Vicki Holland, of Villa Maria College, asks :-

Why, when you telephone someone overseas, is it sometimes a good line and sometimes a bad line?

Des Taylor, a communications engineer, responded.

There are two major causes of bad telephone connections. These are noise and echo distortion.

Noise is due to electrical interference and occurs mainly on either copper cables or satellite communication links which are another form of radio. When it occurs, it sounds like static and tends to mask out the other person's voice. It occurs only rarely on connections in modern telephone networks.

The second major cause of bad connections is echo. This is caused primarily by delay effects If only cables and/or optical fibre are involved in your connection, then usually the end-to-end delay is small enough (almost always less than 0.07 seconds) that conversation is similar to that on a local call. However, if a satellite link forms part of the connection, there is an inherent quarter of a second delay in each direction. This can cause very annoying echo problems, where if you try to interrupt or break into a conversation without waiting for the other person to finish, you cause their voice to break up making the connection sound very noisy.

Echo effects can be minimized by the use of complicated electronics called echo cancellers, but not all international trunks are so equipped. You should also note that even if your call goes out via fibre optic cable it may, within North America say, end up on a satellite connection giving you the echo problem.