PDH Technology

By Ayodeji Morakinyo 1

PLESIOCHRONOUS DIGITAL HIERARCHY (PDH)

By Ayodeji Morakinyo

…At the Speed of Ideas

AGENDA

1. Introduction

2. Pulse Code Modulation

3. Time Division Multiplexing


4. PDH

5. ALU PDH Solutions

INTRODUCTION

In modern transmission systems, a particular wave characteristic

such as phase, frequency and amplitude is varied to achieve modulation

and then time, frequency, space or wavelength is used as a point of

reference for multiplexing several signals before sending them along a

transmission link.


Plesiochronous digital hierarchy (PDH) is one of the most widely used

transmission techniques. It employs the Pulse Code Modulation technique to attain

a basic data rate of 2Mbps (E1) consisting of 32 multiplexed base channels.

Analog Signal

This presentation, being the

output of a recently conducted study

by the presenter, will first consider the

pulse code modulation and time

division multiplexing techniques

before discussing the PDH technology.


It will also showcase some

PDH equipments with a focus on ALU

products.

PULSE CODE MODULATION

MODULATION

This is the technique by which information in their analog or digital forms

(usually baseband signals) are converted to RF signals that can be transmitted. In

modulation, a wave characteristic is varied such that the output conforms to a

desired pattern.

For instance, waves produced from the human vocal cords can be

modulated based on frequency (FM), amplitude (AM) or phase (PM). FM and AM are


modulated based on frequency (FM), amplitude (AM) or phase (PM). FM and AM are

mainly used in commercial radio broadcasting while phase modulation is employed in

applications like GPS receivers. Other ways of modulating information include: FSK,

PSK, ASK, OFSK, DBPSK etc.

Analogue signal transmission has relatively high susceptibility to EMI which

reduces the quality of signal transferred. To avoid this, the analogue signal is usually

digitized before transmission.

Pulse code modulation is a technique used to convert analogue signals to

their digital equivalent by characterizing the original signals in discrete pulses. To

achieve PCM modulation, the signals must be subjected to certain processes such as

sampling, quantizing and encoding.

Advantages of digital transmission over analogue method


•Error detection and correction

•Data integrity when transmitting over long distances

•Multiplexing of voice, video and digital data over one channel

•Larger bandwidth

•Easier to encrypt and compress data

•Reduced hardware cost

SAMPLING

This is the process of reconstructing the continuously changing analogue

signal into approximate samples which have a range of voltages that can be

referenced.

The sampled signals will have various amplitude at different time periods. A

signal range which must cover all possible input levels is chosen. For PCM, 8bits is

chosen for sampling the telephone circuit system.

The bit values sampled are representations of the voltage sampled at a

time. Each digit represents a power of 2.


The telephone system digitizes our voice signal for digital transmission

Full range of human hearing is within 20 – 20000Hz and the human voice

frequency occupies the ultra-low frequency band of the EM spectrum at 300 –

3400Hz.

So, how fast should we sample and obtain discrete values to prevent data loss and

aliasing?


QUANTIZATION

This involves the conversion of samples to discrete values. A comparator

compares the actual voltage to the reference (quantized) voltages.

000 010 100 110

001 011 101 111


From Nyquist Theorem,

Perfect reconstruction of a signal is possible when Fs is > twice the signal being

sampled, i.e. :

Fs= 2 x Fmax

Therefore, bandwidth allocation for single voice frequency transmission channel is

4KHz including guard bands.

Fs= 2 x Fmax = 2 x 4000Hz

Sampling frequency or rate = 8000Hz or 8KHz or 8000samples/bit. And, the time

interval between 2 consecutive samples is 125microsecondsinterval between 2 consecutive samples is 125microseconds

Since 8bits are used for PCM for telephone system, we will need 8 channels for the

8 quantized bits.

Therefore, 8KHz (or 8000samples/segment ) x 8bits/sample

= 64Kbit/s (or 64bits/segment)


ENCODING

This involves the introduction of additional transmission bits (ones and

zeros) to the quantized values by an encoder to ensure proper sending of the digital

signal.

