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ASON/WSON Fundamentals
Welcome to the ASON / WSON Fundamentals e-Learning Course.
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Objectives
During this course we are going to focus on:
An introduction toASON/WSON technology
A description of theASON/WSON Control Plane
A description of theASON/WSON Protections
On completion of this course the participants will be able to:
1.Describe whatASON/WSON is and which are its main characteristics.
2.Differentiate betweenASON and WSON
3.Understand how WSON works and the relevant characteristics in terms of control plane and
protection schemes introduced into the WDM layer
During this course we are going to focus on:
An introduction to the ASON/WSON technology
A description of the ASON/WSON Control Plane
A description of the ASON / WSON Protections
On completion of this course the participants will be able to:
Describe what ASON is and which are its main characteristics.
Differentiate between ASON and WSON
Understand how WSON works and the relevant characteristics in terms of control plane
and protection schemes introduced into the WDM layer
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Introduction
Lets introduce ASON / WSON main general concepts.
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WDMSDH
Terminology
ASTN(ITU-T)
WSON(IETF)
ASON(ITU-T)
GMPLS : Generalized Multi-Protocol Label Switching (IETF)
ASON : Automatic Switched Optical Network (ITU-T / General Optical Domain)
MPLS TP : Multi-Protocol Label Switching Transport Profile
OTN (ODU Switching): Optical Transport Network (Optical Data Unit Switching)
ASTN : Automatic Switched Transport Network (ITU-T / SDH Domain)
WSON : Wavelength Switched Optical Network (IETF / WDM Domain)
Optical Domain
SDH Domain WDM Domain
GMPLS
(IETF)
MPLS-TP
(ITU-T / IETF)
OTN
(ODU Switching)
(IETF)
ASON, ASTN, WSON, GMPLS
Before beginning with the details about the ASON/WSON technology and
according to the fact that its not unusual to listen to people speaking about
ASON, ASTN, WSON, GMPLS, as if all these technologies were the same, itseems to be useful to clarify some points about the terminology.
The acronym GMPLS, that stands for Generalized Multi-Protocol Label
Switching, refers to a suite of protocols developed by IETF to extend the MPLS
ideas outside the context of the IP world.
The acronym ASON, that stands for Automatic Switched Optical Network, is a
Recommendation developed by ITU-T that specifies the requirement to apply the
GMPLS technology to a generic optical network; in this context, with the term
generic optical network we refer both to an SDH or WDM network, or even to
OTN (ODU switching) an MPLS-TP packet network.
In the SDH domain, the specific used acronym is ASTN, that stands forAutomatic Switched Transport Network, and, also in this case, it is a
Recommendation developed by ITU-T. Its a framework that represents the ITU-T
ideas about how the GMPLS technology should be applied to the SDH world (fast
rerouting of Virtual Containers).
In the WDM domain, instead, the specific used acronym is WSON, that stands for
Wavelength Switched Optical Network, and this a draft, not yet a
Recommendation, developed by IETF, that represent the IETF and ITU-T vision
of how the GMPLS should be applied to the optical part of the WDM world (fast
rerouting of wavelength).
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WSON evolution - advantages
WSON
Advantages
Advanced NetworkResilience
Automatic Circuit
Provisioning
Reduced Costs
Why do we need WSON?
What are the main advantages of the WSON network compared with a traditional
network?There are three main advantages:
The most important advantage is that, in WSON, new protection schemas allow
advanced network resilience mechanisms, that can react, in an automatic
way, also in case of multiple failures.
The second advantage is the possibility for WSON to realize the automatic circuit
provisioning: as a strategic point of evolution, WSON will be able, in the
future, to provide circuits on demand, allowing an IP router, for example, to
ask for a circuit directly to the WDM node that is at the ingress of the WSON,
without passing through the NMS and the human operator.The third advantage is the strategic cost reduction mainly due to the fact that it is
possible to share the protection bandwidth among a group of WSON circuits.
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a Traditional Network
Ethernet Node
WDM Node
Network Management System (NMS)
Ethernet Node
Data Communication Network (DCN)
To implement these advantages, WSON is quite different compared with a
traditional WDM network.
To understand what are these differences, lets take a look at what happens in atraditional WDM Network.
Lets consider the WDM network in the picture: this network has 6 interconnected
WDM nodes.
In a traditional WDM network the nodes are not aware of the fact that they are
part of a network.
For example, the nodes dont know the topology of the network and are not able
to take any decision about where to put the customer circuits; even the entity
circuit has no meaning for a single WDM node; in other words, the nodes are not
network aware.Who has the ability to understand how the nodes are interconnected? Who has
the intelligence to understand that a list of cross-connections between two
termination points is a circuit? In other words, who is the brain of the network?
In a non WSON environment the intelligence of the network only resides inside
the NMS and the human operator that sits in front of it.
The decision taken by NMS and the human operator are sent individually to each
node by the DCN.
The nodes, passively execute the commands coming from the NMS; they dont
take part in any routing decision; they are not able to interact with the other nodes
in order to create or delete circuits.
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Ethernet Node
WDM Node
Network Management System (NMS)
Ethernet Node
Data Communication Network (DCN)
a wson Network
In a WSON network the situation is different: the intelligence of the network is
now distributed among the nodes, the NMS and the human operator.
Specific WSON processes run inside the controller cards of each node, thatimplement, together with the WSON processes running inside the NMS, the
distributed Control Plane, that is the distributed intelligence that manages the
customer traffic.
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Evolution: from a centralized to a
distributed control plane
CENTRALIZED
CONTROL PLANE
DITRIBUTED
CONTROL PLANE
WSON
So the first point of distinction between a traditional WDM network and WSON is
the evolution from a centralized intelligence to a distributed intelligence, therefore
the evolution from a centralized control plane to a distributed control plane.
