WLAN Design
Bandwidth and throughput

Many people think that a product with an 11-Mbps aggregate data rate can fully support many 2-Mbps radios at their full data rate. The problem is that the 2 Mbps units transmit at 2 Mbps, which takes five times as long to transmit the same data as an 11 Mbps product would. This means the data rate is only 2 Mbps for any given remote, and the total that the 11 Mbps unit could see is 2 Mbps.

In order to achieve a total aggregate 11 Mbps data rate, all devices will have to be set to 11 Mbps. This is also the case with 802.11a and g operating at 54 Mbps. If a single unit is less than the maximum, the overall rate will be somewhat less than the maximum. This is because the base or central unit has to service the slower remote.

Remember the following:

  • If all devices operate at the same data rate, they will all take the same amount of time to send the same size packets.
  • If some devices are operating at higher speeds, then the packet will transfer faster. This will allow the RF to be available more quickly for the next device waiting to send data.
  • If an attempt is made to reduce throughput to a given site by lowering the bridge speed, this will also affect the high-speed bridges.

The amount of user data that is passed by the media is the throughput . When comparing true throughput to the capacity of the pipe, the data rate is the amount of all data that the media can pass. This includes overhead packets such as ACKs, association packets, and retries. Throughput is typically 50 to 60 percent of the data rate for a wireless system.

When comparing the dedicated pipes to a shared pipe, a point-to-point bridge configuration is an example of a dedicated pipe. If the RF link is set to 11 Mbps, then the data throughput between those sites is 11 Mbps. A shared pipe consists of a point-to-multipoint RF network. If the RF link is set to 11 Mbps, all the remote sites share that 11-Mbps pipe. This sharing can be compared to the sharing of an Ethernet segment. When there are multiple Ethernet devices on a wired segment, they share the pipe they reside on. The more devices added to the pipe, the slower the overall throughput.