SONET Survivability Mechanisms
CSC/ECE 772: Survivable Networks
Spring, 2009, Rudra Dutta
SONET APS
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Automated Protection Switching
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Switches traffic from working to protection resource
upon failure
– No manual intervention - first such mechanism
– Manual intervention necessary for repairs
– Revertive or non-revertive operation
– Two-fiber and four-fiber variants
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OAM protocols expected to detect abnormal
conditions
 Similar techniques later used for ATM
Copyright Rudra Dutta, NCSU, Spring, 2009
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SONET Operation
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Plesiochronous to Synchronous
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Original T-carrier for voice adopted “base rate”
(single voice digitized)
Successive higher rates formed by multiplexing
Synchronized (within limits), but not synchronous
Synchronous - input/output clocks synchronized
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Jitter, wander, phase difference limited to ingress to
network, synchronized inside
Multiplexing and interleaving considered functions of
network
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Early native optical transport
 Ring or non-ring operation
Copyright Rudra Dutta, NCSU, Spring, 2009
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SONET Layers
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STS-1 Frame
K1
K2
(POH)
(SOH + LOH)
Copyright Rudra Dutta, NCSU, Spring, 2009
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Automatic Protection Switching
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APS takes place at the SONET line level
Very complex task
ANSI APS document (T1.105.01-998) 100
pages long!
Only basic operation explained here
Emphasis on role of K1, K2 bytes of LOH
ATM APS protocol similar, K1, K2 bytes in APS
ATM cells
Copyright Rudra Dutta, NCSU, Spring, 2009
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APS Objective
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Whether traffic is received over the working or
protection fiber is determined by:
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The status of the bridge at source node
The status of the selector at destination node
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Objective: establish agreement between source
and destination regarding the status of
bridge/selector
 K1, K2 LOH bytes used by APS protocol for this
purpose
Copyright Rudra Dutta, NCSU, Spring, 2009
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APS Events

Protection switching: a change in the current
position of the bridge/selector
 Initiated due to certain events:
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Externally initiated commands, e.g., forced switch,
manual switch, lockout of protection, etc.
Automatically initiated command, e.g., loss of signal
(LOS), loss of frame (LOF), signal degrade (due to
parity errors), etc.
Copyright Rudra Dutta, NCSU, Spring, 2009
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APS Protocol: K1, K2 bytes
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K1 byte:
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K2 byte:
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Switch request (protection switching event) –4 bits
Destination node –4 bits (max 16 SONET nodes)
Source node –4 bits
Long/short bit
Status (of bridge/selector) –3 bits
Source/destination use K1, K2 bytes to
coordinate protection switching actions
Copyright Rudra Dutta, NCSU, Spring, 2009
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APS Protocol Operation
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Each node:
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Uses local priority logic to rank (possibly many) local
events
– Encodes highest priority event E1 into K1 byte to be
sent
– Extracts event E2 last received by remote entity
– Uses global priority logic to rank events E1, E2
– Let E be the highest priority event among E1, E2:
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Sets the status of local bridge/selector based on E
Encodes status in the K2 byte to be sent
If status != status of last K2 byte received, mismatch
alarm
Copyright Rudra Dutta, NCSU, Spring, 2009
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Underlying concepts
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From non-ring APS
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1+1 switching
1:1 switching
1:n switching
Further generalizations
Physical diversity
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1+1 Switching
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1:1 Switching
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1:n Switching
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Access Protection
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SONET Rings
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Self-healing rings:
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Services automatically restored following a failure or
signal degradation
Restoration times less than 60 ms
Deploy fiber for loop diversity:
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Separate fiber sheath
Separate conduits
Route diversity: take different physical routes from
source to destination
Copyright Rudra Dutta, NCSU, Spring, 2009
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SONET Ring Types
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Choices for various attributes
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Number of fibers per link: 2 or 4
Direction of signal: unidirectional or bidirectional
Level of switching: line or path
All 8 ring types are possible
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But three have become common in practice
– UPSR (2 fiber)
– BLSR/2
– BLSR/4
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Direction
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Unidirectional
