PDH Network Basics

PDH Network Basics

 The invention of the telephone in 1876 (A.G.
Bell) has completely changed the world. Today it would be difficult for us to
imagine our life without the telephone.

Even these days with the ever-growing number of
computer networks and the booming Internet, voice communication still forms the
major part of the total volume of the communication traffic.

In the early days each telephone connection required a
dedicated link all the way between two users. Shortly after the steadily
growing number of subscribers led to the development of various methods and
technologies, which enabled several telephone connections to be transmitted
over a single cable. Most of the systems at that time used Frequency Division
Multiplexing (FDM) technology. Here the idea was to modulate each telephone
channel with a different carrier frequency to shift the signals into different
frequency ranges.

With ever increasing demands for higher transmission
rates with better quality and the advent of semiconductor circuits other
techniques were developed. In the 1960s, digital systems started to appear. Here
the telephone channels are separated by time using a new type of transmission
method known as Pulse Code Modulation (PCM).

The analogue signal
(speech) from the telephone is first converted to a Pulse Amplitude Modulated
(PAM) signal using a process called sampling. Then using quantization and
encoding this sampled analogue (PAM) signal is converted to a digital PCM
signal. Based on the principle above the Plesiochronous
Digital Hierarchy (PDM) has been developed.

Here the 300-3400 Hz band-limited analogue signal is
sampled at 8000 Hz. Then the PAM signals are quantized using a 13-segment
compression characteristic known as A-law (in T1 systems 15-segment m-law characteristic is used). Finally the signal is encoded using an 8
bit code word format. This source coding produces 8-bit code words at a rate of
8 kHz, giving 64 kbps data rate. To improve the utilization of the transmission
medium, the signals are transmitted by time division multiplexing, where the
code-words are interleaved and contained in a PCM frame.

A primary frame consists of 32 code words called
timeslots, which are numbered 0 to 31. A PCM31 frame comprises of 31 timeslots
used for traffic and 1 timeslot used for synchronization.

Frame Synchronization

In a PCM30 system
the frame comprises of 30 timeslots used for traffic and 2 code words that are
used for synchronization and signaling purposes.

Frame
Synchronization                        
Signaling

PDH has two primary communication systems as its
foundation. These are the T1 system based on 1544 kbps that is recommended by
ANSI and the E1 system based on 2048 kbps that is recommended by ITU-T. The T1
system is used mainly in the USA, Canada and Japan. European and certain non-European countries use the
E1 system.

In the following table the characteristics of the PDH
system are summarized.

 

Characteristics

E1

T1

a

Sampling frequency

8000 Hz

8000 Hz

b

Number of samples per telephone signal

8000 per second

8000 per second

c

Length of PCM frame

1/b=1/8000/s=125 ms

1/b=1/8000/s=125 ms

d

Number of bits in each code word

8

8

e

Telephone channel bit rate

b x d=8000/s x 8 bit

= 64 kbit/s

b x d=8000/s x 8 bit

= 64 kbit/s

f

Encoding/decoding

Number of segments in characteristic

A-law

13

m-law

15

g

Number of timeslots per PCM frame

32

24

h

Number of bits per PCM frame

d x g = 8 x 32

= 256 bits

d x g + 1 = 8 x 24 + 1 = 193 bits

i

Length of an 8 bit timeslot

(c x d) / h

(125 us x 8) / 256

approx. 3.9 ms

(c x d) / h

(125 us x 8) / 193

approx. 5.2 ms

j

Bit rate of time-division multiplexed signal

b x h

8000/s x 256 bits

2048 kbit/s

b x h

8000/s x 193 bits

1544 kbit/s

 

PDH transmission rates and the relations between the
different tiers can be examined in the picture below.

 

GLOSSARY

 

A bit

remote (or distant) alarm indicator

AIS

Alarm Indication Signal

AMI

Alternate Mark Inversion

ATM

Asynchronous Transfer Mode

BER

Bit Error Ratio

CAS

Channel Associated Signaling

CRC-4

Cyclic Redundancy Check for 2048kbit/s Systems

E1

2048kbit/s PCM communication system mainly used in Europe

E&M

Exchange and Multiplex signaling

FAS

Frame Alignment Signaling

G.703

ITU-T Recommendation for Physical/Electrical Characteristic for
Hierarchical Digital Interfaces

HDB3

High Density Bipolar code with a maximum of 3 zeros

ISDN

Integrated Services Digital Network

ITU-T

International Telecommunication Union Telecommunications
Standardization Section

MFAS

Multiframe
Alignment Signal

NFAS

Not Frame Alignment Signal

NMFAS

Not Multiframe Alignment Signal

PAM

Pulse Amplitude Modulation

PCM

Pulse Code Modulation

PCM30

30 channels with CAS signaling in timeslot 16

PCM30C

30 channels with CAS signaling in timeslot 16 and CRC error checking

PCM31

31 channels

PCM31C

31 channels with CRC error checking

PDH

Plesiosynchronous Digital Hierarchy

SDH

Synchronous Digital Hierarchy

S/Q

signal-to-quantizing noise

SONET

Synchronous Optical Network

T1

1544kbit/s PCM communication system mainly used USA, Canada and Japan

Y bit

distant multiframe alarm bit