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Evolution: from a centralized to a
distributed control plane
DATA PLANE
DISTRIBUTED CONTROL PLANE
The distributed control plane controls the data plane, that is the logical entity
formed by all the resources available in the network to transport customer traffic.
The data plane transports the customer traffic; the control plane decides wherethis traffic should pass through and sends the right commands to the involved
data plane resources.
The distinction between Control Plane and Data Plane is important in a WSON
context.
Please notice that, the fact the control plane is distributed among different
entities, means that these entities must be coordinated: the complexity of the
WSON Control Plane is mainly due to its distributed nature.
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WSON internodes Communication network
Ethernet Node
WDM Node
Network Management System (NMS)
Ethernet Node
Data Communication Network (DCN)
WSON Inter-node
Communication
Network
In order to be able to implement a distributed intelligence, the WSON equipments
must have , first of all, the capability to speak with each other, not only with the
NMS.This is obtained by an inter-node communication network that is realized using
specific WSON reserved communication channels; in GMPLS terminology, these
special channels are called Control Channels.
Its important to remark that, this new intra-nodes communication network doesnt
substitute the traditional Data Communication Network, but works together with it.
The DCN must be present in any case, to allow the communications between the
nodes and the NMS for both WSON and non WSON related operations. For
example, the performance data collection is realized using the traditional DCN;
the control channels are not involved in this process.
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WSON internodes Communication network
Logical Control Channel
that runs inside the fiber
together with the customer traffic
IN-FIBER-IN-BAND
IN-FIBER-OUT-OF-BAND
Depending on the physical implementation its possible to classify the control
channels in different categories.
An in-fiber-in-band control channel is a logical communication channel thatshares the bandwidth with the customer traffic and that runs in a fiber. The
bandwidth available for the communication is taken by the bandwidth available for
the customer traffic; if there is no bandwidth consumption for the communication,
it means that more bandwidth is available for the customer traffic. Typically, the
routers in the IP world use in-fiber-in-band communication: control information is
sent together with the customer traffic.
In in-fiber-out-of-band control channels, the bandwidth reserved for the traffic and
the bandwidth reserved for the communication are completely not overlapping.
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WSON internodes Communication network
LOGICAL CONTROL CHANNELS IMPLEMENTED
USING THE ETHERNET Q INTERFACE
(NORMALLY RESERVED FOR THE COMMUNICATION WITH THE NMS)
OUT-OF-FIBER-OUT-OF-BAND
Ethernet INTERFACE Ethernet INTERFACE
The other type of control channel is the out-of-fiber-out-of-band control channel:
the typical example is when we use the Ethernet interface, that is normally
reserved for the communication with NMS, to transport the logical controlchannel; in this case, the channel is out of fiber, because it runs outside the fiber
transporting the customer traffic and it is out of band, because the bandwidth on
the Ethernet interface is reserved for the communication and cannot be used to
transport traffic.
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WSON internodes Communication network
123
80
.
..
Optical Supervisory Channel (OSC)
Bandwidth reserved for the in-fiber-out-of-band control channel
As far as DWDM systems are concerned, in particular, the relevant standards
define an additional low rate lambda that is specifically designed to transport
management information.The logical channel that uses this special lambda is called Optical Supervisory
Channel or OSC.
Inside the OSC, a configurable amount of bandwidth can be reserved for WSON
internodes communication.
This type of control channel is in-fiber, because it shares the physical fiber with
the customer traffic, and it is also out-of-band, because the amount of bandwidth
reserved for the control channel inside the OSC cant be used to transport any
other customer traffic.
In special applications, like the single hop long distance, this OSC can also beimplemented on one of the wavelength part of the standard grid for working
traffic, loosing therefore a traffic channel, but achieving longer distance coverage
for the OSC itself.
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Requirements For a WSON
What are
the general
requirements
for a WSON
?
The nodes must be able to exchange
information about the status of the network
The nodes must be able to send commands
each other
The nodes must be able to re-route the traffic
from one line to another line even changing the
lambda of the circuit
But, what are the general requirements to realize a WSON able to take
advantage of the advanced new protection schemas?
A WSON is based on the following assumptions:The nodes are able to exchange information about the status of the network;
The nodes are able to send commands to each other;
The nodes are able to re-route the traffic from one line to another line even
changing the lambda of the circuit.
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Requirements For a WSON
LINE 1
LINE 2
LINE3
LINE
5
LINE6
LINE 7
LINE8
LINE 4
LINE 9
CUSTOMER
TRAFFIC
ROADM
A WDM node that is able to reconfigure the direction of the traffic via software
commands is called multi-directional Reconfigurable Optical Add and Drop
Multiplexer or ROADM: all WSON capable nodes are multi-directional ROADMs.But how is the capability to change the lambda used by a circuit using only one
transponder obtained?
This is obtained using a special kind of transponder that is called tunable
transponder: this transponder can change the lambda of the circuit changing its
internal configuration via software.
Please notice that, its always the controller of the WSON node, guided by the
distributed control plane processes, that triggers the change of direction and, if
necessary, lambda for a circuit.
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Requirements For a WSON
WSON NODENMS
WHERE TO RE-ROUTE
THE TRAFFIC ?
OR
THE DECISION CAN BE TAKEN
BY
PCE
PCE
One of the WSON requirements is that the WSON nodes should be able to re-
route traffic when a failure occurs.
In order to be able to create or re-route WSON circuits, the equipment must knowwhich is the best path to choose (click);
How is this knowledge obtained?
