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Shivkumar KalyanaramanRensselaer Polytechnic Institute
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SONET: Broadband Convergence at Layer 1
Shivkumar KalyanaramanRensselaer Polytechnic Institute
shivkuma@ecse.rpi.edu http://www.ecse.rpi.edu/Homepages/shivkuma
Based in part on slides of Nick McKeown (Stanford)
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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Telephony: Multiplexing
Telephone Trunks between central offices carry hundreds of conversations: Can’t run thick bundles!Send many calls on the same wire: multiplexingAnalog multiplexing
bandlimit call to 3.4 KHz and frequency shift onto higher bandwidth trunk
Digital multiplexing: convert voice to samples8000 samples/sec => call = 64 Kbps
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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Telephony: Multiplexing HierarchyPre-SONET:
Telephone call: 64 kbpsT1 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 problemHave to resort to bit-stuffing and complex extraction => costly “plesiochronous” hierarchy
SONET developed for higher multiplexing aggregatesUse of “pointers” like C to avoid bit-stuffing
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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Digital Telephony in 1984
Key System Aspects:Key System Aspects:•• M13 Building BlocksM13 Building Blocks•• AsynchronousAsynchronous OperationOperation•• Electrical DS3 SignalsElectrical DS3 Signals•• Proprietary Fiber Systems Proprietary Fiber Systems •• Brute ForceBrute Force Cross ConnectCross Connect•• 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
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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Digital Carrier Hierarchy (contd)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 informationToo many 0s => synch loss (max number = 15)“yellow alarm”. 1s density etc => usable b/w = 7bits/frame => 56kbps
Europe: E1; more streamlined framing & 2.048 MbpsVariants: Concatenated T1, Un-channelized (raw) T1
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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Digital Hierarchy (Contd)1980s: demand for bandwidth. But > T3s not available except in proprietary form
Fiber-optic interface for T3 was proprietaryPrimitive online OAM&P capabilities (eg: robbed bits…)Fewer operators: interoperability/mid-span meet not criticalChanged dramatically after 1984 deregulation!
Public vs Private Networks:Private: Customer operates n/w (eg: w/ private leased lines): developed from PBX & SNAPublic: Provider operates n/w for subscribersMore public networks (eg: X.25) outside US
Drivers of SONET:IBM SNA/mainframes => hub-and-spoke networkingIncrease of PCs => client-server & p2p computing => more demands on long-distance trunksT-carrier evolution rate much slower than computing trends
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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Digital Hierarchy (Contd)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 bankVery 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 stuffingPartly 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
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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US Telephone Network Structure (after 1984 divestiture)
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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Post-AT&T Divestiture Dilemmas
Needs:Needs:•• Support Faster Fiber Support Faster Fiber •• Support New ServicesSupport New Services•• Allow Other TopologiesAllow Other Topologies•• Standardize RedundancyStandardize Redundancy•• Common OAM&PCommon OAM&P•• Scalable Cross ConnectScalable Cross Connect
DifferentDifferentCarriers,Carriers,VendorsVendors
•• SwitchesSwitches•• Leased LineLeased Line•• LAN ServicesLAN Services•• Data ServicesData Services
M13M13
SupportSupportOtherOther
Topologies,Topologies,Protect FibersProtect Fibers
DS1DS1
InternalInternalDS3 CrossDS3 CrossConnectConnect
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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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 Bellcore ProposedProposedSONET 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 Byte/Bit International Changes For Byte/Bit Interleaving, Frames, Data RatesInterleaving, Frames, Data Rates•• Phase I, II, III Separate APS, etc.Phase I, II, III Separate APS, etc.•• ITU’s ITU’s SDH initiative…SDH initiative…
SONET Concept Developed By SONET Concept Developed By BellcoreBellcore
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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SONET Standards StorySYNTRAN: pre-divestiture effort, no pointer concept. SONET: primarily US (divestiture) drivenAT&T vs Bellcore debate: 146.432 Mbps vs 50.688 Mbps: compromise at 49.94 Mbps
Virtual tributary concept to transport DS-1 services1986: CCITT (ITU) starts own effort (SDH)June 1987: change SONET from bit-interleaved to byte-interleaved; and rate from 49.92 to 51.84 MbpsPhased rollouts:
1988 = Phase 1: signal level interoperabilityPhase II: OAM&P functions: embedded channel & electrical I/f specification, APS work initiatedPhase III: OSI network management adopted
Seamless worldwide connectivity (allowed Europe to merge its E-hierarchy into SDH)
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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SONET: Achievements1. Standard multiplexing using multiples of 51.84 Mbps (STS-1 and STS-N) as building blocks2. Optical signal standard for interconnecting multiple vendor equipment3. Extensive OAM&P capabilities4. Multiplexing formats for existing digital signals (DS1, DS2 etc)5. Supports ITU hierarchy (E1 etc)6. Accomodates other applications: B-ISDN etc
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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SONET LingoOC-N: Optical carrier Nx51.84 Mbps
