sonet survivability mechanisms csc/ece 772: survivable networks spring, 2009, rudra dutta
TRANSCRIPT
SONET Survivability MechanismsSONET Survivability Mechanisms
CSC/ECE 772: Survivable Networks
Spring, 2009, Rudra Dutta
Copyright Rudra Dutta, NCSU, Spring, 2009 2
SONET APSSONET APS Automated Protection Switching
– 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
OAM protocols expected to detect abnormal conditions
Similar techniques later used for ATM
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SONET OperationSONET Operation Plesiochronous to Synchronous
– 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– Jitter, wander, phase difference limited to ingress to
network, synchronized inside– Multiplexing and interleaving considered functions of
network Early native optical transport Ring or non-ring operation
Copyright Rudra Dutta, NCSU, Spring, 2009 4
SONET LayersSONET Layers
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STS-1 FrameSTS-1 Frame
(SOH + LOH)
(POH)
K1 K2
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Automatic Protection SwitchingAutomatic Protection Switching 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 7
APS ObjectiveAPS Objective Whether traffic is received over the working or
protection fiber is determined by:– The status of the bridge at source node– The status of the selector at destination node
Objective: establish agreement between source and destination regarding the status of bridge/selector
K1, K2 LOH bytes used by APS protocol for this purpose
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APS EventsAPS Events Protection switching: a change in the current
position of the bridge/selector Initiated due to certain events:
– 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.
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APS Protocol: K1, K2 bytesAPS Protocol: K1, K2 bytes K1 byte:
– Switch request (protection switching event) – 4 bits– Destination node – 4 bits (max 16 SONET nodes)
K2 byte:– Source node – 4 bits– Long/short bit– Status (of bridge/selector) – 3 bits
Source/destination use K1, K2 bytes to coordinate protection switching actions
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APS Protocol OperationAPS Protocol Operation Each node:
– 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:
Sets the status of local bridge/selector based on EEncodes status in the K2 byte to be sent
– If status != status of last K2 byte received, mismatch alarm
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Underlying conceptsUnderlying concepts From non-ring APS
– 1+1 switching– 1:1 switching– 1:n switching– Further generalizations– Physical diversity
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1+1 Switching1+1 Switching
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1:1 Switching1:1 Switching
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1:n Switching1:n Switching
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Access ProtectionAccess Protection
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SONET RingsSONET Rings Self-healing rings:
– Services automatically restored following a failure or signal degradation
– Restoration times less than 60 ms
Deploy fiber for loop diversity:– Separate fiber sheath– Separate conduits– Route diversity: take different physical routes from
source to destination
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SONET Ring TypesSONET Ring Types Choices for various attributes
– 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– But three have become common in practice– UPSR (2 fiber)– BLSR/2– BLSR/4
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DirectionDirection Unidirectional
– Only one direction around the ring usedfor two-way communication
Asymmetric delays– All working traffic travels in clockwise direction– Opposite direction used for protection
Bidirectional– Physically indistinguishable from unidirectional rings; difference
is in direction of traffic flow– Under normal routing, both directions of a connection:
Travel along ring through same ring nodes Travel in two opposite directions - Symmetric delays
– 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
ADM
ADM
ADM
ADM
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Number of FibersNumber of Fibers 2 or 4 fibers between each pair of SONET nodes in the
ring– 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 2-fiber ring
– 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
– Working and protection pairs carried over different fibers– Twice as much fiber cable, but each fiber can be used to
capacity
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Switching LevelSwitching Level Concepts valid in general mesh networks (not just rings) Path switching:
– 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:– Restoration of traffic is handled by the nodes at the ends of failed link,
not the sources/destinations– Two ways to implement:
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
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UPSRUPSR 1+1 protection: traffic from A to B sent simultaneously on
working/protection fibers B monitors both fibers, selects the better signal Fast restoration:
– Action required only at receivers– No need for complicated signaling (APS) protocol
But: asymmetric delays– 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:
– A bidirectional connection uses capacity on each link of ring– Max traffic on ring equal to link speed
No limit on number of nodes, length of ring Simple, easy to implement, low cost Popular in lower-speed local exchange and access networks
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UPSRUPSR
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BLSR/4BLSR/4 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:
– Each connection uses capacity only on shortest path– Aggregate traffic can significantly exceed link speed– Shortest path routing maximizes spatial reuse
Extra traffic capability (1:1 protection) Span protection: traffic switched to protection ber between two
nodes where failure occurred– Transmitter/receiver failures on a working fiber– Working fiber cuts
Line protection: traffic rerouted around the ring on protection fibers– Cuts of both protection and working bers along a link– Node failures
Copyright Rudra Dutta, NCSU, Spring, 2009 24
BLSR/4BLSR/4
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BLSR/2BLSR/2 Protection fibers embedded within working bers
– Both fibers used to carry working traffic– Half the capacity on each fiber reserved for
protection
Span protection not possible Line protection similar to BLSR/4:
– 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)
Not discussing details