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Only one direction around the ring used
for two-way communication
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ADM
Asymmetric delays
ADM
All working traffic travels in clockwise direction
Opposite direction used for protection
ADM
ADM
Bidirectional
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Physically indistinguishable from unidirectional rings; difference
is in direction of traffic flow
Under normal routing, both directions of a connection:
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Travel along ring through same ring nodes
 Travel in two opposite directions - Symmetric delays
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Working traffic in both clockwise and counter-clockwise direction
If links between NE1-NE2 fail, protection switching uses spans
between NE2-NE3, NE3-NE4, and NE4-NE1
Copyright Rudra Dutta, NCSU, Spring, 2009
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Number of Fibers
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2 or 4 fibers between each pair of SONET nodes in the
ring
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2-fiber ring
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2-fiber rings robust enough for small geographical area (within
city), may survive single failure, will partition with two or more
4-fiber rings used for regional, national backbones, may survive
multiple failures
Each fiber span carries both working-traffic channels and
protection channels
At most half the channels on each fiber can carry working traffic
4-fiber ring
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Working and protection pairs carried over different fibers
Twice as much fiber cable, but each fiber can be used to
capacity
Copyright Rudra Dutta, NCSU, Spring, 2009
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Switching Level
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Concepts valid in general mesh networks (not just rings)
Path switching:
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Restoration of traffic handled by source/destination of each affected
traffic stream
Source/destination reroute traffic in the event of a failure somewhere in
the route
Affected traffic streams may take different protection routes
Also called path protection
Implemented in a 1+1 or 1:n arrangements
Line switching:
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Restoration of traffic is handled by the nodes at the ends of failed link,
not the sources/destinations
– Two ways to implement:
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Span protection: if a fiber is cut between two nodes, traffic is switched to
another fiber between same two nodes
 Line protection: traffic is switched to another route through the network
between the same two nodes
–
All affected traffic streams take same protection route
Copyright Rudra Dutta, NCSU, Spring, 2009
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UPSR
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1+1 protection: traffic from A to B sent simultaneously on
working/protection fibers
B monitors both fibers, selects the better signal
Fast restoration:
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Action required only at receivers
– No need for complicated signaling (APS) protocol
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But: asymmetric delays
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Not a problem for voice traffic
Potentially problem for TCP window flow control
Line switching not a natural choice with unidirectional ring
No spatial reuse:
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A bidirectional connection uses capacity on each link of ring
– Max traffic on ring equal to link speed
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No limit on number of nodes, length of ring
Simple, easy to implement, low cost
Popular in lower-speed local exchange and access networks
Copyright Rudra Dutta, NCSU, Spring, 2009
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UPSR
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BLSR/4
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Two fibers for working traffic, two fibers for protection
Working traffic carried on both directions along the ring
Traffic routed on shortest path between end nodes
Spatial reuse:
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Each connection uses capacity only on shortest path
– Aggregate traffic can significantly exceed link speed
– Shortest path routing maximizes spatial reuse
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Extra traffic capability (1:1 protection)
Span protection: traffic switched to protection ber between two
nodes where failure occurred
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Transmitter/receiver failures on a working fiber
– Working fiber cuts
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Line protection: traffic rerouted around the ring on protection fibers
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Cuts of both protection and working bers along a link
– Node failures
Copyright Rudra Dutta, NCSU, Spring, 2009
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BLSR/4
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BLSR/2
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Protection fibers embedded within working bers
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Both fibers used to carry working traffic
Half the capacity on each fiber reserved for
protection
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Span protection not possible
 Line protection similar to BLSR/4:
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Upon link failure, traffic rerouted along other part of
ring using protection capacity on two fibers
– Traffic mapping a tricky problem
– Extra traffic capability (1:1 protection)
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Not discussing details
Copyright Rudra Dutta, NCSU, Spring, 2009
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