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S O N E T 1 0 1

This document is intended for anyone with an interest in advanced networks.

It serves as an introductory guide to the Synchronous Optical Network (SONET)

standard and presents an overview of associated techn ology and applications.

The first sections define SON ET and explain synchron ization, the benefits of

SON ET, the frame format, and SO NET enh ancemen ts to OAM& P. Next, the

focus sh ifts to n etworking aspects of SON ET with particular emph asis on networkapp lications not covered by the stand ards literature. Top ics such as traditional

networks, th eir evolution, and how SON ET can b e dep loyed, are explored.

A section is also included describing the international Synchronous Digital

Hierarchy (SDH ) and its relationsh ip to SON ET. Some of the new concepts h ave

been d eveloped throu gh in-depth stu dies und ertaken by No rthern Telecom an d

its research and developm ent arm, Bell-No rthern Research.

N ote: The in form at ion con ta in ed in th is docum en t describes S O N ET as an in du stry

standard. It should not be construed as N orthern Telecom s implementation of SO N ET.

This document replacesSON ET 101: An Introduction to SON ET (56118 .11/06-92

Issue 2).

H D S L I XC B IS D N C C I TT D I D N A N P L AT A M F J O S I U N I V SAT W AT S A D SL F T TC G S M H D S L R H C S O N E T T C P / IP A bou t th is D ocu m en t

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S O N E T 1 0 1

SON ET (Synchron ous O ptical Netwo rk) is a standard for optical transpo rt formu -

lated by the Exchange Carriers Standards Association (ECSA) for the American

National Stand ards Institute (ANSI), which sets indu stry standards in the U .S. for

telecommunications and other industries.

The stand ard has also been incorporated into th e Synch ronou s Digital Hierarchy

recomm end ations of the Con sultative Com mittee on International Telegraph andTeleph one (CCITT) (now called the International Telecomm un ications U nion

[ITU]), wh ich sets stand ards for international telecomm un ications.

The standard was initiated by Bellcore on behalf of the Regional Bell Operating

Com pan ies (RBO Cs) and others for the following key pu rposes:

Com patibi li ty of equipment by all vendors who manu facture to the standard

often referred to as m id-span meet

Synchronous ne twork ing

Enhanced O AM& P (operat ions, adm inistrat ion, maintenance, and provisioning)

More efficient add /drop mu lt iplexing (ADM)

Standards-based survivable rings

The recent p ositioning of the netwo rk for transp ort of new services, such as

Asynchronous Transfer Mode (ATM)

In b rief, SO N ET defines op tical carrier (OC ) levels an d electrically equ ivalent

synchronous transport signals (STSs) for the fiber-optic based transmission

hierarchy. The stand ard SON ET line rates and STS-equivalent form ats are shown

in Figure 1-1. The fiber-optic rates shown in bold seem to b e the m ost widely

supp orted by both network providers and vend ors.

NEW STANDARD FOR FIBER-OPTIC TRANSPORT

H D S L I XC B IS D N C C I TT D I D N A N P L AT A M F J O S I U N I V SAT W AT S A D SL F T TC G S M H D S L R H C S O N E T T C P / I P W h at is S O N ET ?1

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W h a t is S O N E T ?

Figure 1-1 SONET Optica l Line Rates

Optical Carrier Level Electrical Equivalent Line Rate Mbps

OC-1 STS-1 51.84OC-3 STS-3 155. 52OC-12 STS-12 622.08OC-24 STS-24 1244.16OC-48 STS-48 2488.32OC-192 STS-192 9953. 28

NOTE:The higher line rates are integer multiples of th e base rate of 51.84 M bps. For example,OC-12 = 12 x 51.84 Mbps = 622.08 M bps. The highlighted fiber-optic rates are themost w idely supported.

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Figure 1-2 SONET Multiplexing

Rate = n x 51. 84 M bps

VT: Virtual TributarySTS: Synchron ous Transport SignalOC: Optical Carrier

DS- 1 (2 8 ma x pe r STS- 1)

DS- 3 (1 ma x pe r STS- 1)

Future ServiceExamples: Video FDDI Broadband ISDN ATM

M PE6-2

STS-151.84Mbps

M ux(Byte Interleave

Synchronous Mux)

Service Adapters

(map to STS-1)

Service Adapters(map to VTs)

Service Adapters

STSNc(concatenated STS-1s)

STS-n OC-n

E/0

Electricalto OpticalConverter

Timing

S T S -

n

1

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S O N E T 1 0 1

W h a t i s S O N E T ?

Cu rrently m any separate networks provide the comm un ication services availableto sub scribers. Plain old teleph on e service (POTS) is provided over a voice chan -

nel conn ected to a circuit-switched netwo rk. A totally indepen den t packet-

switched n etwork might be u sed for data comm un ication, offering faster metho ds

for data delivery.

An Integrated Services Digital Network (ISDN) provides a single network that can

han dle voice, data, and video. W hereas ISDN has traditionally applied to th e

narrowband telephony world, a broadband ISDN network consists of broadband

ISDN switches an d terminals (Figure 1-4) wh ich can tie high-speed local area

networks (LANs), digital TV and other video services, data co mm un icationdevices, telemetry equipment (for example automatic electrical meter reading),

and voice into on e d igital network.

SONET provides the necessary bandwidth to transport information from one

BISDN switch (or terminal) to anoth er. For examp le, an O C-3 (15 5 Mb ps) rate

may be used to transport an H4 digital broadband channel carrying a broadcast

qu ality TV signal. Broad ban d ISDN will use asynch ron ou s transfer m od e (ATM)

techn ology (see also th e section: N ew S ervices U sin g AT M ).

In the future, public and private networks will be based on ATM/cell-relay tech-no logy. ATM is the CC ITT stand ard that su pp orts cell-based voice, data, video ,

and multimedia communication in a public network under Broadband ISDN.

SONET provides sufficient payload flexibility that it will be used as the underlying

transport layer for BISDN ATM cells.

The ATM Forum is aligning with SONET as the transport layer for cell-based

traffic.

TRANSPORT FOR BROADBAND ISDN

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Figure 1- 3 ISDN Channels (CCITT)

Channel Name Type Bit Rate Applications

A Voice Frequency 3. 2 M Hz analog voice/voice band data (modems)/facsimileB Digital 64 kbps PCM voice/wideband speech (7 KHz)/dataD Digital 16 or 64 kbps signaling channel/packet type data/telemetryE Digital 64 kbps signaling for high-capacity users equipment (PABX)H0 Digital Broadband 384 kbps high-quality audio/high-speed digital infoH11 Digital Broadband 1.536 Mbps video for teleconferencing/high-speed digital infoH12 Digital Broadband 1.92 M bps video for teleconferencing/high-speed digital infoH4 Digital Broadband 150 M bps high-definition TV (approximate rate)

Figure 1-4 Broadband ISDN Network Scenario

Multi-BandwidthDigital Access

Voice ServiceLANHigh Speed Packet

Switch ServiceHigh-Definition TVVideoPrivate LinesCAD/CAMNew ServicesTelex/TeletexMessage SwitchingFaxVideotexElectronic MailTelemetryHome Consumer ServicesElectronic BankingTeleshoppingDistance LearningMedical ImagingRemote Arraignment

Broadband ISDNCustomer Terminal

Broadband ISDNCustomer Terminal

BroadbandISDN Switch

SONETTransportNetwork

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S O N E T 1 0 1


SONET FRAM E FORM AT

SON ET uses a basic transmission rate of STS-1 equivalent to 51.8 4 Mb ps.H igher level signals are integer mu ltiples of the b ase rate. For exam ple, STS-3 is

three times the rate of STS-1 (3 x 51.84 = 155 .52 Mbps). An STS-12 rate would

b e 1 2 x 5 1 .8 4 = 6 2 2 .0 8 M bp s .

STS-1 Building BlockThe fram e form at of the STS-1 signal app ears in Figure 1-5. In general, the frame

can be divided into two main areas: transport overhead and the synchronous

payload envelope (SPE).

The synchron ous payload en velope can also by divided into two p arts: the STSpath overhead and the p ayload. The payload is the revenue-produ cing traffic

being transpo rted and routed over the SON ET network. O nce the payload is

multiplexed into the synchronous payload envelope, it can be transported and

switched through SONET without having to be examined and possibly demulti-

plexed at interm ediate nod es. Thus, SON ET is said to be service-indepen den t

or transparent.

