Synchronous Digital Hierarchy_SDH

8/14/2019 Synchronous Digital Hierarchy_SDH

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Synchronous Digital Hierarchy (SDH or

SONET)

Mercury Communications Ltd. - August 1992

SDH, the great survivor, 20072007 network writings

My TechnologyInside blog

The introduction of any new technology is usually preceded by much hyperboleand rhetoric. In many cases, the revolution predicted never gets beyond this. In

many more, it never achieves the wildly over optimistic growth forecasted bymarket 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 newtechnology, that synchronous digital transmission does not fall into this

category. The fundamental benefits to be gained from its deployment by PTOsseem to be so overwhelming that, bar a catastrophe, the bulk of today's

plesiochronous transmission systems used for high speed backbone links will bepushed aside in the next few years. To quote Dataquest:, "It has been claimed

by many industry experts that the impact of synchronous technology will equalthat 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 today's single-mode fibre-optics links. This first issue of TechnologyWatch looks at synchronous digital transmission and evaluates its potential

impact. Following issues of TW will look at customer oriented broad-bandservices that will ride on the back of SDH deployment by PTOs. These will

include:

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

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 willsupport 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 asmuch to do with simplifying access to links and with bringing the full 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 tothe electronics industry when hard wired logic was replaced by the

microprocessor. As with that revolution, synchronous digital transmission willnot 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 benefitswill be the PTOs themselves, as demonstrated by the technology's early uptake

by many operators including BT. Only later will customers directly benefit withthe 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 late1990s 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 (Figure 2).

Users who extensively use PCs and workstations with LANs, graphic layout, CAD

and remote database applications are now looking to the telecommunicationservice 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


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colleague sitting at the next desk.

Figure 2 - European Revenue Growth of Transmission Equipment

Plesiochronous Transmission.

Digital data and voice transmission is based on a 2.048Mbit/s bearer consistingof 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 an accuracy of 1 in 1011 by synchronising to a master

Caesium clock. Increasing traffic over the past decade has demanded that moreand more of these basic E1 bearers be multiplexed 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 fibreoptic links is 565Mbit/s, with each link carrying 7,680 base channels, and now

even 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 tributaries in SDH speak) to a higher aggregate rate (e.g. 4 x 8Mbit/s

to 1 x 34Mbit/s) requires the padding of each tributary by adding bits such thattheir 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).


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Figure 3 - A typical Plesiochronous Drop & Insert

Because of the large investment in earlier generations of plesiochronoustransmission equipment, each step increase in capacity has necessitated

maintaining compatibility with what was already installed by adding yet anotherlayer of multiplexing. This has created the situation where each data link has a

rigid physical and electrical multiplexing hierarchy at either end. Oncemultiplexed, there is no simple way an individual E1 bearer can be identified 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

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 individualchannel or add a lower rate branch to a

backbone link a complete hierarchy of MUXs is required as shown in

figure 3.

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 1980s, it was clear that a new standard was required toovercome the limitations presented by PDH networks, so the ANSI (American

National Standards Institute) SONET (synchronous optical network) standardwas born in 1984. By 1988, collaboration between ANSI and CCITT produced an


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international standard, a superset of SONET, called synchronous digitalhierarchy (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 toaccommodate a US T-3 plesiochronous payload to maintain backwards

compatibility with PDH. Higher data rates are multiples of this up to STS-48,

which is 2,488Gbit/s.

SDH is based on an STM-1 (155.52Mbit/s) rate, which is identical to the SONETSTS-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 iscurrently trialing STM-1 and STM-16 rate equipment.

SDH supports the transmission of all PDH payloads, other than 8Mbit/s, andATM, SMDS and MAN data. Most importantly, because each type of payload is

transmitted in containers synchronous with the STM-1 frame, selected payloadsmay 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 largelyobviates 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 Transmission

SDH transmission systems have many benefits over PDH:


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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 andmaintenance of the whole network.

Efficient drop and insert. SDH allows simple and efficient cross-

connect without full hierarchical multiplexing or de-multiplexing. A singleE1 2.048Mbit/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 opticalstandards 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, otherwiseknown 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 willeventually 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 in1992 throughout the world prior to introduction in the 1993 - 1995 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 extensivesoftware control will impact positively all parts of the business. It is not so much

a question ofwhetherthe 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 whereconfusion 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 practicaleffect 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 theirSDH development budgets on management software rather than hardware.

Such was the situation in the computer industry in the early 1980s. Not least, it

will have a great impact on such issues as staffing levels and required personal


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skills of personnel within PTOs.

SDH deployment will take a great deal of investment and effort since it replaces

the very infrastructure of the world's core communications networks. But itmust 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 1990s, are so high that

deployment of SDH is just a question of time.

