Why is my wireless not up to speed?

A 540-Mbps peak rate of transfer is quite an achievement for 802.11n, being about 10 times as fast as 'a' or 'g' and almost 50 times as fast as 'b'. So why don't these APs ever get that level of speed in practice? Well, there are many factors outside of the actual operation of the device that will bring down any wireless transfer speed.

First, there is of course the layout of your floor plan as it relates to the AP and the client machines. While most other materials are largely invisible to WiFi, most metals are not.

A large enough piece of metal, whether a large vault or a wall stud that is close to the AP, will cause reflections to the device and constantly repath the signal, decreasing both functional bandwidth and range.

Of course, how much this will happen is entirely dependent on the environment and impossible to predict before actual implementation.

Since your AP will most likely be connected to a LAN, there are several components of that setup that can slow your transfer rate.

Most networks nowadays have 100Base-TX switches, hubs and client adapters. This essentially means that any traffic through most LANs is limited to 100 Mbps.
Nearly all networks use TCP/IP as the way for components to pass information to each other.

In order to reduce collisions and ensure that all connections get an even amount of bandwidth, TCP breaks all data into small packages before sending it along.
Also, all data sent via TCP has to be confirmed before the next package is sent. These factors can certainly reduce transfer rates from the optimal 100 Mbps and will make them even lower with more activity on the network.

We minimized this as much as possible by having a closed network with just the AP, wireless client and wired PC, but TCP/IP is no doubt still largely responsible for the lower-than-100-Mbps transfer rates we measured in our tests.

Of course, you may be accessing the AP through a USB or PC Card on your notebook PC. USB 2.0 devices have 480 Mbps throughput, while PCMCIA can peak between 270 Mbps to over 1 Gbps (depending upon what mode it is in). So even if you are managing to get your data directly from the AP without going through a LAN, your adapter will still slow it down somewhat.

About the Author

Greg Crowe is a former GCN staff writer who covered mobile technology.

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