Lecture Note on Synchronous Optical Network (SONET)

Telephony: Multiplexing

Telephone Trunks between central offices carry hundreds of conversations: Can’t run thick bundles!

Send many calls on the same wire: multiplexing Analog multiplexing

bandlimit call to 3.4 KHz and frequency shift onto higher bandwidth trunk Digital multiplexing: convert voice to samples

8000 samples/sec => call = 64 Kbps

Telephony: Multiplexing Hierarchy Pre-SONET:

Telephone call: 64 kbps T1 line: 1.544 Mbps = 24 calls (aka DS1) T3 line: 45 Mbps = 28 T1 lines (aka DS3)

Multiplexing and de-multiplexing based upon strict timing (synchronous) At higher rates, jitter is a problem Have to resort to bit-stuffing and complex extraction => costly

“plesiochronous” hierarchy

SONET developed for higher multiplexing aggregates Use of “pointers” like C to avoid bit-stuffing

Digital Telephony in 1984

Key System Aspects:Key System Aspects:• M13 Building BlocksM13 Building Blocks• AsynchronousAsynchronous OperationOperation• Electrical DS3 SignalsElectrical DS3 Signals• Proprietary Fiber Proprietary Fiber Systems Systems • Brute ForceBrute Force Cross Cross ConnectConnect• AT&T Network/Western AT&T Network/Western Electric EquipmentElectric Equipment

CentralCentralOfficeOffice



FiberFiber

Fiber OpticFiber OpticTransmissionTransmission

SystemsSystems• SwitchesSwitches• Leased LineLeased Line

M13M13 M13M13

DS3DS3

DS1DS1

DS3DS3

DS1 CrossDS1 CrossConnectConnect

M13M13

DS1DS1

DS1DS1

No GuaranteedNo GuaranteedTimingTiming

SynchronizationSynchronization

Digital Carrier Hierarchy (cont’d)

• Multiplexing trunk networks: called “carrier” systems (eg: T-carrier): – allowed fast addition of digital trunk capacity without expensive layout of new

cables

• Time frames (125 us) and a per-frame bit in the T-carrier for synchronization => TDM– Each phone call (DS0) occupies same position in the frame

• Overhead bits: error control– “robbed” bits in voice call for OAM information– Too many 0s => synch loss (max number = 15)– “yellow alarm”. 1s density etc => usable b/w = 7bits/frame => 56 kbps

• Europe: E1; more streamlined framing & 2.048 Mbps• Variants: Concatenated T1, Un-channelized (raw) T1

Digital Hierarchy (Cont’d)

• 1980s: demand for bandwidth. But > T3s not available except in proprietary form

– Fiber-optic interface for T3 was proprietary– Primitive online OAM&P capabilities (e.g: robbed bits…)– Fewer operators: interoperability/mid-span meet not critical– Changed dramatically after 1984 deregulation!

• Public vs Private Networks: – Private: Customer operates networks (e.g: w/ private leased lines): developed from PBX &

SNA– Public: Provider operates networks for subscribers– More public networks (e.g: X.25) outside US

• Drivers of SONET:– IBM SNA/mainframes => hub-and-spoke networking– Increase of PCs => client-server & p2p computing => more demands on long-distance

trunks– T-carrier evolution rate much slower than computing trends

Digital Hierarchy (Cont’d)• Digital streams organized as bytes (eg: voice samples, data)• Byte interleaving: (eg: 24 DS0 -> DS1)

– service one byte from each input port into a transmission frame – Simple device: T1 mux a.k.a channel bank– Very convenient for processing, add-drop multiplexor (ADM) or Digital Cross-connect

System (DCS) functions (fig 3.8/3.10)– ADM/DCS does both mux (“add”) and demux (“drop”) functions => need to do this with

minimal buffering, fast/scalable processing

• Bit-interleaving (eg: DS1 -> DS2 etc)– Cant use buffers to mask jitter! => bit stuffing– Partly because high speed memory was costly then!– “Plesiochronous hierarchy” => harder to ADM/DCS because full de-stuffing/de-

multiplexing necessary before these functions – DS3s used to be muxed using proprietary optical methods (eg: M13 mux): SONET solves

all these problems

US Telephone Network Structure (after 1984 divestiture)

