. . __ __._____._____:_____._____._____._____:_____._____._____ : Y : _ : _ Y_ _ : _ : . : ___Y __ : . : ____: --------| _ | _ | | _/ ___j _ | | l___ | \_ | | | \_ |-------- _\\//_ | : : : : : | L : | :__: : | | / : | : | : _\\//_ (_o,O) `--'--'--'--'--'--^-----'--'--' `--'-----^-----'-----'-----' (O.o_) " " CCS and The 30 Channel PCM System " " by Psyclone/MED This file will go into a high degree of specific detail about digital switching systems so be ready to get your hands dirty. Basic background information about System X is also provided. Please note the table of signalling codes at the end of the file is also found in 'Part (v)' of ĝline's telefon2.txt. All information is accurate to the best of my knowledge. Introduction ============ BT use a system called Pulse Code Modulation (PCM) to transfer call data to, from and around digital exchanges, it works by taking all the information from several calls and separating it into packets, (packetizing it), and sending the packets each in turn down a single line. Also amongst all this information are the signalling codes which control all of the calls in question. The method that PSTN uses to send the signalling information down the same line is known as Common Channel Signalling (CCS). In System X the Control Processing Subsystem (CPS) software is responsible for call control. The areas of infomation handled by the CPS are the subscriber infomation, call routing, routes and circuits, and the call charging information. The Message Transmission Subsystem (MTS) is the terminal part of the CCS link. It transmits and receives messages sent between the Call Processing Subsystems of the distant exchanges. The MTS message format is : Error Check, Message, Routing label, Header. The header contains the message number in case a resend is needed. The routing label includes the Originating Point Code (OPC) which identifies the originating exchange and the Destination Point Code (DPE) which identifies the destination exchange. The message contains the information which tells CPS what it has to do. And the error check is used to check the message for corruption. All links from the subscriber to the exchange are of course analogue (unless ISDN or other). The Signalling and Interworking Subsystem (SIS) converts the analogue lines to 30 channel PCM for connection to the Digital Switch Subsystem (DSS) which carries out the switching functions. PCM is a digital method of data transmission which uses '1's and '0's, which are transmitted by varying voltage over a metallic path, or by other similar methods eg flashing light if the transmission medium is optical fibre. It can be used in a number of different formats. The 30 Channel PCM System ========================= The 30 channel PCM system provides a transmission path for 30 telephone channels or audio bandwidth data links over a single cable pair using time division multiplexing. The instantaneous speech amplitude of a channel is sampled and converted (by an analogue to digital converter) into an 8 bit word which is transmitted serially over the cable. Each channel is sampled in turn and transmitted, this process repeats itself continuously in order for continuous speech to be possible. Each 8 bit word, which corresponds to a sample, occupies one timeslot in time. A frame consists of 32 timeslots, including a complete set of 30 samples, plus a further two timeslots which are used for alignment and signalling purposes. To provide adequate channel bandwidth the frame repetition rate is 8000 per second. Therefore, in the period of one frame 256 (ie 32 x 8) bits are transmitted. This gives a transmitted bit rate of 2.048 Mbit/second, ie. [ (8 x 32) x (8000) ]. At the receiving end of the cable the timeslots for each channel are segregated and digital to analogue converted to form a voltage which is proportional to the original waveform sample. This voltage is therefore updated each frame for every channel and the resultant audio waveform before filtering is a histogram with 8000 steps per second which follows the original waveform. Finally, it is worth noting that the bandwidth is limited to the band 300 Hz to 3.4 kHz so that the histogram ripple is removed. Frame Alignment =============== In order to align the segregation sequence one of the extra timeslots is used for frame alignment. This is designated Timeslot 0 and in alternate frames it contains a distinctive 8 bit word, the Frame Alignment Word, (FAW). After its arrival the next 8 bits received are Timeslot 1, (containing Channel 1 traffic), followed by the other timeslots in numerical order. The receiving multiplexer enumerates these by synchronising a counter to the FAW, and this process constitutes frame alignment. The channel sequence is broken at Timeslot 16, which is the second extra timeslot and is used for signalling as we shall see in the next paragraph. Channel 16 traffic is therefore transmitted in Timeslot 17, and so on in order until Channel 30 is transmitted in Timeslot 31. This is shown in the table below. +----------+---------+ +----------+---------+ | TIMESLOT | CHANNEL | | TIMESLOT | CHANNEL | +----------+---------+ +----------+---------+ | 0 | - | | 16 | - | | 1 | 1 | | 17 | 16 | | 2 | 2 | | 18 | 17 | | 3 | 3 | | 19 | 18 | | 4 | 4 | | 20 | 19 | | 5 | 5 | | 21 | 20 | | 6 | 6 | | 22 | 21 | | 7 | 7 | | 23 | 22 | | 8 | 8 | | 24 | 23 | | 9 | 9 | | 25 | 24 | | 10 | 10 | | 26 | 25 | | 11 | 11 | | 27 | 26 | | 12 | 12 | | 28 | 27 | | 13 | 13 | | 29 | 28 | | 14 | 14 | | 30 | 29 | | 15 | 15 | | 31 | 30 | +----------+---------+ +----------+---------+ Timeslot 16 and Multiframe Alignment ==================================== Signalling for the 30 channels must also be provided for as well as the actual speech/data, but the bandwidth or information rate needed for signalling is much less than that needed for the traffic itself. The signal path for all channels, or Signal Highway, is provided entirely by Timeslot 16. This system is known as Signalling System TS16, and is the one used by CCS for PSTN (CCITT SS No 7) signalling. So, we already know that each timeslot contains 8 bits in the form of an 8 bit word which consists of part of a channel's traffic - except in the cases of timeslots 0 and 16, where the 8 bits have different purposes. In timeslot 16, the 8 bits are divided into two halves of 4 bits each and these 4 bits contain the signalling codes. Now, these 8 bit words (each containing two 4 bit codes) are transmitted in a cycle - once per frame and over a period of 16 frames. This allows enough time for thirty 4 bit signalling codes to be transmitted - (one for each channel), together with another 8 bits for a multiframe alignment word (MAW) and alarms. This 16 frame cycle is a called a Multiframe, and occupies a period of 2 ms. In the multiframe each frame has a designated number, 0 through to 15. In Frame 1, the first half (4 bits) of Timeslot 16 contains the signalling for Channel 1, Frame 2 for Channel 2 and so on. The second half of each Timeslot 16 contains the signalling for the remaining 15 channels in order. This is kinda difficult to explain, but the following table should clarify things a little. NOTE: Some of the frames and most of the channel-traffic carrying timeslots are not shown in the table as they would not fit, but fear not as they should play no great part in your understanding. FRAME No. TIME-------> | | +----------+---------- | ----------+----------+---------- | ----------+ | TIMESLOT | TIMESLOT | TIMESLOT | TIMESLOT | TIMESLOT | TIMESLOT | | 0 | 1 | 15 | 16 | 17 | 31 | +==========+========== | ==========+==========+========== | ==========+ 0 | FAW | CHAN 1 | CHAN 15 | MAW | CHAN 16 | CHAN 30 | |1(0011011)| TRAFFIC | TRAFFIC |(0000)1011| TRAFFIC | TRAFFIC | +----------+---------- | ----------+----+-----+---------- | ----------+ 1 | FNW | " | " |CH 1|CH 16| " | " | |1(1)0XXXXX| | |SIG | SIG | | | +----------+---------- | ----------+----+-----+---------- | ----------+ 2 | FAW | " | " |CH 2|CH 17| " | " | | | | |SIG | SIG | | | +----------+---------- | ----------+----+-----+---------- | ----------+ 3 | FNW | " | " |CH 3|CH 18| " | " | | | | |SIG | SIG | | | +----------+---------- | ----------+----+-----+---------- | ----------+ 4 | FAW | " | " |CH 4|CH 19| " | " | | | | |SIG | SIG | | | | | | | | | | | | | ------------------------+----------------------------------+------------- | | | | | | | | | | 13 | FNW | " | " |CH13|CH 28| " | " | | | | |SIG | SIG | | | +----------+---------- | ----------+----+-----+---------- | ----------+ 14 | FAW | " | " |CH14|CH 29| " | " | | | | |SIG | SIG | | | +----------+---------- | ----------+----+-----+---------- | ----------+ 15 | FNW | " | " |CH15|CH 30| " | " | | | | |SIG | SIG | | | +----------+---------- | ----------+----+-----+---------- | ----------+ | | FAW = Frame Alignment Word MAW = Multiframe Alignment Word FNW = Frame "Not" Word The FNW or NOT FAS (Not Frame Alignment Signal) 1(1)0XXXXX is transmitted in Timeslot 0 of the frames that intervene the frames containing the FAW. X denotes that the bit state is immaterial. Another point worth noting is that although the FAW and FNW occur in alternate frames, they have no respect for the frame number in which they occur in the Multiframe. For instance the FAW could occur in frames 1, 3, 5, etc. In the same way that frame alignment is necessary in order to enumerate the timeslots and allocate them to the to the channels, multiframe alignment is necessary to enumerate the frames in order to assign the signalling words to their respective channels. To do this the Multiframe Alignment Word, (MAW), is transmitted in the first half of Timeslot 16, Frame 0. This position is not needed for other signalling as there is no Channel 0, and the word, 0000, is distinctive because all signalling codes contain at least one 1. If the receiving multiplexer has established frame alignment it is able to segregate Timeslot 16 and to look in it for the MAW. It aligns on the MAW by synchronising a 16 stage counter, so that the number in this counter is the frame number. Multiframe alignment therefore permits segregation of a channel's signalling, just as frame alignment permits segregation of the channel's traffic. All CCS codes are shown below in ascending mathematical order: Circuit Signalling Condition Four-bit Signalling Forward Direction Backward Direction Code - - 0000 Trunk Offering Manual Hold 0001 - - 0010 Circuit Seized Called Sub Answer 0011 - - 0100 Earth Earth 0101 - - 0110 - Circuit Free 0111 - - 1000 - Coin Fee Check 1001 - - 1010 Dial Break - 1011 - - 1100 Disconnection Disconnection 1101 - - 1110 Circuit Idle Circuit Busy 1111 '-' signifies that the code is not used/has no meaning. The forward direction codes are the ones transmitted from the CPS handling the end from which the call originates, to the CPS which handles the destination of the call, and the backward direction is the reverse. If the call is dealt with by one exchange (ie it is local) then there will only be one CPS involved. Dont phry too many brain cells now. 03/98.