There are many reasons why your modem will not operate at 56K when connecting to an ISP.
The first reason why you will not connect at the 56K speed is that this 56K rate is a theoretical maximum speed based on the phone line networks and the network switch characteristics. Like everything else in life there is a difference between theoretical best case and real world implementation. The first barrier to 56K speeds is that the FCC has imposed a limit on the actual power that may be transmitted over the phone lines. This limit effectively limits the data rate to a value a few K lower than 56K (53K).
The second reason that you may be unable to reach this 56K limit is the quality of the phone lines that connect your house (and neighborhood) to the telephone companies Central Office and then the lines that connect your CO to your ISP. The 56K standards require that the data path from your local CO to your ISP must be entirely in the digital domain and that the only analog section be from your modem, to the local Telephone Company CO. If there is more than one analog-digital conversion in the path from your home to your ISP, then 56K speeds are impossible. Also as the quality of phone line or the path to your ISP varies so will the maximum speeds. The various 56K protocols allow for speeds that range from approximately 28K up to the practical maximum speed of 53333 bps.
The third reason why you may not be able to achieve 56K rates is the protocol that is used by your modem and ISP may be different. There are currently 3 56K standards in use in the world: K56FLEX, X2 and V.90. Before the V.90 standard existed there were two conflicting 56K protocols, X2 and K56FLEX. These two protocols were capable of speaking 56K speeds but not to each other. If a K56FLEX modem called a X2 headend (and vice-versa), then the modems could only communicate using their common V.34 speeds (up to 33600).
A fourth reason that preclude 56K rates is the companding scheme that is employed by your phone network. There are two different companding algorithms in use across the world, A-law and u-law. The X2 and K56FLEX algorithms have to be told the companding scheme that is in use (done by telling the modem the Country it is operating in) so if this is set incorrectly it will cause a problem. The V.90 standard allows the modem to ask the headend which method it is using.
The last reason why your modem could fail to communicate at a 56K connection rate is when the computer system in which the modem is installed is not of sufficient power. If the modem is a software modem (aka WinModem) it needs to use the processing power of the PC's CPU to compensate for the lack of a Digital Signal Processor (DSP) on the modem board itself. The modem may not have the capability to reach its maximum capable speeds if it is installed on an old or slow system.
|