Sadiq SONET

Synchronous Optical Network (SONET)

Sadiq Abdulkarim Page 1

Synchronous Optical Network

SONETTechnical Work

By

Sadiq Abdulkarimwww.sadiqfitted.blogsport.com

www.ajsadiq.wordpress.com

Email: [email protected]

http://www.sadiqfitted.blogsport.com/

http://www.ajsadiq.wordpress.com/

mailto:[email protected]



DEDICATION I dedicate this Book to my father in person of Alh. Abdulkarim

Muhammad and my late mother Haj. Khadija Abdulkarim Muhammad

may her soul rest in perfect peace Amen and the entire Aj’s Family.



Table of ContentsA. Introduction................................................................................................................................ 5

List of Figures ................................................................................................................................. 5

OBJECTIVES................................................................................................................................. 7

INCEPTION OF SONET ............................................................................................................... 9

SONET DEPLOYMENT................................................................................................................ 9

SONET Basics........................................................................................................................... 10

HISTORY OF SYNCHRONOUS OPTICAL NETWORK (SONET)......................................... 11

Background ............................................................................................................................... 11

Asynchronous............................................................................................................................ 12

Synchronous (SONET) ............................................................................................................. 13

Optical Networks....................................................................................................................... 13

CURRENT TECHNOLOGY........................................................................................................ 15

SONET Technology Compeer to 10 Gigabit Ethernet.............................................................. 15

SONET network management protocols................................................................................... 16

Craft interface ........................................................................................................................ 16

Data communication channels (DCCs) ................................................................................. 16

Facture Generation of SONET...................................................................................................... 18

Line............................................................................................................................................ 19

Path............................................................................................................................................ 19

SYNCHRONIZATION ................................................................................................................ 19

Importance of Synchronized Timing......................................................................................... 19

SONET Framing ........................................................................................................................... 21

The Benefits of SONET................................................................................................................ 23

B. Applications and Network Configuration ................................................................................ 26

Applications Area ......................................................................................................................... 27

Technology................................................................................................................................ 27

SONET Topology ......................................................................................................................... 29

Point-to-Point Configuration..................................................................................................... 31



Hubbed Configuration............................................................................................................... 31

Network architectures ................................................................................................................... 32

The Background ........................................................................................................................ 32

SONET Layers .......................................................................................................................... 33

� Photonic,......................................................................................................................... 34

� Section,........................................................................................................................... 34

� Line,................................................................................................................................ 34

� Path,................................................................................................................................ 34

Linear Automatic Protection Switching.................................................................................... 34

Unidirectional path-switched ring............................................................................................. 35

Bidirectional line-switched ring ................................................................................................ 35

TheSTS-1Frame ........................................................................................................................ 36

Paying the Freight ..................................................................................................................... 39

Synchronization ..................................................................................................................... 40

Local external timing............................................................................................................. 40

Line-derived timing ............................................................................................................... 40

Holdover ................................................................................................................................ 40

Timing loops.......................................................................................................................... 40

Frame Structure of SONET .......................................................................................................... 42

C. Implementation......................................................................................................................... 44

Technical Content ......................................................................................................................... 45

SONET Topologies. .............................................................................................................. 46

Availability of SONET.......................................................................................................... 46

OPERATIONS, ADMINISTRATION & MAINTENANCE (OA&M) ...................................... 47

Cost and Benefit analysis.............................................................................................................. 49

Synchronous Optical Network Market...................................................................................... 51

Conclusion .................................................................................................................................... 52

Reference ...................................................................................................................................... 54

Appendix....................................................................................................................................... 55

GLOSSARY OF TERMS............................................................................................................. 56

Acronym Guide............................................................................................................................. 58



A. Introduction



List of Figures

1. Figure 1. :- SONET Topology

2. Figure 2. :- SONET system Hierarchy

3. Figure 3. :- STM-1 Frame Structure

4. Figure 4. :- Frame Structure of SONET

5. Figure 5. :- SONET Multiplexing Mapping

6. Figure 6. :- SONET Market Revenue Forecast

7. Figure 7. :- Synchronous Optical Network



Introduction to SONETThe Synchronous optical network (SONET) is a customary for optical telecommunications

transport formulated by the Exchange Carriers Standards Association (ECSA) for the American

National Standards Institution (ANSI), which sets industry standards in the U.S. for

telecommunications and other industries. The widespread SONET standard is expected to

present the transport communications for worldwide telecommunications for at least the next two

of three decades.

In this day’s production world, each manufacturing is looking for diverse ways to create

aggressive compensation to distribute information, products and services in a more timely and

cost effectual method. End-to-end SONET (Synchronous Optical Network) network solutions

are one significant constituent in creating a aggressive edge. As a crossroads technology,

SONET provides for the combination of voice, data and video over the same transport service.

This guide is projected to offer an equipped overview of SONET (Synchronous Optical

Network) for those who are not familiar with the standard, or those who want to invigorate their

knowledge. At the beginning of this document, an important point to remember is this: SONET

is a powerful, highly scalable technology. Although it may appear to be complex, most of what

goes on in a SONET network is translucent to the user. Another note of significance: This

Abstract briefly discusses Wave Division Multiplexing (WDM) for awareness purposes only,

since WDM is another high routine transport technology that also leverages fiber optics. SONET

is a transport technology, considered to present enterprise and government users – as well as

service providers – a network communications with survivability distinctiveness, so that business

operations keep on continuous. SONET’s self-healing fiber optic ring functionality enables

automatic network recovery due to failures that can be caused by a fiber optic cable cut, lost

signal, or besmirched signal (e.g. due to aging laser) or node/system failure. SONET is also a

technology that is premeditated to ensure network traffic is restored within 60 milliseconds in the

event of a failure. SONET is the North American standard for telecommunications transmission

using fiber optic cables. SONET provides a set of protocols for the administration and control of

high bandwidth optical networking transmission. The SONET standard includes definitions for a

multiplexing structure, optical parameters, service mappings, and network administration



(operations) support for existing and future services. SONET uses standardized interfaces, which

allows multivendor interconnection of terminal (SONET Multiplexers) and subsystems.

OBJECTIVES<“The Synchronous optical network (SONET) is a customary for optical telecommunications





of three decades.

The greater than before configuration flexibility and bandwidth of SONET provides significant

improvement over the older telecommunications system.”>



INCEPTION OF SONET

<“SONET was apprehended of and written about back in the initial 1980’s, when acquiesced to

the members of American National Standard Institute (ANSI) T1 Committee as a universal

transport system. In the mid-1980’s the T1 Committee supplementary heightened the standard to

arrive at the Synchronous Transport Signal One (STS-1) as the base-signaling rate. Around this

time, the ITU-T (International Telecommunication Union- Telecommunications standard)

(formerly CCITT) adopted SONET as the basis for its international standard referred to as SDH

(Synchronous Digital Hierarchy) transport system, where the STS-1 rate (51.84Mbps) was to be

a factor of 3 in terms of the European base rate of 155.52Mbps.”>

SONET DEPLOYMENT <“SONET paraphernalia is actuality deployed in momentous numbers into the arena by

most of the major Service Providers (e.g. SPRINT-SONET Sphere, U.S. West Network 21, and

MCI). Many utility, civil government and great innovativeness productions have also designated

to situation SONET on their innovativeness mainstays to take enhancement of its scalability,

great presentation, and indispensable dependability and survivability appearances. This

government and innovativeness productions either bypass their local juggernauts and self-

manage the complete SONET prearrangement or they tie into a Central Office (CO) to allow

their local service breadwinner to offer WAN connectivity or network administration or

organizations services. In North America, SONET equipment is being situated and configured to

transport DS-1, DS-3, ATM, FDDI, Frame Communicate and IP traffic among other services,

using ring-based, hub-based and point-to-point based configurations. The ring-based

configuration is the most widespread SONET topology, where mission-critical applications

demand network survivability. For additional survivability, dual-ring configurations are also

being implemented (discussed later in this document).”>



SONET Basics

SONET defines optical signals and a synchronous border arrangement for multiplexed digital

traffic. It is a set of principles that define the rates and formats for optical networks specified in

ANSI T1.105, ANSI T1.106, and ANSI T1.117.”>

<“A similar standard, Synchronous Digital Hierarchy (SDH), is used in Europe by the

International Telecommunication Union Telecommunication Standardization Sector (ITU-T).

SONET equipment is generally used in North America, and SDH equipment is generally

accepted everywhere else in the world.

Both SONET and SDH are based on a assembly that has a basic border format and speed. The

border format used by SONET is the Synchronous Transport Signal (STS), with STS-1 as the

base-level signal at 51.84 Mbps. An STS-1 frame can be carried in an OC-1 signal. The frame

format used by SDH is the Synchronous Transport Module (STM), with STM-1 as the base-level

signal at 155.52Mbps. An STM-1 frame can be carried in an OC-3 signal.”>

<“Both SONET and SDH have a hierarchy of signaling speeds. Multiple lower-level signals can

be multiplexed to form higher-level signals. For example, three STS-1 signals can be

multiplexed together to form an STS-3 signal, and four STM-1 signals multiplexed together to

form an STM-4 signal.”>

SONET and SDH are technically comparable standards. The term SONET is often used to refer

to either.



