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Turin Networks, Inc. Chapter 4-1: Voice Network Applications
Page 3
The interchange of signaling between the voice systems connected to the Turin interfaces is carried
using four bits (a, b, c, d) which are formed by the analog voice IM (E&M, FXS, FXO) or by external
digital switching equipment connected to E1 IM.
16 Timeslots 1 to 15 0 Timeslots 17 to 31
Frame
Signaling
Synchronization
Timeslot 1 Timeslot 17
a,b,c,d a,b,c,d
Timeslot 2 Timeslot 18
Timeslot 3 Timeslot 19
Timeslot 4 Timeslot 20
Timeslot 5 Timeslot 21
Timeslot 6 Timeslot 22
Timeslot 7 Timeslot 23
……………………….
Timeslot 15 Timeslot 31
Figure 1 – PCM30 Frame
In Turin equipment, the Control bus is converted into the signaling information transmitted in time
slot 16 of the E1 signal. For incoming voice traffic, the reverse process occurs converting the timeslot
16 information into the control bus which can then be cross connected in the “shadow” DACS.
The interchange of signaling uses the a, b, c, d bits that change state slowly. The slow rate of change
of signaling only requires a signaling rate of 2Kbps for each voice channel. As a result, the capacity
of timeslot 16 is divided between the 30 voice circuit timeslots. Each timeslot 16 frame carries the
signaling bits for two voice circuits and is repeated every 16 normal PCM (E1) frames (or Multiframe).
The PCM30 and PCM31 data streams use a unique frame alignment bit pattern in timeslot 0 for to
initiate the Frame Synchronization. If the PCM (E1) data stream consists of both digitized voice and
of user-defined data in multiples of 64Kbps timeslots, there is a risk the user may transmit the Frame
Alignment pattern. This may cause incorrect frame alignment and the incorrect assignment of all
PCM timeslots. To avoid this malfunction, network administrators are advised to use the optional
CRC-4 cyclic redundancy check for all E1 systems. PCM systems that use CRC-4 may also be
referred to as PCM30C and PCM31C.