Руководство По Проектированию для Cisco Cisco Aironet 350 Mini-PCI Wireless LAN Client Adapter
3-19
Enterprise Mobility 4.1 Design Guide
OL-14435-01
Chapter 3 WLAN Radio Frequency Design Considerations
Planning for RF Deployment
Client Density and Throughput Requirements
Wireless APs have two characteristics that make actual client data throughput slower than the data rate:
•
APs have an aggregate throughput less than the data rate because 802.11 provides a reliable
transport mechanism that ACKs all packets, thereby halving the throughput on the channel.
transport mechanism that ACKs all packets, thereby halving the throughput on the channel.
•
APs are similar to shared hubs. That is, the channel is shared by all the clients associated to that AP
on that channel, thus collisions slow data throughput.
on that channel, thus collisions slow data throughput.
With this in mind, you must have an estimate of the maximum number of active associations (active
clients). This can be adjusted more or less according to the particular application.
clients). This can be adjusted more or less according to the particular application.
Each cell provides an aggregate amount of throughput that is shared by all the client devices that are
within the cell and associated to a given AP. This basically defines a cell as a collision domain. After
deciding on the minimum data rate, be sure to consider how much throughput should, on average, be
provided to each user of the wireless LAN.
within the cell and associated to a given AP. This basically defines a cell as a collision domain. After
deciding on the minimum data rate, be sure to consider how much throughput should, on average, be
provided to each user of the wireless LAN.
Take the example of a simple barcode scanning application; 25 Kbps may be more than sufficient
bandwidth for such an application because using an 802.11b AP at 11 Mbps of data rate results in an
aggregate throughput of 5–6 Mbps. A simple division results in a maximum number of 200 users that
can theoretically be supported. This number cannot in fact be achieved because of the 802.11
management overhead associated with the large number of clients and packet collisions. For a 1 Mbps
system, 20 users can use the same AP for similar bandwidth results.
bandwidth for such an application because using an 802.11b AP at 11 Mbps of data rate results in an
aggregate throughput of 5–6 Mbps. A simple division results in a maximum number of 200 users that
can theoretically be supported. This number cannot in fact be achieved because of the 802.11
management overhead associated with the large number of clients and packet collisions. For a 1 Mbps
system, 20 users can use the same AP for similar bandwidth results.
You can increase the potential per-user throughput by decreasing the number of users contending for the
aggregate throughput provided by a single AP. This can be done by decreasing the size of the coverage
area, or adding a second AP on a non-overlapping channel in the same coverage area. To reduce the
coverage area, the AP power or antenna gain can be reduced, resulting in fewer clients in that coverage
area. This means you need more APs for the same overall area, increasing the cost of deployment. An
example of this is shown in
aggregate throughput provided by a single AP. This can be done by decreasing the size of the coverage
area, or adding a second AP on a non-overlapping channel in the same coverage area. To reduce the
coverage area, the AP power or antenna gain can be reduced, resulting in fewer clients in that coverage
area. This means you need more APs for the same overall area, increasing the cost of deployment. An
example of this is shown in
.