Synchronous Digital Hierarchy

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The introduction of any new technology is usually preceded by much hyperbole and rhetoric. In many cases, the revolution predicted never gets beyond this. In many more, it never achieves the wildly over optimistic growth forecasted by market specialists - home computing and the paperless office to name but two. It is fair to say, however, by whatever method you use to evaluate a new technology, that synchronous digital transmission does not fall into this category. The fundamental benefits to be gained from its deployment by PTOs seem to be so overwhelming that, bar a catastrophe, the bulk of todays plesiochronous transmission systems used for high speed backbone links will be pushed aside in the next few years. To quote Dataquest, It has been claimed by many industry experts that the impact of synchronous technology will equal that of the transition from analogue to digital technology or from copper to fibre optic based transmission.

For the first time in telecommunications history there will be a world-wide, uniform and seamless transmission standard for service delivery. Synchronous digital hierarchy (SDH) provides the capability to send data at multi-gigabit rates over todays single-mode fibre-optics links. This first issue of Technology Watch looks at synchronous digital transmission and evaluates its potential impact. Following issues of TW will look at customer oriented broad-band services that will ride on the back of SDH deployment by PTOs. These will include

· Frame relay

· SMDS (Switched Multi-Megabit Data Service)

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· ATM (asynchronous transfer mode)

· High speed LAN services such as FDDI

Overview

The use of synchronous digital transmission by PTOs in their backbone fibre-optic and radio network will put in place the enabling technology that will support many new broad-band data services demanded by the new breed of computer user. However, the deployment of synchronous digital transmission is not only concerned with the provision of high-speed gigabit networks. It has as much to do with simplifying access to links and with bringing the full benefits of software control in the form of flexibility and introduction of network management.

In many respects, the benefits to the PTO will be the same as those brought to the electronics industry when hard wired logic was replaced by the microprocessor. As with that revolution, synchronous digital transmission will not take hold overnight, but deployment will be spread over a decade, with the technology first appearing on new backbone links. The first to feel the benefits will be the PTOs themselves, as demonstrated by the technologys early uptake by many operators including BT. Only later will customers directly benefit with the introduction of new services such as connectionless LAN-to-LAN transmission capability.

According to one market research company it will take until the mid or late 10s before 70% of revenue for network equipment manufacturers will be derived from synchronous systems. Remembering that this is a multi-billion $ market, this constitutes a radical change by any standard Users who extensively use PCs and workstations with LANs, graphic layout, CAD and remote database applications are now looking to the telecommunication service suppliers to provide the means of interlinking these now powerful machines at data rates commensurable with those achieved by their own in-house LANs. They also want to be able to transfer information to other metropolitan and international sites as easily and as quickly as they can to a colleague sitting at the next desk.

Plesiochronous Transmission.

Digital data and voice transmission is based on a .048Mbit/s bearer consisting of 0 time division multiplexed (TDM) voice channels, each running at 64Kbps (known as E1 and described by the CCITT G.70 specification). At the E1 level, timing is controlled to an accuracy of 1 in 1011 by synchronising to a master Caesium clock. Increasing traffic over the past decade has demanded that more and more of these basic E1 bearers be multiplexed together to provide increased capacity. During this time rates have increased through 8, 4, and 140Mbit/s. The highest capacity commonly encountered today for inter-city fibre optic links is 565Mbit/s, with each link carrying 7,680 base channels, and now even this is insufficient.

Unlike E1 .048Mbit/s bearers, higher rate bearers in the hierarchy are operated plesiochronously, with tolerances on an absolute bit-rate ranging from 0ppm (parts per million) at 8Mbit/s to 15ppm at 140Mbit/s. Multiplexing such bearers (known as tributaries in SDH speak) to a higher aggregate rate (e.g. 4 x 8Mbit/s to 1 x 4Mbit/s) requires the padding of each tributary by adding bits such that their combined rate together with the addition of control bits matches the final aggregate rate. Plesiochronous transmission is now often referred to as plesiochronous digital hierarchy (PDH).

