第5讲 网络生存性
Network Survivabilityy
Agenda
•
•
•
•
•
•
Basic Concepts
Protection in SONET/SDH
Protection in IP Networks
Protection in Optical Layer
Interworking between Layers
Summary
6.1 Basic Concepts
Basic Conception
• Network Survibability
What?
The ability of continue providing services in the present of failures, commonly supplying 99.999% availability of the network
Why?
Millions of users can be disrupted and millions of dollars can be lost for some network‐related outage
g
How?
Providing redundant capacity and automatically rerouting traffic
Basic Concepts
• Distinguish some protection (保护) schemes
ƒ Dedicated or Shared
‐‐‐‐Assignment of protection bandwidth one to one or multiple to one.
ƒ Revertive or nonrevertive
‐‐‐‐switching back or not after the failure is repaired.
Dedicated protection may be Revertive or nonrevertive whereas shared must be Revertive.
ƒ Unidirectional switching and bidirectional switching
Unidirectional switching and bidirectional switching
(see next)
Basic Concepts
working
( )
(a)
Source
(b)
Dest.
protect
working
Source
traffic
no traffic
Dest.
protect
p
Figure 10.1 (a)Unidiretional protection switching
(b)Bidiretional protection switching
Basic Concepts
• How the traffic is rerouted in case of a failure
How the traffic is rerouted in case of a failure
¾ Path switching ¾ Span switching ¾ Ring switching
• Different protection schemes operate at different layers in the network and at different sublayers within a layer.
Basic Concepts
( )
(a)
(b)
Path switching
Normal connection
(c)
Span switching
(d)
Ring switching
Figure 10.2 switching schemes
6.2 Protection in SONET/SDH
Protection Schemes in SONET and SDH
• SONET and SDH have the similar protection schemes,different nomenclatures
h
diff
t
l t
• Each protection scheme associated with a specific layer in the network
• Summary of protection schemes in SONET and SDH
SONET Term
1+1
1:N
SDH Term
1+1
1:N
Type
Topology
Layer
UPSR
BLSR
SNCP MS-SPRing
Dedicated Shared Dedicated Dedicated Shared
Point-point Point-point Ring Ring/mesh Ring
Line/MS
Line/MS Line/MS Path/-/Path
Protection Schemes in SONET and SDH
Notes:
N : number of working interface sharing a single protection interface UPSR : Unidirectional Path‐Switched Ring scheme
SNCP : 1+1 SubNetwork Connection Protection
BLSR : Bidirectional Line‐Switched Ring
MS‐SPRing
MS
SPRing : Multiplexed Section
: Multiplexed Section‐Shared
Shared Protection Protection
Ring
Protection Schemes in SONET and SDH
•
Restoration time (恢复时间）
Restoration time (恢复时间）
In SONET/SDH standards,service must be within 60ms
detect the failure(10ms)
signal to other nodes,including propagation delay
restoration
actual switching time
time
reacquire the frame synchronization after
the switch-over has occurred
Point‐to‐Point Links
（点对点链路）
• 1+1 protection
Traffic is transmitted simultaneously on working and Traffic
is transmitted simultaneously on working and
protection fibers from source to destination.When working fiber is cut,the destination switches over to the other fiber to receive data.
• 1:1 protection,1:N protection
Traffic is transmitted over only the working fiber,when working ki
fiber is cut,both source and destination switch over to the protection fiber.
