1. 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 Metro Long Haul Metro Access Access
DWDM
SONET
Data
Center
DWDM
SONET
BPS2000, Baystack 450 & Passport 8600 (edge ethernet distribution & aggregation)
OPTera Packet Edge on OPTera Metro 3000 series & OPTera OC-48 (ethernet over metro optical/Sonet)
OPTera Metro 5000 series (ethernet over DWDM)
Preside (provisioning & management software)
Juniper & Shasta (collateral IP services & routing platforms)
Contivity (VPN/secure, encrypted tunnel services)
SONET
SONET
Metro
Long Haul
Metro
Access
Access

3. Overview Optical Transmission
Dense Wavelength Division Multiplexing (DWDM)
Synchronous Optical Network (SONET)
Generic Framing Procedure (GFP)

4. Optical Transmission Transmitted data waveform Waveform after 1000 km
PEAKING
CROSS-OVER POINTLOWERED
ZERO LEVEL BROADENED

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
1550
window
1310
window l

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

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 λ1 λ2 λ3 λ4 λ5 λ16 16 uncorrelated vawelengths
2.488 Gbps (1)
1310/1510 nm
2.488 Gbps (16) λ1 λ2 λ3 λ4 λ5
λ16
nm ramge
1310/1510 nm
16*2.488 Gbps = 40 Gbps
16 uncorrelated vawelengths
16 stabilized, correlated
vawelengts

10. DWDM System Design
40-80 km
Terminal
Terminal
Regenerator - 3R (Reamplify, Reshape and Retime)
120 km
Terminal
Terminal
Optical Amplifiers (OA)
Terminal
Terminal
Terminal
Terminal
Terminal
Terminal
OA amplifies all ls

11. DWDM System Design 1 1 Optical Combiner 2 DWDM Filter 2 3 3 4 4 5 5
1550 1
1551 1 2
1552 2 3
1553
Optical Combiner
DWDM Filter 3 4
1554 4 5
1555 5
Amplify 6
1556 6 7
1557 7
1310 nm Rx
External Modulator
Laser
15xx nm
1310 nm
Reamplify
Reshape
Retime Rx Tx
15xx nm

12. All-Optical Switching
Natively switch s while they are still multiplexed
Eliminate redundant optical-electronic-optical conversions
DWDM
Fibers in
Demux
Fibers out
Mux
All-optical OXC

13. 1-D MEMS MEMS: Micro-electromechanical systems
1-Dimensional array of micro-mirrors
1 mirror per wavelength
Digital control; no motors

14. Input & Output fiber array Wavelength Dispersive Element
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
Input Fiber
Output Fiber 1
Output Fiber 2
Input & Output fiber array
Wavelength Dispersive Element
1-D MEMS
Micro-mirror Array
Digital Mirror Control Electronics
1011

15. Optical Switch

16. Optical Add-Drop Multiplexer
Add-drop one l
Each l is associated with a fixed add/drop port
Used to implement ring topologies
DEMUX
MUX

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
Data (payload)
overhead
9 rows
Synchronous Payload Envelope (SPE)
90 columns
SONET STS-1 Frame

20. 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)

21. 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
Section
Section
Section
Section
Intermediate
Multiplexer
(ADM or DCS)
DEMUX
MUX
Regenerator
Regenerator
Line
Line
Path

22. STS-1 Frame Format Two-dimensional: 9*80 = 810 bytes
Or “Columns” 9
Rows
Small Rectangle =1 Byte
Two-dimensional: 9*80 = 810 bytes
Time Frame: 125 µsec
Rate: 810*8 bit/125 µsec = Mbps
For STS-3 only the number of columns changes (3*80 = 270)

23. STS-1 Headers Section Overhead (SOH) Path Overhead (POH):
90 Bytes
Or “Columns” 9
Rows
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) A1
=0xF6 A2
=0x28
J0/Z0
STS-ID B1
BIP-8 E1
Orderwire F1
User D1
Data Com D2
Data Com D3
Data Com
SOH
3 B
87 B

25. Line Overhead (LOH) Last 3 lines in the header Main functions
Pointer H2
Pointer H3
Pointer Act
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 maintenance B2
BIP-8 K1
APS K2
APS D4
Data Com D5
Data Com D6
Data Com D7
Data Com D8
Data Com D9
Data Com
D10
Data Com
D11
Data Com
D12
Data Com S1
Sync M0
REI E1
Orderwire
LOH
3 B
87 B

