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Building a 100Mbps wireless bridge - page 3

Finding the right hardware

It always helps to list what you want from the hardware before you start, so you can start eliminating what doesn't meet the cut. My initial list was
  1. Removeable Antenna with common antenna connector, so I can fit an external antenna.
  2. Variable transmit power, so I can keep within legal limits after attaching an external antenna
  3. WPA or WPA2 encryption so I can increase the security of the link
  4. Extended Range technology, so it will have increased receive sensitivity
  5. Bridging or at least WDS support
  6. Accelerated 54Mbps (i.e. 108Mbps or 125Mbps) support to enable near lan speeds
  7. Cost within clients budget
After going through the data sheets for lots of wireless kit, I'd managed to find just 2 accesspoints that met the requirements, the D-Link DWL-2200AP and the U.S. Robotics 5461 MAXg router. I was quite surprised to find so little kit that fitted. The criteria that seemed to cut out most products were removeable antenna, WPA encryption and Extended Range Technology.

In the end I chose the USR kit, as the d-link was not readily available from any of my suppliers.

The Results

A common mistake when looking at lan or wireless connections is to actually expect to get the speed listed on the box. The speed shown is really the communication speed, not the maximum speed you can transfer data. Usually the maximum speed you can transfer data is less than half the listed speed. Having scoured the web for data and reviews on wireless kit, I'd started to see a pattern in the results, which gave me an idea of what to expect. Speed tests from early 54Mbps kit (2003), had shown it worked at around 12-15Mbps without encryption and 8-10Mbps with WEP encryption. Later hardware seemed to manage around 20Mbps with or without encryption, which was more promising. For real lan speeds I was looking for 30-40Mbps from the hardware, but I wasn't expecting to get those kind of figures once encryption was turned on, as it takes quite a bit processing power to encrypt and decrypt data at those rates.

Some reviews from 2005 testing extended range wireless kit were claiming 35-40Mbps ( with acceleration ) for the hardware, but no details were given as to whether encryption was turned on or not, or whether they were transfering hard to compress files such as mp3 files, or easy to compress text files. This lack of accurate details seemed to be a common feature of both reviews and data sheets.

Once the hardware arrived, it was important to test how well it worked before installing it. Disappointingly the USR routers didn't support the 125Mbps acceleration listed on the box when running in bridging mode, and only very weakly supported it when just in WDS mode. I was hardly suprised that this shortcoming was not advertised in the data sheets but left feeling more than a little cheated. Thankfully it did have a second supported form of 54Mbps acceleration that worked in both WDS and bridging mode.

The actual figures I got testing the U.S. Robotics 5461 MAXg router with WPA encryption turned on were

  • No acceleration - 20.2Mbps in bridging mode, 20.4Mbps in WDS mode
  • 54+ Xpress acceleration - 24.8Mbps in bridging mode and WDS mode
  • MAXg acceleration - 22.2Mbps in WDS mode
I didn't bother testing the speeds without encryption as I can't imagine anyone sensibly running a bridge without some form of encryption. The speeds I got certainly weren't equivalent to a 100Mbps lan, but I'd probably put that down to the encryption the routers are having to do. At almost 25Mbps I was certainly happy enough with the kit, and it was over 10 times faster than the 11Mbps hardware it was replacing , and close enough to the 30-40Mbps transfer rates of 100Mbps ethernet.

The moment of truth

When installing the hardware at the client site, I initially left both routers transmitting on maximum power, to ensure I got a connection, so I could turn the power down afterwards and see how low I could turn it while still getting a solid 54Mbps connection.

It's always a bit nerve wracking when you first connect the antenna (I was using two USR 9db panel antenna) and turn on the hardware, while you see if you can ping across the connection. Thankfully all the hard work payed of, and the routers connected first time and the ping went across the routers in just 1.05 ms, which is 3 times quicker than the kit it was replacing.

Re-checking the maximum transfer speed is the easiest way to check how far you can turn down the transmit power, as while the router's status page gave details of connected clients, it didn't give an details or signal strengths for connected bridges. Bit of a short coming there. I was able to drop the transmit power 3db and still comfortably get 25Mbps across the link.


As wireless acceleration matures more, I'll probably upgrade the hardware again but with something that supports higher speeds, as whilst the USR MAXg router does the job, not supporting the 125Mbps mode when bridging, and not giving signal strength details were its two shortcomings I'd like to rectify. However I was very impressed with how far it managed to get a connection. During the testing I'd removed the antenna, dropped the transmit power to minimum and put the routers 30m apart and on different floors in a brick walled building, with 4 walls between them, and they still connected - now that did impress me.

Doing the sums << Prev page

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