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© Trend Communications

The PDH hierarchy

by JM Caballero

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The PDH hierarchy 2/60© Trend Communications

The telecommunication networks

Information (1)

only meaningful for the end user

Signals (2)modification of a physical characteristic: electricity, light, magnetism...relative to time

Transmission media (3)

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

Nodes (4)

relay the signals maintaining their characteristics.

there are three basic types: regenerators, switches/routers and multiplexers

POTS

11 22 3 32, 3 ,4

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Signals & Information

Information

Analog Digital

Signals

Analog

Modulation

- AM/FM radio

- broadcast TV

Digital Modulat ion

- ADSL

- digital TV

Digital

Digital ization

- audio CD

- ISDN (voice)

Codi f icat ion

- ISDN (data)

- Internet

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Transmission media

- Conductors

- Dielectrics

Twisted pair

Coaxial

Optical Fiber

Space

- Attenuation (loss of signal power)

- Noise

- Distorsion (modification of the signal format)

· 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

Transmission types Transmission obstruction

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Telecommunication in evolution

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The arrival of digital technology

The telephone networks have moved to the digitalization. At the beginning on the localexchanges, backbones. The last step is the local loop.

Modem

digitaldigital

digital

digital

digital

analog

analog analog

analog

analog

analog

Modem

: 1900

: 1960

: 1990

Central Central

Central Central

Central Central

digital

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The digitalization of signals

It is a process in order to transport analog information through a digital network

t0+T ···t

0

t

001 011 001 101 100

t0+T ···t

0

SAM PLING

ENCODING

011

010

001

000

100

101

110

111

QUANTISATION

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Nyquist Sampling Theorem

in order to convert an analog signal to digital it isnecessary to use a sampling frequency (f s) at least twotimes the highest frequency”

• f s ≥≥≥≥ 2BW (in Hertzs)

i.e.) a phone channel BWc = 4000 Hz in 8 bits each

sample it would be necessary:

• f s = 2*4000=8000 Hz

T= 125µµµµs: this is the base period for all digital networkscodifying:

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

64kbit/s is the basic rate, or the unit rate, in digital telecommunications

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Capacity of a channel: the Shannon Law

The capacity of a noisy channel is :

C= Bw log2 (1 + P/N)C: Capacity of a channel in bit/s

Bw: Bandwidth in Hz.

P: Signal power

N: Media noise

Show a maximum capacity for a noisy channel

for transmitting digital information

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Types of digital modulation

Pulse Code Modulation (PCM) the most used for voice

tt0t1t2

t3

t4

t5

t6

t7

t8t9

M O DULATIO N

Delta

M odul.

PULSEDIGITAL

t0t1 t2 t3 t4 t5t6

t7

t8t9

tt0

t0+T t

0+2T

t0+3T t

0+4T

011

010

001

000

100

101

110

111

(3)

(2)

(1)

(0)

(4)

(5)

(6)

(7)

7 V

5 V

3 V

V

- V

-3 V

-5 V

-7 V

tt0+T ···t

0

t

1 3 1 5 4

t0+T ···t

0

3V 3V

- 3V

- V

7V

PAM

PDM M ODULATIONPULSE

ANALOG

t

001 011 001 101 100

t0+T ···t

0

tt0+T ···t

0

1 3 1 5 4

PPM

PCM

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Line Codifications

Facts:

•••• An increase in data rate increases bit error rate

•••• An increase in S/N decreases bit error rate

•••• An increase in bandwidth allows increase in data rate

Evaluation factors:

•••• Avoid high frequency components for less bandwidth

•••• Avoid DC component, just AC allows transformers & media isolation

•••• Signal Synchronization embedded in the bit sequency avoids separate clock

•••• Signal Error Detecting Capability provided by the nature of the codification

•••• Signal Interference and Noise Immunity

•••• Cost and Complexity

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Line Codifications (ii)

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

NRZ

AMI

HDB3

CMI

0

+V

-V

0

+V

-V

0

+V

-V

0

+V

-V

0 0 0 V

B 0 0 V

B 0 0 V

AlternateMarkInversion

Non

ReturnZero

HighDensityBipolar ThreeZeroes

CodedMarkInverted

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

B: balancing

V: violation

2 Mbit/s8 Mbit/s

2 Mbit/s34 Mbit/s

140 Mbit/s155 Mbit/s

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Multiplexing

Allows the use of several communications channels through a transmission media

DTE-ABWs1

DTE-B BWs2

DTE-F

BWs1

.