HISTORY OF SYNCHRONOUS OPTICAL NETWORK (SONET)

Background

<“Before SONET, the first generations of fiber-optic systems in the public telephone network

rummage-sale proprietary architectures, paraphernalia, streak codes, multiplexing formats, and

preservation measures. The uses of the equipment-regional Bell operating corporations and

interexchange carriers (IXCs) in the United State, Canada, Korea, and Hong Kong-wanted

standards so that they could mix and match paraphernalia from modification contractors. The

task of generating such a standard was taken up in 1984 by the ECSA to inaugurate a standard

for connecting one fiber system to another. This standard is called SONET.”>

<”Synchronous optical networking (SONET) is standardized multiplexing protocols that transfer

multiple digital bit streams over optical fiber using lasers or light-emitting diodes (LEDs). Lower

data rates can also be transferred via an electrical interface. The method was developed to

substitute the Plesiochronous Digital Hierarchy (PDH) system for transferring greater quantities

of telephone calls and data traffic over the same fiber without synchronization difficulties.

SONET nonspecific measures are comprehensive in Telcordia Technologies Nonspecific

Necessities document GR-253-CORE. Generic criteria appropriate to SONET and other

transmission systems (e.g., asynchronous fiber optic systems or digital radio systems) are found

in Telcordia GR-499-CORE.”>

<“SONET, which is fundamentally the same, were initially premeditated to conveyance course

mode communications (e.g., DS1, DS3) from a variation of changed foundations, but they were

principally designed to sustenance real-time, uncompressed, circuit-switched voice encoded in

PCM format. The principal struggle in performing this prior to SONET was that the

synchronization sources of this various circuits were different. This predestined that each journey

was essentially functional at a somewhat dissimilar rate and with dissimilar phase. SONET

permissible for the synchronized transport of many dissimilar circuits of contradictory origin

within a single framing protocol. SONET is not itself a communications protocol per se, but a

transport protocol.”>



<”Due to SONT's important protocol impartiality and transport-oriented features, SONET was

the obvious choice for transporting Asynchronous Transfer Mode (ATM) frames. It quickly

evolved mapping structures and concatenated payload ampules to transport ATM influences. In

other words, for ATM (and eventually other protocols such as Ethernet), the interior multifaceted

construction beforehand used to transport circuit-oriented acquaintances was removed and

interchanged with a hefty and concatenated frame (such as OC-3c) into which ATM cells, IP

packets, or Ethernet frames are placed.”>

<“SONET is extensively used today: SONET in the United States and Canada, and in the rest of

the world. It is measured a difference of SDH because of SDH's superior worldwide market

dissemination.”>

<”The SDH standard was initially defined by the European Telecommunications Standards

Institute (ETSI), and is formalized as International Telecommunications Union (ITU) standards

G.707, G.783, G.784, and G.803. The SONET standard was defined by Telcordia and American

National Standards Institute (ANSI) standard T1.105”>.

<”Telecommunication networks have evolved during a century-long history of technological

advances and social changes. The networks that once provided basic telephone service through a

friendly local operator are now transmitting the equivalent of thousands of encyclopedias per

second. Throughout this history, the digital network has evolved in three fundamental stages:

asynchronous, synchronous, and optical.”>

Asynchronous

The first digital networks were asynchronous networks. In asynchronous networks, each network

component's interior clock source scheduled its transmitted signal. Because each clock had a

certain amount of variation, signals arriving and transmitting could have a large variation in

timing, which often occasioned in bit errors.

More importantly, as optical fiber disposition augmented, no principles existed to command how

network fundamentals should format the optical signal. A myriad of proprietary methods

seemed, creating it to be problematic for network providers to interconnect equipment from

different vendors.



Synchronous (SONET)

The essential for optical standards led to the formation of the synchronous optical network

(SONET). SONET standardized line rates, coding schemes, bit-rate hierarchies, and maneuvers

and conservation functionality.

SONET also defined the types of network elements required, network architectures that

merchants could contrivance, and the functionality that each node must perform. Network

providers could now use different vendor's optical equipment with the self-assurance of at least

basic interoperability.

Optical Networks

The one feature of SONET that has permitted it to continue throughout a time of incredible

vicissitudes in network capacity needs is its scalability. Based on its open-ended growth plan for

higher bit rates, hypothetically no greater boundary exists for SONET bit rates. Nevertheless, as

sophisticated bit rates are used, corporeal boundaries in the laser sources and optical fiber begin

to make the repetition of boundlessly snowballing the bit rate on each signal an unreasoning

solution. Furthermore, construction to the networks through access rings has also had augmented

necessities. Customers are challenging more services and options, and are resounding more and

dissimilar categories of data traffic. To deliver full end-to-end connectivity, a new archetype was

needed to encounter all the high volume and varied needs. Optical networks deliver the

mandatory bandwidth and suppleness to enable end-to-end wavelength services.

Optical networks initiated with wavelength division multiplexing (WDM), which ascended to

deliver supplementary volume on prevailing fibers. Like SONET, defined network elements and

architectures

Deliver the basis of the optical network. However, different SONET, somewhat than using a

defined bit-rate and border construction as its basic construction block, the optical network will

be based on wavelengths. The machineries of the optical network will be defined conferring to

how the wavelengths are communicated, turned-out, or instigated in the network. Inspecting the



network from a layered methodology, the optical network necessitates the accumulation of

an”optical layer." To help define network functionality, networks are divided into several

different physical or virtual layers. The first layer, the services layer, is where the services—such

as data traffic—enter the telecommunications network. The next layer, SONET, delivers

refurbishment, presentation monitoring, and provisioning that is translucent to the first layer.

Developing with the optical network is a third layer: the optical layer. Standards organizations

are still defining the optical layer, but it will ultimately deliver the same functionality as the

SONET layer while functioning completely in the optical domain. The optical network also has

the supplementary prerequisite of resounding wide-ranging types of high bit-rate non-SONET

optical indications that bypass the SONET layer altogether. Just as the SONET layer is

transparent to Alcatel: Optical Networks Tutorial the services layer, the optical layer will

preferably be transparent to the SONET layer, providing renovation, presentation monitoring,

and provisioning of separate wavelengths instead of electrical SONET signals.



CURRENT TECHNOLOGY

SONET Technology Compeer to 10 Gigabit Ethernet

Alternative type of high-speed data networking circuit is 10 Gigabit Ethernet (10GbE). The

Gigabit Ethernet Association shaped two 10 Gigabit Ethernet variants: a local area variant (LAN

PHY) with a line rate of 10.3125 Gbit/s, and a wide area optional (WAN PHY) with the same

line rate as OC-192/STM-64 (9,953,280 kbit/s). The WAN PHY variant encapsulates Ethernet

data using a lightweight SDH/SONET frame, so as to be compatible at a low level with

apparatus intended to carry SDH/SONET signals, whereas the LAN PHY optional condenses

Ethernet data using 64B/66B line coding.

Conversely, 10 Gigabit Ethernet does not obviously deliver any interoperability at the bitstream

level with other SDH/SONET systems. This fluctuates from WDM system transponders,

including both coarse and dense wavelength-division multiplexing systems (CWDM and

DWDM) that currently support OC-192 SONET signals, which can customarily support thin-

SONET–framed 10 Gigabit Ethernet

User amount must also subtract path overhead from the shipment bandwidth, but path-overhead

bandwidth is variable based on the types of cross-connects built across the optical system.

Note that the data-rate progression starts at 155 Mbit/s and increases by multiples of four. The

only exception is OC-24, which is standardized in ANSI T1.105, but not a SDH standard rate in

ITU-T G.707. Other rates, such as OC-9, OC-18, OC-36, OC-96, and OC-1536, are defined but

not commonly deployed; most are considered orphaned rates.

The next logical rate of 160 Gbit/s OC-3072/STM-1024 has not yet been standardized, due to the

cost of high-rate transceivers and the ability to more cheaply multiplex wavelengths at 10 and 40

Gbit/s.



SONET network management protocols

SONET equipment is often managed with the TL1 protocol. TL1 is a telecom language for

management and reconfiguring SONET network rudiments. The knowledge language used by a

SONET network component, such as TL1, must be approved by other organization protocols,

such as SNMP, CORBA, or XML. SDH has been mostly achieved using the Q3 interface

protocol suite defined in ITU recommendations Q.811 and Q.812. With the convergence of

SONET and SDH on switching matrix and network elements architecture, newer

implementations have also offered TL1.

Most SONET NEs have a limited number of management interfaces defined:

Electrical interface

The electrical interface, often a 50-ohm coaxial cable, sends SONET TL1 instructions from a

local organization network substantially contained in the central office where the SONET

network component is located. This is for local organization of that network element and,

possibly, inaccessible organization of other SONET network rudiments.

