Q12Mobile Computing
Question
Describe the MAC protocol IEEE 802.11 with its frame format.
Answer
IEEE 802.11 uses CSMA/CA for MAC, which avoids collisions using RTS/CTS. Its frame format includes frame control, duration, four addresses, sequence control, frame body, and FCS.
IEEE 802.11 defines the MAC layer for wireless LANs. Because wireless radios cannot easily detect collisions while transmitting (a station's own strong outgoing signal drowns out the weak signal it would need to sense to detect a collision), the standard CSMA/CD scheme used in wired Ethernet does not work. Instead, 802.11 uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA).
Before transmitting, a station senses the channel. If idle for a period called the Distributed Inter-Frame Space (DIFS), it may transmit; if busy, it waits and then backs off for a random interval chosen from a contention window before retrying. To combat the hidden terminal problem, 802.11 supports an optional Request to Send / Clear to Send (RTS/CTS) exchange: the sender transmits a short RTS frame, the receiver replies with a CTS frame, and both frames carry a duration field that instructs all other stations hearing either frame to set a Network Allocation Vector (NAV) and defer transmission for that duration, effectively reserving the channel.
The general 802.11 MAC frame consists of the following fields.
- Frame Control: Encodes the protocol version, frame type (management, control, or data), and flags such as ToDS, FromDS, and retry.
- Duration/ID: Specifies the channel reservation time used for NAV updates, or a station ID in some control frames.
- Address 1-4: Up to four address fields are used depending on whether the frame is traveling to or from a distribution system, representing source, destination, transmitter, and receiver addresses across an access point.
- Sequence Control: Contains a sequence number and fragment number to detect duplicate frames and reassemble fragmented data.
- Frame Body: A variable-length field carrying the actual payload, up to 2312 bytes.
- Frame Check Sequence (FCS): A 32-bit CRC used by the receiver to detect transmission errors.
This combination of collision avoidance and a robust frame structure allows 802.11 networks to operate reliably despite the shared, error-prone nature of the wireless medium.