonomy
three broad classes:
channel partitioning
divide channel into smaller “pieces” (time slots, frequency, code)
allocate piece to node for exclusive use
random access
channel not divided, allow collisions
“recover” from collisions
“taking turns”
nodes take turns, but nodes with more to send can take longer
turns
Time multiplex
A channel (assigned to one user) 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
Disadvantages:
precise synchronization required c
Guard bands in time domain f
to avoid interference due to
delay spread of channel
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TDMA: time division multiple access
access to channel in "rounds"
each station gets fixed length slot (length = pkt
trans time) in each round
How many users it supports?
unused slots go idle
Example: 6-station TDMA LAN, 1,3,4 have pkt,
slots 2,5,6 idle
6-slot 6-slot
frame frame
1 3 4 1 3 4
Fixed TDMA vs Dynamic TDMA slot allocation
Frequency multiplex
Separation of the whole spectrum into smaller frequency bands. A channel/user gets
a certain band of the spectrum for the whole time.
Advantages:
loose coordination
Simple
k1 k2 k3 k4 k5 k6
c
Disadvantages: f
wastage of bandwidth
if the traffic is
distributed unevenly
inflexible
guard spaces to avoid
interference
8
FDMA: frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band
How many users it supports?
unused transmission time in frequency bands go idle
example: 6-station FDMA LAN, 1,3,4 have pkts, frequency
bands 2,5,6 idle
frequency bands
FDM cable
Time and frequency multiplex
Combination of both methods.
A channel/user gets a certain frequency band for a certain amount of time.
Example: GSM (2G)
Advantages:
more flexibility
But: very precise coordination
required k1 k2 k3 k4 k5 k6
c
How many users it supports?
f
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GSM Frequencies
Originally designed on 900MHz range, now also available on
800MHz, 1800MHz and 1900 MHz ranges.
Separate Uplink and Downlink frequencies
ARFCN for GSM Systems
ARFCN: Absolute Radio Frequency Channel Number is a unique number
given to each radio channel in GSM
TDMA/FDD Example: GSM
• F dl = F ul + 45 MHz to avoid adjacent channel interference
• Handset is assigned both uplink and downlink channels, but half-duplex and
hence uplink and downlink frames are time-shifted to be able to assign
same slot in both directions.
• Some slots are reserved for Signaling. 20 MHz is kept for Guarding.
• Each GSM frame of 4.615 ms contains 8 slots of 577 microsec each
carrying 114 info bits. GMSK: one bit per symbol
• 124*8 slots (channels) for communication of user data and signaling msgs
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GSM Downlink & Uplink Frames
Offset of 3 slots so that MN getting same uplink & downlink slot #
need not to tx and rx at the same time effectively half-duplex
like in Wi-Fi, reduction in handset cost
Source: http://www.rfcafe.com/references/electrical/gsm-specs.htm
GSM Channels
Physical Channel: Each timeslot (aka burst period) on a carrier per
TDMA frame is referred to as a physical channel
Logical Channel: Variety of information is transmitted between the
MS and BTS on physical channels.
Different types of logical channels:
Traffic channel
Control Channel
Use of Slots in Downlink & Uplink
http://www.sharetechnote.com/html/FrameStructure_GSM.html
Mixed GSM/GPRS Timeslot Usage
Example of one static allocation of slots in a TDMA Frame
TS0-TS1: GSM/GPRS signaling
TS2: GSM calls
TS3-6: GPRS sessions
TS7: GSM calls
Operator can also dynamically change slot allocation for GSM
and GPRS traffic
TDMA/TDD Example: DECT
• DECT: Digital Enhanced Cordless Telecommunications; works in unlicensed bands (1.88 to 1.9MHz)
• DECT is the second most successfully ETSI standard after GSM (73% market share)
• Frame length is 10 ms having 24 slots, half-duplex thru TDD
• Supports Voice/Data: GFSK, 4/8PSK, 16/64 QAM
• How many users does it support simultaneously?
