7.11.7 What Are SONET and SDH, and Why Were They Developed?
- What Are SONET and SDH?
- Why Were New Standards Needed?
- What Problems Did PDH Create?
- What Does "Synchronous" Mean?
- How Does Synchronization Improve Multiplexing?
- What Is the Basic SONET Hierarchy?
- What Is the SDH Hierarchy?
- Are SONET and SDH Compatible?
- What Is a SONET or SDH Frame?
- Why Is the Overhead Important?
- What Are Add-Drop Multiplexers?
- Why Was This Such an Improvement?
- Why Are SONET and SDH Well Suited to Optical Fiber?
- Were SONET and SDH Used Only for Telephone Calls?
- Are SONET and SDH Still Used Today?
- Did SONET and SDH Influence Modern Networks?
- Why Are SONET and SDH Important?
Description
Understand why the limitations of plesiochronous digital hierarchies led to synchronous optical transport systems. Learn how SONET and SDH simplified multiplexing, enabled very high data rates, and became the backbone of global telecommunications networks.
Introduction
By the late 1970s, digital transmission had largely replaced analog communication systems in long-distance telephone networks. Time-Division Multiplexing (TDM) allowed many voice and data channels to be combined efficiently, while optical fiber was beginning to replace copper and microwave links as the preferred transmission medium. Although these developments dramatically increased network capacity, they also exposed serious shortcomings in the way digital signals were multiplexed.
Different countries had developed their own digital hierarchies. North America adopted the T-carrier system, beginning with the T1 format operating at 1.544 Mb/s, while Europe, Australia, and many other countries adopted the E-carrier system beginning with E1 at 2.048 Mb/s. As transmission rates increased, these systems became increasingly difficult to manage. Different standards were incompatible, multiplexing equipment became more complex, and extracting a single low-speed channel from a high-speed transmission often required several stages of demultiplexing.
The solution was to develop an international transmission standard based upon precise synchronization and a common hierarchy. This led to two closely related standards:
- Synchronous Optical Network (SONET), developed in North America; and
- Synchronous Digital Hierarchy (SDH), developed internationally.
Together, these standards transformed global telecommunications by providing a simple, flexible, and highly reliable method of transporting enormous quantities of digital information over optical-fiber networks.
What Are SONET and SDH?
SONET and SDH are standardized digital transmission systems designed primarily for high-capacity optical-fiber networks.
Both standards define:
- transmission rates;
- multiplexing structures;
- frame formats;
- synchronization methods; and
- network management procedures.
Although developed by different standards organizations, SONET and SDH are closely related and largely compatible.
Why Were New Standards Needed?
Before SONET and SDH, digital transmission relied on the Plesiochronous Digital Hierarchy (PDH).
The word plesiochronous means "almost synchronized." Individual digital systems operated at nearly, but not exactly, the same clock frequency. Although the frequency differences were very small, they complicated multiplexing and network management considerably.
As communication networks expanded, these limitations became increasingly significant.
What Problems Did PDH Create?
Several practical difficulties arose.
These included:
- incompatible international standards;
- independent clock sources;
- complex multiplexing equipment;
- difficult channel extraction; and
- limited network flexibility.
As transmission speeds increased into the hundreds of megabits per second, these problems became increasingly difficult to manage.
What Does "Synchronous" Mean?
Unlike PDH, SONET and SDH operate using highly accurate network timing.
All equipment within the network is synchronized to common reference clocks. Consequently:
- frames occur at precisely defined intervals;
- multiplexing becomes much simpler;
- channels remain accurately aligned; and
- network management is greatly simplified.
Synchronization is therefore one of the defining features of both SONET and SDH.
How Does Synchronization Improve Multiplexing?
When every network element operates from the same timing reference, incoming digital signals no longer require continual adjustment to compensate for clock differences.
This greatly simplifies multiplexing. Instead of repeatedly inserting and removing justification bits to accommodate small timing differences, synchronized signals can be combined directly into higher-rate transmission streams.
The resulting equipment is simpler, more reliable, and easier to manage.
What Is the Basic SONET Hierarchy?
SONET defines a hierarchy based upon the Synchronous Transport Signal Level 1 (STS-1) which operates at 51.84 Mb/s. Higher transmission rates are formed by combining multiple STS-1 signals.
Examples include:
- STS-3;
- STS-12;
- STS-48; and
- STS-192.
Each higher level provides proportionally greater transmission capacity.
What Is the SDH Hierarchy?
SDH uses a similar hierarchy to SONET but begins with a different base rate.
