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Can you help me clear a dispute about signal loss, and a rebroadcast site for a 802.11b network?

I had a major argument at work about signal loss, and a rebroadcast site for a 802.11b network.

We have a Cisco 340 bridge with a directional dish 24 DB on a 40-foot storage silo. Base elevation is 1186 feet. The source antenna central node is on a tower of about 130 feet and the base elevation is 1235 feet. Distance is 4.3 miles central node is an Omni at about 10 DB. The signal is weak about -85 DB, but it works most of the time.

My solution is to place a 100-foot tower in the middle of the path at 2.4 miles from the central node with two Cisco's at the tower base. A re-transmission site if you like, it will also serve a PC there that is now a dial-in client. Base elevation 1202 feet. New antenna configuration will be from what I got kicking around. The 24 DB from the storage silo pointing back to the central node would be relocated to the tower and continue to point to the central node. A 10 Db omni would be placed on the tower Top and a 15 Db directional would replace the 24 Db antenna at the storage silo.

What I expect is that it would fix all the problems at the distant site, and provide LAN services at the new mid-point site. I've budgeted about $10K CDN.

The central site is down town... but it's a small city no large buildings biggest obstructions are trees and the system acts up from May till Oct. The remote sites are on the edge of the city, nothing but farmland and gravel pits.

I've been told it won't work and that it's not worth the money. I know the rebroadcasts will reduce performance by 50% but what are my lost packets and retransmissions costing me?

I've had great luck till now with these Cisco units, I wish they could go on the tower instead of a box at the base, but there not meant for out door use.
I do love a good semantic argument. When you're talking about re-transmission sites there are a few different ways people tend to implement them; relay sites, repeater sites and point-multipoint hubs.

I'll go through the fundamentals of each type of implementation and the pros and the respective pros and cons.

Relay Site / Active Repeater Site
A relay site is usually used where you do not want to half your throughput and can afford to have two RF links instead of one.
Link 1: Central Node -> New Site
Link 2: New Site -> Storage Silo

At the relay site you would have two Cisco 340 or 350 series bridges (using the assumption that you're staying with Cisco) and a switch. This way you reduce the RF path loss by shortening each link and gain a much higher possibility of success.

The current dialup client could be patched into the switch that sat between the 2 Cisco Bridge Units and operate as part of the network.

However, as you're now managing two different links using different frequencies, you don't lose any throughput – however you may add about 2-4 ms worth of delay to your network link.

Passive Repeater Site
A passive repeater site is one where you place two directional antennae at the new site with a single cable between the two.

Central Site -> New Site Antennae A (cable) New Site Antennae B -> Silo

In this circumstance there is no Cisco units placed at the bottom of the new site and the dialup client would have to remain dialup. The advantage of this is the lack of hardware at the central site.

The beautiful thing about this solution is the simplicity of its design. Be aware that you loose about 3-6 dB using a passive repeater but you do not lose any throughput. You may also experience an additional delay of up to 2 ms on your network link but I really wouldn't imagine it to be that high.

This solution is usually used when you have excellent signal strength but no line of sight between the two end-points.

Point-Multipoint A point to multipoint solution could be a possibility for this scenario however this is the case where you would reduce your throughput by (more than) 50% due to rebroadcasts.

A single Cisco bridge would be placed at the new site (to which the existing user could be attached via the Ethernet port) and an omni-directional antenna could be placed atop the new site.

The layout for this would look something like:
Central Site -> Omni Antenna -> Storage Silo

The advantage of this solution is that it allows you to use minimal hardware whilst breaking the long link into two shorter links. The shorter links will help to reduce your path loss but it would take a site survey and some calculations to determine whether the 10 db omni-directional antenna would be enough for a successful link.

Personally I think your best option would be an Active Repeater site if you have the budget but I also believe that a point-multipoint configuration with the Cisco Bridge Units would also work.

Unfortunately for you, whilst I have done some preliminary calculations using the information that you gave me, I'm not going to give you any hard numbers or a definite answer. This is primarily due to that fact that even if there was nothing between the two sites, I have no way of knowing what types of EMI and other RF Interference exist on each site, tree levels, etc etc etc.

However, feel free to tell your co-workers that you were right – as I believe you probably are – but without a proper site survey I can't give you a definite yes.

This was last published in August 2003

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