.

.

MULTIPLEXER

Transmission media

A A

B

CD

EF

BCDEF AB

TDMAFDMA

BWC

frequency

time

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

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

code Bit

CDMA

Radio, TV, GSM ISDN, Frame Relay,GSM UMTS

Frequency Division Multiplexing Access Code Division Multiplexing AccessTime Division Multiplexing Access

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Digital switching

Analog switching & transmission: Inefficient, expensive

•••• Requires continuous modulation/demodulation

•••• Noise is always present

Digital switching & transmission

•••• Integrates in one operation the demultiplexing and switching

•••• Easy to manage

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

A(f1)

B(f2)

C(f3)

A(f1)

B(f2)

C(f3)

D(f4)D(f4)

A(f1), B(f2)

C(f3), D(f4)

Demodulator demultiplexer

4 channels at the

same frequency

Analogswitch

Modulator multiplexer

ABCDABCDABCDABCD

ABABABABAB

CDCDCDCDCD

Digital switch

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Typical analog arrangement

The swictching capabilities are between subribers and digital multiplexors

LTE

SUBSCR IBERS

SUBSCRIBERS

2 M bit/s

2 M bit/s

LTE

DIGITAL

TRANSM ISSION

LINE

REGENERATOR

REGENERATOR

PCM M UX

ANALO G

EXCHANGE

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Typical digital arrangement

The swictching capabilities use to be inside and integrated with the digital network

LTE

DIGITAL

EXCHANGE

SUBSCRIBERS

2 M bit/s

2 M bit/s

LTE

DIGITAL

TRANSM ISSION

LINE

REGENERATOR

REGENERATOR

PCM M UX

SUBSCRIBERS

PCM M UX

2 M bit/s 2 M bit/s

2 M bit/s 2 M bit/s

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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

...but

•••• Requires more Bandwidth

•••• Needs synchronization

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Digital milestones

•••• Telex (Germany 1935) first digital network

•••• Digitalization (France 1942)

•••• Fax (Japan 1950)

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

•••• Digital switching AT&T (USA 1962)

•••• T-Carrier (USA 1965) CM 24 channels Western Electric

••••

RSAN (Spain 1968) first public packet Network Telefonica•••• PDH (Europe 1975)

•••• IDN (USA 70s) first full digital network

•••• ISDN (Europe 1984) standarized voice and data metwork

••••

SONET (USA 1988) first installations•••• B-ISDN (Europe 1990) SDH+ATM broadband networks

•••• GSM (France 1994) digital wireless telephony

•••• UMTS (Europe 2001) broadband wireless network

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Identify Digital Technology areas

Switching Symplifies demultiplexing and switchingoperation

Allows network managementTransmission Allows TDMA to transmit several

Allows error detection and quality

measurements

Mandatory for data cammunications

Signalling Allows the development of advanced features

when stablishing, maintaining or realease

connections

Local loop Allows advanced features for any applicationsbased on voice, data, hypermedia or

multimedia

End-to-end digital quality

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Section

The PDH standards

IT U -T T e le com m un icatio nS ta n d ard iza tio n S e ctor of th e

In tern atio n al

T eleco m m u n icatio n

U n io n

R E C O M M E N D A T IO N S

G S E R IE S : T ran sm issio n system s a n dM u ltip le xa tion eq uipm e nt

O S E R IE S : M e asu ring e qu ip m ent sp e cifica tio n s

M S E R IE S : T ran sm issio n system s m ain te n ance

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Multiplexing Hierarchies

Provides an standarized way for transmission and multiplexing in terms of rates and formats

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PDH is the European hierarchy

•••• It is digital

•••• It is a hierachy because define four standarized layers for 2, 8, 34, and 140 Mbit/s

