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P300H P300 Series Modem Installation and Operating Handbook Page 210
Signalling Systems Introduction: CCS, CAS & RBS
Signalling systems are used to convey call status on PCM systems, call status is information such as Busy,
Idle, Ring tone, Dialling etc.
Common Channel Signalling (CCS)
Some PCM systems us a`Common Channel Signalling` (CCS), where a whole timeslot is allocated to carry
the signalling function between the ends of the link (with protocols such as `Signalling system No 7` etc
running within that 64kbps channel). Common Channel Signalling is used on both 1544kbps (T1) and
2048kbps (E1) systems, although different signalling systems run on the common channel (of course !)
Channel Associated Signalling
Some E1 PCM systems use Channel Associated Signalling (CAS), where a low rate data signalling channel
is provided uniquely for each timeslot. ITU-G.704 defines these signalling channels as comprising of four
bits (named `abcd`) which are sent every 16th frame, resulting in a channel of 2kbps. There is an individual
2kbps signalling channel for each of the 30 PCM channels (timeslots 1-15 and 16-31) and these are carried
sequentially in pairs in TS16 (there is no signalling channel for TS0 nor TS16, as TS0 is reserved for the
Frame Alignment Word, and TS16 carries the signalling for the other timeslots).
Robbed Bit Signalling (RBS)
Robbed bit signalling is a technique used on T1 PCM bearers whereby signalling is conveyed by stealing
the signalling bandwidth from the 64kbps timeslot to which it relates. Once every 6 frames one bit (the least
significant bit) of the 8 bits per timeslot is `stolen` and used to convey signalling. In T1-D4 (which has a 12
frame terrestrial multiframe) this gives two signalling bits `ab`, and in T1-ESF (which has a 24 frame
terrestrial multiframe) this gives four signalling bits `abcd`. The frames in which the bit is robbed are
synchronised to the terrestrial multiframe as follows: T1-D4: 6th & 12th, T1-ESF 6th, 12th, 18th, 24th.
Stealing the LSB every sixth frame was acceptable when PCM signals conveyed primarily PCM audio as
it made a negligible difference to the audio quality. However when the PCM network was later used to
convey data not just PCM encoded voice, only 56kbps of data could be conveyed (using 7 bits per timeslot),
as it was difficult to guarantee the signal would not pass through a link using Robbed Bit Signalling where
the 8th bit would be overwritten every 6th frame, causing errors on the 64kbps data.
Signalling Over Satellite
Common Channel Signalling (CCS)
With Common Channel Signalling, the whole 64kbps signalling channel (timeslot) must be conveyed from
one end of the link to the other and typically this is done on sub primary order satellite circuits (thin routes)
by selecting the timeslot carrying the CCS for Drop/Insert as with other data timeslots. Special action is
required with CCS, as the equipment at both ends of the link needs to be configured for the ` thin route`
over the satellite so it does not dynamically allocate traffic to a timeslot which is not passed over satellite
link !
Channel Associated Signalling
The IBS specification includes an option for equipment to support CAS, and defines a mechanism whereby
the CAS (the 2kbps signalling channel per timeslot) can be carried within the 6.7% IBS satellite overhead.
This option is fully implemented within the P300 equipped with the Extended D/I feature (standard on P300
TCM), and is further explained in the following section on "CAS Multiframe".
Robbed Bit Signalling (RBS)
Historically PCM bearers using Robbed Bit Signalling could not be used with Drop/Insert, as over standard
IBS links the location of the signalling information (ie which timeslots have a bit overwritten with signalling)
was lost. Even with Timeslot ID Maintenance (which preserved timeslot sequence and timeslot start / end
boundaries) RBS was still lost. To preserve RBS not only is TS ID maintenance required, but also the
timeslots required placing back into the correct frames within the terrestrial multiframe (so in T1-D4 for
example the timeslots containing the RBS are placed in frames 6 & 12 of the terrestrial multiframe).