The PDH hierarchy

© ICT electronics 

The PDH hierarchy


Information 

Signals 

The telecommunication networks 

only meaningful for the end user 

modification of a physical characteristic: electricity, light, 

magnetism...relative to time 

Transmission media 

Nodes 

allow the movement of a signal from a source to a target 

relay the signals maintaining their characterictics. 

There are three basic types: 

regenerator, switches/routers and multiplexers 

Plesiochronous Digital Hierarchy 

2 /32


signals 

Signals & Information 

Information 

Analog Digital 

Analog Modulation Digital Modulation 

Digital Digitalization Codification 


3 /32


Transmission media 

Transmission types Transmission obstruction 

- Conductors 

- Dielectrics 

Twisted pair 

Coaxial 

Optical Fiber 

Space 

- Attenuation 

- Noise 

- Distortion 

· proportional to the distance 

· the signal loses power 

· must have a good relation with noise 

· thermic 

· intermodulation (sum total of frequencies) 

· noise point 

· different propagation speeds 


4 /32


Telecommunication in evolution 


5 /32


The arrival of digital technology 

digital 

analog 

Modem 

digital 

analog 

LE LE 

digital 

Modem 

analog digital analog 

LE LE 

LE LE 

analog 

digital 

digital 

: 1900 

: 1960 

: 1990 


6 /32


t0 

t0 

The digitalization of signals 

SAMPLING 

t0 +T ··· 

ENCODING 

001 011 001 101 100 

t0+T ··· 

t 

t 

011 

010 

001 

000 

100 

101 

110 

111 

QUANTISATION 

t 


7 /32


Nyquist Sampling theorem 

“In order to convert an analog signal to digital it is necessary to use a sampling 

frequency (f s ) at least two times the highest frequency” 

• f s ≥ 2BW (in Hertzs) 

i.e. to digitalize a phone channel BW c = 4000 Hz in 8 bits each sample it 

would be necessary: 

• f s =2*4000=8000 Hz 

T= 125μs: this is the base period for all digital networks 

codifying: 

• 8000 samples/seg* 8 bits/sample = 64.000 bits/seg 

this is the basic speed for digital channels 


8 /32


Capacity of a channel: the Shannon Law 

The capacity of a noisy channel is : 

• C= Bw log 2 (1 + P/N) 

C: capacity of a channel in bit/s 

Bw: Band width in Hz. 

P: Signal power 

N: media noise 

Show a max capacity for a noisy channel for transmitting digital information 


9 /32

7 V 

5 V 

3 V 

V 

- V 

-3 V 

-5 V 

-7 V 


(3) 

(2) 

(1) 

(0) 

(4) 

(5) 

(6) 

(7) 

011 

010 

001 

000 

100 

101 

110 

111 

t 0 t0+T t0+2T 

t0 t1 t 2 t 3 t4 t5 t6 

Types of digital modulation 

t 0+3T t 0+4T 

t7 t 8 t 9 

t 

t 

PAM 

PDM 

PPM 

PCM 

Delta 

Modul. 

t 0 

t 0 

t0+T ··· 

1 3 1 5 4 

t 0 

t0 

7V 

3V 3V 

t 0+T ··· 

t 0+T ··· 

001 011 001 101 100 

t0+T ··· 

- 3V 

- V 

1 3 1 5 4 

t 0 t 1 t2 t3 

t 4 t5 t 6 t 7 

t8 t9 

t 

t 

t 

t 

t 

ANALOG 

PULSE 

MODULATION 

DIGITAL 

PULSE 

MODULATION 


10 /32

NRZ 

AMI 

HDB3 

CMI 


+V 

0 

-V 

+V 

0 

-V 

+V 

0 

-V 

+V 

0 

-V 

Line Codification 

1 0 1 0 0 0 0 1 1 0 0 0 0 0 0 0 0 1 0 

0 0 0 V 

B 0 0 V 

B 0 0 V 

1 0 1 0 0 0 0 1 1 0 0 0 0 0 0 0 0 1 0 


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FDMA 

DTE-A BW s1 

DTE-B BW s2 

. 

A 

B 

C D 

F 

E 

DTE-F 

f 

BW s1 

B 

Multiplexing 

MULTIPLEXER 

A 

F 

E 

t 

TDMA 

D 

C B 

A 

frame 

BW C 

Transmission channel 

CDMA 

110100010110111001 

001011101110111001 

Bit Bit 


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a) Analog switching 


A(f1), B(f2), C(f3), D(f4) 

b) Digital switching 

Digital switching 

Demodulator 

demultiplexer 

ABCDABCDABCDABCD 

A(f1) 

B(f2) 

C(f3) 

D(f4) 

4 channels at the 

same frequency 

Analog 

switch 

Digital switch 

A(f1) 

B(f2) 

C(f3) 

D(f4) 

Modulator 

multiplexer 

ABABABABAB 

A(f1), B(f2) 

CDCDCDCDCD 

C(f3), D(f4) 


13 /32


Advantages of digital technology 

• Reduces hardware cost 

• Simplifies swtiching 

• Improves reliability, maintenance and quality 

• Allows you to offer Quality of Service (QoS) 

