PDH vs SDH used in telecom

PDH (Plesiochronous Digital Hierarchy) and SDH (Synchronous Digital Hierarchy) are both digital transmission technologies used in telecommunications to transport voice and data signals over high-capacity communication links. PDH systems operate with slight timing differences between different network elements and its networks require sophisticated synchronization mechanisms to maintain proper timing alignment. In contrast, SDH systems maintain strict synchronization across all network elements and its networks are designed to ensure that all devices are synchronized to a common clock reference.

PDH

In PDH, digital multiplexer's inputs(bit streams) are of same bit rate and are derived from different clocks from different oscillators. Each will differ within tolerance of few clock periods. The term "plesiochronous" means "almost synchronous."

Bit Interleaving is used in PDH to combine digital signals.

In PDH there are two main standards i.e. 30 channel one used in Europe and 24 channel one used in North America/Japan. Basic rate is 64 Kbps in North America(designated as DS0) and in Europe (designated as E0). Rates derived from 2.048 Mbps basic rate including bit stuffing in 30 channel case are mentioned below.
2.048 x 4 gives 8.448 Mbps (120 channels)
8.448 x 4 gives 34.368 Mbps (480 channels)
34.368 x 4 gives 139.264 Mbps (1920 channels)
139.264 x 4 gives 564.992 Mbps (7680 channels)

The figure-1 depicts PDH level hierarchy used in North America while figure-2 depicts PDH levels used in Europe. Rates derived from 1.544 Mbps basic rate including bit stuffing in 24 channel case are mentioned below.
1.544 x 4 gives 6.312 Mbps (96 channels)
6.312 x 7 gives 44.736 Mbps (672 channels)
44.736 x 6 gives 274.16 Mbps (4032 channels)
44.736 x 3 gives 139.264 Mbps
44.736 x 12 gives 564.992 Mbps

The table below mentions PDH rates used in Japan.

Following are the limitations or disadvantages of PDH:
•  In PDH, different frame is used for transmission and in data layer. Hence multiplexing and de-multiplexing is very complex.
• Accessing lower tributary requires the whole system to be de-multiplexed.
• The maximum capacity for PDH is 566 Mbps, which is limited in bandwidth.
• Tolerance is allowed in bit rates.
• PDH allows only Point-to-Point configuration.
• PDH does not support Hub.
• Every manufacturer has its own standards; PDH also has different multiplexing hierarchies making it difficult to integrate interconnecting networks together.

SDH

As PDH was not scalable to support high capacity bandwidth and hence was not suitable to accomodate growing traffic need. SONET was developed as American Standard while SDH as European Standard.

SDH supports various topologies such as point to point, ring, star, linear bus etc. It uses TDM and octet multiplexing. It uses extremely precise timings. It employs both electrical and optical specifications.

In SDH, digital multiplexer's inputs are of same bit rate and are derived from common clock, hence are in phase. Hence it is synchronous.

Word(group of bits) Interleaving is used in SDH to combine digital signals.

SDH uses basic rate of 155.52 Mbps and four times multiples of this i.e. 622.08 Mbps and 2488.32Mbps. Basic SDH rate of 155.52 is also known as STM-1. STM stands for Synchronous Transport Module. SDH is referred as SONET (Synchronous Optical Network) in USA due to optical interfaces used. Figure-3 depicts table mentioning SONET and SDH levels with their respective data rates.

As per CCITT, any of PDH rates upto 140 Mbps can be integrated with SDH rates up to 155.52 Mbps.

Following are the merits or advantages of SDH:
• A more simplified multiplexing and demultiplexing technique.
• Synchronous networking and SDH supports multipoint networking.
• Capability of transporting existing PDH signals.
•  Easy growth to higher bit rates which enhances the administration and maintenance process.
• It is capable of transporting broadband signals.
• It is multivendor and supports different operators.
• It provides network transport services on LAN such as video conferencing, and interactive multimedia.
• Optical fibre bandwidth can be increased without limit in SDH.
•  Switching protection to traffic is offered by rings.
• SDH allows quick recovery from failure.

Difference between PDH and SDH

Following table compares PDH vs SDH with respect to various parameters and mention difference between PDH and SDH in the form of table below.

Conclusion In summary, SDH is characterized by strict synchronization where as PDH operates with slight timing differences. SDH, with its superior synchronization, error performance, and flexibility, has largely replaced PDH in modern telecom networks, offering a more efficient and standardized solution for high-capacity communication.

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