PDH vs SDH: Understanding Telecommunication Hierarchies

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 (Plesiochronous Digital Hierarchy)

In PDH, digital multiplexer’s inputs (bit streams) are of the same bit rate and are derived from different clocks from different oscillators. Each will differ within the tolerance of a 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: a 30-channel system used in Europe and a 24-channel system used in North America/Japan. The basic rate is 64 Kbps in both North America (designated as DS0) and Europe (designated as E0).

Rates derived from the 2.048 Mbps basic rate (including bit stuffing in the 30-channel case) are mentioned below:

• 2. 048 x 4 = 8.448 Mbps (120 channels)
• 3. 448 x 4 = 34.368 Mbps (480 channels)
• 3. 368 x 4 = 139.264 Mbps (1920 channels)
• 4. 264 x 4 = 564.992 Mbps (7680 channels)

Figure 1 depicts the PDH level hierarchy used in North America, while Figure 2 depicts PDH levels used in Europe.

Rates derived from the 1.544 Mbps basic rate (including bit stuffing in the 24-channel case) are mentioned below:

• 1. 544 x 4 = 6.312 Mbps (96 channels)
• 2. 312 x 7 = 44.736 Mbps (672 channels)
• 3. 736 x 6 = 274.16 Mbps (4032 channels)
• 4. 736 x 3 = 139.264 Mbps
• 5. 736 x 12 = 564.992 Mbps

The table below mentions PDH rates used in Japan.

Limitations/Disadvantages of PDH

• In PDH, different frames are used for transmission and in the data layer. Hence, multiplexing and de-multiplexing are very complex.
• Accessing a lower tributary requires the entire 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 configurations.
• PDH does not support Hubs.
• Every manufacturer has its own standards; PDH also has different multiplexing hierarchies, making it difficult to integrate interconnecting networks together.

SDH (Synchronous Digital Hierarchy)

As PDH was not scalable to support high-capacity bandwidth and hence was not suitable to accommodate growing traffic needs, SONET was developed as an American Standard, while SDH was developed as a European Standard.

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

In SDH, digital multiplexer’s inputs are of the same bit rate and are derived from a common clock, hence they are in phase and synchronous. Word (group of bits) Interleaving is used in SDH to combine digital signals.

SDH uses a basic rate of 155.52 Mbps and four times multiples of this i.e., 622.08 Mbps and 2488.32 Mbps. The basic SDH rate of 155.52 is also known as STM-1. STM stands for Synchronous Transport Module. SDH is referred to as SONET (Synchronous Optical Network) in the USA due to the optical interfaces used.

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

Merits/Advantages of SDH

• A more simplified multiplexing and demultiplexing technique.
• Synchronous networking and SDH support 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 LANs such as video conferencing and interactive multimedia.
• Optical fiber 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

The following table compares PDH vs SDH with respect to various parameters and mentions the difference between PDH and SDH in the form of a table below.

Conclusion

In summary, SDH is characterized by strict synchronization, whereas 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.