ITU-T G.703 2048 kbit/s (PCM, E1, CEPT) interface

Application, services, topology, change of data, electrical characteristics

Application:

The ITU-T G.703 2048 kbit/s interface (in other names: CEPT-, E1, European, - PCM interface ) is a 4-wire, total duplex interface multiplexed according to the time-sharing principle, with transmitting abilities and with 2048 kbit/s transmission speed (thirty 64 kbit/s PCM voice or data channels, one 64 kbit/s signalling channel, 64 kbit/s synchronization, control and service). This interface is used widespread (e.g. for connecting together different network elements (centres, multiplexes, line adapters). ISDN received this standard, it is the basis of the primary rate ISDN network access.

This interface consists of two transceiver circuits, and of 2 x 2 metal wires connecting them to each other.

Elements of the standard or recommendation describing the interface:

• ITU-T G.703 physical/electrical characteristics
• ITU-T G.704 frame organization
• ITU-T G.706 synchronization, development of a CRC multiframe

Interface provides the following services for the higher layers:

• Transmission capatibilty
• Signaling transmission capability
• Bit synchronization
• Frame, multiframe synchronization
• Test and control functions (Maintenance)

Interface realizes point-to-point connection.

Data change cables:

It is necessary for the realization of PCM interface a metallic transmission medium consisting of a symmetric (twisted pair) or asymmetric (coaxial) 2 x 2 wires, with 2048 kbit/s transmission capacity to both directions.

    Pin positions in the jack (numbering from left to right):

    RJ-45 (EIA/TIA-464-B)

        funkció        TE
    1    RxA         input
    2    RxB         input
    3    signal gnd
    4    TxA         output
    5    TxB         output
    6    signal gnd
    7    nc
    8    nc

    DB9                             RJ11

    1    Rx-         6   Rx+        1
    2    nc          7              2    Rx      input
    3    nc          8              3    Tx      output
    4    nc          9   Tx+        4    Tx      output
    5    Tx-                        5    Rx      input
                                    6

Oscillogram of one part of the PCM signal flow is shown on the following figure:

• bit rate: 2048 kbit/s +/- 50 ppm, bit time: 488 ns
• line coding: HDB3 / RZ

• 	asymmetic	symmetric
  nominal impedance	75 ohm	120 ohm
  amplitude of the pulse ("mark")	2,37 V	3 V
  voltage belonging to the "space"	0 +/- 0.237 V	0 +/- 0.3 V

• pulse width: 244 ns
• pulse shape
• reflexion:
  • 12 dB (51 - 102 kHz)
  • 18 dB (102 - 2048 kHz)
  • 14 dB (2048 - 3072 kHz)
• jitter

How large is the distance superable with the interface? It depends on the attenuation for length unit of the transmission medium. It is the rule, that at the half of the bit rate (at 1MHz) the attenuation of the transmission medium must be smaller, than 6 dB. For bridging a larger distance, we have to install a digital line section amongst the two interfaces.