Media Access Methods

1999 Cabletron Systems


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MAC FunctionalityCSMA/CA with ACKCSMA/CA with RTS/CTS protocolAutomatic data-rate fall-backIn cell relay operationsAd-Hoc mode of operation


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Accessing the Medium CSMA/CD (Standard Ethernet)• In a CSMA/CD system the media is sensed to verify that it

is not in use

• In CSMA/CD a station will:

– Defer if carrier is in use

– Transmit if the carrier is not in use

– When collision is detected station all stations involved will

• back off and

• retransmit


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CSMA/CA with MAC - level Acknowledgment• In a wireless environment the network adapters cannot

detect collisions:

– Carrier Sensing - listen to the media to determine if it is free

– If media is free, then one of two methods is used to verify proper transmission

• MAC - Level Acknowledgment

• Request To Send / Clear To Send (RTS/CTS)


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Message

ACK

CSMA/CAMAC - level Acknowledgment

• Collisions still can occur (interference; incapability of sensing other carrier)

– IEEE 802.11 defines “low-level” ACK protocol

– Provides faster error recovery

– Makes presence of high level error recovery less critical

Following a transmission, the receiving station must transmit an ACK signal back to the originating station within a specified time frame to verify proper frame transmission


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Message

ACK

CSMA/CA MAC - level Acknowledgment

• An ACK signal

– will only be generated upon reception of a valid frame

– will allow the transmitting station to know that the frame was not involved in a collision or corrupted

– is a low overhead approach to verifying proper transmission

ACK is the recommended mode of operation in a small wireless network. The alternative would be using the RTS/CTS mode in a larger cell size which is discussed in the next few slides.


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BA C

The Problem “Hidden stations”

• Situation that occurs in larger physical areas

(typically outdoors)

– Loss of performance

– Error recovery required

A sends to BC doesn’t detect that, so C might also start sending a transmissionCollision of messages occurs: both messages lost


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RTS: I want to send for 10 micro seconds

BA C

The Solution CSMA/CAWith Request To Sent/Clear To Send Protocol

• In this scenario station A wants to transmit to station B

– A transmits a Request To Send to B which tells B how long the transmission will take

A to B


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BA C

CSMA/CA WithRequest To Sent/Clear To Send Protocol

• In this scenario station A wants to transmit to station B– B receives the RTS and transmits a Clear To Send to A which all other

stations receive. This cause them to defer all attempts to transmit

CTS: OK A, go ahead send for 10 micro seconds. Everybody else be quiet for 10 microseconds

B to A (all other stations take note)


A to B


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BA C


• In this scenario station A wants to transmit to station B– A now transmits to B and since all other stations are aware of the

transmission, via the previous CTS, no collisions should occur.




A to B

Transmit: 10 micro seconds

A to B


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BA C


• In this scenario station A wants to transmit to station B– Following the transmission B will reply with an ACK signal which all other

stations regard as meaning the media is clear




A to B

Transmit: 10 micro seconds

A to B

ACK: I received the transmission

B to A


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RTS/CTS Issues

– Defaults to the off state

– Can be switched on when a problem arises with hidden nodes

– Increased overhead

– No interruptions when large files are transmitted

– Big picture:

• Since most communication is from the AccessPoint (during normal operations) hidden nodes even when present will probably NOT be an issue. This is why the RTS/CTS mode should only be used when absolutely required.


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Interference in a large frame requires re-transmission of the frame.Fragmenting reduces the frame size and the required time to re-transmit since only a portion of the original frame will need to be retransmitted.

interference

Message Fragmentation (not available yet but planned for the future)

• MAC level function to fragment large messages into smaller frames (user definable)

– Improves performance in RF polluted environments

– Can be switched off to avoid the overhead in RF clean environments


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Multi-rate operations

• Fall back to lower data-rate when communications quality decreases

– Out of range situations

– Interference

• Operating bandwidth

– High 11Mbps

– Medium 5.5Mbps

– Standard 2Mbps

– Low 1Mbps


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Why the Need for Data Rate Fall Back?

• As the Signal to Noise Ratio falls to unacceptable levels, to support the data rate that is being used, the system will automatically fall back to the next lower speed.

• Operating with a lower data rate makes the system better able to deal with a poor signal quality.


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Data Rate Fall Back

• 802.11B 11Mbps High Speed device will fall back in the following order:

– High 11Mbps

– Medium 5.5Mbps

– Standard 2Mbps

– Low 1Mbps

• 802.11 2Mbps device will fall back in the following order:

– Standard 2Mbps

– Low 1Mbps


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Advantages of Multiple Data Rates

• User configurable option to use a fixed data rate if desired.

• Wireless Network adapter in AccessPoint is capable of supporting different data-rates “simultaneously”

– e.g. operates at “High” speed in communication to nearby station and at “Low” speed to station that is further away

– This provides a simple reliable method to automatically regulate the wireless network!!!


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Issues With Multiple Data Rates

High Data Rate

11Mbps

Low DateRate

2Mbps

Note: both stations are in the same cell but are operating at different speeds. The AccessPoint is capable of communication to each station. Stations can communicate with each other through the AccessPoint.


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What is In-cell relay

• AccessPoint acts as wireless repeater to relay all intercell station-to-station communication

• Communication from one station to another station in the same wireless cell is via the AccessPoint.

• Implications of all intercell traffic going to the AccessPoint:

– Effective size of the cell increases Because Stations only need to be in the range of the AccessPoint and not in the range of all other stations in the wireless network

– Reduced performance in peer to peer communication within one cell since all communications have to involve the AccessPoint.


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Using Cell Relay

• Station A wants to communicate with Station B

– A transmits to B but the communication goes to the AccessPoint

RoamAbout AccessPoint

Station A

Station B

Station E

Station D


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Using Cell Relay

• Station A wants to communicate with Station B

– A transmits to B but the communication goes to the AccessPoint

RoamAbout AccessPoint

Station A

Station B

Station E

Station D


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Cell Relay

• In the previous example, the AccessPoint relayed the transmission from the source to all the other stations in the cell

– Note that even though station B was closer to A than was the AccessPoint was used to relay the transmission.


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Ad-Hoc network

Ad-Hoc Mode of Operation

• In the Ad-Hoc mode of operation, the wireless network is comprised of a peer-to-peer network without the aid of an AccessPoint.


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Using the AccessPoint

AccessPoint network

Wired LAN (Ethernet)


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Building to Building

• Used to connect two separate locations with a wireless bridge connection

• Link is dedicated to the point-to-point connection

• Directional antennas are used for extended distances

Documents

Media Access Methods