• Optimizes the use of resources 

• Supports audio, data, video under a unified media 

• Makes it easier to build computer networks 

• Requires more Band Width 

• Needs synchronization 

...but 


14 /32


Digital milestones 

• Telex (Germany 1935) first digital network 

• Digitalization (France 1942) 

• Fax (Japan 1950) 

• Integration (USA 50´s) of transmission and switching 

• AT&T (USA 1962) first PTT with digital switching 

• Western Electric (USA 1965) first digital transmission PCM 24 channels 

• Telefonica (Spain 1968) first packet network 

• IDN (USA 70s) first full digital network 

• ISDN (Europe 1984) standarized voice and data metwork 

• SONET (USA 1988) first installations 

• B-ISDN (1989) broadband networks 

• ATM (1994) first public ATM net 


15 /32


Plesiochronous Digital Hierarchies 

5 th level 

4 th level 

3 rd level 

2 nd level 

1 st level 

single channel 

Europe (CEPT) USA Japan 

564992 Kbit/s 

139264 Kbit/s 

x 4 

34368 Kbit/s 

x 4 

8448 Kbit/s 

2048 Kbit/s 

x 4 x 3 

64 Kbit/s 

274176 Kbit/s 

44736 Kbit/s 

x 30 x 24 

6312 Kbit/s 

1544 Kbit/s 

397200 Kbit/s 

x 4 x 4 

x 3 

x 6 

x 7 x 5 

x 4 

97728 Kbit/s 

x 3 

32064 Kbit/s 


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2 Mbit/s 

MUX 

2 

2 

2 

2 

8 

8 

8 

8 

PDH is a hierarchy 

8 Mbit/s 

MUX 

8 

8 

34 

34 

34 Mbit/s 

MUX 

34 

140 

140 Mbit/s 

LTE 

Four standarized mux levels 2, 8, 34 and 140 Mbit/s 

TRANSMISSION MEDIA 

COAXIAL CABLE 

SATELLITE 

RADIO LINK 

OPTICAL FIBRE 


17 /32


PDH 





PDH is plesiochronous 


clock 

alignment 

PDH islands with their own clock 

SWITCH 


18 /32


The PDH hierarchy 

A 

S 

T 1 

J 11 

R 1 

a i b i c i d i 

C 1 

E 

1 

0 

C 2 C 3 C 4 

Remote Alarms Indicator (FAS and MFAS) 

Spare bits (national use) 

