Some modems (older Alcatel and Cayman routers) are manufactured to receive the DSL signal on the "outer pair" (line 2) in the jack. Most modems however use the "inner pair" (line 1). To make the older modem work someone may have installed a line swapper or rewired the jack itself. One quick way to check if the jack was rewired is to plug a regular phone in to jack and see if you get dial tone. If you have dial tone then the signal is on the inner pair (pins 3 & 4). If you do not have dial tone then the jack has been rewired and the signal is on the outer pair (pins 2 & 5).

Reference these links for more information about:
Swapping the pairs at the wall plate
RJ-11 wall jack schematic
RJ-11 pin outs
line swapper.

Additionally, some modems ship with specialized RJ-11 cables that have a built in pin swap or are a reverse polarity cable. When swapping modems use the cables supplied with the new modem to avoid potential problems. An incorrectly wired jack may still provide enough connectivity to allow synch but the attenuation and signal to noise margin will be degraded and you will probably have connectivity issues.

Note: Not having the correct VPI/VCI on a modem will not prevent a modem from synching.

Important Note: The RJ-11 standard was designed for 3 pairs. However most telephone cords and wall plates only have 2 pairs (pins 2-5). So when most people say "inner pair" they are technically talking about line pair #1 (pins 3 & 4). When they say "outer pair" they are technically talking about line pair #2 (pins 2 & 5).


Schematic and picture by Andy Houtz

Andy Houtz DSL