Point‐to‐Point Links
splitter
switch
(a)
switch
switch
(b)
switch
switch
switch
switch
switch
switch
Low-priority data
(c)
Figure 10.3 1+1,1:1,1:N protection
Point‐to‐Point Links
Comparison of 1+1 and 1:1 protection
Comparison of 1+1 and 1:1 protection
• 1+1 protection is faster and requires no signaling protocol,while 1:1 needs protocol
• Protection fiber in 1:1 scheme can transmit lower‐
priority traffic under normal operation
• 1:1 protection can be extended to share a single protection fiber among many working fibers (1:N) Self‐Healing Rings
（自愈环）
•
Concept of ring
Concept of ring
•
Self‐healing Rings
Simplest topology that is 2‐connected Rings can automatically detect failures and reroute traffic away from failed links or nodes onto other routes rapidly
routes rapidly
•
Ring architectures widely deployed
ƒ Two‐fiber unidirectional path‐switched rings(UPSR)
ƒ Four‐fiber bidirectional line‐switched rings(BLSR/4)
ƒ Two‐fiber bidirectional line‐switched rings(BLSR/2)
Unidirectional Path‐Switched Rings
Working connection A to B
g
working connection B to A
ADM
B
path
protect
connection B to A
ADM A
C ADM
protect connection
A to B
D
protection fiber
working fiber
ADM
Figure 10.4
UPSR
Unidirectional Path‐Switched Rings
1+1 scheme operating at the path layer
1+1
scheme operating at the path layer
Easily handle failures of links,nodes
Simple to implement,thus,low cost
requiring no protocol and communication between nodes,thus,short restoration time
• Drawback: low fiber utilization
• Popularly used in lower‐speed local exchange and access networks
•
•
•
•
Unidirectional Path‐Switched Rings
• Limit
Limit on the number of nodes or ring length
on the number of nodes or ring length
Limited by restoration time affected by the different delays associated with the clockwise and couterclockwise path taken by a signal
Bidirectional Line‐Switched Rings
• Operating
Operating at the line layer (SONET) or multiplex at the line layer (SONET) or multiplex
section layer (SDH)
• Two types of BLSR:
BLSR/4
BLSR/2
Bidirectional Line‐Switched Rings
Working
g connection
ADM
B
working fibers
protection fibers
ADM A
protect connection ADM
C
D
BLSR/4
ADM
Figure 10.5
Bidirectional Line‐Switched Rings
ADM
Working connection
Working/protection fiber
?
Working/protection fiber
ADM
ADM
ADM
Figure 10.6
BLSR/2
Bidirectional Line‐Switched Rings
• BLSR/2 is similar with BLSR/4 • Types of protection schemes
BLSR/4
•
span switching (protection for working fiber failed)
• ring switching (protection for fiber or cable failed)
BLSR/2
ring switching
• Priority of protection schemes span switching is prior to ring switching
Bidirectional Line‐Switched Rings
Working
g connection
ADM
B
working fibers
protection fibers
ADM A
protect connection ADM
C
D
ADM
Figure 10.7 span switching
Bidirectional Line‐Switched Rings
Working
g connection
ADM
B
working fibers
protection fibers
ADM A Protect connectionC ADM
D
ADM
Figure 10.8 ring switching
Bidirectional Line‐Switched Rings
• Priority of protection schemes Priority of protection schemes
ƒ span switching is prior to ring switching
ƒ span switching can be cancelled if span fails both on working and protection path
Bidirectional Line‐Switched Rings
• Difference between BLSR/4 and BLSR/2
Difference between BLSR/4 and BLSR/2
¾ BLSR/4 deployed in long‐haul carriers,while BLSR/2 in metro carriers
¾ BLSR/4 can handle more failures than BLSR/2 ¾ BLSR/4 is easier to service than BLSR/2
¾ Ring management is more complicated in BLSR/4 than BLSR/2
6.3 Protection in IP Networks
Protection in IP Networks
• Characteristic of IP protection
ƒ Packets rerouted incorrectly and possibly loop
Destination:
E
D
A
B
C
D
next hop:B next hop:C next hop:D normal operation
next hop:B next hop:C next hop:B after failure,before convergence
next hop:E next hop:A next hop:B after convergence
Figure 10.13
An example of illustrating routing loops
Protection in IP Networks
• Slow for all the routing table have been updated • Packets can rerouted on the newly set up LSPs with Packets can rerouted on the newly set up LSPs with
the use of MPLS
• Detecting failures slowly
• Another option is relying on the underlying SONET or optical layer to detect the failure and inform the IP layer(not usually used)
6.4 Why Optical Layer
Protection
Why Optical Layer Protection
1 way is
considered
Save 1 wavelength
光层可以动态提供一条
Lightpath
Why Optical Layer Protection
Benefit of optical layer protection:
e e o op ca aye p o ec o
• Can provide extensive protection functions for client layers(IP,SDH).