We can have two alternative approaches:
The decision is taken by some process running inside the NMS; The NMS tells to
the nodes which is the best path to create;
The decision is taken by the WSON node itself; its able to calculate where to
route o re-route the traffic without passing through the NMS.
In both cases, the engine that is in charge to calculate the best worker and all the
best protections path, according to some specific constraints, that its possibleto explicitly define, is generically called Path Computation Engine or PCE.
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The Control Plane
Lets start describing in deeper details the main features of a WSON
Control Plane.
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Ideal situation
1. INFORMATION ABOUT
THE NETWORK TOPOLOGY:
GOSPF-TE
2. CALCULATION OF THE BEST PATHS: PCE
3. SENDS COMMANDS TO OTHER NODES
TO CREATE AND PROTECT THE CIRCUITS:
GRSVP-TE
WSON NODE
Lets now consider an ideal situation for a while.
The ideal situation is the one in which the PCE runs inside each single WSON
node: this would allow each node to take real-time decision very fast,independently from other nodes and from the NMS. As a consequence, more
efficient protection schemas could be implemented.
To make it work, the first step is to allow the WSON nodes to know the topology
of the entire WSON network.
This information can be acquired using one of the GMPLS supporting protocols,
the GOSPF-TE.
GOSPF-TE is a routing protocol and its purpose is to allow WSON nodes to
exchange information about the network topology. This information is: the
administrative costs of the links all around the network, that are assigned viaconfiguration; the bandwidth available or the bandwidth used for each link of the
network; the status of each link: is the link up or is it down?; other additional
administrative information that could have been assigned to the links for different
purposes.
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Ideal situation
1. INFORMATION ABOUT
THE NETWORK TOPOLOGY:
GOSPF-TE
2. CALCULATION OF THE BEST PATHS: PCE
3. SENDS COMMANDS TO OTHER NODES
TO CREATE AND PROTECT THE CIRCUITS:
GRSVP-TE
WSON NODE
The second step is to calculate the best path: having the picture of the network,
every WSON node is able to find out what is the best path to follow to create or to
protect a circuit; the tool that is in charge of calculating best paths is the PCE.
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Ideal situation
1. INFORMATION ABOUT
THE NETWORK TOPOLOGY:
GOSPF-TE
2. CALCULATION OF THE BEST PATHS:
PCE
3. SENDS COMMANDS TO OTHER NODES
TO CREATE AND PROTECT THE CIRCUITS:
GRSVP-TE
WSON NODE
Step number 3 is the actual creation or re-routing of a WSON circuit: once
decided the best path, a WSON node will send requests to the other nodes on
the selected path in order to create, re-route or release WSON circuits; this isdone using another GMPLS supporting protocol: the GRSVP-TE.
GRSVP-TE is a signaling protocol and its purpose is to allow WSON nodes to
send commands, and receive feedbacks from the other nodes, in order to create,
re-route or delete WSON circuits.
Notice that WSON nodes always exchange topology information, using the
GOSPF-TE, even if there are no active circuits in the network;
On the other side, the signaling protocol, the GRSVP-TE, is active only at the
moment when a circuit is created, re-routed or deleted and only between the
involved nodes.
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THE Path computation engine
PCE
CALCULATE
THE BEST PATHS
PERFORM
THE RWA
VALIDATE
THE PATHS
Probably, the path computation engine is the most critical component of a
WSON.
In general the functions of a PCE are:Calculating the best paths;
Performing the Routing and Wavelength assignment;
Validating the paths.
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THE Path computation engine
PCE
CALCULATE
THE BEST PATHS
PERFORM
THE RWA
VALIDATE
THE PATHS
In a WSON, the work of the PCE is complicated by the fact that calculating thebest path in the photonic world is not as easy as it would be in SDH world.
In the photonic world there are some additional physical constraints to be takeninto consideration, such as Polarization Mode Dispersion, non-linear effects, notperfect signal amplification and so on; these constraints have a more criticalimpact in WDM world compared with SDH world.
For example, in the photonic world, the fact that its possible to create a circuitbetween a node A and a node B and that its possible to create a circuit betweennode B and a node C, doesnt imply that its always possible to create a circuitbetween node A and node C.
Another critical point is that, in WDM world, given the fact that it is not possible tochange the frequency of a circuit without going in the electrical domain, itsnecessary to try to minimize the usage of the lambdas all around the network; the
problem to find out the best path in terms of administrative cost, minimizing, atthe same time, the number of lambdas used in the network is known in literatureas the Routing and Wavelength Assignment problem (RWA). The RWA has beendemonstrated to be an hard problem to solve; practically, a real PCE will solvethe problem using approximated solutions, based on heuristic, that aretechniques designed to solve a problem that ignores whether the solution can beproven to be correct, but which usually produces a good solution.
In conclusion, the work of the PCE is to resolve, for each circuit that itsnecessary to create o protect, the RWA problem, taking into account all thepossible physical impairments specific for the network: every path must becalculated and validated, checking if the path is physically feasible despite all the
physical impairments.
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THE Path computation engine
PCE
CALCULATE
THE BEST PATHS
PERFORM
THE RWA
VALIDATE
THE PATHS
Its not easy to realize an efficient engine that is able to perform all these tasks:
especially the validation step can be a long process in a big network.
This is the reason why its difficult to have PCE running inside the single WSONnode.
More commonly, WSON nodes interact with a PCE that works off-line, to have
the time to plan all the paths performing calculation, RWA and validation.
The circuits that pre-calculate and pre-validate the protection paths before a
failure happens are called pre-planned protected circuits.