Craft interfaceLocal "craftspersons" (telephone network engineers) can access a SONET network component

on a "craft port" and issue instructions finished a dumb terminal or terminal emulation program

running on a laptop. This interface can also be devoted to a comfort server, allowing for remote

out-of-band organization and logging.

Data communication channels (DCCs)SONET and SDH have dedicated data communication channels (DCCs) within the section and

streak above for organization traffic. Commonly, section overhead (regenerator section in SDH)

is used. Rendering to ITU-T G.7712, there are three modes used for management.

• IP-only stack, using PPP as data-link



• OSI-only stack, using LAP-D as data-link

• Dual (IP+OSI) stack using PPP or LAP-D with tunneling functions to communicate

between stacks.

To handle all of the possible management channels and signals, most modern network elements

contain a router for the network commands and underlying (data) protocols.

The main functions of network management include:

1. Network and network-element provisioning

In demand to apportion bandwidth throughout a network, each network element must be

configured. Although this can be done locally, through a craft interface, it is normally done

through a network organization system (sitting at a higher layer) that in turn operates through the

SONET/SDH network organization network.

2. Software upgrade

Network-element software upgrades are completed frequently through the SONET/SDH

organization network in contemporary apparatus.

3. Performance management

Network fundamentals have a definite great set of principles for demonstration organization. The

performance-management standards permit not only treatment the health of separable network

nitty-gritties, but separating and recognizing most network inadequacies or outages. Higher-layer

network checking and association software certifications the proper filtering and troubleshooting

of network-wide performance organization, so that imperfections and outages can be hurriedly

acknowledged and determined.



Facture Generation of SONETSONET development was initially driven by the need to conveyance multiple PDH signals—like

DS1, E1, DS3, and E3—along with other groups of multiplexed 64 kbit/s pulse-code modulated

voice traffic. The capability to transport ATM traffic was additional premature submission. In

order to sustenance huge ATM bandwidths, concatenation was developed, whereby smaller

multiplexing containers (e.g., STS-1) are contrariwise multiplexed to build up a larger container

(e.g., STS-3c) to support large data-oriented pipes.

“One badly-behaved with outdated concatenation, however, is stubbornness. Contingent on the

data and voice traffic mix that must be approved, there can be a huge quantity of unused

bandwidth left over, due to the immovable sizes of concatenated containers. For example, fitting

a 100 Mbit/s Fast Ethernet connection inside a 155 Mbit/s STS-3c container leads to

considerable waste. More significant is the need for all transitional network fundamentals to

provision newly-introduced concatenation sizes. This problem was overwhelmed with the

introduction of Virtual Concatenation.”

Virtual concatenation (VCAT) allows for a more arbitrary assembly of lower-order multiplexing

containers, structure superior containers of fairly random size (e.g., 100 Mbit/s) without the need

for transitional network rudiments to sustenance this specific form of concatenation. Simulated

concatenation influences the X.86 or Generic Framing Procedure (GFP) protocols in order to

map payloads of arbitrary bandwidth into the virtually-concatenated container.

The Link Capacity Adjustment Scheme (LCAS) allows for enthusiastically changing the

bandwidth via energetic virtual concatenation, multiplexing containers based on the short-term

bandwidth requirements in the network.

The set of next-generation SONET protocols that enable Ethernet transport is referred to as

Ethernet over SONET (EoS).

SONET Transport Hierarchy

Each level of the hierarchy terminates its corresponding fields in the SONET payload, as such:

Section



A section is a single fiber run that can be terminated by a network element (Line or Path) or an

optical regenerator.

The main function of the section layer is to properly format the SONET frames, and to convert

the electrical signals to optical signals. Section Terminating Equipment (STE) can originate

access, modify, or dismiss the subdivision header overhead. (A standard STS-1 frame is nine

rows by 90 bytes. The first three bytes of each row embrace the Section and Line header

overhead.)

Line

Line-Terminating Equipment (LTE) originates or terminates one or more sections of a line

signal. The LTE does the synchronization and multiplexing of information on SONET frames.

Multiple lower-level SONET signals can be mixed together to form higher-level SONET signals.

An Add/Drop Multiplexer (ADM) is an example of LTE.

Path

Path-Terminating Equipment (PTE) interfaces non-SONET equipment to the SONET network.

At this layer, the payload is mapped and demapped into the SONET frame. For example, an STS

PTE can assemble 25 1.544 Mbps DS1 signals and insert path overhead to form an STS-1 signal.

This layer is concerned with end-to-end transport of data.

SYNCHRONIZATION

Importance of Synchronized Timing

Network timing between SONET devices is an integral part of maintaining accurate information

transmitted over a SONET network. In the earlier days of networking, the method used in timing

was Asynchronous. In Asynchronous timing each switch runs its own clock. In Synchronous



timing, switches can use a single common clock to maintain timing. This single common clock is

referred to as a Primary Reference Source (PRS) or Master Clock. Synchronous timing maintains

better accuracy than Asynchronous timing because it uses a single common clocking scheme to

maintain timing. This accurate timing often becomes important to government and enterprise

applications, particularly when they are running time sensitive applications (e.g. video streaming

applications).

There are three methods typically used in obtaining synchronous timing in SONET multiplexers

(e.g. Lucent DDM-2000 Multiplexer), they include:

•�Timing from an onboard internal oscillator

•�Timing from an incoming optical signal from a high-speed interface

•�Timing from an external source coming from a DS1 timing reference that can be stratum 3 or

higher clocking

SONET defines a timing hierarchy known as the Stratum Clock hierarchy. Table 3 below shows

the long-term accuracy requirements within the Stratum Clock hierarchy, which ranges from

Stratum 1 through 4.

Although Synchronous networks display accurate timing, some dissimilarity can transpire

amongst dissimilar network devices or amongst networks. This modification is known as phase

dissimilarities. Phase variations are defined as Jitter or Wander. Jitter is defined as short-term

phase variations above10Hz.Wander is defined as long-term phase variations below 10Hz. In

digital networks, Jitter and Wander are handled by buffers found in the interfaces within diverse

network devices. One example is a Slip Buffer. The Slip Buffer is used to handle frequency

differences between read and write operations. To prevent against write operations happening

faster than read operations, read operations are handled at a slightly higher rate. On a periodic

basis, read operations are paused, while a bit is stuffed into a stream to account for any timing

differences between the read and write operations. This bit-stuffing scheme is referred to as a

controlled slip (frame). In terms of Wander, Bit Slips and Controlled Slips, the Stratum Clocking



accuracy timing requirements are defined in table 4. The timing accuracy requirements increase

as the Stratum hierarchy increases. As you will see, the Stratum 1 Clock must meet the highest

degree of accuracy.

SONET FramingA standard STS-1 frame is nine rows by 90 bytes. The first three bytes of each row represent the

Section and Line overhead. These overhead bits comprise framing bits and pointers to different

parts of the SONET frame.

There is one column of bytes in the payload that represents the STS path overhead. This column

frequently "floats" throughout the frame. Its location in the frame is determined by a pointer in

the Section and Line overhead.

The combination of the Section and Line overhead comprises the transport overhead, and the

remainder is the SPE.

For STS-1, a single SONET frame is transmitted in 125 microseconds, or 8000 frames per

second. 8000 fps * 810 B/frame = 51.84 Mbs, of which the payload is roughly 49.5 Mbs, enough

to encapsulate 28 DS-1s, a full DS-3, or 21 CEPT-1s.

An STS-3 is very similar to STS-3c. The frame is nine rows by 270 bytes. The first nine columns

contain the transport overhead section, and the rest is SPE. For both STS-3 and STS-3c, the

transport overhead (Line and Section) is the same.

For an STS-3 frame, the SPE contains three separate payloads and three separate path overhead

fields. In essence, it is the SPE of three separate STS-1s packed together, one after another.

In STS-3c, there is only one path overhead field for the entire SPE. The SPE for an STS-3c is a

much larger version of a single STS-1 SPE.

STM-1 is the SDH (non-North American) equivalent of a SONET (North American) STS-3

frame (STS-3c to be exact). For STM-1, a single SDH frame is also transmitted in 125

microseconds, but the frame is 270 bytes long by nine rows wide, or 155.52 Mbs, with a nine-

byte header for each row. The nine-byte header contains the Multiplexer and Regenerator



overhead. This is nearly identical to the STS-3c Line and Section overhead. In fact, this is where

the SDH and SONET standards differ.

SDH and SONET are not directly compatible, but only differ in a few overhead bytes. It is very

unlikely that Cisco will ever use a framer that does not support both.

SONET is very widely deployed in telco space, and is frequently used in a ring configuration.

Devices such as ADMs sit on the ring and behave as LTE-layer devices; these devices strip off

individual channels and pass them along to the PTE layer.

