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CS 268:Optical Networks
Ion Stoica
April 21, 2004
(Based in part on slides from Ed Bortolini (Network Photonics), Ling Huang (UC Berkeley), Shivkumar Kalyanaraman (RPI),Larry McAdams (Cisco))
2
Big Picture
SONET
DataCenter SONET
SONET
SONET
DWDM DWD
M
Access
Long HaulAccess
MetroMetro
3
Overview
Optical Transmission Dense Wavelength Division Multiplexing (DWDM) Synchronous Optical Network (SONET) Generic Framing Procedure (GFP)
Optical Transmission
Waveform after 1000 kmTransmitted data waveform
5
Fiber Attenuation
Telecommunications industry uses two windows: 1310 & 1550 nm
1550 window is preferred for long-haul applications
- Less attenuation
- Wider window
- Optical amplifiers
1310window
1550window
6
Dispersion
Dispersion causes the pulse to spread as it travels along the fiber
Chromatic dispersion- Light propagation in material varies with the wavelength
- Degradation scales as (data-rate)2
(Figure from http://lw.pennnet.com/)
7
Dispersion
Modal dispersion- Only for fiber that carry multiple light rays (modes)
- Different modes travel at different speeds
- Multimodal fiber used only for short distances
8
Overview
Optical Transmission Dense Wavelength Division Multiplexing (DWDM) Synchronous Optical Network (SONET) Generic Framing Procedure (GFP)
9
DWDM
1310/1510 nm
1310/1510 nm
16 uncorrelated vawelengths
λ1λ2 λ3λ4 λ5 λ16
2.488 Gbps (1)
2.488 Gbps (16)
16*2.488 Gbps = 40 Gbps
1530-1565 nm ramge
16 stabilized, correlated vawelengts
DWDM System Design
40-80 km
Terminal
Regenerator - 3R (Reamplify, Reshape and Retime)
Terminal
120 km
TerminalTerminal
Optical Amplifiers (OA)
Terminal
OA amplifies all s
Terminal
Terminal
Terminal
Terminal
Terminal
DWDM System Design
1550
1551
1552
1553
1554
1555
1556
1557
01234567
01234567
Amplify DW
DM
Filt
er
Op
tic
al C
om
bin
er
15xx nm 1310 nmReamplifyReshapeRetime
Rx Tx1310 nm
Rx
Ex
tern
al
Mo
du
lato
r
Laser
15xx nm
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All-Optical Switching
Natively switch s while they are still multiplexed
Eliminate redundant optical-electronic-optical conversions
DWDMFibers
inDWDMDemux
DWDMDemux
DWDMFibers
outDWDM
Mux
DWDMMux
All-optical
OXC
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1-D MEMS
MEMS: Micro-electromechanical systems
1-Dimensional array of micro-mirrors
- 1 mirror per wavelength Digital control; no motors
14
Optical Switch
1-input 2-outoput illustration with four wavelengths
1-D MEMS with dispersive optics - Dispersive element separates the ’s from inputs
- MEMS independently switches each - Dispersive element recombines the switched ’s into outputs
1-D MEMSMicro-mirror
Array
Digital MirrorControl
Electronics1011
Wavelength Dispersive Element
Input Fiber
Output Fiber 1
Output Fiber 2
Input & Output fiber array
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Optical Switch
2 in
4 in
5 in
7 in
N in
1 out
2 out
4 out
5 out
7 out
N out
1drop
2drop
3drop
4drop
5drop
6drop
7drop
Mdrop
1add
2add
3add
4add
5add
6add
7add
Madd
...
...
......
Tunable lasers
1 in
Wavelength-multiplexer N x M
four-portoptical matrix
switch
3 in 3 out
6 out6 in
16
Optical Add-Drop Multiplexer
Add-drop one Each is associated with a fixed add/drop port Used to implement ring topologies
... ...
...
...
...
DE
MU
X
MU
X
17
Overview
Optical Transmission Dense Wavelength Division Multiplexing (DWDM) Synchronous Optical Network (SONET) Generic Framing Procedure (GFP)
18
SONET
Encode bit streams into optical signals propagated over optical fiber
Uses Time Division Multiplexing (TDM) for carrying many signals of different capacities
- A bit-way implementation providing end-to-end transport of bit streams
- All clocks in the network are locked to a common master clock
- Multiplexing done by byte interleaving
19
Synchronous Transport Signal (STS)
First two bytes of each frame contain a special bit pattern that allows to determine where the frame starts
Receiver looks for the special bit pattern every 810 bytes
- Size of frame = 9x90 = 810 bytes
90 columns
9 ro
ws
Data (payload)overhead
SONET STS-1 Frame
Synchronous Payload Envelope (SPE)
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Encoding
Overhead bytes are encoded using Non-Return to Zero- high signal 1; low signal 0
To avoid long sequences of 0’s or 1’s the payload is XOR-ed with a special 127-bit patter with many transitions from 1 to 0
- Duration of a frame is 125 µsec (51.84 Mbps for STS-1)
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SONET Overhead Processing
Three layers of overhead- Path overhead (POH): end-to-end transport
- Line overhead (LOH): mux-to-mux transport
- Section overhead (SOH): adjacent network element
MU
X
IntermediateMultiplexer(ADM or DCS)
Regenerator Regenerator
Section Section Section Section
Line Line
Path
DE
MU
X
22
STS-1 Frame Format
Two-dimensional: 9*80 = 810 bytes Time Frame: 125 µsec Rate: 810*8 bit/125 µsec = 51.84 Mbps For STS-3 only the number of columns changes
(3*80 = 270)
90 Bytes90 BytesOr “Columns”Or “Columns”
99RowsRows
Small Rectangle =1 Byte
23
STS-1 Headers
90 Bytes90 BytesOr “Columns”Or “Columns”
99RowsRows
Section Overhead (SOH)
Line Overhead (LOH)Path Overhead (POH): Floating; can begin anywhere