In a first phase, all the WSON protections are pre-planned;
In a second phase, when the PCEs will run directly inside the WSON nodes
controllers, other kinds of protection will be available, like the so called on the fly
protections, in which the protection path is calculated and validated on the fly,real-time, only when a failure occurs.
Notice that, even in case in which the PCE works off-line, the routing protocol is
still required to be running to monitor the real time status of the network
resources; otherwise, the nodes would never know if a link that is part of a pre-
calculated protection path is still available or not.
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acquiring topology information
Control Plane configuration: costs assignment
COST 10 COST 20
COST 5 COST 15 COST 20
COST 30COST 10
When setting-up a WSON, its always necessary to configure some parameters
that will be used by the GMPLS processes running in each WSON node; this
phase is known as configuration of the control plane parameters.During the configuration of the control plane, one of the steps is to assign some
administrative constraint to each link on the network.
This operation can be done manually, or using some automatic tool that can help
to avoid mistakes.
Once configured, these parameters will be used by the GMPLS routing protocol,
the GOSPF-TE.
Now, lets see and example of how WSON nodes acquire information about the
topology of the network, that is the first step to calculate the best path.
One of a parameter to assign to the links is the administrative cost; it representsthe cost to send traffic out of the interface connected to that link.
This information will be used by the PCE, because it will try to minimize the total
cost of a path represented by the sum of the cost of the links it passes through.
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acquiring topology information
COST 10 COST 20
COST 5 COST 15 COST 20
COST 30COST 10
1 2 3
4 5 6
LINK 1 LINK 2
LINK 3
LINK 4
LINK 5
LINK 6
LINK 7
LSULSU
LSU
LSU
LSU
LSU
LSU
Im the node 1
Im connected to the node 2
through link 1,
the cost of which is 10;
Im connected to the node 4
through link 7,
the cost of which is 5
Information
packet
LSU
How does it work?
Every node in the WSON is responsible for sending detailed information about its directly
connected link to all the other nodes in the network.
Consider node 1 in the picture: when the GOSPF-TE process is enabled, after an initial
handshake to its adjacent neighbors, the node 1 builds a packet containing at least the following
information in order to define some common parameters: Im the node 1, Im directly connected
with node 2 through the link 1, that has the cost of 10 and Im connected to the node 4, through
the link 7, the cost of which is 5.
In the context of GOSPF-TE, this information packet is called LSU, Link State Update and the
piece of information describing each link is called Link State Advertisement or LSA.
Once this information packet is created, its sent out to all interfaces of node 1.
When the node 2 and the node 4 receive the packet coming form node 1, they store the
information inside a local topology database and send an updated copy of the packet to all their
own adjacent neighbors, but not the one from which the information packet has been received.
When the node 3 and the node 5 receive the packet coming from node 2 and node 4, respectively,
they store the information inside a local topology database and send an updated copy of the
packet to all their own adjacent neighbors, but not the one from which the information packet has
been received.
Finally, also the node 6 receives the information coming from the node 1 and stores this
information inside its local database.
The final result is that the piece of network described by node 1, regarding how link 1 and link 7
are interconnected to the other nodes, is known by all the nodes in the network.
In a similar way, also the other nodes in the network create an information packet regarding their
own directly attached links, and flood it to all the other nodes in the network.
At the end of this flooding process, every node will have exactly the same picture of the network.
On the base of this picture, the PCE will apply its algorithm to calculate the best paths.
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GOSPF-TE: TE information
Additional Traffic Engineering information:
In GMPLS context, with the term Traffic Engineering, we refer to the capability to
take routing decision based on some additional information that is not possible to
express only with a fixed cost given to a link.What we have seen till now, regards only the part of the GOSPF-TE that is not
related to the traffic engineering functionalities.
To support the traffic engineering functionalities, the LSU packet must transport
some additional information regarding the links, not only the administrative cost.
For example, we can think of assigning an higher cost to links that have a very
high percentage of used bandwidth, so we can imagine a dynamic cost
associated to the link that can vary with the bandwidth occupancy.
Every time that there is a variation on the bandwidth, for example, when a new
circuit is created or is released, new LSU are sent by the nodes that are adjacentto the links involved in the paths to all the other nodes.
This functionality is difficult to use when the PCE works off-line, because all the
path calculation is done in advance and so the dynamic cost cannot be taken in
account.
This functionality is important when the PCE run directly inside the nodes and
can calculate and validate the paths real time.
Notice that, even if the PCE works off-line the status of the network must be
monitored because the WSON nodes must always know which of the links are
available and which are not.
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GOSPF-TE: TE information
Additional Traffic Engineering information:
Total Reserve-able Bandwidth
Bandwidth Available
The total reserve-able bandwidth: for each link, how many channels can I reserve
for WSON paths? All? Only a part? Is this an 80 channels link o a 40 channels
link?The Bandwidth available: how many lambda are free in a moment to create new
paths?
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GOSPF-TE: TE information
Additional Traffic Engineering information:
Colors
Shared Risk Link Groups (SRLGs)
Two optional additional information can be transported by the LSU:
Colors
Shared Risk Link Groups (SRLGs)
A color is an attribute that can be assigned to a link, so to distinguish a group of
links from the others.
For example, a red color can be assigned to a group of military links: when we
create a circuit, it possible to request that the circuit pass only through the links
that have the red color as attribute configured during the control plane
configuration phase. If its not possible to satisfy the color constraint, the circuit in
not created at all. Among the links respecting the color constraint, the one with
the lowest cost path is selected.
Notice that the color constraint, not only is stronger than the administrative costconstraint, but it is also blocking: if there is no way to connect the two termination
point of a circuit, the circuit is not created at all and a message is sent to the
operator.
