SONET: Broadband Convergence at Layer 1

1
SONET Broadband Convergence at Layer 1

• Shivkumar Kalyanaraman
• Rensselaer Polytechnic Institute
• shivkuma_at_ecse.rpi.edu
• http//www.ecse.rpi.edu/Homepages/shivkuma
• Based in part on slides of Nick McKeown (Stanford)

2
Telephony Multiplexing

• Telephone Trunks between central offices carry
  hundreds of conversations Cant 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 gt call 64 Kbps

3
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 gt costly plesiochronous hierarchy
• SONET developed for higher multiplexing
  aggregates
• Use of pointers like C to avoid bit-stuffing

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Digital Telephony in 1984
Fiber Optic Transmission Systems
DS1
M13
M13
DS1
DS1 Cross Connect

• Switches
• Leased Line

Fiber
Central Office
DS3
M13
Central Office
DS3

• Key System Aspects
• M13 Building Blocks
• Asynchronous Operation
• Electrical DS3 Signals
• Proprietary Fiber Systems
• Brute Force Cross Connect
• ATT Network/Western Electric Equipment

DS1
Central Office
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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 gt 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 gt synch loss (max number 15)
• yellow alarm. 1s density etc gt usable b/w
  7bits/frame gt 56 kbps
• Europe E1 more streamlined framing 2.048 Mbps
• Variants Concatenated T1, Un-channelized (raw) T1

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Digital Hierarchy (Contd)

• 1980s demand for bandwidth. But gt T3s not
  available except in proprietary form
• Fiber-optic interface for T3 was proprietary
• Primitive online OAMP capabilities (eg robbed
  bits)
• Fewer operators interoperability/mid-span meet
  not critical
• Changed dramatically after 1984 deregulation!
• Public vs Private Networks
• Private Customer operates n/w (eg w/ private
  leased lines) developed from PBX SNA
• Public Provider operates n/w for subscribers
• More public networks (eg X.25) outside US
• Drivers of SONET
• IBM SNA/mainframes gt hub-and-spoke networking
• Increase of PCs gt client-server p2p computing
  gt more demands on long-distance trunks
• T-carrier evolution rate much slower than
  computing trends

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Digital Hierarchy (Contd)

• Digital streams organized as bytes (eg voice
  samples, data)
• Byte interleaving (eg 24 DS0 -gt 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 gt need to do this with minimal
  buffering, fast/scalable processing
• Bit-interleaving (eg DS1 -gt DS2 etc)
• Cant use buffers to mask jitter! gt bit stuffing
• Partly because high speed memory was costly then!
• Plesiochronous hierarchy gt 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

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US Telephone Network Structure (after 1984
divestiture)
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Post-ATT Divestiture Dilemmas

• Switches
• Leased Line
• LAN Services
• Data Services

Different Carriers, Vendors
DS1
M13
Internal DS3 Cross Connect

• Needs
• Support Faster Fiber
• Support New Services
• Allow Other Topologies
• Standardize Redundancy
• Common OAMP
• Scalable Cross Connect

Support Other Topologies, Protect Fibers
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The SONET Standards Process
Divestiture
CCITT Expresses Interest in SONET
SONET/SDH Standards Approved
Exchange Carriers Standards Associate (ECSA) T1
Committee Formed
British and Japanese Participation in T1X1
ANSI T1X1 Approves Project
CCITT XVIII Begins Study Group
CEPT Proposes Merged ANSI/CCITT Standard
Bellcore Proposed SONET Principles To ANSI T1X1
1984
1985
1986
1987
1988
SONET Concept Developed By Bellcore
US T1X1 Accepts Modifications

• gt400 Technical Proposals
• Rate Discussions ATT vs. Bellcore
• (resolved w/ virtual tributary concept)
• International Changes For Byte/Bit Interleaving,
  Frames, Data Rates
• Phase I, II, III Separate APS, etc.
• ITUs SDH initiative

ANSI Approves SYNTRAN
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SONET Standards Story

• SYNTRAN pre-divestiture effort, no pointer
  concept.
• SONET primarily US (divestiture) driven
• ATT 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 OAMP 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)

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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 OAMP capabilities
• 4. Multiplexing formats for existing digital
  signals (DS1, DS2 etc)
• 5. Supports ITU hierarchy (E1 etc)
• 6. Accomodates other applications B-ISDN etc

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SONET Lingo

• OC-N Optical carrier Nx51.84 Mbps
• Approximate heuristic bit rate N/20 Gbps (eg
  OC-48 gt 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 OAMP 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

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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
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STS-1 Frame Format
90 Bytes Or Columns
9 Rows
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 ms/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
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STS-1 Headers
90 Bytes Or Columns
9 Rows
Line Section overhead Transport Overhead (TOH)
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SONET Equipment Types
PTE
Repeaters
SONET End Device - I.e. Telephony Switch, Router
PTE
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SONET Overhead Processing
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Headers Section Overhead (SOH)
A1 0xF6
A2 0x28
J0/Z0 STS-ID
Rcv SOH
Xmt SOH
B1 BIP-8
E1 Orderwire
F1 User
D1 Data Com
D2 Data Com
D3 Data Com

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

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

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Headers Line Overhead (LOH)
H1 Pointer
H2 Pointer
H3 Pointer Act
B2 BIP-8
K1 APS
K2 APS
Xmt LOH
Rcv LOH
D4 Data Com
D5 Data Com
D6 Data Com
Rcv SOH
Xmt SOH
D7 Data Com
D8 Data Com
D9 Data Com
Xmt SOH
Rcv SOH
D10 Data Com
D11 Data Com
D12 Data Com
S1 Sync
M0 REI
E1 Orderwire

