31-medium access control for ieee 802

8/3/2019 31-Medium Access Control for IEEE 802

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Medium Access Controlfor IEEE 802.11



By.

P. Victer Paul

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Functional Areas

MAC layer covers three functional areas:

Reliable data delivery

Access control

Security



Reliable Data Delivery

A wireless LAN using the IEEE 802.11physical & MAC layers is subject tounreliable.

Noise, interference & other propagationeffects result in loss of significant no. offrames.

This situation can be dealt with by reliabilitymechanisms at a higher layer, such as TCP.




For this purpose, IEEE 802.11 includes aframe exchange protocol.

Frame exchange protocol

Source station transmits data

Destination responds with acknowledgment (ACK)

If source doesn’t receive ACK, it retransmits frame




To further enhance reliability, a four-frameexchange may be used.

Four frame exchange

Source issues request to send (RTS)

Destination responds with clear to send (CTS)

Source transmits data

Destination responds with ACK



C FB ED

RTS

RTS = Request-to-Send

Pretending a circular range



C FA B ED

RTS

RTS = Request-to-Send

NAV = 10

NAV = remaining duration to keep quiet



C FA B ED

CTS

CTS = Clear-to-Send



C FA B ED

CTS

CTS = Clear-to-Send

NAV = 8



C FA B ED

DATA

•DATA packet follows CTS. Successful data receptionacknowledged using ACK.



C FA B ED

ACK



Functional Areas

MAC layer covers three functional areas:

Reliable data delivery

Access control

Security



MEDIUM ACCESS CONTROL

IEEE 802.11 considered 2 types of MACalgorithm:

Distributed Access protocols

Centralized Access protocols.

End result for 802.11 is a MAC algorithmcalled DFWMAC (Distributed Foundation

Wireless MAC).



Distributed Coordination Function

DCF makes use of simple CSMA algorithm.

If a station has MAC frame to transmit, it listens to themedium.

If the medium is idle, station may transmit.

Otherwise it must wait until current transmission iscomplete.

DCF does not include a Collision detection function.



Distributed Coordination Function

To ensure smooth & fair functioning of thisalgorithm, DCF includes a set of delays thatamounts to a priority scheme.

Let us consider a single delay known as anInter Frame Space (IFS).



3 Different IFS values

SIFS (Short IFS): The Shortest IFS, Used forimmediate response actions

PIFS (Point Coordination Function IFS): A

mid-length IFS, used by centralized controllerin the PCF scheme.

DIFS (Distributed Coordination Function IFS):

The longest IFS, used as a minimum delayfor asynchronous frames.



IFS Usage

SIFS

Acknowledgment (ACK)

Clear to send (CTS)

Poll response

PIFS

Used by centralized controller in issuing polls

Takes precedence over normal contention traffic

DIFS

Used for all ordinary asynchronous traffic



Point Coordination Function



MAC Frame Fields

Frame Control – frame type, control information

Duration/connection ID – channel allocation time

Addresses – context dependant, types include

source and destination

Sequence control – numbering and reassembly

Frame body – MSDU or fragment of MSDU

Frame check sequence – 32-bit CRC



Frame Control Fields

Protocol version – 802.11 version

Type – control, management, or data

Subtype – identifies function of frame

To DS – 1 if destined for DS

From DS – 1 if leaving DS

More fragments – 1 if fragments follow

Retry – 1 if retransmission of previous frame



Frame Control Fields

Power management – 1 if transmitting station is

in sleep mode

More data – Indicates that station has more data

to send

WEP – 1 if wired equivalent protocol is

implemented

Order –

1 if any data frame is sent using theStrictly Ordered service



Thank You

31-medium access control for ieee 802

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