IEEE 802.11 Medium Access Control
The IEEE 802.11 MAC layer covers three functional areas: reliable data delivery, medium access control, and security.
Reliable Data Delivery
- As with any wireless network, a wireless LAN using the IEEE 802.11 physical and MAC layers are subject to considerable unreliability.
- Noise, interference, and other propagation effects result in the loss of a significant number of frames.
- Even with error-correction codes, a number of MAC frames may not successfully receive.
- This situation can dealt with by reliability mechanisms at a higher layer, such as TCP.
- However, timers used for retransmission at higher layers are typically on the order of seconds.
- It is, therefore, more efficient to deal with errors at the MAC level. For this purpose, IEEE 802.11 includes a frame exchange protocol.
- When a station receives a data frame from another station, it returns an acknowledgment (ACK) frame to the source station.
This exchange treated as an atomic unit, not to be interrupted by a transmission from any other station.
- If the source does not receive an ACK within a short period of time, either because its data frame damaged or because the returning ACK damaged, the source retransmits the frame.
- Thus, the basic data transfer mechanism in IEEE 802.11 involves an exchange of two frames.
- To further enhance reliability, a four-frame exchange may use. In this scheme, a source first issues a request to send (RTS) frame to the destination.
- The destination then responds with a clear to send (CTS). After receiving the CTS, the source transmits the data frame, and the destination responds with an ACK.
- The RTS alerts all stations that are within reception range of the source that an exchange is underway; these stations refrain from transmission in order to avoid a collision between two frames transmitted at the same time.
- Similarly, the CTS alert all stations that are within reception range of the destination that an exchange is underway.
- The RTS/CTS portion of the exchange a required function of the MAC but may disable.
Medium Access Control
- The 802.11 working group considered two types of proposals for a MAC algorithm:
- Distributed access protocols, which, like Ethernet, distribute the decision to transmit over all the nodes using a carrier-sense mechanism.
- Centralized access protocols, which involve regulation of transmission by a centralized decision maker.
- A distributed access protocol makes sense for an ad hoc network of peer workstations (typically an IBSS) and may also be attractive in other wireless LAN configurations that consist primarily of busty traffic.
- A centralized access protocol natural for configurations in which a number of wireless stations are interconnected with each other and some sort of base station that attaches to a backbone wired LAN; it is especially useful if some of the data is time sensitive or high priority.