24
Section Overhead (SOH)
First 3 lines in the header Main functions
- Framing (A1, A2)
- Monitor performance
- Local orderwire (E1): select repeater/terminal within communication complex
- Proprietary OAM (Operation, Administration, and Maintenance) (F1)
SOH
3 B 87 B
A1=0xF6
A2A2=0x28=0x28
J0/Z0J0/Z0STS-IDSTS-ID
B1B1BIP-8BIP-8
E1E1OrderwireOrderwire
F1F1UserUser
D1D1Data ComData Com
D2D2Data ComData Com
D3D3Data ComData Com
25
Line Overhead (LOH)
Last 3 lines in the header Main functions
- Locating payload (SPE) in the frame (H1, H2)
- Muxing and concatenating signals
- Performance monitoring
- Automatic protection switch (K1, K2)
• Switchover in case of failure
- Line maintenanceLOH
3 B 87 B
H1H1PointerPointer
H2H2PointerPointer
H3H3Pointer ActPointer Act
B2B2BIP-8BIP-8
K1K1APSAPS
K2K2APSAPS
D4D4Data ComData Com
D5D5Data ComData Com
D6D6Data ComData Com
D7D7Data ComData Com
D8D8Data ComData Com
D9D9Data ComData Com
D10D10Data ComData Com
D11D11Data ComData Com
D12D12Data ComData Com
S1S1SyncSync
M0M0REIREI
E1E1OrderwireOrderwire
26
Path Overhead (POH)
1st column in SPE Main functions
- Info about SPE content
- Performance monitoring
- Path status
- Path trace SPE can start anywhere in
the current frame and span the next one
- Avoids buffer management complexity & artificial delays 1st STS-1
2nd STS-1
POH
9 rows
J1J1TraceTrace
B3B3BIP-8BIP-8
C2C2Sig LabelSig Label
G1G1Path StatPath Stat
F2F2UserUser
H4H4IndicatorIndicator
Z3Z3GrowthGrowth
Z4Z4GrowthGrowth
Z5Z5TandemTandem
SPE
27
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
28
STS-N: Generic Frame Format
STS-1 STS-N
29
STS-Nc Frame Format
Concatenated mode:Concatenated mode:• Same header structure and data rates as Same header structure and data rates as STS-NSTS-N• Not all header bytes usedNot all header bytes used• First H1, H2 Point To POHFirst H1, H2 Point To POH• Single PayloadSingle Payload In Rest Of SPE In Rest Of SPE
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
30
Practical SONET Architecture
ADM – Add-Drop MultiplexerDCS – Digital Crossconnect
31
Protection Technique Classification
Restoration techniques can protect the network against:- Link failures
• Fiber-cables cuts and line devices failures (amplifers)
- Equipment failures
• OXCs, ADMs, electro-optical interface. Protection can be implemented
- In the optical channel sublayer (path protection)
- In the optical multiplex sublayer (line protection) Different protection techniques are used for
- Ring networks
- Mesh networks
32
1+1 Protection
Traffic is sent over two parallel paths, and the destination selects a better one
In case of failure, the destination switch onto the other path Pros: simple for implementation and fast restoration Cons: waste of bandwidth
33
1:1 Protection
During normal operation, no traffic or low priority traffic is sent across the backup path
In case failure both the source and destination switch onto the protection path
Pros: better network utilization Cons: required signaling overhead, slower restoration
34
1:1 Ring Protection
Each channel on one ring is protected by one channel on the other ring
When faults loop around
ADM
ADM
ADM ADM
ADM
ADM
ADM ADM
35
Protection in Ring Network
1+1 Path Protection
Used in access rings for traffic aggregation
into central office
1:1 Line Protection
Used for interoffice rings
1:1 Span and Line Protection
Used in metropolitan or long- haul rings
(Unidirectional Path Switched Ring)
(Bidirectional Line Switched Ring)
36
Protection in Mesh Networks
Working Path
Backup Path
Network planning and survivability design - Disjoint path idea: service working route and its backup route are
topologically diverse.
- Lightpaths of a logical topology can withstand physical link failures.
37
Path Switching: restoration is handled by the source and the destination.
Normal Operation
Line Switching: restoration is handled by the nodes adjacent to the failure. Span Protection: if additional fiber is available.
Line Switching: restoration is handled by the nodes adjacent to the failure.
Line Protection.
Path Protection / Line Protection
38
Shared Protection
1:N Protection
Backup fibers are used for protection of multiple links Assume independent failure and handle single failure. The capacity reserved for protection is greatly reduced.
Normal Operation
In Case of Failure
39
Overview
Optical Transmission Dense Wavelength Division Multiplexing (DWDM) Synchronous Optical Network (SONET) Generic Framing Procedure (GFP)
40
Generic Framing Procedure (GFP)
GFP provides a generic mechanism to adapt traffic from higher-layer client signals over an octet synchronous transport network (e.g., SONET)
GFP – Common Aspects (Payload Independent)
GFP – Client Specific Aspects (Payload Dependent)
Ethernet IP/PPP Other Client Signals
SONET/SDH VC-n Path
OTN ODUk Path
Other octet-synchronous
paths
41
Generic Framing Procedure (GFP)
Frame-mapped- Need to know the client protocol
- Associate a length to each higher level frame
- Efficient: eliminate the need for byte stuffing or for block encoding (e.g., 8B/10B)
Transparent- No need to know the client protocol
- Less efficient; can transmit signal even when the client is idle