Wireless Frame Transmission

When people talk about wireless networks, they often say that they are just like wired 802.3 LANs. This is actually incorrect, aside from the fact that they use MAC addresses. Wireless LANs use the 802.11 frame structure, and you can encounter multiple types of frames. To get a better understanding, you can begin by learning the three types of wireless frames. Once you are familiar with the three types of wireless frames, you can further your knowledge by taking a deeper look at interframe spacing (IFS) and why it is necessary.


Wireless Frame Types

Wireless LANs come in three frame types:

• Management frames: Used for joining and leaving a wireless cell. Management frame types include association request, association response, and reassociation request, just to name a few. (See Table 7-2 for a complete list.)

• Control frames: Used to acknowledge when data frames are received.

• Data frames: Frames that contain data.

Now that you have an idea of what frames are used, it is helpful to see how these frames are sent. For this, you need to understand a few more terms that might be new to you. Because all the terms meld together to some degree, they are explained in context throughout the next section.


Sending a Frame

Recall that wireless networks are half-duplex networks. If more than one device were to send at the same time, a collision would result. If a collision occurs, the data from both senders would be unreadable and would need to be resent. This is a waste of time and resources. To overcome this issue, wireless networks use multiple steps to access the network. Wireless LANs use carrier sense multiple access collision avoidance (CSMA/CA), which is similar to the way 802.3 LANs work. The carrier sense part means that a station has to determine if anyone else is sending. This is done with clear channel assessment (CCA), and what it means is that you listen. You can, however, run into an issue where two devices cannot hear each other. This is called the hidden node problem. This issue is overcome using virtual carrier sense (VCS). The medium is not considered available until both the physical and virtual carrier report that it is clear.

Each station must also observe IFS. IFS is a period that a station has to wait before it can send. Not only does IFS ensure that the medium is clear, but it ensures that frames are not sent so close together that they are misinterpreted. The types of IFS periods are as follows:

• Short interframe space (SIFS): For higher priority and used for ACKs, among other things

• Point-coordination interframe space (PIFS): Used when an AP is going to control the network

• Distributed-coordination interframe space (DIFS): Used for data frames and is the normal spacing between frames

Each of these has a specific purpose as defined by the IEEE.

SIFS is used when you must send a frame quickly. For example, when a data frame is sent and must be acknowledged (ACK), the ACK should be sent before another station sends other data. Data frames use DIFS. The time value of DIFS is longer than SIFS, so the SIFS would preempt DIFS because it has a higher priority.

Figure 7-1 illustrates the transmission of a frame. In the figure, Station A wants to send a frame. As the process goes, both the physical and virtual carrier need to be free. This means the client has to listen. To listen, the client chooses a random number and begins a countdown process, called a backoff timer. The speed at which the countdown occurs is called a slottime and is different for 802.11a, b, and g.


It works like this:

1. Station A selects the random timer value of 29.

2. Station A starts counting at 29, 28, 27, 26, and so on. While Station A is counting down, it is also listening for whether anyone else is sending a frame.

3. When the timer is at 18, Station B sends a frame, having a duration value in the header of 45.

4. The duration of 45 that is in the header of the frame sent by Station B is called a network allocation vector (NAV) and is a reservation of the medium that includes the amount of time to send its frame, wait for the SIFS, and then receive an ACK from the AP.

5. Station A adds 45 to the 18 that is left and continues counting down, 63, 62, 61, and so on. The total time that Station A waits before sending is called the contention window.

6. After the timer on Station A reaches 0, it can send its frame as illustrated in Figure 7- 2. At this point, the medium should be clear.

If Station A sends but fails, it resets the backoff timer to a new random number and counts down again. The backoff timer gets larger as the frames fail in transmission. For example, the initial timer can be any number between 0 and 31. After the first failure, it jumps to any number between 0 and 127. It doubles for the next failure, then again, then again.


This entire process is known as the distributed coordination function (DCF). This simply means that each station is responsible for coordinating the sending of its data. The alternative to DCF is point coordination function (PCF), which means the AP is responsible for coordination of data transmission.


If the frame is successful, an ACK must be sent. The ACK uses the SIFS timer value to make sure it is sent quickly. Some amount of silence between frames is natural. The SIFS is the shortest period of silence. The NAV reserves this time. A normal silence time is the DIFS. Again, the ACK uses SIFS because you want it to be sent immediately. The station that sends the ACK waits for the SIFS and then ACKs with the duration of 0. This is how the end of the transmission is indicated.


Frame Types

For the most part, all frames are going to have the same type of header. The difference is in the body of the frame. The body is more specific and indicates what the frame is all about. Table 7-2 shows some frame types.