•••• It is plesiochronous because each multiplexer can use its clock

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PDH is plesiochronous

Plesio- means “almost” but truth is that each PDH island has its own clock: the result is anunsynchronized network

PDH

PDH

PDH

PDH

PDH

PDH

SWITCH

clock

PDH

PDH

PDH

PDH

PDH

PDH

alignmentPDH circuits

Lines Input Synchronization Switched lines

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PDH standard by ITU-T

hirarchy standard binary rate line code amplitude attenuation

1 G.704/732 2048kbit/s±50ppm HDB3 2.37V ó3.00V 6dB

2 G.742 8448kbit/s±30ppm HDB3 2.37V 6dB

3 G.751 34368kbit/s±20ppm HDB3 1.00V 12dB

4 G.751 139264kbit/s±15ppm CMI 1.00V 12dB

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PDH Frame stream sequence

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The PDH hierarchy

A

S

T1

J11

R 1

E

1

0

C1

ai bi ci di

Remote Alarms Indicator (FAS and MFAS)

Spare bits (national use)

i - Tributary bits

Justification control bits

Justification bits

i - Channel CAS bits

C2 C3 C4

CAS multiframe alignment

CRC-4 Multiframe alignment

Frame alignment bits

Frame alignment supervision bits

Cyclic Redundancy Checksum bits

CRC-4 Error signaling bits

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Frame alignment

•••• Allows targetting of synchronization to find the beginning of the frame

•••• It needs the FAS word at the beginning of each odd framefor the 2 Mbit/s or at the beginningof the frame for the rest of the hierarchies

FAS FAS

tim e slots

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The 2048Mbit/s basic frame

•••• Multiframe composed by 16 frames, each one has 32 bytes

•••• The first time slot is for the control, the 16 channel is for signalling

•••• The frame period is 125 µ s then 1byte is a 8 bit/125 µ s= 64 kbit/s channel

•••• The transmission rate is (32channel x 8bit/channel) / 125 µ s = 2,048 Mbit/s

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The 2 Mbit/s basic frame (ii)

•••• It is the basic frame and the most used

•••• All the european network equipment support

•••• Most of the narrow band networks are built over this frame: POTS, Frame Relay, GSM, N-ISDN, and some leased lines, and ATM access networks

Binary rate = 2048.0 Kbit/s ± 50 ppm

Line Code = HDB3

Nom inal am plitude = 2.37 V (coaxial cable)

Im pedance = 75 (coaxial cable)

Tolerated input level attenuation = 0 to 6 dB at 1024 Khz according to √fFram e length = 256 bits

Available bits per tim eslot= 8 bits

M ultiplexing m ethod = octet interleaving

Fram e rate = 8000 fram e/s

FAS bits rate = 28000 bit/s

Ω

(including supervision bit) = 32000 bit/s

120 (balanced cable)Ω

3.00 V (balanced cable)

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The FAS for the alignment

• FAS =0011011

• FAS is only transmitted on odd frames the

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

NFAS N F Ali S (i)

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NFAS: Non Frame Alignment Sequence (i)

The second bit of the NFAS is equal to “1” and it is used to avoid aleatory coincidences withthe FAS

NFAS N F Ali t S (ii)

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NFAS: Non Frame Alingment Sequence (ii)

•••• The A bits are used for alarm management

•••• The S bits are reserved space for opertators that want to implement management andmaintenance protocols

Ch k R d d C d CRC 4

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Check Redundancy Code CRC-4

•••• It detects block errors. Each 4 bits CRC corresponds to the previous sub-multiframe

•••• The receiver compute the submultiframe CRC and compares it with the code received onthe next frame

•••• If it does not match then an indition is sent using the E bit

E it i

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Error monitoring

•••• This two bits indicate block errors detected by the CRC. First for the upper submultiframe

and the second for the II submultiframe•••• “1” is the defect value

•••• If multiplexer detects block errors then sets to “0” the bit E to the frame which is sent to theother side

Multiframe alignment

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Multiframe alignment

•••• The “001011” sequence is the alignment which is inserted on the odd frames

•••• They must identify the CRC-4 submultiframe

Distance alarm indication (bit A)

