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.