Telecommunications Signaling
Highlights the conventions of signalling systems and provides a detailed assessment of their architecture.
TABLE OF CONTENTS Telecommunications Signaling
Appendix 2: Channel Associated Signalling
A2.1 Introduction
Channel Associated Signalling (CAS) systems were once the mainstay of telecommunications networks. Over time, they have been replaced by CCS Systems within networks, but some CAS systems are still common for gaining access to networks. Chapter 2 describes the basic tenet of CAS systems, i.e. dedicated signalling capacity is provided for each traffic circuit. This Appendix describes the principles of six categories of CAS system. Examples are given to illustrate the techniques used. Details are well documented elsewhere1.
One point of terminology needs to be clarified. Signals can be sent in one of two directions. A 'forward signal' is sent in the direction from the calling customer to the called customer. A 'backward signal' is sent in the reverse direction.
A2.2 Loop-disconnect Signalling
A2.2.1 Loop Conditions
In Loop Disconnect (LD) signalling systems, the signalling information pertaining to an analogue traffic circuit is transferred by modifying the electrical conditions applying to the traffic circuit. The status of a direct current (DC) loop on the traffic circuit identifies the information being transferred. Signals from a calling customer are defined by make and break conditions on the loop. Signals from a called customer are transmitted through the network by changing line polarity.
Consider a link between a customer and a local exchange, which is based on a DC loop. When a customer's telephone is in an idle state (i.e. the handset is in the cradle, 'on-hook'), the loop is disconnected (open) and current does not flow. When the calling customer commences...
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