All current Cisco line cards and Port Adapters (PAs) act as PTE-layer devices; these devices

terminate the full SONET session and L2 encapsulation. They are Packet Over SONET (POS)

cards, which indicate serial transmission of data over SONET frames. There are two RFCs that

describe the POS process: RFC 1619, PPP over SONET, and RFC 1662, PPP in HDLC-like

Framing.

These Cisco products cannot sit directly on a SONET or SDH ring. One of them must hang off

of some LTE-layer device, such as an ADM. Equipment such as an Integrated SONET Router

(ISR) has both PTE and LTE functionality, so it can terminate and pass through data.



The Benefits of SONETThe transport network using SONET affords much more influential networking capabilities than

existing asynchronous systems. The key benefits provided by SONET are as follows:

1. Pointers, MUX/ DEMUX

As a result of SONET transmission, the network’s clocks are referenced to as extremely steady

reference point. Consequently, the requirement to align the data watercourses or synchronize

clocks is superfluous. Consequently, a lower rate signal such as DS1 is accessible, and

demultiplexing is not needed to access the bitstreams. Also, the signals can be weighted

composed without bit stuffing. For those situations in which reference frequencies may vary,

SONET uses pointers to allow the streams to “float” within the payload envelope. Synchronous

clocking is the key to pointers. It allows a very flexible allocation and alignment of the payload

within the transmission envelope.

2. Reduced Back-to-Back Multiplexing

Separate M13 multiplexers (DS1 to DS3) and fiber optic transmission system terminals are used

to multiplex a DS1 signal to a DS2, DS2 to DS3, and then DS3 to an optical line rate. The next

stage is a mechanically integrated fiber/multiplex terminal. In the existing asynchronous format,

care must be taken when routing circuits in order to avoid multiplexing and demultiplexing too

many times since electronics (and their associated capital cost) are required every time a DS1

signal is processed. With SONET, DS1s can be multiplexed directly to the OC-N rate. Because

of synchronization, an entire optical signal doesn’t have to be demultiplexed, only the VT or STS

signals that need to be accessed.



3. Optical Interconnect

Because of different optical formats among vendors’ asynchronous products, it’s not possible to

optically connect one vendor’s fiber terminal to another. For example, one manufacturer may use

417 Mb/s line rate, another 565 Mb/s. A major SONET value is that it allows mid-span meet

with multi-vendor compatibility. Today’s SONET standards contain definitions for fiber-tofiber

interfaces at the physical level. They determine the optical line rate, wavelength, power levels,

pulse shapes, and coding. Current standards also fully define the frame structure, overhead, and

payload mappings. Enhancements are being developed to define the messages in the overhead

channels to provide increased OAM&P functionality.

4. Multipoint Configurations

The difference between point-to-point and multipoint systems was shown previously in Figures

25 and 26. Most existing asynchronous systems are only suitable for point-to-point, whereas

SONET supports a multipoint or hub configuration. A hub is an intermediate site from which

traffic is distributed to three or more spurs. The hub allows the four nodes or sites to

communicate as a single network instead of three separate systems. Hubbing reduces

requirements for back-to-back multiplexing and demultiplexing, and helps realize the benefits of

traffic grooming. Network providers no longer need to own and maintain customer-located

equipment. A multi-point implementation permits OC-N interconnects or mid-span meet,

allowing network providers and their customers to optimize their shared use of the SONET

infrastructure.

5. Convergence, ATM, IP, Video, and SONET

Convergence is the trend toward delivery of audio, data, images, and video through diverse

transmission and switching systems that supply high-speed transportation over any medium to

any location. Tektronix is pursuing every opportunity to lead the market providing test and

measurement equipment to markets that process or transmit audio, data, image, and video signals

over high-speed networks. With its modular, service-independent architecture, SONET provides



vast capabilities in terms of service flexibility. Some broadband services use Asynchronous

Transfer Mode (ATM) – a fast packet-switching technique using short, fixed-length packets

called cells. Asynchronous Transfer Mode multiplexes the payload into cells that may be

generated and routed as necessary. Because of the bandwidth capacity it offers, SONET is a

logical carrier for ATM. Also, as local and wide area networks converge,

Packet over SONET (PoS) technologies allows the transport of IP packets of SONET rates.”>



B. Applications and Network Configuration



Applications AreaSONET was originally designed for the public telephone network. In the early 1980's, the forced

breakup of AT&T in the United States created numerous regional telephone companies, and

these companies quickly encountered difficulties in networking with each other. Fiber optic

cabling already prevailed for long distance voice traffic transmissions, but the existing networks

proved unnecessarily expensive to build and difficult to extend for so-called long haul data

and/or video traffic.

The American National Standards Institute (ANSI) successfully devised SONET as the new

standard for these applications. Like Ethernet, SONET provides a "layer 1" or interface layer

technology (also termed physical layer in the OSI model). As such, SONET acts a carrier of

multiple higher-level application protocols. For example, Internet Protocol (IP) packets can be

configured to flow over SONET.

Technology

SONET commonly transmits data at speeds between 155 megabits per second (Mbps) and 2.5

gigabits per second (Gbps). To build these high-bandwidth data streams, SONET multiplexes

together channels having bandwidth as low as 64 kilobits per second (Kpbs) into data frames

sent at fixed intervals.

Synchronous Optical Network (SONET) is a standard for optical telecommunication transport. It

was defined by the American National Standard Institute (ANSI) for US standards. The

international version of SONET is synchronous digital hierarchy (SDH). The two standards

provide the transport infrastructure for worldwide telecommunication network systems. SONET

defines optical carrier (OC) levels and electrically equivalent

In view of that the PoS is a Layer 2 technology that uses PPP in HDLC encapsulation, using

SONET framing. The PoS solution lowers the cost per megabyte when compared to other Wide

Area Networking architectures. The PoS interface supports SONET level alarm processing,

performance monitoring, synchronization, and protection switching. This support enables PoS

systems to seamlessly interoperate with existing SONET infrastructures and provides the

capability to migrate to IP+Optical networks without the need for legacy SONET infrastructures.



PoS is used in a point-to-point environment, much like the legacy T-carrier architectures, but

without the need for TDM.

PoS efficiently encapsulates IP traffic with a low-overhead PPP header. When encapsulated, the

traffic is placed inside an HDLC-delimited SONET SPE and transported across SONET. Voice,

video, and data can be carried within the IP packets using Layer 3 QoS mechanisms to control

priority when bandwidth contention occurs.

PoS can be used in tandem with other technologies carried over SONET architectures. PoS is not

compatible with these other technologies, but is not aware of them because they are being

transported over different time slots. PoS, TDM voice, ATM, and Dynamic Packet Transport

(DPT) can each use their required synchronous transport signals, not interacting with each other.

PoS interfaces are available in concatenated and non concatenated (channelized) options.

Channelized interfaces are more costly than concatenated interfaces.



SONET TopologyShows a typical topology for a SONET network. This topology is a dual ring. Each ring is an

optical fiber cable. One ring is the working facility. The other ring is the protection facility,

which acts as a standby in the event of fiber or system failure on the working facility.

SONET topology.

Figure 1.

End-user devices operating on LANs and digital transport systems (such as DS1, E1, etc.) are

attached to the network through a SONET service adapter. This service adapter is also called an

access node, a terminal, or a terminal multiplexer. This machine is responsible for supporting the

end-user interface by sending and receiving traffic from LANs, DS1, DS3, E1, ATM nodes, etc.

It is really a concentrator at the sending site because it consolidates multiple user traffic into a

payload envelope for transport onto the SONET network. It performs a complementary, yet

opposite, service at the receiving site.

The user signals (such as T1, E1, and ATM cells) are converted (mapped) into a standard format

called the synchronous transport signal (STS), which is the basic building block of the SONET

multiplexing hierarchy. The STS signal is an electrical signal. The notation STS-n means that the

service adapter can multiplex the STS signal into higher integer multiples of the base rate. The

base rate is 51.84 Mbit/s in North America and 155.520 Mbit/s in Europe. Therefore, from the

perspective of a SONET terminal, the SDH base rate in Europe is an STS-3 multiplexed signal

(51.84 x 3 = 155.520 Mbit/s).

The terminal/service adapter (access node) shown in the above diagram is implemented as the

end-user interface machine, or as an add-drop multiplexer (ADM). The ADM implementation

multiplexes various STS input streams onto optical fiber channels. The optical fiber channels are



now called the optical carrier signal and designated with the notation OC-n, where n represents

the multiplexing integer. OC-n streams are demultiplexed as well as multiplexed with the ADM.

The term add-drop means that the machine can add or drop payload onto one of the fiber links.

Remaining traffic that was not dropped passes straight through the multiplexer without additional

processing.

The digital cross-connect (DCS) machine usually acts as a hub in the SONET network. It can not

only add and drop payload, but it can also operate with different carrier rates, such as DS1, OC-

n, E1, etc. The DCS can make two-way cross-connections between the payload and can

consolidate and separate different types of payloads.

