bmk@mvuxi.att.com (Bernard Mckeever) (09/26/89)
Here's a subject I haven't seen here before. I'll start it off and
hope that [insert your name here] has the time to provide us with
another of his informative articles. BTW, The traffic on this group is
picking up and I notice that MANY of the readers have extensive
knowledge of the "early days". Keep up the good work! Please feel free
to correct any mistakes in the following.
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The first commercially available digital channel bank for use by the
telephone company was introduced in the early 60's and went by the
name of the D1 channel bank, also known, at that time, as T1 carrier.
Some time latter T1 carrier would be used to describe only the
interoffice facility used for transport, and D1 would refer to the
D1-A bank. The system used Pulse Code Modulation [PCM] and channels
shared the facility by means of Time Division Multiplexing [TDM]. Each
channel bank could provide 24 time slots [channels] and the the
channel units and related common equipment were called a digroup. The
banks function was to convert voice frequency and channel signaling
information into a string of 1's and 0's and take this same string at
the far end and turn it back into its' original form.
As someone mentioned D1-A [D1-B] used A-law encoding a 100 step linear
encoding scheme to "digitize the voice signal. Signals were first
sampled and converted to a Pulse Amplitude Modulated [PAM] signal and
passed to the COder/DECoder [CODEC] for conversion to 7 bit digital
codes, an additional signaling bit was added to each byte to form an 8
bit byte. Each channel was sampled 8,000 times per second and at the
end of 24 bytes, a framing bit was added. This gave us a multiplexed
line rate of 1.544,000 bits per second. Also as mentioned D2 used
u-255 [Mu-255] a non linear encoding scheme that used 8 bit voice
encoding in 5 of 6 frames and 7 bit encoding every 6th frame. In every
6th frame a signaling bit was added to each byte. The frames were now
aligned into a 12 frame Super Frame [SF], during frame 6 signaling
information was sent on the "A highway" and in frame 12 on the "B
highway". This required the frame bit to "share" information so the
terminal and the signaling equipment knew when thing were in sync.
D2 banks did not last long but were the 1st Toll grade digital
terminals. Next in the series was D3 terminal banks. About 1/2 the
size of a D1 bank D3 used the same line coding [SF] as D2 and provided
additional Special Service Applications. Each system mentioned used
the same line rate [1.544.000 b/s] but were not end to end compatible.
Each used a different channel counting sequence. In theory you could
connect D2 and D3 but why bother, most of the systems by that time
were either D1-B or D3. D1-B was not compatible with D2 or D3 because
of the different encoding schemes. Next came D1-D a major retrofit to
D1-B that used the same encoding and framing scheme as D3. This
allowed the VERY large base of older D banks to be upgraded to Toll
grade service and to provide additional special service applications.
One of the first uses of D1-D to D3 caused some embarrassment to the
testers who turned up the system. They said everything is OK and the
Circuit Provisioning Center [CPC] started to assign service. The 1st
circuit turned up without a problem but they could not tone out
channel 2 for the second circuit. After hours of head scratching
someone [me] remembered that D1 banks count channels 1, 13, 2,
14,....12, 24, and that D3 counted 1, 2, 3,....23, 24. when the
system was accepted they only toned out channels 1 and 24, quite a
common practice. They forgot to install the adapter at the D3 end of
the span because at that time they did not know a D1 would be at the
other end.
After D3 came D4 [what else] a 2 digroup system [48 channels] that
could operate at several line rates DS-1 DS-1C DS2 and provided twice
as much service in the same amount of space as a D3 bank. Today, by
far, D4 banks are the dominant system in North America. D4 also is the
basis for SLC-96 carrier systems.
Many other digital terminals still exist in the network and a new
generation of them are starting to take over where remote operating
centers are in place.