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Networks Part 6: SDH, the great

survivorMarch 2007

When I first wrote about Synchronous Digital Hierarchy (SDH) and

SONET (SDH is the European version SONET) back in 1992, it was seento be truly transformational for the network service provider industry. It

marked a clear boundary from just continually enhancing an oldasynchronous technology belatedly called Plesiochronous Digital

Hierarchy (PDH) to a new approach that could better utilise andmanage the ever increasing bandwidths then becoming available

through the use of optical fibre. An up-to-date overview of SDH /SONET technology can be found in Wikipedia.

SONET was initially developed in the USA and adapted to the rest of

world a little later which called SDH. This was needed as the rest of the

world used different data rates to those used in the USA - this latercaused interesting inter-connect issues when connecting SONET to SDH

networks. For the sake of this post, I will only use the term SDH fromnow on as, by installation base, SDH far outweighs SONET.

Probably even more amazing was that when it was launched, followingmany years of standardisation efforts, it was widely predicted that

along with ATM it would become a major transmission technology. Ithas achieved just that. Although ATM hit the end stop pretty quickly

and the dominance of IP was unforeseen at that time, SDH and SONETwent on to be deployed by almost all carriers that offered traditional

Public Switched Telephone Network (PSTN ) voice services.

The benefits that were used to justify rollout of synchronous networking

at the time pretty much panned out in practice.

Clock rates tightly synchronised within a network through the

use of atomic clocks Synchronisation enabled easier network inter-connect between

carriers Considerably simplified and reduced costs of extracting low data
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Making SDH, DWDM packet friendlyMarch 2007

Back in 1993, I wrote about the advances taking pace in fiber optic

technologies and optical amplifiers. At that time, technology

development was principally concerned with improving transmissiondistances using optical amplifier technology and increasing data rates.

These optical cables a single wavelength and hence provided provided asingle data channel.

Wide area traffic in the early 1990s was principally dominated by Public

Switched Telephone Network (PSTN) telephony traffic as this was wellbefore the explosion in data traffic caused by the Internet. When additional

throughput was required, it was relatively simple to lay down additionalfibres in a terrestrial environment. Indeed, this became standard procedure

to the extent that many fibres were laid in a single pipe with only a fewbeing used or lit as it was known. Unlit fibre strands were called dark

fibre. For terrestrial networks when increasing traffic demanded additionalbandwidth on a link, it was simple job to simply add additional ports the

appropriate SDH equipment and light up an additional dark fibre.
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Wave Division Multiplexing (Picture credit:

photeon)

In undersea cables adding additional fibres tosupport traffic growth was not so easy so the

concept ofWave Division Multiplexing(WDM) came into common usage for point to

point links (the laboratory development of

WDM actually went back to the 1970s). Theuse of WDM enabled transoceanic carriers to upgrade the bandwidths of

their undersea cables without the need to lay additional cables which would

cost multiple billions of Dollars.

As shown in the picture, a WDM based

system uses multiple wavelengths thusmultiplying the available bandwidth by the

number wavelengths that could be

supported. The number of wavelengthsthat could be used and the data rate oneach wavelength were limited by the

quality of the optical fibre that was beingupgraded and the current state-of-the-art

of the optical termination electronics.Multiplexers and de-multiplexers at either

end of the cable aggregated and split thecombined data into separate channels by

converted to and from electrical signals.

A number ofWDM technologies or architectures were standardised overtime. In the early days, Course Wavelength Division Multiplexing(CWDM) was relatively proprietary in nature and meant different things to

different companies. CWDM combines up to 16 wavelengths onto a singlefibre and uses an ITU standard 20nm spacing between the wavelengths of

1310nm to 1610nm. With CWDM technology, since the wavelengths arerelatively far apart compared to DWDM, the are generally relatively cheap.

One of the major issues at the time was that Erbium Doped FibreAmplifiers (EDFAs) as described in optical amplifierscould not be utilised

due to the wavelengths selected or the frequency stability required to beable de-multiplex the multiplexed signals

In the late 1990s there was an explosion of development activity aimed at

deriving benefit of the concept of Dense Wavelength DivisionMultiplexing (DWDM) to be able to utilise EDFA amplifiers that operatedin 1550nm window. EDFAs will amplify any number of wavelengths

modulated at any data rate as long as they are within its amplificationbandwidth.

DWDM combines up to 64 wavelengths onto a single fibre and uses an ITUstandard that specifies 100GHz or 200GHz spacing between the

wavelengths, arranged in several bands around 1500-1600nm. With DWDM
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technology, the wavelengths are close together than used in CWDM,resulting in the multiplexing equipment being more complex and expensive

than CWDM. However, DWDM allowed a much higher density ofwavelengths and enabled longer distances to be covered through the use

of EDFAs. DWDM systems were developed that could deliver tens of

Terabits of data over a single fibre using up to 40 or 80 simultaneouswavelengths e.g. Lucent 1998.