The DCS is designed to eliminate devices called back-to-back multiplexers. As we learned

earlier, these devices contain a plethora of cables, jumpers, and intermediate distribution frames.

SONET does not need all these physical components because cross-connection operations are

performed by hardware and software.

The topology can be set up either as a ring or as a point-to-point system. In most networks, the

ring is a dual ring, operating with two or more optical fibers. As noted, the structure of the dual

ring topology permits the network to recover automatically from failures on the channels and in

the channel/machine interfaces. This is known as a self-healing ring and is explained in later

chapters.

SONET technology enables a number of different network topologies to solve networking

requirements, including survivability, cost, and bandwidth efficiencies. The following provides a

description of 3 different SONET configurations, which are deployed in a variety of enterprise

situations. The SONET configurations include:

Point-to-point configuration

Hubbed configuration

Linear Add/Drop configuration

Ring configuration



Point-to-Point Configuration

SONET point-to-point configurations (see figure 2) create a simple topology that terminates a

SONET payload at each point of a fiber optic cable span. Point-to-point configurations are

typically deployed in transport applications, which require a single SONET multiplexer in a

single route. Point-to-point configurations can be enhanced to increase survivability by to

deploying a protection path (second fiber span) over a different path between two or more

SONET multiplexers.

Hubbed Configuration

Hubbed configurations (see figure 3) consolidate traffic from multiple sites onto a single optical

channel, which then can be forwarded to another site. This topology is often used in applications

where the user wants to consolidate traffic from multiple satellite sites to a single site such as

corporate headquarters, before extending it, in some cases to a central office. This topology helps

to reduce the number of hops as well as the equipment required to create a multisite topology.



Network architecturesDescription of the distributed synchronous optical network (SONET) architecture. Today's

asynchronous network architecture is based on fixed centralized bandwidth management with a

hierarchy of hubs targeted to keep system fill high. This architecture is highly vulnerable to

transport system and hub failures. Cross connects can be used to implement survivability, but

with high cost, considerable planning effort, and poor restoration time performance. SONET

rings permit a distributed network architecture which provides survivability without cross

connects disadvantages and hub vulnerability. A target distributed network is shown to have a

25% to 30% cost advantage over the hubbed network. Transition to this target can occur as

SONET technology is introduced

11/20/2002 -- If you have a background in packet-switched data communications, many of the

terms and concepts used in the traditional voice telecommunications world are foreign. In an

earlier article, I gave an overview of wide area networks (WANs) that touched on SONET as the

principal backbone technology for common-carrier WANs. A few years ago SONET may have

been considered to have a limited future as all-Ethernet and other packet-based overlay networks

took over the world. The economics of the game have changed, however, and now it looks as if

SONET and its successors will be with us for the foreseeable future.

This article is designed to give a more in-depth look at SONET architecture and its unique terms

in a way that those with a previous understanding of datacomm networks can comprehend. If

you've spent some time trying to do this research yourself, you know that most articles on

SONET read like incomprehensible gibberish, so I'll try to provide a telecom/datacom Rosetta

Stone to help bridge the gap.

The Background

Traditional digital telecommunications services such as T1/DS1s were designed to aggregate

analog telephone lines for more efficient transport between central offices. Twenty four digitized

voice lines (DS0s) were carried over a DS1 using time-division multiplexing (TDM).

To review, in a TDM architecture, multiple channels -- in this case 24 -- share the circuit

basically in rotation, with each DS0 having its own assigned time slot to use or not as the case



may be. As each channel is always found in the same place, no address is needed to extract

(demultiplex) that channel at the destination. 28 DS1s are TDM aggregated into a DS3 in the

same manner. Contrast this with a packet-based network such as Ethernet where there are no

assigned time slots; instead, the packets are multiplexed onto the media via an arbitration

mechanism (CSMA/CD) and demultiplexed via MAC address.

The older DS1/DS3 system is known as the Plesiochronous Digital Hierarchy (PDH), as the

timing of signals across the network is plesiochronous, which means almost but not precisely

synchronous (now you have a new word to use in cocktail conversation). Data communications

networks such as Ethernet are asynchronous, as there is not a centralized timing source and each

node has its own clock.

As more and more channels are multiplexed together into higher layers of the PDH hierarchy, a

number of problems arise. Since the timing on various DS1s going into a DS3 may differ

slightly, padding of some channels (bit-stuffing) is required to align all within the DS3 frame.

Once this is done, the individual DS1s are no longer visible unless the DS3 is completely

demultiplexed. In other words, you cannot just pick off an individual channel but must tear the

whole DS3 frame down, take the DS1 you want out, then rebuild the DS3; the equipment

required to do this is expensive. Another problem comes where different networks with relatively

wide differences in timing meet, such as Europe and the U.S.; expensive equipment that also

adds latency is required for the interface.

To alleviate these problems, in the mid-'80s the Synchronous Optical NETwork (SONET) was

designed. The International Telecommunications Union later generalized SONET into the

Synchronous Digital Hierarchy (SDH) in order to accommodate the PDH rates in use outside

North America.

SONET Layers

Each SONET network node ultimately derives its timing from an exceedingly precise and stable

cesium atomic clock somewhere on the network, leading to the "synchronous" part of the name.

The SONET model is totally different from the familiar OSI datacomm 7-layer model. The

SONET model defines four layers:



Photonic, which corresponds to the OSI's physical layer, defines the optical equipment's

attributes (OC-n.) The tolerances in areas such as signal timing, jitter, phase shift, etc.

required to maintain a synchronous network over a wide area are exacting, much more so

than in asynchronous networks such as Ethernet, and we won't get into them here; those

interested can order up the inches-thick Bellcore specifications for their reading

enjoyment.

Section, the frame format and certain low-level signal definitions, roughly corresponding

to the OSI link layer.

Line, the way in which lower-level frames are synchronized and combined into higher

levels; you can sort of look at this as parts of the network and transport layers. The line

layer also defines data channels carrying operations, administration, maintenance and

provisioning (OAM&P) information, which would be an application layer (like SNMP)

in an OSI modeled network.

Path, the end-to-end transport of a circuit, which also has application information

(performance monitoring, status, tracing) for management.

SONET and SDH have a limited number of architectures defined. These architectures allow for

efficient bandwidth usage as well as protection (i.e. the ability to transmit traffic even when part

of the network has failed), and are fundamental to the worldwide deployment of SONET and

SDH for moving digital traffic. Every SDH/SONET connection on the optical Physical layer

uses two optical fibers, regardless of the transmission speed.

Linear Automatic Protection Switching

Linear Automatic Protection Switching (APS), also known as 1+1, involves four fibers: two

working fibers (one in each direction), and two protection fibers. Switching is based on the line

state, and may be unidirectional (with each direction switching independently), or bidirectional

(where the network elements at each end negotiate so that both directions are generally carried

on the same pair of fibers).



Unidirectional path-switched ring

In unidirectional path-switched rings (UPSRs), two redundant (path-level) copies of protected

traffic are sent in either direction around a ring. A selector at the egress node determines which

copy has the highest quality, and uses that copy, thus coping if one copy deteriorates due to a

broken fiber or other failure. UPSRs tend to sit nearer to the edge of a network, and as such are

sometimes called collector rings. Because the same data is sent around the ring in both

directions, the total capacity of an UPSR is equal to the line rate N of the OC-N ring. For

example, in an OC-3 ring with 3 STS-1s used to transport 3 DS-3s from ingress node A to the

egress node D, 100 percent of the ring bandwidth (N=3) would be consumed by nodes A and D.

Any other nodes on the ring could only act as pass-through nodes. The SDH equivalent of UPSR

is subnetwork connection protection (SNCP); SNCP does not impose a ring topology, but may

also be used in mesh topologies.

Bidirectional line-switched ring

Bidirectional line-switched ring (BLSR) comes in two varieties: two-fiber BLSR and four-fiber

BLSR. BLSRs switch at the line layer. Unlike UPSR, BLSR does not send redundant copies

from ingress to egress. Rather, the ring nodes adjacent to the failure reroute the traffic "the long

way" around the ring. BLSRs trade cost and complexity for bandwidth efficiency, as well as the

ability to support "extra traffic" that can be pre-empted when a protection switching event

occurs.

BLSRs can operate within a metropolitan region or, often, will move traffic between

municipalities. Because a BLSR does not send redundant copies from ingress to egress, the total

bandwidth that a BLSR can support is not limited to the line rate N of the OC-N ring, and can

actually be larger than N depending upon the traffic pattern on the ring. In the best case, all

traffic is between adjacent nodes. The worst case is when all traffic on the ring egresses from a

single node, i.e., the BLSR is serving as a collector ring. In this case, the bandwidth that the ring

can support is equal to the line rate N of the OC-N ring. This is why BLSRs are seldom, if ever,

deployed in collector rings, but often deployed in inter-office rings. The SDH equivalent of

BLSR is called Multiplex Section-Shared Protection Ring (MS-SPRING).



