It is not widely advertised, but a number of problems can effect higher speed modem communications. They include:
Noisy phone lines: These can degrade communications a little or a lot. In the worst case, they can make it pretty much impossible.
Incompatible technologies: Four different technologies are used for "56K" connections -- X2, K56Flex, V.90 and V.92. While the technologies are interoperable, for the most part, they do not interoperate at full speed. Thus, an X2 modem connecting to a K56Flex modem may negotiate a speed more like 32K than 56K. Some modems support more than one technology and may be able to achieve faster connections to some destinations if directed in the initialization string to connect using a different technology. Similar problems may be encountered with slower nominal data rates.
Asymmetry: Of the four technologies, only V.92 can approach the advertised 56K bandwidth in both directions.
Robbed Bit Signaling: Phone signals are digitized except for the customer to Central Office links. Some phone companies use one bit out of every 48 bits in the digitized messages for link control. This has no significant effect on voice communications, and has only minimum impact on high speed communications (2% slowdown) ... if the modem can correctly identify which 8 bit frame contains the missing bit. If the frame is improperly identified, the effect can be substantial. If two or more different RBS systems are involved in the link, satisfactory communications may be established only when the robbed bit frames on the phone systems synchronize by chance, and then only if the modem guesses the frame with the missing bit properly.
Digital Padding: Phone systems use attenuation pads to match levels and possibly to suppress echoes. The echo suppression (if it works) is based on desired signals making only one trip though the pad whereas echoes make two trips through and thus are attenuated more than desired signals. Modems must compensate for padding. Compensating for analog padding is generally not an issue, but compensation for digital padding sometimes does not work well. This situation is exacerbated because there are no general standards for the amount of padding on circuits of different types, and a signal may pass through several systems with unknown padding en route between two modems.
Pair Gain Lines: Pair Gain is a company that makes digital line multiplexors that are widely used by telephone companies. These devices convert analog signals to digital signals and multiplex multiple signals onto one copper wire. They are used in areas where there are insufficient physical wires in place to handle all the traffic. Pair Gain devices effectively divide the potential 56K bandwidth by the number of multiplexed lines.
Multiple A/D conversions: High Speed modems assume that there will be only one Analog to Digital conversion performed in the signal path. This is usually done at the phone company Central Office. Performance may be degraded -- often severely -- if a second A/D conversion takes place as with a Private Branch Exchange (PBX) as is commonly used in offices. Any phone line that needs a special character -- e.g. 9 -- to get an outside line is likely to perform poorly with a modem.
DSL capability: Lines configured to support DSL may (purportedly) include filtering that degrades performance of conventional 56K modems. While it is not commonly done, it is often possible to connect via an analog modem over a digital line using DSL technology, This is sometimes done in order to connect to an alternative ISP.
Loading coils: Inductances used by the phone company to tailor the line frequency response. These can interfere with modem performance. Phone companies have been known to remove these for customers experiencing modem problems.
Most telephone companies in the US and elsewhere take the position that they are obligated only to deliver acceptable voice service on basic phone lines. Some companies may choose to look at modem problems. Some will not.
Return To Index Copyright 1994-2002 by Donald Kenney.