Bridge Topologies
Bandwidth

Many people think that the 11-Mbps products will support many 2-Mbps radios. It is also thought that it will provide a total, or aggregate data rate of 11 Mbps, with each remote unit getting a full 2 Mbps. The problem is that the 2-Mbps units transmit at 2 Mbps. This will take five times as long to transmit the same amount of data, as an 11-Mbps product would. This means that the data rate is only 2 Mbps, for any given remote site. The total the 11-Mbps unit will see at any time is only 2 Mbps as shown in Figure .

In order to achieve an aggregate data rate of 11 Mbps, all remote units have to be using an 11-Mbps rate. If a single unit is less than 11 Mbps, the overall rate will be somewhat less than 11 Mbps. The base or central unit has to service the slower remote sites also.

If all devices are operating at the same data rate, they will all take the same amount of time to send the same sized packets. If some devices are operating at higher speeds, then they will transmit the packet faster. This will allow the RF to be available more quickly, for the next device that is waiting to send data.

Notice, in Figure , the differences between the values for data rate and those for throughput. For example, a data rate of 1.6 Mbps may only yield 500 Kbps of throughput. This would represent only a 31 percent efficiency. This difference is because the term data rate does not take any overhead into account. Overhead includes the protocol headers and trailers, acknowledgements, retransmissions, and more. In the case of WLANs, the RTS/CTS exchange can also add to this overhead.

Some 802.11b vendors claim a full 1 Mbps, but coverage may be limited to about 9 m (30 ft). At the maximum rated distance, some of these systems may only see about 300 Kbps of throughput.

As shown in Figure , the Cisco Aironet equipment ratings are at the maximum distance, unless specifically stated otherwise.

While the Cisco Aironet APs will allow 2007 associations, with each AP in the system, the limiting factor is the bandwidth needs of the applications. The Cisco Aironet 802.11b APs act like 10-Mbps Ethernet hubs. If the system is used for minimal bandwidth usage applications, such as e-mail, then up to 50 users per AP can be easily supported. For high data-rate applications, fewer users can be supported.

Although this section has focused on 802.11b and its 11-Mbps maximum data rate, the concepts also apply to the higher data rates of 802.11a. The maximum aggregate date rate can only be achieved in a cell, if all remote units are operating at the highest rate. The number of users that can be supported by a single AP is dependent upon the bandwidth the application needs.