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Distance alarm indication (bit A)

Used to send alarms to the remote side:

•••• Alarm bit used to indicate a power fault, loss of incoming signal, loss of frame, coder/

decoder fault, a very high bit error rate (>10-3

) that do not allows recover the channels•••• Then the receiver sets the bit A=‘1’ on the frames travelling on the other direction

•••• When transmitter realizes on the alarm state then send an AIS setting all the frame bits to ‘1’

Spare bits

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Spare bits

•••• The bits S are reserved for the Network Operator internal use only

•••• Usually are application, maintenance or monitoring of performance

•••• If they are not used, or in international links, must be set to “1”

The signalling channel

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The signalling channel

Used to interchange information between Local Exchanges (LE)

•••• Allows to establish, maintain an release end user connections.

•••• Uses the time-slot TS16 of the 2 Mbit/s frame

•••• Si is a four bits channel (a1, a2, a3, a4) i values goes from 1 to 30, one per channel

Signalling channel methods

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Signalling channel methods

•••• Channel Associated Signalling CAS

Each 64 kbit/s channel (TS1-TS15 and TS17-TS30) has a 2 kbit/s channel, as fast as each oneof the 30 signalling channel can be found at predefined positions

•••• Common Channel Signalling (CCS)

Byte oriented protocol. There is not a predefined position for each information channel

because the protocol messages can be identified by means of an specific field

Multiframe Alignment Signal (MFAS)

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Multiframe Alignment Signal (MFAS)

•••• To synchronize the CAS an alignment signal

•••• 0000 sequence is found on the first bits of the multiframe

No Multiframe Alignment Signal (NMFAS)

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No Multiframe Alignment Signal (NMFAS)

Used to send alarms to the remote side:

•••• Alarm bit used to indicate a power fault, loss of incoming signal, loss of multiframe CAS,coder/decoder fault, a very high bit error rate (>10-3) that do not allows recover the channels

•••• Then the receiver sets the bit A=‘1’ on the frames travelling on the other direction

•••• When transmitter realizes on the alarm state then sets all the bits of the CAS multiframe to

indicate the alarm on the response from the remote side is to set CAS bits to ‘1’

FAS - higher hierarchies

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FAS higher hierarchies

Uses some bits more depending on the bit rate

T 1T 2T3 T4

FA S

T1T2T3T4

140 M bit/s

34 M bit/s

8 M bit/s

AS

S

T1T2T3T4

AS

34 M bit/s tributaries bits

FA S


FA S


A111110100000

1111010000

1111010000

Frame synchronization criteria

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Frame synchronization criteria

2048 Kbit/s

8448 Kbit/s

34368 Kbit/s

139264 Kbit/s

G.704/732

G.742

G.751

3 consecutiveG.751

FAS, NFAS(bit 2), FAS

correct FAS

3 consecutivecorrect FAS

3 consecutivecorrect FAS

3 consecutiveerrored FAS




Bit rate CCITT standard Frame LossFrame Alignment

8 Mbit/s channel structure

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T1T2T3T4 T1T2T3T4 T1T2T3T4 T1T2T3T4T1 T4

1 1213 2121 2124 5

FAS AS Ji

T1T2T3T4 T1T2T3T4T1 T4 T1T2T3T4T1 T1T2T3T4T4

1 212 1 2128 94 5

Ji Ji Ri

4 5

B inary rate = 8448.0 K bit/s ± 30 ppm

Line C ode = HD B3

N om inal am plitude = 2.37 V

Im pedance = 75

Tolerated input level attenuation = 0 to 6 dB at 4224 Khz according to √f N um ber of tributaries = 4

Justification : P ositive

bits Jij = 1 →→→→ R i= fill-in (justification)

(decision is based on m ajority count of bits Jij)

Fram e length = 848 bits

A vailable bits per tributary per fram e = 206 bits

M ultiplexing m ethod = bit interleaving

Fram e rate = 9962.264 fram e/s

FA S bits rate = 99622.64 bit/s

M axim um justification rate per tributary = 10000 bit/s approx.

bits Jij = 0 →→→→ R i= inform ation (no justification)

