..::::: |-----> Basic CCS   : ....:::::|||lllSSSS$$$$$$$$$$$$$$$$$$$$$$$$$$$$
..::::: |-----> By Hybrid   ...::::::::::::|||lllSSSS$$$$$$$$$$$$$$$$$$$$$$$$
..::::: |-----> th0rn@coldmail.com   ...::::::::::::|||lllSSSS$$$$$$$$$$$$$$$



CCS stands for Common Channel Signalling and forms the basis of our SS7 phone
networks. SS7 is different from previous switching systems because it has a
unique feature. The data used to control the system is seperate from the
voice circuit, so boxing an SS7 system is not possible. Modems are employed
to transmit and recieve the serial binary data over *analogue* transmission
channels, effectivly seperating the switching from the speech, so it would be
imposible to do a captain crunch down the phone with your whistle.

CCS has it's advantages for the phone companys. For example:

*Faster signalling rates
*CCS can accomodate a larger volume of signals
*The system can handle siganals during the speech period.
*Signalling patterns, reates etc, can be cahnged at anytime.
*It's cost effiective for large networking hierarchys.

CCS, however, gives rise to requirements which do not arise with signalling
on speech-path systems such as:

*High order error rate performance
*Signalling link security backup
*Assurance of speech-path continuity as, unlike speech-path signalling, CCS.
does not establish speech path integrity.

General features of CCS:

*Information may be transferred as signal messages of varying bit length, or
by defined signal units. 

*Each signalling channel operates in the synchronous mode with its continuous
bit stream divided into contiguous signal units which all contain the same
number of bits for the same application or service. The two signalling
channels of a link need not necessarily be synchronised to each other, and if
not, drift may arise between the signal units in the respective directions,
which, in certain implementations of CCS (e.g. CCITT 6 system) may require
compensation arrangements.

*Synchronisation (idle) units are transmitted when message signals are not
being transmitted, to maintain signal unit synchronism on the signalling
channel.

*The signal unit is divided into a number of constituent bit fields each
having its own function in the system, typically, heading, signal
information, circuit label, parity check, etc.

*As the CCS link is time-shared, each message signal requires identification
of the speech circuit (and thus the call) to which it belongs. This is by
means of a circuit label bit-field of size depending on the number of speech
circuits to be identified.

*As CCS signals do not prove the continuity of the speech path selected,
other arrangements (e.g. per-call continuity check, routine testing of idle
paths) must be made.

*Errors are liable to occur in the signal-transfer process and some form of
error control is required as the uncorrected error-rate of transmission plant
is usually unacceptable for c.c s. Error detection is based on redundant
coding, the parity check bits being, typically, part of each signal unit.
Error correction can be, and usually is in telephony CCS, by retransmission.
Despite the incorporation of error control, undetected errors may arise. Even
with a high degree of error correction a signalling link could be unusable
for varying periods, which requires signalling security backup.

*For signalling 'security', signals may be directed, by automatic procedures,
from a regular signalling link to an alternative signalling facility when an
excessive error rate, or complete failure, of the regular link is detected.

*On multilink connections, signalling information is transmitted on a link-by
-link basis, the signals being transferred from one link to the next only
after processing. This is a majour feature of CCS.

*Since the circuit label may require a substantial proportion of the bits
available in a signal unit, comparatively few bits may remain for coding the
information a message has to convey. It follows that when the information is
even moderately extensive, and particularly when it has a data content which
can vary from one message to another, it cannot be transmitted efficiently by
a succession of single signal unit messages. Instead, the initial circuit
label carrying signal unit may be augmented by one or more subsequent signal
units in which all the available bits can be used for carrying the
information, thus producing an efficient multiunit message. Thus lone signal
units (LSU) and multi-unit messages (MUM) may apply with CCS, and in the
latter case the initial signal unit (ISU) carries the circuit label bit-field
for the whole MUM. Each SU in a MUM carries its own check bit-field.


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