What Is A Wireless Bridge?
A wireless bridge is a connection between two end points, those end points can be: buildings, CCTV cameras, telecommunication masts or just about any IP device that you want to link.
One of the endpoints becomes a ‘master’ and the other a ‘slave’. Often the master will determine the frequency and other attributes the link is established using – as it assumes the role of a controller.
Unlike an access point which serves multiple end points or devices in a 30 – 360 degree spread, a wireless bridge is purely directional between point A and point B. They can be used over both short (just a few meters) and long (over many miles) distances, making them ideal for a variety of connectivity solutions.
A wireless bridge is often directional using 10 degree beam widths, allowing all of the RF (radio frequency) energy to be focussed in one direction. The fact it is directional minimises its potential to hear or pick up interference from other transmitters, and the fact that it is focussed in one direction is a positive in a noisy RF environment because the receiver and transmitters are deaf to what is around it outside of that focussed 10 degree beam width zone.
Fresnel Zone & RF Line Of Sight
Over long distances the 10 degrees beam width can get quite large and is called a “Fresnel zone”. A Fresnel zone is rugby ball shaped and its size will depend on the frequency used.
When investigating whether you have line of sight between two end points the mistake can be made to just look at optical line of sight, and not RF line of sight. Due to the Fresnel zone, this is two completely different things.
If anything encroaches on the Fresnel zone, you have a near or none line of sight link – this can often occur when you have optional line of sight. Depending on the type of bridge used (frequency or hardware vendor), it will not be detectable using a WiFI device. While you may be able to detect the frequency in use with a WiFi spectrum analyser, you will not be able to join the network or obtain any of the data from it. Above all else, most wireless bridges will use encryption to keep the data secure.
A wireless bridge is layer 2, this means it is using wired switches to communicate between either end of the link and NOT a router, a bridge is not a routed (layer 3) deployment. This is sometimes called a ‘flat’ network, all of this means you can in effect create what BT once called a LES circuit – this is a LAN (local area network) extension.
Wireless Bridge Speeds
The speeds available to anyone looking for a wireless bridge are now up to 20Gbps. To achieve this you would bond 2 x 10Gbps links together achieving 20Gbps full duplex. Full Duplex means the same speed in both directions (uplink and downlink).
The speeds achieved will not always meet what it states on the box or marketing material, as lots of factors can impact the achieved results. We have discussed RF line of sight, for example a 300Mbps link can achieve just 14Mbps (or no link at all) if the line of sight is infringed on too much or there are too many interfering channels.
Sometimes a building in the way can be your friend, as you can bounce (or to use the correct term ‘multipath’) the signal off the building. This means the receiver is going to receive multiple data streams and is going to combine them to offer one single usable link. Buildings in the link path do this better than trees, but it’s important to point out that high capacity requirements will always need a strict line of sight.
Radio Frequency and Microwave Wireless Bridges
There are various frequencies wireless bridges operate over, and in this section we are going to discuss some of the most common ones.
The following frequencies will be deployed in line of sight conditions, but can work in near to none line of sight:
3GHz: Requires licensing and offers good penetration of foliage for near to none line of sight links
2.4GHz: License Exempt and used for short links but this is also used for legacy WiFI so it often isn’t a good idea to use.
5GHz: Can be licensed exempt (Band A or B) or Light Licensed (Band C) and used for links of up to 800Mbps (using an 80Mhz channel)
The following frequencies are normally higher capacity and require RF line of sight to work correctly, this is often a condition of the license from Ofcom (that they are deployed in line of sight):
6-38GHz: Licensed and can provide over 2Gbps extending long distances. These are licensed in the UK and these licenses can be expensive when operating in the UK especially when you want higher capacity over long distance.
60GHz: License Exempt and usable for links of up to 1Gbps – 2.5Gbps over distances of around 800m – 1600m
70GHz: Light Licensed and usable for links of up to 1Gbps – 2.5Gbps. Slightly greater range than 60GHz owing to atmospherics.
80GHz: Light Licensed and capable of 10Gbps over much longer distances than 60/70GHz
Each of the frequencies above require line of sight to work at their full capability.
Bridges Using Laser/FSO (Free-Space Optics)
Laser links offer a really good high speed and interference free wireless bridge, the downsides however include fog and sun sometimes blocking the pencil thin beam of light that connects both ends. A laser link will not modulate down (go from 1Gbps to 100Mbps) in the event of something entering the line of sight path, it will just stop working until the obstruction is cleared. This can be extremely problematic for businesses that require constant uptime to fully function.
Theoretically some wireless bridge hardware can be configured so it acts as an access point serving more than one end point, while it may still bridge traffic over layer 2 to multiple locations it would no longer be a ‘wireless bridge’. In most cases you will need to use different antennas and the solution is designed differently in the first instance.
If you just have two end points then you need a bridge, if you have more than two end points you want to serve then you need a fixed wireless access point, this is also called point to multipoint.
Wireless bridges use point to point technology to wirelessly connect two end points together. By using high frequency radio waves, wireless bridges can now transfer data up to 20Gbps (full duplex).
Most commonly, wireless bridges are used to connect: buildings, CCTV cameras, sensors, telecommunication masts, corporate WAN connections, internet access or anything else that requires a data connection.
Yes wireless bridges can work without line of sight but don’t expect hundreds of Mbps through a non-line of sight connection, it all depends on distance and the amount of intrusion there is into the Fresnel zone
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