pdh

© Trend Communications

The PDH hierarchyby JM Caballero

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


Signals & Information

InformationAnalog Digital

Signals

AnalogModulation- AM/FM radio - broadcast TV

Digital Modulation - ADSL- digital TV

DigitalDigitalization- audio CD - ISDN (voice)

Codification- ISDN (data)- Internet


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


Telecommunication in evolution


The arrival of digital technology

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

Modem

digitaldigital

digital

digitaldigital

analog

analog analog

analog

analog

analog

Modem

: 1900

: 1960

: 1990

Central Central

Central Central

Central Central

digital


The digitalization of signals

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

t0+T ···t0t

t

001 011 001 101 100

t0+T ···t0

SAM PLING

ENCO DING

t

011010001000100101110111

Q UANTISATIO N


Nyquist Sampling Theorem

in order to convert an analog signal to digital it is necessary to use a sampling frequency (fs) at least two times the highest frequency”

• fs ≥≥≥≥ 2BW (in Hertzs)

i.e.) a phone channel BWc = 4000 Hz in 8 bits each sample it would be necessary:

• fs = 2*4000=8000 Hz

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

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

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


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/sBw: Bandwidth in Hz.P: Signal powerN: Media noise

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


Types of digital modulation

Pulse Code Modulation (PCM) the most used for voice

tt0t1 t

2 t3

t4 t5 t6 t7

t8 t9

M O DULATIO N

DeltaM odul.

PULSEDIG ITAL

tt0t1 t

2 t3 t4 t5 t6

t7 t8 t9

tt0

t0+T t

0+2T

t0+3T t0+4T

011010001000100101110111

(3)(2)(1)(0)(4)(5)(6)(7)

7 V5 V3 V V- V-3 V-5 V-7 V

tt0+T ···t0

t

1 3 1 5 4

t0+T ···t0

3V 3V

- 3V

- V

7V

PAM

PDM M ODULATIO NPULSE

ANALO G

t

001 011 001 101 100

t0+T ···t0

tt0+T ···t0

1 3 1 5 4

PPM

PCM


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


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

HighDensityBipolarThreeZeroes

CodedMarkInverted

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

B: balancingV: violation

2 Mbit/s8 Mbit/s

2 Mbit/s34 Mbit/s

140 Mbit/s155 Mbit/s


Multiplexing

Allows the use of several communications channels through a transmission media

DTE-A BWs1

DTE-B BWs2

DTE-FBWs1

.

..

MULTIPLEXER

Transmission media

AA

BC

DE

F BCDEFAB

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


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)

Demodulatordemultiplexer

4 channels at thesame frequency

Analogswitch

Modulatormultiplexer

ABCDABCDABCDABCD

ABABABABAB

CDCDCDCDCD

Digital switch


Typical analog arrangement

The swictching capabilities are between subribers and digital multiplexors

LTE

SUBSCRIBERS

SUBSCRIBERS

2 M bit/s

2 M bit/s

LTE

DIG ITAL TRANSM ISSIO N

LINE

REG ENERATOR

REG ENERATOR

PCM M UX

ANALO GEXCHANG E


Typical digital arrangement

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

LTE

DIGITALEXCHANGE

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


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


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


Identify Digital Technology areas

Switching Symplifies demultiplexing and switching operation

Allows network management

Transmission 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 applications based on voice, data, hypermedia or multimedia

End-to-end digital quality

Section

The PDH standardsIT U -T T eleco m m un ica tio n

S ta nd ardiza tion S e ctor of th eIn tern atio n alT elec o m m u n ic atio nU 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 d M u ltip le xa tion e q uipm e n t

O S E R IE S : M e asu rin g e qu ip m ent spe cifica tio n s

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


Multiplexing Hierarchies

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


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


PDH is plesiochronous

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

PDH

PDH

PDH

PDH

PDHPDH

SWITCH

clock

PDH

PDH

PDH

PDH

PDHPDH

alignmentPDH circuits

Lines Input Synchronization Switched lines


PDH standard by ITU-T

hirarchy standard binary rate line code amplitude attenuation

1 G.704/732 2048kbit/s±50ppm HDB3 2.37V ó 3.00V

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


PDH Frame stream sequence


The PDH hierarchy

A

S

T1

J11

R1

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


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 beginning

of the frame for the rest of the hierarchies

FAS FAS

tim e slots


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 µs then 1byte is a 8 bit/125 µs= 64 kbit/s channel•••• The transmission rate is (32channel x 8bit/channel) / 125 µs = 2,048 Mbit/s


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 ppmLine 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 √f Fram e length = 256 bitsAvailable bits per tim eslot = 8 bits

M ultiplexing m ethod = octet interleavingFram 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)