• Line Overhead
• 18 Bytes Total
• Originated And Terminated By All Line Devices
  (Multiplexers, CPE)
• LOHSOHTOH (Transport OH)

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

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b
Floating Payload SONET LOH Pointers
SPE is not frame-aligned overlaps multiple
frames! Avoids buffer management complexity
artificial delays Allows direct access to
byte-synchronous lower-level signals (eg DS-1)
with just one frame recovery procedure
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SPE Synchronous Payload Envelope

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

• 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

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Headers Path Overhead (POH)
J1 Trace
PTE
PTE
STE
B3 BIP-8

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

Frame N
Frame N
C2 Sig Label
Frame N1
Frame N1
G1 Path Stat

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

F2 User
H4 Indicator

• POH Beginning Floats Within Frame

Z3 Growth

• 9 Bytes (1 Column) Spans Frames

Z4 Growth

• Originated And Terminated By All Path Devices
  (I.e. CPE, Switches)
• End-to-end OAM support

Z5 Tandem
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STS-1 Headers Putting it Together
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Accommodating Jitter
Positive 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

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Clock Synchronization
BITS
BITS

• Level 1 10-11
• Level 2 1.6x10-8
• Level 3 4.6x10-6
• Level 4 32x10-6

PTE
Primary Reference
Backup Reference

• Building Integrated Timing System
• Hierarchical Clocking Distribution
• Normally All Synchd To Stratum 1 (Can Be
  Cesium/Rubidium Clock)
• Dedicated Link Or Recovered
• Fallback To Higher Stratum In Failure
  (Temperature Controlled Crystal)

BITS
PTE
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STS-N Frame Format
90xN Bytes Or Columns
N Individual STS-1 Frames
Examples STS-1 51.84
Mbps STS-3 155.520 Mbps STS-12 622.080
Mbps STS-48 2.48832 Gbps STS-192 9.95323 Gbps

• Composite Frames
• Byte Interleaved STS-1s
• Clock Rate Nx51.84 Mbps
• 9 colns overhead

Multiple frame streams, w/ independent payload
pointers Note header columns also interleaved
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STS-N Generic Frame Format
STS-N
STS-1
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Example STS-3 Frame Format
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STS-Nc Frame Format
90xN Bytes Or Columns
Transport Overhead SOHLOH

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

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

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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!

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SONET Transmission Encoding

• Electrical Transmission Standard
• STS-1 B3ZS (BPV), 450
• STS-3 Coded Mark Inversion, 225
• Useful Intra-Office Connection

• Scrambling
• Ensures Ones Density
• Does Not Include A1, A2, C1 Bytes
• Output Is NRZ Encoded

E O
1x6x7
OC-N Is Optical Carrier STS-N Long Reach 40
km 1310 or 1550 nm SM Intermediate Reach 15
km 1310 or 1550 nm SM Short ReachLong Reach 2
km 1310 nm MM
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SONET Scrambling
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Packet Over SONET (POS)

• Special Data Scrambler
• 1 x43 Polynomial
• Protects Against Transmitted Frames Containing
  Synch Bytes Or Insufficient Ones Density

• Standard PPP Encapsulation
• Magic Number Recommended
• No Address and Control Compression
• No Protocol Field Compression

PPP
Byte Stuff
FCS
Scrambling
SONET Framing

• Standard CRC Computation
• OC3 May Use CRC-16
• Other Speeds Use CRC-32

• SONET Framing
• OC3, OC12, OC48, OC192 Defined
• C2 Byte 0x16 With Scrambling
• C2 Byte oxCF Without (OC-3)

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Practical SONET Architectures
Today multiple stacked rings over DWDM
(different ?s)
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SONET Network Elements
DR
MN
MN
ADM
DCC
TM
DS1s
DR
MN
MN
DR
DS1s
Nonstandard, Functional Names TM Terminal Mux
(aka LTE ends of pt-pt links) ADM Add-Drop
Mux DCC Digital Cross Connect (Wideband and
Broadband) MN Matched Node DR Drop and Repeat
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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 functions
• Grooming
• Grouping traffic with similar destinations, QoS
  etc
• Muxing/extracting streams also
• Narrow-/wide-/broad-band and optical crossconnects

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Topology Building Blocks
ADM
ADM
2 Fiber Ring
4 Fiber Ring
DCC
ADM
DCC
ADM
Each Line Is Full Duplex
Each Line Is Full Duplex
ADM
ADM
ADM
ADM
DCC
ADM
DCC
ADM
ADM
ADM
Uni- vs. Bi- Directional
All Traffic Runs Clockwise, vs Either Way
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APS
ADM
ADM
ADM
ADM
ADM
ADM
Line Protection Switching
Path Protection Switching
Uses TOH Trunk Application Backup Capacity Is
Idle Supports 1n, N1-14
Uses POH Access Line Applications Duplicate
Traffic Sent On Protect 11

• Automatic Protection Switching
• Line Or Path Based
• Revertive vs. Non-Revertive
• Mechanism For Intentional Cutover
• Restoration Times 50 ms
• K1, K2 Bytes Signal Change
• Common Uses 2 Fiber UPSR or ULSR, 4 Fiber BPSR