The STS-1 payload h as the capacity to transp ort up to:

2 8 D S-1 s

1 DS-3 21 CEPT-1s (2.048 Mbp s CCITT type signal) or combinations of above.

Type Digital Bit Rate Voice T1 DS-3

DS-1 1. 544 M bps 24 1

CEPT 2. 048 M bps 30

DS-3 44. 736 M bps 672 28 1

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The STS-1 frame format is usually depicted as a matrix of nine rows of 90 bytes

each as show n in Figure 1 -5. The signal is transm itted b yte-by-byte beginn ing

with byte one, scann ing left to right from row on e to row n ine. The entire frame

is transmitted in 125 microseconds.

Figure 1-5 SONET Frame Format

1

911

91 3 9 0

Line Overhead

Payload49.5 M bps

90

18 0

STS PathOverhead

SectionOverhead

Synchronous Payload Envelope (SPE)TransportOverhead

90 Bytes

51.84 Mbps Building Block (STS-1)

12 5 ms

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SON ET provides su bstantial overhead information, allowing simpler m ultiplexing

and greatly expan ded OAM& P capabilities. The overhead information h as several

layers, which are shown in Figure 1-6. Path-level overhead is carried from en d-to -

end ; it is add ed to DS-1 signals wh en th ey are m apped into virtual tribu taries and

for STS-1 p ayloads th at travel end -to-end . Line o verhead is for th e STS-n signal

between STS-n m ultiplexers. Section overhead is used for com m un ications

between ad jacent n etwork elemen ts, such as regenerators.

From Figure 1-5, transp ort overhead is com posed of section overhead and line

overhead. The STS-1 p ath overhead is part of the synchron ous p ayload envelope.

Enou gh information is contained in th e overhead to allow the netwo rk to op erate

and allow OAM&P communications between an intelligent network controller

and the individual nodes.

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Figure 1-6 SONET Multiplexing

Path

TerminatingEquipment

Line


Line


Path

TerminatingEquipment Regenerator Regenerator

SONETTerminalor Switch

SONETHub

Section

Line

Path(end-to-end)

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Specifically, th e o verhead con sists of:

Section Overhead:

performance m onitoring (STS-n signal)

local o rderwire

data comm un ication channels to carry information for OAM& P

fram in g

Line Overhead:

performan ce monitoring of the individual STS-1s

express o rderwire

da ta channels fo r OAM&P

pointer to the start of the synchronous payload envelope

protect ion switching information

line alarm ind ication signal (AIS)

line far-end receive failure (FERF) ind ication

STS Path O verhead :

performance mon itoring of the STS SPE

signal label (equipped or unequipped)

p at h st at us

p at h trace

VT Path O verhead:

performan ce mon itoring (virtual tribu tary level)

signal label (equipped or unequipped)

p at h st at us poin ter (depending on VT type)

In add ition to th e STS-1 b ase format, SO NET also defines synchronou s formats at

sub -STS-1 levels. The STS-1 payload m ay be su bd ivided into virtual tribu taries,

which are synchron ous signals used to transpo rt lower-speed transmissions.

There are three sizes of VTs:

S O N E T 1 0 1


VIRTUAL TRIBUTARIES

Type Transport for (typically) VT Rate

VT-1.5 1 DS-1 1.544 M bps 1.728 M bps

VT-2 1 CETP 1 2.048 M bps 2. 304 M bps

VT-6 1 DS-2 6. 312 M bps 6.912 M bps

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VT payloads an d VT path overhead comp rise the virtual tribu tarys synchron ous

payload en velop e and are similar to th e STSs payload. W ithin an STS-1 frame,

each VT occupies a num ber of column s as shown in Figure 1-7. W ithin the

STS-1, many VT groups can be mixed together to form an STS-1 payload.

VT GroupTo accom m od ate d ifferent m ixes of VTs in an efficient m ann er, th e STS-1 SPE is

divided into seven group s. A conceptual view of the VT groups ap pears in Figure

1-8. (The VT grou ps are actually interleaved.) A VT group m ay con tain one

VT-6, three VT-2s, or fou r VT-1.5 s. A VT group m ust co ntain o nly one size of

VTs, but different types of VT groups may be mixed into one STS-1 SPE.

DS-1 VisibilityFigure 1-7 show s the SO N ET STS-1 frame form at. Ind ividual DS-1s are visible

(neatly stacked together for easy iden tification ). Because th e mu ltiplexing is

synchronou s, the low-speed tribu taries (inpu t signals) can b e m ultiplexed

togeth er bu t are still visible at higher rates. An ind ividu al VT contain ing a DS-1

can be extracted withou t dem ultiplexing the en tire STS-1. This imp roved

accessibility im proves sw itching an d groom ing at VT or STS levels.

In an asynch ronou s DS-3 frame, the DS-1s h ave gone th rough tw o levels of

multiplexing (DS-1 to DS-2; DS-2 to DS-3) which include the addition of stuffing

and framing bits. The DS-1 signals are mixed somewh ere in th e information b itfields and cannot be easily identified without completely demultiplexing the

entire frame.

Different synch ron izing techn iques are used for mu ltiplexing. In existing asyn-

chrono us systems, the tim ing for each fiber op tic transm ission system (FO TS) ter-

m inal is no t locked on to a com m on clock. Therefore, large frequ ency variation s

can occu r. Bit stuffing is a techn ique u sed to synch ron ize th e variou s low-

speed signals to a com mo n rate before mu ltiplexing.

N ote: A m ore deta iled expla nat ion of syn chron izat ion is provided in S ect ion 2.

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PointersSONET uses a concept called pointers to compensate for frequency and phase

variations. Pointers allow th e transparent transp ort of synch ronou s payload

envelopes (either STS or VT) across plesiochronous boundaries, i.e., between

nod es with separate network clocks having almo st the same timing. The use of

pointers avoids the delays and loss of data associated with the use of large

(125-microsecond frame) slip buffers for synchronization.

Pointers provide a simple means of dynamically and flexibly phase-aligning STS

and VT payloads, th ereby permitting ease of dropp ing, inserting, and cross-

conn ecting th ese payloads in th e network. Transmission signal wander and jitter

can also be readily m inim ized with p ointers.

Figure 1 -9 shows an STS-1 pointer wh ich allows the SPE to be sep arated from th e

transport overhead. The po inter is sim ply an o ffset value that po ints to the b yte

wh ere the SPE begins. The d iagram d epicts th e typical case of th e SPE overlap-

pin g onto tw o STS-1 frames. If there are any frequ ency or ph ase variations

between the STS-1 frame and its SPE, the pointer value will be increased or

decreased accordingly to m aintain synch ronization.

VT MappingsThere are several options for how payloads are actually mapped into the VT.

Locked-mo de VTs bypass th e po inters with a fixed byte-oriented map ping of lim-ited flexibility. Floating mod e map pin gs use the po inters to allow the payload to

float within th e VT payload. There are th ree different floating mo de m app ings

asynchronous, bit-synchronous, and byte-synchronous.

Concatenated PayloadsFor future services, the STS-1 may not have enough capacity to carry some ser-

vices. SON ET offers the flexibility of con catenatin g STS-1s to p rovide the n eces-

sary band width . Figure 1-10 illust rates SON ET flexibility by concaten ating three

STS-1s to 155 .52 Mbp s to p rovide th e capacity to transport an H4 digital-video

chann el. STS-1s can be concatenated u p to STS-3c. Beyond STS-3, concatena-tion is don e in mu ltiples of STS-3c. For ATM services, con catenation is don e in

multiples of STS-12c.

Virtual tributaries can be concatenated up to VT-6 in increments of VT-1.5, VT-2,

or VT-6 (see Figure 1 -7).

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Figure 1-7 DS-1 Visibility

1

91 (Byt es) 9 0

S T S P a t

h

O v e r h e a

d

F i r s

t D S

- 1

51.84 Mbps Building Block (STS-1)

12 5 ms

SectionOverhead

Line Overhead

VT1. 5

VT1. 5

VT1. 5

VT1. 5

VT6

S e c o n

d D S

- 1

T h i r d D S

- 1

F o u r t

h D

S - 1

D S

- 2

Because all virtualtributaries areidentifiable, it is easyto tell the bytes thatbelong to each DS-1.