Common characteristics of transmission codes are:

•To increase possible transmission distance, the average DC line component of the

code has to be zero volt

•To recover the bit clock, frequent transitions of the digital signal must be provided

Examples of the transmission coding types include but are not limited to:Examples of the transmission coding types include but are not limited to:

HDB3, AMI, RZ, NRZ and CMI coding.

In HDB3, where there more than 3 zeros consecutively, a violation pulse is

added by the encoder and the decoder deletes it at the receiving end.


In AMI, the encoder represents a “0” by zero volt and a “1” by an alternate

positive or negative pulse.

In NRZ, a “0” is represented by a negative pulse and a “1” is represented by

a positive pulse.

In CMI, a “0” is represented by half a positive and half a negative pulse

within one time interval. A “1” is represented by a positive or negative pulse for a

full unit time interval.


TIME DIVISION MULTIPLEXING

MULTIPLEXING

This involves the combination of two or more signals for transmission

along a single channel. This is done to maximise bandwidth and save overall cost

when huge amounts of information are involved.

Data from Combined signal

De

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By Ayodeji Morakinyo 13M

ux

Data from

difference

sources

Combined signal

De

mu

x Separation

back into

different

components

Multiplexing can be of different types:

TDM, FDM, SDM and WDM. In this presentation, we will discuss TDM as a technique

that is compatible with PDH technology.

TIME DIVISION MULTIPLEXING

In TDM, multiplexing is a function of time slot. Two or more bit streams or

signals are transferred simultaneously as sub-channels in one communication

channel, but are physically taking turns on the channel. The time domain is divided

into several recurrent timeslots of fixed length, one for each sub-channel. A sample

byte or data block of sub-channel 1 is transmitted during timeslot 1, sub-channel 2

during timeslot 2, etc.

A time slot is an interval within which an 8-bit PCM word is transmitted and

a frame is a set of consecutive time slots.


TDM can be performed through Bit or Byte Interleaving. In Byte interleaved

type, the input signal occupies one complete byte of the input signal while in bit

interleaved format, one bit of input signal occupies a time slot.

For example,

A1 A2 A3 A4 A5 A6 A7 A8

B1 B2 B3 B4 B5 B6 B7 B8

C1 C2 C3 C4 C5 C6 C7 C8

TDM Multiplexer

A1 A2 A3 A4 A5 A6 A7 A8

Byte Interleaving

Sub-

channel A

Sub-

channel B

Sub-

1 byte of 1 channel per time slot

or

C1 C2 C3 C4 C5 C6 C7 C8

D1 D2 D3 D4 D5 D6 D7 D8

TDM Multiplexer

A1 B1 C1 D1 E2 F2 G2 H2

H1 H2 H3 H4 H5 H6 H7 H8

.

.

.

Bit Interleaving

8 bits

8 bits of 8 channels per time slot

Sub-

channel C

Sub-

channel D

Sub-

channel H

8 bytes per sub-channel


PLESIOCHRONOUS DIGITAL HIERARCHY

“Plesiochronous” means, almost the same. Plesiochronous signals are

signals that have the same nominal frequency but are not synchronised to one

another. This is because they originate from different multiplexers which have

different clocking.

As such, in PDH, the rise and fall time of the pulses in each tributary (or

bit stream) do not coincide unlike in SDH where all the tributaries have same clock

frequency and are synchronised to a master clock.


PDH Hierarchy in Japan, North America and Europe


Digital MUX

Level

No.of 64Kb/s

Channels

North America

(Mbits/s)

Europe

(Mbits/s)

Japan

(Mbits/s)

0 1 0.064 0.064 0.064

1 24 1.544 1.544

30 2.048

48 3.152 3.152

2 96 6.312 6.312

120 8.448

PDH MUX Levels


120 8.448

3 480 34.368 32.064

672 44.376

1344 91.053

1440 97.728

4 1920 139.264

4032 274.176

5760 564.992 397.200

BIT/PULSE STUFFING

In PDH, the small variations in frequency about the nominal value must be

accounted for when multiplexing four tributaries to the higher order level on the

hierarchy. To achieve this, a process termed stuffing must occur.

