S interface

The S0 bus is an interface within an ISDN basic access installation. It serves the premises wiring and connecting ISDN terminals to the NT. The S0 bus is not to be confused with the S0 interface for the transmission of measured values ​​in building automation.

Some cable modems that provide phone services via cable TV network, also offer an S0 bus for the connection of ISDN devices.

Technically, the bus from a four-wire cabling (twisted pair) are used for each communication direction ( send and receive ) at the two wires. The cores "a1" and "b1", when viewed from the terminal, the receive wire pairs, the veins " a2 " and " b2 ", the transmitting wire pair (see below connection diagram). So the bus is operating in full -duplex mode ( through spatial multiplexing ). As an encoding method of the modified AMI code will be used. All useful signals and control signals are combined in a 48-bit frame in time multiplex transmitted 4000 times per second. Thus, an S0 frame has the time length of 250 microseconds, a bit of about 5.2 microseconds. By using Ausgleichsbits ensures that each full frame is dc-free. The individual frames are separated by code violations within the AMI code. The nominal data rate up to 192 kbit / s

This data rate is spread over two B channels for user data transfer (each 64 kbit / s), one D-channel (16 kbit / s), which is used for signaling and different synchronization and organization of data. Thus, the D- channel from the NT is mirrored and sent back to the so-called echo channel to the terminal to implement a protocol for concurrent access (see also: CSMA / CA).

Execution for ISDN multipoint connection

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

In a multi- device connection it connects multiple devices simultaneously ( point-to- multipoint). For scheduling, terminating resistors must be used in this case, the bus can be up to 150 m long and be wired with a maximum of twelve ISDN jacks (UAE or IAE). A maximum of eight devices simultaneously be connected to the S0 bus of which can be a maximum of four supplied by the NT with electricity.

Construction for ISDN system connection

The basic connection in the system connection mode, only one terminal device may (usually a PBX with DID number) to be connected.

Bus types (cabling types )

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

Difference "internal " / "external " bus

For smaller telephone systems for the Soho area, a distinction between the internal and external S0 buses. An external S0 bus is looped through the S0 bus of the NT. This all calls, and the calls between phones in the same household / office, via the exchange in the public network are handled. Thus usually cost also in-house conversations connection fees. In addition, in an external S0 bus are in -house discussions both B- channels are busy, so for the time of the chargeable call no more line -free and therefore no one can be reached.

If an internal S0 bus, the telephone system will provide the experts; the house calls cause a connection to the public telephone network and are therefore free of charge.

Activation procedure

Before a terminal can initiate or accept a connection, the S0 interface must be enabled. The S0 interface is disabled when 10 seconds with no connection exists for a time and no data exchange (exception are permanently enabled ports, for example, system connections ). For the activation procedure, there are five states, called INFO, which are passed on layer 1 of the interface.

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

Upon activation by the subscriber's terminal equipment (TE ) all five states are run through. Upon activation by the network termination (NT) only the states INFO S2 through S4 to INFO. Activation and deactivation take place within milliseconds automatically TE and NT without additional activity of the participant.

Security

Due to the construction of a bus, each led to an S0 bus telephone or overheard data connection at any other junction box principle; it is used no special encryption. By mirroring the D channel on the E- bit connection establishment and release ( signaling ) itself can be captured with an ISDN card normally connected and logged. For the user data sent by other terminals, this ISDN hardware that can receive to the transmit data wires requires (for example, some ISDN cards ). Alternatively, it is also often used to one not compatible ISDN card and connected by wires are reversed. In both cases a complete screen all over the line of transmitted data is possible.

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