The other day I was doing some speed tests and found I’m only getting about 2.5 mbps down on my laptop from Time Warner’s 4mbps service. Before calling I decided to hook the laptop directly up to the cable modem and try the test again and to my surprise I got 3.98mbps down.
I then hooked the laptop to my wired router (linksys RTP300 from Vonage) and power cycled everything and still get the 3.98 down.
I then hooked my wireless AP back up (linksys WAP54G) and had the laptop connect to that. The speed went right back down to 2.5.
Laptop is a Dell Inspiron 6000 with built in wireless G. I was sitting about 10 feet from the access point when I did the second test. I’ve tried swapping out the cat5 between the router and the access point.
Most people want to know how fast it goes, their first surprise after benchmarking is that they can’t get the speed written on the box, and that the number seems low. Even when converting the byte per seconds to bit per seconds, it is obvious that the TCP throughput is much lower than the signalling rate. This is because the Wireless LANs are slower to start with, may not use their fastest speed and on top of that have higher overhead.
Most Wireless LANs products only specify their highest signalling rate (also called bitrate). The signalling rate is the speed the bits are send over the air (802.11b is up to 11 Mb/s, 802.11g is up to 54 Mb/s) and it doesn’t account of all the overhead of the various protocols.
Most protocols have multiple rates and adapt the signalling rate depending on the quality of the link (for example 802.11b can run at 1, 2, 5.5 and 11 Mb/s). When the link is clear and reception is strong, it uses the fastest rate, but when there is noise/interference or the devices are further away, it downgrades to a more robust rate, which is slower. The throughput that you will get will depend on which rate you are using, for example the highest speed might be only usable in line of sight. The environment conditions constantly change, so most products adjust their rate continuously, and the bandwith available can be unpredictable.
The Wireless LAN protocols also have usually a higher overhead than their wired counterpart (such as Ethernet) because of some technological limitations and to improve the reliability and the coverage of the Wireless LAN (optimisation trade-offs). Wireless protocols have a lot more protocol overhead, such as contention, large headers, encryption, management frames… Retransmissions are needed to overcome various radio conditions. Part of the protocol overhead is fixed and does not scale with the bit rate, which means that at the highest rate this overhead is becoming proportionally larger. At 11 Mb/s the TCP throughput can be almost reduced to 50%.
One thing to do on your router to increase your speed would be a) change from mixed b/g mode to just g. Other than that you won’t see much faster through a wireless device until the n standard is released. Right now there are draft n products which work great, but unless upgrading equipment you won’t see much faster speed tests wirelessly (ps 15/2 is only $10 more and so worth it IMO).