Some phone lines are inherently incapable of 56k.

While all phone lines look pretty much the same - a pair of wires - there is a variety of ways the phone company sends the phone signal down them, and sometimes the result is a phone line simply unable to support connections using the concepts behind the 56k protocols.

Nobody really knows how many lines there are that cannot support 56k, but estimates run from a low of 15% up to as high as 40%.  Our experience suggests that around here the number is toward the middle of this range for residential lines.  However, the dispersed rural nature of most of our area means many of the lines around here are the second type described below.  These lines can support 56k protocols but not at the highest speeds.

Most phone lines (but not all phone lines) are one of three kinds:
 
• Simple wire pair - 3.5 miles or less

This is the simplest kind of phone line and the best to have for 56k. It consists of just a pair of wires that runs from your phone to the telephone company's switching office. Like many simple things it works very well. If you have this kind of phone line, you have a good chance of getting good 56k connections.
 

• Loaded wire pair - more than 3.5 miles

As the length of the wires gets longer, it becomes harder for a telephone to push a high quality signal down the wires. When the wire length reaches 18,000 feet (about three and a half miles), you can no longer use a simple pair.

"Loading" is something the phone company can do to a wire pair to make it carry a telephone signal for miles. If you live many miles from where your telephone calls are switched, you may have a loaded pair. Loaded pairs are fine for voice, but loading reducing your chances of getting the best modem connections.

Loading does not guarantee that you cannot do 56k connections, but it does reduce your chances and normally limits your top speed. The longer your line, the more loading it has and the less well it will work for all modem connections, not just 56k.
 

• Wire pair from a remote (SLC) - short or long

When a telephone company needs to provide a significant amount of phone service in a location distant from the nearest exchange switch, they could just run a lot of long lines, or they can run a trunk to a remote distribution unit.  These systems for delivering many phone lines to a remote location are called SLCs (Subscriber Line Carriers), and they are generally bad news for modem connections, especially 56k connections.

At first, deployment of remotes was largely in response to the advent of suburban housing developments. These created the need for a lot of phone service in small areas distant from telephone switches.

Fiber has made such remotes very popular with telephone companies, because so much phone service can be carried on a single fiber bundle.  These days any new installation of a significant amount of phone service, such as for a new office building, is likely to be done with a remote.

People receiving such phone service are often lead to believe they are getting special high quality because the service is delivered to the remote on a fiber, and this is more or less true for voice.  Unfortunately, in most cases the way the phone service is fed to the fiber at the telephone switch makes it unusable for 56k.  With older remotes there is no alternative to using this kind of head-end feed.  Newer remotes do not require this kind of configuration, but it is still commonly used.

Larger business locations and other larger sites often have their own internal phone system called a PBX.  Some PBXs - those receiving digital phone service from the phone company - are similar to the phone company remotes, and they are nearly always connected to the phone company in a way that cannot support 56k.  In fact, we suspect that the reason the estimates of how many phone lines cannot support 56k vary so much is that some estimates count the large number of office PBX phone lines while others count only lines from the phone companies.

Small-office phone systems are basically just distribution systems for non-digital phone service from the phone company.  Some of these are very good and do not cause significant problems for 56k, while others cause major problems for 56k and sometimes for other modem protocols.

• General phone line quality All the usual things that degrade phone service and hinder normal modems connections will interfere with 56k modems also.

 

Many of these degradations are introduced by users themselves. See our page on connections problems for info on how to avoid causing your own problems.

V.90 and K56Flex have shown a surprising robustness in the face of some of these "conventional" degradations. This robustness in the presence of audible noise and interference is all the more remarkable given the sensitivity of 56k to other much more subtle effects.
 

• Call routing

Calls are carried on analog "lines" from telephone switches to homes and offices, but they are carried on digital "trunks" between switches in different towns. Trunks use a wide range of different digital technologies, so they come in many kinds. Regardless of how they are implemented, they are divided into two classes.

One class is adapted for long distance calls, while the other is used exclusively for "local" calls, roughly meaning calls that aren't going to travel very far. However, there aren't enough "local-only" trunks to carry all the local calls at peak times, and some local calls travel the long-distance trunks.

The signals carrying the calls are processed differently for the two kinds of trunks. The difference is very slight but has a big effect on 56k calls.  One difference is in what is called "digital pad."  Calls switched onto trunks that can carry long distance calls have no digital pad, while those on local-only trunks do get a digital pad.

The digital phone service ISPs use to provide 56k dialup is delivered to them on what are essentially trunks. The phone companies normally treat these like local-call trunks. When 56k first came out, it was noticed by many users and ISPs that users got better connections if they called long distance. Having the phone companies change the ISP's trunks so they are treated like long-distance trunks improved things greatly for local callers.

However, when users call from areas served by switches other than the one serving the ISP, the users' calls can still travel from switch to switch on either "local-only" or "long-distance" trunks. Which kind of trunk a call takes is pot luck and causes connections to vary from call to call in ways nobody has any control over.

In addition, trunks and other equipment carrying calls differ in a variety of other ways. Some trunks use "robbed-bit signaling", and if that sounds to you like it's less than ideal for modem connections, you're correct. How much it matters depends on how many bits are robbed, and that depends on the route a particular calls takes.

While the route a call takes through the network is luck of the draw, the most probable routing can depend on time of day.  Generally, the better trunks are used first, and at times of heaviest phone usage the best trunks are likely to be in use.  At these times your call is more likely to be routed via an older trunk and to have more bit-robbing.

Even if a premium path is available, your call may not get it.  ISDN (digital phone service) calls can only be carried on premium paths, and at times of high ISDN usage, some of the premium paths must be kept open for possible ISDN calls.  When plenty of these premium paths are idle, some voice/modem calls will be routed through them.  The phone network is gradually being upgraded, and eventually the entire phone network will be ISDN-grade.

Few people know the phone system well enough to give a complete explanation of all the ways one call can vary from another. We certainly do not. What we do know is that calls are subject to an amazing variety of treatments and that some of these have severe influences on 56k modems connections. (Some also have a significant influence on V.34 connections.)