What is PDH Technology?

online_learn_earn February 01, 2020

What is Plesiochronous Digital Hierarchy?


The Plesiochronous Digital Hierarchy(PDH) is a technology used in telecommunications networks to transport large quantities of data over digital transport equipment such as fiber optics and microwave radio system. The term Plesiochronous is derived from Greek Plesio,meaning near and refers to the fact that PDH networks run in a state where different parts of the network are almost,but quite perfectly,Synchronized.

PDH allows transmission of data streams that are nominally running at the same rate, but allowing some alteration on the speed around a nominal rate

PDH Signal Formats

The multiple PDH signal formats are:-
1. D1 (32 x 64kHz=2.048 Mbps)
2. D2 (4 × 2.048 +stuffing bits=8.448 Mbps)
3. D3 (4 × 8.448 + stuffing bits=34.368 Mbps)
4. D4 (4 × 34.368 + stuffing bits=139.264 Mbps)
5. D5/Super frame (4 × 139 +stuffing bits=565 Mbps) 

Digital data and voice transmission based on a 2.048Mbps consisting of 30 time division multiplexed (TDM) voice channels, each running at 64Kbps known as E1 and described by the CCITT G.702,G.703 specification and two additional channels carrying control information.

The additional bits are called justification or stuffing bits.

1. D1 Capacity is 30 base channels.
2. D2 Capacity is 120 base channels.
3. D3 Capacity is 480 base channels.
4. D4 Capacity is 1920 base channels.
5. D5 Capacity is 7680 base channels.

D0 is 64 Kbps channels in PDH

The Different  PDH bit rates used in world:-

 Europe       North America    Japan

 564,992 Mbps    -----Nil----        397,200Mbps
139,264 Mbps    274,176 Mbps    97,728 Mbps
34,368 Mbps       44,436 Mbps       32,064 Mbps
8,448 Mbps          6,312 Mbps          6,312 Mbps
2,048 Mbps            1,544 Mbps        1,544 Mbps
64 Kbps                   64 Kbps                64 Kbps
  

Limitation of PDH Network:

1. Incapability to identify several channels in a higher-order bit stream.
2. Insufficient capacity for network management.
3. The multiplexing and demultiplexing process is time-consuming.
4. Most PDH network management is Proprietary.
5. There are different hierarchies in use around the world.specialized interface equipment is required to inter work the two hierarchies.
6. Lack of performance monitoring capability.
7. Standard equipment from different vendor are not compatible.
8. As PDH networks topologies are mostly point to point,there is very little provision for  network resilience in order to protect subscriber channels from equipment failure or cable damage.
9. Speed can not exceed 565 Mbps.

Types of Distribution Frame used in Telecommunication .

1.DDF (Digital Distribution Frame)
2.FDF (Fiber Distribution Frame)
3.MDF(Main Distribution Frame)

What is DDF (Fiber Distribution Frame)?

Digital Distribution Frame is a cross connection panel to facilitate checking ,isolation and cross patching of digital transmission tributaries during testing or maintenance. Transmission circuits get distributed/diverted towards transmission facility through station strapping on DDF.

Types of Digital Distribution  Modules

1. DDM 120 ohms
2.DDM  75 ohms

DDM 120 ohms.

Digital Distribution Frame for 2 Mbps interface has the facility of a number of  DDM (120ohms) mounted on a single bay. The DDF has the facility for isolating line side and equipment side by 4 Pin Plug/Socket arrangement. Through connections are made by 4 pin U-link which also provides facility for on line testing.

1. Mechanical Specifications

  • Bay Size -- 2350 mm Height
  • Module Size-- 234.2 × 89.0 × 255.0 mm
  • Bay Capacity-- Nine modules(max)
  • Modules Capacity-- 120 ohms 8 set of trans & rece termination of 2 Mbps.
  • Termination type-- Wire Wrapping 

2. Electrical Specifications

  • Impedance --120 ohms balanced
  • Return loss-->25dB

3. Accessories

  • Slim rack--Patch cords
  • Slim rack--U-links

DDM 75 ohms.