Approximate heuristic: bit rate = N/20 Gbps (eg: OC-48 => 48/20 = 2.4 Gbps)Overhead percentage = 3.45% for all N (unlike PDH!)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 portionConcatenation: “un-channelized” (envelope can carry “super-rate” data payloads: eg: ATM): Eg: OC-3c
Method of concatenation different from that of T-carrier hierarchy…
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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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
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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90 Bytes90 BytesOr “Columns”Or “Columns”
99RowsRows
Small Rectangle =1 Byte
STS-1 Frame Format
Two-dimensional frame representation (90 bytes x 9 bytes)…Frame Transmission: Top Row First, Sent Left To Right
• Time-frame: 125 µs/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
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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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)
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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SONET Equipment Types
PTEPTE
SONET EndSONET EndDevice Device -- I.e.I.e.TelephonyTelephonySwitch, RouterSwitch, Router
•• Section Termination (STE)Section Termination (STE)
SectionsSections
RepeatersRepeaters
PTEPTE
LineLine•• Line Termination (LTE)Line Termination (LTE)
PathPath
•• Path Termination (PTE)Path Termination (PTE)
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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A1=0xF6
A2A2=0x28=0x28
J0/Z0J0/Z0STSSTS--IDID
B1B1BIPBIP--88
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: Section Overhead (SOH)
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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RcvRcvSOHSOH
XmtXmtSOHSOH
XmtXmtSOHSOH
XmtXmtLOHLOH
RcvRcvSOHSOH
RcvRcvLOHLOH
Headers: Line Overhead (LOH)H1H1
PointerPointerH2H2
PointerPointerH3H3
Pointer ActPointer ActB2B2
BIPBIP--88K1K1
APSAPSK2K2
APSAPSD4D4
Data ComData ComD5D5
Data ComData ComD6D6
Data ComData Com
D7D7Data ComData Com
D8D8Data ComData Com
D9D9Data ComData Com
D10D10Data ComData Com
D11D11Data ComData Com
D12D12Data ComData Com
S1S1SyncSync
M0M0REIREI
E1E1OrderwireOrderwire
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
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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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
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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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 …
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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J1J1TraceTrace
B3B3BIPBIP--88
C2C2Sig Sig LabelLabel
Path Overhead• H1,H2 fields of LOH points to Beginning of POH
G1G1Path StatPath Stat
F2F2UserUser
H4H4IndicatorIndicator
PTEPTE
Z3Z3GrowthGrowth
Z4Z4GrowthGrowth
Z5Z5TandemTandem
PTEPTESTESTE
Frame NFrame NFrame N+1Frame N+1
Frame NFrame NFrame 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
Headers: Path Overhead (POH)
Selected fields:•BIP-8 - Parity• C2 - Payload Type Indicator• G1 - End End Path Status
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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Positive Stuff Positive Stuff Negative Stuff Negative Stuff
Accommodating Jitter
• 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
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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Building Integrated Timing SystemBuilding Integrated Timing System•• Hierarchical Clocking DistributionHierarchical Clocking Distribution•• Normally All Normally All Synch’d Synch’d To Stratum 1 To Stratum 1 (Can Be Cesium/Rubidium Clock)(Can Be Cesium/Rubidium Clock)•• Dedicated Link Or RecoveredDedicated Link Or Recovered•• Fallback To Higher Stratum In Failure Fallback To Higher Stratum In Failure (Temperature Controlled Crystal)(Temperature Controlled Crystal)
PTEPTE
PTEPTE
BITSBITS BITSBITS
BITSBITS
PrimaryPrimaryReferenceReference
BackupBackupReferenceReference
••Level 1: 10Level 1: 10--1111
••Level 2: 1.6x10Level 2: 1.6x10--88
••Level 3: 4.6x10Level 3: 4.6x10--66
••Level 4: 32x10Level 4: 32x10--66
Clock Synchronization
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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STS-N Frame Format
Composite Frames:Composite Frames:•• Byte InterleavedByte Interleaved STSSTS--1’s1’s•• Clock RateClock Rate = Nx51.84 Mbps= Nx51.84 Mbps•• 9 9 colns colns overheadoverhead
90xN Bytes90xN BytesOr “Columns”Or “Columns”
N Individual STSN Individual STS--1 Frames1 Frames
ExamplesExamplesSTSSTS--11 51.84 Mbps51.84 Mbps
STSSTS--33 155.520 Mbps155.520 MbpsSTSSTS--1212 622.080 Mbps622.080 MbpsSTSSTS--4848 2.48832 2.48832 GbpsGbpsSTSSTS--192 9.95323 192 9.95323 GbpsGbps
Multiple frame streams, w/ independent payload pointersNote: header columns also interleaved
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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Concatenated mode:Concatenated mode:•• Same TOH Structure And Data Rates As STSSame TOH Structure And Data Rates As STS--NN•• 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 SPEIn Rest Of SPE•• Accommodates FDDI, E4, dataAccommodates FDDI, E4, data
90xN Bytes90xN BytesOr “Columns”Or “Columns”
Transport Overhead: Transport Overhead: SOH+LOHSOH+LOH
STS-Nc Frame Format
Current IP over SONET technologies use concatenatedmode: OC-3c (155 Mbps) to OC-192c (10 Gbps) ratesa.k.a “super-rate” payloads
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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Virtual Tributaries (Containers)
Opposite of STS-N: sub-multiplexingSTS-1 is divided into 7 virtual tributary groups (12 columns ea), which can be subdivided furtherVT groups are byte-interleaved to create a basic SONET SPEVT1.5: most popular quickly access T1 lines within the STS-1 frameSDH uses the word “virtual containers” (VCs)
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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Virtual Tributaries: Pointers
VT payload (a.k.a VT SPE) floats inside the VTOne more level of pointer used to access it.
