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THE NETWORK - THE BIG PICTURE

Before we go on to more detailed troubleshooting, let's examine the network in greater detail. We will examine the USA network, but you will find similar topology in other parts of the world.

Long distance access

Tandem switches and trunks, Figure 5, add the network facilities to allow A to make long distance calls from CO1. USA telecommunications policy requires that users be permitted "equal access" to various competing long distance carriers. Therefore, the local Telcos have something called an "Access Tandem Switch" that allows for this flexibility. This means that you may observe the somewhat paradoxical situation where a problem that occurs only with long distance calls is actually due to the local Telco.

Figure 5 - Local central offices with connection to the Interexchange carriers’ long distance networks

All long distance calls are routed by the CO switch to the Tandem Switch over "Tandem Trunks". These trunks are never used for local calls, but are used for all long distance calls, regardless of the long distance carrier. The local CO informs the Tandem Switch of the user's PIC (Primary Interchange Carrier - the established long distance carrier) when the call is placed. Or, if a "casual access" carrier identification code is included when dialing (a 101xxxxx code), the tandem routes the call to a carrier based on that information. The tandem switch then examines the routing tables for that carrier, and routes the call over a trunk to their long distance network. At the far end, the reverse process occurs; the long distance carrier routes the call to a tandem switch, which then routes it over tandem trunks to the destination CO of the called party.

Multiple trunk groups and overflow trunks

As we have seen, trunks are arranged as groups, and each group has members. When a call is routed to a particular group, hunting dynamically determines the member to be used for the call. In our examples so far, we have shown only a single trunk group for each route.

Often, multiple trunks groups are present between two locations. In this case, there is a hunt for an available trunk group, and then hunting within the members of that group. Note, this is not a "concentrating" function, just a "grouping" function. This arrangement is shown in Figure 6. If all trunks are busy there may be an "overflow" or "alternate" trunk group. In some cases, those alternate paths may be indirect routes that go through a third switch. Or, frequently, a two-way trunk group will be the overflow route when none of the one-way trunks groups are available.

Figure 6 - Hunting across multiple trunk groups

More on Routing Tables

As mentioned earlier, once a switch determines the destination of a call, it consults a routing table to determine where to route the call. This can be relevant for several reasons. In cases where a new area code or exchange has been added to the network, your local switch may not have an entry for this type of call to this destination.

Separate routing tables exist for Voice calls, 56 kbps Circuit Switched Data (CSD) calls, and 64 kbps Circuit Switched Data calls. The fact that the routing tables are not the same for Voice calls, and the fact that Circuit Switched Data calls are a small percentage of calls over the network, implies that that data calls are more prone to network problems. And, they will be less likely to be noticed by routine network surveillance by the Telcos.

Note that in some cases the trunks for these different types of calls may be the same, but not necessarily. Digital 64 kbps trunks can handle all three types of calls. However, each type of call is routed according to separate routing tables. Therefore, 56kbps CSD calls may take one route, while 64kbps channels are reserved for 64 kbps CSD calls. In cases where older robbed bit facilities are still in place, these trunks can handle voice and 56 kbps CSD, but not CSD at the higher rate.

TYPICAL NETWORK PROBLEMS

The following symptoms are typical of network-related problems:

Circuit Switched Data calls (e.g. codecs)

• Clean data in one direction. Dropouts or corrupted data in the other direction (intermittent loss of, or no, codec "lock" on one end).

• Having your own data looped back to you. This is surprisingly common, and is caused by some piece of gear accidentally being left in a diagnostic mode. In some cases, codec A will receive its own audio back, while codec B receives A's audio. Or, in other cases, codec A will receive its own audio back while codec B receives its own audio back. (If either side gets a mix of both audio sources, the problem is not a network problem).

• Corrupted data in both directions (neither codec locks).

• Calls fail to complete, and the ISDN cause code indicates "no route to destination" or "incompatible bearer cap" (we've seen other cause codes as well).

• Calls fail to complete and calls sits at "proceeding".

Voice calls (including modems)

• Audio in one direction only.

• Distorted, very loud, or very soft, audio in one direction.

• Echo, singing, howling on one or both ends.

• Poor hybrid performance/leakage.

• Unusually low modem connect speeds.

• Calls fail to complete. Busy, fast busy (reorder), silence, or an intercept message, is received.

Note that some of these symptoms can be caused by a bad local line, or equipment, so you need to eliminate those causes first. The key to a network problem is not so much the symptoms, but the fact that they have certain additional characteristics that can only be explained by the nature of the network. For example, if the problem occurs only when A calls B, but never occurs when B calls A.

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