Medium Access Control for 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 802.11 physical & MAC layers is subject to unreliable.

Noise, interference & other propagation effects result in loss of significant no. of frames.

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

Reliable Data Delivery

For this purpose, IEEE 802.11 includes a frame exchange protocol.

Frame exchange protocol Source station transmits data Destination responds with acknowledgment (ACK) If source doesn’t receive ACK, it retransmits frame

Reliable Data Delivery

To further enhance reliability, a four-frame exchange 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 EDRTS

RTS = Request-to-Send

Pretending a circular range

C FA B EDRTS

RTS = Request-to-Send

NAV = 10

NAV = remaining duration to keep quiet

C FA B EDCTS

CTS = Clear-to-Send

C FA B EDCTS

CTS = Clear-to-Send

NAV = 8

C FA B EDDATA

•DATA packet follows CTS. Successful data reception acknowledged using ACK.

C FA B EDACK

Functional Areas

MAC layer covers three functional areas:

Reliable data delivery

Access control

Security

MEDIUM ACCESS CONTROL

IEEE 802.11 considered 2 types of MAC algorithm: Distributed Access protocols Centralized Access protocols.

End result for 802.11 is a MAC algorithm called 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 the medium.

If the medium is idle, station may transmit.

Otherwise it must wait until current transmission is complete.

DCF does not include a Collision detection function.

Distributed Coordination Function

To ensure smooth & fair functioning of this algorithm, DCF includes a set of delays that amounts to a priority scheme.

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

3 Different IFS values

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

PIFS (Point Coordination Function IFS): A mid-length IFS, used by centralized controller in the PCF scheme.

DIFS (Distributed Coordination Function IFS): The longest IFS, used as a minimum delay for 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 the Strictly Ordered service

Thank You