VT Path Overhead

Sizes of Virtual Tributaries

VT-1.5 VT-2 VT-2 VT-6

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Figure 1-8 VT Groups (Conceptual View)

STS-1 Frame

TransportOverhead

STS PathOverhead

7 VT Groups

1 2 3 4 5 6 7

STS-1 SynchronousPayload Envelope (SPE)

19

VT Group

1 (Bytes) 12

147

101316

1922

258

111417

2023

369

121518

2124

S/F

Bytes for Overhead

Bytes for Pointer VT 1.5

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Figure 1-9 Pointer

Byte 1

Transport Overhead

STS-1 Pointer indicates where SPE begins

F r a m e

1

F r a m e 2

STS-1 Synchronous Payload Envelope (SPE)STS-1 Synchronous Payload Envelope (SPE)

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S O N E T 1 0 1


Figure 1-1 0 Concatenated STS-1s

1

91 9 1 0 2 7 0

SectionOverhead

Line Overhead

Example: STS-3c (155.52 Mbps) Formed by Three Concatenated STS-1s (51.84 Mbps)

STS-3c Synchronous Payload Envelope (SPE)Transport Overhead(TOH)

87

TOH STS-1 #3

3

87

TOH STS-1 #2

3

87

TOH STS-1 #1

3

STS Path

Overhead

Capacit y to ca rry Broadband ISDN H4Digital Video Channel

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Althou gh n etwork elemen ts (NEs) are compatible at the OC -n level, they maydiffer in features from vend or to vend or. SON ET do es not restrict m anu facturers

from providing a single type o f produ ct, nor require them to p rovide all types.

For examp le, on e vendor might o ffer an ad d/d rop m ultiplexer with access at DS-1

only, whereas another might offer simultaneous access at DS-1 and DS-3 rates.

See Figure 1-11.

Add/Drop MultiplexerA single-stage multiplexer/demultiplexer can multiplex various inputs into an

O C-n signal. At an add /drop site, only those signals that need to be accessed

are drop ped o r inserted. The remaining traffic continu es through th e networkelemen t withou t requ iring special pass-throu gh u nits or oth er signal processing.

In rural applications, an ADM can b e dep loyed at a terminal site or any interme-

diate location for consolidating traffic from widely separated locations. An ADM

can also be configured as a survivable ring.

Drop and RepeatSO NET enables drop and repeat (also known as drop and continue) a key

capability in both telepho ny and cable TV applications. W ith drop and repeat,

a signal terminates at one n ode, is du plicated (repeated), and is then sent to th enext node and to subsequent nodes (see Figure 1-11).

In ring-survivability applications, drop and repeat provides alternate routing for

traffic passing th rough interconnecting rings in a m atched-nod es configuration.

If the connection cannot be made through one of the nodes, the signal is repeat-

ed and passed along an alternate route to the destination node.

In m ulti-nod e d istribution ap plications, one transport ch annel can efficiently

carry traffic between mu ltiple distribu tion no des. W hen transporting video, for

example, each programming channel is delivered (dropped) at the node and

repeated for delivery to the next and su bsequ ent nod es. No t all band width (pro-

gram ch ann els) need be terminated at all the nod es. Ch annels not terminating at

a node can be passed through without physical intervention to other nodes.

SONET NETWORK ELEMENTS

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Broadband Digital Cross-ConnectA SONET cross-connect accepts various optical carrier rates, accesses the STS-1

signals, and switches at this level. It is ideally used at a SO NET hu b. O ne m ajor

difference between a cross-conn ect and an ad d-drop mu ltiplexer is that a cross-

conn ect may be used to interconnect a mu ch larger nu mb er of STS-1s. The

broadb and cross-conn ect can be used for grooming (consolidating or segregating)

of STS-1s or for broad ban d traffic m anagem ent. For examp le, it m ay be used to

segregate h igh-band width from low-band width traffic and sen d them separately to

the h igh-band width (for examp le video) switch an d a low-band width (voice)

switch. It is th e synch rono us equ ivalent of a DS-3 digital cross-conn ect and

supp orts hub bed network architectures.

Wideband Digita l Cross-ConnectThis type is similar to the b roadban d cross-connect except that th e switching is

do ne at VT levels (sim ilar to DS-1/D S-2 levels). It is sim ilar to a DS-3/1 cross-

conn ect because it accepts D S-1s, D S-3s, and is equipped with op tical interfaces

to accep t op tical carrier signals. It is suitable for DS-1 level groom ing app lications

at hub locations. O ne m ajor advantage of wideband digital cross-conn ects is that

less dem ultiplexing an d mu ltiplexing is required b ecause on ly the required tribu-

taries are accessed and switched.

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S O N E T 1 0 1

Traditionally, transm ission systems have been asynch ronou s, with each term inal

in the n etwork run ning on its own clock. In digital transm ission, clocking is

one of the most imp ortant considerations. Clocking m eans using a series of

repetitive pulses to keep th e bit rate of data constan t and to ind icate where the

ones an d zeroes are located in a d ata stream.

Since these clocks are totally free-running and not synchronized, large variationsoccur in th e clock rate and th us th e signal bit rate. For exam ple, a DS-3 signal

specified at 44 .736 Mbp s + 20 p pm (parts per million) can p rodu ce a variation

of up to 17 89 b ps between one incoming DS-3 and another.

Asynch ronou s mu ltiplexing uses m ultiple stages. Signals such as asynchron ous

DS-1s are multiplexed, extra bits are added (bit-stuffing) to account for the varia-

tions of each individual stream and are combined with oth er bits (framing bits)

to form a DS-2 stream . Bit-stuffing is used again to m ultiplex up t o DS-3. The

DS-1s are neither visible no r accessible within a DS-3 frame. DS-3s are mu lti-

plexed u p to h igher rates in the same m anner. At the higher asynch ronou s rate,

they cannot be accessed without demultiplexing.

In a synchronous system, such as SONET, the average frequency of all clocks in

the system will be th e same (synchrono us) or n early the same (p lesiochronou s).

Every clock can be traced back to a highly stable reference sup ply. Thu s, the

STS-1 rate remains at a n om inal 51.84 Mbp s, allowing m any synchrono us STS-1

signals to be stacked together when m ultiplexed withou t any bit-stuffing. Thus,

th e STS-1s are easily accessed at a h igher STS-n rate.

Low-speed synchronous virtual tributary (VT) signals are also simple to interleave

and transport at higher rates. At low speeds, DS-1s are transpo rted by synch ro-

nou s VT-1.5 signals at a constant rate of 1.728 Mbp s. Single-step m ultiplexing

up to STS-1 requires no bit stuffing and VTs are easily accessed.

Pointers accom m odate d ifferences in th e reference source frequencies, ph ase

wander, and prevent frequency differences during synchronization failures.

H D S L I XC B IS D N C C I TT D I D N A N P L AT A M F J O S I U N I V SAT W AT S A D SL F T TC G S M H D S L R H C S O N E T T C P / IP W h y S y n ch ron iz e?

SYNCHRONOUS VERSUS ASYNCHRONOUS

2

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Digital switches and digital cross-conn ect system s are comm only emp loyed in thedigital net work synch ronization hierarchy. The netw ork is organized with a m as-

ter-slave relationship with clocks of the higher level nodes feeding timing signals

to clocks of th e lower level nod es. All no des can b e traced up to a primar y refer-

ence sou rce, a Stratum 1 atomic clock with extremely high stability and accuracy.

Less stable clocks are adequate to su pp ort the lower nod es.

The network of Figure 2-1 illustrates the digital network synchronization hierar-

chy, with all clocks normally operating at the same frequency as the reference

source. A large network can com prise the interconn ection o f man y such clusters

of nod es, each o perating p lesiochronou sly.

SYNCHRONIZATION HIERARCHY

Stratum 1

Nodes

Figure 2-1 Digital Network Synchronization Hierarchy

Stratum 2

Stratum 3

Stratum 4

1

3

22

444

TimingSignals

24

W h y S y n c h ro n i z e?

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Figure 2- 2 Synchronizing a SONET Network

SwitchingNode

SONET

SwitchingNode

SONET

BITS BITS

InternalClock

InternalClock

SONET

SONETADM

(Line Timed)

Customer Premise s

InternalClock

Incoming OC-n providestiming for internal clockand outgoing OC-n

Timing foroutgoing OC-n

External Reference

Timing (Master M ode)

W h y S y n c h ro n i z e?

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S O N E T 1 0 1

H D S L I XC B IS D N C C I TT D I D N A N P L AT A M F J O S I U N I V SAT W AT S A D S L F T TC G S M H D S L R H C S O N E T T C P / I P W hat a re the Ben efit s of S O N ET ?

