T- Carriers
In telecommunications, T-carrier, sometimes abbreviated as T-CXR,
is the generic designator for any of several digitally multiplexed telecommunications
carrier systems originally developed by Bell Labs and used in North America
and Japan.
DS0 is the basic unit of the T-carrier
system is the, which has a transmission rate of 64 kbit/s, and is commonly
used for one voice circuit.The E-carrier system, where 'E' stands for European,
is incompatible with the T-carrier and is used most places in the world
outside of North America and Japan. It typically uses the E1 line rate and
the E3 line rate. The E2 line rate is less commonly used. See the table
below for bandwidth comparisons.
A "T1" now seems to mean any data circuit that runs at the original
1.544 Mbit/s line rate. Originally the T1 format carried 24 pulse-code modulated,
time-division multiplexed speech signals each encoded in 64 kbit/s streams,
leaving 8 kbit/s of framing information which facilitates the synchronization
and demultiplexing at the receiver.
T2 and T3 circuit channels carry multiple T1 channels multiplexed,
resulting in transmission rates of up to 44.736 Mbit/s. Supposedly, the
1.544 Mbit/s rate was chosen because tests done by AT&T Long Lines in
Chicago were conducted underground. To accommodate loading coils, cable
vault manholes were physically 6600 feet apart, and so the optimum rate
was chosen empirically--the capacity was increased until the failure rate
was unacceptable, then reduced to leave a margin.
Companding allowed acceptable audio performance with only seven bits per
PCM sample. The code did not allow an all zero sample, thus avoiding a string
of binary zeros which would cause the repeaters to lose bit synch.
TD microwave radio systems were also fitted with high rate modems to allow
them to carry DS3 and DS4 signals. None of these were widespread, because
optical fiber overtook them. Bell Labs developed higher rate systems in
the 1960’s and thus created a T-1C with a more sophisticated modulation
scheme carried 3 MBPS. T-2 carried 6.2 MBPS, requiring a special low-capacitance
cable.
