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A 192 kbps digital trunk or tie-line offering 2 user channels (timeslots) of 64 kbps each. A
16 kbps channel is used for the signalling for both user channels. There is 48 kbps of
overhead (synchronisation etc.).
Digital links are used to connect the ISPBX to other digital public or private networks. The
physical interface between the digital trunk line and the ISPBX is formed by the Digital Trunk
Unit (DTU) or Digital Trunk and Extension (DTX). There are a number of different DTUs,
supporting various signalling systems. Some DTUs performs the conversion between the the
digital link signalling and the internal message protocol (IMP) as used in the SOPHO ISPBX.
For other DTUs this task is performed by an ISDN Protocol Handler (IPH). Several digital
signalling systems are discussed in the following sections.
The DTU/DTX link is projected as a route to which a CV is assigned. Any internal party can
be allowed to use a timeslot in the link for data communication or speech when the CV
connection allowance is 3 (allowed).
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For an outgoing call. After dialling the access code to a destination which can be reached
via the digital route, the ISPBX accesses a free timeslot in the DTU/DTX link and
establishes a transparent connection between the switching network and the external
network. The access code will be a trunk access code for connection to the public
network or a network access code for connection within a private network.
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For an incoming call. For an incoming call (DDI) the number is analysed in the DDI-tree
related to the digital route. If the number indicates an internal number, ringing current is
sent to the extension. After answering the incoming call, a transparent connection
between the external network and the internal party is established.
For more information on the routing aspects: see Facility Implementation Manual; Networks
and Routing.
3.3.1. DASS 2 Route
A Digital Access Signalling System No 2 (DASS 2) link is a 2Mbps line, 30B+D (timeslot 16 is
used for signalling, timeslot 0 is used for synchronisation). It is used for connection to a public
network. The signalling protocol is handled by an IPH or DTU-PH.
For determining if the related users in both private networks are compatible, a special
parameter is transferred over the DASS 2 link with incoming or outgoing calls. This parameter
is the Service Indicator Code (SIC). The SIC is a two byte code which is derived from the IBSC
of the ISPBX internal party. The SIC can be assigned as voice or data together with some
additional values for data such as speed and parity.
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For an incoming call, the SIC is translated into an Internal Service Identifier (ISI). The ISI
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