Introduction to Computer

Networking
ICN511S

Loini Iiyambo liiyambo@nust.na

Department of Computer Computer Networking: A Top Down
Approach
Science 6th edition
Jim Kurose, Keith Ross
Addison-Wesley
March 2012

Introduction 1-1
 Chapter 6: Physical layer
our goals: Wireless links
understand characteristics
principles behind
IEEE 802.11
physical layer.
CSMA/CA
learn about copper
cables: Association
Unshielded Twisted
Pair
Shielded Twisted Pair
Coaxial cables
Fiber optic cables

Transport Layer 3-2

 Physical layer
Concerns how signals are used to transfer
message bits over a link
Links can be guided or unguided media

10110 10110

Signal
 Communication Media
Copper cable: The
signals are patterns of
electrical pulses.
Fibre-optic cable: The
signals are patterns of
light.
Wireless: The signals
are patterns of radio
waves.
 Physical layer functions
Physical components
- Design of hardware devices
- Design of connectors
Data Encoding
- Converting data bits to a pre-defined format
Modulation
- Combining a carrier signal with a signal to be
transmitted over the medium
Data Encoding Techniques
Modulation Techniques
 Physical layer Characteristic
All physical media have:
Bandwidth (Mbps)
Maximum segment length
Maximum number of devices per segment
Interference susceptibility
 Ethernet Cables
Unshielded twisted pair Shielded twisted pair
 Ethernet Cables
Ethernet cables are used to transfer data in computer
networks.
Unshielded Twisted Pair (UTP)
- Cat 1, Cat 2, , Cat 7
Shielded Twisted Pair (STP)
- Cables are shielded to prevent against electromagnetic
interference (EMI) and radio frequency interference (RFI)
Coaxial cables
- Older cables
- Commonly used to connect televisions
 UTP Ethernet cables categories
Category 1 (Cat1): Traditional telephone cable used for voice only.
Category 2 (Cat2): Cabling for bandwidth up to 4 Mpbs, consisting
of four pairs of wire.
Category 3 (Cat3): Speed capability of 10 Mbps, with cable
segments up to 100 meters. Consists of four pairs of wire.
Category 4 (Cat4): The first data-grade cable. Certified for
bandwidth up to 16 Mpbs. Consists of four pairs of wire.
Category 5 (Cat5): Speed capability of 1 Gbps, with cable segments
up to 100 meters. Consists of four pairs of wire.
Category 6 (Cat6): Consists of four pairs of wire wrapped in foil
insulation. The insulation provides shielding against crosstalk and
allows for support up to at least six times the throughput of Cat5.
Category 7 (Cat7): Speed capability of 1 Gbps, with two layers of
shielding. Due to the additional shielding, special connectors are
needed.
Coaxial Cables
 Other Ethernet Cables
Crossover
- Computer to computer
- Switch to switch
Straight through
- Computer to switch/hub
- Router to switch/hub
Rollover?
 Ethernet Cables
Easy to install
Easy to expand
Less susceptible to crosstalk and other
interferences
Affordable

Disadvantages?
Fiber Optic Cables
Fiber optic cables have
a much greater
bandwidth.
This means that they
can carry more data.
Less susceptible to
interference.
Much thinner and
lighter.
Disadvantages?
Wireless Networks
Background:
# wireless (mobile) phone subscribers now exceeds #
wired phone subscribers (5-to-1)!
# wireless Internet-connected devices equals #
wireline Internet-connected devices
laptops, Internet-enabled phones promise anytime untethered
Internet access
two important (but different) challenges
wireless: communication over wireless link
mobility: handling the mobile user who changes point of
attachment to network

Wireless, Mobile Networks 6-16

Elements of a wireless network

network
infrastructure

Wireless, Mobile Networks 6-17

Elements of a wireless network
wireless hosts
laptop, smartphone
run applications

network
infrastructure

Wireless, Mobile Networks 6-18

Elements of a wireless network
base station
typically connected to
wired network
relay - responsible for
sending packets between
network
wired network and
infrastructure
wireless host(s) in its
area
e.g., cell towers,
802.11 access points

