What Is a Wireless Bridge?

I thought I would use my latest blog to get back to basics on the subject of wireless bridges as there are often a lot of misconceptions about exactly what they are and how they work. Please note some of the above licensing information may not apply outside of the UK.

So what is a wireless bridge?

In its most basic form a wireless bridge is created by linking 2 access points together. One access point adopts the role of being an 'access point' and the other the role of being a 'client' or 'station'. The client access point connects to the other access point in a very similar way to how a client device such as an iPad or laptop would connect to a normal everyday access point. The difference with a point-to-point wireless bridge is that the connection is an exclusive one between the 2 devices.

Another key difference is that wireless bridges are very directional. In most access points the RF energy is transmitted in a 360degree coverage pattern. This is useful in wireless networking as usually the client devices are either mobile or there are multiple of them. This means that the access point needs to be able to connect to them wherever they may be in relation to it. With a wireless bridge however, in most cases both points are fixed so any RF energy not directed towards the other device forming the bridge is effectively wasted.

Wireless bridges are generally required to provide links over long distances. In order to do this the devices must focus the RF energy. To explain why this is the case, think about what you do when you want to shout at someone who is quite far away from you generally you would cup your hands around your mouth. This allows you to throw your voice further and be heard at greater distances. On the flip side if you are struggling to hear someone who is quite far away from you, then you may choose to cup your hands around your ear in the direction of the person speaking. This allows you to block out other sounds and focus on the person talking to you. This is exactly what a wireless bridge does.

Typically a wireless bridge is a Layer 2 Connection between 2 wired ports. If a link is Layer 2 it means that it uses switching rather than a combination of switching and routing in order for the packets to get to their destination. Both ends of the link will exist within the same Subnet. You should effectively treat most wireless bridges as if they were simply a long ethernet cable. In fact, it actually doesn't matter what IP addresses you give the radio units at each end of the link, the link will still work, you may however not be able to log into them to manage the link unless they are addressed correctly.

Line of Sight Requirements

Typically for a wireless bridge to work at full throughput you will require perfect line of sight. With the lower frequency bridges it may be possible to obtain a link in near line of sight situations e.g if there are a few tree branches in the way. In these cases it is unlikely that you will get the advertised full throughput as the link may have to drop its modulation in order to provide a stable connection. For a 'non line of site' or 'near line of site' links you will need to operate in the lower frequencies (the lower the better). Lower frequencies have longer wavelengths so are better at penetrating obstacles (see my blog on wavelength calculations here). There is a common misconception about 'non line of sight' and often people think it is feasible to put in a wireless link that has its Fresnel Zone blocked by solid buildings. In most cases this is Game Over for the link however sometimes the wireless may multipath off surrounding buildings and a weak link at a lower data rate may be possible to achieve.

Radio Frequency (RF) or MicroWave (MW) Bridges

These bridges operate in a wide range of frequencies. The most common are:

  • 3GHz: Requires licensing and usually used by WISPs (Wireless ISPs) to provide low throughput connection to end users
  • 2.4GHz: License Exempt and used for short links providing speeds of <300Mbps
  • 5GHz: Can be licensed exempt (Band A or B) or Light Licensed (Band C) and used for links of <300Mbps
  • 6-38GHz: Licensed and can provide up to 1Gbps over long distances. You need to be careful when operating in the UK as not all of this spectrum is usable.
  • 60GHz: License Exempt and usable for links of up to 1Gbps over distances of around 800m - 1600m
  • 70GHz: Light Licensed and usable for links of up to 1Gbps. Slightly greater range than 60GHz
  • 80GHz: Light Licensed and capable of 1Gbps over much longer distances than 60/70GHz

The above bridge frequencies require line of sight to work to their full capability. Due to the nature of RF there is an area of wireless propagation between the links known as the Fresnel Zone which looks like a very elongated football and potentially another Blog topic!

Laser or Free Space Optics (FSO) Bridges

These are incredibly low latency and provide links of up to 1Gbps over distances of up to around 3.5km (1Gbps) or 5km 100Mbps). They are the license free and require absolutely perfect line of sight to operate.

Summary

Hopefully this gives you a quick insight into what a wireless bridge is and how it differs from a standard Access Point

Topic
Point-to-Point