MAC

802.11 MAC Overview

Uses variant of Carrier Sense Multiple Access with Collision Avoidance (CS/MACA)

• RTS/CTS used for addressing hidden-nodes.

Automatic Repeat Request (ARQ)

• Error control method for reliability.

• All frames have to be properly ACK, or timeout occurs.

Two operating modes:

• Infra-structured network (Access point).

• Ad-Hoc networks (without access point).

Power saving support.

Wired Equivalent Privacy (WEP).

MAC management.

Independent of the physical layer or of operating mode.

Features of 802.11 MAC protocol

Fair control access.

• Supports Media Access Control functionalities

  • Addressing.

  • CSMA/CA

Protection of date.

• Error detection (FCS – Frame Check Sequence).

  • Compares number with received values.

• Error correction (ACK frame).

Reliable data delivery.

• Fragmentation.

• Flow control: stop-and-wait (the next frame is only sent after an ACK from the previous one is received).

Frames

Three types of frames:

• control: RTS, CTS, ACK.

• Management.

• Data.

Header depends on the frame type.

Format

Packet Types

Type/sub-type field is used to indicate the type of the frame.

Management:

• Association/Authentication/Beacon.

Control.

• RTS, CTS, CF-end, ACK.

Data.

• Data only, or Data + CF-ACK, or Data + CF-Poll or Data + CF-Poll + CF-ACK

Some More Fields

Duration/ID: Duration in DCF mode/ID is used in PCF mode.

More Frag: 802.11 supports fragmentation of data.

More Data: In polling mode, station indicates it has more data to send when replying to CF-POLL.

RETRY is 1 if frame is a retransmission.

WEP (Wired Equivalent Privacy) is 1 if frame is WEP coded.

Power Mgmt is 1 if in Power Save Mode.

Order = 1 for strictly ordered service.