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))
Big Picture
Data
Center
SONET
SONET
DWD
M
DWD
M
SONET
SONET
Access
Metro
Long Haul
Metro
Access
2
Overview




Optical Transmission
Dense Wavelength Division Multiplexing (DWDM)
Synchronous Optical Network (SONET)
Generic Framing Procedure (GFP)
3
Optical Transmission
Transmitted data waveform
Waveform after 1000 km
Fiber Attenuation


1550
window
Telecommunications
industry uses two windows:
1310 & 1550 nm
1550 window is preferred
for long-haul applications
- Less attenuation
- Wider window
- Optical amplifiers
1310
window
l
5
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
6
(Figure from http://lw.pennnet.com/)
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
7
Overview




Optical Transmission
Dense Wavelength Division Multiplexing (DWDM)
Synchronous Optical Network (SONET)
Generic Framing Procedure (GFP)
8
DWDM
2.488 Gbps (1)
1310/1510 nm
2.488 Gbps (16)
λ1 λ2 λ3 λ4 λ5
λ16
1530-1565 nm ramge
1310/1510 nm
16 uncorrelated vawelengths
16*2.488 Gbps = 40 Gbps
16 stabilized, correlated
vawelengts
9
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
1552
1553
1554
1555
1556
0
1
2
3
4
5
6
7
Amplify
1557
Rx
Laser
External
Modulator
1310 nm
1551
DWDM Filter
1550
0
1
2
3
4
5
6
7
Optical Combiner
DWDM System Design
15xx nm
15xx nm
Rx
Reamplify
Reshape
Retime
1310 nm
Tx
All-Optical Switching


Natively switch ls while they are still
multiplexed
Eliminate redundant optical-electronicoptical conversions
DWDM
Demux
DWDM
Fibers
in
DWDM
Mux
DWDM
Fibers
out
All-optical
DWDM
Demux
OXC
DWDM
Mux
12
1-D MEMS


MEMS: Microelectromechanical systems
1-Dimensional array of micromirrors
- 1 mirror per wavelength

Digital control; no motors
13
Optical Switch

1-input
2-outoput
illustration
with four wavelengths
Input &
Output fiber
Wavelength
1-D MEMS
array
Dispersive Element
Micro-mirror
Array
Input Fiber
Output Fiber 1
1011
Digital Mirror
Control
Electronics
Output Fiber 2

1-D MEMS with dispersive optics
- Dispersive element separates the l’s from inputs
- MEMS independently switches each l
- Dispersive element recombines the switched l’s into outputs
14
Optical Switch
1
3
5
7
2
4
6
add
add
add
add
add
add
add
...
M
add
Tunable lasers
1 out
2 in
2 out
3 in
3 out
4 in
4 out
5 in
5 out
6 in
6 out
7 in
7 out
...
...
1 in
N in
N out
Wavelengthmultiplexer
...
1
3
5
7
2
4
6
drop
drop
drop
drop
drop
drop
drop
M
drop
NxM
four-port
optical matrix
switch
15
Optical Add-Drop Multiplexer
l
...
...
MUX
l
...
DEMUX

...

Add-drop one l
Each l is associated with a fixed add/drop port
Used to implement ring topologies
...

16
Overview




Optical Transmission
Dense Wavelength Division Multiplexing (DWDM)
Synchronous Optical Network (SONET)
Generic Framing Procedure (GFP)
17
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
18
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
overhead
9 rows

Data (payload)
Synchronous Payload Envelope (SPE)
90 columns
SONET STS-1 Frame
19
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)
20
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
Regenerator
Section
Intermediate
Multiplexer
(ADM or DCS)
Line
Section
Regenerator
DEMUX
Section
MUX

Line
Path
21
STS-1 Frame Format
90 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 = 51.84 Mbps
For STS-3 only the number of columns changes
(3*80 = 270)
22
STS-1 Headers
Section Overhead (SOH)
90 Bytes
Or “Columns”
9
Rows
Line Overhead (LOH)
Path Overhead (POH):
Floating; can begin anywhere
23
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
B1
BIP-8
A2
=0x28
J0/Z0
STS-ID
E1
Orderwire
F1
User
D1
D2
D3
Data Com Data Com Data Com
SOH
3B
87 B
24
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 maintenance
H1
Pointer
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
M0
REI
E1
Orderwire
LOH
3B
87 B
25
Path Overhead (POH)


1st column in SPE
Main functions
-

J1
Trace
B3
BIP-8
Info about SPE content
Performance monitoring
Path status
Path trace
C2
Sig Label
G1
Path Stat
F2
User
SPE can start anywhere in
the current frame and
span the next one
- Avoids buffer
management complexity
& artificial delays
H4
Indicator
Z3
Growth
1st STS-1
2nd STS-1
P
O
H
Z4
Growth
SPE
9 rows
Z5
Tandem
26
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
27
Note: header columns also interleaved
STS-N: Generic Frame Format
STS-1
STS-N
28
STS-Nc Frame Format
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
29
Practical SONET Architecture
ADM – Add-Drop Multiplexer
DCS – Digital Crossconnect
30
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
31
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
32
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
33
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
34
Protection in Ring Network
(Unidirectional Path
Switched Ring)
(Bidirectional Line
Switched Ring)
1+1 Path Protection
1:1 Span and Line Protection
Used in access rings for
traffic aggregation into
Used in metropolitan or longhaul rings
central office
1:1 Line Protection
Used for interoffice
rings
35
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
36
Path Protection / Line Protection
Normal Operation
Path Switching:
restoration is handled
by the source and the
destination.
Line Switching: restoration is
handled by is
restoration
thehandled
nodes by
adjacent
the
nodestoadjacent
the failure.
to the
Span Protection: if additional
failure.
fiber is available.
Line Protection.
37
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
38
Overview




Optical Transmission
Dense Wavelength Division Multiplexing (DWDM)
Synchronous Optical Network (SONET)
Generic Framing Procedure (GFP)
39
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)
Ethernet
IP/PPP
Other Client Signals
GFP – Client Specific Aspects
(Payload Dependent)
GFP – Common Aspects
(Payload Independent)
SONET/SDH
VC-n Path
Other octetsynchronous
paths
OTN ODUk Path
40
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
41