Post-AT&T Divestiture Dilemmas

Needs:Needs: • Support Faster Fiber Support Faster Fiber • Support New ServicesSupport New Services• Allow Other Allow Other TopologiesTopologies• Standardize Standardize RedundancyRedundancy• Common OAM&PCommon OAM&P• Scalable Cross Scalable Cross ConnectConnect

DifferentDifferentCarriers,Carriers,VendorsVendors

• SwitchesSwitches• Leased LineLeased Line• LAN ServicesLAN Services• Data ServicesData Services

M13M13

SupportSupportOtherOther

Topologies,Topologies,Protect FibersProtect Fibers

DS1DS1

InternalInternalDS3 CrossDS3 CrossConnectConnect

The SONET Standards Process

19841984 19851985 19861986 19871987 19881988

SONET/SDHSONET/SDHStandardsStandardsApprovedApproved

ANSI ApprovesANSI ApprovesSYNTRANSYNTRAN

DivestitureDivestiture

Exchange CarriersExchange CarriersStandards Associate (ECSA)Standards Associate (ECSA)T1 Committee FormedT1 Committee Formed

ANSI T1X1ANSI T1X1ApprovesApprovesProjectProject

Bellcore ProposedBellcore ProposedSONET PrinciplesSONET PrinciplesTo ANSI T1X1To ANSI T1X1

CCITT ExpressesCCITT ExpressesInterest in SONETInterest in SONET

British and JapaneseBritish and JapaneseParticipation in T1X1Participation in T1X1

CCITT XVIIICCITT XVIIIBegins StudyBegins StudyGroupGroup

CEPT ProposesCEPT ProposesMerged ANSI/CCITTMerged ANSI/CCITT

StandardStandard

US T1X1 AcceptsUS T1X1 AcceptsModificationsModifications

>400 Technical Proposals>400 Technical Proposals• Rate Discussions AT&T vs. Rate Discussions AT&T vs. BellcoreBellcore(resolved w/ (resolved w/ virtual tributaryvirtual tributary concept)concept)• International Changes For International Changes For Byte/Bit Interleaving, Frames, Byte/Bit Interleaving, Frames, Data RatesData Rates• Phase I, II, III Separate APS, etc.Phase I, II, III Separate APS, etc.• ITU’s SDH initiative…ITU’s SDH initiative…

SONET Concept Developed By BellcoreSONET Concept Developed By Bellcore

SONET Standards Story

• SYNTRAN: pre-divestiture effort, no pointer concept. • SONET: primarily US (divestiture) driven• AT&T vs Bellcore debate: 146.432 Mbps vs 50.688 Mbps: compromise at

49.94 Mbps– Virtual tributary concept to transport DS-1 services

• 1986: CCITT (ITU) starts own effort (SDH)• June 1987: change SONET from bit-interleaved to byte-interleaved; and

rate from 49.92 to 51.84 Mbps• Phased rollouts:

– 1988 = Phase 1: signal level interoperability– Phase II: OAM&P functions: embedded channel & electrical I/f specification, APS work

initiated– Phase III: OSI network management adopted

• Seamless worldwide connectivity (allowed Europe to merge its E-hierarchy into SDH)

SONET: Achievements

1. Standard multiplexing using multiples of 51.84 Mbps (STS-1 and STS-N) as building blocks

2. Optical signal standard for interconnecting multiple vendor equipment

3. Extensive OAM&P capabilities

4. Multiplexing formats for existing digital signals (DS1, DS2 etc)

5. Supports ITU hierarchy (E1 etc)

6. Accommodates other applications: B-ISDN etc

SONET Lingo

• OC-N: Optical carrier Nx51.84 Mbps– Approximate heuristic: bit rate = N/20 Gbps (e.g: OC-48 => 48/20 = 2.4 Gbps)– Overhead percentage = 3.45% for all N – OC signal is sent after scrambling to avoid long string of zeros and ones to enable clock

recovery

• STS-N: Synchronous Transport Signal (electronic equivalent of OC)• Envelope: Payload + end-system overhead

