Guest Writer: Roy Kruger

Synopsis
The synchronous transmission standards, known as SDH (Synchronous Digital Hierarchy) in Europe/ITU and SONET (Synchronous Optical NETwork) in North America, offer significant benefits when compared to the older plesiochronous or asynchronous transmission specifications. Not only do they improve the capabilities of transmission products, they also provide a basis on which to build next-generation network infrastructure. This is particularly true when the SDH/SONET standards are coupled with the emerging standards supporting multivendor high speed copper and synchronous fibre transmission interfaces to digital switches.

Let us look at how the capabilities of SDH/SONET and the new generation of Multiservice Digital Switches can be applied to address the needs of carrier and overlay service network providers.

Market Drivers
During the last few years, global deregulation has encouraged a host of new and potential alternative carrier and overlay network service providers in many developing and developed telecommunication markets. Furthermore, in these newly deregulated markets, the presence of these emerging carriers is provoking traditional PTTs to respond competitively to the demand for advanced service overlay networks, and the pressure to serve the market need is mounting.

Consequently, the opportunities and challenges facing the emerging carriers and overlay network providers are enormous. For instance, how should they scale their capital investment to revenue growth? Which services should they offer? How can they build market-place confidence – particularly in the premium business market? How can they best assess the impact of the demand for emerging broadband services? Furthermore, hiring and training skilled personnel to deliver and operate high quality services from “Day One” will not be an easy exercise, and a poor entry record could irrevocably damage their acceptance in the market place.

The ideal network to suit this situation is highly flexible and highly scaleable – flexible so that a mix of services and bandwidth can be provided quickly and easily in response to market demand; and scaleable so that start-up investment is kept to a minimum, yet incremental capacity can easily be added as needed.

The Synchronous Digital Hierarchy
At this point it is worth considering in more detail the synchronous optical transmission standards underlying this ideal network. SDH is the name given to the set of European/ITU standards for synchronous transmission. A similar set, called SONET and defined by the North American standards body ANSI, applies in North America, and certain Asian and South American countries.

The Principles
The above standards define a synchronous fibre optic network capable of accepting plesiochronous electrical tributary signals (called “asynchronous” signals in North America) and carrying them through the network in the form of “payloads.” To accommodate the difference in clock rates between the synchronous fibre optic network and the plesiochronous electrical network, the content of the payload, known as the “tributary,” is allowed to “float.” When the tributary needs to be extracted for termination at a network destination, a way must be found of pinpointing its exact location within the payload. This is done by attaching pointer tables to the payload in the form of overhead bytes which identify the first byte of the tributary. The concept of using pointers and payloads in a higher bit-rate signal allows individual lower bit-rate signals to be inserted or extracted without having to demultiplex the entire higher-rate signal.

The lowest rate SDH signal is STM-1 (Synchronous Transport Module) and is defined at 155.52 Mbps, including payload and overhead. Higher rate signals are exact multiples of the STM-1 signal and operate at the following rates:
STM-4 622.08 Mbps
STM-16 2488.32 Mbps
STM-64 10Gbps
STM-256 40Gbps

Each signal has the same structure: payloads carrying floating tributary traffic, and an overhead section carrying pointers and management information. The overhead section also provides information channels over which SDH-compliant network elements can communicate throughout the network. As a result, the network elements can exchange maintenance information such as alarms, error rates, and protection switching, and widespread network management and control can be exercised from a single point.

Each Synchronous Transport Module contains a number of elements broken up as follows:
Container (C) - A Container is a defined unit of payload capacity, for carrying plesiochronous tributaries.

Virtual Container (VC) - A Virtual Container comprises a single container, including system information (Path Overhead - POH).

Administrative Unit (AU) – An Administrative Unit comprises a VC together with a payload pointer.

Administrative Unit Group (AUG) - An Administrative Unit Group is an assembly of one or more multiplexed AU’s.

Synchronous Transport Module (STM) - A Synchronous Transport Module comprises AUG’s together with system information (Section Overhead - SOH)

For example four AUG’s can be multiplexed into a STM-4, which has a bit rate of 622.08 Mbit/s and 16 AUG’s can be multiplexed into a STM-16, which has a bit rate of 2.488Gbit/s and similarly 64 AUG’s can be multiplexed into a STM-64, which has a bit rate of 10Gbit/s.

The Benefits
When compared to plesiochronous standards, SDH brings significant new capabilities and greater benefits to the network. These include multivendor inter-working, more flexibility, greater management and control, higher reliability, and increased transmission capacity.

