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Difference Between PDH and SDH

PDH Vs SDH: In this tutorial, we will learn about the PDH (plesiochronous digital hierarchy) and SDH (synchronous digital hierarchy) and the difference between PDH and SDH. By IncludeHelp Last updated : June 27, 2023

Introduction

In today's fast-paced world of telecommunications, understanding the differences between various network technologies is crucial for both novices and professionals. PDH (plesiochronous digital hierarchy) and SDH (synchronous digital hierarchy) are two widely used multiplexing techniques in optical networks that play a significant role in efficient data transmission.

While both have their roots in fiber optic networking, they differ considerably in terms of synchronization, data transfer rates, and network structure. This article aims to provide an insightful comparison between PDH and SDH, highlighting why SDH has emerged as the more advanced and preferred technology over its predecessor.

After reading this tutorial, you will learn -

• PDH and SDH are two widely used multiplexing techniques in optical networks that differ considerably in terms of synchronization, data transfer rates, and network structure.
• SDH technology is more advanced than PDH due to its synchronous communication between all elements in the network, higher data transmission rates, and flexible hierarchical system known as Optical Transport Network (OTN).
• Understanding key concepts like frame synchronization and multiplex technology becomes increasingly crucial for industry professionals working with digital communication systems such as fiber optic networking or SONET.
• As the demand for high - speed internet access continues to increase rapidly, telecommunications providers are opting for scalable and efficient technologies like SDH over traditional hierarchical systems like PDH.

What is PDH?

PDH stands for Plesiochronous Digital Hierarchy which is an older technology that was widely used in transmitting digital information over fiber-optic networks. PDH was initially developed in the 1960s as a time-division multiplexing (TDM) method, PDH facilitated the transportation of multiple voice channels through a single line by dividing the time into slots for different channels.

What is SDH?

SDH stands for Synchronous Digital Hierarchy which evolved from PDH as a more advanced approach to digital transmission specifically designed to address its limitations. SDH was developed during the 1980s with its original version called SONET (Synchronous Optical Network), SDH brought significant improvements to network protocols while supporting higher data transfer rates and providing greater flexibility for cross-connecting and demultiplexing tributaries within optical fiber communication networks.

Differences Between PDH and SDH

PDH and SDH differ in synchronization and timing, with PDH having no synchronization between payload and frame, while SDH has strict synchronous communication. Additionally, their data transmission rates vary significantly, with SDH capable of handling higher speeds than PDH networks.

Synchronization and Timing

One of the key differences between PDH and SDH is synchronization and timing. In a PDH network, there is no guarantee that different tributary streams are synchronized to each other, which can cause problems such as jitter or delay in data transfer.

For example, consider a circuit-switched network where multiple users share a single channel for voice transmission. With PDH technology, there is a possibility that one user's speech reaches another user with delays due to unsynchronized transmission times.

However, with SDH technology, this problem is eliminated by using precise clocking mechanisms at every node in the network.

As telecommunications systems continue to evolve towards digital technologies like fiber optical networking and Synchronous Optical Network (SONET^wikipedia), understanding the importance of synchronization and timing becomes increasingly crucial for industry professionals working with multiplexing protocols such as T-carrier systems or time-division multiplexing (TDM).

Data Transmission Rates

One of the key differences between PDH and SDH is their data transmission rates. PDH offers a fixed hierarchy of bit rates, which range from 1.544 Mbps for T1 lines to 45 Mbps for T3 lines.

For example, an OC-48 SONET circuit operates at approximately 2.5 Gbps, while an STM-16 SDH circuit runs at about 2.4 Gbps.

As fiber optic technology continues to advance rapidly, the demand for higher data transmission rates will only continue to increase.

Network Structure

One of the key differences between PDH and SDH is their network structure. In PDH, all signals are combined into a single stream before being sent over the network, which can lead to bottlenecks and slower transmission speeds.

On the other hand, SDH uses a hierarchical system known as Optical Transport Network (OTN), which breaks down data into smaller, more manageable streams known as tributaries.

This flexible network structure makes SDH ideal for large-scale fiber optical networking where multiple clients need access to various services simultaneously. For example, an internet service provider using SDH could allocate specific bandwidths to different customers depending on their needs, without disrupting other users' traffic flows or causing congestion on the network.

Conclusion

In conclusion, the differences between PDH and SDH are significant in the telecommunications industry. While both technologies are used in fiber optical networking, SDH proves to be a better and more advanced technology compared to PDH.

The synchronization and timing, data transmission rates, network structure, flexibility in cross-connecting, and demultiplexing of tributaries offer numerous advantages over its predecessor.

By overcoming the limitations of PDH, SDH has become an essential tool for digital communication systems. With the increasing demand for high-speed internet access and reliable data transfer speeds, it's no wonder that SDN is becoming increasingly popular among professionals in this field.

PDH Vs. SDH FAQs

The following are the frequently asked questions with answers based on the difference between PDH and SDH.

1. What is PDH and how does it differ from SDH?

PDH stands for Plesiochronous Digital Hierarchy, it is an older digital transmission technology that uses fixed time slots to transmit voice and data signals at various rates. Whereas SDH stands for Plesiochronous Digital Hierarchy, it is a more recent digital transmission technology that uses synchronous transport modules to efficiently transmit large amounts of data at high speeds.

2. How does PDH affect the quality of transmissions compared to SDH?

PDH can cause variations in timing due to its use of non-synchronous bit rates while SDH provides reliable synchronization between different network elements resulting in better-quality transmissions. The result is that SDH offers faster connectivity, higher reliability, and greater flexibility than PDH.

3. Which networks still use PDH technology?

Although most carriers have switched over to using SDH as their primary network infrastructure, particularly for long-haul transmission across countries/continents; certain specialized industries such as the aviation industry continue using both technologies depending upon specific requirements and cost limitations within their operational boundaries.

4. Is it necessary for businesses to upgrade from PDH to SDN?

It may not be necessary for all businesses since this depends on factors such as bandwidth needs, budget availability, and usage patterns among others but generally speaking, upgrading from a legacy system like PDN would offer greater benefits including increased capacity, improved scalability& management features ensuring resiliency within complex networking environments with less downtime exposure making overall operations smoother & efficient leading towards competitive advantages in the business sector.