It depends. When communicating from a wired device to a wireless device, a good rule of thumb is that you can expect to average roughly a third of the datarate. That may surprise you, or disappoint you, but it is true of most two-way wireless networks including Wi-Fi networks. The difference in throughput is because of the nature of the technology, network protocols, and radio transmissions.

For example, suppose you have a desktop computer that is connected by Ethernet cable to a 802.11g "54 Mbps" wireless router. When sending a large file to a wireless laptop, you can expect roughly 18 Mbps of actual throughput.

Similarly, suppose you have a 1XRTT network-capable wireless telephone and are attempting to connect to a video stream. Although it reports that you are connected at 230.4 Kbps, you can expect throughput of roughly 76.8 Kbps.

If your transfer is taking multiple wireless "hops," such as from a wireless computer to an AP, and then again from the AP to your computer, expect to cut the throughput by roughly half again.

I emphasize roughly because it can be considerably higher or lower depending upon...

•The strength and clarity of the wireless signals
•The interference caused by non-WiFi devices that share the same radio frequencies as the wireless network
•The activity of neighboring WiFi devices that are sharing the radio frequency
•The number and activity of clients in your own network
•The efficiency of the transfer protocol that you are using
•Any proprietary enhancements you might be using in your network (such as those that purport to go up to 108 Mbps or 125 Mbps on an 802.11g device)
•The output speed of the device from which you are downloading the file
•Other activity on your local computer or the sending computer
•Other factors

This is not an evil plot by the networking manufacturers (although a little marketing is certainly involved). Network devices are always described by their maximum data rate (signalling) rate -- the speed at which a network device can send or detect the electrical pulses (1's and 0's). But because of all the factors above, the maximum throughput of any network device is always going to be less.

If you're still awake at this point, here are some terms that will make you sound smart:

•The port speed is what the hardware and driver reports to the Operating System (OS). You can think of this as the maximum data rate (sometimes spelled datarate). The port speed is also the speed that the OS will talk to the driver. While it usually stays at the maximum, the driver may adjust the port speed lower if it detects errors when communicating to the OS. An example of this is when a USB 2.0 network device is inserted into a USB 1.1 connection. In several versions of Windows, when you hover over the icon in the system tray for your wireless network card (the Windows icon, not the manufacturer's icon), you will see the port rate in a pop-up window.

The Windows icon looks like this:

•The data rate is the current signalling rate of the wireless transmitters and receivers. In 802.11, and other protocols, this rate often varies up and down along a set of agreed-upon supported rates. This increase and decrease is based on the error rate of the wireless link. If the errors are increasing, then the link is slowed down (because slower packets are easier to decipher). Many of the Wi-Fi utilities provided by the manufacturers will report both the TX and RX rate in their monitoring utilities, You often see that they are somewhere below the port speed.

•Finally, the throughput is the rate of the delivery of the actual cargo of the network -- your desired data. This is what really matters to you! As a rule, the faster the data rate, the faster the throughput unless there are too many errors. Errors cause retransmissions, which takes bandwidth. It is more efficient to slow the datarate to the point where fewer errors occur. (This is why it's a bad idea to lock the Transmit rate or "TX Rate" instead of setting it to automatically adjust.)

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by funchords edited by No_Strings
last modified: 2005-12-26 19:04:48