Date: Tue, 10 Nov 92 17:44:37 CST
From: varney@ihlpl.att.com (Alan L Varney)
Newsgroups: comp.dcom.telecom
Subject: Re: What's a T1?
Organization: AT&T Network Systems, Lisle, IL
X-Submissions-To: telecom@eecs.nwu.edu
X-Administrivia-To: telecom-request@eecs.nwu.edu
X-Telecom-Digest: Volume 12, Issue 838, Message 2 of 5

In article <telecom12.836.8@eecs.nwu.edu> vixen!jadams@uunet.UU.NET
(22475-adams) writes:

> In article <telecom12.830.4@eecs.nwu.edu>, John C. Fowler <fowlerc@
> magellan.colorado.edu> writes:

>> I've seen the abbreviation 'T1' or 'T-1' here in the Digest many times
>> over the past few years, but I don't think I've ever seen anyone
>> define exactly what a T1 was.  

> Although many good treatments abound, allow me to offer a "Reader's
> Digest" version:

> T1 is a type of digital carrier system which was originally deployed
> in 1962 to solve a number of problems inherent in congested
> metropolitan area short haul (less than 50 mile) carrier needs.  T-1
> (Or T1 as you like) specifies a four wire circuit transmitting signals
> at 1.544 megabits/second (Mbps).  T1 carries its information in the
> format known as DS1 (Digital Signal 1).

   Hi, Jack -- Well, you beat me to it!  But I was going to offer the
"Scientific American" version ... anyway, here's some added details on
"T1".

   While 1962 was the beginnings of T1, the current almost-8-bit PCM
encoding wasn't used until the D2 system was deployed.  D1 used 7-bit
samples and simple framing patterns (alternating 0 & 1). The AT&T D4
digital channel banks were introduced in 1976.

  I've never seen exact descriptions of the "T1" vs. "DS1" terms, so
I'll mention my version of them:

   T1 refers to a (bidirectional) digital transmission FACILITY
      operating at a bit rate of 1.544 Mb/s, originally copper,
      with all the physical/electrical/maintenance interfaces.
      D4 channel banks (and digital switches, etc.) interface
      with the T1 facility, but are not part of the facility.

   DS1 refers to the digital BIT STREAM format at the 1.544 Mb/s
      rate -- in this case, 24 PCM voice channels (or data).  The
      DS1 stream "flows" through D4's and switches interconnected
      with T1 facilities.

   DS1 operates with a "sub-frame" of 193 bits, carrying 24 PCM voice
channels, arranged with a framing bit and 8 bits/channel (192 bits).
Each sub-frame takes 125 micro-seconds to transmit, yielding 8000
sub-frames/sec.  For a given channel, the net rate is 8000 x 8 bits or
64,000 bits/second of sampled voice.  The PCM samples are formed by
converting the sampled analog voltage to a binary code using
non-linear mu=255 companding, better known as "mu-law" encoding.

   In order to detect where sub-frames begin (so that channels and
8-bit PCM bits can be located), the D4 channel bank organizes the DS1
stream into "Frames" of six sub-frames each and "Superframes" of 12
frames each. (I believe the D1 system first called these "frames", and
D4 stayed with the same terminology).  Each Superframe thus contains
an "A-frame" and a "B-frame".  The lone framing bit of each sub-frame
is set to 0 or 1 in a way that allows the system to detect "loss of
synchronization" with a far-end transmitter.

   The framing bit for the first and subsequent "odd" sub-frames
alternates in a "1 0 1 0 1 0" pattern.  The framing bit for even
sub-frames follows a "0 0 1 1 1 0" pattern.  The resulting bit pattern
is {10001101110}.  When the "odd" sub-frame pattern isn't consistently
detected by the receiver, "sync" has been lost.  Sync is restored by
forcing the far end to transmit a fixed pattern in the 24 "channel"
positions and the full 12-bit framing pattern.  Once in sync, only the
"odd" sub-frame pattern is checked.

   The DS1 stream does not have "room" to carry the DC on-hook and
off-hook signals of the analog trunks it replaces, so several means
have been used to substitute for those signals.  The most common is
"robbed-bit" or "AB bit" signaling, which uses the lowest bit in each
PCM sample of the 6th and 12th frame as an "A" and "B" bit.  In many
cases, A=1 means off-hook -- B is used in schemes where FX "lines" are
provided via T1 and power ringing or such must be signalled.  (The 6th
and 12th frames are where the framing bit of the "even" sub-frames
change from "0" to "1" or "1" to "0".)

   Since the "even" sub-frame's framing bit isn't needed to maintain
sync, it can also be used as a slow-speed data link, at about 4Kb/s.
When the D4 bank is used in the SLC(tm)-96 loop carrier system, the
data-link is used for both call signaling and alarm/maintenance data.
The D4 bank can also be used in a "48-channel" mode (DS1C=3.152Mb/s),
and in a paired 96-channel arrangement (DS2=6.312Mb/s).  These can be
connected to T1C and T2 facilities, respectively.


Al Varney