Broadband Access Networks WLAN coverage is usually quite
Broadband Access Networks of each other. They may both transmit at the same time causing a collision at the AP, but which they will be unaware of. The terminals are hidden from each other. To manage the hidden terminal problem, WLAN uses Carrier Sense Multiple Access/ Collision Avoidance (CSMA/CA). CSMA/CA works by requiring an acknowledgement from the destination of every frame transmitted. While this is effective in detecting collisions, it has the effect of reducing the available bit rate substantially. Simulations have suggested that for 802.11b, if communication is primarily TCP-based, 5.9 Mbps is available to be shared and if communication is primarily UDP, 7.1 Mbps is available, substantially less than the raw 802.11b bit rate of 11 Mbps. Most 802.11 networks use the Distributed Coordination Function (DCF), with CSMA/CA to manage contention for the shared channel. In the DCF, a station that wishes to transmit must detect that the medium has been idle for a specified period of time, which is referred to as the Distributed Control Function Interframe Space (DIFS). If another station also tries to transmit at the same time, there will be a collision which will be detected through the CSMA/CA mechanism. The station now waits an Extended Interframe Space (EIFS). The EIFS is a randomly selected length of time whose maximum value increases as the number of failed attempts to transmit increases. Although DCF is simple, it causes random variations in delay and bit rate. With DCF, once a station gains access to the medium it may keep the medium for as long as it chooses. The number of users sharing the medium and interference from other devices operating in the ISM band will further affect the bit rate available to each user while the CSMA/CA contention mechanisms reduces it even further. Consequently, WLAN access networks can be subject to random variations in delay and bandwidth depending on the number of users and other devices operating in the ISM band. 8.5.3 Recent Developments in WLAN Quality of Service Because WLAN channels are shared, Quality of Service can be difficult to guarantee. The available capacity is not only shared by all users but also users whose traffic has different round trip times (as the result of one user accessing a local server and another a more remote server) it can experience quite different values in latency [NGUY2004b]. For example, where an 802.11b WLAN is shared between clients accessing data from a local server while others accessing it from a remote server, latencies of up to 100 ms have been observed. For game players, latencies of this order are a serious problem. To provide some guarantees of Quality of Service (QoS) for delay-sensitive applications, the 802.11e Working group has been investigating enhancements to the 802.11 MAC layer. The work of 802.11e builds upon the little used Point Coordination Function (PCF) defined, but rarely implemented, in the 802.11 MAC layer. The purpose of the PCF is to allow access to the medium in a fair manner controlled by the AP. When PCF is used, time on the medium is divided into a Contention Free Period (CFP) and a Contention Period (CP). During the Contention Period, access is controlled by the DCF. During the Contention Free Period, access is controlled by the PCF. Each station is polled by the AP for any data to be transmitted. If no response is received within a Point Control Function Interframe Spacing (PIFS) then the next station is polled. The PIFS is shorter than the DIFS, ensuring that PCF has priority over DCF. This mechanism provides guaranteed bandwidth for delay-sensitive applications such as voice and games.
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