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Mobile Computing COE 446

Wireless Multiple AccessTarek Sheltami

KFUPMCCSECOE

hthttp://faculty.kfupm.edu.sa/coe/tarek/coe446.htm

Principles of Wireless Networks

K. Pahlavan and P. Krishnamurth

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Outline CSMA/CA BTMA DSMA

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IEEE 802.11 Wireless LAN 802.11b

2.4-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

all hosts use same chipping code

widely deployed, using base stations

802.11a 5-6 GHz range up to 54 Mbps

802.11g 2.4-5 GHz range up to 54 Mbps

All use CSMA/CA for multiple access

All have base-station and ad-hoc network versions

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802.11 LAN architecture

wireless host communicates with base station

base station = access point (AP)

Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains:

wireless hosts access point (AP):

base station ad hoc mode: hosts

only

BSS 1

BSS 2

Internet

hub, switchor routerAP

AP

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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

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Listen Before Talk (LBT) MT1 senses the channel first and then sends a

packet During the second transmission of MT1, MT2

senses the channel and discovers that another terminal is using the medium

MT2 delays the transmission using a pack-off algorithm

CSMA reduces packet collision probability significantly compared with ALOHA

The longer the propagation time, the less the efficient the CSMA

IEEE 802.11 MAC Protocol CSMA/CA

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CSMA..

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CSMA.. Several variations of CSMA have been employed in

LANs, however, ALOHA is still preferred in WANs

CSMA

a) Persistent

b) Non-Persistent

After sensing the busy channel:

a) The MT continues sensing the channel until the channel becomes free

b) The MT attempts to another sensing after a random wait period

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CSMA..

a) 1-Persistent

b) P-PersistentAfter the channel becomes free:

a) The MT transmits its packet immediately

b) The MT runs a random number generator and based on the outcome, transmits its packet with probability p

Persistent

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CSMA..

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CSMA.. Hidden Terminal Problem:

Two terminal within the range of intended third node, but out of range of each other

Busy Tone Multiple Access (BTMA) (for Ad hoc) BW is divided into two channels, message channel

and busy channel Whenever an MT sends signal on the message

channel, it transmits a simple busy tone signal on its busy tone channel

When any MT senses the busy tone signal, it runs on its own busy tone channel

An MT with packet ready to send, first senses the busy tone channel to determine if the network is occupied

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It is possible that a node senses the channel idle, but should not transmit (the hidden terminal problem)

Also, It is possible that a node senses the channel busy, but should transmit

(In the examples, only nodes connected by a straight line can listen to each other’s transmissions)

Carrier Sense Multiple Access (CSMA)

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Busy Tone Multiple Access (BTMA)

Receiver transmits a busy tone in another channel (control channel)

Nodes sense the control channel before transmitting

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Disadvantages of BTMA

Some bandwidth is sacrificed, and nodes must be full-duplex (harder than half-duplex)

If all receivers transmit BT, some transmitters are unnecessarily stopped

If only intended receiver transmits BT, there are collisions elsewhere

In first case, B can not transmit a packet to A (he should have been allowed)

In second case, B transmits a packet to C and there is collision (bandwidth is wasted)

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CSMA.. Most cellular mobile data use different

frequencies for downlink and uplink The BS is not hidden from MTs, whereas

MTs may be hidden from on another In this situation, one may use the

downlink channel to announce the availability of the MT

This concept is used in a protocol referred to as digital or data sense multiple access (DSMA)

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CSMA.. DSMA

Widely used in mobile networks The downlink broadcasts a periodic busy-

idle bit announcing availability of downlink channel for data transmission of its packet

An MT checks the busy-idle bit prior to transmission of its packet

As soon as the MT starts the transmission of its packet, BS changes the busy-idle bit to the busy-state to prevent other MTs from transmission

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CSMA/CA avoid collisions: 2+ nodes transmitting at same time 802.11: CSMA - sense before transmitting

don’t 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)

can’t sense all collisions in any case: hidden terminal, fading

goal: avoid collisions: CSMA/C(ollision)A(voidance)

AB

CA B C

A’s signalstrength

space

C’s signalstrength

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Carrier Sense Multiple Access Collision Avoidance (CSMA/CA)

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CSMA/CA..802.11 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK, increase random backoff

interval, repeat 2802.11 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

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CSMA/CA..

Packet

ACK

Next Packet

Source

Destination

Other nodes

DIFS Contention Window

Back-off after deferDefer Access

CSMA/CA Back-off Algorith

SIFS

DIFS

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CSMA/CA.. IEEE 802.11b has four types of Inter

Frame Space (IFS): Short IFS (SIFS): is the period between the

completion of packet transmission and the start of the ACK frame. (The minimum IFS)

Point Coordination IFS (PIFS): is SIFS plus a Slot Time, which is optional and we do not use

it in our study Distributed IFS (DIFS): is PIFS plus a Slot Time

Extended IFS (EIFS): is a longer IFS used by a station that has received a packet that it could

not understand. This is needed to prevent collisions

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RTS/CTS

idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames

sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but they’re short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

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Collision Avoidance: RTS-CTS exchange

APA B

time

RTS(A)RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

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Performance Analysis

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Performance Analysis..