The transport network using SONET provides much more powerful networking

capabilities than existing asynchron ous systems. The key benefits provided by

SON ET are sum m arized in th e table of Figure 3-1.

Figure 3-1 Key SONET Benefit s

Reduced Lower AddedFeature Capital Cost Revenue

1. Mult i -point configurat ions Grooming X Reduced back-to-back terminals and M uxes X Reduced cabling and DSX panels X

2. Enhanced OAM&P OAM &P integration X Enhanced performance monitoring X X

3. Concurrent NMA* and OPS/INE**

operation X4. New services X5. Optical interconnect X X X

* NMA: Network Monitoring and Analysis** OPS/INE: Operations System/Intelligent Network Element

3

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28

Synchronous Systems Such as SONET Allow Ease ofImplementing M ulti-Point Configurations

Node D

Node C

Node A

Node B

Node D

Node C(Hub)

Node A

Node B

Figure 3-2 Point-to-Point Versus Multi-Point and Grooming

Most Existing Asynchronous Systems are Point-t o-Point

Traffic terminating at B (OC-n)

Traffic terminating at A (OC-m)

Combined traff ic Se gre ga ti on Consol ida ti on

Grooming Direction

W h a t a r e t h e B e n e f i t s o f S O N E T ?

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30

Grooming refers to either consolidating or segregating traffic to make more effi-cient use of the facilities. Co nso lidation m eans com binin g traffic from different

locations on to on e facility. In Figure 3-2, for examp le, fiber link s from N od e A to

Node C and from N ode B to Node C are bo th underu t ilized . Ins tead of connect -

ing two fibers from A to D, traffic is consolidated along a single fiber toward

N od e D, m aximizing ut ilization of facilities.

Segregation is the sep aration o f traffic. For examp le, traffic from N od e D m ust go

to Nod es A and B. W ith exist ing systems, the cumbersome technique of back-

hauling might be u sed to redu ce the expense of repeated m ultiplexing and

dem ultiplexing. Thus, traffic for Node B is carried past No de C toward Nod e A.The traffic for Nod e A is dropp ed and the traffic for Nod e B is routed b ack

toward N ode C u sing the excess capacity. It then reaches its final destination at

N o d e B.

Groo m ing eliminates inefficient tech niqu es like back-hau ling. It is possible to

groom traffic on asynchron ous systems, how ever to d o so requires expen sive

back-to-back configurations and m anual D SX pan els or electronic cross-connects.

By contrast, a SO N ET system can se gregate traffic at either an STS-1 or VT level

to send it to the appropriate nodes.

Groo m ing can also provide segregation of services. For examp le, at an int ercon-

nect point, an incoming SONET line may contain different types of traffic, such

as switched voice, data, or video . A SON ET net work can con veniently segregate

the switched and non -switched traffic.

GROOMING


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S O N E T 1 0 1

ProprietaryOptical Format

DS-1DS-1 Visibility

Multi-stagemultiplexing /

Separate networkelements

ProprietaryOptical Format

SONETOptical Carrier

Format

DS-3 t oOptical

M uxDS-1 to DS-3

Multi-stagemultiplexing /

Single networkelement

DS-3 t oOptical

M uxDS-1 to DS-3

Figure 3-3 Fiber Terminal Evolution

DS-1 t oOC-n

DS-1DS-1


The evolution of a fiber mu ltiplex terminal is illustrated in Figure 3-3. SeparateM13 m ultiplexers (DS-1 to D S-3) and FO TS terminals are used to m ultiplex a

DS-1 signal to a DS-2, DS-2 to DS-3, and then DS-3 to an optical line rate. The

next stage is a mechanically integrated fiber/multiplex terminal.

In th e existing asynch ronou s format, care mu st be taken when routing circuits in

order to avoid mu ltiplexing and dem ultiplexing too m any times since electronics

(and their associated capital cost) are required every time a DS-1 signal is

processed. W ith SON ET, DS-1s can be m ultiplexed d irectly to the O C-n rate.

Because of synchronization, an en tire op tical signal does n ot h ave to be

dem ultiplexed, o nly the VT or STS signals that need to b e accessed.(Refer to Figure 3-4.)

REDUCED BACK-TO- BACK M ULTI PLEXING

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32


Asynchron ous systems are dom inated by back-to-back terminals because the

asynch ronou s FO TS architecture is inefficient for other th an point-to-point

netw orks. (See Figure 3-5.) Excessive m ultiplexing and dem ultiplexing are used

to transp ort a signal from one end to ano ther, and m any bays of DSX-1 cross-

conn ect and D SX-3 panels are requ ired to interconn ect the systems. Associatedexpenses are the p anel, bays, cabling, th e labor installation, and the inconven-

iences of increased floor space and congested cable racks.

The correspond ing SO NET system allows a hu b con figuration, reducing the

need for back-to -back terminals. Groo m ing is performed electron ically so DSX

panels are not u sed except wh en requ ired to interface with existing asynchrono us

equipment.

REDUCED CABLING AND ELIM INATION OF DSX PANELS

DSX-3DSX-1DSX-3

Existing Asynchronous Systems

FOTSA

FOTSC

M 13M ux

M 13M ux

Some DS-1sDropped / Inserted

DSX-1

SONET System

SONETAdd / Drop

Only DS-1s / DS-3s /STS-1s that are added /dropped need to bemultiplexed /demultiplexed

Figure 3-4 Reduction of Back-to-Back Multiplexing

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S O N E T 1 0 1


SON ET allows integrated network O AM&P, in accordance with th e ph ilosoph y of single-end ed m ainten ance. In other words, one conn ection can reach all network

elemen ts; separate links are not requ ired for each network element. Remote pro-

visioning provides cen tralized m ainten ance and reduced travel for m ainten ance

personn elwh ich translates to expense savings.

Note: OAM&P is sometimes referred to as OA&M.

Substantial overhead information is provided in SON ET to allow q uicker troub le-

shooting and detection of failures before they degrade to serious levels.

ENHANCED OAM& P

ENHANCED PERFORM ANCE M ONITORING

DSX-3

Existing Asynchronous Systems

Grooming at DSX-3:Combining underfilledfibers from FOTS A andFOTS B to transport on asingle fiber on FOTS C.

SONET System

FOTSC

DS-3 Coax Cables

OC-n

OC-n

OC-nInternal Grooming

at STS Level

565 M bps

565 M bps

565 Mbps

FOTSB

FOTSA

SONETHub

Figure 3-5 Reduction of Cabling and DSX Panels

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34


O ne o f the impo rtant ben efits of SON ET is its ability to position the network forcarrying new revenue-generating services. W ith its m od ular, service-ind epen den t

architecture, SON ET p rovides vast capab ilities in term s of ser vices flexibility.

High-speed packet-switched services, LAN transport, and high-definition televi-

sion (HDTV) are examp les of new SON ET-sup ported services.

Many of these broadb and services may use asynchron ous transfer mod e (ATM)

a fast-packet-switching technique using short, fixed-length packets called cells.

Asynchron ous transfer mod e m ultiplexes the p ayload into cells that m ay be gener-

ated and ro uted as neces sary. Because of th e band width cap acity it offers,

SO N ET is a logical carrier for ATM.

In principle, ATM is quite similar to other packet-switching techniques; however,

the d etail of ATM op eration is somewh at different. Each ATM cell is made u p of

53 octets, or bytes. O f these, 48 octets make up the user-information field and

five octets make up the h eader. The cell head er iden tifies th e virtual path to be

used in routing a cell throu gh the netwo rk. The virtual path d efines the connec-

tions throu gh wh ich th e cell is routed to reach its destination.

An ATM-based network is b andw idth-transp arent, which allows hand ling of a

dynamically variable mixture of services at different bandwidths (see Figure 3-6).ATM also easily accomm od ates traffic of variable speed s. An examp le of a service

that iden tifies the ben efits of a variable-rate interface is that o f a video cod ec-

based ser vice. The video signals can be coded in to digital signals and p acketized

within ATM cells.

During periods of high activity within the video cameras field of view, the rate of

packet tran sfer will increase. Hen ce, an average rate of transfer may take place

below th e maxim um capacity of the p hysical layer. But du ring peaks in activity,

the rate of packetization goes above the no rmal maximum .

NEW SERVICES THAT USE ATM

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S O N E T 1 0 1


The rate at which cells can be transmitted through the network is dependent

up on th e ph ysical layer of the network u sed for transpo rt of the cells. The inter-

face rate presented to the user may vary between a minimum and a maximum

rate. For periods o f time, this maximum may be greater than th e aggregate rate of

the physical transport mechanism.

