What is a network loop?
Let’s take a look.
There are different types of loops but basically a network loop occurs in networks when the data is going from the router/switch and returns back to it. For example if a Desk Phone has a connection to the network via one LAN cable it’s fine. However for example if it is then connected back to the network via a second port a loop is formed. This is because the Desk Phone now has two active paths to the network. The information now loops and amplifies itself instead of stopping at its destination, this propagates throughout the network and more devices are affected until most devices fail to communicate effectively.
It should be noted that what appears to be a network loop may not be as simple as two cables going to the same device. The problem can also be caused by damaged cables that are causing a short circuit on the network; whereby the transmit and receive lines are touching even intermittently can cause serious network issues.
Because of their communication needs, VoIP phones are particularly susceptible to network loops, and will fail very quickly when a loop occurs. With some of the resulting actions being quite bizarre such as attempts to reboot, blank screens or flashing screens for example. However whatever the symptom the phone will be next to useless.
The situation needs to be rectified as soon as possible to regain service, but any action needs to be methodical and controlled. While it’s tempting to rip out all the networking cables at the same time its not the most productive method.
Let’s take a particularly annoying scenario as an example, which isn’t as simple as two cables causing the loop.
We have a building with offices served by separate cables from the main router to individual devices, hubs or switches. Most devices are working correctly around the building; one office has a switch serving several VoIP Extensions. Periodically one or two of these Extensions will start to have crackling connections and poor service. Then they start to disconnect as they try to make connection with the PBX over the network. This gets worse until such time as all of the Extensions are unusable, at this point the users start moving round the office because they can’t work. In doing this they move the handsets and in turn the cables and miraculously the Extensions start to work again. I know at this point everyone wants to rip all the cables out, however unless you plan to replace all of the cables this action is counterproductive because you won’t identify the offending cable(s) (unless you have relevant cable test equipment of course). Assuming that there’s nothing else in the room and each Extension has a labelled dedicated cable back to the switch, a controlled logical approach is best.
- Inspect all the cables for any physical signs of damage, paying particular attention to any bends and the connectors at each end. if any damage is found replace the cable immediately and check the status of the Extensions.
Hopefully you found a faulty cable and everything starts to work again. However, I’m sorry to say that we all know things aren’t that easy. If you didn’t find a faulty cable but the Extensions are now working you’ve got an intermittent fault somewhere and its the most annoying thing to find. You’ll now need to wait for the next issue to raise its ugly head and proceed as follows.
- It’s important to keep all cables in the same position around the office to minimise the risk of hiding any intermittent fault that may exist. When removing a cable from the switch try to minimise movement of any other cables.
- Firstly can you identify the Extension(s) that started to experience the poor signal quality or other problems? If you can, disconnect the cable(s) for the Extension(s) from the switch, otherwise disconnect the first Extension cable at the switch.
- With the cable(s) disconnected do the other Extension(s) start to come back online? Have the users lift their handsets and see if the dial-tone is present and clear.
- If at this point the other Extension(s) appear to be fine you may have found the offending cable(s). However if the fault was intermittent the offending cable(s) could still be in use but in a position that its not causing an issue at the moment.
- A quick and easy test is to plug the offending cable(s) back in (one at a time if more than one) and see if the problem returns, if it does replace the cable(s).
- Otherwise go through all the cables at the switch (repeating the process above) until you’ve tested each one.
Its time consuming and annoying but there’s not really a lot of choice, unless you have test equipment or replace all the cables. Remember if one of the cables is damaged any number of others could also be damaged but not showing an error as yet. My advice would be to route cables where they can’t be damaged wherever possible; under the floor or over the ceiling is way better than underfoot.
For reference all cables have what is called a minimum bend radius which refers to the lowest radius to which a cable can be bent. Which basically means how tight a corner the cable can get round. For comparison think how tight a corner a bicycle can turn in comparison to an articulated lorry.
We hope that you don’t experience the issues above but if this helps someone to overcome a particularly annoying situation its been more than worthwhile.
If you can think of an article on VoIP, VoIP equipment or have an idea for a VoIP feature please let us know in a comment below or contact us via the website. While we can’t promise to post something immediately we do read all your comments and will act on them when time permits.