tef@CGL.UCSF.EDU (02/18/88)
I stand corrected on a couple of points in my recent email message: 1) AT&T's USA-wide master T1 clock is located in Hillsboro, Missouri, not Atlanta. Hillsboro was chosen because it is the "geographic center" of the country. (Does this mean if there was a giant H-bomb it would be dropped there? Never mind...) There are several backup master clocks arranged in a hierarchal fashion in case of failures in the primary synchronization system. An article about this recently appeared in Data Communications. I'm told it is not necessarily easy to slave CSUs to the master clock. The clock is used by telco COs, but it may not be easy for you to get at it. 2) The ZBTSI ANSI standard is part of T1X1 committee and will be balloted on shortly. Several telco's are already using the current ZBTSI document as a defacto standard. The standard does, in fact, require ESF. It uses 2 kbps of the 4 kbps ESF data channel for transmitting "Z" control bits used to flag encode control information. The properitary Verilink 551VCC product does not require ESF. Other differences between ZBTSI and Verilink VCC are more substantial than I orignally implied. They include: (a) 500 microsecond delay on xmit and recv for Verilink, 500 microsecond delay on xmit only with ZBTSI, (b) no modification of the T1 bit stream if it already meets density requirements for Verilink, channel 96 time slot always exchanged with channel 1 time slot for ZBTSI, (c) no bit scrambling with Verilink, 5-bit scrambler added to ZBTSI data stream to minimize error multiplication.