WLAN Performance

With higher bandwidths supported by the standards, WLANs are deployed for larger numbers of users and involve applications of e-mail, Web browsing, and database access. The current highest data rate supported by 802.11a is 54 Mbps for each of 12 (maximum) nonoverlapping channels with freedom from most potential RF interference.

However, the need for higher data rates and techniques to improve performance of WLANs is crucial for many reasons, including the rise of multimedia and MPEG traffic in videoconferencing and mobile 3G and 4G applications.

The WLAN standard is drawn up by the IEEE 802.11 committee. 1 It consists of a family of specifications, with major definitions being 802.11, 802.11a, 802.11b, 802.11e, and 802.11g. All use Ethernet running CSMA/CA (Collision Avoidance as opposed to Collision Detection on Ethernet).

The original 802.11 provides 1- or 2-Mbps transmission in the 2.4-GHz band, using either frequency hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS). The 802.11a specification is an extension to 802.11 and provides up to 54 Mbps in the 5-GHz band. It uses an OFDMencoding scheme rather than FHSS or DSSS.

The actual available data rates for 802.11a are 6, 9, 12, 18, 24, 36, 48, and 54 Mbps, where the support of transmitting and receiving at data rates of 6, 12, and 24 Mbps is mandatory. The 802.11b 2 specification (also referred to as 802.11 High Rate or Wi- Fi 8 ) is also an extension to 802.11 that provides an 11-Mbps transmission (with a fallback to 5.5, 2, and 1 Mbps) in the 2.4-GHz band.

The 802.11b specification uses only DSSS. The 802.11g 3 specification offers wireless transmission over relatively short distances at 54 Mbps in the 2.4-GHz band. The 802.11g specification also uses the OFDM-encoding scheme.

The 802.11e specification is the newest addition to the 802.11 family, which is based upon 802.11a and includes quality-of-service (QoS) issues. Although 802.11b provides 11-Mbps data rates, with only three nonoverlapping channels, it can be extended to 802.11g to have 54-Mbps operation.

However, the three nonoverlapping channels limitation still exists. With currently available technologies, 802.11a provides maximum performance, but more APs are needed because of the weaker range it has compared with 802.11b. Hence, the need to develop these standards still remains important.

It is noted that WLAN performance suffers severely when operated in a typical office environment (around 200-ft diameter). The degradation in performance is observed to be due to excessive packet loss and error rates, and results in much lower throughput than expected.

Although most of the performance degradation is blamed on radio interference, other factors may cause limitations due to physical and MAC layers of the protocol. In general, these factors are modulation techniques and standards, the hardware used, quality of radio signals, processing speed of the stations, environmental effects such as path loss and echoes, radio interference, software design, and interfacing with high-level protocols.

One major factor affecting WLAN performance is adequate RF coverage. If APs are too far apart, then some users will be associating with the WLAN at less than the maximum data rate. For example, users close to an 802.11b AP may be operating at 11 Mbps, whereas a user at a greater distance may only have 2-Mbps capability.

Performance may be maximized by ensuring that RF coverage is as spread out as possible in all user areas, especially the locations where the bulk of users reside. Another factor affecting WLAN performance is RF interference. Mobile phones and other nearby WLANs can interfere with signals, degrading the operation of an 802.11b WLAN.

These external sources of RF energy may block users and APs from accessing the shared air medium. As a result, the performance of 802.11b-based WLAN suffers when RF interference is present. However, deploying 802.11a networks using 5 Ghz may control this problem.

WLAN performance is heavily dependent upon the performance of underlying physical layer technologies. The access method relies on the physical carrier sensing. The underlying assumption is that every station can hear all other stations. This is not always the case, as one of the models presented in the next section illustrates.

A problem occurs when the AP is within range of one station, but is out of range of another downstream station. In this case, the two stations would not be able to detect transmissions from each other, and the probability of collision is greatly increased. This is known as the hidden-node problem.

To combat this problem, a second carrier sense mechanism is available, which enables a station to reserve the medium for a specified period of time through the use of request-to-send or clear-to-send (RTS or CTS) frames. In the case described in the preceding text, a distant station sends an RTS frame to the AP.

The RTS will not be heard by the second station. The RTS frame contains a duration or ID field that specifies the period of time for which the medium is reserved for a subsequent transmission. Upon receipt of the RTS, the AP responds with a CTS frame, which also contains a duration or ID field.

Although the nearer station cannot detect the RTS, it will detect the CTS and adjust accordingly to avoid collision, even though some nodes are hidden from other stations. The hidden terminals may cause performance degradation in WLANs, which can be controlled by using the RTS/CTS mechanism, as mentioned in the preceding text.

Optimal WLAN performance may be obtained by adjusting the RTS/CTS threshold while monitoring the impact on throughput. The 802.11 standard requires that a station must refrain from sending a data frame until it completes a RTS/CTS handshake with an AP.

The “listen before talk,” or CSMA/CA access method used in 802.11, guarantees equal duration of channel access to all devices irrespective of their needs. In other words, when a device with a low bit rate captures the channel, it penalizes other devices using a higher rate by degrading the speed of their connections.

For example, when one wireless device connects to a WLAN at a lower bit rate than other devices due to being too far from the AP, performance of other network devices degrades. A possible remedy is 802.11e, which defines QoS mechanisms to support bandwidth-sensitive applications such as voice and video.

WLAN performance may also be improved by using a fragmentation option that divides the 802.11 data frames into smaller pieces sent separately to the destination. Each fragment consists of a MAC layer header, FCS (frame check sequence), and a fragment number indicating its position within the frame.

With properly set values, fragmentation can reduce the amount of data that needs retransmission because the station only needs to retransmit the fragment containing the bit errors.