Telecommunications Signaling
Highlights the conventions of signalling systems and provides a detailed assessment of their architecture.
TABLE OF CONTENTS Telecommunications Signaling
Appendix 4: ITU-T Signalling System No. 6
A4.1 Introduction
ITU-T Signalling System No. 61 (CCSS6) was the first CCS system to be implemented internationally. It was originally designed for use in the international network, but some flexibility is included to allow its use in national networks. It is an inter-exchange signalling system that was implemented widely, but it has now been superseded by ITU-T CCSS7.
CCSS6 offers a wide range of features associated with CCS systems, including operation in the quasi-associated mode, error detection and correction mechanisms and re-routing capabilities in fault conditions. However, its main drawback is its limited evolutionary potential, caused by its lack of a tiered architecture. In a dynamic environment, CCSS7 offers far greater flexibility and evolutionary capability, particularly due to its structured architecture.
A4.2 General Description
A4.2.1 Transmission
CCSS6 was originally optimised to be used over analogue transmission paths, but digital paths can also be accommodated. It is operated on a link-by-link basis. Communication between two exchanges is via a datastream over a CCS link.
A4.2.2 Signal Units
The information sent over the link is divided into 'Signal Units' of 28 bits in length. All Signal Units are of the same length, the last eight bits being used for error detection. The Signal Units are grouped into 'blocks' of 12, the last Signal Unit in each block being used for acknowledgement purposes. Each Block is allocated a sequence number as part of the error detection and correction mechanism.
Signal Units within a block are either 'Message Signal Units' or 'Synchronisation Signal Units'.
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