Multivendor Interworking
Both the SDH and SONET standards define multivendor interface specifications which allow different vendor’s’ equipment to be interconnected on the same fibre span. Specifications for managing multiple vendors’ equipment are also included in these standards.

Flexibility
Plesiochronous equipment requires transmission signals to be demultiplexed to access lower-rate signals, so back-to-back multiplexers must be introduced to add and drop the signals as they join and leave the transmission network. To keep equipment costs to a minimum, network designers have tended to avoid this solution, only to complicate the network engineering rules to such an extent that service responsiveness to end-users became limited. With the advent of SDH-compliant products, network designers can now reach into any multiplexed signal and extract only the information they need, thereby ridding themselves of considerable equipment costs.

Management and Control
The increased flexibility inherent in the SDH network and the greater control available through the overhead maintenance channels empower the network operator to respond immediately to customer service needs. The software-intensive design of these SDH/SONET-based products also means that network operators are able to communicate with individual terminals from a centralised location to obtain status reports on performance.

Furthermore, if the SDH capabilities are extended to the switching and access portion of the network (including Fibre-in-the-Loop products), a network-wide SDH/SONET network management infrastructure can be created to support integrated management on an end-to-end basis across all network elements.

Reliability
Since an SDH terminal performs several levels of multiplexing and optical conversion all in one unit, the number of network elements required is greatly reduced, so the network itself is greatly simplified. If a simplified network is combined with a sophisticated control capability, and the intelligence in each terminal is able to report on its ”health” and correct its own faults as necessary, the network then becomes significantly more reliable.

The intelligent SDH network elements are capable of recognising loss of signal and can reroute traffic in tens of milliseconds. As a result, “self-healing” ring structures can be built to guarantee uninterrupted service, even in the event of a fibre cable cut.

Transmission Capacity
SDH currently defines rates up to 40 Gbps capacity - far greater than that of plesiochronous products (565 Mbps). But even 40 Gbps is not the limit, since the standard could support as much as 80 Gbps on a single-mode fibre cable. Therefore service on the network need no longer be restricted by bandwidth bottlenecks. (And of course Dense Wave Division Multiplxers (DWDM’s) can be added to increase bandwidth even more).

Applying SDH in the Network
Before all the benefits of SDH can be fully realised, a vision of the ideal network must first be defined. Then step by step, as product deployment decisions are made, that same vision should be used as a guide. It is important not to limit the vision to transmission planners only, but to throw it open so that everyone involved in planning the network infrastructure and building the network’s future may share it.

The Ideal Network
An ideal network would be completely fibre-based with all business and residential connections made on fibre. Active or passive components could be used to taper the fibre count from a large number of users down toward the serving central office. Or instead, users could be connected to large fibre rings - a solution which offers the advantage of survivability in the case of cable cuts.

Since bandwidth in this ideal network can now be managed by software because of the “payload-independent” nature of SDH, the fibre optic cable can be connected directly to the switch. There is no longer any need for patch panels or cross connects where the fibre enters the central office because, throughout the entire network all bandwidth assignment and capacity grooming is performed at each individual network element. Nor is it necessary, for management purposes, to segregate traffic in the switch according to service type. All the fibres connect to one switch capable of handling all the services whether they are analogue or digital voice, data or video; narrowband, wideband or broadband; connection oriented or connectionless.

Traffic needing to pass to the network does so on very high capacity fibres – again, preferably, configured as rings for survivability. At this point, service provisioning becomes no more than a keyboard entry that is immediately interpreted and transmitted to all network elements, resulting in instant response to the end user.
A network infrastructure of this nature:
• offers gigabits of bandwidth on demand;
• switches all types of traffic from Plain Old Telephone Services (POTS) to new multimedia broadband services;
• simplifies the network by reducing the number of physical interfaces and network elements;
• offers virtual 100 percent survivability from equipment failures and cable cuts;
• gives all businesses, whether large or small, access to a full range of advanced services from narrowband through broadband; and
• is based on open architectures, standard protocols and multivendor connectivity.
Conclusion
With its virtually unlimited bandwidth capabilities SDH will remain the choice architecture for high speed data transmission for many years to come.

Kruger is Managing Director, BCX Networks Ltd., Nigeria, a subsidiary of Business Connexion (Pty) Ltd., South Africa. He can be reached at: roykruger_2@yahoo.com