06/14/22 1 Mobile Computing COE 446 Wireless Multiple Access Tarek Sheltami KFUPM CCSE COE ht http://faculty.kfupm.edu.sa/coe/tarek/co e446.htm Principles of Wireless Networks K. Pahlavan and P. Krishnamurth
Jan 07, 2016
04/20/23 1
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..