Stuffing involves intentionally making the output bit rate of a channel higher

than the input by inserting additional bits in the incoming bit stream. Consequently,

the output channel contains all the input data plus variable number of “stuffed bits”

that are NOT part of the incoming subscriber information.


-- --Higher

Order

Multiplexer

Frame no.1

Lower Bit Rate

Higher Bit Rate

Stuffing

Control bit Stuffing

bit is a

data bit

Stuffing bit is

a stuff bit

Frame no: 2

PDH Frame

The basic frame rate for PDH (using the European standard) is 2.048Mbps

where 32 voice channels are used. But only 30 channels are utilized for communication

because one is used for signalling and another for synchronisation.



A multiframe is composed of 16 frames and a sub-multiframe is

composed of 8 frames. The first time slot is used for frame alignment or control

while timeslot 16 is used for signalling (if not, it can be used to carry user

information). Each frame has 32bytes.

PDH Frame period is 125µs then,

1byte is a 8bit/125µs = 64Kbit/s

And the transmission rate is:

(32channel X 8bit/channel) X 125µs = 2.048Mbps

FRAME ALIGNMENT SIGNAL (FAS)

•allows targeting of synchronisation to find beginning of the frame

•comprises the 7bits of the first time slot of each numbered frame

which are responsible for the CRC (C1,C2,C3,C4 bits) or CRC-


which are responsible for the CRC (C1,C2,C3,C4 bits) or CRC-

4multiframe and E bits

•the CRC-4 bits detect block errors and the E bits indicate the block

errors

•FAS = 0011011

NON-FRAME ALIGNMENT SIGNAL (NFAS)

The second bit of NFAS is “1” and it is

used to avoid coincidences with the FAS.

The “A” bits (distance alarm indication

bits) are used for alarm management. They

indicate power fault, LOS, LOF and codec fault.


The “S” bits (spare bits) are reserved

space for network operator’s use i.e. application,

monitoring and maintenance of performance. If

the S or spare bits are not used, they should be

set to 1.

ERRORS

Common errors associated with PDH transmission include the LOF which is

indicated via an AIS and the REBE which is detected by the CRC4 bits.


Other Associated Errors


Types:

1. CAS which uses the multiframe structure (TS1-TS15 and

TS17-TS30) as predefined positions to carry the

SIGNALLING CHANNEL

This is used to exchange information between LEs. It uses the timeslot 16 of

the 2Mbps frame where Si is a four bits (a1,a2,a3,a4 ) channel and i values go from 1 to 30

per channel.


TS17-TS30) as predefined positions to carry the

signalling multiframe alignment signal.

2. CCS which is a byte-oriented responsible for carrying

signalling information for several communications.

There are no predefined positions for the

communication channels .

SYNCHRONIZATION

When multiplexing to the basic rate, synchronisation is put in place by the

PCM MUX through bit interleaving and if they connected to the same master clock, the

signals become synchronous.

But when multiplexing to higher rates (second, third and fourth orders), the

MUXes must provide a mechanism for accommodating rate impairments.

This mechanism is called justification/stuffing and the bits that perform this

are the justification bits and justification control bits (JB and JC bits).


are the justification bits and justification control bits (JB and JC bits).

•If bits Jik=1 then, Ri is justification, no information

•If bits Jik= 0 then, Ri contains tributary information

•If not all are 0s or 1s, then decision is based on majority count of Jik

Where:


Where:

A = alarm indication bit

R = reserved for internal use

T = bits from tributary i

JC = justification control bit for tributary i

JB = justification bit for tributary i

LIMITATIONS OF PDH

•Lack of efficiency which makes it necessary to fully demultiplex before low

tributaries can be extracted from high order aggregates.

•Insufficient capacity for network management and monitoring because the system is

limited to a few bits (NFAS, NMFAS).

•There is no standardised definition of PDH bit rates greater than 140 Mbp e.g.

564.992Mbps.


564.992Mbps.

•Every clock is different and therefore synchronisation errors show up.

•Not Transparent (lack of compatibility between European, Japanese and North

American standards.

•Protection schemes are not available hence, ring, and hub configurations are not

possible.