Wireless, Mobile Networks 6-19

Elements of a wireless network
wireless link
typically used to connect
mobile(s) to base station
multiple access protocol
coordinates link access
network various data rates,
infrastructure transmission distance

Wireless, Mobile Networks 6-20

Types of a wireless network
infrastructure mode
base station connects
mobiles into wired
network
handoff: mobile changes
network
base station providing
infrastructure
connection into wired
network

Wireless, Mobile Networks 6-21

Types of a wireless network
ad hoc mode
no base stations
nodes can only
transmit to other
nodes within link
coverage
nodes organize
themselves into a
network

Wireless, Mobile Networks 6-22

 Wireless Link Characteristics (1)
important differences from wired link .
decreased signal strength: radio signal attenuates as it
propagates through matter (path loss)
interference from other sources: standardized wireless
network frequencies (e.g., 2.4 GHz) shared by other
devices (e.g., phone); devices (motors) interfere as
well
multipath propagation: radio signal reflects off
objects ground, arriving ad destination at slightly
different times

. make communication across (even a point to point)

wireless link much more difficult
Wireless, Mobile Networks 6-23
 IEEE 802.11 Wireless LAN
802.11b 802.11a
5-6 GHz range
2.4-5 GHz unlicensed spectrum
up to 54 Mbps
up to 11 Mbps
802.11g
2.4-5 GHz range
up to 54 Mbps
802.11n: multiple antennae
2.4-5 GHz range
up to 200 Mbps

all use CSMA/CA for multiple access

all have base-station and ad-hoc network versions
Wireless, Mobile Networks 6-24
802.11 LAN architecture
wireless host
communicates with base
Internet
station
base station = access point
(AP)

hub, switch Basic Service Set (BSS) (aka

or router cell) in infrastructure
mode contains:
wireless hosts
BSS 1 access point (AP): base
station
ad hoc mode: hosts only

BSS 2
Wireless, Mobile Networks 6-25
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
APs name (SSID) and MAC address
selects AP to associate with
may perform authentication
will typically run DHCP to get IP address in APs
subnet
Wireless, Mobile Networks 6-26
 802.11: passive/active scanning
BBS 1 BBS 2 BBS 1 BBS 2

1
1 1 2 2 AP 2
AP 1 AP 2 AP 1
2 3
3 4

H1 H1

passive scanning: active scanning:

(1) beacon frames sent from APs (1) Probe Request frame broadcast
(2) association Request frame sent: H1 to from H1
selected AP (2) Probe Response frames sent
(3) association Response frame sent from from APs
selected AP to H1 (3) Association Request frame sent:
H1 to selected AP
(4) Association Response frame sent
from selected AP to H1
Wireless, Mobile Networks 6-27
IEEE 802.11: multiple access
avoid collisions: 2+ nodes transmitting at same time
802.11: CSMA - sense before transmitting
dont collide with ongoing transmission by other node
802.11: no collision detection!
difficult to receive (sense collisions) when transmitting due to weak
received signals (fading)
cant sense all collisions in any case
goal: avoid collisions: CSMA/C(ollision)A(voidance)

space

Wireless, Mobile Networks 6-28

IEEE 802.11 MAC Protocol: CSMA/CA
802.11 sender
1 if sense channel idle, then
transmit entire frame (no CD)
2 if sense channel busy then
start random back off time
timer counts down while channel idle
transmit when timer expires
if no ACK, increase random backoff interval,
repeat 2

Wireless, Mobile Networks 6-29

Chapter 6 summary
Physical layer
wired links:
Ethernet cables
Fiber optic cables
Coaxial cables
Signal representation
IEEE 802.11 (Wi-Fi)
802.11 standards
Active/passive scanning
CSMA/CA
Wireless channel
characteristics

Wireless, Mobile Networks 6-30