The fundamental SDH transmission format is the Synchronous Transport Module Level 1 (STM-1) which operates at 155.52 Mb/s. Higher-rate signals include:
- STM-4;
- STM-16;
- STM-64; and
- STM-256.
These standards support transmission capacities ranging from hundreds of megabits per second to many tens of gigabits per second.
Are SONET and SDH Compatible?
Yes.
Although they originated independently, the two standards were deliberately designed to interoperate. For practical purposes:
- STS-3 corresponds closely to STM-1;
- higher-rate hierarchies align naturally; and
- international gateways can interconnect the two systems efficiently.
This compatibility greatly simplified international telecommunications.
What Is a SONET or SDH Frame?
Information is transmitted in fixed-length frames that repeat continuously.
Each frame contains:
- user information;
- synchronization data;
- network management information; and
- maintenance overhead.
Because every frame has the same well-defined structure, network equipment can process traffic rapidly and reliably.
Why Is the Overhead Important?
Unlike earlier transmission systems, SONET and SDH include extensive management information.
The overhead supports functions such as:
- synchronization;
- fault detection;
- performance monitoring;
- network management; and
- maintenance.
These capabilities allow operators to monitor network performance continuously and identify faults rapidly.
What Are Add-Drop Multiplexers?
One of the major innovations introduced by SONET and SDH was the Add-Drop Multiplexer (ADM).
An ADM allows selected communication channels to be:
- removed from a high-speed transmission stream;
- replaced with new channels; and
- retransmitted without fully demultiplexing the entire signal.
This capability greatly simplifies network operation and reduces equipment complexity.
Why Was This Such an Improvement?
Under the older PDH hierarchy, extracting a single low-speed channel often required several complete stages of demultiplexing.
For example, recovering one E1 circuit from a high-speed transmission might require successively demultiplexing through multiple intermediate levels. SONET and SDH largely eliminate this inefficiency.
Individual channels can often be accessed directly using add-drop multiplexers, reducing both cost and complexity.
Why Are SONET and SDH Well Suited to Optical Fiber?
Optical fibers provide enormous transmission bandwidth with exceptionally low attenuation.
SONET and SDH were specifically designed to exploit these characteristics. Their synchronized frame structures, standardized transmission rates, and efficient multiplexing techniques make them ideally suited to long-distance optical transmission.
Together, they enabled the rapid expansion of national and international fiber-optic networks.
Were SONET and SDH Used Only for Telephone Calls?
No.
Although originally developed to transport digital telephone traffic, they soon became the transport backbone for many communication services.
These included:
- Internet traffic;
- leased data circuits;
- mobile-phone backhaul;
- television distribution; and
- enterprise networking.
In practice, SONET and SDH became general-purpose digital transport systems.
Are SONET and SDH Still Used Today?
Yes, although their role has evolved.
Many telecommunications operators continue to use SONET and SDH infrastructure because it is:
- extremely reliable;
- highly standardized;
- well understood;
- easy to manage; and
- capable of supporting very high availability.
However, much new network construction increasingly employs Ethernet-based optical transport technologies and Optical Transport Networks (OTN), particularly for carrying packet-switched Internet traffic.
Did SONET and SDH Influence Modern Networks?
Very much so.
Many concepts introduced by SONET and SDH remain central to modern optical networking.
These include:
- standardized frame structures;
- synchronized transmission;
- comprehensive network management;
- rapid fault recovery; and
- high-capacity optical transport.
Even where newer transport technologies have replaced SONET and SDH, many of their design principles continue to be used.
Why Are SONET and SDH Important?
SONET and SDH transformed global telecommunications by replacing numerous incompatible digital hierarchies with internationally standardized synchronous transport systems. They dramatically simplified multiplexing, improved reliability, and enabled optical-fiber networks to carry vast quantities of information over continental and intercontinental distances.
For several decades they formed the backbone of the world's telecommunications infrastructure, supporting everything from international telephone calls to Internet traffic. Although networking technology continues to evolve, their influence on modern communication systems remains profound.
Summary
SONET and SDH are standardized synchronous digital transmission systems developed to overcome the limitations of earlier plesiochronous digital hierarchies. By employing precise synchronization, standardized frame structures, and efficient multiplexing techniques, they greatly simplified high-capacity digital transport over optical-fiber networks.
These technologies became the foundation of modern telecommunications backbones, enabling reliable, flexible, and internationally compatible transmission of voice, data, video, and Internet traffic at rates ranging from tens of megabits per second to many gigabits per second.
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