The 802.11 MAC is very similar in concept to 802.3, in that it is designed to support
multiple users on a shared medium by having the sender sense the medium before accessing it. For 802.3 Ethernet LANs, the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol regulates how Ethernet stations establish access to the wire and how they detect and handle collisions that occur when two or more devices try to simultaneously communicate over the LAN. In an 802.11 WLAN, collision detection is not possible due to what is known as the “near/far” problem: to detect a collision, a station must be able to transmit and listen at the same time, but in radio systems the transmission drowns out the ability of the station to “hear” a collision.
To account for this difference, 802.11 uses a slightly modified protocol known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) or the Distributed Coordination Function (DCF). CSMA/CA attempts to avoid collisions by using explicit packet acknowledgment (ACK), which means an ACK packet is sent by the receiving station to confirm that the data packet arrived intact.
CSMA/CA works as follows. A station wishing to transmit senses the air, and, if no activity is detected, the station waits an additional, randomly selected period of time and then transmits if the medium is still free. If the packet is received intact, the receiving station issues an ACK frame that, once successfully received by the sender, completes the process. If the ACK frame is not detected by the sending station, either because the original data packet was not received intact or the ACK was not received intact, a collision is assumed to have occurred and the data packet is transmitted again after waiting another random amount of time.
CSMA/CA thus provides a way of sharing access over the air. This explicit ACK mechanism also handles interference and other radiorelated problems very effectively. However, it does add some overhead to 802.11 that 802.3 does not have, so that an 802.11 LAN will always have slower performance than an equivalent Ethernet LAN.
Another MAC-layer problem specific to wireless is the “hidden node” issue, in which two stations on opposite sides of an access point can both “hear” activity from an access point, but not from each other, usually due to distance or an obstruction. To solve this problem, 802.11 specifies an optional Request to Send/Clear to Send (RTS/CTS) protocol at the MAC layer. When this feature is in use, a sending station transmits an RTS and waits for the access point to reply with a CTS. Since all stations in the network can hear the access point, the CTS causes them to delay any intended transmissions, allowing the sending station to transmit and receive a packet acknowledgment without any chance of collision. Since RTS/CTS adds additional overhead
to the network by temporarily reserving the medium, it is typically used only on the
largest-sized packets, for which retransmission would be expensive from a bandwidth standpoint.
Finally, the 802.11 MAC layer provides for two other robustness features: CRC checksum and packet fragmentation. Each packet has a CRC checksum calculated and attached to ensure that the data was not corrupted in transit. This is different from Ethernet, where higher-level protocols such as TCP handle error checking. Packet fragmentation allows large packets to be broken into smaller units when sent over the air, which is useful in very congested environments or when interference is a factor, since larger packets have a better chance of being corrupted. This technique reduces the need for retransmission in many cases and thus improves overall wireless network performance. The MAC layer is responsible for reassembling fragments received, rendering the process transparent to higherlevel protocols.
Association, Cellular Architectures, and Roaming
The 802.11 MAC layer is responsible for how a client associates with an access point. When an 802.11 client enters the range of one or more APs, it chooses an access point to associate with (also called joining a Basic Service Set), based on signal strength and observed packet error rates. Once accepted by the access point, the client tunes to the radio channel to which the access point is set. Periodically it surveys all 802.11 channels in order to assess whether a different access point would provide it with better performance characteristics. If it determines that this is the case, it reassociates with the new access point, tuning to the radio channel to which that access point is set (Figure 4).
Figure 4. Access Point Roaming
This process of dynamically associating and reassociating with APs allows network managers to set up WLANs with very broad coverage by creating a series of overlapping 802.11b cells throughout a building or across a campus. To be successful, the IT manager ideally will employ “channel reuse,” taking care to set up each access point on an 802.11 DSSS channel that does not overlap with a channel used by a neighboring access point (Figure 5). As noted above, while there are 14 partially overlapping channels specified in 802.11 DSSS, there are only three channels that do not overlap at all, and these are the best to use for multi-cell coverage. If two APs are in range of one another and are set to the same or partially overlapping channels, they
may cause some interference for one another, thus lowering the total available bandwidth in
the area of overlap.
Figure 5. Unlimited Roaming
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