Many protocols and RF concepts support the operation of a wireless LAN, but don’t forget that data frames actually get the job done. Learn how data frames carry information over a Wi-Fi network.

Wireless LANs perform many functions, but the main idea is to move information from one point to another over an air medium. There are lots of networking protocols and physics that come into play to make this happen, but 802.11 data frames are responsible for getting the job done.

Data Frame Basics

An 802.11 data frame consists of a frame control field, address fields, frame body, and frame check sequence field. The frame control field has the same structure as other 802.11 frames. The Type subfield with the control field distinguishes the data frame from other frames.

Via MAC addresses, data frames can be directed to one particular station or sent to multiple stations through multicasting. Data frames travel directly from one ad hoc wireless client to another. With infrastructure wireless LANs, which include access points, data frames do not travel directly between clients. Instead, a wireless client sends the data frame to an access point, and the access point then sends the contents of the original data frame in a different data frame to the receiving client.

The body of the data frame carries information from higher layers, such as TCP/IP and UDP packets. The payload size of the data frame body can be up to 2,312 bytes, which means that most information requires multiple data frames to carry the entire load. In fact, streaming video demands a continuous flow of data frames to transport the moving pictures.

Network analyzers attempt to identify the type of protocols that the data frame is transporting. When looking at a packet trace, you’ll often see many data frames flowing over the network in addition to user information. Besides emails and web traffic, data frames often appear carrying other protocols, such as DHCP requests and 802.2 frames.

As with other frames, the frame check sequence is part of the error control process. Upon reception of a frame, the receiving station performs calculations along with this field to determine if errors are present in the data frame.

Null Data Frames

Some wireless LAN vendors use null data frames, which contain an empty frame body, to carry special control information to another station. For example, wireless clients commonly use a null data frame sent to the access point to indicate a change in sleep state by setting the power management bit in the frame control field appropriately. This most often occurs after a wireless client implementing power save mode has been a wake receiving buffered frames from the access point. The null data frame tells the access point to start buffering frames again for that client station because the client is going back to sleep.

Another use of null data frames is part of active scanning. The client station sends a null data frame to the access point to indicate sleep state prior to performing active scanning, which is the process of looking for access points on different channels for the purpose of possible roaming. The access point then buffers frames for the client station while the client scans other RF channels. While on another channel, the client can’t receive frames from the access point.

Once the client station comes back to the associated access point channel, the client sends another null data frame to the access point with the power management bit reset to indicate that the client is ready to receive frames again. This maneuver somewhat fools the access point to think that client is in sleep mode; however, it works very well to reduce frame retransmissions while the client is busy scanning other channels.

Error Correction Mechanisms

After performing medium access, a station can send a data frame. Only one station can send a frame at any given time; therefore, the stations (including access points) set to the same channel share the air medium.

The 802.11 medium access mechanisms, however, don’t fully preclude collisions from occurring. For example, two stations may listen to the medium at the same time and find that no one is transmitting. Both of the station can then transmit, and a collision may occur. This causes both data frames to receive errors. In addition, RF interference or excessive attenuation may occur while the data frame is traveling from the source to the destination. Thus, a data frame may incur errors while in flight.

In order to recover from these situations, the 802.11 protocols incorporate the use of acknowledgement frames. If a destination station receives a directed data frame without errors, then the destination station always sends an acknowledgment frame to the prior sending station. The receiving stations do not acknowledge multicast frames, though.

If the sending station doesn’t receive an acknowledgement for a directed frame within a specific period of time, the sending station will attempt to retransmit the data frame. Retransmissions will take place several times (actual number depends on vendor) before the sending station gives up. If the sending station is not able to successfully send the data frame and receive an acknowledgement, then higher level protocols (such as TCP) can provide error recovery.

Something else that relates to data frame error control is 802.11’s automatic rate control. If excessive retransmissions occur, stations can operate at a lower data rate in order to increase the range boundary, mainly for the purpose of decreasing the signal-to-noise requirements at the receiver. The inability to successfully send a data frame (that is, no acknowledgements) automatically prompts the sending station to lower its data rate. The receiving station will know that the data rate is lower when analyzing the physical layer header of the data frame, which includes rate information.

An important point, though, is that data rate is not always a good measure for performance. The data rate is the number of data bits sent per second, but it only applies during the time that the frame is actually sent. There is lots of time when other stations are using the shared medium and possibly RF interference is causing delays. As a result, throughput corresponding to the data frame varies widely depending on utilization and is much lower than the data rate (often less than fifty percent lower).

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