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Synchronous Digital Hierarchy (SDH orSONET) Mercury Communications Ltd. - August 1992

SDH, the great survivor, 2007

2007 network writingsMy TechnologyInside blog

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 - homecomputing 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 doesnot 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 today's

plesiochronous transmission systems used for high speed backbone links will be pushedaside 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 fromanalogue 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 today's single-mode fibre-

optics links. This first issue of Technology Watch looks at synchronous digital transmissionand 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 willinclude:

Frame relay

SMDS (Switched Multi-Megabit Data Service) ATM (asynchronous transfer mode) High speed LAN services such as FDDI

Figure 1 shows the relationship between these technologies and services.

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Figure 1 - The Relationship Between Services

OverviewThe use of synchronous digital transmission by PTOs in their backbone fibre-optic and radionetwork 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 ofsynchronous 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 thefull benefits of software control in the form of flexibility and introduction of networkmanagement.

In many respects, the benefits to the PTO will be the same as those brought to theelectronics industry when hard wired logic was replaced by the microprocessor. As with that

revolution, synchronous digital transmission will not take hold overnight, but deploymentwill 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

technology's early uptake by many operators including BT. Only later will customers directlybenefit with the introduction of new services such as connectionless LAN-to-LANtransmission capability.

According to one market research company it will take until the mid or late 1990s 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 changeby any standard (Figure 2).

Users who extensively use PCs and workstations with LANs, graphic layout, CAD and remotedatabase applications are now looking to the telecommunication service suppliers to providethe 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 transferinformation to other metropolitan and international sites as easily and as quickly as theycan to a colleague sitting at the next desk.

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Figure 2 - European Revenue Growth of Transmission Equipment

Plesiochronous Transmission.Digital data and voice transmission is based on a 2.048Mbit/s bearer consisting of 30 time

division multiplexed (TDM) voice channels, each running at 64Kbps (known as E1 and

described by the CCITT G.703 specification). At the E1 level, timing is controlled to anaccuracy 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 bemultiplexed together to provide increased capacity. During this time rates have increased

through 8, 34, 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 noweven this is insufficient.

Unlike E1 2.048Mbit/s bearers, higher rate bearers in the hierarchy are operatedplesiochronously, with tolerances on an absolute bit-rate ranging from 30ppm (parts per

million) at 8Mbit/s to 15ppm at 140Mbit/s. Multiplexing such bearers (known as tributariesin SDH speak) to a higher aggregate rate (e.g. 4 x 8Mbit/s to 1 x 34Mbit/s) requires thepadding 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 nowoften referred to as plesiochronous digital hierarchy (PDH).

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Figure 3 - A typical Plesiochronous Drop & Insert

Because of the large investment in earlier generations of plesiochronous transmissionequipment, each step increase in capacity has necessitated maintaining compatibility with

what was already installed by adding yet another layer of multiplexing. This has created thesituation 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 beidentified in a PDH hierarchy, let alone extracted, without fully demultiplexing down to theE1 level again as shown in Figure 3.

The limitations of PDS multiplexing are:

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

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

Incompatibilities at the optical interfaces of two different suppliers can cause majorsystem integration problems.

To add or drop an individualchannel or add a lower rate branch to a backbone link acomplete hierarchy of MUXs is required as shown in figure 3.

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

Synchronous TransmissionIn the USA in the early 1980s, it was clear that a new standard was required to overcome

the limitations presented by PDH networks, so the ANSI (American National StandardsInstitute) SONET (synchronous optical network) standard was born in 1984. By 1988,collaboration between ANSI and CCITT produced an international standard, a superset ofSONET, 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-3 plesiochronous payload to

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maintain backwards compatibility with PDH. Higher data rates are multiples of this up toSTS-48, which is 2,488Gbit/s.

SDH is based on an STM-1 (155.52Mbit/s) rate, which is identical to the SONET STS-3 rate.Some higher bearer rates coincide with SONET rates such as: STS-12 and STM-4 =

622Mbit/s, and STS-48 and STM-16 = 2.488Gbit/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 andMAN 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 withPDH systems.

Further, all SDH equipment is software controlled, even down to the individual chip, allowingcentralised management of the network configuration, and largely obviates the need for

plugs and sockets. A future SDH network could look like Figure 4.

Figure 4- An Example Future SDH Digital Network

Benefits of SDH TransmissionSDH transmission systems have many benefits over PDH:

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

Survivability. With SDH, ring networks become practicable and their use enablesautomatic 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 withoutfull hierarchical multiplexing or de-multiplexing. A single E1 2.048Mbit/s tail can be

dropped or inserted with relative ease even on Gbit/s links.

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Standardisation enables the interconnection of equipment from different suppliersthrough 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 transfermode), of broad-band transmission, otherwise known as B-ISDN or the precursor of

this service in the form of Switched Multimegabit Data Service, (SMDS).

ConclusionsThe introduction of synchronous digital transmission in the form of SDH will eventuallyrevolutionise 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 1992 throughout the world prior tointroduction in the 1993 - 1995 time scale.

There is still a lack of understanding of the ramifications of the introduction of SDH withintelecommunications operations. In practice, the use of extensive software control will

impact positively all parts of the business. It is not so much a question ofwhetherthetechnology will be taken up, but when.

Introduction of SDH will lead to the availability of many new broad-band data servicesproviding users with increased flexibility. It is in this area where confusion reigns withpotential 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 forceequipment 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 ratherthan hardware. Such was the situation in the computer industry in the early 1980s. Notleast, 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 veryinfrastructure of the world's core communications networks. But it must not be forgottenthat there are still many issues to be resolved.

The benefits to be gained in terms of improving operator profitability, and helping them tocompete in the new markets of the 1990s, are so high that deployment of SDH is just a

question of time.