Primary rate connection

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The primary rate interface ( PMxAs ), English Primary Rate Interface (PRI) , is an interface definition in ISDN that is mainly used by companies to connect telephone systems to the ISDN. At the end of 2006 there were 113,000 primary rate access lines in Germany; at the end of 2015 there were 86,000. Another type of connection in ISDN is the basic connection .

Names and abbreviations

Other names for the primary rate connection are PMx connection and S 2M connection . Common abbreviations are PMx, PMxA, PMxAs, PRI, CEPT 1, E1 (in Europe), T1 (in the USA ), J1 (in Japan ) and S 2M .

Channel structure

The primary rate connection is structured in channels and consists of at least 16 and a maximum of 30 user channels ( B channels ), as well as a signaling channel ( D channel ) and a synchronization channel . There are different channel structures:

  • E1 - in Europe 2048 kbit / s gross data transmission rate divided into 32 channels of 64 kbit / s each as follows:
    • 30 bidirectional user channels with 64 kbit / s each (together 1920 kbit / s)
    • 1 signaling channel (17th with CCS) with 64 kbit / s
    • 1 synchronization channel with 64 kbit / s
  • T1 - in the USA 1544 kbit / s gross data transmission rate depending on the signaling method is divided as follows:
    • Common Channel Signaling (CCS)
      • 23 bidirectional user channels with 64 kbit / s each (together 1472 kbit / s)
      • 1 signaling channel (the 24th) with 64 kbit / s
      • 1 synchronization channel (the 1st) with 8 kbit / s
    • Channel Associated Signaling (CAS)
      • 24 bidirectional user channels with 64 kbit / s each
      • One bit is used for signaling in every 6th frame of each channel (robbed bit).

B channel

A useful channel is referred to as a B channel (from Bearer ). In terms of understanding, a useful channel corresponds to a communication channel ; it is used, for example, for a telephone call connection.

H channel

Several of the user channels of a primary rate connection can be merged in order to achieve a higher data transmission rate (up to 1920 kbit / s). These are called H channels . The primary rate connection for the downstream thus offers approximately the same data transmission rate as a 2 Mbit / s ADSL connection, but with the same high data transmission rate for the upstream .

Transmission method

With the primary rate connection, the channels are transmitted using the time division multiplex method (TDM / TDMA), i.e. the channels are transmitted one after the other in what are known as time slots .

The time slots of a primary rate access E1 are used as follows:

  • Time slot 0 is used for synchronization and error handling with the checksum of the cyclical redundancy check (ZRP, CRC).
  • User channels are transmitted in the fifteen time slots 1 to 15.
  • In time slot 16, the data for controlling the ISDN communication are transmitted ( D channel ).
  • User channels are transmitted in the fifteen time slots 17 to 31.

A new frame then begins with time slot 0. The frame period is 125  µs , the frame repetition rate is 8  kHz .

Line interface between local exchange and network termination

U K2 (electrical)

The line interface between the local exchange and the network termination (engl. Network Termination ) is in the house when using copper cable as " U K2 interface" hereinafter. As a rule, two copper pairs are used. To bridge larger distances between the exchange and the network termination must repeaters are used.

U G2 (optical)

Instead of the electrical U K2 interface, the optical U G2 interface is also used in some cases . A fiber optic cable is connected for the outward and backward direction. Due to the significantly more favorable attenuation values , longer distances to the exchange can be implemented with this connection variant.

Line interface between network termination and telephone system

The NTPM is used for the network termination with the primary rate connection . The line interface between the NTPM and the telephone system is called "S 2M ".

S 2M (electric)

For the S 2M interface, either two wires (one pair) or one coaxial cable are required for each transmission direction. An S 2M cable therefore often consists of four cores (two twin cores) that can have a common shield. The line coding for the S 2M interface is carried out with the ternary HDB3 code , i.e. there are three electrical states. As a result, the S 2M interface requires a bandwidth of only around 1  MHz in each transmission direction , while the data transmission rate is 2048 kbit / s.

Individual evidence

  1. ↑ Federal Network Agency Annual Report 2006. p. 60 , accessed on July 31, 2016 .
  2. ↑ Bundesnetzagentur annual report 2016. March 7, 2017, p. 54 , accessed on June 7, 2017 .
  3. ↑ a b ITU-T I.703 SERIES G: Transmission Systems And Media, Digital Systems And Networks Digital terminal equipments - General, Physical / electrical characteristics of hierarchical digital interfaces, Chap. 9
  4. ↑ ITU-T I.431 Integrated Services Digital Network (ISDN) - Primary Rate User-Network Interface - Layer 1 Specification, chap. 7th