This is possible by buffering ATM cells at the user interface and transmitting them

du ring periods of user inactivity. This metho d en sures mu ch m ore efficient uti-

lization of the bandwidth made available to the end userhence, the adoption of

ATM as the transfer mode for broadband ISDN and other broadband applications.

Voice Sets

PBX

ATMMultiplexer/Switch

MultimediaTerminal

SONETInterface

OC-3cOC-12c

File Server

Video

Figure 3-6 ATM Network Handles Various Bandwidths/Services

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Because of different op tical formats am ong vendors asynch ronou s p rodu cts, it isno t possible to optically con nect on e vend ors fiber terminal to anot her. For

example, one manufacturer may use 570 Mbps line rate; another 565 Mbps.

A major SO N ET value is th at it allows mid-span m eet with m ulti-vend or com pati-

bility. See Figure 3-7. Tod ays SO N ET standard s contain d efinitions for fiber-to -

fiber interfaces at the p hysical (ph oton ic) level. They determ ine the op tical line

rate, wavelength, power levels, pu lse shapes, and coding. The current stand ards

also fully define the frame structu re, overhead, and payload map pings.

Enhancements are being developed to define the messages in the overhead chan-

nels to provide increased OAM& P fun ctionality.

OPTICAL INTERCONNECT

SON ET allows op tical interconn ection between network providers regardless of

who m akes the equipm ent. The network provider can purchase one vend ors

equipment and conveniently interface with other vendors SONET equipment at

either the different carrier location s or custo m er premises sites. U sers may no w

obtain the OC-n equipment of their choice and meet with their network provider

of choice at that OC-n level.

Switching Node

Vendor A

Vendor A Carrier #1

Vendor B

CustomerPremises

Vendor D

Carrier #2

Vendor C

Multi-VendorEnvironmentMid-Span Meet

Figure 3-7 Optical Interconnect

Vendor A

36

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S O N E T 1 0 1

OAM&P and network management are major concerns among network providers.

Since th e transmission network is continu ously growing and there are many ven-

dors and types of equipment, network providers must be able to administer,

m onitor, provision, and con trol the n etwork from a central location. The

Telecommunications Management Network (TMN) Standards Committee has

agreed to an OAM& P architecture and is now formulating imp lementation

standards for th at architecture.

SONET improves network management by providing extra bandwidth and func-

tionality in the overhead structu re. It sup ports OAM& P data channels, enabling

com mu nications b etween intelligent con trollers and individual network no des as

well as inter-nod e comm un ications. These OAM& P chann els are called data

comm un ications chann els.

SON ET allows circuits to be remotely enabled or d isabled to carry or remove

traffic. This flexibility allows th e netw ork to b e dynam ically reconfigured d ue to

trouble situations (network restoration), traffic variations, or customer needs (like

time-o f-day service). O ther ben efits includ e: faster inst allation of circuits, which

is im portant for special services needs; redu ced n eed to dispatch p ersonnel for

circuit provisioning; an d ease of reconfiguration, which makes th e system m ore

responsive to chan ges.

H D S L I XC B IS D N C C I TT D I D N A N P L AT A M F J O S I U N I V SAT W AT S A D SL F T TC G S M H D S L R H C S O N E T T C P / IP S O N ET O A M & P

REM OTE PROVISI ONING AND RECONFIGURATION

4

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Figure 4-1 SONET Integrated OAM&P

Existing Asynchronous System

SONET Network

M uxFOTSB

FOTSFOTS

DSXor Digital

Cross-Connect

A

Separate c raft interfac es required.

Overheadcommunications areseparated betweensystem A and B.

M uxFOTSB

FOTSFOTS

SONETDigitalCross-

Connect

A

Only one craft interface required.

Overheadcommunications areintegrated a mongsystem A, B, andcross-connect.

OAM&PCommunications

OAM&PCommunications

S O N E T O A M & P

38

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S O N E T 1 0 1

SONET supports the evolution of operations systems (OSs), such as those beingdeveloped by Bellcore. It supp orts the TMN s architecture for OAM& P data

communicat ions.

In a synchronous net works: separate O Ss are used to p rovide centralized single-end ed m ainten ance

a separate data l ink n etwork is used to conn ect the O S to each NE

OAM&P band width and information are l imited

To address the se proble ms, SONET: makes OAM&P an integral part of the transm ission standard b y broadening

bandwidth and information for OAM&P

provides m ore op erations-level information to simp lify such activities as p erfor-

mance m onitoring

consists of an integral data commu nicat ions/LAN network

makes it easier to imp lement centralized O AM& P

Figure 4-1 illustrates one im po rtant elemen t of integrated OAM& P. In asynch ro-

nou s networks, there are many n etwork elemen ts provided by different vendors.

The internal overhead chann els for each system are not carried from one system

to anoth er. Typically, FO TS A and FO TS B behave as two separate systems.

Thus, sep arate craft interfaces are need ed to perform OAM& P functions, and

overhead communications are not carried through the cross-connect.

In a SON ET network, as long as the SO NET signals are passed th rough, overhead

com mu nications are maintained from o ne system to the n ext. This ph ilosoph y,

called single-end ed m ainten ance perm its an entire network to b e controlled

from a single n etwork element.

COM PATIBILIT Y WI TH THE OS EVOLUTION

INTEGRATED OAM & P

S O N E T O A M & P

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S O N E T O A M & P

Figure 4-2 Improved Surveillance

Limited Surveillanc e i n Existing Asynchronous System

SONET offers improved surveillance with extra alerting indications.Even site A is aware of the problem downstream.

Network ProviderInterexchange Carrier

M uxFOTSDS-3

AIS(blue signal)

ProprietaryKeep Alive Signal

FOTSFOTS

Network ProviderInterexchange Carrier

DS-3AIS

STS Path Yellow Alarm

STS Path AIS

SONETHub

SONETTerminal

A

SONETHub

SONETTerminal

Detects Failureat Hub

Line FERF AIS: Alarm Indication SignalFERF: Far End Receive Fa il Signal

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S O N E T 1 0 1

S O N E T O A M & P

SON ET overhead is organized into th ree layers defined as section, line, an d pathoverhead. They provide abu nd ant inform ation for alarm sur veillance and perfor-

m ance m onitoring. Many bytes in th e SON ET overhead are allocated for Bit

Interleave Parity-8 (BIP-8), a m eth od of error mon itoring. This BIP-8 is actually a

byte (eight bits) which creates an even parity over a sequence of bits in the

transm itting signal. A full set of performan ce mo nitoring statistics can be

generated with the format, h elping to locate degraded con ditions before they

become serious.

BIP-8 allows for sectionalization of alarms and single-ended maintenance.

Perform ance information is available for all levels dow n to t he VT path levels,which means parity error information is available for DS-1s on an end-to-end

basis. At the VT level, perform ance m on itoring is accomp lished by a BIP-2.

Present FOTS systems provide parity error information only for line rate and

DS-3 rate.

An example of improved facility maintenance and surveillance appears at the

bo ttom of Figure 4-2. In case of a failure ind icated by an X in the d iagram, an

alarm ind ication signal (AIS) is sent to su pp ress down stream alarms. The termi-

nal up stream at the left side (A) may be u naware that a failure has occurred.

SONET provides extra information such as a yellow alarm indication signal at thepath level from th e far end t ermin al to site A. In add ition , a far-end receive failure

(FERF) signal at the line layer overhead is sent from the hub detecting the failure

to tell site A that a failure has occurred downstream.

With SONETs enhanced performance monitoring and surveillance, the time it

takes to restore service is reduced . Also, fewer mainten ance actions are required

since faults can be m ore easily sectionalized. Added to this are th e advantages of

SON ETs bu ilt-in d ata comm un ications networking.

ENHANCED PERFORM ANCE M ONITORING

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S O N E T O A M & P

The architecture for OAM& P comm un ications as d efined by th e InternationalTelecomm un ications Un ion, an agency of the U nited N ations, is shown in Figure

4-3. It provides a framework for an O S to comm un icate with th e network and

vice versa. An OS is a soph isticated app lication software that man ages the who le

network.

An O S can gather information from the n etwork, as well as from alarm, status,

and p erform ance statistics. It can also perform remo te-circuit testing or net work-

adm inistration task s. An O S may also con tain information o n facilities available

for provisioning and m ay be used to p rovision by send ing remote-control

comm ands to enable or disable circuits.

