BSS-transition。 

This is defined as a station movement from one BSS to another BSS within the same ESS。 In this case, delivery of data to the station requires that the addressing capability be able to recognize the new location of the station。

ESS-transition。 

This is defined as a station movement from a BSS in one ESS to a BSS within another ESS。 This case is supported only in the sense that the station can move。 Maintenance of upper-layer connections supported by 802。11 cannot be guaranteed。 In fact, disruption of service is likely to occur。 details of this option are for further study。

The 802。11 working group considered two types of proposals for a MAC algorithm: distributed-access protocols which, like CSMAICD, distributed the decision to transmit over all the nodes using a carrier-sense mechanism; and centralized access protocols, which involve regulation of transmission by a centralized decision maker。 A distributed access protocol makes sense of an ad hoc network of peer workstations and may also be attractive in other wireless LAN configurations that consist primarily of bursty traffic。 A centralized access protocol is natural for configurations in which a number of wireless stations are interconnected with each other and with some sort of base station that attaches to a backbone wired LAN; it is especially useful if some of the data is time-sensitive or high priority。

The end result of the 802。11 is a MAC algorithm called DFWMAC (distributed foundation wireless MAC) that provides a distributed access-control mechanism with an optional centralized control built on top of that。 Figure 13。20 illustrates the architecture。            The lower sublayer of the MAC layer is the distributed coordination function (DCF)。 DCF uses a contention algorithm to provide access to all traffic。 Ordinary asynchronous traffic directly uses DCF。 The point coordination function (PCF) is a centralized MAC algorithm used to provide contention-free service。 PCF is built on top of DCF and exploits features of DCF to assure access for its users。 Let us consider these two sublayers in turn。

Distributed Coordination Function

The DCF sublayer makes use of a simple CSMA algorithm。 If a station has a MAC frame to transmit, it listens to the medium。 If the medium is idle, the station may transmit; otherwise, the station must wait until the current transmission is complete before transmitting。 The DCF does not include a collision-detection function (i。e。, CSMAICD) because collision detection is not practical on a wireless network。 The dynamic range of the signals on the medium is very large, so that a transmitting station cannot effectively distinguish incoming weak signals from noise and the effects of its own transmission。 To ensure the smooth and fair functioning of this algorithm, DCF includes a set of delays that amounts to a priority scheme。 Let us start by considering a single

delay known as an interframe space (IFS)。 In fact, there are three different IFS values, but the algorithm is best explained by initially ignoring this detail。 Using an IFS, the rules for CSMA access are as follows:

1。 A station with a frame to transmit senses the medium。 If the medium is idle, the station waits to see if the medium remains idle for a time equal to IFS, and, if this is so, the station may immediately transmit。

2。 If the medium is busy (either because the station initially finds the medium busy or because the medium becomes busy during the IFS idle time), the station defers transmission and continues to monitor the medium until the current transmission is over。

3。 Once the current transmission is over, the station delays another IFS。 If the medium remains idle for this period, then the station backs off using a binary exponential backoff scheme and again senses the medium。 If the medium is still idle, the station may transmit。