– Synchronous payload envelope (SPE): 9 rows, 87 columns in STS-1

• Overhead: management OAM&P portion• Concatenation: “un-channelized” (envelope can carry “super-rate” data

payloads: eg: ATM): Eg: OC-3c– Method of concatenation different from that of T-carrier hierarchy…

SONET Multiplexing Possibilities

•Asynchronous DS-3

•Virtual Tributaries for DS1 etc

•STS-3c for CEPT-4 and B-ISDN

STS-1s are mutually synchronized irrespective of inputs

STS-1 Frame Format

90 Bytes90 BytesOr “Columns”Or “Columns”

99RowsRows

Small Rectangle =1 Byte

Two-dimensional frame representation (90 bytes x 9 bytes)…

Frame Transmission: Top Row First, Sent Left To Right• Time-frame: 125 s/Frame• Frame Size & Rate: 810 Bytes/Frame * 8000 Frames/s * 8 b/byte= 51.84 Mbps• For STS-3, only the number of columns changes (90x3 = 270)

STS = Synchronous Transport Signal

STS-1 Headers

90 Bytes90 BytesOr “Columns”Or “Columns”

99RowsRows

Section Overhead (SOH)

Line Overhead (LOH)Path Overhead (POH): Floating => can begin anywhere

Line + Section overhead = Transport Overhead (TOH)

SONET Equipment Types

PTEPTE

SONET EndSONET EndDevice - I.e.Device - I.e.TelephonyTelephonySwitch, RouterSwitch, Router

• Section Termination Section Termination (STE)(STE)

SectionsSections

RepeatersRepeaters

PTEPTE

LineLine

• Line Termination (LTE)Line Termination (LTE)

PathPath

• Path Termination (PTE)Path Termination (PTE)

SONET Overhead Processing

Headers: Section Overhead (SOH)

A1=0xF6

A2A2=0x28=0x28

J0/Z0J0/Z0STS-IDSTS-ID

B1B1BIP-8BIP-8

E1E1OrderwireOrderwire

F1F1UserUser

D1D1Data ComData Com



Section Overhead• 9 Bytes Total• Originated And Terminated By All Section Devices (Regenerators, Multiplexers, CPE)• Other Fields Pass Unaffected

RcvRcvSOHSOH

Selected Fields: •A1,A2 - Framing Bytes•BIP-8 - Bit Interleaved Parity• F1 User - Proprietary OAM Management

XmtXmtSOHSOH

Headers: Line Overhead (LOH)

H1H1PointerPointer

H2H2PointerPointer

H3H3Pointer ActPointer Act

B2B2BIP-8BIP-8

K1K1APSAPS

K2K2APSAPS




Line Overhead• 18 Bytes Total• Originated And Terminated By All Line Devices (Multiplexers, CPE)• LOH+SOH=TOH (Transport OH)

Selected Fields: •H1-3 - Payload Pointers•K1, K2 - Automatic Protection Switching• D4-D12 - 576 kbps OSI/CMIP







S1S1SyncSync

M0M0REIREI

E1E1OrderwireOrderwire

RcvRcvSOHSOH

XmtXmtSOHSOH

XmXmtt

SOSOHH

XmXmtt

LOLOHH

RcvRcvSOHSOH

RcvRcvLOHLOH

Floating Payload: SONET LOH Pointers

SPE is not frame-aligned: overlaps multiple frames! Avoids buffer management complexity & artificial delaysAllows direct access to byte-synchronous lower-level signals (eg: DS-1) with just one frame recovery procedure

