SONET Survivability Mechanisms CSC/ECE 772: Survivable Networks Spring, 2009, Rudra Dutta

Copyright Rudra Dutta, NCSU, Spring, SONET 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

Copyright Rudra Dutta, NCSU, Spring, SONET 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, SONET Layers

Copyright Rudra Dutta, NCSU, Spring, STS-1 Frame (SOH + LOH) (POH) K1 K2

Copyright Rudra Dutta, NCSU, Spring, Automatic Protection Switching APS takes place at the SONET line level Very complex task ANSI APS document (T ) 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, APS 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

Copyright Rudra Dutta, NCSU, Spring, APS 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.

Copyright Rudra Dutta, NCSU, Spring, APS 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

Copyright Rudra Dutta, NCSU, Spring, APS 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 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, Underlying concepts From non-ring APS – 1+1 switching – 1:1 switching – 1:n switching – Further generalizations – Physical diversity

Copyright Rudra Dutta, NCSU, Spring, Switching

Copyright Rudra Dutta, NCSU, Spring, :1 Switching

Copyright Rudra Dutta, NCSU, Spring, :n Switching

Copyright Rudra Dutta, NCSU, Spring, Access Protection

Copyright Rudra Dutta, NCSU, Spring, SONET 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

Copyright Rudra Dutta, NCSU, Spring, SONET 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

Copyright Rudra Dutta, NCSU, Spring, Direction Unidirectional – Only one direction around the ring used for 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

Copyright Rudra Dutta, NCSU, Spring, Number 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

Copyright Rudra Dutta, NCSU, Spring, Switching 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

Copyright Rudra Dutta, NCSU, Spring, UPSR 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

Copyright Rudra Dutta, NCSU, Spring, UPSR

Copyright Rudra Dutta, NCSU, Spring, BLSR/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, BLSR/4

Copyright Rudra Dutta, NCSU, Spring, BLSR/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