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Introduction to Data Communications
31b. TDM - T1 Carrier System (cont'd)
It samples the analog signal 8000 times a second (2x 4 kHz - makes Nyquist happy!). It is a serial data stream so we transmit the 8 bit data 1 bit at a time. This means that for a digitized Voice Channel, the data rate is:
8 bits x 8000 samples = 64 Kbps
The basic Carrier used in the T1 Carrier System is called the T1 (sometimes called DS-1) and it carries 24 Voice Channels. The Bit Rate for the T1 Carrier is 1.544 Mbps. If we multiply:
24 Voice Channels x 64 Kbps per Voice Channel = 1.536 Mbps
The missing 8 K is used to "frame" the data. It is information used for the Start Frame bytes, End Frame, Error Checking, Routing information, etc..
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Digital Circuit |
Voice Channels |
Bit Rate |
# of T1 Circuits |
|
T1 (DS-1) |
24 |
1.544 Mbps |
1 |
|
T2 (DS-2) |
96 |
6.312 Mbps |
4 |
|
T3 (DS-3) |
672 |
44.736 Mbps |
28 |
|
T4 (DS-4) |
4032 |
274.176 Mbps |
168 |
Typically:
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T1 - Twisted Pair or Coax Cable
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T2 - Coax Cable
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T3 - Coax, Fibre Optics or Light Route Radio
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T4 - Coax or Fibre Optics
You can rent any quantity of a T1 line, you don't have to rent the complete circuit. You basically rent a time-slot on the line based on 64 kbps channels. This is called Fractional T-1.
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