Plesiochronous Digital Hierarchy

PDH Frequently Asked QuestionsQ : What PDH stands for?A : PDH stands for Plesiochronous Digital Hierarchy. Plesiochronous means almost synchronous (same bitrate but not synchronized to a common master clock). As you can see in the graphic, the multiplexer inputs havesignals (tributaries) with different clock sources. Then, it uses its internal timing to generate a higher rate signalwhich contains the different tributaries.

MUXMUX

Q : What is the primary rate on a PDH system?A : Primary rate refers to the basic rate on which the multiplexing system is based on. For ITU transmission networks,the primary rate is 64 Kbps. This rate will be multiplexed to the first level of the PDH hierarchy and transmitted at a higher rate. Then, it an be multiplexed to the fourth level of the hierarchy.

Q : What are the hierarchy levels on a PDH system?A : Depends on the country.

Level North Ameria Japan International1 1.544 Mbps (T1/DS1) 1.544 Mbps(J1) 2.048 Mbps (E1)2 6.312 Mbps (DS2) 6.312 Mbps (J2) 8.448 Mbps (E2)3 44.736 Mbps (DS3) 32.064 Mbps (J3) 34.368 Mbps (E3)4 139.264 Mbps (DS4) 97.728 Mbps (J4) 139.264 Mbps (E4)

The multiplexing struture for the ITU-PDH system is shown in the next page

Doc. No. A112

PDH Frequently Asked Questions

64 KbpsCEPT1

2.048 MbpsCEPT2

8.448 MbpsCEPT3

34.368 Mbps 139.264 Mbps

Ch 00Ch 01Ch 02Ch 03

Ch 31Ch 30

Ch 28Ch 29

E1

E1

E1

E1

E2

E2

E2

E2

E3

E3

E3

E3

140 Mbps

MUXMUX

MUXMUX

MUXMUX

MUXMUX

x4x4x32x32 x4x4 x4x4

Q : What is the line coding for higher rates?A : ITU-T G.703 specifies HDB3 as a line coding for 8M and 34M. CMI (Coded Mark Inversion) is the line codingfor 139.264 Mbps as well as for 155 Mbps (STM-1 eletrical, SDH). HDB3 stands for High Density Bipolar-3 whichbasically means that you see a signal with high density of pulses. No more than three onsecutive zeroes will befound when this type of line coding is used. See 2.048 Mbps application note for more details regarding HDB3.ITU-T G.703 defines CMI as follows:CMI is a 2-level non-return-to-zero code in which binary 0 is coded so that both amplitude levels, A1 and A2, are

attained consecutively, each for half a unit time interval (T/2). Binary 1 is coded by either of the amplitude levels A1 or A2, for one full unit time interval (T), in such a way that the level alternates for successive binary 1s. An example is given in the figure 3 Note 1 – For binary 0, there is always a positive transition at the midpoint of the binary unit time interval. Note 2 – For binary 1, a) there is a positive transition at the start of the binary unit time interval if in the preceeding time interval the level was A1 b) there is a negative transition at the start of the binary unit time interval if the last binary 1 was encoded by level A2.

Q : How is the frame structure at higher rates:A : The signal is divided in frames. There are three different modes for higher rates : Frame, Unframe and Structured.Unframe mode only carries the payload which an be live date (in-service) or a test pattern (out of service). In Framemode, each frame carries a Frame Alignment Signal (FAS) word and the payload. In structured mode, framing isrequired. Besides the FAS word and the payload, the structured mode frame carries additional bits which are calledjustification and justification control bits, which are basically used for the multiplexing/demultiplexing of the tributariesinside a high rate signal. The frame structure for higher rates containing tributaries is as follows: 8M


C13,C23,C33,C43

Just. Bits (3)

SET IVBitBit BitBitBit BitBitBitBitBit Bit BitBitBitBit BitBitBit BitBit BitBitBitBit

264

BITS FROM TRIBUTARIES

Available bits for NegativeJustification

1 5 9

SET IIIBitBit BitBitBit BitBitBitBitBit Bit BitBitBitBit BitBitBit BitBit BitBitBitBit

264C12,C22,C32,

C42Just. Bits (2)