The OS communicates with the network elements either directly or through

med iation devices. Mediation d evices can su pp ort mu ltiple fun ctions such as

consolidation of com mu nication links to th e various n etwork elemen ts at the

same location, con version of protocols back and forth, and man agem ent of

performan ce mon itoring data.

The advantage of SONET is that the data communication channel can be used as

part of the Data Communications Network, as shown in the TMNs architecture.

TELECOM M UNICATIONS M ANAGEM ENT NETWORK (TM N) ARCHITECTURE

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S O N E T 1 0 1


Figure 4-3 Telecommunications M anagement Network OAM& P Communications

DataCommunications

Network

OperationsSystems

MediationDevice

DataCommunications

Network

Network

Element

NetworkElement

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S O N E T 1 0 1

SONET NETWORKING TRENDS

Increases in dem and for services such as non -switched private lines m ean m ore

and mo re T1 m ultiplexers (intelligent chann el banks) are being dep loyed by large

end users. Some are now imp lem enting T3 (DS-3) fiber services to meet these

demands.

ISDN and Broadban d ISDN will allow a mu ltitud e of new services to p rovide

band width on dem and . Users of an integrated services network, which offersvoice, data, and video onto a single network, h ave new requirements includ ing

variable band width, h igh band width, user con trol, m easurement of traffic perfor-

m ance, and rapid service provisioning. To p rovide th e necessary bandwidth , fiber

has b een d eployed increasingly in the access area (connections b etween a service

providers node and the user).

H D S L I XC B IS D N C C I TT D I D N A N P L AT A M F J O S I U N I V SAT W AT S A D SL F T TC G S M H D S L R H C S O N E T T C P / I P S O N E T N et work in g5

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S O N E T N e t w o r k i n g

Figures 5-1 and 5 -2 illustrate th e way a tradition al netw ork app ears. Traditionalnetworks can be d ivided into two m ain areasaccess and inter-nod e.

The inter-nod e netwo rk consists of trun ks used to interconn ect service-provider

nod es or to conn ect those n odes with inter-exchange carrier locations. Fiber

routes con sist of p oint-to-point systems with cross-conn ections at the DSX-3

panel. The fiber system is symm etrical with on e terminal end of the system a

m irror image of th e other. DS-3s enter at one end , leave at the oth er, and occa-

sionally may be accessed at intermediate drop/insert sites. Cu rrent deployments

are SO NET fiber terminals configured as point-to-point, ring, or 1:n systems.

The access network (local loop) is mostly comp osed o f twisted cop per p airs

conn ecting a switch to a teleph one or other terminating equipm ent. In some

areas, T1 lines or fiber are used to connect a switch to a remote (digital loop

carrier) terminal (such as the N orth ern Telecom DM S-1 U RBAN). The digital

loop carrier (DLC) terminal in turn conn ects copp er pairs to th e individu al users

in a carrier serving area (CSA).

SON ET next-generation digital loop carriers (NG DLCs), such as N orthern

Telecoms S/DMS AccessNod e, are being d eployed tod ay. NG DLCs com bine the

access functionality available from DS-0s with SONET transmission capabilitiesfrom D S-1 th rough O C-n.

The transport n etwork is evolving towards a m ulti-point architecture and SON ET

enables new configurations to supp ort this evolution. SON ET add/ drop m ulti-

plex and hub configurations can provide efficient traffic management capabilities,

both in the inter-node and access environments.

THE TRADITIONAL NETWORK

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S O N E T 1 0 1


InterexchangeCarrier (IEC)

To Users

SwitchingNode

SwitchingNode

SwitchingNode

Switching Node

Drop-and-InsertSwitchingNode

DSX-3Cross-Connect

Switching Node

Switch

Figure 5-1 Traditional Inter-Node Network

Inter-Node Network Access Network

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T1 Repeater

Consolidation of Switchedand Non-switched Linesat DSX-1 Cross-Connect

Switching

Node

Figure 5-2 Traditional Access Network

FOTS

M 13M ux

CSA (Carrie rServing Area)

T1 LineDigital Carrier* *

End Users

POTS Lines*

* Mostly copper pairs in the loop* * Copper twisted pair

DLC

Customer-Located

Equipment

Fiber MuxTerminal

DS-1

Special Service Lines(non-switched)**

Switching Node

FOTS

Inter-Node Fiber

150 Mbps

DigitalSwitch

ChannelBank

Fiber MuxTerminal

Recent Fiber Deployment

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50


Point-to-pointconfigurations are

supporte d i n SONET.Ma y be used to separate

between different carriers.

HUB

SwitchingNode

ADMSwitching Node

ADMSwitching Node

SONETTerminal

HUBSwitching Node

SurvivableRingMid-Span

Meet CraftTerminal

OC-n

OC-m

OC-n

OC-n

OC-n

OC-n

OC-n

OC-n

POP

Embedded OverheadChannels for OAM&P

ADM: SONET Add/Drop MuxHUB: SONET Hub (ADM, DCS)POP : P oi nt of P re se nc e

AccessInter-Node NetworkInterexchange Carrier

Figure 5-3 SONET Inter-Node Network

SurvivableRing

HUBSwitching Node

OC-n

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S O N E T 1 0 1


Local Inter-Node Deployment ExampleFigure 5 -4 illustrates ho w SO NET can pen etrate th e local inter-node network.

The encircled area is the urban corean area that could d eploy SO NET first

because of growth in th e core area, exhausted cap acity in the cu rrent optical

chann els, and T1 m odernization p rograms. Here, SO NET allows inter-nod e

trunk ing with room for further growth.

In the longer term, SONET deployment can expand to subu rban and rural

switching nod es. Hu b topo logies becom e comm on as circuits are consolidated to

route towards the core area. SON ET add /drop mu ltiplexers are used along the

way to pick up or drop off traffic.

Long-Haul Deployment ExampleFigure 5-5 depicts a typical SONET network for inter-exchange carrier applica-

tions. A long-haul SO NET network dep loys an OC -n high-speed transpo rt over-

laying an existing asynch ronou s network, using a 565 Mbp s FOTS. The main

SONET network elements are the SONET add/drop multiplexer used in terminal

and repeater configuration and the SONET broadband cross-connect used for

traffic management.

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52


Short-term SONET route s

Longer term SONETexpansion to peripheralareas

Hub

Figure 5- 4 Local Inter-Node SONET Deployment

Add/Drop

IEC

OC-n OC-n

OC-n

OC-n

OC-n

OC-n

OC-n

Add/Drop

IEC

Hub

Hub Topology inSuburban a ndRural Areas

M esh Topology inMe tropolitan Core for

Potential Restoration Routes

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S O N E T 1 0 1


SONET Termina l

SONET Add/ DropMux used inrepeaterconfiguration

Existing fiber opticterminal

Existing fiber opticrepeater

SONET Hub

Figure 5-5 Long-Haul SONET Deployment (Overlay on Existing 56 5-M bps Network)

OC-n

CityA

CityB

CityC

CityD

CityG

CityH

CityF

CityE

OC-n

OC-n

OC-n

OC-n

OC-n

OC-n

OC-n

OC-n

OC-n

OC-n

OC-n

565 Mbps 565 M bps

565 M bps

565 Mbps

City

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54


DEPLOYM ENT OF SONET IN THE ACCESS NETWORK

As shown in Figures 5-6 and 5-7, SONET can be deployed in the access network,particularly where new fiber is being installed and/or existing routes are being

exhausted, to provide new networking capabilities and transport of n ew services.

Different application scenarios are described below:

Simple Point-to-PointA po int-to-po int app lication is relatively simp le to imp lemen t. Instead o f a pair-

gain link such as twisted pair copper, fiber is used to con nect a con trol terminal

from a switching nod e switch to a remote terminal. SON ET will transport m ulti-

ple DS-1s to th e CSA with plenty of room for growth . The overhead chann el

link s all systems togeth er for int egrated O AM& P.

Add/Drop Applicati onThe SON ET system transports traffic from the switching n ode to a term inal at a

CSA or cust om er prem ises location. To pick up or drop off add ition al traffic,

intermediate SO NET add /drop mu ltiplexers are used for CSAs located along the

way. ADMs could also be con nected in a closed OC -n loop to provide a surviv-

able ring to po logy.

