Sonet and SDH standards push for worldwide interoperability

Dec. 1, 1995

Sonet and SDH standards push for worldwide interoperability

To provide a seamless global communications infrastructure, Sonet and SDH transmission disparities are overcome by dual signal-handling circuits



The need for a seamless global communications network dictates that the telecommunications infrastructure be flexible, reliable, efficient and worldwide compatible. To meet that need, two standard transmission schemes are currently being developed. The synchronous optical network, or Sonet, standard serves North America. The synchronous digital hierarchy, or SDH, standard dominates Europe, Japan and the rest of the world. Both standards provide a robust scheme for transporting all types of voice, video and data services.

Not only do telecommunications networks differ among countries, but they also differ within the same country. For instance, they provide different transmission schemes for voice and data services. North American carriers still widely use transmission schemes based upon the DS-0 rate of 64 kilobits per second for voice and the X.25 packet switch protocol for data. Voice transmission schemes are multiplexed to form higher rate services, such as DS-1 at 1.544 megabits per second and DS-3 at 44.736 Mbits/sec. In addition to different rates, different transmission schemes are generally provided for various services in North America.

European carriers also employ old communications infrastructures that furnish different and incompatible services. The voice channels in Europe use plesiochronous digital hierarchy, or PDH, technology based upon the 64-kbit/sec channel. These signals are multiplexed in formats that are different from the North American DS-1 and DS-3 signals. The most prevalent European signals are based on E1, or 2.048-Mbit/sec, and E4, or 139.264-Mbit/sec, signals.

These European signals are incompatible with North American signals and require conversion when passed across international boundaries. Such conversion requires additional equipment in the networks and incurs signal time delays. As a result, varying transmission schemes complicate a common worldwide communications infrastructure.

Tomorrow`s telecommunications systems must provide diverse features, increased reliability and easier implementation to accommodate the expected variety of user needs. Network planners and providers, therefore, need a standard transmission architecture so that network equipment from different manufacturers will work together seamlessly.

This transmission scheme must support all types of voice, data and video services. and provide improved reliability, remote provisioning and minimum maintenance. Moreover, carriers need a scheme that can be easily expanded to higher rates to meet future demands. The scheme must also cost less than the current architecture.

The Sonet and SDH standard transmission schemes have been designed to satisfy all of these demanding requirements. The aim of these schemes is to allow carriers to purchase equipment from different companies that can communicate with each other, known as mid-span meet. The equipment must accommodate both old and new services such as DS-1 and ATM, respectively. The overhead in both Sonet and SDH provide error monitoring for higher reliability and data channels, order wires and user channels so that carriers can perform extended operations, administration, maintenance and provisioning. The two standards can also be expanded to the highest rates that current equipment technology allow.

Basic specifications

In North America, Sonet has a base rate of 51.84 Mbits/sec and can carry payloads to 48.384 Mbits/sec. The frame structure for this rate is a synchronous transport signal level 1, or STS-1. The payload within STS-1, known as a synchronous payload envelope, has the capacity to carry a DS-3 signal. It can, however, also carry multiple multiplexed lower-rate signals, such as 28 multiplexed DS-1 signals. This payload floats within the STS-1 frame to allow for jitter, wander and frequency differences. Sonet carries higher-rate signals by combining the payloads (concatenating) to provide expandability. It can thus carry fiber distributed data interface, or FDDI; switched multimegabit data service; asynchronous transfer mode, or ATM; and E4/DS-4NA signals.

Sonet overheads are also divided into different parts and terminated in different places to provide increased system resolution and performance. These overheads are divided into section, line, tandem connection and path.

End-to-end connectivity demonstrates how a Sonet signal travels its path to an end destination; the route is known as the path. The signal generally moves through different locations and remote terminals and travels across a line between buildings, which is known as the line layer. As it travels across the line layer, the signal may need to be regenerated if the transmission distance is long. In that case, the layer is divided into different sections termed the section layer.

The tandem connection layer monitors the signal across multiple line layers and can be set up by the carrier. The path layer can also provide more resolution to the lower rate tributaries (such as DS-1), and lower order path overhead can be applied to each of these signals.

The SDH base rate is 155.52 Mbits/sec and can carry payloads to 149.760 Mbits/sec. The basic frame structure for this rate is a synchronous transport module level 1, or STM-1. This transport module has the capacity to carry an E4 signal, but can also carry multiple multiplexed lower-rate signals, such as 63 multiplexed E1 signals. SDH frames can also be concatenated to support higher rate signals, such as for ATM.

Gradual switchover

Of course, North American carriers cannot immediately switch over to the Sonet transmission system. Their vast infrastructure of equipment already in place would take huge amounts of time, resources and money to replace. Therefore, the transition must be done gradually. Consequently, the new and the old systems must be able to communicate with each other. Fortunately, the existing transmission schemes can be mapped into Sonet and SDH to ease the transition.

