MODMER / Interfészek / G.703 2048 kbit/s

The ITU-T G.704 (2048 kbit/s) (PCM) frame

This chapter is about the primer PCM time-frame of the European standard (CEPT, E1). Classical digital connection centres, and PCM multiplex transmission systems connecting them with each other can operate with the help of this framing. ISDN received this standard, it is the basis of the ISDN network availability with primer speed.
With the development of telecommunication technologies many of systems based on PCM ?retired?, but the PCM time-frame lives on as a universal digital interface.

Main parameters of the frame, frame types
Build-up of the primer time-frame
Build-up of CAS multiframe
Build-up of CRC-4 multiframe

Main parameters of the frame, frame types

- frame time: 125 us
- bit time: 488 ns (2048 kbit/s)
- number of timeslots: 32 (serial numbering: 0...31) (from this 30 are used for receiving information, 2 ( 0. and 16.) are used for synchronizing, controlling, and signalling tasks).
- number of bits in a timeslot: 8 (serial numbering 1...8)

There are two (earlier four) types of the European (CEPT) PCM time frame, depending on, what kind of multiframes in the frames were generated. A multiframe could be:

_{The so-called CAS multiframe (Channel Associated Signalling), which unifies the channels used for transmission of marks, first was used for transmitting the digitalized line signaling between electromechanical (crossbar) centres of PSTN networks (reservation, dissociation). It is not a curriculum so far, because ISDN networks do not already use these marking channels.}
The so-called CRC-4 multiframe, which is used for forwarding CRC bits, that indicate errors occured via transmission, which was developed at the 1. bitposition of the 0. time-gap.

Frame types:

Name:	Description:
_{PCM30 in museum}	_{CAS multiframe: yes} _{CRC multiframe: none} _{Channel associated signalling (CAS)}
_{PCM30_CRC in museum}	_{CAS multiframe: yes} _{CRC multiframe: yes} _{Channel associated signalling (CAS)}
PCM31	CAS multiframe: none CRC multiframe: none No nhannel associated signalling (CAS)
PCM31_CRC	CAS multiframe: none CRC multiframe: yes No nhannel associated signalling (CAS) This frame type is the physical layer frame of the primary rate ISDN interface

Build-up of a primer time frame

Build-up of a primer PCM time frame is shown on the following figure:

Channel capacity of an information timeslot is 64 kbit/s. If we need a larger capacity, we can obtain it with unifying more timeslots

Build-up of the CAS multiframe

_{A CAS multikeretet a primer PCM keretek 16. időréseiben építették fel.
CAS multiframe was built up in the 16. timeslot of the primer PCM frames. The multiframe contains 30 marking channel assigned to the information receiving channels of the PCM frame - capacity of the individual marking channels is 2 kbit/s. (This solution is a good example of how to generate channels with a speed, that is smaller, then 64 kbit/s in a PCM frame.

Content of 16. timeslot in a multiframe:

serial number of signalling
channel: 1 16 ... 15 31

A signalling channel consists of 4 bits, a, b, c, d, which are assigned to a speech channel.

Build-up a CRC-4 multiframe
A CRC-4 multiframe is built up in the 1. bit position of the 0. timeslot.
The 4-bit CRC is generated by subframes (128 bits) with the
x^4 + x + 1 generator polynom.

Content of the 0. timeslots in a multiframe:
subframe frame 1 2 3 4 5 6 7 8
number number
0 C1 0 0 1 1 0 1 1 FRAME
1 0 1 A SA4 SA5 SA6 SA7 SA8 SERCIVE
2 C2 0 0 1 1 0 1 1 FRAME
3 0 1 A SA4 SA5 SA6 SA7 SA8 SERCIVE
I 4 C3 0 0 1 1 0 1 1 FRAME
5 1 1 A SA4 SA5 SA6 SA7 SA8 SERCIVE
6 C4 0 0 1 1 0 1 1 FRAME
7 0 1 A SA4 SA5 SA6 SA7 SA8 SERCIVE

8 C1 0 0 1 1 0 1 1 FRAME
9 1 1 A SA4 SA5 SA6 SA7 SA8 SERCIVE
10 C2 0 0 1 1 0 1 1 FRAME
11 1 1 A SA4 SA5 SA6 SA7 SA8 SERCIVE
II 12 C3 0 0 1 1 0 1 1 FRAME
13 Si1 E 1 A SA4 SA5 SA6 SA7 SA8 SERCIVE
14 C4 0 0 1 1 0 1 1 FRAME
15 Si2 E 1 A SA4 SA5 SA6 SA7 SA8 SERCIVE

001011 - CRC multiframe synchron code
C1...4 - CRC bits
The 4-bit CRC is generated on the transmitting side by subframes
(128 bits) with the x^4 + x + 1 generator polinom and the result
is forwarded in this bits to the PCM receiver.
Si1, Si2, E (E13,E15) - error indicator bits (by subframes)
The PCM receiver determines the CRC by subframes on an equally way
with the transmitter and compares the CRC-bits with the given C1-C4 CRC
bits. Then it indicates the result of the comparison in Si1, Si2 bits
for the transmitter.
Si1, Si2 - 0 there is no CRC multiframe synch 1->0 CRC error.
A - remote alarm (RAI) indicator bit.
RAI=1 alarm,
RAI=0 no alarm
SA4...SA7 - spare bits (e.g. a service channel can be generated in them.)

When giving a PCM frame, we generate a control amount by sub-multiframes
and we insert in the 0. timeslot its bits.
Receiving the PCM frame we evaluate the control amount by sub-multiframes.
Result of the evaluation we can forward in the bits of 0. timeslot towards
the transmission unit.
In the case of a difference the receiver will generate an error signal.}