N om inal justification ratio = 0.424

Ω

Fram e duration =848 bits

8448 kbit/s= 100.4 µs

Tributary R ate =bits per tributary (per fram e)

fram e duration= 2051,7 kbit/s

206 bits

100.4 s=


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T1T2T3T4 T1T2T3T4 T1T2T3T4 T1T2T3T4T1 T4

1 1213 384 1 3844 5

FAS AS Ji

T1T2T3T4 T1T2T3T4T1 T4 T1T2T3T4T1 T1T2T3T4T4

1 384 1 3848 94 5

Ji Ji Ri

4 5

Binary rate = 34368.0 Kbit/s ± 20 ppm

Line Code = HDB3

Nom inal am plitude = 1 V

Im pedance = 75

Tolerated input level attenuation = 0 to 12 dB at 17.184 M hz according to √ Num ber of tributaries = 4Justification : Positive

bits Jij = 1 →→→→ R i= fill-in (justification)

(decision is based on m ajority count of bits Jij)

M ultiplexing m ethod = bit interleaving


FAS bits rate = 223750.0 bit/s

M axim um justification rate per tributary = 22735 bit/s approx.

bits Jij = 0 →→→→ R i= inform ation (no justification)

Nom inal justification ratio = 0.436

Ω


Available bits per tributary per fram e = 378 bits


34368 kbit/s= 44.7 µs

Tributary Rate =

bits per tributary (per fram e)

fram e duration= 8456,4 kbit/s

378 bits

44.7 s=


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T1T2T3T4 T1T2T3T4

T1T2T3T4T1 T1T2T3T4T4

1 16 17 488

1 4888 9

FAS A S

Ji Ri

4 5

T1T2T3T4 T1T2T3T4T1 T4


1 488

1 4884 5

4 5

Ji

Ji



1 488

1 4884 5

4 5

Ji

Ji

Binary rate= 139264.0 Kbit/s ± 15 ppm

Line Code = CMI

V pp nom inal = 1 V

Im pedance= 75

Tolerated input level attenuation = 0 to 12 dB at 70 Mhz according to √f Number of tributaries= 4

Justification : Positive

bits Jij = 1 →→→→ Ri= fill-in (justification)

(decision is based on majority count of bits Jij)

Multiplexing m ethod = bit interleaving


FAS bits rate = 570754.098 bit/s

Maxim um justification rate per tributary = 47563 bit/s approx.

bits Jij

= 0 →→→→ Ri

= information (no justification)

Nominal justification ratio = 0.419

Ω


Available bits per tributary per fram e = 723 bits


139264 kbit/s= 21.02 µs

Tributary Rate =bits per tributary (per frame)

frame duration

= 34394,2 kbit/s723 bits

21.02 s

=

Synchronization problems

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•••• The standard allows some offsets from the nominal bit rates because it is assumed the lackof synchronization on PDH networks

•••• The problem appears when multiplexing to higher rate

•••• In order to avoid errors the second, third and fourth hierachies provides mechanisms toaccommodate the rate impairments

8448 Kbit/s (+5 ppm)

8

34



8448 Kbit/s (-10 ppm)

34368 Kbit/s

Majority criteria for justification

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•••• If the tributary were absolutely synchronized with the multiplexed frame the it would use theR bit about the 50% of the opportunities

•••• Then the multiplexer must set on all the Jik bits that belong to that tributary i.e.) if it is thesecond tributary would set J21, J22, J23 = 1 and R2=1

•••• At the reception site a majority criteria is applied to identify if R bit contains information ofthe tributary or not. If it does the bits must be insert on the bit sequence when demultiplexing

The justification mechanism

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•••• Bits Jik=1 then Ri is justification, no information

•••• Bits Jik= 0 the Ri contains tributary information

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

Maximum justification rate. 2nd hierarchy: 9962,264 bits/s, 3rd hierarchy: 22375,0 bits/s, 4th.

hierarchy: 47562,842 bits/s

Alarms - higher hierarchies

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The same functionality than 2 Mbit/s frame uses the full duplex capabilities of a link.