The transmission equipment must include interfaces for both the old and the new signals. For example, an add/drop multiplexer might have STS-1, STS-3, DS-1 and DS-3 interfaces. Furthermore, digital crossconnects must be able to connect to different interfaces and make several signal conversions.

Sonet and SDH schemes were designed concurrently. They were specifically structured to be compatible, implement the same major features and operation, and incorporate the same base frame structure and similar overhead functionalities, such as mapping features and multiplexing structures for future expandability.

Although Sonet uses less overhead, when the signal is multiplexed to the same STS-3 signal, the frame structure is exactly the same as for SDH. The bytes and the names are the same; the only variances encompass the implementation of a few features.

Once the multiplexing of the Sonet signal is at the STS-3 level, matching the STM-1, then the multiplexing structure is identical to SDH at the higher rates. Even though the initial rate of the SDH signal is 155.52 Mbits/sec, the SDH specification does have a multiplexing structure for the lower rates similar to Sonet. The recommended methodology for SDH multiplexing is to use a virtual container-4, or VC-4, structure; this multiplexing structure, which uses three virtual channel-3, or VC-3, structures, was put into the specification for compatibility.

The majority of overhead functionalities for Sonet and SDH are the same. Minor differences come in the actual implementation of selected functions, and because SDH offers more functionality at the higher and lower path layers. Both Sonet and SDH offer the following:

Identical framing words

Bit interleaved parity at all levels

Trail traces

Order wires

User channels

Data communication channels

Synchronization status

Automatic protection switching

Performance feedback to far end

Signal labels.

Although the signals to be mapped into the transmission schemes are generally different for Sonet and SDH (for example, DS-1 mapping into Sonet versus E1 mapping into SDH, in most cases), mapping for all the signals is the same in both Sonet and SDH. Moreover, the mappings for ATM are also identical in both standards.

Starting at the multiplexed signal rate of 155.52-Mbits/sec, the higher specified rates are identical for both Sonet and SDH. The signals can be multiplexed to as high as current technology can process the signal. The standards are designed so they can be expanded even further, as they are currently synchronously multiplexed by a factor of four each time.

Sonet/SDH differences

Sometimes, issues emerge where the standards committees developing Sonet and SDH differ in interpretation. To handle these issues, the committees try to compromise and find a middle ground that both entities can accept; however, some specifications remain different.

For example, Sonet has added features for which SDH has a different implementation, such as trace identifiers, tandem connection maintenance and bit interleaved parity versus block accumulation of bit interleaved parity errors. The SDH committee has modified these definitions but has inserted the option of using the Sonet specifications.

On the other hand, SDH has added features to its specification that Sonet does not use, such as some lower-order path functions--trail trace, tandem connection maintenance and automatic protection switching--an additional user channel and path automatic protection switching for higher-order path. Sonet has set aside its overhead, which is not to be used for another purpose in the future. Sonet also has a feature that SDH does not use in payload defect indication.

SDH`s signal construction terminology is unlike Sonet in having more defined steps. This representation makes discussing the various steps easier. In addition, because SDH has a higher base rate, the recommended multiplexing scheme to the SDH base rate differs from that of Sonet. The SDH recommended scheme does not have three separate paths at the base level (an STM-1/AU-4 has one path and an STS-3 has three paths). This difference causes vendor complications in partitioning the design and manufacture of integrated circuits and systems to be both Sonet- and SDH-compatible.

However, Sonet and SDH are compatible, and both signal types can be terminated in the same equipment. A possible transition configuration uses an international boundary interface that employs broadband transmission support chips.

By using integrated circuits that are Sonet- and SDH-compatible, a system can be structured efficiently with a minimum amount of devices. Sonet tributary signals are based on STS-1 synchronous payload envelopes in their mappings. SDH`s equivalent AU-3s have the same structure as Sonet`s STS-1 synchronous payload envelope, but should be received only at an international boundary and not transmitted into an SDH environment. Thus, the compatible integrated circuits must be able to support both AU-3 and AU-4 multiplexing structures.

The section, line, tandem connection and path overheads can be terminated in a two-chip solution. The signals can then be switched at an synchronous payload envelope/AU level. In Sonet integrated circuits, the modes are set according to Sonet modes, and the SDH chips receive the international-boundary-switched data. If lower order tributaries need to be switched, then a tributary unit pointer rejustifier must also be used.

Variances in standards result in equipment that must be both Sonet- and SDH- compatible and must carry extra circuitry to handle the differences. Integrated circuits for both switch and transmission equipment are, therefore, more complicated in order to handle both types of signals. However, on the positive side, these complex integrated circuits allow worldwide interoperability (mid-span meet) for communications carriers. They also permit future expansion and flexibility to meet tomorrow`s needs. u

Peter Chadbourne is senior circuit design engineer at National Semiconductor Corp. in South Portland, ME.

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