X2/K56Flex Modem Technology: X2 modem technology is an asymmetrical modem technology coming into use in mid 1997. Asymmetric means the speed is different in different directions -- 33.6Kbit per second upstream, 53Kbps downstream. These 33.6/53K modems are identified as "56K modems" even though they can not operate at 56Kbps even under ideal conditions. 2X modems gain their increased bandwidth by using unused uplink capacity for downlink. They deliver speeds greater than 33.6K only when talking to special modems connected directly to high speed digital telephone lines and only if the signal path does not include digital to analog conversions. Thus, 2X modems are primarily useful for downloading from communication service providers of one sort or another and only if there is no intervening PBX or Subscriber Local Concentrator (SLC).

Some modem vendors provide test lines to determine if a phone line will support 56K. One is at 1-888-877-9248. Log on as "x2".

53Kbps is the maximum speed and can only be achieved with especially good telephone lines. Initial testing (PC Magazine May97) in early 1997 indicates that typical speeds will be about 47K.

In some cases 33.6Kbps (V.34) modems are software upgradable to X2.

To further complicate things, a competing, similar, incompatible, technology called K56Flex is being marketed by a group of modem manufacturers. K56Flex capabilities are similar to X2. Compuserve and AOL have announced plans to support X2. It is not clear if, and which, data suppliers will support K56Flex.

To summarize, X2 or K56Flex modems can be used to achieve speeds somewhat above 33.6Kbps in one direction when talking to a modem designed to support the specific technology over a communication line that is compatible.

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