Can access a T1 with just two pointer operationsVery complex to do the same function in DS-3Eg: accessing DS0 within DS-3 requires FULL de-multiplexing: a.k.a stacked multiplexing or mux-mountains!
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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SONET Transmission Encoding
OCOC--N Is Optical Carrier STSN Is Optical Carrier STS--NNLong Reach: 40 kmLong Reach: 40 km
1310 or 1550 nm SM1310 or 1550 nm SMIntermediate Reach: 15 kmIntermediate Reach: 15 km
1310 or 1550 nm SM1310 or 1550 nm SMShort Reach:Long Reach 2 kmShort Reach:Long Reach 2 km
1310 nm MM1310 nm MM
E OE O
Electrical Transmission Electrical Transmission StandardStandard•• STSSTS--1: B3ZS (BPV), 450’1: B3ZS (BPV), 450’•• STSSTS--3: Coded Mark 3: Coded Mark Inversion, 225’Inversion, 225’•• Useful IntraUseful Intra--Office Office ConnectionConnection
1+x1+x66+x+x77
ScramblingScrambling•• Ensures Ones DensityEnsures Ones Density•• Does Not Include A1, A2, C1 BytesDoes Not Include A1, A2, C1 Bytes•• Output Is NRZ EncodedOutput Is NRZ Encoded
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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Packet Over SONET (POS)
PPPPPP ByteByteStuffStuffFCSFCS ScramblingScrambling SONETSONET
FramingFraming
Standard PPP Encapsulation• Magic Number Recommended• No Address and Control Compression• No Protocol Field Compression
Standard CRC Computation• OC3 May Use CRC-16• Other Speeds Use CRC-32
Special Data Scrambler• 1+ x43 Polynomial• Protects Against Transmitted Frames Containing Synch Bytes Or Insufficient Ones Density
SONET Framing• OC3, OC12, OC48, OC192 Defined• C2 Byte = 0x16 With Scrambling• C2 Byte = oxCF Without (OC-3)
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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Practical SONET Architectures
Today: multiple “stacked” rings over DWDM (different λs)
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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ADMADMTMTMDS1sDS1s
DS1sDS1s
MNMN MNMN
MNMNDCCDCC
D+RD+R
D+RD+R
D+RD+RMNMN
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
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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Digital Cross Connects (DCS)
Cross-connects thousands of streams under software control (replaces patch panel)Handles perf monitoring, PDH/SONET streams, and also provides ADM functionsGrooming:
Grouping traffic with similar destinations, QoS etcMuxing/extracting streams also
Narrow-/wide-/broad-band and optical crossconnects
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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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
UniUni-- vs. Bivs. Bi--DirectionalDirectional
All Traffic Runs Clockwise, All Traffic Runs Clockwise, vsvs Either WayEither Way
Topology Building Blocks
Shivkumar KalyanaramanRensselaer Polytechnic Institute
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APS
ADMADM
Line Protection SwitchingLine Protection SwitchingUses TOHUses TOHTrunk ApplicationTrunk ApplicationBackup Capacity Is IdleBackup Capacity Is IdleSupports 1:n, N=1Supports 1:n, N=1--1414
Automatic Protection SwitchingAutomatic Protection Switching•• Line Or Path BasedLine Or Path Based•• RevertiveRevertive vs. Nonvs. Non--RevertiveRevertive•• Mechanism For Intentional CutoverMechanism For Intentional Cutover•• Restoration Times ~ 50 msRestoration Times ~ 50 ms•• K1, K2 Bytes Signal ChangeK1, K2 Bytes Signal Change•• Common Uses: 2 Fiber UPSR or ULSR, Common Uses: 2 Fiber UPSR or ULSR, 4 Fiber BPSR4 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
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