The section, line and path layers correspond to types of equipment (known as network elements)

as shown in Figure 1, above. A basic element is the SONET terminal, or terminal multiplexer,

which concentrates DS1s or other non-SONET signals into a SONET link.

A very simple SONET network could consist of two terminals with length of fiber between

them. If the distance is too long for one fiber link, regenerators are used to amplify and

reconstruct the physical signal. An add/drop multiplexer provides two fiber connections with the

ability to access the internal structure of the SONET frame to remove or insert individual

channels as required for that node while passing the rest of the traffic on through. Cross-connects

are used to switch, combine, redirect, and otherwise groom traffic, with varying degrees of

granularity. Some operate only on the STS-n level (see below), while others can switch or

multiplex down to the VT (see below) or DSn level. All of these elements are section terminating

equipment; all except regenerators are line terminating equipment. Network elements where non-

SONET signals are attached to the SONET network are path terminating equipment. All

elements are intelligent, accessing in-band management information dedicated to each layer

within the SONET frame.

Within metropolitan areas, SONET networks are typically configured physically as rings (see

Figure 2, above). A ring topology provides a single level of redundancy, allowing restoration of

service if one fiber link is broken. The SONET mechanism for restoration takes less than 50

milliseconds to recover from a break, but is considered somewhat inefficient as half the total ring

bandwidth is reserved. Note that even though the physical topology may be a ring, the individual

channels (which are manually provisioned) are point-to-point -- SONET has no equivalent of

Ethernet/IP broadcast or multicast service.

TheSTS-1Frame

The basic building block of the SONET protocol is the Synchronous Transport Signal level 1

(STS-1). Unlike Ethernet or IP where the frame structure is usually illustrated linearly, the large

frame sizes involved in SONET are depicted as two dimensional matrices. The STS-1 frame is

illustrated as an array of bytes 90 columns wide by nine rows high. Read left to right, then top to

bottom, this works out to 810 bytes or 6480 bits per frame, transmitted at a rate of 8,000 frames

per second resulting in the basic SONET signal rate of 51.840 Mbit/sec. All higher level signals



are multiples of this rate. Note that although the STS-n hierarchy defines the signal rates, these

are carried on optical fiber using the equivalent OC-n hierarchy.

Optical Signal Name SONET Level Name Line Rate(Mbit/sec)

Optical Signal

Name

SONET Level

Name

Line

Rate(Mbit/sec)

OC-1 STS-1 51.840

OC-3 STS-3 155.520

OC-12 STS-12 622.080

OC-48 STS-48 2488.320

OC-192 STS-192 9953.280

OC-768 STS-768 39813.120

The STS-1 frame consists of overhead and payload. The first three columns comprise the

transport overhead; the remaining 87 columns are called the Synchronous Payload Envelope

(SPE) (see Figure 3, below). The transport overhead section has the framing, performance

monitoring, pointers, alarms and other OAM&P information used by the section and line layers.

Within the SPE of 9 rows by 87 columns is the path overhead, found in column 1, and what is

known as "fixed stuff" in columns 30 and 59; this has end-to-end monitoring and more

performance information. The remaining 84 columns (756 bytes) are the revenue-generating part

of the whole frame, known as the STS-1 Payload Capacity. A little quick math shows that the

total overhead of six columns (three transport and three SPE overhead) within an STS-1 frame is

9.5 percent; looked at another way, the 51.840 Mbit/sec signal can carry 48.38 Mbit/sec of

payload.

While it is easiest to envision the Synchronous Payload Envelope as a 9×87 rectangle beginning

in the top row of the STS-1 frame just after the three bytes of transport overhead as shown in

Figure 2, in fact it can start anywhere in the frame and extend into the next with the SPE start

indicated by a pointer (see Figure 4, below).



For higher levels in the STS-n hierarchy, STS-1 building blocks are byte-interleaved to produce

an STS-n frame consisting of n times 90 columns by 9 rows, still transmitted at 8,000 frames per

second. For instance, an STS-3 has 270 columns × 9 rows resulting in a signal rate of 155.52

Mbit/sec. The overhead columns of each STS-1 are aligned with the STS-n frame, but the SPE

payloads need not be as pointers indicate the start of each. The STS-1s may be independent of

one another, or concatenated to provide greater channel capacity than the basic building block

provides. Concatenated frames are indicated by a "c" suffix; an STS-3c has three concatenated

STS-1s for a payload capacity of 145.15 Mbit/sec. higher levels may also be concatenated in

multiples of STS-3c's.

It is important to always remember the big difference between time division multiplexed

networks such as SONET and packet-switched networks such as Ethernet. In a packet network, a

node only packages up a frame and sends it out when there is some sort of payload; the wire may

be "dead" between packets. In SONET, the frames are sent back-to-back, continuously,

regardless of whether the payload portion is occupied or not. Visualize the STS-1 frame as a long

(and really fast!) freight train. It has three engines (transport overhead), then 87 freight cars

(payload envelope), then another three engines, 87 more freight cars, etc. for a total length of 810

combined engines and freight cars. Within each group of 87 freight cars the 1st, 30th, and 59th

(path overhead) are used by the railroad and not available to carry freight. This train is moving

really fast, with all 810 cars passing by in 125 µsec. Hitched up to the last of the 810 cars in the

train is another one that is identically configured, and so on to infinity.

We can even extend this analogy to higher levels in the SONET hierarchy. Three of these

infinitely long and very fast STS-1 trains converge on a switchyard. Both engines and freight

cars on the first are all painted red, the second all white, and the third all blue. When the trains

reach the switchyard, the first engine from each of the trains is unhitched, then the three instantly

interleaved (multiplexed) into a single train leaving the switchyard; then follow the second and

third engines from each and the 87 freight cars. So what you have leaving is a train three times as

long, starting with 9 engines sequentially red/white/blue/red/white/blue/red/white/blue followed

by 783 freight cars alternating colors the same way. Then come the next nine engines, and so on.

This sequence repeats nine times to complete the STS-3 express freight train, 2,430 total engines

and cars -- and note that it still takes the same 125 µsec for the longer train to pass, so each car is



moving three times as fast. This shows the necessity for exact synchronism of the three feeder

STS-1 trains, so that the interleaving can take place without a train wreck! And note also that

each of the original trains' cars may be located within the longer train, as they are always at a

fixed position from the front; down the line another of our hypothetical switchyards can readily

disassemble (demultiplex) the STS-3 into three STS-1s going to different destinations. The rest

of the SONET hierarchy can be built up the same way, with four STS-3s being combined into an

STS-12, four STS-12s into an STS-48, and so on. In every case, the time for one train (frame) to

pass by remains 125 µsec while the train length multiplies so the speed of each car increases

proportionately.

Paying the Freight

O.K., so an STS-1 payload can handily accommodate one 45 Mbit/sec DS3. As a DS3 carries

672 voice calls, this is a pretty coarse level of granularity. However, the STS-1 payload can also

be divided up into a number of Virtual Tributaries (VTs) ( known as Virtual Containers, VCs in

SDH terminology) designed to provide synchronous transport for lower-speed PDH channels.

Virtual tributaries come in three sizes corresponding to three PDH signals:

VT Type Transports VT Rate No. of STS-

1 Columns

VT Group Contains

VT-1.5 One DS1 (1.544 Mbit/sec) 1.728 Mbit/sec 3 4

VT-2 One E1 (2.048 Mbit/sec) 2.304 Mbit/sec 4 3

VT-6 One DS2 (6.312 Mbit/sec) 6.912 Mbit/sec 12 1

Each STS-1 SPE payload can carry a mix of virtual tributary types. The SPE is divided into

seven VT groups of 12 columns each; each group may contain four VT-1.5s, three VT-2s or one

VT-6 (but not mixed within the group). Hence if all DS1s are involved, an STS-1 can carry

4×7=28 DS1s. Because of the way VTs utilize defined column locations within the SPE,

individual VTs can be identified and picked out without breaking down the entire STS-1.

Contrast that with the plesiochronous DS3, where the entire frame must be broken down and

reassembled if a DS1 is to be extracted.



Within the virtual tributary, there is VT path overhead and VT payload. In order to map the

plesiochronous signals into the synchronous virtual tributary, pointers are used so that the

tributary signals may float within the VT. Also null bits may be added ("stuffed") or removed to

accommodate differing frequencies or jitter. Because of this flexibility within the rigid

boundaries of the STS-1 frame, it seems to me that the SDH "Virtual Container" name is a more

apt description. To take our freight train analogy to an extreme, the VT (or VC) is a set of tanker

cars located at the same position in each STS-1 train; as the gauges on the pumps that fill each

tanker car are not quite accurate, the pumps are set to fill each car not quite to capacity to allow

for this variance.

SynchronizationClock sources used for synchronization in telecommunications networks are rated by quality,

commonly called a stratum. Typically, a network element uses the highest quality stratum

available to it, which can be determined by monitoring the synchronization status messages

(SSM) of selected clock sources.

