Comparison Between Data Rate and Throughput (Including Simplex Versus Duplex Throughput)
There is a common misunderstanding regarding the bandwidth, the data rate, and the throughput of a wireless device:
* Bandwidth refers to the raw data rate of the device.
* Throughput refers to the actual amount of end user data that the device can transfer in a given time interval.
The result of this misunderstanding is that wireless network users are frequently disappointed in the wireless throughput (data transfer speeds) that they experience.
Understandably, wireless equipment manufacturers want their equipment to look as attractive as possible to potential buyers. For this reason, they usually use the raw data rate in their sales and advertising material. An 802.11b AP, for example, provides a raw data rate of 11 Mbps.
Wireless users have a different expectation; they are interested in how fast a web page or a file downloads. They are interested in the capability of the wireless device to deliver their data. When the wireless users’ 802.11b AP delivers just 5.5 Mbps of data throughput, they feel that there must be a problem with the equipment.
Most frequently, the real data throughput potential of a half-duplex wireless network is approximately 50 percent of the raw data rate. An 802.11b AP operating at the maximum 11-Mbps raw data rate has a maximum throughput potential of about 5.5 Mbps. This difference between raw data rate and actual throughput has several causes, including these:
* The framing and signaling overhead
* The half-duplex turnaround time between transmit and receive
* The lower efficiency inherent in the transmission of small packets
Collisions between wireless users and interference from other networks can reduce the throughput below 50 percent. Chapter 8, "Solving Noise and Interference Problems," discusses this issue in more detail.
Remember that your end users rely on you to set their throughput expectations realistically. When they measure their throughput and discover that it meets or slightly exceeds the throughput that you told them to expect, they will judge your wireless network performance to be good.