SPE: Synchronous Payload Envelope

Synchronous Payload Envelope• Contains POH + Data• First Byte Follows First Byte Of POH• Wraps In Subsequent Columns• May Span Frames• Up To 49.536 Mbps for Data:

•Enough for DS3

Defined Payloads• Virtual Tributaries (For DS1, DS2)• DS3• SMDS• ATM• PPP …

Headers: Path Overhead (POH)

J1J1TraceTrace

B3B3BIP-8BIP-8

C2C2Sig LabelSig Label

Path Overhead• H1,H2 fields of LOH points to Beginning of POH

Selected fields:•BIP-8 - Parity• C2 - Payload Type Indicator• G1 - End End Path Status

G1G1Path StatPath Stat

F2F2UserUser

H4H4IndicatorIndicator

PTEPTE

Z3Z3GrowthGrowth

Z4Z4GrowthGrowth

Z5Z5TandemTandem

PTEPTESTESTE

Frame NFrame N

Frame N+1Frame N+1Frame NFrame N

Frame N+1Frame N+1

•POH Beginning Floats Within Frame• 9 Bytes (1 Column) Spans Frames• Originated And Terminated By All Path Devices (I.e. CPE, Switches)

• End-to-end OAM support

STS-1 Headers: Putting it Together

Accommodating Jitter

Positive Stuff Positive Stuff Negative Stuff Negative Stuff

• To Shorten/Lengthen Frame:• Byte After H3 Ignored; Or H3 Holds Extra Byte

• H1, H2 Values Indicate Changes - Maximum Every 4 Frames• Requires Close (Not Exact) Clock Synch Among Elements

Clock Synchronization

Building Integrated Timing Building Integrated Timing SystemSystem• Hierarchical Clocking Hierarchical Clocking DistributionDistribution• Normally All Synch’d To Stratum Normally All Synch’d To Stratum 1 (Can Be Cesium/Rubidium Clock)1 (Can Be Cesium/Rubidium Clock)• Dedicated Link Or RecoveredDedicated Link Or Recovered• Fallback To Higher Stratum In Fallback To Higher Stratum In Failure (Temperature Controlled Failure (Temperature Controlled Crystal)Crystal)

PTEPTE

PTEPTE

BITSBITS BITSBITS

BITSBITS

PrimaryPrimaryReferenceReference

BackupBackupReferenceReference

•Level 1: 10Level 1: 10-11-11

•Level 2: 1.6x10Level 2: 1.6x10-8-8

•Level 3: 4.6x10Level 3: 4.6x10-6-6

•Level 4: 32x10Level 4: 32x10-6-6

STS-N Frame Format

Composite Frames:Composite Frames:• Byte InterleavedByte Interleaved STS-1’s STS-1’s• Clock RateClock Rate = Nx51.84 Mbps = Nx51.84 Mbps• 9 colns overhead9 colns overhead

90xN Bytes90xN BytesOr “Columns”Or “Columns”

N Individual STS-1 FramesN Individual STS-1 Frames

ExamplesExamples STS-1STS-1 51.84 Mbps 51.84 Mbps

STS-3STS-3 155.520 Mbps 155.520 MbpsSTS-12STS-12 622.080 Mbps 622.080 MbpsSTS-48STS-48 2.48832 Gbps 2.48832 GbpsSTS-192 9.95323 GbpsSTS-192 9.95323 Gbps

Multiple frame streams, w/ independent payload pointersNote: header columns also interleaved

STS-N: Generic Frame Format

STS-1 STS-N

Example: STS-3 Frame Format

STS-Nc Frame Format

Concatenated mode:Concatenated mode:• Same TOH Structure And Data Rates As Same TOH Structure And Data Rates As STS-NSTS-N• Not All TOH Bytes UsedNot All TOH Bytes Used• First H1, H2 Point To POHFirst H1, H2 Point To POH• Single PayloadSingle Payload In Rest Of SPE In Rest Of SPE• Accommodates FDDI, E4, dataAccommodates FDDI, E4, data