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GOSPF-TE: TE information
Additional Traffic Engineering information:
Colors
Shared Risk Link Groups (SRLGs)
The concept of Shared Risk Link Group in used by the PCE when calculating the
protection path for a circuit. It represents a group of links that, somehow, share
some risk. For example two links passing through the same physical conduit insome point share a risk, in the sense that, if someone damages the conduits,
probably the two links will be broken at the same time. During the configuration of
the control plane is possible to specify the SRLGs of which a link is part of: its an
additional topological information that cant be given only with the administrative
cost metric.
When are SRLGs used? An operator can specify that he would like to have the
worker and protection paths passing through completely disjoint SRLGS. If, for
example, the worker path pass through links that are part of SRLG 1, SRLG 2
and SRLG 3, the PCE will try to avoid the usage of links that are parte of SRLG1,
SRLG 2 or SRLG 3.As it was for the colors constraint, the SRLGs constraint is stronger than the
Administrative cost. But, unlike the case of the colors constraint, the SRLG
constraint is not blocking: even if it is not possible to respect the SRLGs diversity
between worker and protection paths, the circuit is protected, if a path is
available.
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GOSPF-TE: colors
Control Plane configuration: colors assignment
COST 10 COST 20
COST 5 COST 15 COST 20
COST 30COST 10
COLOR RED
COLOR RED
COLORS RED
AND GREEN
LINK 1 LINK 2
LINK 3
LINK 6
LINK 7LINK 5
LINK 4
COLOR GREEN
NODE 1
NODE 4
Failure on a link
NO PROTECTION PATH AVAILABLE: THE CIRCUIT IS NOT PROTECTED
The PCE, before calculating the best path, deletes from the picture of the network
the links not respecting the color red constraint: link 2, link, 3, link 6 and link 7 are
deleted from the topology.Now the PCE calculate the best path taking into account the administrative costs;
there is only one path available: the one passing through link 1, link 4 and link 5;
node 1 sends commands to the nodes involved in the path and the circuit is
created.
Notice that, once a color attribute has been assigned to the links, it can be used
or not; if the operator doesnt specify a color during a creation of the circuit, the
color attribute is not taken into consideration by the PCE.
Another point is that, in case the operator wanted this circuit to be protected by
another path passing only through red links, if there is a failure on one of the links
of the working side, its not possible to protect the circuit anymore, because thereis no other path that can join node 1 and node 4 passing only through red links.
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GOSPF-TE: srlgs
COST 5 COST 15
COST 30COST 10
COST 10/SRLG 1/LINK 1
SRLG 3
LINK 7
COST 10/SRLG 1/LINK 2
COST 20/SRLG 2/LINK 3
COST 20/SRLG 2/LINK 4
COST 20SRLG 4
LINK 5
SRLG 5
LINK 9
SRLG 6/ LINK 8
Control Plane configuration: SRLGs assignment
SRLG 7/ LINK 6
Lets now see an example for SRLGs.
As we said, it can be used to inform the PCE that two links pass through the
same physical conduit in a point of the network; this information can be useful,because we can suppose that, probably, if there is a damage on the conduit, both
the links will go down.
When is this information used?
An operator can request to the PCE to satisfy an SRLG diversity between worker
and protection path during the creation of a circuit.
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GOSPF-TE: srlgs
COST 5 COST 15
COST 30COST 10
COST 10/SRLG 1/LINK 1
SRLG 3
LINK 7
COST 10/SRLG 1/LINK 2
COST 20/SRLG 2/LINK 3
COST 20/SRLG 2/LINK 4
COST 20SRLG 4
LINK 5
SRLG 5
LINK 9
SRLG 6/ LINK 8
Control Plane configuration: SRLGs assignment
SRLG 7/ LINK 6
Failure on a linkTHE CIRCUIT IS PROTECTED TRYING
TO RESPECT THE SRLG DIVERSITY
Failure on a second link
THE CIRCUIT IS PROTECTED EVEN IF
IT IS NOT POSSIBLE TO RESPECT
THE SRLG DIVERSITY
Lets take a look at the picture;
Suppose the worker path of a circuit pass through links 1, 3 and 5, that are part of
SRLG 1, SRLG 2 and SRLG 4, respectively. If the operator specifies that hewants to have SRLG diversity between worker and protection, this means that the
PCE will try to avoid the usage, for the protection path, of links that are part of
SRLG 1, 2 and 4.
This is done by the PCE, assigning a very high cost to links not respecting the
diversity during the calculation of the best path.
Anyway, this constraint is not blocking, like it would have been with colors: if
there is a path available, even not respecting the diversity, the circuit is protected
in any case; in this situation, the cost of the protection path will be very high.
Lets see an example: If there is a failure on link 3, the traffic is switched on theprotection path passing through the SRLG 5, 6 and 7: the SRLG diversity
requirement is respected.
Now, suppose there is a second failure on the link 6. The traffic is switched on a
second protection path passing through SRLG 1, 2 and 4: the SRLG diversity
requirement cant be respected but, anyway, the traffic is protected.
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GRSVP-TE: signaling a path
OFF-LINE
PCEWSON
VALIDATED
LSPs
CIRCUIT REQUEST
NMSCIRCUIT REQUEST
WITH PATH SPECIFICATION
Once the PCE has calculated and validated a circuit, worker and protection
paths, the next step will be to setup the circuits, that, in a WSON, must always be
bidirectional.The validates paths are therefore sent by the PCE to the NMS, that passes these
information on to the ingress node for the specific circuit.
