S 0 bus

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The S 0 bus is an interface within an ISDN basic connection installation. It is used for in-house cabling and connects ISDN terminals with the NTBA . The S 0 bus is not to be confused with the S0 interface for the transmission of measured values ​​in building automation.

Some cable modems that enable telephony via the cable television network also offer an S 0 bus for connecting ISDN terminal devices. Some IADs , such as B. FRITZ! Boxes and speed ports , an S 0 bus to enable the operation of ISDN telephones on the next generation network . This is a simulated S 0 bus that does not offer the functionality of an ISDN basic connection , e.g. B. there are problems with some ISDN fax machines.

Structure of the S 0 - frame
The starting point of the S 0 bus is the NTBA

Technically, the bus consists of four-wire cabling in which a pair of wires is used for each communication direction (send and receive) . The wires "a1" and "b1" are, as seen from the end device, the receiving pair, the wires "a2" and "b2" are the sending pairs (see connection diagram below). The bus therefore works in full duplex mode ( using room multiplex ). As a coding method which is modified AMI code used. All useful signals and control signals are summarized in a 48-bit long frame that is time-multiplexed 4000 times per second. An S 0 frame thus has a time length of 250 microseconds, a bit of about 5.2 microseconds. The use of equalization bits ensures that every full frame is DC-free. The individual frames are separated by code rule violations within the AMI code. The nominal data rate is therefore 192 kbit / s.

This data rate is distributed over two B channels for the transmission of user data (64 kbit / s each), a D channel (16 kbit / s) which is used for signaling and various synchronization and organizational data. The D channel is mirrored by the NTBA and sent back to the end devices on the so-called echo channel in order to implement a protocol for simultaneous access ( see also: CSMA / CA ).

Execution with ISDN multi-device connection

Connection diagram for the S 0 bus

Alternatively, the following terminal designations are possible in the connection diagram shown on the right when using UAE sockets : 1a = 4; 1b = 5; 2a = 3; 2b = 6

With a multi-device connection, it connects several devices at the same time (point-to-multipoint). In this case, terminating resistors must be used for termination; the bus can be up to 150 m long and can be connected to a maximum of twelve ISDN connection sockets ( UAE or IAE ). A maximum of eight end devices can be connected to the S 0 bus at the same time , of which a maximum of four can be supplied with power by the NTBA.

Execution with ISDN system connection

With the basic connection in the system connection operating mode , only one terminal device (usually a telephone system with an extension number ) may be connected.

Bus types (types of cabling)

  • Short passive bus: distance of the end devices from the NTBA maximum 150 m. Maximum of 12 connection sockets, maximum of 8 end devices. Use in the multi-device connection operating mode . This is the usual variant.
  • Extended passive bus: The extended passive bus is at least 100 m and max. 500 m long. The end devices may only be connected to the end of the bus within the last 50 m. Maximum of 12 connection sockets, maximum of 4 end devices. Use in the multi-device connection operating mode
  • Point-to-point connection (long passive bus): Distance of the end device from the NTBA maximum 1000 m. Only one terminal device may be connected. Use in the system connection operating mode and in the multi-device connection operating mode (special case: only one terminal device).

Difference between “internal” and “external” bus

In the case of smaller telephone systems for the Soho area, a distinction is made between internal and external S 0 buses. An external S 0 bus is the looped S 0 bus of the NTBA . With this, all calls, including calls between telephones within the same household / office, are handled via the exchange in the public network. As a rule, internal calls also cost connection charges. In addition, with an external S 0 bus, both B-channels are occupied during internal calls , so that no other line is free and no one can be reached while the call is charged.

With an internal S 0 bus , the telephone system takes over the switching; In-house calls do not create a connection to the public telephone network and are therefore free of charge.

Activation procedure

Before a terminal can establish or accept a connection, the S 0 interface must be activated. The S 0 interface is deactivated if there is no connection for a period of 10 seconds and no data exchange takes place (exceptions are permanently activated connections, for example system connections). There are five states for the activation procedure, called INFO , which are passed through on layer 1 of the interface.

  • INFO S0: No signal exchange on the lines.
  • INFO S1: TE → NT subscriber terminal sends a continuous signal
  • INFO S2: NT → TE network termination sends S 0 frames in which all channels are set to logical "0" (A bit = 0).
  • INFO S3: TE → NT subscriber terminal sends S 0 frames which are offset by 2 bits from the frame received with INFO S2 and in which all channels are set to logic "1" (A bit = 1).
  • INFO S4: NT → TE network termination sends the same frame (frame valid).

When activated by the subscriber terminal (TE), all five states are run through. When activated by the network termination (NT), only the states INFO S2 to INFO S4 are run through. Activation and deactivation take place automatically within a few milliseconds by TE and NT without additional activity on the part of the participant.


Due to the construction as a bus, every telephone or data connection on an S 0 bus can in principle be overheard at every other junction box; no special encryption is used. Due to the data mirroring of the D-channel on the E-bit, connection establishment and termination ( signaling ) can be recorded and logged even with a normally connected ISDN card . For the user data sent by other end devices, this requires ISDN hardware that can also receive data on the send wires (for example, some ISDN cards). Alternatively, an ISDN card that is not capable of this is often used and connected by means of exchanged wires. In both cases it is possible to listen in to all of the data transmitted over the line.

Individual evidence

  1. a b I.430 Basic user-network interface - Layer 1 specification .


  • Hubert Zitt: ISDN & DSL for PC and telephone . Pearson Education, 2005, ISBN 3-827-26987-3 .
  • Rudolf Huttary: Successfully installing and repairing telephone technology yourself . Franzis Verlag, 2002, ISBN 3-772-35106-9 .
  • Andreas Kanbach, Andreas Körber: ISDN - The Technology . Hüthig, 1999, ISBN 3-7785-2288-4
  • Telecommunication with ISDN and ADSL . PUBLISHING EUROPA-LEHRMITTEL, 2003, ISBN 3-8085-3513-X

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