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SONETSwitch

Interfaceor SONET

Cross-Connect

Figure 5-6 SONET Access Network

DigitalLoopCarrier

CustomerPremises

Cable TV

AsynchronousFOTS

HUB Switching Node

EndUsers

Video

TV StudioCSA

Switch

OC-n

RadioRD-6C/11C

OC-nHigh CapacityInter-Node Route

VideoCodec

HUB

SONETTerminal

ADM

DS-3 or DS-1

DS-1/DS-3

ExistingTerminal Special Services

PBX

OC-n

OC-n

DS-1

DS-1

150 M bps

Consolidat ion ofServices at Hub

OC-mSONET

TerminalSwitching

Node

DSX-3

T1 Mux

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OC-n

ADM: SONET Add/Drop MuxCLE: Customer Located EquipmentCSA: Carr ie r Se rv ing Ar eaHUB: SONET Hub (ADM, DCS)RT: Remote Terminal /Digital Loop CarrierTERM: SONET Terminal

Figure 5-7 SONET Access Applica tions

SwitchingNode

CSA CSA

OC-m

OC-nOC-n

ADM ADM

TERM

RTTo Users

POTS Lines

DS-1DS-3

TERM

CSA

ADM

TERMOC-m

CSA

CLE

HUB

OC-n

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Hub Applicat ionUn derfilled fibers feed into a SON ET hu b w hich con solidates th e traffic onto one

fiber for transp ort to the switchin g nod e. At the switchin g nod e, the traffic is seg-

regated into switched and non -switched (special service dedicated lines), DS-1

and DS-3, or any com bination thereof.

ATM InterconnectSON ET enables network p roviders, includ ing teleph one com panies or other ser-

vice providers, to efficiently deliver wideband and broadband service to their busi-

ness custom ers. W ith an ATM mu ltiplexer or switch d eployed at the provider

location, DS-1, DS-3, and O C-3 b andw idths can b e delivered d irectly to business

custom ers locations.

The ATM device can interface transport products offered by several vendors.

Narrowban d service, such as ISDN , can b e delivered on various NG DLC access

products using an O C-3 or OC -12 feed to the h ost switching nod e.

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Following ANSIs developm ent of the SO N ET stand ard, the C CITT und ertook to

define a synchronization standard that would address interworking between the

European an d N orth American transm ission hierarchies. That effort culminated

in 19 89 with CC ITTs pu blication of the Synchronou s Digital Hierarchy (SDH )

standards. Synch ronou s Digital Hierarchy is a world stand ard, and as such ,

SONET can be considered a subset of SDH.

Transmission standards in N orth America and Europe evolved from different

basic-rate signals in th e non -synch ron ou s hierarchy. Tim e Division Mu ltiplexing

(TDM) in No rth America comb ines twenty four 6 4-kbps ch annels (DS-0s) into

one 1 .54-Mbp s DS-1 signal. European TDM m ultiplexes thirty-two 64 -kbps

chann els (E-0s) into one 2.048 Mbp s E-1 (CEPT format) signal. (The Europ ean

standard has becom e comm on in many parts of the world.) Figure 6-1 com pares

the North American and European non-synchronous transmission hierarchies.

The issues between N orth American an d C CITT stand ards-m akers involved how

to efficiently accommodate both the 1.5-Mbps and the 2-Mbps non-synchronous

hierarchies in a single synch ronization stand ard. The agreemen t reached sp ecified

a basic t ransmission rate of 51 Mbps for SO NET and a basic rate of 155 Mbps

for SDH.

H D S L I XC B IS D N C C I TT D I D N A N P L AT A M F J O S I U N I V SAT W AT S A D SL F T TC G S M H D S L R H C S O N E T T C P / IP Synchronous Digi tal Hierarchy

A WORLD STANDARD OF SYNCHRONIZATION

6

Figure 6-1 Non-Synchronous Hierarchies

North American Rate European Rate

Signal Speed Channels Signal Speed Channels

DS-0 64 kbps 1 DS-0 E-0 64 kbps 1 E-0

DS-1 1. 54 M bps 24 DS-0s E-1 2.048 M bps 32 E-0s

DS-2 6. 3 M bps 96 DS-0s E-2 8.45 M bps 128 E-0s

DS-3 44. 8 Mbps 28 DS-1s E-3 34 Mbps 16 E-1s

not defined E-4 140 M bps 64 E-1s

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Convergence of European and North American Hierarc hiesSON ET and SDH converge at SDHs 155 -Mbps b ase level, defined as STM-1 o r

Synch rono us Transp ort Mod ule-1. The base level for SON ET is STS-1 (or

O C-1) and is equivalent to 51.84 Mbp s. Thus, SDHs STM-1 is equivalent to

SON ETs STS-3 (3 x 5 1 .8 4 M b p s = 1 5 5 .5 M b p s). Hig h er SDH rat es o f STM-4

(622 Mbps), STM-16 (2.4 Gbp s), and STM-64 (9.9 Gbp s) have also been

defined . Figure 6-2 com pares the transmission h ierarchies of SON ET and SDH.

Multiplexing is accomp lished by com bining or interleaving m ultiple lower-

order signals (1.5 Mbps, 2 Mbps, etc.) into h igher-speed circuits (51 Mbps,

15 5 Mbp s, etc.). By changing the SON ET standard from bit-interleaving to byte-

interleaving, it became p ossible for SDH to accom mo date both the Europ ean andN orth American transm ission h ierarchies. This mod ification allows an STM-1

signal to carry mu ltiple 1.5-Mbp s or 2 -Mbps signals and mu ltiple STM signals

to b e aggregated to carry higher orders o f SO NET o r SDH tribu taries.

Asynchronous and Synchronous TributariesSDH d oes away with a n um ber of the lower m ultiplexing levels, allowing no n-syn-

chrono us 2-Mbp s tributaries to b e m ultiplexed to th e STM-1 level in a single

step. SDH recomm end ations define m ethod s of subd ividing the payload area of

an STM-1 frame in various ways so that it can carry com binations of synchronou s

and asynchronou s tribu taries. Using this m ethod , synchrono us transmissionsystems can accomm odate signals generated b y equipm ent op erating from various

levels of the non-synchronous hierarchy.

ANSI, C CITT, and the European Transmission Stand ards Institute (ETSI) continu e

to collaborate on SDH-SON ET interwo rking. Areas of ongoing stud y for

SDH-SONET products include market requirements, standards alignment and

compatibility, and global standardization.

60

S y n c h r o n o u s D i g i t a l H i e r a r ch y

Figure 6- 2 SONET/SDH Hierarchies

SONET Bit Rate SDH

STS-1 / OC-1 51. 84 Mbps

STS-3 / OC-3 155. 52 M bps STM -1

STS-12 / OC-12 622.08 M bps STM -4

STS-24 / OC-24 1244. 16 M bps

STS-48 / OC-48 2488. 32 M bps STM -16

STS-192 / OC-192 9953.28 M bps STM -64

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SON ET stand ardization efforts have been u nd erway since 19 85 , mainly from the

T1X1 sub comm ittee of the ECSA. No rthern Telecom h as helped define th e

fundamental concepts of payload, pointers, and STS-1 format, all of which form

the b ackbone o f the SO NET stand ard. They are briefly explained below:

Payload: an en velope used to transp ort signals of many types, giving SON ET

inherent flexibility to accomm odate D S-1, DS-2, DS-3, video, b roadband data,or any future services.

Payload pointers:used for frequency justification in order to maintain syn-

chronization without requ iring huge bu ffers. They allow payloads to b e con-

catenated, perm itting SON ET to also transpo rt band width signals greater than

50 Mbps.

The STS-1 base format:pro vides flexible signal transp ort with sufficient over-

head for OAM&P surveillance, performance monitoring, and communication

channels.

Network stra tegy and protocol prop osals for efficient u se of OAM& P data

channels: recomm endations h elped define u sage comm ands to com mu nicate

alarms, controls, and p erforman ce information at bo th system an d n etwork

(inter-system) levels.

Rationalization of section, line, and path performance monitoring to meet

future n etwork objectives:selecting an d determining th e right thresholds for

achieving desired qu ality of ser vice levels.

Determination of alarm and control methods.

Autom atic protection switching:helped define usage and implementation of

the protection bytes in the SONET overhead.

Op tical interfaces:proposals to the EIA (Electronics Industry Association) in

the areas of central wavelength, reflection penalty, and spectral width measure-

ments to account for mode partition noise.

