Structure of the PCM-30 (E1) frame

Figure 5 – Stages at the forming of a group signal

Н. Н. Моисеев. Универсум. Информация. Общество. — М.: «Устойчивый мир», 2001. — 199 с.

Н. Н. Моисеев. Математика ставит эксперимент. — М.: «Наука», 1979. — 223 с.

T _ch – channel time slot (channel interval),

T _clk – clock interval,

m – quantity of bits of PCM code.

Continuous signal is converted to a sequence of samples (individual PAM signal) (Fig 5 a).

Further individual PAM signals of all sources combined into a group PAM signal (Fig 5 b).

After that, the group PAM signal is coded by the PCM coder (Fig 5 c).

Now give a definition of the main parameters of the transmission cycle (frame).

PAM signal transmission frame on duration is equal to the discrete sampling period. It consists of:

- Pulses of channels 1, 2,…N;

- Service pulses.

On a following time interval an arrangement of pulses is similarly 1, 2,…N and service pulses. This periodically repeating information structure consisting of information and service pulses is called as a transmission frame (cycle). Sometimes the transmission cycle is called as a time spectrum.

As channel time slot (channel interval) is called the time interval between samples of the adjacent channels. An equation for calculation is:

T _ch= T _frame/ N _ch

Clock interval is time of transmission of one (binary) digit. A calculation formula is:

T _clk= T _ch/ m = T _frame/(m * N _ch)

Clock frequency f _clkin DTS is called the quantity of clock intervals per a time unit (1 second). For the ADE at fulfillment of conditions T _frame= T _s and f _frame= f _s we obtain the formula for a clock frequency (clock rate):

f _clk= f _s* m * N _ch

where f _s is a sampling frequency (f _s=8 kHz);

N _ch – quantity of channel intervals (N _ch= N _vbc for the approximate calculation or N _ch= N _vbc+ N _serv for accurate calculation).

Transmission bit rate B is called the quantity of the binary symbols which are transmitted during unit time interval (1 second).

The DTS frequency band Δ f _PCM is called a frequency band which contains a basic power part (>90%) of binary signal.

The next equality is carried out for DTS:

f _clk= B = Δ f _PCM=

Hz bit/s Hz

kHz kbit/s kHz

MHz Mbit/s MHz

Consider the principle of formation of the DTS frame for the example PCM-30

PCM-30 line signal (primary digital bit stream (PDB or PDS or E1)) is organized into the multiframes (MF) and every multiframe consists of 16 frames (F). Each PCM-30 frame consists of 32 channel time slots (TS) with numbers 0, 1,…31. Each channel time slot consists of eight bits.

Figure 6 – Frame of PCM-30

30 TS are informational and serve for transmission of sampled signal by means of PCM code combinations. Two others (TS-0 and TS-16) serve for transmission of service information.

Every even frame contains the frame alignment signal (FAS) in the TS-0 (0011011). In odd frames transmitted the alarm FAS.

First bit in TS-0 of all frames transmits the signal of discrete information (DI).

All others TS (with the exception of TS-16) transmits PCM code combinations (or organized into the data channels E0 with bitrate 64 kbps in each).

TS-16 contains the signaling bits (with Command and Interaction Signals, CIS) for two voice channels. For CIS transmission of all 30 information (voice) channels are necessary 30/2=15 frames, and so frames are organized into the multiframes from 16 frames each.

Multiframes alignment signal (MFAS) is transmitted (0000 codeword) in frame-0.

Frames 1…15 contain the CIS of 30 voice channels.