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CSC/ECE 791B – Survivable NetworksSONET Protection Switching
George N. Rouskas
Department of Computer Science
North Carolina State University
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.1
Outline
1. Protection Switching Architectures
2. SONET Ring Types
3. Two-Fiber Protection
4. Four-Fiber Protection
5. Automatic Switching Protocol (APS)
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.2
Protection Switching
Network must continue to provide reliable serviceseven in the presence of
failures
errors
poor signal quality
Protection techniques:
ensure survivability
involve the provision of redundant capacity
reroute traffic when failures occur
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.3
APS Protocol
OAM protocol detects abnormal conditions
Automatic protection switching (APS) protocol:
switches traffic from working to protection entity upon failure
no manual intervention
Manual intervention necessary for repairing failed entity
Revertive or non-revertive operation
ATM protection techniques and APS protocol very similar
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.4
Non-Ring APS
Four architectures:
1. 1+1 switching
2. 1:1 switching
3. 1:n switching
4. m:n switching
T-carrier employed protection switching
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.5
1+1 Switching
Source Sink
Working path
Protection path
Bridge
Protection Domain
Selector
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.6
1:1 Switching
Source Sink
Working path
Protection path
Protection Domain
SelectorBridge
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.7
Extra Traffic Capability
Extra traffic:
low priority, not protected traffic
occupies protection entity under normal operation (no failures)
preempted (dumped) when working entity fails and protectedtraffic switched over to protection entity
sold at deep discount
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.8
1:n Switching
Source 1
Sink n
Sink 2
Sink 1
Working path
Switch
Switch SwitchProtection path
Source 2
Source n
Protection Domain
Selector
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.9
Protection in the Access Network
CO
Business
CO
Business
Same feeder,Diverse conduit
Same feeder,Diverse sheath
CO
Business
Different feeders,Diverse route
CO CO
Business
Different feeders,Different COs
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.10
SONET Rings
Self-healing rings:
services automatically restored following a failure or signaldegradation
restoration times less than 60 ms
Deploy fiber for loop diversity:
1. separate fiber sheath
2. separate conduits
3. route diversity: take different physical routes from src to dest
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.11
SONET Ring Types
Attribute Choices
Number of fibers per link 2-fiber
4-fiber
Direction of the signal Unidirectional
Bidirectional
Level of protection switching Line switching
Path switching
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.12
Unidirectional Rings
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
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.13
Unidirectional Rings (cont’d)
Span #7
Span #3
Span #1
Span # 5
Span
#4 Span #8 Sp
an #
6 Span #2
NE 2
NE 3
NE 1
NE 4
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.14
Bidirectional Rings
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 spansbetween NE2-NE3, NE3-NE4, and NE4-NE1
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.15
Bidirectional Rings (cont’d)
Span #3
Span
#4 Span #8 Sp
an #
6 Span #2
NE 2
NE 3
NE 1
NE 4
Span #1
Span # 5
Span #7
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.16
2-Fiber vs. 4-Fiber Rings
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
4 fiber unidirectional rings: quite uncommon
2-fiber vs. 4-fiber bidirectional rings
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.17
2-Fiber Bidirectional Ring
NE1 NE2Span #1 Working channels
Span #1 Protection channels
Span #5 Working channels
Span #5 Protection channels
Each fiber span carries both working-traffic channels and protectionchannels
At most half the channels on each fiber can carry working traffic
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.18
4-Fiber Bidirectional Ring
NE1 NE2Span 1A Working channels
Span 1B Protection channels
Span 5A Working channels
Span 5B Protection channels
Working and protection pairs carried over different fibers
Twice as much fiber cable, but each fiber can be used to capacity
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.19
Path Switching vs. Line Switching
Concepts valid in general mesh networks (not just rings)
Path switching:
restoration of traffic handled by source/destination of each affectedtraffic stream
source/destination reroute traffic in the event of a failuresomewhere in the route
affected traffic streams may take different protection routes
also called path protection
implemented in a 1+1 or 1:n arrangements
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.20
Path Switching vs. Line Switching (cont’d)
Line switching: restoration of traffic is handled by the nodes at theends of failed link, not the sources/destinations
Two ways to implement:
1. span protection: if a fiber is cut between two nodes, traffic isswitched to another fiber between same two nodes
2. line protection: traffic is switched to another route through thenetwork between the same two nodes
