THE INTERLEAVING PROCESS

We have seen already that once message signals have been reduced to a digital format, they may be most easily combined, that is multiplexed, with other digital signals by interleaving them in the time domain. The technology involved in this operation is to be the subject of this chapter, and Chap. 7. We shall concern ourselves here, however, only with the basic principles involved while omitting those details concerning asynchronous multiplexing (Chap. 7).

6.1 THE INTERLEAVING PROCESS

The fundamental circuit element within any time division multiplexer (TDM) is a serializer. This circuit accepts the parallel channel inputs and allows each, taken in turn, access to the output. Thus each channel input is allotted a time slot which uniquely defines the channel information within the serialized output stream. This is illustrated in Fig. 6-1, together with the relevant timing waveform.

We know that the channel messages, in the case of PCM signals, occur as a sequence of fixed-length code words. This gives rise to two different multiplexing procedures.

If the channel time slot is long enough to accommodate one complete code word, the multiplexed output signal is termed word interleaved. Alternatively each channel code word may be scanned one binary digit (bit) at a time to produce a bit-interleaved multiplexed signal. The two processes are illustrated within Fig. 6-2.

The term character interleaved is also used, and describes multiplexed telex signals.

Each character refers to a code word used to specify the various internationally agreed upon alphanumeric symbols.

The serialization timing is derived from a high stability oscillator, as shown in Fig. 6-1. Typically, the frequency reference used within the oscillator is a quartz crystal of tolerance in the range 10 to 50 ppm (see Chap. 7). This oscillator provides the master clock which specifies all the timing functions within the multiplexer (see Sec. 6.4).

If all the channel signals are derived from the same master clock, there will be a fixed phase relationship between them, and they are termed synchronous. Multiplexing such signals is straightforward. For example, in the case of word interleaving, since the channel word rate always remains equivalent to the information time-slot rate, no allowance for clock tolerances need be made.

117

118 Multiplexing

__________

^~rl~

__________

^~rL_

---I

f--L

Time slot dedicated to channel 3

---

^Time

Fig. 6-1 The basic principles of time division multiplexing.

Timing signal (1)

Obviously the demultiplexer must be able to identify which time slot is associated with which channel. For this reason a predetermined recognizable binary sequence is periodically interleaved with the information time slots. This sequence is used greater than c times the channel rate if the frame alignment signal is to be accommo-dated; where c refers to the number of channels. The most convenient frame structures to implement have a regular format, and for this reason it is desirable to define the frame alignment word (FA W) such that it occupies a finite number of time slots

Synchronous Time Division 119 exactly. Thus, the number of time slots per frame are typically chosen as c

+

1, or c

+

2, etc.

We shall now obtain an expression for the multiplex output bit rate

to

for a frame structure based on c

+

2 time slots. Let the analog channel messages be sampled at a frequency

= Is.

Let the number of digits used to represent each coded sample

= w. Therefore,

fo

⁼ wls(c

+

2) (6.1)

It is important to note that the two time slots allocated for frame alignment may be positioned anywhere within the frame. For example frame formats as specified in Figs. 6-3a, b, or c are equally valid. However, the frame format should be chosen in such a way as to reduce the average time taken for frame alignment, and to simplify the physical implementation. The term bunched framing is used to describe Fig. 6-3a, while the term distributed framing is reserved for Figs. 6-3b and c.

Some interleaving schemes do not require additional framing digits, since the tribu-tary signals themselves contain the necessary alignment information. In this case the demultiplexer must scan the incoming digit stream until it detects and synchronizes to some recognizable systematic feature particular to each tributary. Thus the tribu-tary signals define their own channel number and allocated time slot within the digit stream.

Normal pulse code modulated speech does not fulfill the above criterion. However, some television signals converted to PCM, and certain data message switching systems allow the frame to be identified by analysis of the tributary signal.

Channell

Bit: Al Bl Cl Dl

Channel 2

!

^{I Bit: A2 B2 C2 D2}

Channel 3

Bit: A3 B3 C3

---r

^D3

Channel 4

Bit: A4 B4 C4 D4

I'

Word, of length 4 bits

-I

Word interleaved

I All Bll C11 Dll A21B21 c21D21A31 B31 C31 D31A4IB41 C41 D41

f--Word---.j

I---Time slot ---+1 Bit interleaved

Fig.6-2 Word and bit interleaving.

120 Multiplexing

l!:"b;;7;>F1'77J<'7;>F7;>²77r"-:o---::-r--::-:--:-r-:-:-:,---I CHn

L~w/;/~/'?

~ CHl

I

CH2

I

CH31 CH41 . ~//@L1

(a)

CH(n!2) CH n!2 + 1

(b)

~

^F2

~C~~C-H-2~I-CH-3~I-c-H4-1.-r-I'~---One frame---~·I (c)

Fl, F2: Time slots allocated to unique frame synchronization words, which are exactly repeated each frame

CH 1, 2,3, etc: Time slots allocated to channel message words

Fig. 6-3 Frame formats based on bunched (a), and distributed (b, c), frame synchronization words.

I---

Frame 1 - _ .

+--1.>----

Frame 2 -~'I~'--Frame 3

--·If..,.--

Frame 4 ~

FAW I, FAW FAW 1 13 FAW 14 FAW

---t-"-1c-

^-c--

-rr---"",-~==:~=-~~1--L-1-=-_-P'--~~_~----,,-,,--2

--'.."--1;S ----+-'P=L--

_~t;J"___. ----+~~"---'

^_

t

I----One f r a m e - - . . j : I :

i t ! ⁱ

ⁱ

: y : : :

x

: :Stateel : ^I

State e ~--State f ^{- - - l •} ..I;' ... ^{- - - l •} ..I;' •• - -State f ___ ,''''', .t---State g - -.... : ... - State a

f f I I

. i : Search

I : '

Search - , I. "

I---FAW:

11,2,3,4 :

States a- g:

Frame alignment word

Information bits that due to their random nature happen to

"imitate" the FAW See Fig. 6-5

Fig. 6-4 An example of frame synchronization.

Im Dokument and DIGITAL TRANSMISSION SYSTEMS (Seite 132-135)