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
DS1
M13
• Switches
• Leased Line
DS3
Fiber Optic
Transmission
Systems
M13
DS1
DS1 Cross
Connect
Fiber
Central
Office
Key System Aspects:
• M13 Building Blocks
• Asynchronous Operation
• Electrical DS3 Signals
• Proprietary Fiber Systems
• Brute Force Cross Connect
• AT&T Network/Western
Electric Equipment
M13
Central
Office
No Guaranteed
Timing
Synchronization
Central
Office
DS3
DS1
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/demultiplexing 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
• Switches
• Leased Line
• LAN Services
• Data Services
Different
Carriers,
Vendors
Needs:
• Support Faster Fiber
• Support New Services
• Allow Other Topologies
• Standardize Redundancy
• Common OAM&P
• Scalable Cross Connect
M13
DS1
Internal
DS3 Cross
Connect
Support
Other
Topologies,
Protect Fibers
The SONET Standards Process
Divestiture
CCITT Expresses
Interest in SONET
British and Japanese
Participation in T1X1
Exchange Carriers
Standards Associate (ECSA)
T1 Committee Formed
ANSI T1X1
Bellcore Proposed Approves
SONET Principles
Project
To ANSI T1X1
1984
1985
CCITT XVIII
Begins Study
Group
1986
SONET/SDH
Standards
Approved
CEPT Proposes
Merged ANSI/CCITT
Standard
1987
1988
SONET Concept Developed By Bellcore
US T1X1 Accepts
>400 Technical Proposals
Modifications
• Rate Discussions AT&T vs. Bellcore
(resolved w/ virtual tributary concept)
• International Changes For Byte/Bit ANSI Approves
SYNTRAN
Interleaving, Frames, Data Rates
• Phase I, II, III Separate APS, etc.
• ITU’s SDH initiative…
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 BISDN
STS-1s are mutually synchronized irrespective of inputs
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
STS-1 Headers
Section Overhead (SOH)
90 Bytes
Or “Columns”
9
Rows
Path Overhead (POH):
Line Overhead (LOH) Floating => can begin anywhere
Line + Section overhead = Transport Overhead (TOH)
SONET Equipment Types
Path
Sections
PTE
Repeaters
• Section Termination (STE)
Line
• Line Termination (LTE)
• Path Termination (PTE)
SONET End
Device - I.e.
Telephony
Switch, Router
PTE
SONET Overhead Processing
Headers: Section Overhead (SOH)
Rcv
SOH
Xmt
SOH
A1
=0xF6
B1
BIP-8
A2
=0x28
J0/Z0
STS-ID
E1
Orderwire
F1
User
D1
D2
D3
Data Com Data Com Data Com
Section Overhead
• 9 Bytes Total
• Originated And Terminated By All
Section Devices (Regenerators,
Multiplexers, CPE)
• Other Fields Pass Unaffected
Selected Fields:
•A1,A2 - Framing Bytes
•BIP-8 - Bit Interleaved
Parity
• F1 User - Proprietary
OAM Management
Headers: Line Overhead (LOH)
H1
Pointer
Xmt
LOH
Xmt
SOH
Rcv
LOH
Rcv
SOH
Xmt
SOH
Rcv
SOH
B2
BIP-8
H2
H3
Pointer Pointer Act
K1
K2
APS
APS
D4
D5
D6
Data Com Data Com Data Com
D7
D8
D9
Data Com Data Com Data Com
D10
D11
D12
Data Com Data Com Data Com
S1
Sync
Line Overhead
• 18 Bytes Total
• Originated And Terminated By All
Line Devices (Multiplexers, CPE)
• LOH+SOH=TOH (Transport OH)
M0
REI
E1
Orderwire
Selected Fields:
•H1-3 - Payload Pointers
•K1, K2 - Automatic
Protection Switching
• D4-D12 - 576 kbps
OSI/CMIP
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
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)
PTE
PTE
STE
Frame N
Frame N+1
Frame N
Frame N+1
Path Overhead
• H1,H2 fields of LOH points to
Beginning of POH
•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
J1
Trace
B3
BIP-8
C2
Sig Label
G1
Path Stat
F2
User
H4
Indicator
Z3
Growth
Z4
Growth
Z5
Tandem
Selected
fields:
•BIP-8 - Parity
• C2 - Payload
Type Indicator
• G1 - End End
Path Status
STS-1 Headers: Putting it Together
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
Clock Synchronization
BITS
BITS
PTE
•Level
•Level
•Level
•Level
1: 10-11
2: 1.6x10-8
3: 4.6x10-6
4: 32x10-6
Primary
Reference
Building Integrated Timing System
• Hierarchical Clocking Distribution
• Normally All Synch’d To Stratum 1
(Can Be Cesium/Rubidium Clock)
• Dedicated Link Or Recovered
• Fallback To Higher Stratum In Failure
(Temperature Controlled Crystal)
Backup
Reference
BITS
PTE
STS-N Frame Format
90xN Bytes
Or “Columns”
N Individual STS-1 Frames
Composite Frames:
• Byte Interleaved STS-1’s
• Clock Rate = Nx51.84 Mbps
• 9 colns overhead
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
Multiple frame streams, w/ independent payload pointers
Note: header columns also interleaved
STS-N: Generic Frame Format
STS-1
STS-N
Example: STS-3 Frame Format
STS-Nc Frame Format
90xN Bytes
Or “Columns”
Transport Overhead: SOH+LOH
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
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
D+R
DS1s
TM
ADM
DS1s
MN
MN
MN
MN
DCC
D+R
D+R
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
D+R: Drop and Repeat
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
ADM
DCC
ADM
ADM
ADM
2 Fiber Ring
DCC
Each Line Is
Full Duplex
ADM
ADM
ADM
DCC
Each Line Is
Full Duplex
ADM
ADM
ADM
4 Fiber Ring
DCC
Uni- vs. BiDirectional
All Traffic Runs Clockwise,
vs Either Way
ADM
ADM
APS
ADM
ADM
ADM
Line Protection Switching
Uses TOH
Trunk Application
Backup Capacity Is Idle
Supports 1:n, N=1-14
ADM
ADM
ADM
Path Protection Switching
Uses POH
Access Line Applications
Duplicate Traffic Sent On Protect
1+1
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