Because of the large investment in earlier generations of plesiochronous transmission equipment, each step increase in capacity has necessitated maintaining compatibility with what was already installed by adding yet another layer of multiplexing. This has created the situation where each data link has a rigid physical and electrical multiplexing hierarchy at either end. Once multiplexed, there is no simple way an individual E1 bearer can be identified in a PDH hierarchy, let alone extracted, without fully demultiplexing down to the E1 level again as shown in Figure .

The limitations of PDS multiplexing are

· A hierarchy of multiplexers at either end of the link can lead to reduced reliability and resilience, minimum flexibility, long reconfiguration turn-around times, large equipment volume, and high capital-equipment and maintenance costs.

· PDH links are generally limited to point-to-point configurations with full demultiplexing at each switching or cross connect node.

· Incompatibilities at the optical interfaces of two different suppliers can cause major system integration problems.

· To add or drop an individual channel or add a lower rate branch to a backbone link a complete hierarchy of MUXs is required as shown in figure .

· Because of these limitations of PDH, the introduction of an acceptable world-wide synchronous transmission standard called SDH is welcomed by all.

Synchronous Transmission

In the USA in the early 180s, it was clear that a new standard was required to overcome the limitations presented by PDH networks, so the ANSI (American National Standards Institute) SONET (synchronous optical network) standard was born in 184. By 188, collaboration between ANSI and CCITT produced an international standard, a superset of SONET, called synchronous digital hierarchy (SDH).

US SONET standards are based on STS-1 (synchronous transport signal) equivalent to 51.84Mbit/s. When encoded and modulated onto a fibre optic carrier STS-1 is known as OC-1. This particular rate was chosen to accommodate a US T- plesiochronous payload to maintain backwards compatibility with PDH. Higher data rates are multiples of this up to STS-48, which is ,488Gbit/s.

SDH is based on an STM-1 (155.5Mbit/s) rate, which is identical to the SONET STS- rate. Some higher bearer rates coincide with SONET rates such as STS-1 and STM-4 = 6Mbit/s, and STS-48 and STM-16 = .488Gbit/s. Mercury is currently trialing STM-1 and STM-16 rate equipment.

SDH supports the transmission of all PDH payloads, other than 8Mbit/s, and ATM, SMDS and MAN data. Most importantly, because each type of payload is transmitted in containers synchronous with the STM-1 frame, selected payloads may be inserted or extracted from the STM-1 or STM-N aggregate without the need to fully hierarchically de-multiplex as with PDH systems.

Further, all SDH equipment is software controlled, even down to the individual chip, allowing centralised management of the network configuration, and largely obviates the need for plugs and socketsBenefits of SDH Transmission

SDH transmission systems have many benefits over PDH

· Software Control allows extensive use of intelligent network management software for high flexibility, fast and easy re-configurability, and efficient network management.

· Survivability. With SDH, ring networks become practicable and their use enables automatic reconfiguration and traffic rerouting when a link is damaged. End-to-end monitoring will allow full management and maintenance of the whole network.

· Efficient drop and insert. SDH allows simple and efficient cross-connect without full hierarchical multiplexing or de-multiplexing. A single E1 .048Mbit/s tail can be dropped or inserted with relative ease even on Gbit/s links.

· Standardisation enables the interconnection of equipment from different suppliers through support of common digital and optical standards and interfaces.

· Robustness and resilience of installed networks is increased.

· Equipment size and operating costs are reduced by removing the need for banks of multiplexers and de-multiplexers. Follow-on maintenance costs are also reduced.

· Backwards compatibly will enable SDH links to support PDH traffic.

· Future proof. SDH forms the basis, in partnership with ATM (asynchronous transfer mode), of broad-band transmission, otherwise known as B-ISDN or the precursor of this service in the form of Switched Multimegabit Data Service, (SMDS).

Conclusions

The introduction of synchronous digital transmission in the form of SDH will eventually revolutionise all aspects of public data communication from individual leased lines through to trunk networks. Because of the state-of-the-art nature of SDH and SONET technology, there are extensive field trials taking place in 1 throughout the world prior to introduction in the 1 - 15 time scale.

