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Trunk group behavior when a trunk is bad

Trunk hunting is key to understanding the patterns of behavior typically experienced when the network is the cause of the problems. Using the example shown in Figure 2, lets note what would happen if different "members" of the trunk group were having problems. Recall that this is a bottom up trunk group. First assume that the first member of this group has a problem where it has audio in one direction only - trunks are nearly always 4-wire circuits, so problems that occur in only one direction are the usual case. Line A dials B, and gets no response. S/he then hangs up and dials again. Since this is the first member of the group, this will happen repeatedly until, by chance, another call is on member 1 while A is dialing, causing A's call to hunt up to member 2 or higher.

In this case the, symptoms will happen quite often, and will occur frequently regardless of how busy the network is. During the busiest time of day (called the "busy hour") the problem will happen slightly less, since the odds will be better that someone else will have the bad channel when A makes the call.

Now, let's assume that instead of member 1 that has the problem, member 6 has the problem. In this case, during slow times of the day, the problem will not occur; it will only occur once 5 other calls are up. Once this occurs, the symptoms will be similar to the first case. The average incidence will be lower than the previous example, even during the busy hour, since whenever a party on members 1-5 hangs up, the call will go through rather than hunting all the way to member 6.

Figure 3 -Trunk groups between local central offices

One-way versus Two-way trunk groups

In Figure 3 we see our example again, however this time we have included provisions for B to call A. The most common case is where two "one-way" hunt groups are used between CO1 and CO2. Trunk group 1-2 is used for calls from CO1 to CO2, and trunk group 2-1 is used for calls in the opposite direction. In this case, a bad trunk in group 1-2 would result in some percentage of failures when A calls B. However, since trunk group 2-1 is unaffected, 100% of calls from B to A will succeed. This behavior, where the direction of the call consistently affects the results, is a classic symptom of network-related problems.

Figure 4 -Local central offices with two-way trunks

If traffic between CO1 and CO2 is small enough, a "two-way" trunk group may be used instead of two one-way groups as shown in Figure 4. This increases network efficiency. In this case, calls in the two directions hunt in opposite directions: Calls from CO1 to CO2 hunt bottom up and calls from CO2 to CO1 hunt top down. If member 2 were bad, we will see, on average, many more failed calls when A calls B versus B calling A. On the other hand, if member 3 were bad, we'd see similar failure rates in both directions. Fortunately, two-way trunk groups are rare, which makes life simpler when troubleshooting.

Introduction to troubleshooting network problems

The key to troubleshooting network problems is persistence. If we make enough calls, and we eventually get one that does not fail, this tells us several things:

• The problem is not our equipment. Terminal equipment (such as a telephone or codec) should not care how many calls you make - it should act similarly in each case. 

• The problem is "acting like" a "network" (e.g. a "trunk") problem in that it is non-absolute; rather, it is probabilistic.

Generally, we will want to make 15 calls, carefully keeping track of the number of calls where the problem occurs (we can then calculate a "success rate" from this raw data). Next, we reverse the direction of the call, and place 15 calls. If the success rates are markedly different, we can be very suspicious this is a network problem. The logic for this conclusion is as follows: On each call the same customer equipment and same Central Office switches will be used. However, as we have seen, trunk selection is dynamic. Another clue that the problem may be network related, is if the success rate varies substantially depending on the time of day. You will also sometimes note that the success of Circuit Switched Data (CSD) calls at 56 kbps may differ versus CSD calls at 64 kbps, and both will usually act differently versus voice calls.

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