Synchronization sources available to a network element are:

Local external timing This is generated by an atomic Caesium clock or a satellite-derived clock by a device in the same

central office as the network element. The interface is often a DS1, with sync-status messages

supplied by the clock and placed into the DS1 overhead.

Line-derived timing A network element can choose (or be configured) to derive its timing from the line-level, by

monitoring the S1 sync-status bytes to ensure quality.

Holdover As a last resort, in the absence of higher quality timing, a network element can go into a holdover

mode until higher-quality external timing becomes available again. In this mode, the network

element uses its own timing circuits as a reference.

Timing loopsA timing loop occurs when network elements in a network are each deriving their timing from

other network elements, without any of them being a "master" timing source. This network loop

will eventually see its own timing "float away" from any external networks, causing mysterious



bit errors—and ultimately, in the worst cases, massive loss of traffic. The source of these kinds

of errors can be hard to diagnose. In general, a network that has been properly configured should

never find itself in a timing loop, but some classes of silent failures could nevertheless cause this

issue.

Figure 2.

Figure 3.



Frame Structure of SONET

9 R o w s(1 Octet Per

90 Octets (Columns)

9 R o w s(1 Octet PerR ow )

3 Octets (1 Octet per column)

PATHOverhead

STS-1 SONET Payload Envelope

1

2

3

4

5

6

7

8

9

S T SLine Overhead(18 Octets)

STS SHO

(9 OCTELS) 9 R o w s(1 Octet PerR ow )

Figure 4.

vSTS-1 SONET Payload Envelope

STS SOH

(9 Octets

S T SLine Overhead(18 Octets)

PATHOverhead



DS1s mapped to VT1.5s, which are mapped to STS-1 SPE which are

Converted to Optical Signal (OC)

SONET Multiplexer Mapping

DS1

DS1

DS1

DS3

DS1

4 x VT1.5s

VT-GByteInterleaving(group of 4VT1.5s)

STS-1

(Up to 7 VT-Gscan be ByteInterleaved intoan STS-1)

OC-n SONETRing

Figure 5.



C. Implementation



Technical Content

Technical Content SONET is a grouping of physical layer specifications based on a signaling

speed hierarchy called STS or synchronous transport signals. SONET also defines sub levels of

the STS-1 format. It is possible for STS-1 signals to be subdivided into segments called virtual

tributaries. Virtual tributaries are synchronous signals that are used for the transport of lower-

speed transmissions. Table 1 below contains a listing of virtual tributaries and their sizes.

Table 1. - Virtual Tributaries

Virtual Tributary Type Bit Rate Size of Virtual Tributary

VT1.5 1.728 Mb/s 9 rows, 3 columns

VT 2 2.304 Mb/s 9 rows, 4 columns



In order to compensate for frequency and phase variations, a concept known as "pointers" is

used. Pointers allow the transparent transport of synchronous payload envelopes (either STS or

virtual tributaries) across plesiochronous boundaries, which are between nodes with separate

network clocks having almost the same timing). Pointers are useful in helping avoid delays and

data loss.

SONET. When SONET was originally developed by Bellcore Labs in 1984, it was designed for

use in domestic U.S. networks. However, SONET has been implemented for private LANs and

WANs as well. SONET is a standard for the United States and Canada. It should be pointed out

that although SONET and SDH are similar there are some fundamental differences; therefore the

two standards don’t really interoperate. SONET is based on the STS-1 at 51.84 Mbps, which

makes it an affective carrier of T3 signals. There is no STS-1 level for SDH. SDH starts at STS-



3, which is also known as STM-1 (Synchronous Transport Module-1) equal to 155.52 Mbps,

which makes SDH more suited for the carrying of E4 signals. The fundamental differences in

SONET and SDH are mainly a result of different current rates in Europe and North America.

SONET/SDH Hierarchies

SONET BIT Rate SDH

STS-1/OC-1 51.84 Mbps -

STS-3/OC-3 155.52 Mbps STM-1

STS-12/OC-12 622.08 Mbps STM-4

STS-24/OC-24 1244.16 Mbps -

STS-48/OC-48 2488.32 Mbps STM-16

STS-192/OC-192 9953.28 Mbps STM-64

SONET Topologies. The most common architecture for the deployment of SONET is the ring. Multiple ADMs can be

placed in a ring configuration. A primary benefit of the ring architecture is its survivability. For

example, if a fiber cut were cut the multiplexers will automatically send the signals through an

alternative path through the ring without interruption. Because of the built in redundancy, rings

are the most popular architecture for SONET implementation.

The two primary architecture types for the deployment of SONET are Unidirectional path-

switched rings (UPSR) and Bidirectional line-switched rings (BLSR). In the UPSR architecture,

all users share transmission capacity around a ring. BLSR is a four fiber topology with built in

redundancy that protects the network against network failures. In the case of a failure the user’s

traffic is rerouted without compromising the data.

Availability of SONET. SONET services are mostly available only in major metropolitian areas.

In order to access SONET services a local carrier must bring the fiber-based ring directly to the



specified location and assign dedicated bandwidth to the customer. Most organizations with high

bandwidth requirements are currently employing T-3s. SONET can be viewed as a possible

upgrade for these services.

GTE Corp is currently involved with a project that will provide SONET services to more cities

by the end of 1999. GTE has purchased on $150 million worth of Northern Telecom Inc’s fiber-

optic equipment to like more than 100 U.S. cities through a synchronous optical network

backbone. Most of the 100 cities will be turned up on the new backbone by the end of 1998.

GTE plans to begin the project on the coasts and move in to the Midwest. MCI is also planning

to offer a similar SONET service. The service would be offered in numerous metropolitan areas

and would be on the OC-3 level.

The research for this paper was conducted for the purpose of explaining the benefits of

implementing the Synchronous Optical Network. In short, what are the benefits that SONET

provides?

OPERATIONS, ADMINISTRATION & MAINTENANCE (OA&M)

SONET specifies network management support through the Operations, Administration

&Maintenance (OA&M) functions. Additional octets (bytes) known as “overhead” are added to

SONET frames to provide network management control and monitoring capabilities (see

SONET Frame Structure section of this document for additional information on SONET

overhead). The network management overhead consumes approximately 4% of available

bandwidth. Network monitoring functions are primarily alarm generation functions, which

include the following:

Loss of Frames

Loss of Pointers

Loss of Signals

System failures

Loss of synchronization



Parity errors

Bit error rates



Cost and Benefit analysisIn spite of the increasing interest towards newer and innovative technologies such as Ethernet

and Internet protocol/multi-protocol label switching (IP/MPLS), synchronous optical network

(SONET) still remains to be a preferred for metro and long distance services. This is expected to

maintain its position as the leading transport technology in North America for some time. Even

though most opportunities for SONET technology are likely to emerge from wireless carrier

customers, Internet service providers (ISPs), and content delivery providers, the industry faces a

significant challenge i.e. competing services from IP/Ethernet. Eventually, many customers will

slowly migrate to such next-generation technologies. This shift, for the most part, is driven by

the quick growth of data applications.

The world SONET/SDH related test equipment market generated revenues of $156.4 million in

2007, with a growth rate of 2.8 percent over 2006. In 2014, the revenues are expected to reach

$199.8 million. The compound annual growth rate (CAGR) from 2007 to 2014 is estimated at

3.6 percent.

Figure 6.



Ethernet as a current alternative to SONET reduces the demand for SONET services. In the

metro, Ethernet has many opportunities since standards development has been happening in that

area, and carrier class features and reliability have been developed as well. Packet traffic

continues to increase, but is still largely transported over legacy networks designed for voice

traffic, meaning over SONET. IP VPNs are being offered through SONET also. However, since

IP is better suited to Ethernet, the demand for Ethernet as a transport layer, replacing SONET, is

growing. Ethernet offers the lowest cost interface for customers to connect to data services.

Ethernet is widely used for business transparent LANs, Internet access, and VPNs and is slowly

replacing SONET elements in access networks.

Ethernet services provisioned today in North America reveal, however, that they are still

delivered over SONET. Using Generic Framing Procedure (GFP), Link Capacity Adjustment

Scheme (LCAS) and Virtual Concatenation (VCAT) now allows SONET equipment to add

native Ethernet interfaces to SONET multi-service provisioning platforms (MSPPs). As Ethernet

standards are continuingly developing, increasing migration to carrier Ethernet or Ethernet over

WDM is expected, as packetized traffic such as VoIP, data and video become a greater part of

the traffic mix. Metro Ethernet networks currently have this capability, as do Ethernet access

networks.

SONET's longevity is also largely due to its quality, reliability and performance, which have still

not been entirely replicated even with the advances in Ethernet technology. With SONET

deployments in North America, Asia, and Europe, and the large ATM and Frame Relay market

that continues to exist (although declining), carriers are not expected to replace their many

SONET network elements anytime soon. However, Ethernet services provisioning will continue

to grow, in access and metro markets especially. Ethernet over SONET will continue to be the

preferred method of providing such services and it is viewed as a step to Ethernet or Ethernet

over WDM migration in the future.

