3 - Wireless Medium Access

7/31/2019 3 - Wireless Medium Access

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Applications of Wireless

Communication

Student Presentations and

Research Papers

Wireless Communication

TechnologiesWireless Networking and

Mobile IPWireless Local Area

Networks

Wireless Communication and Networks

Wireless Medium Access

http://web.uettaxila.edu.pk/CMS/AUT2012/teWCNms/


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Medium Access Control CSMA

CSMA-CD

CSMA-CA

Random Contention Access

802.11 Multiple Access schemes PCF

DCF HCF

BA and WMM

Virtual Carrier Sense

Physical Carrier Sense

Multiple Access Problems

Outline


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Similar to multi-path or noise

Two transmitting stations will

constructively/destructively interfere with each other atthe receiver

Receiver will hear the sum of the two signals (which

usually means garbage)

Multi-transmitter Interference

Problem


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Medium Access Control

Protocol required to coordinate access

i.e. transmitters must take turns

Similar to talking in a crowded room

Also similar to hub based Ethernet


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Carrier Sense Multiple Access

(CSMA)

Procedure Listen to medium and wait until it is free

(no one else is talking)

Wait a random back off time then start talking Advantages

Fairly simple to implement

Functional scheme that works

Disadvantages Can not recover from a collision

(inefficient waste of medium time)


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Medium Access Control

Media Access Control (MAC) describes how the

media (wired or wireless) is used

Polling, token passing, contention based

CSMA/CD is for ethernet wired networks

CSMA/CA for wireless 802.11

Stations using either access method must first

listen to see whether any other device is

transmitting. If another device is transmitting, the

station must wait until the medium is available.


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Collision Detection Problem

Transmit signal is MUCH stronger thanreceived signal

Due to high path loss in the wirelessenvironment (up to 100dB)

Impossible to listen while transmittingbecause you will drown out anything you hear

Also transmitter may not even have much of asignal to detect due to geometry


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with Collision Avoidance

CSMA-CA Procedure

Similar to CSMA but instead of sending packetscontrol frames are exchanged

RTS = request to send

CTS = clear to send

DATA = actual packet

ACK = acknowledgement


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Advantages

Small control frames lessen the cost of collisions(when data is large)

RTS + CTS provide virtual carrier sense whichprotects against hidden terminal collisions (where Acant hear B)

A B


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Disadvantages

Not as efficient as CSMA-CD

Doesnt solve all the problems of MAC inwireless networks


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CSMA-CD vs CSMA-CA

The difference is when the coast is clear

CSMA/CD node can immediately begintransmitting. If a collision occurs while a CSMA/CD node is

transmitting, the collision will be detected and thenode will temporarily stop transmitting.

802.11 wireless stations are not capable of

transmitting and receiving at the same time, sothey are not capable of detecting a collisionduring their transmission. 802.11 wireless networking uses CSMA/CA instead of

CSMA/CD to try to avoid collisions.


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CSMA-CA

IF CSMA/CA station sees no

transmissions it will wait a random

interval

Keep watching the medium

If still clear after interval, transmit

If not, start over

Because only 1 station can use the

frequency at a time

Half duplex


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CSMA-CA

Standard Distributed Coordination

Function (DCF) is defined to allow access

of multiple stations Checks and balances to keep the line clear

Minimize chances of collision


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CSMA-CA

Since 802.11 stations cannot detect collisions,

how do they know when they happen?

Every unicast frame requires an

acknowledgement

Broadcast and multicast dontACK Frame

When ACK Frame is received, original

station knows the frame is received


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Collision Detection

Receiving device will also check the CRC

If frame is corrupt, no ACK frame

IF no ACK frame, sender assumes deliveryfailure


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Random Contention Access

Slotted contention period

Used by all carrier sense variants

Provides random access to the channel

Operation

Each node selects a random back off number

Waits that number of slots monitoring the

channel If channel stays idle and reaches zero then

transmit

If channel becomes active wait until

transmission is over then start counting again


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Distributed Coordination Function (DCF)

Fundamental method of 802.11 communication

Mandatory access method Point Coordination Functions (PCF) is optional 802.11 also has Hybrid Coordination Function (HCF)

Four Main parts

Interframe Space



Random back-off timer

Pg 254


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Interframe Space (IFS)

The amount of time between transmissions

Actual length of time depends on network speed

Short interframe space (SIFS), highest priority

PCF interframe space (PIFS), middle priority

DCF interframe space (DIFS), lowest priority

Arbitration interframe space (AIFS), used by

QoS stations

Extended interframe space (EIFS), used with

retransmissions Pg 254


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Interframe Space (IFS)

Only certain types of frames are sent after

certain interframe spaces

Only ACK and CTS after SIFS

Pg 254


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802.11 DCF Example

B1 and B2 are backoff intervals

at nodes 1 and 2cw = 31

data

wait

B1 = 5

B2 = 15

B1 = 25

B2 = 20

data

wait

B1 and B2 are backoff intervals

at nodes 1 and 2cw = 31

B2 = 10


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802.11 Contention Window

Random number selected from [0,cw]

