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Designing for Mesh and bridge links



Mesh

Mesh networks are networks that are implemented at locations where wired access may not exist. The access points reach the wired network by hops. The access points are directly connected to each other with one or more access points for redundancy. There are some guidelines for implementing a mesh network. Each link adds more latency to the network, and since you can expect some missing frames those need to retransmission and this incurs delays as well. For access point type, you should select dual-band access points. One radio is used for clients and the other radio for connecting to other mesh access points.

For the backhaul, that is similar to PtP/PtMP, the link budget must be determined based on the data rate that is needed and the distance of the link. For the backhaul band most of the time 5 GHz is selected since the clients connect to 2.4 GHz. However, nowadays you see that more clients will connect to the 5 GHz. In this case you can select the 2.4 GHz for the backhaul and the 5 GHz for the clients or mix them together. Mixing together means some access points with the backhaul on 2.4 GHz and some access points with 5 GHz backhaul. With some vendors, you can do dual-band 5 GHz radios. In this case, select channels for 5 GHz that do not support clients and 5 GHz channels for supporting clients.

Channel plan for the 2.4 GHz network can be hard since there are only 3 non-overlapping channels. For small-scale mesh networks it is possible, but for larger-scale, it is better to use the 5 GHz. You can use the non-DFS channels for client access and the DFS channels for the backhaul. Also with that, ACI can still exist when you pick 5 GHz channels that are close to each other.

Bridge links

Like with mesh links, with designing for bridge links you need to select the right output power and antenna and mount them line of sight. There are two categories, PtP (point-to-point) and PtMP (point-to-multipoint). With PtMP, use high-gain omnidirectional antennas or semi-directional antenna for the root bridge. The branch offices will use a high-gain directional antenna to get back to the central location. When the branch offices cannot hear each other, you have kind of a hidden node situation and it can cause collisions at the central location. Like with mesh network, you should use different channels, not channels that are too close to each other because this can cause CCI and ACI.

For bridge links, you need a visual line of sight (LoS). The visual LoS is what you can see when you are standing on the building, this is a straight line. RF are waves and they can refract or reflect, so this line is most of the time not straight. This is the RF LoS. The extra space can be calculated and it is called the Fresnel Zone (Frah-nell). When there is no visual LoS, then there will be no RF LoS. For wireless, the first Fresnel Zone (1FZ) is important. To calculate the 1FZ you take the 72.2 x (D/(4xF)). Where the D is the distance in miles and F is the frequency in GHz. So, for a 1.5 mile distance in the 2.4 GHz band it will be 28.54 feet (72.2 x (1.5/(4 x 2.4))). You need a 60% clearance radius—when it is blocked by more than 40% it is a non-functional link. So, 60% of 28.54 is 17.12 feet (or using the formula 43.3 x (D/4xF))). Since 60% is on the edge, some wireless engineers use 80% clearance to have some room for error, since, for example, trees grow and can block the 1FZ easily. In this case the formula will be 57.8 x (D/4xF)), and the answer should be rounded up for some extra room of error. We calculate the radius, since the blockage is most of the time downwards only. The answer is the height that the antenna needs to be, again in feet. When the link is over a greater distance, the earth bulge can interfere as well. We are talking here about distances greater than 7 miles. For this the antenna need to be raised, and the extra height can be calculated by D2/8 where the distance is in miles and the height in feet. For 11 miles, the antenna needs to be raised by 15.12 feet (also round up here as it’s better to be safe than sorry). To put both together in one formula, the minimal antenna height is (57.8 x (D/(4xF))) + (D2/8). So, with 11 miles the answer will be 77 feet high for the antenna placement.

For data rate requirements, you should have SNR value to achieve the data rate that you want. This should be written in the vendor specific documentation. You need to select the band that you want. 2.4 GHz works better over a longer distance, but has more ACI and CCI issues than the 5 GHz band. As mentioned in other blogs, the received signal strength indicator (RSSI) is a measurement that is different per vendor. The RSSI_MAX is not the same for all the vendors, and the range is between 0 and 255. Where Cisco uses 0-100, Atheros chipset use 0-60. For example, 75% of the signal strength with Cisco is 75, while with Atheros it is 45. Because of this complexity you should compare the chipset with the same chipset and when you do a survey using the device that is the weakest.

For power, you should use the same power on both ends of the link. Also, when you use two different types of antenna, for example, an omnidirectional and a directional. There should be no mismatch in power.

As stated in the beginning, there are different types of antenna. If you want to use a PtMP you will probably select an omnidirectional at the central location. Different types of antennas have different beamwidth. The beamwidth is measured on the main lobe, the primary RF energy that is transmit from the access point. The beamwidth is measured on the horizontal plane (Azimuth) and the vertical plane (Elevation). The measurement is taken where the signal is decreased by 3 dB. In the below table, you see from some antenna types the different beamwidth (horizontal and vertical).

antenna

To align antenna, there are several methods to use: Laser Sighting—good for long distances and establishing the initial connection RSSI alignment—good for short distances and fine tuning the links Level alignment—good for close antennas at the same or similar elevations Sight alignment—good for short to medium range alignments

All this leads to the link budget to implement quality bridge connection. The link budget results in system operating margin (SOM) that takes power, gain, loss receiver sensitivity and fade margin into account. Consider, what is the minimal signal strength that you need at the other side of the link (receive sensitivity), because cables, connectors and the free space path loss weaken the signal during transmitting. The receive sensitivity is the weakest signal the wireless radio can reach at a specific data rate. In the below table, the weakest signal strength is -94 dBm, and like the RSSI, also the receive sensitivity is different per vendor. To calculate the SOM you subtract the signal strength (S) from the receive sensitivity (RS) (SOM = RS – S) (29 dBm = (-94) - (-65)). The 29 dBm means that the signal can be weakened by 29 dBm and that the link will still be maintained. This is an estimated number; the link can still be up at a loss of 30 dBm or be lost at a loss of 27 dBm. This formula is only for outdoor access points because indoor access point is most of the time not direct in LoS.

sensitivity

The link budget from below image will be calculated as follows: 100 mW equals to 20 dBm.
The signal strength = 20 dBm – 3 dBm + 7 dBi – 83 dB = -59 dBm
SOM = (-94) – (-59) = 35 dBm. So, the signal strength can be weakened by 35 dBm and the link should be, in theory, still up and running.

example