Wireless LANs—IEEE 802.11(Wi-Fi)
One of the most popular technologies being used to access the Internet today is wireless fidelity (Wi-Fi), also known by its IEEE standard name 802.11, effectively a wireless version of Ethernet. Wi-Fi has developed to become an inexpensive, highly convenient way to provide connectivity and performance levels acceptable for most applications. Wi-Fi networks are easy to set up, and most portable computers and smartphones now include the necessary hardware to access Wi-Fi infrastructure. Many coffee shops, airports, hotels, and other facilities include Wi-Fi “hot spots,” and Wi-Fi is even seeing considerable advancement in developing countries where other infrastructure may be difficult to obtain. The architecture of an IEEE 802.11 network is shown in Figure 3-17.
802.11 Frames
There is one common overall frame format for 802.11 networks but multiple types of frames. Not all the fields are present in every type of frame. Figure 3-18 shows the format of the common frame and a (maximal-size) data frame.
Management Frames
Management frames are used for creating, maintaining, and ending associations between stations and access points. They are also used to determine whether encryption is being used, what the name (SSID or ESSID) of the network is, what transmission rates are supported, and a common time base. These frames are used to provide the information necessary when a Wi-Fi interface “scans” for nearby access points.
Control Frames: RTS/CTS and ACKs
Control frames are used to handle a form of flow control as well as acknowledgments for frames. Flow control helps ensure that a receiver can slow down a sender that is too fast. Acknowledgments help a sender know what frames have been received correctly. These concepts also apply to TCP at the transport layer (see Chapter 15). 802.11 networks support optional request-to-send (RTS)/clear-tosend (CTS) moderation of transmission for flow control. When these are enabled, prior to sending a data frame a station transmits an RTS frame, and when the recipient is willing to receive additional traffic, it responds with a CTS. After the RTS/CTS exchange, the station has a window of time (identified in the CTS frame) to transmit data frames that are acknowledged when successfully received. Such transmission coordination schemes are common in wireless networks and mimic the flow control signaling that has been used on wired serial lines for years (sometimes called hardware flow control).
Data Frames, Fragmentation, and Aggregation
Most frames seen on a busy network are data frames, which do what one would expect—carry data. Typically, there is a one-to-one relationship between 802.11 frames and the link-layer (LLC) frames made available to higher-layer protocols such as IP. However, 802.11 supports frame fragmentation, which can divide frames into multiple fragments. With the 802.11n specification, it also supports frame aggregation, which can be used to send multiple frames together with less overhead.
Power Save Mode and the Time Sync Function (TSF)
The 802.11 specification provides a way for stations to enter a limited power state, called power save mode (PSM). PSM is designed to save power by allowing an STA’s radio receive circuitry to be powered down some of the time. Without PSM, the receiver circuitry would always be running, draining power. When in PSM, an STA’s outgoing frames have a bit set in the Frame Control Word. A cooperative AP noticing this bit being set buffers any frames for the station until the station requests them. APs ordinarily send out beacon frames (a type of management frame) indicating various things like SSID, channel, and authentication information. When supporting stations that use PSM, APs can also indicate the presence of buffered frames to a station by setting an indication in the Frame Control Word of frames it sends. When stations enter PSM, they do so until the next AP beacon time, when they wake up and determine if there are pending frames stored at the AP for them.
802.11 Media Access Control
In wireless networks, it is much more challenging to detect a “collision” than in wired networks such as 802.3 LANs. In essence, the medium is effectively simplex, and multiple simultaneous transmitters must be avoided, by coordinating transmissions in either a centralized or a distributed manner. The 802.11 standard has three approaches to control sharing of the wireless medium, called the point coordination function (PCF), the distributed coordinating function (DCF), and the hybrid coordination function (HCF). HCF was brought into the 802.11 specification [802.11-2007] with the addition of QoS support in 802.11e and is also used by 802.11n. Implementation of the DCF is mandatory for any type of station or AP, but implementation of the PCF is optional and not widespread (so we shall not discuss it in detail). HCF is found in relatively new QoS-capable Wi-Fi equipment, such as 802.11n APs and earlier APs that support 802.11e. We turn our attention to DCF for now and describe HCF in the context of QoS next.