i - Tributary bits 

Justification control bits 

Justification bits 

i - Channel CAS bits 

CRC-4 Error signaling bits 

CAS multiframe alignment 

Frame alignment bits 

CRC-4 Multiframe alignment 

Frame alignment supervision bits 

Cyclic Redundancy Checksum bits 


19 /32


Frame 0 

Frame 1 

Frame 15 

The 2048Mbit/s basic frame 

FAS 

NFAS 

FAS 

NFAS 

FAS 

NFAS 

ch 1 

CAS: 30 channels/frame 

CCS: 31 channels/frame 

ch 2 ch 15 

BITS PER SAMPLE: 8 bits 

ch 16 ch 17 ch 30 

ch 1 ch 2 ch 15 ch 16 ch 17 ch 18 ch 31 

MFAS SA S 

s1 s16 s 2 

s 3 

s 14 

s 15 

SAMPLING RATE (of every channel): 8000 samples/s 

CHANNEL BW: 0-3400 Hz 

s 17 

s 18 

s 29 

s 30 

COMPRESSION LAW: A 

INVERSION OF EVENT BITS 

Frame duration: 125 μs 


20 /32


Frame alignment 

FAS FAS 

tributaries bits 

Allows targetting of synchronization to find the beginning of the frame 


21 /32

Frame 0 

Frame 1 

Frame 2 

Frame 3 

Frame 14 

Frame 15 


TS0 

C 10 

0 1 1 0 1 1 

0 1 A S 

C 20 

0 1 1 0 1 1 

0 1 A S 

C 40 

0 1 1 0 1 1 

E1 

A S 

Also called NFAS 

FAS 

TS2 TS1 

TS15 

FAS 

TS16 

0 0 0 0 S A S 

Frame Alignment Supervision bit 

FAS is only transmitted on odd frames 

NFAS uses a bit equal to “1” to avoid coincidences 

s 1 

s 2 

s 3 

s 14 

s 15 

s 16 

s 17 

s 18 

s 29 

s 30 

TS17 TS18 TS31 


22 /32


Sub-Multiframe 

I 

Sub-Multiframe 

II 

Cyclic Redundancy Checksum CRC-4 

Frame 0 

Frame 8 

Frame 15 

C 1 

0 

C 2 

0 

C 1 

C4 E 

TS0 

FAS 

NFAS 

FAS 

NFAS 

FAS 

FAS 

NFAS 

Allows the detection of errors 

TS1 TS2 TS15 TS16 

C 1 0 C 2 0 C 31 C 4 0 C 11 C 21 C 3 E C 4 E 

C 1C 2C 3C 4: CRC-4 check bits for the previous sub-multiframe 

001011: CRC alignment signal 

EE: CRC distant error indicating bits 

MFAS SA S 

s1 s16 s 2 

s 3 

s 8 

s 14 

s 15 

s 17 

s 18 

s 23 

s 29 

s 30 



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Frame 0 

Frame 1 

Frame 2 

Frame 3 

Frame 14 

Frame 15 

8 Mbit/s 

TS0 

FAS 

NFAS 

FAS 

NFAS 

FAS 

NFAS 

NFAS: No FAS 

TS1 

Used to manage alarms and errors 

DISTANT 

ALARM 

LTE LTE 

8 Mbit/s 

FRAME 

LOSS 

TS2 TS31 


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Frame 0 

Frame 1 

Frame 2 

Frame 3 

Frame 14 

Frame 15 

bit A: Remote Alarms Indication 

TS0 

C 1 FAS 

0 1A 

S 

C 2 FAS 

0 1A 

S 

C 4 FAS 

E1 

A S 

TS1 TS15 TS16 

MFAS 

SA 

S 

s1 s16 s 2 

s 3 

s 14 

s 15 

s 17 

s 18 

s 29 

s 30 

TS17 TS31 


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Frame 0 

Frame 1 

Frame 2 

Frame 3 

Frame 14 

Frame 15 


TS0 

C 1 FAS 

0 1 AS 

SS SS 

C 2 FAS 

0 1 AS 

SS SS 

C 4 FAS 

E1 

AS 

SS SS 

bits S for PTT use 

TS1 TS15 TS16 

MFAS SA 

SS 

s1 s16 s2 s17 s3 s18 s14 s15 s 29 

s 30 

TS17 TS31 

can be used for application, maintenance or monitoring of performance 


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Frame 0 

Frame 1 

Frame 2 

Frame 3 

Frame 14 

Frame 15 


TS0 

FAS 

NFAS 

FAS 

NFAS 

FAS 

NFAS 

CAS Multiframe 

TS1 TS2 TS15 TS16 

ai bi ci di 0MFAS 0 0 0 S A S 

b 1 

a 1 

b 2 

a 2 

b 3 

a 3 

c1 d1a16b 16c 

16d 

16 

c2 d2a17b 17c 

17d 

17 

c3 d3a18b 18c 

18d 

18 

a14b 14c 

14d 

14a 

29b 

b 29 

c29d 29 

a15b 15c15d 

15a 

30b 

b 30 

c 30 

d 30 

: i-channel signalling bits (CAS) 

MFAS for CAS 


Each of the 30 channels have associated 2 kbit/s, bits a i ,b i ,c i , d i in TS16 


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Multiframe Alignment Signal (MFAS) 

b 1 

a 1 

b 2 

a 2 

b 3 

a 3 

TS16 

0MFAS 0 0 0 SA 

S 

c1 d1 a16 b16 c16 d16 c2 d2 a17 b17 c17 d17 c3 d3 a18 b18 c18 d18 Used to synchronize the CAS 

No MFAS 

(NMFAS) 


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Frame 0 

Frame 1 


TS0 

bit A in NMFAS (TS16) 

TS1 TS15 TS16 

C 1 FAS MFAS SA 

S 

0 1A 

S 

s1 s16 TS17 TS31 

Alarm bit used to indicate a power fault, loss of incoming signal, or loss of 

multiframe alignment 

then A=‘1’ the response from the remote side is to set CAS bits to ‘1’ 

A=1 

CAS=1 


29 /32

Frame 0 

Frame 1 

Frame 15 


FAS 

NFAS 

FAS 

NFAS 

FAS 

NFAS 

2 Mbit/s channel structure 

ch 1 ch 2 ch 15 ch 16 ch 17 ch 18 ch 31 

8 bit per sample 

8000 samples per second 

30 or 31 channels of information 

125μs frame period 

3.400 Hz bandwidth per channel 

MFAS SA S 

s1 s16 s 2 

s 3 

s 14 

s 15 

s 17 

s 18 

s 29 

s 30 


30 /32

RTB 

Frame 

Relay 

Rented 

ISDN 

Internet 

GSM 

ATM 


2 

2 

2 

8 

8 

8 

PDH as circuit provider 

34 

8 

PDH networks provide circuits to switched public networks. 

They are also used to build synchronization networks 

8 

34 

8 

8 

8 

2 

2 

2 

Rented 

ISDN 

Internet 

GSM 

ATM 

RTB 


Frame 

Relay 

31 /32


PDH network, some problems 

• The supervision and maintenance functions are limited (just a few 

bits for alarms in NFAS, NMFAS and E bit (2 Mbit/s frame) 

• In order to get low speed channel (i.e. 2 Mbit/s) from a hight 

hierarchy (i.e. 140 Mbit/s) a full demultiplexing is need 

• Loss of compatibility between European, Japanese and North 

American hierachies 

• There are no standards for speeds over 140 Mbit/s 

• Low management capabilities 


32 /32

The PDH hierarchy

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