Small value for cw

Less wasted idle slots time Large number of collisions with multiple senders (two or more

stations reach zero at once)

Optimal cw for known number of contenders & know

packet size Computed by minimizing expected time wastage (by both

collisions and empty slots)

Tricky to implement because number of contenders is difficultto estimate and can be VERY dynamic


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802.11 Adaptive Contention

Window

802.11 adaptively sets cw Starts with cw = 31

If no CTS or ACK then increase to 2*cw+1 (63, 127, 255)

Reset to 31 on successful transmission

802.11 adaptive scheme is unfair Under contention, unlucky nodes will use larger cw than lucky

nodes (due to straight reset after a success)

Lucky nodes may be able to transmit several packets while

unlucky nodes are counting down for access

Fair schemes should use same cw for all contending

nodes (better for high congestion too)


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802.11 PCF (CSMA-CA)

Optional access control method Polling

AP is point coordinator Only work in a BSS

Not in ad-hoc/IBSS

Both AP and client station must support PCF

AP will switch between DCF and PCF PCF time is contention free period (CFP)

DCF is contention period (CP)


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802.11 DCF (CSMA-CA)

B

Full exchange with virtual carrier sense

(called the Network Allocation Vector)

RTS

CTS

DATA

ACK

Sender

Receiver

Sender Receiver

A B

A

B

NAV (RTS)

NAV (CTS)


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Hybrid Coordination Function (HCF)

Added in 802.11e

Enhanced Distributed Channel Access (EDCA)

HCF Controlled Channel Access (HCCA) DCF and PCF require contention for each frame

HCF defines ability to send multiple frames

Transmit Opportunity (TXOP)

During TXOP period, client station can send a frameburst Uses Short Interframe Space (SIFS)

Pg 259


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Provides differentiated access using eight user

priority levels

Extension of DCF

Uses frame tags similar to 802.1D standard QoS standard at MAC (layer 2) level

Define priority values

With priority queuing

Data waiting in higher priority queues transmitsbefore lower priority queues

Pg 259


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Pg 259


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Defines four access categories base on UserPriority level

From lowest to highest priority access category: AC_BK (Background)

AC_BE (Best Effort)

AC_VI (Video)

AC_VO (Voice

For each category, Enhanced DistributedCannel Access Function (EDCAF) is used Frames with higher category have lower back off

values and are more likely to get a TXOPPg 259


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Block Acknowledgement (BA)

Part of 802.11e

Allows for a single acknowledgement for

multiple frames

Reduces overhead

Two Types Immediate

For low latency traffic

Delayed

For latency tolerant traffic Pg 261


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Block Acknowledgement (BA)-Immediate

Originator sends a block of QoS data

frames to station

Originator requests acknowledgement of

all outstanding QoS data

Block AckReq frame Recipient can send a single ack frame for allreceived frames

Can request a single frame from block beretransmitter

Pg 261


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Block Acknowledgement (BA)-Immediate

Pg 261


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W i-Fi Multimedia (WMM)

For latency sensitive data Real time voice and video

Voice, video, audio, have less tolerance of

latency (cumulative delay) 802.11e had layer 2 MAC methods to meet QoS

requirements WiFi Alliance created WMM

Because WMM is based on EDCA mechanisms,802.1D priority tags from the Ethernet side areused to direct traffic to four access-category

priority queues. The WMM certificationprovides for traffic prioritization via four accesscategories Pg 262


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W i-Fi Multimedia (WMM)

Pg 262


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Provided by RTS & CTS

Designed to protect against hidden terminal collisions

(when C cant receive from A and might start transmitting) However this is unnecessary most of the time due to

physical carrier sense

A B C

RTS CTS



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Physical Carrier SenseMechanisms

Energy detection threshold Monitors channel during idle times between packets to

measure the noise floor

Energy levels above the noise floor by a threshold trigger carriersense

DSSS correlation threshold Monitors the channel for Direct Sequence Spread Spectrum

(DSSS) coded signal

Triggers carrier sense if the correlation peak is above athreshold

More sensitive than energy detection (but only works for 802.11transmissions)

High BER disrupts transmission but not detection


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Physical Carrier Sense Range

Carrier can be sensed at lower

levels than packets can be

received Results in larger carrier sense

range than transmission range More than double the range in

NS2 802.11 simulations

Long carrier sense range helps

protect from interferenceReceive Range

Carrier Sense Range


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Hidden Terminal Revisited

Virtual carrier sense no longer needed in

this situation

A B C

RTS CTS

RTS CTS Still Useful


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RTS CTS Still UsefulSometimes

Obstructed hidden terminal situation

Fast collision resolution for long data

packets

A B


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Exposed Terminal Problem

Hidden terminal is not the only challenge for a

distributed wireless MAC protocol

A blocks B, and C doesnt know what is happening (B isexposed)

A B CD


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