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GRSVP-TE: signaling a path
LINK 1 LINK 2
LINK 7 LINK 5 LINK 3
LINK 4LINK 6
NODE 1
NODE 4
NODE 2 NODE 3
NODE 5NODE 6
Control Channel Control Channel
ControlChannel
PCE
The management of WSON circuits is delegated to the GMPLS signaling
protocol: the GRSVP-TE.
GRSVP-TE is used to setup and tear down circuits.A circuit is the entity that is formed by both worker and protection paths.
In GMPLS terminology, a path is called Label Switched Path or LSP, because,
during the setup, the nodes exchange messages about resource reservations
identified by labels.
A circuit is formed by one or more LSPs.
But what is a label? A GMPLS label represent a traffic resource on a link: there is
one label for each lambda that is possible to reserve in a link; for example, if ,
after the Routing and Wavelength Assignment phase, the PCE has decided to
reserve the first channel of the eighty available for a circuit, it indicates thischannel with a label, that represents the channel 1.
The information about the chosen label is transported inside GRSVP-TE
messages.
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GRSVP-TE: signaling a path
LINK 1 LINK 2
LINK 7 LINK 5 LINK 3
LINK 4LINK 6
NODE 1
NODE 4
NODE 2 NODE 3
NODE 5NODE 6
Control Channel Control Channel
ControlChannel
PCE
Now, lets consider an example: an operator wants to setup a protected circuit
between node 1 and node 4; he inserts his request inside the NMS.
At this point two things can happens:1. if the PCE process runs inside the nodes, the NMS sends its request of circuit
creation to the node 1, that represents the ingress node for the circuit; the
PCE calculates the best path for both worker and protection side, performs
Routing and Wavelength Assignment and validates the traffic; the output of
the PCE is a list of nodes, links and lambdas for both worker and protection
side, that will be used by the GRSP-TE to signal the circuit.
2. if the PCE runs off-line, not inside the nodes, the PCE calculates the best path
for both worker and protection side, performs Routing and Wavelength
Assignment and validates the traffic. The output of the PCE is sent to the
ingress node, and the GRSVP-TE process will use this information to signalthe circuit.
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GRSVP-TE: signaling a path
LINK 1 LINK 2
LINK 7 LINK 5 LINK 3
LINK 4LINK 6
NODE 1
NODE 4
NODE 2 NODE 3
NODE 5NODE 6
Control Channel Control Channel
ControlChannel
PCE
PATH PATH
PATH
RSV
RSVRSV
The circuit is created: the status of the circuit is transmitted to the NMS
Please, notice that, in both cases, the signaling phase is performed directly by
the ingress node, not by the NMS: in the first case, we can think about a
centralized routing and a distributed signaling behavior (CD); in the secondcase, we can think about a distributed routing and a distributed signaling
behavior (DD).
Now, suppose that the output of the PCE were: node 1 - link1 - node 2 - link 2 -
node 3 - link 3 - node 4 lambda 1. The ingress node prepares a GRSVP-TE
message, that is called Path message, and sends it to next node in the list; in
the example, node 1 sends a path message to node 2. The Node 2 checks
the lambda 1 on the link 2: if its still available, it forwards the request to the
next node in the list, that is, the node 3; otherwise, it sends a path error
message to the node 1 to inform that is not possible to satisfy the request. If
all is ok, the node 3 performs the same operations as the node 2 and forwardthe path message to the node 4.
The node 4 recognizes that its the egress node of the LSP, and, if all its ok,
builds a new message, the reservation message (RSV message) to send
backward to the ingress node, the node 1.
When the reservation message reaches the node 1, the LSP is created and the
information is sent to the NMS to align it about the status of the circuit.
Optionally, the ingress node can send a confirm message to the egress node to
inform it that all the LSP creation operation has been completed successfully.
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Other GMPLS related concepts
Other GMPLS concepts:
Link Component (LC)
Link Cluster (LK)
Control Channel (CC)
Related to the GMPLS technology, there are some other important concepts:
the concepts of link component or LC;
the concepts of link cluster or LK;
the concept of control channel or CC.
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Link Component
Link Component (LC)
What is a Link Component?
In GMPLS terminology a link component represents, in the control plane, a
physical link.A GMPLS Control Plane can use a physical bi-directional link if and only if it has
been declared as a link component; otherwise, for the WSON control plane, that
link doesnt exist.
In the picture, two WSON nodes are interconnected by a couple of fiber; from a
WSON point of view, they are interconnected by a single link component.
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Link Cluster
LC 1
LC 2
Link Cluster (LK)
What is a Link Cluster?
ITU-T and IETF were worried about scalability problems that could have risen
due to the amount of links that a generic transport network could have.If there are many links in a WSON, more GOSPF-TE LSU messages are
required;
More LSU messages mean more load on the node controllers and more time for
PCEs to find out and validate best paths.
The idea has been to reduce the number of advertised links, by grouping a
number of physical links together, that is, a number of link components, in a
logical entity called link cluster; a link cluster is a cluster of link components.
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Link Cluster
LC 1
LC 2
Link Cluster (LK)
Two link components can be inserted in the same link cluster only if they share
some common characteristics: same capacity, same set of SRLGs, colors,
administrative cost; same nodes as termination points; same kind of link levelprotection.
Its always necessary to create the logical entity link cluster, even if only one link
component is defined, because, the GOSPF-TE exchange information about link
clusters and not about link component.
During the control plane configuration phase, the normal procedure is:
Step 1: Define an empty link cluster, assigning to it all the GOSPF-TE
parameters, like administrative cost, SRLGs, colors, etc.
Step 2. Define one or more link components;
Step 3: Associate one or more link component to the already created link cluster.When all the link clusters are defined, even the one containing only one link
component, the GOSPF-TE process will only take them into consideration.