Digital Distribution Frame for 8Mbps / 34 Mbps /140Mbps / 155 Mbps consists of a number of modules mounted on a single bay. The modules are of metal plate type on which 75ohms coaxial connectors are mounted.

1. Mechanical Specifications

  • Bay Size--2350 mm Height.
  • Module Size--234.2 × 89.0 × 255.0 mm
  • Bay capacity--nine modules(max)
  • Module capacity--75 ohms 4 set of trans & rece.
  • Termination type--Cable termination on coaxial socket.

2. Electrical Specifications

  • Impedance  75 ohms unbalanced.
  • Return Loss >25dB

3. Accessories

  • Slim rack--Patch cords
  • Slim rack--U-links

Installation and Wiring of DDF

1. Installation of Bay Frame

DDF can be erected on ground or on wooden base plate as per requirement.

2. Mounting of Modules on Bay Rack

120 ohms DDM, 75 ohms DDM are mounting as per required quantity on the rack.

3. Cabling and Wiring

i. Cabling
Cable to be terminated on the DDM is laid and brought in two bunches to appropriate location with sufficient length providing the slack for movement of DDM. Cable are anchored at regular intervals by cable tie brackets.

ii. Wiring
a.DDM 2Mbps (120 ohms balanced)
The capacity of the module is to accept 8 PCM systems of 2 Mbps each . The signals interconnected in this module use symmetrical 120 ohms cable. The  wiring installation for this module is done by wire wrapping tools with sufficient slackness in cable. Wrap two symmetric wires to top(T) and lug (R) pins a screen wire to ground (G) pin. Trans and receive signals are separated in two vertical halves of a module.

b. DDM 8Mbps, 34Mbps, 140Mbps & 155 Mbps (75 ohms Unbalanced)
Digital Distribution Module for terminating higher order bit stream is having 75 ohms characteristic impedance and uses suitable coaxial connectors. The through link is made by spinner type U-Link. This module provides for terminating four sets of trans & receive terminals.RG 59 B/U signal screen cable is used for 8Mbps, 34Mbps signals through connectors.RG59 B/U double screen cable is used for 140Mbps, 155 Mbps signals through connectors.

4. Earthing

Exchange earthing wire is connected to the earth terminal providing on either side of the frame and modules are also connected to the frame for earthing.

5. Labelling

After exchange wiring has been completed and checked address labels and designation labels are put  in the label holder indicating direction of signal flow.

Operation and Maintenance of DDF

Through Digital Distribution Frame has been installed and wiring has been completed and checked the following operational facilities are available.

1. DDM 2Mbps (120 ohms symmetrical pair)

i. Standard Mode
This Module has been provided for through connections. Break in through path is possible by removing the corresponding 4 pin U links. Temporary patching of PCM is done by using a pair of balanced cord and plug  at the 16 pin socket port by removing the  U-link Tx ,Rx port.
ii. Permanent Patching
Permanent patching is done by changing straight cable connection to crossed wire wrapping on suitable Tx/Rx signal pairs.

iii. Loop Back
For maintenance and PCM testing Tx/ Rx loop back can be provided on a channel by plugging balanced patch cord to appropriate position on the 16 pin socket removing the corresponding  U-link.

2. DDM 8/34/140/155 Mbps(75 ohms coaxial)

i. Standard Mode
This module has coaxial sockets mounted at appropriate send /receive location . It caters for 4 sub-system . Insertion of coaxial links provide through path . Break in through path is done by the removal of coaxial U-link. Temporary patching is done by using  a copper patch  cord and plug assy to patch Tx/Rx signals on u-link port.


 ii. Permanent Patching
When signal route has to be permanently diverted,it is possible to remove the coaxial cable with socket termination and remount it on the DD panel with sufficient slack in the coaxial cable so that the Tx Rx sockets are aligned  and standard coaxial  U link is inserted for providing through path.

 iii. Loop Back
Loop back of sen and receive signal is done by inserting the coaxial U-Link horizontally across Tx-Rx ports.


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