1 5 9SPAREBITS

Available bits for PositiveJustification

13

TRIBUTARY BIT RATE : 2048 KbpsNUMBER OF TRIBUTARIES : 4FRAME LENGTH : 1056 BITS

SET IBitBit BitBitBitBitBitBitBit

0Bit

0Bit

ZBit

YBitBitBitBitBit

1Bit

1Bit

1Bit

0Bit

0Bit

1Bit

1Bit

0

264FAS

BITS 1 - 8BITS FROM TRIBUTARIES

1 9

SET IIBitBit BitBit BitBitBitBitBitBit BitBitBit BitBitBitBitBit BitBitBitBit BitBit

264C11,C21,C31,

C41Just. Bits (1)


1 5 9BITS FOR SERVICE FUNCTIONS

Z : RAIY : National Use



SET IIBitBit Bit BitBitBit BitBitBitBit BitBitBit BitBitBitBitBit BitBitBit BitBitBit

384

C11,C21,C31,C41

Just. Bits (1)


1 5


0Bit

0BitBitBitBitBitBitBit

1Bit

1Bit

1Bit

1Bit

0Bit

1Bit

0Bit

0

384

FASBITS 1 - 10

BIT 11: ALARMBIT 12 : RESERVED


1

13

1

SET IIIBitBit BitBit BitBitBitBitBitBit Bit BitBitBit BitBitBitBit Bit BitBitBit BitBit

384

C12,C22,C32,C42

Just. Bits (2)


5

C13,C23,C33,C43

Just. Bits (3)

SET IVBitBit BitBit BitBitBitBitBitBit BitBitBit BitBitBitBitBit BitBitBitBit BitBit

384


Available bitsfor

Justification

1 5 9

34M


SET IIBitBit Bit BitBitBit BitBitBitBit BitBitBit BitBitBitBitBit BitBitBit BitBitBit

488

C11,C21,C31,C41

Just. Bits (1)


1 5


0Bit

0Bit

0Bit

0BitBitBitBitBit

1Bit

1Bit

1Bit

1Bit

0Bit

1Bit

0Bit

0

488

FASBITS 1 - 12

BIT 13: ALARMBITs 14-16 : RESERVED


1

17

1

SET IIIBitBit BitBitBitBitBitBitBitBit BitBitBitBitBitBitBitBit BitBitBitBitBitBit

488

C12,C22,C32,C42

Just. Bits (2)


5

SET IVBitBit BitBitBitBitBitBitBitBit BitBitBitBitBitBitBitBit BitBitBitBitBitBit

488

C13,C23,C33,C43

Just. Bits (3)


1 5

1

SET VBitBit BitBitBitBit BitBitBitBit BitBit BitBitBit BitBitBit BitBitBit BitBitBit

488

C14,C24,C34,C44

Just. Bits (4)


5

C15,C25,C35,C45

Just. Bits (5)

SET VIBitBit BitBit BitBitBitBitBitBit BitBitBit BitBitBitBitBit BitBitBitBit BitBit

488


Available bitsfor

Justification

1 5 9

139M


PDH Frequently Asked QuestionsQ : What types of equipment are involved in PDH transmission?A : There are several types of equipment involved in PDH transmission. The Primary Multiplexer which acceptsdifferent 64Kbit/s channels, and transmit them at 2.048 Mbps. The 2 Mbps tributaries will be grouped and multiplexedto higher rate signals by the high order multiplexers. Then, you will see the transmission terminal with a digital outputthat can be transmitted through different medias : coaxial cable, fiber optic. satellite links or mirowave digital radios.PDH test equipment can be used to test the multiplexing process, the transmission terminal equipment and themedia by generating a digital test pattern that can be multiplexed and sent through the different media.

PRIMARY MULTIPLEX

SECTION PBX or SWITCH

HIGHER ORDER

MULTIPLEX

TRANSMISSIONTERMINAL

TRANSMISSIONTERMINAL

HIGHER ORDER

MULTIPLEX

PRIMARY MULTIPLEX

SECTION PBX or SWITCH

MEDIAMEASUREMENTS

FIBER OPTIC

COAXIAL CABLE

DIGITAL RADIO

SATELLITE

MODEM DATA SERVICEUNIT MODEM DATA SERVICE

UNIT

Plesiochronous Digital Hierarchy

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