lecture note on synchronous optical network (sonet)
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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 - PowerPoint PPT PresentationTRANSCRIPT
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
CentralCentralOfficeOffice
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
D2D2Data ComData Com
D3D3Data 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
D4D4Data ComData Com
D5D5Data ComData Com
D6D6Data ComData Com
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
D7D7Data ComData Com
D8D8Data ComData Com
D9D9Data ComData Com
D10D10Data ComData Com
D11D11Data ComData Com
D12D12Data ComData Com
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