PCM Systems

Block Diagram of a PCM

First the register signals and voice samples are collected from subscribers 1-32 except 16.The TS0 is generated separately.The supervisory signals which are carried inTS16 are also being collected separately. Theseare taken from subscribers 1-32 except 16.These separately collected time slots are multiplexed and are transmitted.
The de-multiplexer of the other exchange separates the samples into Voice and TS16. The frame get synchronized using TS0.Hence the sent frame order is re-done and voice samples are sent to the appropriate subscribers.This same procedure is done reversely when samples are sent from the second exchange to the first

Higher Order PCM

1.1 Characteristics of Primary PCM

In Primary PCM there are 32 channels (CEPT system) where each channel will have 8
bits. Each bit will be of 488ns. The primary PCM will lead to generate 2.048 Mbps and
carried from one end to another end in a given transmission medium. There are two
fundamental questions to be addressed i.e. why not use 8 media to send each time slot 8
bits in parallel from one end to another end. Another question will be why we need 488ns
to represent a binary bit where this is the fundamental micro information of a sample
(where sample information will have 8 bits).
To answer the first question it is obvious that the cost of having media from one end to
another end will be very high when compared to a single medium. Another disadvantage
will be synchronization of 8 bits at the receiver end for a given sample deriving from 8
different media resulting in the reception of wrong sample values.
To answer the second question it is the maximum period that can be given for a bit where
the sampling theorem has to be followed with the 32 channel Time Division Multiplexing
Primary PCM system.

1.2 Characteristics of Higher Order PCMs

In higher order multiplexing system further extension of a bit period will be considered.
Do we really need 488ns to identify a bit? No. Then 488ns for a bit is needed to satisfy
the sampling theorem and 32 Channel multiplexing. In the higher order multiplexing the
transmitter and the receiver of the primary bit stream will be of 488ns bit period while in
between the transmitter and receiver with using the same media another bit streams of the
same nature will be multiplexed and transmitted through one transmission media and at
the receiver first it will be de-multiplexed to the original 488ns bit period then sent to the
Primary receiver.





In this Diagram four forward Primary PCM are arriving from different originating
points to a single location of station X where all these PCMs are traveling to station Y
and from station Y each Primary PCM will follow a different destination. Here from
station X to station Y one transmission media is used to transport all Primary PCMs
information to station Y. The bit information of Primary PCM is carried in 488ns while in
the secondary PCM same information will be carried in 488/4 ns and at station Y this
488/4 ns is further expanded back to 488ns. Hence the original bit period will be seen at
the receiver. From station X to station Y to represent a bit a period is not defined and can
use any period to represent a bit which is normally less than 488ns. In this case it has
become 144ns. At station Y a bit will be identified at one side with 144ns and sent to the
primary side representing a bit with 488ns. This phenomena is possible due to the fact
that a bit to be identified in a station does not need much time only a moment is enough
to identify whether it is a bit or not.
The above example can be further illustrated as follows.
Assume the PCM streams are P0, P1, P2 and P3 and further assume all the streams are
synchronized. If the 1 bit of all the PCMs are 1,1,0,1 and will be carried in NRZ format
the above example can be observed as follows.















1.3 Multiplexing and De-multiplexing of PCM Systems

Telecommunication is a bi-directional transmission. From Station X to Station Y thereshould be two transmission media to carry the voice of two parties connected with StationX and Station Y. Hence at a given Station X multiplexing and de-multiplexing will occurand at the other given Station Y De-multiplexing and Multiplexing will occur which is shown as follows














1.4 Concepts to be analyzed for higher Order PCMs

In the primary PCMs the starting of a frame for every 256 bits will be identified by 8 bits out of 256 bits. Hence primary PCM multiplexing and De-multiplexing can be achieved with Time Slot 0 synchronization byte. Now from Station X to Station Y (vise versa) the four PCM streams are being treated to carry more bits. Hence at Station Y Demultiplexing to be carried out additional synchronization byte needed in addition to the Primary PCM synchronization. This will be true even for Station X for the backward bits stream. In addition to this synchronization byte Station X maintenance personnel need to contact the maintenance personnel of Station Y for the management of this high order PCM . How to accommodate all these requirements?


Further overhead bits to be added in addition to the primary PCM Time Slot information
at Station X and these bits to be extracted at Station Y for the forward higher order PCM.
Same phenomena applied from Station Y to Station X of backward higher order PCM.