all affected traffic streams take same protection route
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.21
Path Switching vs. Line Switching (cont’d)
Connection
(a) Normal operation (b) Path switched restoration (path protection)
(c) Span protection, a form of line switching (d) Line protection, another form of line switching
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.22
Common SONET Ring Types
All 8 ring types are possible, but three have become common:
1. UPSR: two-fiber unidirectional path-switched rings
2. BLSR/2: two-fiber bidirectional line-switched rings
3. BLSR/4: four-fiber bidirectional line-switched rings
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.23
UPSR
1+1 protection: traffic from A to B sent simultaneously onworking/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
problem for TCP window flow control
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.24
UPSR (cont’d)
NE 2NE 1
NE 4 NE 3
Protection fiberProtection trafficWorking traffic Working fiber
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.25
UPSR (cont’d)
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
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.26
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)
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.27
BLSR/4 (cont’d)
NE 1
NE 4 NE 3
NE 2
Working fiber Protection fiber
Working traffic
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.28
BLSR/4 Protection Mechanisms
Span protection: traffic switched to protection fiber between two nodeswhere 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 fibers along a link
node failures
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.29
BLSR/4 – Normal Operation
NE 4
NE 3NE 2NE 1
NE 5NE 6
Working
Protection
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.30
BLSR/4 – Span Protection
NE 4
NE 3NE 2NE 1
NE 5NE 6
Working
Protection
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.31
BLSR/4 – Line Protection
NE 5NE 6
Working
Protection
NE 4
NE 3NE 2NE 1
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.32
BLSR/2
Protection fibers embedded within working fibers
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 usingprotection capacity on two fibers
traffic mapping a tricky problem
extra traffic capability (1:1 protection)
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.33
BLSR/2 – Normal Operation
OC-12 BLSR/2 with 12 STS-1s
W: 1-6P: 7-12
W: 1-6P: 7-12
W: 1-6P: 7-12
W: 1-6P: 7-12
W: 1-6P: 7-12
W: 1-6P: 7-12
W: 1-6P: 7-12
W: 1-6P: 7-12
W: 1-6P: 7-12
W: 1-6P: 7-12
NE 1 NE 2 NE 3
NE 4NE 5
STS-1 #3STS-1 #3
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.34
BLSR/2 – Line Switching
W: 1-6P: 7-12
W: 1-6P: 7-12
W: 1-6P: 7-12
W: 1-6P: 7-12
W: 1-6P: 7-12
W: 1-6P: 7-12
W: 1-6P: 7-12
W: 1-6P: 7-12
NE 1 NE 2 NE 3
NE 4NE 5
STS-1 #3STS-1 #3
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.35
BLSRs
More efficient than UPSRs for distributed traffic patterns
Protection capacity shared among all connections
Example: 10-node ring, 1.5 Mbps between adjacent nodes
UPSR requires 15 Mbps protection capacity on each fiber
BLSR/2 requires 1.5 Mbps protection capacity on each fiber
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.36
BLSRs (cont’d)
Maximum number of nodes: 16
Maximum ring length: 1200 Km (6 ms propagation delay)
BLSRs deployed in regional/national high-speed (OC-48, OC-192)networks
BLSR/4 can handle more failures than BLSR/2
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.37
SONET Matched Nodes
ADM
ADM
ADM
ADM
ADM
ADM
ADM
ADM
MN
MN MN
MN
1+1 protection
SONET Ring #1 SONET Ring #2
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.38
Automatic Protection Switching (APS)
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
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.39
APS Objective
Whether traffic is received over the working or protection fiberis determined by:
1. the status of the bridge at source node
2. the status of the selector at destination node
Objective: establish agreement between source and destinationregarding the status of bridge/selector
K1, K2 LOH bytes used by APS protocol for this purpose
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.40
APS Events
Protection switching: a change in the current position of thebridge/selector
Initiated due to certain events:
1. externally initiated commands, e.g., forced switch, manual switch,lockout of protection, etc.
2. automatically initiated command, e.g., loss of signal (LOS), loss offrame (LOF), signal degrade (due to parity errors), etc.
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.41
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 protectionswitching actions
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.42
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 6= status of last K2 byte received, mismatch alarm
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.43
WDM SONET Rings
1. W point-to-point rings, each on one of W wavelengths
high cost: W OADMs, SONET ADMs at each node
independent rings
severe electro-optic bottleneck
2. Static virtual topology, based on traffic pattern
fewer OADMs, SONET ADMs, alleviates bottleneck
3. Dynamic virtual topology
requires sophisticated OXCs, traffic grooming capabilities
CSC/ECE 791B, Spring 2008: SONET Protection Switching Copyright c© 2007 by George N. Rouskas – p.44