3db PAD - Bad for 56k

3db PAD - Bad for 56k

The telephone network is based upon the old model of 2 wires coming into the phone carrying a full-duplex send and receive signal: with most phones you plug in, you can talk while also hearing what someone is saying at the other end (that's full-duplex).

The telephone company carries the signal on 2 wires only a limited distance - either to your Central Office, or to some subscriber carrier equipment near your premises. Here, a line card changes your analog signal to digital - and puts it on its own channel; at the same time, the line card receives a digital signal from a similar line card at the other end, converts it to analog and sends it down the line to you. This combination/separation of 1 channel (the 2-way analog) into 2 channels (send / receive digitized audio ) is done by a hybrid.

One of the things that happens is a certain amount of the signal gets leaked back through the hybrid: some of the audio you send is returned back down the receive line - both at the hybrid on your line card, and the hybrid at the distant end. [With 56k modems, the distant end doesn't use a hybrid, so there is no leakage of your modem's transmit at the distance end.] This leakage is referred to a echo, and many modems report the level of this signal in diagnostic screens.

For voice calls where there are hybrids at each end, the distance between the two ends becomes a factor and can produce unwanted echo. By reducing the level of the distant end's signal before being converted to analog, less signal will be leaked back: less echo. Delay is introduced in the echo signal by both distance and the trunking/channelization and switching processes in the telephone network. More delay makes the echo more noticeable.

With this in mind, as a general rule, telcos in the US place a pad whose value varies depending upon the type of call. (These pad values are determined by software settings programmed into the telco switch.) No pad is used on intra-office calls (both ends of the call are served by the same switch). A 3db pad is used on inter-office 'local' calls (calls that are beyond the office that serves you, but are still local calls); and a 6db pad on toll circuits.

Remember: the pad reduces echo (which isn't a problem for 56k modems anyway), and at the same time reduces the real signal - thereby resulting in a lower signal-to-noise ratio. But, in many instances, the 3db pad is the worst choice for 56k modems! Apparently, there is something about the way in which the pads attenuate the signal that can impair 56k PCM performance.

While it's possible for the telco to change the pad values for your (and other) lines, it's not something they generally do for customers. So, it may not be practical for you to get the best pad for your 56k calls. If you have an ISP who uses the same telco office as your line is on, that ISP may give you better performance that others located further away. Some ISPs using CLEC facilities, or "virtual" local numbers may be routed like a toll call - even though you are calling a local number - and encounter a 6db pad. And finally, you may be able to check to see if a 6db pad gives you better performance by dialing your local ISP as a toll call (1+area code + number, or if your telco blocks the call that way, using a 1010+access+1+ISP).