Network Technology CSE3020 - 2006 1 Network Technology CSE3020 Week 9

Network Technology CSE3020 - 2006

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Network Technology CSE3020

Week 9


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Wireless LAN

Why Wireless LANs?

Mobility

Flexibility

No cables, save cost

Fast installation

Problems:Limited bandwidth

Noisy channel

Multipath

Security

Power consumption


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Wireless LANs100

80

60

40

20

0

1 10 100

IEEE 802.11

HIPERLAN 1

HIPERLAN 2Bluetooth

Ran

ge (

m)

Data rate (Mbps)

focus

• IEEE 802.11 (USA).• HIPERLAN 1 and 2 (Europe).• Bluetooth (Industry: Erricson, Nokia, IBM, Intel,…):

Provides short distance (around 10m), low data rate (1Mbps) communications between devices (speakers, laptops,…).


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Wireless LAN: IEEE 802.11

Physical Layer Topologies MAC Protocol

DCF and PCF

Infrastructureand Ad hoc

Radio (DSSS,FHSS)and Infrared

• DSSS: Direct Sequence Spread Spectrum• FHSS: Frequency Hopping Spread Spectrum• DCF: Distributed Coordination Function• PCF: Point Coordination Function


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IEEE 802.11 Functions

Distributed Coordination Function (DCF):• Based on CSMA with Collision Avoidance• Four-way handshaking access method may be applied

(optional). • Retransmission is based on Binary Exponential Backoff.

Point Coordination Function (PCF):• Similar to a polling system.


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Wireless LAN: IEEE 802.11 family

•IEEE 802.11: First standard in the family (1997). It uses DSSS or FHSS at 2.4GHz ISM band offering up to 2Mbps.

•IEEE 802.11a: It is operated at 5GHz ISM band. It uses OFDM modulation (multi-carrier) scheme. It offers up to 54Mbps.

•IEEE 802.11b (1999): It is operated at 2.4GHz ISM band. It uses CCK & QPSK (with DSSS). It offers 1, 2, 5.5 and 11Mbps.

•Other standards are still being developed.

• ISM band: Industrial,Scientific,Medicine band• OFDM: Orthogonal Frequency Division Multiplexing


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IEEE 802.11: Physical Layer

Radio• Experience gained from

building mobile phone and wireless WAN networks can be reused.

• Large coverage and deep penetration.

• Very limited license-free frequency bands.

• Very noisy.

Infrared• Cheap.• No licenses needed.• Interference by sunlight,

heat sources.• Low bandwidth.• Signals may be blocked by

many surrounding objects.


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IEEE 802.11: Topologies

Ad hoc networks Infrastructure networks

Access Point

LAN WAN


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IEEE 802.11: Architecture of an Infrastructure

NetworkESS

BSSSTA

BSSSTA

802.XLANs

AP APPortal

Radio coverageSTA = StationAP = Access PointBSS = Basic Service SetESS = Extended Service Set


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IEEE 802.11: Infrastructure

Design issues:• Handover and roaming.• Frame Forwarding.• Frame conversion from/to existing LANs’ frame format.

AP

LAN WAN

AP AP


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IEEE 802.11: Ad hoc

Design issue:• Multihop relaying (Routing): How can we connect

cells if we do not have an access point in each cell?

A

B

C

D

E

F G


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IEEE 802.11 MAC Protocol• A protocol is needed to define rules for all stations to access

the common channel without conflict.

• Many proposals were submitted to IEEE 802.11 working group, CSMA/CA was chosen (1997).

• CSMA/CA is similar to CSMA/CD (used in Ethernet). A station must make sure the common channel is clear before any transmission attempt.

• Unlike CSMA/CD (Ethernet), a station cannot detect a collision. The receiver must reply with an acknowledgement immediately after receiving a frame.

• A station must choose a random future time for all its transmission (new or collided frames).


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CSMA/CA• To transmit a frame, a station first picks a random integer, r (or

counter) from range [0,W-1].• The value r is decreased by one when the common channel is

detected idle for a short period of time known as slot time (e.g. 50sec).