There is still a lack of understanding of the ramifications of the introduction of SDH within telecommunications operations. In practice, the use of extensive software control will impact positively all parts of the business. It is not so much a question of whether the technology will be taken up, but when.

Introduction of SDH will lead to the availability of many new broad-band data services providing users with increased flexibility. It is in this area where confusion reigns with potential technologies vying for supremacy. These will be discussed in future issues of Technology Watch.

Importantly for PTOs, SDH will bring about more competition between equipment suppliers designing essentially to a common standard. One practical effect could be to force equipment prices down, brought about by the larger volumes engendered by access to world rather than local markets. At least one manufacturer is currently stating that they will be spending up to 80% of their SDH development budgets on management software rather than hardware. Such was the situation in the computer industry in the early 180s. Not least, it will have a great impact on such issues as staffing levels and required personal skills of personnel within PTOs.

SDH deployment will take a great deal of investment and effort since it replaces the very infrastructure of the worlds core communications networks. But it must not be forgotten that there are still many issues to be resolved.

The benefits to be gained in terms of improving operator profitability, and helping them to compete in the new markets of the 10s, are so high that deployment of SDH is just a question of time.

Benefits of a Synchronous Digital Hierarchy

Synchronous transmission overcomes the limitations experienced in a plesiochronous network. It allows the network to evolve to meet the new demands being placed upon it. Synchronous offers a number of benefits, both to telecoms, network operators, and to end users.

NETWORK SIMPLIFICATION

One of the main benefits seen by a network operator is the network simplification brought about through the use of synchronous equipment. A single synchronous multiplexer can perform the function of an entire plesiochronous multiplexer mountain, leading to significant reductions in the amount of equipment used. Lower operation costs will also result due to the reduction in required spare inventory, simplified maintenance, the reduction in floor space required by equipment and lower power consumptions. The more efficient drop and insert of channels offered by an SDH network, together with its powerful network management capabilities, will lead to greater ease in provisioning of high bandwidth lines for new multimedia services, as well as ubiquitous access to those services. Thus, the simplification of the network, and the new flexibility this brings, opens up the potential for the network operator to generate new revenues.

SURVIVABILITY

The deployment of optical fiber throughout the network and adoption of the SDH network elements makes end to end monitoring and maintenance of network integrity a possibility. The network management capability of the synchronous network will enable immediate identification of link and node failure. Using self-healing ring architectures, the network will be automatically reconfigured with traffic instantly rerouted until the faulty equipment has been repaired.

Thus, failures in the network transport mechanism will be invisible on an end to end basis. Such failures will not disrupt services, allowing network operators to commit to extremely high availability of service figures, and guarantee high levels of network performance.

SOFTWARE CONTROL

Provision of network management channels within the SDH frame structure means that a synchronous network will be fully software controllable. Network management systems will not only perform traditional event management functions such as dealing with alarms in the network, but will also provide a host of other functions, like performance monitoring, configuration management, resource management, network security, inventory management, and network planning and design.

The possibility of remote provisioning and centralized maintenance will result in a great savings in time spent by maintenance personnel in travelling to remote sites, and this of course corresponds to a reduction of expenses.

BANDWIDTH ON DEMAND

In a synchronous network it will be possible to dynamically allocate network capacity, or bandwidth, on demand. Users anywhere within the network will be able to subscribe at very short notice to any service offered over the network, some of which may require large amounts of bandwidth. An example of this is dial-up video-conferencing. Users will be able to obtain the required bandwidth for a video-conferencing link just by dialing the appropriate number, as opposed to the current situation where video-conferencing links must reserved days in advance.

Many other new services become possible in a synchronous network . These will represent new sources of revenue for network operators, and provide increased conveniences for users. Some examples of such services are high speed packet switched services, LAN interconnection, and High Definition TV (HDTV).

