Why your FAX won’t work on a T1 (not PRI ISDN).

You have a T1 into you PBX, voice calls are not a problem, but you can’t send a FAX. The T1 that you are leasing from the LEC is terminated into a Digital Crossconnect System (DSC) at the CO. The LEC uses the DCS to connect the T1 channels to CO carrier channel banks with FXO channel units. This takes the digital channels to analog to connect to the CO switch ports (POTS).

T1 coding has two options ESF/SF (SF is listed as D4 on some test sets) B8ZS/AMI. If the ESF/SF options at each end of a T1 do not match the system will not sync up and the T1 will not work at all. T1 transmission requires at lease a 17% ones density to keep sync. G.711 coding of voice is rich in ones. Data can have a high number of zeros causing a carrier system to have a low ones density. To overcome this B8ZS was introduced. Any time a channel has all zeros; Binary 8 Zero Substitution (B8ZS) will replace the channel with 4 bits with 2 bipolar violations. When the equipment at the other end of the T1 detects the B8ZS code the equipment will replace the four one bits with all zeros.

B8ZS/AMI option must match at both ends of the CO DCS to carrier channel bank. When the DCS is optioned B8ZS and the channel bank is optioned AMI, voice will work ok but Fax’s will fail. This is difficult for the CO to find. If your PBX lines are spit between banks and only one is optioned wrong, sometimes you can send a FX sometimes you can’t, depending on witch co line you uses.

Moore to come.