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Quantum and Evolution Series Installation and Operating Handbook
2) Timeslot identity (ID) maintenance, ie which 8 bits relate to which timeslot (first, second,
third etc) within the data stream.
The data from each timeslot appears as a set of 8 bits from each terrestrial frame. Timeslot
boundary maintenance is provided by placing the data into the IBS frame, timeslot aligned
after TS0. When TS0 is located, the next 8 bits represent 8 bits from one terrestrial timeslot,
the following 8 from the next etc.
Timeslot Identity Maintenance again relies on relative position to TS0. Data from the first
timeslot of the group of timeslots is always placed in the first timeslot after TS0, the second
of the group in the second TS, and so forth. Once the 8 bits of data from every timeslot
have been placed in the frame, the process repeats until the frame is full. For the following
frame to be similarly formatted the timeslot data must fit into the 60 available data timeslots
of the satellite frame exactly (so that the next data is from the first TS again). This brings in
the following complication.
Each IBS frame has 60 timeslots available for data, and so in order to provide timeslot ID
maintenance, the number of timeslots `N` must fit into 60. This limits the values of N to:
1 Frame (ie 60 available data timeslots): N = 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30
Some further values are accommodated by mapping them into two frames:
2 Frames (ie 120 available data timeslots): N = 8, 24
Finally, to support one further rate, four frame are used:
4 Frames (ie 240 available data timeslots): N = 16
The remaining values of N cannot have Timeslot ID Maintained by groups of 1, 2, or 4
frames, these are collectively know as the `Odd values of N`:
`Odd values of N`: N = 7, 9, 11, 13, 14, 17, 18, 19, 21, 22, 23, 25, 26, 27, 28, 29, 31
Most previous equipment could not provide timeslot ID maintenance for these `odd values
of N`, however the modem when equipped with the Extended D/I feature uses a derivative
of the IBS CAS Multiframe (see later section "CAS Multiframe") to provide timeslot ID
maintenance for even these remaining values of N.
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 use 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.
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`) that are sent every 16th frame, resulting
in a channel of 2kbps. There is an individual 2kbps signalling channel for each of the 30
8-13
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