ALU PDH SOLUTIONS

9400 AWY

The 9400AWY is ideal for short-haul digital transmissions in high density

mobile networks especially 2G, 2.5G, 3G and Wimax. It utilizes the utility networks,

regional & local traffic systems and private WANs and LANs.

Features:

• Indoor unit (IDU) supports up to 32 × E1 traffic

interfaces


9400AWY9400AWY

interfaces

• Outdoor unit (ODU) supports 4- and 16-quadrature

analogue multiplier (QAM); software upgradeable as

needed

• One ODU version exists with 32 Mb/s capacity/4-

QAM only

• Can combine with the Alcatel-Lucent 9500

MPR/MXC and 9600 LSY/USY families in the same

network

More Features:

• Provides output power agility: automatic

transmit power control (ATPC) and

static/reactive TPC (RTPC) in all frequency bands

• Offers capacity agility to support up to 32 ×

E1, 1 × E3, or 4 Ethernet ports, with a maximum

capacity of up to 64 Mb/s, including mixed

configurations such as 16 × E1 and 2 Ethernet

ports

• Provides flexible TDM/LAN interfaces

•Software configurable, with easy


•Software configurable, with easy

installation/setup and multilevel loopback/test

facilities

• Offers comprehensive application options e.g.

GSM/GPRS/UMTS, Wireless data access,

WAN/LAN data networks, PABX, ATM,

videoconferencing, Utility networks: pipelines,

electricity,

railways, municipalities.

9400AWY Configurations

In 1+0: 1U optimised Main IDU

minimum configuration 8E1/16Mbps or 16E1/32Mbps

modular slot for expansion up to 32E1/64Mbps

common to all frequency bands


In 1+1: 1U complete Extension IDU

AWY offers both full protection and radio protection

AWY Indoor can be replaced for repair without traffic impact

common to all frequency bands

ODU V1

32/64Mb

ODU V2

32/64MbODU V1

32/64Mb

ODU V2

32/64Mb

32/64 Mb max on the air

Compatibility


9500 MXC

The 9500 MXC provides solution for SDH and “super PDH” applications

going up to 93 E1/100DS1, higher flexibility is given by the integrated cross

connection capabilities. Additionally, the product fits well for fixed applications

including DSL and WiMAX backhauling due to its multiple interfaces: PDH, SDH and

Ethernet with integrated layer-2 switching.

Features

•Cost-effective wireless solution for High

Capacity applications up to 2xSTM-1.


Capacity applications up to 2xSTM-1.

•High Capacity Ethernet transport with

embedded L2 switch

•Intelligent Indoor nodal unit supports up to

six ODUs

•Universal ODUs capacity and modulation

independent

•9500 MXC supports 1+1 protected configurations including Hot Stand-by and

diversity configurations.

•For additional protection of INU functions an NPC can be added to provide power

supply and NCC control redundancy

•Choice of operation as SDH or SuperPDH radio via

software.

•NxE1 rates up to 93xE1 over a single 28MHz carrier

•Choice of E1, E3, STM-1, Ethernet and Gigabit Ethernet


•Choice of E1, E3, STM-1, Ethernet and Gigabit Ethernet

customer interfaces.

•Software-configurable traffic routing, without local

cabling.

•9500 MXC Craft Terminal, an advanced Java-based

maintenance tool presents local and remote node status

with performance monitoring, configuration control and

diagnostics

The INU is available in a standard 1RU and extended 2RU shelf (INUe) to support

up to 3 outdoor units (ODUs) or 6 ODUs respectively.

The wireless node architecture is enabled by the Intelligent Node Unit, or INU. Its

modular design supports either a simple terminal, or a more complex node,

through a variety of hot-swappable plug-in cards.


INUe

INU unit


INUe unit

9500 MPR

9500 MPR is a Service

Aggregator. This means that it is able to

gather different kinds of traffic sources

and to use ETHERNET as a convergence

layer for all of them for uplink into a

Packet Network.

MXC and MPR share the same Outdoor Unit and IF coax cable, allowing

when needed , a smooth and less expensive migration in field, from TDM to packet

(IP), by simply replacing the Indoor unit.