This means that, in general, the nodes all around the network are aware only of
the link clusters that are present in the network; The details about what is inside a
single link cluster are known only by the nodes that are the termination point of
that particular link cluster, through a specific GMPLS protocol called Link
Management Protocol or LMP.
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the lmp protocol
Control Channel (CC) 2
Control Channel (CC) 1LMP Messages
LMP Messages
The Link Management Protocol of LMP has two major functionalities:
Setup and maintenance of the control channels defined between two nodes;
Setup and maintenance of the link cluster defined between two nodes.
Normally more than one control channel is defined between two nodes for
redundancy and load sharing; the LMP is responsible for managing these control
channels, for example, activating one secondary control channel, when the
primary is down.
The other important task of the LMP is related to link clusters: the correct
associations between link components and link clusters is maintained and
checked by the LMP. For example, if we try to associate two link components that
dont have common characteristics in a common link cluster, the LMP running
between the two nodes will forbid the operation.The LMP protocol is the glue that keeps control channel, link component and link
cluster together.
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Protections
Lets now analyze the new advanced WSON protections in details.
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WSON Protections
There are two different kinds of WSON protections:
The Transponder Sharing Protection or Green Protection
The Safe Optical Sub-Network Connection Protection (Safe OSNCP)
There are two different kinds of WSON protections:
The Transponder Sharing Protection or Green Protection;
The Safe Optical Sub-Network Connection Protection (Safe OSNCP).
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transponder sharing
In case of Transponder Sharing Protection, the PCE calculates and validates
two or more LSPs:
One worker LSP, that is immediately signaled and activated by the ingress node;
One or more protection LSPs, that the ingress node do not signal and do not activate
immediately; they are maintained inside the database of the ingress node, associated to the
specific transponder sharing protected circuit.
The first kind of WSON protection is the transponder sharing protection: one
worker LSP is activated and one or more protection LSPs are pre-calculated and
pre-validated but not activated until a failure occurs.Only one transponder is necessary for this kind of protection;
This type of protection in similar to an On the Fly protection, in which, as soon as
a failure occurs, a new protection path is calculated, validated, signaled and
activated: the only difference is that, for the transponder sharing protection, the
paths calculation and validation is done in advance by the PCE off-line; only the
signaling and activation phase are done on the fly, when a failure occurs.
This is a flexible protection, because the protection bandwidth can be shared
among the protection paths of other WSON circuits.
The only disadvantage is the time for switching, not only because its necessaryto signal the new protection path, and this requires an amount of time that
depends on the number of nodes, but mainly because adding a new circuit in a
WDM network requires an amount of time for equalization and PMD
compensation. PMD compensation can be critical especially for high bit rates, like
40 Gigabit per seconds or 100 Gigabit per seconds.
Please notice that, in general, planning a WSON only with transponder sharing
protections, allows to save a lot of resources in terms of power consumption: this
is the reason why this type of protection is also called Green Protection.
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transponder sharing
NODE 1 NODE 2 NODE 3
NODE 4NODE 5NODE 6
Link 1 Link 2
Link 3
Link 4
Link 5 Link6
Link 7
Link 8
Link9 Worker
LSP
Second
Protection
LSP
First
Protection
LSP
Third
Protection
LSP
Lets consider the transponder sharing example in the picture:
Suppose an operator would like to have a circuit from node 1 to node 4 that is
protected with a transponder sharing protection: one decision that he has to takeis how many protection path he wants the PCE to calculate and validate; lets, for
this case, suppose that the operator decided to have one worker LSP and three
protections LSPs.
Suppose also that we are in the case where the PCE runs off-line.
The operator passes his request to the PCE and the output of the PCE is a list of
four LSPs, one worker and three protections, all validated.
Now suppose that the PCE has decided the following paths:
For the worker: node 1- link 1- node 2- link 2- node 3- link 3- node 4 and the
lambda chosen is the green one;For the first protection: node 1- link 9- node 5- link 4- node 4 and the lambda
chosen is the red one;
For the second protection: node 1- link 8- node 6- link 7- node 5- link 4- node 4
and the lambda chosen is the yellow one;
For the third protection: node 1- link 1- node 2- link 5- node 5 link 4 node 4
and the lambda chosen is the purple one.
Please notice that the protection bandwidth is not reserved until a failure happen;
this means that the protection bandwidth can be shared with other protection
paths of other circuits.
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transponder sharing
NODE 1 NODE 2 NODE 3
NODE 4NODE 5NODE 6
Link 1 Link 2
Link 3
Link 4
Link 5 Link
6
Link 7
Link 8
Link9
Now, suppose a failure happens on the link 2; the ingress node, the node 1 in this
case, checks if all the links on the path for the first protection LSP are ok; if all are
ok, the LSP is signaled and activated; if one of the links is not ok, the ingressnode checks the second LSP and, if all the links are ok, this LSP is signaled and
activated; over wise, the ingress node goes on checking all the possible
protection LSPs until one of these has all the links that are ok. In this case, the
first protection LSP is good because all its links are ok.
If, after a while, before repairing the link 2, a second failure occurs, on link 9, for
example, the third LSP is signaled and activated.
If there is an other failure, on link 8, for example, the ingress node is able to react
because it knows also a third protection LSP and the traffic can be protected.
Of course, in case of a fourth failure, the traffic is lost.
This type of protection is called transponder sharing because to implement this
protection only one transponder is necessary; if we would like to have the
capability not only to reroute the circuit, but also to change the lambda, it will be
necessary to use a kind of transponder that has the capability to tune its
frequency. Its common to refer to these transponders as tunable transponders.