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: Non Frame Alignment Sequence (i)

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


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 and

maintenance protocols


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 on

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


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 the

other side


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)

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

•••• 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

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

•••• Channel Associated Signalling CASEach 64 kbit/s channel (TS1-TS15 and TS17-TS30) has a 2 kbit/s channel, as fast as each one of 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)

•••• To synchronize the CAS an alignment signal•••• 0000 sequence is found on the first bits of the multiframe


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

Uses some bits more depending on the bit rate

T 1T 2T3 T4

FA S

T1 T2T 3T 4

140 M bit/s

34 M bit/s

8 M bit/s

AS

S

T1 T2T 3T 4AS

34 M bit/s tributaries bits

FA S


FA S


A111110100000

1111010000

1111010000


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

T1T2T3T4 T1T2T3T4 T1T2T3T4 T1T2T3T4T1 T4

1 12 13 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 Kbit/s ± 30 ppmLine C ode = H D B3

N om inal am plitude = 2.37 VIm pedance = 75

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

Justification : P ositivebits Jij = 1 →→→→ R i = fill-in (justification)

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

Fram e length = 848 bitsA vailable bits per tributary per fram e = 206 bits

M ultiplexing m ethod = bit interleavingFram e rate = 9962.264 fram e/s

FA S bits rate = 99622.64 bit/sM 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/s206 bits

100.4 µs=



T1T2T3T4 T1T2T3T4 T1T2T3T4 T1T2T3T4T1 T4

1 12 13 384 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 ppmLine Code = HDB3

Nom inal am plitude = 1 VIm pedance = 75

Tolerated input level attenuation = 0 to 12 dB at 17.184 M hz according to √fNum ber of tributaries = 4

Justification : Positivebits J

ij = 1 →→→→ R i

= fill-in (justification)

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

M ultiplexing m ethod = bit interleavingFram e rate = 22375.0 fram e/s

FAS bits rate = 223750.0 bit/sM 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

Fram e length = 1536 bitsAvailable bits per tributary per fram e = 378 bits


34368 kbit/s= 44.7 µs

Tributary Rate =bits per tributary (per fram e)


44.7 µs=



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 ppmLine Code = CMIVpp nom inal = 1 V

Im pedance = 75

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

Justification : Positivebits J

ij = 1 →→→→ Ri

= fill-in (justification)

(decision is based on majority count of bits Jij)

Multiplexing m ethod = bit interleavingFram e rate = 47562.842 frame/s

FAS bits rate = 570754.098 bit/sMaxim um justification rate per tributary = 47563 bit/s approx.

bits Jij = 0 →→→→ Ri

= information (no justification)

Nom inal justification ratio = 0.419

Ω

Fram e length = 2928 bitsAvailable bits per tributary per fram e = 723 bits


139264 kbit/s= 21.02 µs

Tributary Rate =bits per tributary (per frame)


21.02 µs=


Synchronization problems

•••• The standard allows some offsets from the nominal bit rates because it is assumed the lack of 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 to

accommodate the rate impairments

8448 Kbit/s (+5 ppm) 8

34

8448 Kbit/s (+7 ppm)

8448 Kbit/s (+2 ppm)

8448 Kbit/s (-10 ppm)

34368 Kbit/s


Majority criteria for justification

•••• If the tributary were absolutely synchronized with the multiplexed frame the it would use the R 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 the second 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 of the tributary or not. If it does the bits must be insert on the bit sequence when demultiplexing


The justification mechanism

•••• 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

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?)

T 1 T 2 T 3 T 4

A SFAS

140, 34 y 8 Mbit/s


Spare channel - higher hierarchies

•••• general purpose bit that defines a channel which can be used by any operator application•••• some samples are maintenance or monitoring of performance

T 1 T 2T 3 T 4

T 1 T 2T 3T 4

140 M bit/s.

34 M bit/s

8 M bit/s

S

A SFAS

AFAS

T 1 T 2T 3T 4SAFAS

S S


PDH events

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

•••• 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

•••• 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 writter4) error indication reader

errors....


PDH as circuit provider

•••• 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

8GSM

34

8

POTS

ATM

ISDN

Alquiladas

Internet

GSM

FrameRelay Frame

Relay

LMDS

ADSL

LMDS

ADSL


PDH some restrictions

•••• 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 high 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

Section

Test & Measurement


How to measure

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

ERROR2 Mbit/s

140 Mbit/s

2 Mbit/s

DETECTORPATTERN

GENERATOR

In Service Measurement

Out Of Service Measurement

(ISM)

(OOS)

test equipment

test equipment


Equalization

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

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

SERIALOUTPUT

% ROUTE ALLOCATION

OK DEGRADED BAD

LIMIT LIMITOK BAD

pdh

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