Challenges for the MAC

Differences between the wireless network environment and the traditional wired environment create challenges for network protocol designers. This section examines a number of the hurdles that the 802.11 designers faced.

RF Link Quality

On a wired Ethernet, it is reasonable to transmit a frame and assume that the destination receives it correctly. Radio links are different, especially when the frequencies used are unlicensed ISM bands. Even narrowband transmissions are subject to noise and interference, but unlicensed devices must assume that interference will exist and work around it. The designers of 802.11 considered ways to work around the radiation from microwave ovens and other RF sources. In addition to the noise, multipath fading may also lead to situations in which frames cannot be transmitted because a node moves into a dead spot.

Unlike many other link layer protocols, 802.11 incorporates positive acknowledgments. All transmitted frames must be acknowledged, as shown in Figure 3-1. If any part of the transfer fails, the frame is considered lost.

Figure 3-1. Positive acknowledgment of data transmissions

The sequence in Figure 3-1 is an atomic operation, which means it is a single transactional unit. Although there are multiple steps in the transaction, it is considered a single indivisible operation. Atomic operations are "all or nothing." Either every step in the sequence must complete successfully, or the entire operation is considered a failure. The sender of the data frame must receive an acknowledgment, or the frame is considered lost. It does not matter from the sender's perspective whether the initial data frame was lost in transit, or the corresponding acknowledgment was lost in transit. In either case, the data frame must be retransmitted.

One of the additional complexities of treating the frame transmission of Figure 3-1 as atomic is that the transaction occurs in two pieces, subject to control by two different stations. Both stations must work together to jointly take control of the network medium for transmissions during the entire transaction. 802.11 allows stations to lock out contention during atomic operations so that atomic sequences are not interrupted by other stations attempting to use the transmission medium.

Radio link quality also influences the speed at which a network can operate. Good quality signals can carry data at a higher speed. Signal quality degrades with range, which means that the data transmission speed of an 802.11 station depends on its location relative to the access point. Stations must implement a method for determining when to change the data rate in response to changing conditions. Furthermore, the complete collection of stations in a network must manage transmissions at multiple speeds. Rules for multirate support are discussed later in this chapter.

The Hidden Node Problem

In Ethernet networks, stations depend on the reception of transmissions to perform the carrier sensing functions of CSMA/CD. Wires in the physical medium contain the signals and distribute them to network nodes. Wireless networks have fuzzier boundaries, sometimes to the point where each node may not be able to directly communicate with every other node in the wireless network, as in Figure 3-2.

Figure 3-2. Nodes 1 and 3 are "hidden"

In the Figure 3-2, node 2 can communicate with both nodes 1 and 3, but something prevents nodes 1 and 3 from communicating directly. (The obstacle itself is not relevant; it could be as simple as nodes 1 and 3 being as far away from 2 as possible, so the radio waves cannot reach the full distance from 1 to 3.) From the perspective of node 1, node 3 is a "hidden" node. If a simple transmit-and-pray protocol was used, it would be easy for node 1 and node 3 to transmit simultaneously, thus rendering node 2 unable to make sense of anything. Furthermore, nodes 1 and 3 would not have any indication of the error because the collision was local to node 2.

Collisions resulting from hidden nodes may be hard to detect in wireless networks because wireless transceivers are generally half-duplex; they don't transmit and receive at the same time. To prevent collisions, 802.11 allows stations to use Request to Send (RTS) and Clear to Send (CTS) signals to clear out an area. Both the RTS and CTS frames extend the frame transaction, so that the RTS frame, CTS frame, the data frame, and the final acknowledgment are all considered part of the same atomic operation. Figure 3-3 illustrates the procedure.

Figure 3-3. RTS/CTS clearing

In Figure 3-3, node 1 has a frame to send; it initiates the process by sending an RTS frame. The RTS frame serves several purposes: in addition to reserving the radio link for transmission, it silences any stations that hear it. If the target station receives an RTS, it responds with a CTS. Like the RTS frame, the CTS frame silences stations in the immediate vicinity. Once the RTS/CTS exchange is complete, node 1 can transmit its frames without worry of interference from any hidden nodes. Hidden nodes beyond the range of the sending station are silenced by the CTS from the receiver. When the RTS/CTS clearing procedure is used, any frames must be positively acknowledged.

The multiframe RTS/CTS transmission procedure consumes a fair amount of capacity, especially because of the additional latency incurred before transmission can commence. As a result, it is used only in high-capacity environments and environments with significant contention on transmission. For lower-capacity environments, it is not necessary.

Hidden nodes have also become less of a problem as 802.11 has grown up. In small, quiescent networks with only a few stations associated to an access point, there is very little risk of simultaneous transmission, and plenty of spare capacity to be used for retransmission. In many larger environments, the coverage is dense enough that the clients are located physically close enough to an access point that they are all within range of each other. (In fact, the range of many client systems is probably too large for most networks, which will be explored in the planning phase of this book.)

You can control the RTS/CTS procedure by setting the RTS threshold if the device driver for your 802.11 card allows you to adjust it. The RTS/CTS exchange is performed for frames larger than the threshold. Frames shorter than the threshold are simply sent.