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Building a 100Mbps wireless bridge
A case study on building a high speed 120m wireless bridge between two corporate offices, including details on which hardware was choosen and why.

The Mission

I've been asked to upgrade a wireless bridge between two offices from the current 11Mbps bridge to at least 100Mbps. The current bridge was one of the first wireless setups I'd done using consumer grade Buffalo access points, and has proven to be very reliable and stable. When it was first installed, the networks at either end of the link were running at 10Mbps, so an 11Mbps bridge was fine, and 802.11b kit was all that was available at the time.

The issues

Upgrading the link from 11 to 100 meg isn't just a simple case of replacing the accesspoints or routers with newer kit and using the existing antenna. I wish it were that simple. The main problem is receive sensitivity. Wifi hardware can pick up a much weaker wireless signal running at 11Mbps than it can at 54 or 100+ Mbps, this usually is exhibited by the link speed dropping as you become further away from the access point, that is, as the signal becomes weaker.

So even putting newer kit in, I'd still only expect about an 11Mbps signal. I also needed to upgrade the antenna as well, but the problem was working out by how much.

If you download the datasheets from manufacturer web sites, you will see that very few give real figures for either the receive sensitivity at different speeds for their kit, or usable range figures. Some will claim things like 'max range 100m' but what does that mean? The maximum range to what? and at what speed? Is that a 2Mbps link to a pc with a large antenna, or a 100Mbps link to a laptop with a pc-card? These bland statements in manufacturers literature really don't shed much light on their hardware's performance, and I guess that is their purpose. Some newer harware is even claiming '30% greater range than standard 802.11g wifi' without explaining what standard harware is, or what the standard range is. Not helpful. There is also newer hardware with a feature called 'MIMO' that boasts increase range, but it manages this by using multiple antenna, which would not help me, as I'm going to be sticking a single external antenna on the access point at each end.

The only figure that most manufacturers to publish is the transmit power of their hardware, but even then they sometimes give this as a range such as '18-20dbi' without explaining when is it 18 and when 20.

Making Sense of the Numbers

When 802.11g wireless kit is claiming speeds of upto 108Mbps or 125Mbps if you read the small print you will see that the kit is actually just applying various acceleration tricks to a 54Mbps link, such as packet bursting and compression. Some of these techniques improve the speed for an individual client connection, others improve the aggregate speed possible with multiple client connections. So the range and power figures for running at 54Mbps should be the same as those when running at 108 or 125Mbps, so I'll just refer to the 54Mbps connections from now on, and assume the figures with acceleration will be the same. The maximum transfer rate we were currently getting with the 11Mbps wireless link was just 2.3Mbps with 128bit WEP encryption.

Lets start with the transmit power figure. When this is given as a range, say 16-18dbi it appears that the higher figure refers to the transmit power at the lowest transmissions rates ( 2Mbps and up ) whilst the lower transmit power refers to the power at the highest transmission speed ( 54Mbps and down ). Given I'm looking to build a fast wireless link, I need to be looking at the lower transmit power.

Now onto receive sensitivity. Whilst not all hardware is the same, or performs as well under different conditions, it seems that the average receive sensitivity at 54Mbps is around -66 to -68db. There is however some newer kit coming to market which supports greater ranges, usually denoted by a term like 'rangemax' or 'maxg' and these seem to have a recieve sensitivty of around -71 to -72db at 54Mbps. This compares to a receive senstivity of -83db at 11Mbps for the bufallo kit currently running the bridge.

And so onto range. Many manufacturers give a max range figure, but only some show the maximum range for different connection speeds, which is much more useful. Whilst they don't explain what it's the maximum range to, I always assume it's a laptop with a pc-card from the same manufacturer, with the same receive sensitivity as the access point, but without an external antenna. Though this is just a guess, it seems to be born out by empirical testing (more on that later).

Looking at a lot of data sheets for a lot of kit from a lot of manufacturers, it seems that most access points with a 2db antenna manage a maximum range of around 50m outside, but thats with a 10% error rate. I assume by error rate, they mean that 10% of the wireless packets had to be retransmitted, which wireless kit does automatically as it copes with interference from phones, microwave ovens, and folk walking between the link. This 50m figure though is the maximum range at 54Mbps in ideal conditions, with nothing at all between the accesspoint and laptop. For a stable, robust and fast connection, we don't want to be too close to this maximum range.

Next Page >> Doing the Sums



As we suspected, US Robotics have come out with a 125Mbps MaxG wireless bridge, which may have been ideal for this upgrade.

Posted 12 Nov 2005 by Bob
commentBetter choice
Zyxel G-6000 is great for fast wireless bridge LAN to LAN.
Posted 20 May 2006 by WaMaR
commentBuffalo AirStation WHR-HP-G54 for bridging?
Thanks for this article. This is the first time I've attempted any bridging between buildings. Would the Buffalo WHR-HP-G54 be a good option?
Posted 12 Jul 2006 by kirkilj
commentRE: Buffalo AirStation WHR-HP-G54 for bridging?
It depends on whether the antenna is detachable or not, and if the AP supports variable transmit power.
Posted 22 Jul 2006 by Bob

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