26. Path Overhead (POH) 1st column in SPE Main functionsJ1
Trace
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 B3
BIP-8 C2
Sig Label G1
Path Stat F2
User H4
Indicator Z3
Growth
1st STS-1 Z4
Growth P O H
SPE
9 rows Z5
Tandem
2nd STS-1

27. N Individual STS-1 Frames
STS-N Frame Format
90xN Bytes
Or “Columns”
N Individual STS-1 Frames
Examples
STS Mbps
STS Mbps
STS Mbps
STS Gbps
STS Gbps
Composite Frames:
Byte Interleaved STS-1’s
Clock Rate = Nx51.84 Mbps
9 colns overhead
Multiple frame streams, w/ independent payload pointers
Note: header columns also interleaved

28. STS-N: Generic Frame Format

29. STS-Nc Frame Format Concatenated mode:
90xN Bytes
Or “Columns”
Transport Overhead: SOH+LOH
Concatenated mode:
Same header structure and data rates as STS-N
Not all header bytes used
First H1, H2 Point To POH
Single Payload In Rest Of SPE
Current IP over SONET technologies use concatenated mode: OC-3c (155 Mbps) to OC-192c (10 Gbps) rates
a.k.a “super-rate” payloads

30. Practical SONET Architecture
ADM – Add-Drop Multiplexer
DCS – 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
(Unidirectional Path Switched Ring)
(Bidirectional Line Switched Ring)
1:1 Line Protection
Used for interoffice rings
1+1 Path Protection
Used in access rings for traffic aggregation into central office
1:1 Span and Line Protection
Used in metropolitan or long- haul rings

36. Protection in Mesh Networks
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.
Working Path
Backup Path

37. Path Protection / Line Protection
Path Switching: restoration is handled by the source and the destination.
Line Switching: restoration is handled by the nodes adjacent to the failure.
Line Protection.
Normal Operation
Line Switching: restoration is handled by the nodes adjacent to the failure Span Protection: if additional fiber is available.

38. Shared Protection
Normal Operation
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.
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)
The GFP is an approach driven to standards by legacy equipment suppliers (e.g., Lucent, Nortel, NEC) in T1X1 and ITU. It intends to introduce a common data-link layer, on top of either SONET or OTN L1, so as to more efficiently accommodate many different L2 data signals. In particular, it allows statistical multiplexing of several bursty sources into a GFP frame. Initial standards for point-to-point GFP allow such multiplexing in the originating node on the ring. Later specification (ring GFP) would also allow other nodes to multiplex data traffic into the same frame. The GFP framing uses HEC and frame length indication for synchronization and delineation, similar to ATM and as opposed to a flag field as used in HDLC frames (e.g., in POS).
Several vendors, and the ITU, envision the use of GFP and virtually concatenated SONET channels to circumvent SONET data deficiencies (granularity and burstiness) without eliminating SONET as a physical layer and for circuit bandwidth management.
Packet-aware features may also be combined with GFP/VC to decouple VC bandwidth from the tributary interface (e.g., GbE) bandwidth, to provide flow control, and packet layer performance monitoring.

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
The GFP is an approach driven to standards by legacy equipment suppliers (e.g., Lucent, Nortel, NEC) in T1X1 and ITU. It intends to introduce a common data-link layer, on top of either SONET or OTN L1, so as to more efficiently accommodate many different L2 data signals. In particular, it allows statistical multiplexing of several bursty sources into a GFP frame. Initial standards for point-to-point GFP allow such multiplexing in the originating node on the ring. Later specification (ring GFP) would also allow other nodes to multiplex data traffic into the same frame. The GFP framing uses HEC and frame length indication for synchronization and delineation, similar to ATM and as opposed to a flag field as used in HDLC frames (e.g., in POS).
Several vendors, and the ITU, envision the use of GFP and virtually concatenated SONET channels to circumvent SONET data deficiencies (granularity and burstiness) without eliminating SONET as a physical layer and for circuit bandwidth management.
Packet-aware features may also be combined with GFP/VC to decouple VC bandwidth from the tributary interface (e.g., GbE) bandwidth, to provide flow control, and packet layer performance monitoring.