The MSS is used to interface the tributaries, to perform service while the

ODU is used to convert the modulated IF signal into RF and allow the transmission in

the air by means of an integrated antenna (directly connected to the ODU) as in the

slide or a not integrated antenna (connected to the ODU with a flex twist/waveguide

section).

9500 MPR is made of two separated parts (split-mount): an indoor part

called MSS (Microwave Service Switch) and an external one called ODU.

To integrate TDM, IP and microwave technologies, the Alcatel-Lucent 9500

MPR platform includes three main components.

Lack of Synchronization could mean call-dropping in handover cases;

avoiding this situation is obviously mandatory in order to improve QoS and

Customer Satisfaction.


The Microwave Packet Transport Unit — High Capacity, Long Haul (MPT-

HL) simplifies the critical part of a microwave system, using the latest RF and modem

technology. It provides two RF slots in 2.5 rack units of space in 1+0, 1+1 Space

Diversity/Frequency Diversity (SD/FD), and 2+0 installations. The MPT-HL is the first

all-packet, all-indoor nodal microwave transmission system.

Feature 9500 MPR 9500 MXC 9400 AWY

Mixed Traffic (TDM + Ethernet) Yes Yes Yes

Point-to-point configuration Yes Yes Yes

Node configuration Yes (*) Yes No

Native Packet Transport (over Air) Yes No No

Adaptive Modulation Yes No No

Internal cross-connections Yes Yes No

9400 AWY – 9500 MXC – 9500 MPR feature comparison


Internal cross-connections Yes Yes No

End-to-end traffic management Yes No No

QoS management Yes Yes Yes

End-to-end QoS management Yes No No

NE pre-configuration (Off-line tool) Yes (*) No Yes (*)

AT

THE

END OF PRESENTATION

Thank you


THE

SPEED

OF

IDEAS

ACRONYMS

ALU = Alcatel-Lucent

AMI = Alternate Mark Inversion

ASK = Amplitude Shift Keyed Modulation

ATM = Asynchronous Transfer Module

AWY = Asynchronous Wireless Second Generation

Codec = Coder/Decoder


Codec = Coder/Decoder

CCS = Common Channel Signal

CRC = Cyclic Redundancy Check

CRC4 = Cyclic Redundancy Check 4 bits

DAC = Digital-Analogue Converter

DBPSK = Differential Binary Phase Shift keyed Modulation

DC = Direct Current

Demux = demultiplexer

E1 = E-carrier signal level 1

EM = Electromagnetism or Electromagnetic

EMI = Electromagnetic Interference

FDM = Frequency Division Multiplexing

Fs = Sampling Frequency

GPRS = General Packet Radio Service

GPS = Global Positioning System


GPS = Global Positioning System

GSM = Global System for Mobile Communications

HDB3 = High Density Bipolar Excess 3

IDU = Indoor Unit

IF = Intermediate Frequency

INUe = Intelligent Node Unit extension

IP = Internet Protocol

LAN = Local Area Network

LE = Local Exchange

LSY = Long-Haul Second Year/Generation

MPR = Microwave Packet Radio

MUX = Multiplexer

MXC = Microwave Cross-Connect

NCC = Node controller Card

NPC = Node Protection Card


NPC = Node Protection Card

NMFAS = Non-Multiframe Alignment Signal

NRZ = Non-Return to Zero

OFSK = Orthogonal Frequency Shift Keyed Modulation

PABX = Private Automatic Branch Exchange

PSK = Phase Shift Keyed Modulation

QoS = Quality of Service

RAC = Radio Access Card

RF = Radio Frequency

RZ = Return to Zero

SDH = Synchronous Digital Hierarchy

SDM = Space Division Multiplexing

STM = Synchronous Transport Module

TDM = Time Division Multiplexing

TS = Time Slot


TS = Time Slot

UMTS = Universal Mobile Telecommunications System

USY = Ultra Long-Haul Second Year (Second Generation)

WAN = Wide Area Network

WDM = Wave Division Multiplexing

WiMAX = Wireless Interoperability for Microwave Access

PDH Technology

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