I wouldn't claim to be an expert in the subject, but I would expect that indense urban environments or over longer runs where access is available to

the fibre runs, it is considerably cheaper to install additional runs of fibre

than to install expensive DWDM systems. An exception to this would be acarrier installing cables across a continent. If dark fibre is available then

it's an even simpler decision.

Although considerable advances were taking place at optical transport with

the advent of DWDM systems, existingSONET and SDH standards of the

time were limited to working with a single wavelength per fibre and werealso limited to working with single optical links in the physical layer. SDHcould cope with astounding data rates on a single wavelengths, but could

not be used with emerging DWDM optical equipment.

Optical Transport Hierarchy

This major deficiency in SDH / SONET led to further standards

development initiatives to bring it "up to date". These are known as the

Optical Transport Network (OTN) working in an Optical TransportHierarchy (OTH) world. OTH is the same nomenclature as used for PDH

and SDH networks.

The ITU-T G.709 (released between 1999 - 2003) standard Interfaces

for the OTN is a standardised set of methods for transporting wavelengthsin a DWDM optical network that allows the use of completely optical

switches known as Optical Cross Connects that does not requireexpensive optical-electrical-optical conversions. In effect G.709 provides a

service abstraction layer between services such as standard SDH, IP,MPLS or Ethernet and the physical DWDM optical transport layer. This

capability is also known as OTN/WDH in a similar way that the term

IP/MPLS is used. Optical signals with bit rates of 2.5, 10, and 40Gbits/s were standardised in G.709 (G.709 overviewpresentation) (G.709 tutorial).

The functionality added to SDH in G.709 is:

Management of optical channels in the optical domain Forward error correction (FEC) to improve error performance

and enable longer optical spans Provides standard methods for managing end to end optical

wavelengths
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Other SDH extensions to bring SDH up to date and make it 'packet

friendly'

Almost in parallel with the development of G.709 standards a number ofother extensions were made to SDH to make it more packet friendly.

Generic Framing Procedure (GFP): The ITU, ANSI, and IETF have

specified standards for transporting various services such as IP, ATM and

Ethernet over SONET/SDH networks. GFP is a protocol for encapsulatingpackets over SONET/SDH networks.

Virtual Concatenation (VCAT): Packets in data traffic such as Packetover SONET (POS) are concatenated into larger SONET / SDH payloads to

transport them more efficiently.

Link Capacity Adjustment Scheme (LCAS): When customers' needs for

capacity change, they want the change to occur without any disruption in

the service. LCAS a VCAT control mechanism, provides this capability.

These standards have helped SDH / SONET to adapt to an IP or Ethernet

packet based world which was missing in the original protocol standards ofthe early 1990s.

Next Generation SDH (NG-SDH)

If a SONET or SDH network is deployed with all the extensions that make itpacket friendly is deployed it is commonly called a Next Generation SDH

(NG-SDH). The diagram below, shows the different ages of SDHconcluding in the latest ITU standards work called T-MPLS ( I cover T-MPLS

in: PBT - PBB-TE or will it be T-MPLS?
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Transport Ages (Picture credit: TPACK)

Multiservice provisioning platform (MSPP)

Another term in widespread use with advanced optical networks is MSPP.

SONET / SDH equipment use what are known as add / drop

multiplexers (ADMs) to insert or extract data from an optical link.Technology improvements enabled ADMs to include cross-connect

functionality to manage multiple fibre rings and DWDM in a single chassis.These new devices replaced multiple legacy ADMs and also allow

connections directly from Ethernet LANs to a service provider's opticalbackbone. This capability was a real benefit to Metro networks sitting

between enterprise LANs and long distance carriers.

There are many variant acronyms in use as there are equipment vendors:

Multiservice provisioning platform (MSPP): includes SDHmultiplexing, sometimes with add-drop, plus Ethernet ports,

sometimes packet multiplexing and switching, sometimes WDM. Multiservice switching platform (MSSP): an MSPP with a large

capacity for TDM switching. Multiservice transport node (MSTN): an MSPP with feature-rich

packet switching. Multiservice access node (MSAN): an MSPP designed for

customer access, largely via copper pairs carrying Digital-SubscriberLine (DSL) services.

Optical edge device (OED): an MSSP with no WDM functions.This has been an interesting post in that it has brought together many ofthe technologies and protocols discussed in the previous posts, inparticular SDH, Ethernet and MPLS and joined them to optical networks. It

seem strange to say on one hand that the main justification of deployingconverged Next Generation Networks (NGNs) based on IP is to simplify

existing networks and hence reduce costs, but then consider thecomplexity and plethora of acronyms and standards associated with that!

I think there is only one area that I have not touched upon and that is theIETF's initiative - Generalised MPLS (GMPLS) or ASON / ASTN, but that

is for another day!

Back to home
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