T-Carrier Systems

1. T-Carrier Description and History
2. T-Carrier Facilities
   1. Wiring - 4 wires
   2. Voltage - 48 volts
   3. Loading Coils
   4. Regenerators
   5. Network Interface Unit/Device (NIU/NID)
   6. Smart Jack
   7. Demarcation Point
   8. Customer Premise Equipment
   9. Channel Service Unit/Data Service Unit
3. T-Carrier Signalling & Line Encoding
   1. Timing
      1. Methods
      2. Source (internal or loop)
      3. Slips
   2. Encoding Methods
      1. AMI (US & Japan)
      2. B8ZS (US & Japan)
      3. HDB3 (Europe)
   3. Error Checking

T-Carrier Description and History

Before 1957, the telephone system was entirely analog and circuit-switched. This meant that one phone conversation tied up one set of wires from end to end within the phone system. This created serious limitations in the growth of the phone system. To connect a new subscriber, a new circuit had to be built to the subscriber's location from the closest central office and additional trunk connections had to be added between central offices. When a subscriber made a phone call the phone system built a circuit from end to end. While the conversation was taking place, the circuit could not be used by anyone else. What Bell Telephone needed was a way to share the physical trunk lines between telephone offices.

AT&T Bell Labs (Wm. Shockley, John Bardeen & Wm. Brattain) invented what became today's transistor in order to build a new equipment capable of multiplixing many calls onto one trunk line. The physical facilities used to carry this new multiplexed communication was the T-Carrier system. T-Carrier systems are digital and packet switched, making them much more scalable than the previous analog and circuit switched systems.

The terms DS-xand T-x (eg T1/DS1or T3/DS3) are often used interchangeably but this usage is incorrect. These two terms refer to different parts of the telecom circuit.

The 'T' usually refers to the physical transmission facilities, that is, the wires, connectors, jacks, repeaters and the signal carried on them. The term 'DS' refers to Digital Signalling refers to both the speed and bit-order (framing) of the data as encoded in the T-carrier signal.

T-Carrier Facilities

Wiring - 4 wires

T-Carrier facilities use two twisted pair of copper wires. The first pair is used for transmission and the second pair is for receipt. Because there are two pair of wires, T-carrier based systems are able to transmit and receive simultaneously allowing full-duplex communication.

Regenerators

Demarcation Point

The demarcation point is the point at which the telecommunications company considers their responsibility to end. Typikcally, this is either the point at which the telecommunications company's line enters the building or the smart jack.

Smart Jack

WIth most T1 facilities today, there is what is called a smart jack used by the telephone company for the purpose of line testing. A smart jack is capable of understanding commands sent on the command channel including test and lookback commands.

 

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