• Intercom b/w DECT basestations, conference b/w phones, multi-lane
http://www.etsi.org/technologies-clusters/technologies/dect 17
Comparison
TDMA/FDD TDMA/TDD
E.g., GSM (WWAN) DECT
Same frame structure for In a frame, some (e.g., half)
both D/L and U/L slots for D/L and rest for U/L
D/L and U/L on different D/L and U/L on same carrier
carrier frequencies frequency
GSM uses lower frequencies
for U/L to reduce tx power
of handset and get better U/L
SNR
Code multiplex
k1 k2 k3 k4 k5 k6
Each user has a unique code aka spreading code
All users use the same spectrum at the same time c
Advantages:
bandwidth efficient
good protection against interference and
eavesdropping
Disadvantage: f
more complex signal regeneration
Implemented using spread spectrum technology,
Used in 3G systems
t
Space Multiplex
Unlike previous schemes
which use omni-directional
antennas, SDMA uses
directional antennas to
cover angular regions
Different regions can be
served using same
frequency
Spectrum reuse is achieved
Used popularly in cellular
systems & satellite
networks
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Comparison of SDMA/TDMA/FDMA/CDMA
Approach SDMA TDMA FDMA CDMA
Idea segment space into segment sending segment the spread the spectrum
cells/sectors time into disjoint frequency band into using orthogonal codes
time-slots, demand disjoint sub-bands
driven or fixed
patterns
Terminals only one terminal can all terminals are every terminal has its all terminals can be active
be active in one active for short own frequency, at the same place at the
cell/one sector periods of time on uninterrupted same moment,
the same frequency uninterrupted
Signal cell structure, directed synchronization in filtering in the code plus special
separation antennas the time domain frequency domain receivers
Advantages very simple, increases established, fully simple, established, flexible, less frequency
capacity per km² digital, flexible robust planning needed, soft
handover
Dis- inflexible, antennas guard space inflexible, complex receivers, needs
advantages typically fixed needed (multipath frequencies are a more complicated power
propagation), scarce resource control for senders
synchronization
difficult
Comment used in all cellular standard in fixed typically combined higher complexity
systems networks, together with TDMA
with FDMA/SDMA (frequency hopping
used in many patterns) and SDMA
mobile networks (frequency reuse)
In practice, several access methods are used in combination
Example :SDMA/TDMA/FDMA for GSM
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Random access protocols
when node has packet to send
transmit at full channel data rate R.
no a priori coordination among nodes
two or more transmitting nodes ➜ “collision”,
random access MAC protocol specifies:
how to detect collisions
how to recover from collisions (e.g., via delayed
retransmissions)
examples of random access MAC protocols:
slotted ALOHA
ALOHA
CSMA, CSMA/CD (Ethernet), CSMA/CA (Wi-Fi)
CSMA (carrier sense multiple access)
CSMA: listen before transmit:
if channel sensed idle: transmit entire frame
if channel sensed busy, defer transmission
human analogy: don’t interrupt others!
CSMA collisions spatial layout of nodes
collisions can still occur:
propagation delay means
two nodes may not hear
each other’s
transmission
collision: entire packet
transmission time
wasted
distance & propagation
delay play role in in
determining collision
probability
CSMA/CD (collision detection)
CSMA/CD: carrier sensing, deferral as in CSMA
collisions detected within short time
colliding transmissions aborted, reducing channel wastage
collision detection:
easy in wired LANs: measure signal strengths, compare
transmitted, received signals
difficult in wireless LANs: received signal strength
overwhelmed by local transmission strength
human analogy: the polite conversationalist
“Taking turns” MAC protocols
channel partitioning MAC protocols:
share channel efficiently and fairly at high load
inefficient at low load: delay in channel access, 1/N
bandwidth allocated even if only 1 active node!
random access MAC protocols
efficient at low load: single node can fully utilize channel
high load: collision overhead
“taking turns” protocols
look for best of both worlds!
“Taking turns” MAC protocols
polling:
master node “invites”
slave nodes to transmit data
in turn poll
typically used with
“dumb” slave devices master
concerns: data
polling overhead
latency
single point of slaves
failure (master)
“Taking turns” MAC protocols
token passing:
T
control token passed
from one node to next
sequentially.
token message (nothing
concerns: to send)
token overhead T
latency
single point of failure
(token)
data
“Taking turns” MAC protocols
Bluetooth:
Master controls the
channel access by salves
Up to 7 active salves
Polling by Master
Summary of MAC protocols
channel partitioning, by time, frequency or code
Time Division, Frequency Division
random access (dynamic),
ALOHA, S-ALOHA, CSMA, CSMA/CD
carrier sensing: easy in some technologies (wire), hard
in others (wireless)
CSMA/CD used in Ethernet
CSMA/CA used in 802.11 (discussed later in course)
taking turns
polling from central site, token passing
Bluetooth, FDDI, token ring
Readings
Schiller book: Ch 3
Kurose and Ross book: MAC chapter
Reading Material Section on Google Classroom
Random_Access_Techniques_for_Data_Transmission_over_Packet_Sw
itched_Radio_Channels.pdf
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