FUTURE-PROOF NETWORKING

The synchronous digital hierarchy offers network operators a future-proof network solution, plus the ability to upgrade software and extensions to existing equipment. They can be confident their investment in equipment is money well spent because synchronous has been selected as the bearer network for the next generation of telecommunication networks, the Broadband ISDN (B-ISDN). Research into implementation of B-ISDN is underway as part of the RACE program in Europe. B-ISDN will enable all users to have access to the network at rates in the order of Mega bits per second.

STANDARDIZATION

The SDH standards mean that for the first time, transmission equipment from different manufacturers can interwork on the same link. The ability to achieve this so-called mid-fiber meet has come about as a result of standards which define fiber-to-fiber interfaces at the physical (photon) level. They determine the optical line rate, wavelength, power levels, pulse shapes and coding. Frame structure, overhead and payload mappings are also defined.

This standardization of equipment and interfaces in SDH means network operators have freedom to choose different equipment from different vendors, and be confident that it will interwork. This means operators can avoid the problems traditionally associated with being locked into proprietary solutions from a single vendor.

SDH standards also facilitate interworking between North American and European transmission hierarchies. Using plesiochronous transmission, this was difficult, due to the different transmission rates used on each side of the Atlantic.

THE IMPACT OF SYNCHRONOUS

In the short term, network operators will adopt synchronous transmission equipment due to its improvement in network quality and its reduction in operating costs compared to plesiochronous transmission. The long term vision of a flexible and efficient network, with full network management facilities will also make this attractive.

Once widespread deployment of synchronous transmission systems occurs, the way in which network are designed, operated, and managed will change completely. Currently, network complexity makes deployment of new services a difficult proposition. In a much simplified synchronous network, service providers will be freed from these complexities. As a result they will be able to offer a rich mixture of services.

Thus, the SDH will eliminate the network complexity which currently restricts the growth of new services. Eventually a truly global telecommunications network will evolve where it will be possible to seamlessly transfer multimedia information almost anywhere at any time.

Limitations of the Plesiochronous Digital Hierarchy

The problem of flexibility in a plesiochronous network is illustrated by considering what a network operator may need to do in order to be able to provide a business customer with a Mbits leased line. If a high speed channel passes near the customer, the operation of providing him with a single Mbits line from within that channel would seem straightforward enough. In practice, however, it is not so simple.

The use of justification bits at each level in the PDH means that identifying the exact location of the frames in a single Mbits line within say a 140 Mbits channel is impossible. In order to access a single Mbits line the 140 Mbits channel must be completely demultiplexed to its 64 constituent Mbits lines via 4 and 8 Mbits. Once the required Mbits line has been identified and extracted, the channels must then be multiplexed back up to 140 Mbits.

Obviously this problem with the drop and insert of channels does not make for very flexible connection patterns or rapid provisioning of services, while the multiplexer mountains required are extremely expensive

Another problem associated with the huge amount of multiplexing equipment in the network is one of control. On its way through the network, a Mbits leased line may have travelled via a number of possible routes. The only way to ensure it follows the correct path is to keep careful records of the interconnection of the equipment. However, as the amount of reconnection activity in the network increases it becomes more difficult to keep records current and the possibility of mistakes increases. Such mistakes are likely to affect not only the connection being established but also to disrupt existing connections carrying live traffic.

Another limitation of the PDH is its lack of performance monitoring capability. Operators are coming under increasing pressure to provide business customers with improved availability and error performance, and there is insufficient provision for network management within the PDH frame format for them to be able to do this.

The STM-1 Frame

As was explained in the last section an STM-1 frame consists of 40 bytes which can be considered as a structure of 70 columns x lines. The frame is divided into three main sections

Payload Area

AU Pointer Area

Section Overhead Area

PAYLOAD

We have seen previously that signals from all levels of the PDH can be accommodated in a synchronous network by packaging them together in the payload area of an STM-1 frame.

The plesiochronous tributaries are mapped into the appropriate synchronous container, and a single column of nine bytes, known as the Path Overhead (POH), is added to form the relevant Virtual Container (VC). The path overhead provides information for use in end-to-end management of a synchronous path.

The Figure bellow describes VC-4 packaging with VC-4 Path Overhead

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