by Rolf Taylor

INTRODUCTION

The Public Switched Telephone Network (PSTN) is truly one of the marvels of the 20th century. Broadcasters rely on this technology extensively; not just for call in programming, but for high quality links over ISDN, and for low cost IFB. Broadcast Engineers are forced to become familiar with this technology to do their jobs.

Many readers will be familiar with the in and outs of troubleshooting ISDN and POTS (plain old telephone service) circuits. However, once your line and equipment have been eliminated as the source of problems, troubleshooting can get very difficult. Only through a thorough understanding of the network is it possible to conclusively locate the source of the problem. Moreover, since the Telco is bound to be skeptical, it is best to be sure of the problem before going to them.

Figure 1 - Simplified view of the telephone network

NETWORK FUNDAMENTALS

Usually we can ignore the network when considering Telephone problems. In figure 1, we see the usual simplified view of the network. Normally, this simplistic view is sufficient to allow us to solve our difficulties - After all, everything in the cloud is the Telco's responsibility, right? True enough, and network problems will frequently resolve of their own accord. However, sometimes we can't wait for someone else to discover and fix the problem. Put on your Sherlock Holmes hat and let's investigate!

Trunk groups & hunting

The key to understanding network behavior, is the understanding of trunk groups and how they function. Lets look at the simplest telephone connection that includes network trunking. Figure 2 shows a diagram of a simple connection between telephone "line A" and telephone "line B". Line A is served by Telephone Central Office Exchange "CO1" while Line B is served by a second Central Office, which we'll call "CO2". The call between A and B is a local call, so there is no long distance carrier involved.

Figure 2 -Local telephone network - one direction shown

When A dials B's telephone number, CO1 consults a routing table to determine a path or "route" to CO2. In this example, there is a "trunk group" directly between these two central offices. The switch's next step is to choose an available channel on that trunk group for this call. This process is called "hunting", a term that dates back to the days when an Operator scanned or "hunted" through the trunks until finding a vacant channel. The usual hunt method is to begin at the lowest numbered channel, and then sequentially check each channel to see if it is in use. Once a vacant channel is located, the call can proceed. This is called "bottom up" hunting. "Top down" hunting works identically, except that the highest numbered trunk is tested first, and the switch hunts downward.

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