These 30 timeslots are available for the transmission of the digitized analogue signal in 8-bit form, with a bandwidth of 64 kbit/s (e.g. the customers’ data).
The European recommended system defines that the Timeslot 0 of each frame is used for synchronization, also known as frame alignment (see the following Figure). This ensures that the timeslots in each frame are aligned between the transmitting station and the receiving station.
The frame alignment word (FAW) is carried in data bits 2 to 8 of each even frame, while the odd frames carry a not frame alignment word (NFAW) in data bit 2 (see the following Figure).
An error check is also available in timeslot 0, using a cyclic redundancy check (CRC) to verify the frame alignment, which is carried in data bit 1 of all frames. There is also the facility of reporting Far End Alarms, which is indicated by a binary 1 being inserted in data bit 3 of all the odd frames. The remaining data bits 4 to 8 of the odd frames can be utilized for national alarms and network management.
Timeslot 16 has 8 data bits available, and by using a variable code of 4 data bits, signaling can be performed for 2 voice channels in each frame.
TTherefore it can be seen that 15 frames are required to complete the signaling for all the voice channels (see the following Figure).
As there are now multiple frames being carried in a logical order, there has to be a device for aligning these. This is achieved by using the frame prior to the frames containing signaling information, known as Frame 0.
Timeslot 16 in Frame 0 contains a multi-frame alignment word (MFAW), using data bits 1 to 4, and are used to indicate the start of a multi-frame, which are checked at the receiving station (see the following Figure).
Data bit 6 can be used to indicate distant multi-frame alignment loss (DLMFA). As can be seen, a multi-frame consists of all the frames required to complete all speech and signaling operations, i.e. 16 frames, and is known as a multi-frame (see the following Figure).
The duration of a multi-frame can be calculated using the following −
Duration of multiframe = Number of frames x duration of frame
= 16 x 125micro seconds
= 2000micro seconds
= 2 milli seconds
The remaining channels are all usable for voice or data transmission, and are known as timeslots 1 to 15 and 17 to 31, and equate to channels numbered 1 to 30.
FAW = Frame Alignment Word
MFAW = Multiframe Alignment Word
DATA = 8 bit data words
SIG = CAS signalling timeslot