H D S L I XC B IS D N C C I TT D I D N A N P L ATA M F J O S I U N I V SAT W AT S A D SL F T TC G S M H D S L R H C S O N E T T C P / I P C on t rib u t ion s t o S O N E T 7

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C o n t r i b u t i o n s t o S O N E T

Working in close consultation with Bellcore, network providers, carriers, and end

users, No rthern Telecom has also becom e the first sup plier to o ffer a com prehen -

sive SON ET-based solution to n etworking.

The S/DMS Product Family imp lem ents th e full power of SO NET, no t just the

basic SON ET interface stand ards. S/DMS prod uct s have bu ilt-in capab ilities to

ensu re the full value of SON ET can be realized in futu re networks . More infor-

mation on the full family of produ cts and N orthern Telecoms vision can b e

obtained from th e sup plier.

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H D S L I X C B I S D N C C I T T D I D N A N P L A T A M FJ O S I U N I V S AT W A T S A D S L F T T C G S M H D S L R H C S O N E T T C P / I P U N I L ist of Term s8

AIS (Alarm Ind icating Signa l)A code sent downstream indicating an upstream

failure has occurred.

ANSI (American National Standards Institute)A membership organization

which develops U .S. indu stry standards and co ordinates U.S. participation in the

International Stand ards O rganization (ISO ).

Asynchronous A network where transmission system payloads are not synch ro-

nized and each network terminal runs on its own clock.

ATM (Asynchronous Transfer Mode) A type o f multiplexing wh ich Broadb and

ISDN will use, where payload is multiplexed into cells.

Backhauling Cum bersome traffic management technique used to reduce

expense of m ultiplexing/dem ultiplexing.

BIP-8 (Bit In terleaved Parity-8) A method of error checking in SO NET which

allows a full set of performance statistics to be generated.

BISDN (Broadban d Integrated Services Digital Network ) A single ISDN

network which can h and le voice, data, and eventu ally video services.

Bit Stuffing In asynch ronou s systems, a techn ique used to synchronize

asynch ronou s signals to a com mo n rate before mu ltiplexing.

Broadband Services requiring 5 0-600 Mbp s transport capacity.

CCITT The techn ical organs of the Un ited Nations specialized agency for

telecomm un ications, now the International Telecomm un ications Union. They

fun ction throu gh international com mittees of teleph one ad ministrations an d

private operating agencies.

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L i s t o f Te r m s

CEPT European Con ference of Postal and Telecom mu nications Adm inistrations.

The CEPT format defines the 2.048-Mbps European T1/E-1 signal made up of 32

voice-frequency channels.

Concatenated The linking together of various d ata structures, for examp le two

bandwidths joined to form a single bandwidth.

Concaten ated STS-1 A Synchron ous Transport Signal (STS-NC), com posed of

n STS-1s com bined. It is used to transp ort signals that do n ot fit into an STS-1

(51 Mbps) payload .

Concatena ted VT A virtual tributary (VT x Nc) wh ich is comp osed of N x VTs

combined. Its payload is transported as a single entity rather than separate signals.

Data Communicat ions Channels OAM& P channels in SON ET that enable

comm un ications between intelligent controllers and individual network n odes as

well as inter-nod e com mu nications.

ECSA (Exchan ge Carrier Standard s Association) An organization that sp eci-

fies telecomm un ications standards for ANSI.

EIA (Electronics Industry Association)

FERF (Far End Receive Failure) A signal to indicate to the transm it site that a

failure has occurred at the receive site.

FCOT (Fiber Central Office Terminal) A generic term for fiber termin al which

can be co nfigured either fully digital, fully analog, or m ixed.

FOTS (Fiber O ptic Tran smission System)

Grooming Co n solidat ing or segregating traffic for efficiency.

Interleave The ability of SO NET to mix together and transport d ifferent types of

input signals in an efficient manner, thus allowing higher-transmission rates.

ISDN (Integrated Services Digital Network) An international telecom mu nica-

tions network stand ard that p rovides end -to-end digital conn ections for both

voice and non-voice service.

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Jitter Short waveform variations caus ed by vibration , voltage fluctu ations, con -

trol system instability, etc.

Line A transmission medium with equ ipm ent at both end s used to transport

information between two network elements.

MesochronousA network wh ereby all nod es are tim ed to a single clock source,

thus all timing is exactly the same (truly synchronous).

NarrowbandServices requiring u p to 1 .5 M bp s transport cap acity.

NE (Network Elements) In SON ET, the five basic netw ork elemen ts are

add /drop m ultiplexer, b roadband digital cross-connect, wideban d digital cross-

connect, digital loop carrier, and switch interface.

OAMO perations, Adm inistration, an d M ainten ance. Also called OAM& P.

OAM&P (Operations, Administration, Maintenance, and Provisioning)

Provides the facilities and p ersonnel required to m anage a network.

OC -1 (Optical Carrier Level 1) The op tical equ ivalent of an STS-1 signal.

OC -n (Op tical Carrier Level n) The op tical equ ivalent of an STS-n signal.

OrderwireA channel used by installers to expedite the provisioning of lines.

OS (Operations System) Sophisticated app lications software that o verlooks the

entire network.

OSI Seven-Layer Model A standard architecture for data comm un ications.

Layers define hardware an d software required for m ulti-vendor information pro-

cessing equ ipmen t to be m utu ally com patible. The seven layers from lowest to

highest are: ph ysical, link, network, transp ort, session, presentation, and applica-tion.

OverheadExtra bits in a digital stream used to carry information besides traffic

signals. O rderwire, for examp le, wou ld be considered overhead information.

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Packet SwitchingAn efficient method for breaking down and handling high-

volume traffic in a network.

Path A logical con nect ion betw een a po int wh ere an STS or VT is mu ltiplexed

to the p oint where it is dem ultiplexed.

PayloadThe portion of the SONET signal available to carry service signals such

as DS-1, DS-2, and DS-3.

Payload Pointers Indicates the beginning of the synch ronou s payload envelope.

PhotonicThe b asic un it of light transmission used to d efine the lowest (physi-

cal) layer in the OSI seven-layer model.

PlesiochronousA network with no des timed by separate clock sources with

almost the same timing.

PollAn individual control message from a central controller to an individual

station on a mu ltipoint n etwork inviting that station to send .

POP (Point-of-Presence) A point in the network w here inter-exchange carrier

facilities like DS-3 or OC-n meet with access facilities managed by telephone

comp anies or oth er service providers.

SDH (Synch ronou s Digital Hierarchy) The CCITT-defined world standard of

synchronization w hose base transm ission level is 15 5 Mbp s (STM-1) and is

equivalent to SO NETs STS-3 or O C-3 transm ission rate. SDH stand ards were

pub lished in 1 989 to address interworking between the European and North

American transmission hierarchies.

SectionAn optical span and its equipment.

SlipAn overflow (deletion) or underflow (repetition) of one frame of a signal ina rece iving bu ffer.

SONET (Synchronous Optical Network) A stand ard for optical transport that

defines optical carrier levels and their electrically equivalent synchronous trans-

port signals. SON ET allows for a mu lti-vend or environm ent and positions the

network for transp ort of new services, synch ronou s networking, and en hanced

O AM& P.

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StratumLevel of clock so urce used to categorize accuracy.

SynchronousA network wh ere transm ission system p ayloads are synchron ized

to a m aster (network) clock and traced to a reference clock.

Synchr onou s Transfer Mod ule (STM) A measure of the SDH transm ission

hierarchy. STM-1 is SDH s base-level transm ission rate equal to 155 Mbp s.

Higher rates of STM-4, STM-16, and STM-64 are also defined.

SPE (Synchron ous Payload En velope) The m ajor portion of the SO NET frame

format u sed to transp ort payload and STS path overhead.

STS-1 (Synchronous Transport Signal Level 1)The basic SONET building

block signal transm itted at 5 1.84 Mbp s data rate.

STS-n (Synchronous Transport Signal Level n) The signal obtained by mu lti-

plexing integer m ultiples (n ) of STS-1 signals togeth er.

T1X1 Subcom mittee A comm ittee within the ECSA that specifies SO NET

optical interface rates and formats.

VT (Virtu al Tribu tary) A signal designed for transport and switching of sub-

STS-1 p ayload s.

VT GroupA 9 row x 12 column structure (108 bytes) that carries one or more

VTs of th e same size. Seven VT grou ps can be fitted in to on e STS-1 p ayload .

Wander Long-term variations in a waveform .

Wideband Services requiring 1 .5-50 Mbp s transpo rt capacity.

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