90xN Bytes90xN BytesOr “Columns”Or “Columns”

Transport Overhead: Transport Overhead: SOH+LOHSOH+LOH

Current IP over SONET technologies use concatenated mode: OC-3c (155 Mbps) to OC-192c (10 Gbps) ratesa.k.a “super-rate” payloads

Virtual Tributaries (Containers)

• Opposite of STS-N: sub-multiplexing• STS-1 is divided into 7 virtual tributary groups (12 columns ea), which can be

subdivided further• VT groups are byte-interleaved to create a basic SONET SPE• VT1.5: most popular quickly access T1 lines within the STS-1 frame• SDH uses the word “virtual containers” (VCs)

Virtual Tributaries: Pointers

• VT payload (a.k.a VT SPE) floats inside the VT• One more level of pointer used to access it.

– Can access a T1 with just two pointer operations– Very complex to do the same function in DS-3– Eg: accessing DS0 within DS-3 requires FULL de-multiplexing: a.k.a stacked

multiplexing or mux-mountains!

Practical SONET Architectures

Today: multiple “stacked” rings over DWDM (different s)

SONET Network Elements

Nonstandard, Functional NamesTM: Terminal Mux: (aka LTE: ends of pt-pt links)ADM: Add-Drop MuxDCC: Digital Cross Connect (Wideband and Broadband)MN: Matched NodeD+R: Drop and Repeat

ADMADMTMTMDS1sDS1s

DS1sDS1s

MNMN MNMN

MNMNDCCDCC

D+RD+R

D+RD+R

D+RD+RMNMN

Digital Cross Connects (DCS)

• Cross-connects thousands of streams under software control (replaces patch panel)

• Handles performance monitoring, PDH/SONET streams, and also provides ADM functions

• Grooming: – Grouping traffic with similar destinations, QoS etc– Muxing/extracting streams also

• Narrow-/wide-/broad-band and optical crossconnects

Topology Building Blocks

DCCDCC

ADMADM

ADMADM

ADMADM

DCCDCC

ADMADM

ADMADM

ADMADM 2 Fiber Ring2 Fiber RingEach Line IsEach Line IsFull DuplexFull Duplex

DCCDCC

ADMADM

ADMADM

ADMADM 4 Fiber Ring4 Fiber RingEach Line IsEach Line IsFull DuplexFull Duplex

DCCDCC

ADMADM

ADMADM

ADMADM

Uni- vs. Bi-Uni- vs. Bi-DirectionalDirectionalAll Traffic Runs All Traffic Runs Clockwise, vs Either Clockwise, vs Either WayWay

APS

ADMADM

Line Protection SwitchingLine Protection SwitchingUses TOHUses TOHTrunk ApplicationTrunk ApplicationBackup Capacity Is IdleBackup Capacity Is IdleSupports 1:n, N=1-14Supports 1:n, N=1-14

Automatic Protection SwitchingAutomatic Protection Switching• Line Or Path BasedLine Or Path Based• Revertive vs. Non-RevertiveRevertive vs. Non-Revertive• Mechanism For Intentional Mechanism For Intentional CutoverCutover• Restoration Times ~ 50 msRestoration Times ~ 50 ms• K1, K2 Bytes Signal ChangeK1, K2 Bytes Signal Change• Common Uses: 2 Fiber UPSR or Common Uses: 2 Fiber UPSR or ULSR, 4 Fiber BPSRULSR, 4 Fiber BPSR

ADMADMADMADM ADMADM

Path Protection SwitchingPath Protection SwitchingUses POHUses POHAccess Line ApplicationsAccess Line ApplicationsDuplicate Traffic Sent On ProtectDuplicate Traffic Sent On Protect1+11+1

ADMADMADMADM

Lecture Note on Synchronous Optical Network (SONET)

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Transcript of Lecture Note on Synchronous Optical Network (SONET)