• Significant cost saving
• Handle some faults more efficiently than the client layer
• Can protect multiple failures
Can protect multiple failures
• A variety of mesh‐based protection schemes are being developed,requiring less protection capacity than ring‐based schemes
Why Optical Layer Protection
SONET connection
ADM
ADM
SONET connection
OXC
ADM
OXC
Lightpath
OXC
ADM
Lightpath
OXC
ADM
OXC
OXC
ADM
rerouted
t d lightpath
li ht th
OXC
OXC
ADM
ADM
Figure 10.15 Optical layer protection used to enhance SONET protection
Why Optical Layer Protection
Limitations of optical layer protection
Limitations of optical layer protection
• Not all failures can be handled by the optical layer
• Optical layer may not be able to detect the appropriate conditions that would cause it to invoke protection switching
• The optical layer protects traffic in units of lightpaths,can’t protect parts of the traffic within a lightpath
Why Optical Layer Protection
• Service Classes Based on Optical Layer Protection Level
of Service
Level of
connection
availablility
Restoration
time
Example of
protection
provider
Platinum
highest
Fastest(around 60ms)
Didicated 1+1
protection
Gold
high
Fast(hundreds
of ms)
Shared mesh
protection
Silver
medium
medium
Providing
g
“best-effort”
Bronze
Low
lowest
Unprotected
lightpaths
Lead
Lowest
Lowest
Used to
protect other
classes of
service
6.5 Optical Layer Protection Schemes
Optical Layer Protection Schemes
• Optical layer protection is lightpath‐based
p
y p
g p
• Optical protection schemes operate in the Och or OMS layers
• Och layer‐‐‐‐‐path layer OMS layer‐‐‐‐‐line layer
Optical Layer Protection Schemes
Optical protection schemes in OMS layer
1+1
OMS-DPRing OMS-SPRing
1:1
Dedicated
Shared
Type
Topology Point-point Point-point
Dedicated
Shared
Ring
Ring
Optical protection schemes in Och layer
Type
Topology
1+1
Och-SPRing
Och-Mesh
Dedicated
Shared
Mesh
Ring
Shared
Mesh
Optical Layer Protection Schemes
• Protection
Protection schemes referred above are schemes referred above are
mostly similar to their counterparts in SONET/SDH ƒ 1+1 OMS Protection‐‐‐‐‐‐1+1 Line Protection
ƒ 1:1 OMS Protection ‐‐‐‐‐‐1:1 Line Protection
ƒ OMS‐DPRing(ULSR) ‐‐‐‐‐‐UPSR in OMS layer
Och-Mesh Protection
ƒ
ƒ
ƒ
ƒ
OMS‐SPRing‐‐‐‐‐‐‐BLSR/4,BLSR/2
1+1 Och Dedicated Protection‐‐‐‐‐‐1+1 Protectionin Och layer Och‐SPRing Protection ‐‐‐‐‐‐BLSR/4 in Och layer
/
Och‐Mesh Protection
• Mesh protection is suitable for dense physical topologies,with traffic being fairly distributed
• Bandwidth efficiency of mesh protection compared to ring protection depends on 3 factors
¾ Network topology
Network topology
¾ Traffic pattern
¾ Type of mesh protection scheme
Och‐Mesh Protection
‐‐‐‐‐‐‐‐‐‐‐‐‐an example
Mesh network
Och-SPRing protection
1+1 dedicated protection
Och-Mesh protection
Protection is
set up
dynamically
Figure 10.17 Illustration of bandwidth efficiency of mesh protection
Och‐Mesh Protection
• Protection
Protection lightpaths of mesh protection are only set up lightpaths of mesh protection are only set up
when there is a failure,not ahead of time.i.e.it use a dynamic restoration.
Och‐Mesh Protection
• The mesh protection schemes were abandoned before for slow restoration time(order of minutes),complexly management and no applicable standards
management and no applicable standards. • Reason for the resurrection of mesh protection schemes
ƒ The processing power available to implement mesh protection has dramatically increased(computing new routes)
ƒ OXC and other optical layer equipment protect bandwidth at much larger lightpaths than DXC ƒ Relatively fast signaling and routing protocols have been developed
ƒ The 60ms protection time requirement is not a hard number
• Mesh
Mesh protection is needed deployed,requiring protection is needed deployed requiring
functions of route computation,topology maintenance and signaling to set up the protection routes 6.6 Interworking
between Layers
Interworking between Layers
• The protection mechanisms in different layers can prevent or delay service restoration.
d l
i
i
• Restoration of network converge should under right assumptions
ƒ A viable protection path exists for each layer
ƒ The server layer is independent of the client layer to detect failure and invoke its protection –switching functions
y p
p
y y
g
ƒ The client layer protection is revertive and will repeatedly try switching to the other path • Coordination between protection mechanisms
ƒ Add a priority among layers
ƒ impose an additional hold‐off time in the higher layer