The previous section identified the different channels in each frequency band which are available for AREDN® networking. Devices on each side of a radio link must use the same frequency band, channel, and channel width. Beyond that requirement, however, you have quite a bit of freedom to select the radio channels that will ensure the highest signal quality for your network.
In a basic AREDN® meshnet with individual nodes spread across a limited geographical area, all of the nodes may use the same frequency, channel, and channel width. This allows them to establish a network and route data to any of the sites as needed. However, as more nodes join the network or when several nodes are collocated, it is possible for them to interfere with each other. In order for an AREDN® network to scale up in size and complexity, channel planning becomes increasingly important.
Some amount of data traffic is required for OLSR to maintain and operate the mesh network. Having a growing number of nodes on the same channel will increase the amount of OLSR handshaking, which can lead to more latency for data traffic across the network. Any application, program, or service that is sensitive to latency (especially voice or video services) may experience difficulty or even become unusable.
Another case is when there is one poor quality link over which all traffic must be routed. The handshaking and data retransmissions may cause all the other links to wait. The entire network can be impacted by one low quality path which becomes a single bottleneck. If at all possible you should increase the signal quality of that vital link, or establish a better link as an alternate path.
In any wireless network there will be nodes which are not within radio range of each other. In the example below, A can hear B but cannot hear C. Since A and C are hidden from each other, they may try to transmit on the same channel at the same time without knowing it. Collision detection mechanisms will not help because the nodes have no way to communicate except through node B, so collision avoidance mechanisms must be used instead.
AREDN® firmware follows IEEE 802.11a/b/g/n standards and uses Carrier Sense Multiple Access / Collision Avoidance (CSMA/CA) for determining whether a channel is busy. Optionally AREDN® firmware also employs Request to Send / Clear to Send (RTS/CTS) messages to negotiate the use of a channel. Decreased network throughput is one side effect of this, because several nodes may be negotiating or waiting to use the channel.
For hidden nodes, two approaches may help to minimize this issue. You may be able to make the hidden nodes visible to each other, for example by increasing their signal strength. The alternative is to isolate the nodes completely by placing them onto different bands or channels. Since nodes using directional antennas are nearly invisible to others not positioned in the antenna’s beam, directional antennas should be used with care when sharing a channel. It may be more appropriate to create a separate link between the sites and to put the radios on a different band or channel.
This is another issue that can lead to performance problems on a network. You may have parallel paths between two Remote Nodes, and these paths have similar ETX values which indicates that the cost of using either route is comparable. These two paths may appear to be functioning well most of the time.
However, when a bandwidth-intensive application such as video conferencing begins sending traffic across one of the paths, you may notice that link getting bogged down and the ETX will drop below that of the other path. At this point OLSR switches to the alternate path which now has a lower cost. The video stream then bogs down its new path, which lowers the ETX, and OLSR switches back to the original link whose ETX is better again. This situation may continue indefinitely, with neither path being able to deliver the traffic adequately.
This issue can happen on multi-hop links with similar ETX which seem to work fine until they are loaded with traffic. Then packet loss begins to occur, connections time out, and neither path is reliable during that cycle. One solution might be to improve the multi-hop link cost by increasing the signal quality of the links along one of the paths. Conversely, you could also turn down the power on the alternate path to increase its cost. If bandwidth-intensive traffic must be passed between two remote endpoints, the best approach would be to design a more robust path between those two endpoints to meet that need.
At some sites there may be several devices mounted on the same building or structure. The photo on the right shows many nodes collocated on a single tower. Network problems can occur if these nodes share an RF band and channel. For example, when two sector antennas are collocated and share the same channel, the network throughput for that site will be reduced by half or more. If you have collocated nodes then it makes sense to allow the devices to pass traffic over their Ethernet interface rather than forcing them to use their radio channel.
Most of the latest AREDN® devices use dual polarity antennas and MIMO features in the radios that exploit multipath propagation. However, if you are using single polarity antennas with “single chain” radios, another way to achieve signal separation for collocated devices is to orient the site’s antennas so that one is vertically polarized and the other is horizontally polarized. This can result in a signal separation of up to 20 dB. Vertical polarization is usually preferred because it tends to be less susceptible to reflections and rain fade, but horizontal polarization still provides adequate signal with clear line of sight. Note that the antennas on both sides of a radio link must be oriented the same way.
Channel Planning Tips¶
You may experience poor network performance if there are too many nodes using the same band and channel. Here is a simple example to illustrate the issue: a three-hop path from QTH1 to Tower1 to Tower2 to QTH2. If all links are using the same channel, then only one link at half-duplex can send data at a time. This instantly cuts the throughput by one-third or more and increases latency with protocol overhead. To improve performance you can configure each link to use a different channel, allowing simultaneous transmissions. In the first case with channel sharing, it might be possible to have one HD video stream and one VoIP call with frequent dropouts. In the second case using different link channels, you could have three HD video streams and several VoIP calls simultaneously with few dropouts.
Based on the purpose for your network, try to create reliable paths to the locations where data is needed. Use channel separation and DtD linking of collocated nodes to avoid RF channel contention. The 3.4 GHz and 5.8 GHz bands provide the most unshared channels for use in AREDN® networks.
- If you need broad local coverage for a high profile area you can install sector antennas on a tower site: for example, three panels with 120 degree beam width each. DtD link the sectors at the tower site, and use different channels for each sector in order to avoid channel contention issues.
- Consider putting each local meshnet on its own channel to minimize the interaction between coverage areas, similar to how cellular network “cells” are planned and deployed.
- If you are installing long distance point to point links to connect mesh islands, be sure to use a separate band or channel for the backbone link. This type of link has a single purpose: to carry as much data as quickly as possible from one end to the other. Eliminate any type of channel contention so that these links can focus on throughput without distractions.
- Remember that a multi-hop path through the network must have good signal quality on each leg of the journey. You cannot expect adequate performance through a series of poor quality links. For example, if you traverse three links having LQ metrics of 65%, 45%, and 58%, your aggregate LQ will be 17% which is unusable. The aggregate LQ should be at least 50% to have a usable path.