4.7 WiFi data rates and operating range
Just as for Ethernet, developments in technology have increased the achievable data rates since the first WiFi standard was developed in 1997. At the time of writing, the latest WiFi standard to be published – IEEE 802.11g – defines a data rate of 54 Mbps.
Activity 17: exploratory
How do you think the rate for transmitting messages between stations is affected by:
the management information that is included with each frame;
the protocols used to enable multiple stations to share the communication channel;
multiple users on a WiFi network?
Both the management information and the sharing protocols impose an overhead that uses up some of the capacity of the communication channel. This effectively reduces the capacity available for sending message data (as compared to management data). The effective data rate will, therefore, be lower than the value quoted.
When thinking about the effects of multiple users on the network, it's also important to realise that the data rate quoted is the maximum that can be achieved, and this has to be shared out between all the users of the network. As the number of users increases, the data rate available to each individual user decreases.
The practical message data rate that can be achieved in a wireless network is often described as its throughput. Even in ideal operating conditions, the throughput may be only 50 per cent to 75 per cent of the maximum data rate. For WiFi, throughput is generally about half the maximum data rate possible on the communication channel, giving about 30 Mbps for 802.11g networks, and this has to be shared between all the stations on the network.
The achievable data rate reduces with distance from the AP (or in the case of an ad hoc network, with distance from other stations). Maximum data rates can be achieved only within about 30 m of an AP, tailing off at distances greater than this. For 802.11g networks the data rate drops to as low as 1 or 2 Mbps at 100 m. Physical barriers such as partitions and walls will further reduce the maximum rate possible at a given distance from the AP.