It is used to indicate for alarms at higher rates:

•••• loss of signal

•••• loss of frame (where the frame starts?)

T1

T2

T3

T4

A SFA S

140, 34 y 8 Mbit/s

Spare channel - higher hierarchies

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•••• general purpose bit that defines a channel which can be used by any operator application

•••• some samples are maintenance or monitoring of performance

T1

T2

T3

T4

T1 T 2 T 3T4

140 M bit/s.

34 M bit/s

8 M bit/s

S

A SFAS

AFAS

T1

T2

T3

T4

SAFAS

S S

PDH events

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hierarchy ID Explanation

All AIS Alarm Indication Signal

LOF Loss Of Frame alarm

LOS Loss Of Frame Signal alarm

RAI (RDI) Remote Alarm Indication

FAS error Alignment error

Bit error Bit sequence mismatch (the patterns is known)

Code error Violation on codification sequence

2Mbit/s CRC-LOM Cyclic Redundancy Checksum - Loss Of Multiframe

CAS-LOM Channel Associated Signalling - Loss Of Multiframe

RLOM Remote Loss Of Multiframe

CRC error Redundancy Check error

REBE Remote End Block Error

RAI (bit A=1)

LOF

Different AIS types

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•••• AIS: all the tributary bits are “1”

•••• Receiver detects it when tries to identify the FAS

•••• TS16 AIS at the signaling channel. The rest of the bits are not modified

: X

= 1

2 Mbit/s AIS 8, 24, 140 Mbit/s AISTS16 AIS

The CRC-4 mechanism

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•••• It is used for error detection as well as synchronization

•••• It is OK for low error rates (< 10-6)

••••

As all CRC It is not perfect the 6,25% of the errors are not detected•••• Each multiplexer informs to the partner the detected errors using the E bit:

•••• Some of the old multiplexers does not implement this capabilities

2 Mbit/s

CRC4

multiplexer

REBE (bit E=1)

multiplexer

1) CRC process 2) error detection3) error indication writter 4) error indication reader

errors....

PDH as circuit provider

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•••• PDH networks provide circuits to public and private networks like POTS, GSM, ISDN, FRL,audio, video, and data.

•••• The 2 Mbit/s frame is used also to build the synchronization network.

POTS

ATM

ISDN

Alquilada

Internet

2

8

2

8

2

8

2

8

2

8

2

8

34

8

GSM34

8

POTS

ATM

ISDN

Alquiladas

Internet

GSM

FrameRelay Frame

Relay

LMDS

ADSL

LMDS

ADSL

PDH some restrictions

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••••

The supervision andmaintenance functions arelimited (just a few bits foralarms in NFAS, NMFASand E bit (2 Mbit/s frame)

•••• In order to get low speed

channel (i.e. 2 Mbit/s) froma high hierarchy (i.e. 140Mbit/s) a fulldemultiplexing is need

•••• Loss of compatibility

between European,Japanese and North American hierachies

•••• There are no standards forspeeds over 140 Mbit/s

••••

Low managementcapabilities

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Section

Test & Measurement

How to measure

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64

2

2

140 64

2

2

140

FRAME

2 Mbit/s 140 Mbit/s 2 Mbit/s

ANALYZER

64

2

2

140 64

2

2

140

ERROR

2 Mbit/s

140 Mbit/s

2 Mbit/s

DETECTORPATTERN

GENERATOR

In Service Measurement

Out Of Service Measurement

(ISM)

(OOS)

test equipment

test equipment

Equalization

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Test equipment provides automatic equalization

•••• attenuation is bigger for high frequencies•••• amplification is a requirement

Attenuation (dB)

f

√√√√f

EQUALIZATION

2

140

8

34

8

140 Mbit/s

2

Quality Measurements

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ITU-T Recommendations

•••• G.821 under 2Mbit/s,

•••• G.826 applies to PDH and SDH,

•••• M.2100 bringing into service and maintenance PDH

•••• M2101.1 bringing into service and maintenance SDH

SERIAL

OUTPUT

% ROUTE ALLOCATION

OK DEGRADED BAD

LIMIT LIMITOK BAD

7231376-Pdh

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