• The station transmits the frame when r reaches zero.• If the channel is detected busy before r reaches zero, the station

stops to decrease r. The countdown will be reactivated when a long period of idle channel is detected (this period is known as DIFS – Distributed Interframe Space).

• After the completion of the transmission, the station must wait for a very short period (known as SIFS – Short IFS) for the acknowledgement (ack).

• The return of the ack confirms the transmission, otherwise, the station must repeat the first step.


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Ack not received

transmission completed

Channel turns busy

CSMA/CA

The station transmits the frame

To transmit a frame, a station first picks a random integer, r (or counter) from range [0,W-1]

The r value is frozen. The station will continue to monitor the channel.The countdown will be reactivated when a long period of idle channel is detected (known as DIFS)

the station must wait for a very short period (known as SIFS) for an acknowledgement (ack).

For every slot time (eg. 50sec) where the common channel is sensed idle, r =r-1

when r =0channelturns idle for a DIFS

START

DONEack received

REPEAT START


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CSMA/CA

• Initially, W is set to a small value (W=8 according to the IEEE 802.11 standard, this is known as the “minimum contention window”).

• As a station experiences collision, W is doubled.

• When W reaches a large value, it stays at that value (W=256 according to the IEEE 802.11 standard, this is known as the “maximum contention window”).


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CSMA/CA Operation: Example-1

B

D

C

Consider this ad hoc WLAN:

Scenario:B is attempting to transmit a frame to D. The transmission is successful.

time

B was ready, B picked r = 3r = 0

B transmitted its frameACK from D

Slot time SIFS


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B

D

C


Scenario:B, C are attempting transmissions to D. Both transmissions are successful.

time

B transmitted its frame. C detected a busy channel, rc was frozen.

ACK from D

Slot time

B, C are ready, B picked rB = 1 and C picked rc = 3

DIFS

rc was reactivated

C transmitted its frame here when rc = 0.


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Scenario:B, C are attempting transmissions to D. A collision occurs before both transmissions are successful.

time

Transmissions of B and C collided here.

Slot time

B, C are ready, B picked rB = 1 and C picked rc = 1

No ACK from D

DIFS

B, C repeated the operation. B picked rB = 1 and C picked rc = 13

B transmitted its frame here when rB =0. rc was frozen here.

ACK from D

...

B

D

C


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Hidden Station Problem

When A is transmitting a frame to B, since D is not in A’s coverage, D is a hidden station that D doesn’t sense a busy channel, thus D may start a transmission that collides with A’s transmission

Hidden Station Problem in ad hoc Wireless LANs:

A BC D

A’s coverage B’s coverage


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Exposed Station problem

While A is transmitting a frame to C, B senses a busy channel and concludes that it may not transmit any frame to D which is incorrect

Exposed Station Problem in ad hoc Wireless LANs:

A BC D

A’s coverage B’s coverage


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Solution to Hidden/Exposed Station Problem

Four-way Handshaking

time

sender receiver

data

ACKBasic

operation

Additional

operation

RTS

CTS

RTS: Ready to send CTS: Clear to send


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RTS/CTS Operation

B

D

C


Scenario:B is attempting to transmit a frame to D. The transmission is successful.

time

B was ready, B picked r = 3r = 0

B transmitted its frame after receiving CTS

ACK from D

Slot time

B transmitted RTSD replied with CTS


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Four-way Handshaking• Four-way handshaking access method is an optional

operation in IEEE 802.11.

• It is also used to improve performance (by reducing the bandwidth wastage due to a collision).

• Short frames are transmitted using Basic access method, and long frames are recommended to transmit using Four-way handshaking access method.


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IEEE 802.11: DCF & PCF

SIFS

PIFS

DIFSSIFS = Short Interframe spacePIFS = Point IFSDIFS = Distributed IFS

PCF (optional) DCF

SIFS

P Data P CD

IFS

PCF Operation:

PIFS

timeBackoffDataBusy

SIFS

PIFS

time


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Required Reading

• W. Stallings, Data and Computer Communications Prentice-Hall.

>> Chapter 13.6 & 14.5 6E

Chapter 17 7E

Documents

Network Technology CSE3020 - 2006 1 Network Technology CSE3020 Week 9