Ethernet is the biggest threat to the SONET/SDH technology. The more Ethernet continues to

develop carrier grade reliability, the more it will affect the growth of the SONET/SDH and OTN

markets. Ethernet has already achieved dominance at the access level. It has not replaced

SONET/SDH on a core network yet, but it expected to pose a threat to that technology over time.

Currently, there only OTN and SDH technologies that can perform higher bit (40G)



transportation. At this point of time, there it is only one choice available for end users. There is a

growing demand for 40G bandwidth from router-to-router connections.

Currently, 40 Gig test equipment costs more than 10 gig test equipment. This cost difference is

hindering the development of the 40 gig test equipment market. The process of implementing 40

gig on routers and SONET switches is currently not that cost-effective, when compared to the 10

Gig application. There are limited investments to upgrade the networks to higher bit rates. Also

operators and manufacturers are uncertain about the future direction of the market, whether

SONET/SDH is to be retained or overcome by pure Ethernet/IP. Moreover, from a field test

equipment perspective, the transition to higher speed on the OTN for a field unit, this adds

technical challenges that have to be taken into consideration in order to make the field equipment

manageable, especially for hand-held equipment types.

Synchronous Optical Network Market

Figure 7.



ConclusionSONET equipment is being deployed in significant numbers into the field by most of the major

Service Providers (e.g. SPRINT-SONET Sphere, U.S. West Network 21, and MCI). Many

utility, civil government and large enterprise customers have also elected to deploy SONET on

their enterprise backbones to take advantage of its scalability, high performance, and inherent

reliability and survivability characteristics. These government and enterprise customers either

bypass their local carriers and self-manage the entire SONET infrastructure, or they tie into a

Central Office (CO) to allow their local service provider to offer WAN connectivity or network

management services. In North America, SONET equipment is being deployed and configured to

transport DS- 1, DS-3, ATM, FDDI, Frame Relay and IP traffic among other services, using

ring-based, hub-based and point-to-point based configurations. The ring-based configuration is

the most popular SONET topology, where mission-critical applications demand network

survivability. For additional survivability, dual-ring configurations are also being implemented

(discussed later in this document).

The Synchronous optical network (SONET) is a customary for optical telecommunications





of three decades.

The greater than before configuration flexibility and bandwidth of SONET provides significant

improvement over the older telecommunications system.

In this day’s production world, each manufacturing is looking for diverse ways to create

aggressive compensation to distribute information, products and services in a more timely and

cost effectual method. End-to-end SONET (Synchronous Optical Network) network solutions

are one significant constituent in creating a aggressive edge. As a crossroads technology,

SONET provides for the combination of voice, data and video over the same transport service.



This guide is projected to offer an equipped overview of SONET (Synchronous Optical

Network) for those who are not familiar with the standard, or those who want to invigorate their

knowledge. At the beginning of this document, an important point to remember is this: SONET

is a powerful, highly scalable technology. Although it may appear to be complex, most of what

goes on in a SONET network is translucent to the user. Another note of significance: This

Abstract briefly discusses Wave Division Multiplexing (WDM) for awareness purposes only,

since WDM is another high routine transport technology that also leverages fiber optics. SONET

is a transport technology, considered to present enterprise and government users – as well as

service providers – a network communications with survivability distinctiveness, so that business

operations keep on continuous. SONET’s self-healing fiber optic ring functionality enables

automatic network recovery due to failures that can be caused by a fiber optic cable cut, lost

signal, or besmirched signal (e.g. due to aging laser) or node/system failure. SONET is also a

technology that is premeditated to ensure network traffic is restored within 60 milliseconds in the

event of a failure. SONET is the North American standard for telecommunications transmission

using fiber optic cables. SONET provides a set of protocols for the administration and control of

high bandwidth optical networking transmission. The SONET standard includes definitions for a

multiplexing structure, optical parameters, service mappings, and network administration

(operations) support for existing and future services. SONET uses standardized interfaces, which

allows multivendor interconnection of terminal (SONET Multiplexers) and subsystems.



Reference

1. Nortel SONET Document. Northern Telecom

2. Goldman, James, E., Applied Data Communications A Business-Oriented Approach,

second edition, 1998 John Wiley & Sons, Inc. pp. 320-323.

3. , SDH & ATM - Q&A http://www.broadband- guide.com/lw/reports/olr0498.html

4. What’s the difference between SONET and SDH:

http://www.ele.auckland.ac.nz/students/tankh/atm/faq/D15.html

5. Ameritech SONET: http://www.ameritech.com/products/custom/product/01t.thml

6. ADV: News: SONET/SDH Technology: http://www.ispo.cec.be/ispo/lists/ispo/0396.html

7. PACIFIC BELL NETWORK "fasTrak on SONET":

8. http://www.pacbell.com/prodcuts/business/fastrak/networking/sonet/features.html

9. ATM Physical Layer Interfaces: http://www.fudan.sh.cn/shnet/ddd/stn/atm002.html.

10. New AT&T chips for digital SONET networks:

http://www.att.com/press/0290.900227.mea.html

11. SONET: Double Digit Growth, Double Digit Price Drop: http://www.insight-

corp.com/2_26_98.html

12. SPRINT COMPLETES FIRST INTERNATIONAL SONET RING:

http://www.sprint.com/sprint/press/releases/9606/9606100264.html

13. SONET FAQ: http://www.sonet.com/docs/faq.htm

14. Bernier, Paula, "MCI Goes End-To-End With New SONET Service"

http://www.zdnet.com/zdnn/content/inwk/0434/inwk0056.html



Appendix

I recommend this Book for IT students to go through and do some research over the Book to

serve as a guide to them during their own leaning, assignments and projects.



GLOSSARY OF TERMS

ANSI (American National Standards Institute) – An organization that defines industrystandards for the U.S., and coordinates standards with the ISO (International StandardsOrganization).

ADM (Add/Drop Multiplexer) - Refers to the adding and dropping of payloads onto one of two channels.

ATM (Asynchronous Transfer Mode) - A set of standards from the ATM Forum that defines multiplexing technique and fixed length cells.

Broadband – A set of transmission services that handle transport capacity ranging from 50Mbps to 600Mbps.

CCITT (Consultative Committee International Telegraph & Telecommunications) –International standards body replaced by the ITU-T.

CEPT (Conference of European Postal and Telecommunications Administrations) –European Standards body responsible for defining telecommunications standards (e.g. E1, E3).

DS1/DS3 – Digital Signals with performance capacities of (DS1) 1.5Mbps and (DS3) 51Mbps.

Grooming – A function that allows traffic to be combined with or segregated from multiple sites, providing multipoint capability.

Interleave - A method used to combine multiple signals into a combined signal (STS-1).

ITU-T (International Telecommunication Union-Telecommunications standard) –nternational standards (formerly the CCITT) body responsible for defining such standards as the SONET standard.

Jitter – Variations in a short waveform caused by voltage fluctuations.

Narrowband – A set of transmission services that handle transport capacity of up to 1.5Mbps.

OA&M (Operations, Administration and Maintenance) – Network management functions to support SONET.

OCn (Optical Carrier) - Optical signals that carry SONET traffic. 22

Overhead – Bits added to the SONET digital stream to carry OA&M network management information.

Path – A logical connection between two end-points.



Payload – The portion of a SONET frame that carries service and signals (e.g. DS1)

Segregation – A function that allows traffic to be groomed, or dropped at certain sites and sent through to specific sites.

SDH (Synchronous Digital Hierarchy) – European defined version of SONET that uses the STS-3 as its standard size payload envelop (155Mbps).

SPE (SONET Payload Envelope) – Standard payload used to transmit SONET frames (STS-1 in North America, STS-3 in Europe).

STS-1 (Synchronous Transport Signal) – Standard frame size used in SONET by the North American market.

SONET (Synchronous Optical Network) – ITU-T standard that defines an optical transport service to carry synchronous transmissions.

VT (Virtual Tributary) – A signaling technique designed to handle sub-STS-1 payloads.

VT-G (Virtual Tributary-Group) – One or more VTs of the same rate bundled into an STS-1 payload.

Wideband - A set of transmission services that handle transport capacity ranging from 1.5Mbps to 50Mbps.



Acronym Guide

Abbreviation Meaning ATM asynchronous transfer mode

DWDM dense wavelength division multiplexing

ETDM electronic time division multiplexing

MAN metropolitan-area network

OADM optical add/drop multiplexer

OBLSR optical rings

OSNR optical signal-to-noise ration

OTDM optical time division multiplexing

OXC optical cross-connect

SOA semiconductor optical amplifier

SONET synchronous optical network

TDM time division multiplexing

WAN wide-area network

WDM wavelength division multiplexing

WP wavelength path

Documents

Sadiq SONET