Data and Telecom Carrier Redundancy
Connecting a redundant circuit frequently does not offer a completely redundant connection. When a redundant connection is purchased it should have as much redundancy as possible. Ideally the entire thing has complete redundancy with no single point of failure. Unfortunately the reality is that this is very difficult to achieve. However with knowledge, and some managing of the network, you can reduce the risk of network failure. The most straight forward way is to have two of everything (complete redundancy): Two switches, two routers, and two internet circuits, and so on. Within the a firm’s building is controlled by the business and therefore is fairly straightforward to arrange. However the business doesn’t own the internet, or even the carrier, so it is very hard to ensure outside your building has no single point of failure. This applies equally as much to telephone communications as it does to data communications.
The connections leaving your building is the farthest you can extend your reach so choose it carefully to provide redundancy. Consider the following example. XYZ inc. has redundant switches and two routers connecting two separate T1s. The T1s leave the building on opposite sides and are contracted through different providers. However, the cables both circle around into the same trench and pass through the same repeaters to the same central office through the same router on the same carrier’s network which has the same non-redundant upstream trunk. Not very redundant is it? Unfortunately this is a reality for many who don’t take the time to manage the communications provider.
The company that bills for a data or phone circuit may not be the same one that actually provides the service. The company providing the service is known as the local exchange carrier (LEC). It is wise to insist that redundant circuits be provided by different LECs if there is more than one in the area. It is even wiser to also insist that each connection is connecting to separate central offices (CO). A CO is the small building that houses the LEC’s communications equipment and generally serves to collect area connections and connect these to larger trunks. Unfortunately in some areas, especially rural areas and small towns, there is only one CO that serves a very large geographic area and redundant circuits are impossible. Many areas also only have one LEC. (Most LECs let other companies resell their connectivity as was done for XYZ’s service..) If there is not a second LEC, service can be acquired through satellite access. It has extremely high latency and therefore is very slow, but offers perfect redundancy. This author has seen locations (using frame relay) over 200 miles apart fail due to the same carrier issue.
Always try to ensure that all critical cabling is through separate trenches and redundant repeaters (if any) to achieve as much redundancy as possible. This is true for power, telephony, and data communications. Considering that running cables is the most expensive things a LEC does, they may resist. It may help if you don’t order the second circuit until after the first one is active and specifically request it be in a separate trench. If the LEC didn’t anticipate or plan a second connection they will be forced to run additional cabling, but it is not a guarantee. Many firms also have the redundant connections to different parts of the building to reduce risk that a future cable trench will not cut both at the same time. Be sure that there are at least two separate breakers supplying power; preferably to the redundant power supplies on every device or at least to twin devices. Also be sure that the providers equipment is connected to your building ups and generator if you have them. Do this even if they provide there own UPS. This author has seen an entire building effectively ran on backup power with the network dropping because the LEC has equipment that was only connected to the LEC’s small ups and commercial power. PBX’s traditionally have quite large UPS battery lifetimes. However once these run out they fail too. There only has to be a single piece of non-redundant communications equipment for the redundancy to fail to protect the business.
Please continue on to: internal network redundancy.