RAF@cu.nih.gov (Roger Fajman) (01/25/91)
> Any time we order a "Data Line" for our computers here, we are given a > device to put inline between the modem and the phone jack. It is a > box about 4 X 2 X 1 inches, has one two-pair modular cord to plug into > the phone jack, and one modular outlet that the modem plugs into. It > also has a switch labeled FLL and PROG. Sounds like an RJ-41S jack, or some sort of adapter to make a regular RJ-11 type jack look to the modem like an RJ-41S jack. FLL means something like Fixed Loss Level. When the RJ-41S jack is installed, the loss is supposed to be measured to the central office. The jack contains a resistor to attenuate the rather high level signal put out by the modem just enough so that it arrives at the central office at the maximum permitted strength. This requires a modem with an 8-wire connector intended for use with FLL. AT&T sells modems, such as the 2224B, 2224CEO, and DL424, that can use this arrangement. With the PROG setting, the modem actually senses from the jack what the transmit level should be and sets itself accordingly. This again requires the 8-wire cable with a modem designed for that. If you plug a modem designed for FLL into the RJ-41S jack with an 8-wire cable and set the switch to PROG, it will not work at all. A regular RJ-11 type cable can also be used with an RJ-41S jack by plugging it into the center. The switch should be set to PROG, so that the transmit level is not unnecessarily reduced by the jack. With an RJ-11 (permissive) connection the modem transmits at a standard level with no compensation for the loss to the central office. This standard level is less than is used with the FLL arrangement. I'm trying to remember what that level is: -11 dBm sticks in my mind, but I'm not certain. Many AT&T modems can be used with either four-wire cable or eight-wire cable. The modular plug on the eight-wire cable has a key on it that forces the modem into the right mode. Roger Fajman Telephone: +1 301 402 1246 National Institutes of Health BITNET: RAF@NIHCU Bethesda, Maryland, USA Internet: RAF@CU.NIH.GOV