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transponder sharing
Line 1
Line 2
Line 9
Multi Direction Directionless Colorless
Line 1 Trp sharing
Protection
Now, lets see an example of a possible internal structure of a WSON node
(ROADM).
The configuration in the picture shows one card for each line direction; this cardis reconfigurable via software and is able to block or to allow to pass a particular
lambda; we refer to this type of card as Wavelength Selective Switch, or WSS,
card. The WSS card must be supported by a real-time equalizer, that can also be
embedded in the WSS card, depending on the particular implementation.
In a particular configuration that is called, directionless configuration, a WSS card
can be used to re-route a traffic coming from transponder.
In the example, we can see a circuit, with a transponder sharing protected circuit
using a tunable transponder, that is able to change the frequency to the WDM
link via software configuration, guided by the node controller.
In the starting situation the circuit uses the orange lambda and passes through
the line 1.
Now, lets concentrate on what happens in case of a fault.
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transponder sharing
Line 1
Line 2
Line 9
Multi Direction Directionless Colorless
Line 1 Trp sharing
Protection
A failure occurs on the line 1: the controller immediately activates its GRSVP-TE
process to signal and to activate the pre-calculated protection LSP passing
through the line 9.Please notice that, in this example, we changed the lambda, because the orange
lambda on the line 9 has been considered as already used by an other circuit; the
change of lambda is obtained using a tunable transponder, in a configuration
called color-less, that means, with no fixed lambda.
If, at this point, a second failure occurs on the line 9, a second pre-calculated
LSP is signaled and activated.
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Safe OSNCP
In case of the Safe OSNCP Protection, the PCE calculates and validates three
LSPs:
One worker LSP and one protection LSP, that are both immediately signaled and activated
by the ingress node;
One more protection LSP, that the ingress node doesnt signal and doesnt activate
immediately; its maintained inside the database of the ingress node, associated to the
specific Safe OSNCP protected circuit.
The second kind of WSON protection is the Safe OSNCP protection: one worker
LSP and one protection LSP are activated and one protection LSP is pre-
calculated and pre-validated but not activated until a failure occurs.Two transponders are necessary for this kind of protection;
The traffic is sent in both the worker and protection side at the same time, as it
would happen for the traditional OSNCP protection.
In case there is a failure on the worker side, the traffic is switched on the
protection side in less than 50 milliseconds and the second protection LSP is
immediately signaled and activated.
Now, in case there is a second failure, this time on the first, original, protection
side the traffic is switched on the second protection LSP in less than 50
milliseconds.
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SAFE OSNCP
NODE 1 NODE 2 NODE 3
NODE 4NODE 5NODE 6
Link 1 Link 2
Link 3
Link 4
Link 5 Link
6
Link 7
Link 8
Link9 Worker
LSP
First
Protection
LSP
Second
Protection
LSP
Lets consider the example in the picture:
Suppose an operator would like to have a circuit from node 1 to node 4 that is
protected with a Safe OSNCP protection.Also in this case, suppose that we are in the case where the PCE runs off-line.
The operator passes his request to the PCE and the output of the PCE is a list of
three LSPs, one worker and two protections, all validated.
Now suppose that the PCE has decided the following paths:
For the worker LSP: node 1- link 1- node 2- link 2- node 3- link 3- node 4 and the
lambda chosen is the green one;
For the first protection LSP: node 1- link 8- node 6- link 7- node 5- link 4- node 4
and the lambda chosen is the orange one;
For the second protection LSP: node 1- link 9- node 5- link 4- node 4 and thelambda chosen is the red one.
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Safe OSNCP
First
Protection
Side
Worker
Side
The traffic switch on this side in less than 50 ms and the pre-calculated secondary protection LSP is signaled and activated
NODE 1 NODE 2 NODE 3
NODE 4NODE 5NODE 6
Link 1 Link 2
Link 3
Link 4
Link 5 Link
6
Link 7
Link 8
Link9The traffic is switched on this side in less than 50 ms
The traffic is LOST
The output of the PCE is passed through the NMS to the ingress node of the
circuit, in this example, the node 1, but only the worker LSP (green) and the first
protection LSP (orange) are signaled and activated; the other protection LSP ismaintained in the database of the node associated to this particular circuit.
Now suppose that a failure happens on the link 2: in less than 50 milliseconds the
traffic is switched on the first protection side (orange); in the meanwhile, the
secondary protection LSP (red) is signaled and activated.
If, after a while, and before repairing the link 2, a second failure happens on link
7: the ingress node reacts switching the traffic on the second protection side in
less than 50 milliseconds.
If a third failure occurs, for example, on the link 9, the traffic is lost until one of the
LSPs is repaired.
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Safe OSNCP
Safe-1+1OSNCP/OUPSR
Multi Direction Directionless ColorlessProtection
Line 1Line 1
Line 2
Line 9
Lets see an example of what happens inside a WSON node in case of a SAFE
OSNCP protection.
In the picture, its possible to see one circuit formed by two LSPs: the orange linerepresents the worker LSP and the green line represents the protection LSP; two
tunable transponders are necessary for the SAFE OSNCP protection.
Suppose that a failure occurs on the line 1: The traffic is switched on the
protection side in less than 50 milliseconds, as it would happen for the normal
OSNCP protection. The original worker path is deleted; a new pre-calculated
protection LSP 2 is signaled and activated passing through the line 2.
Now suppose that an other failure happens on line 9. The traffic is switched on
the LSP 2 in less than 50 ms.
In case a third failure occurs, the circuit is not protected anymore, because onlytwo protection LSPs are pre-calculated by PCE for that specific circuit. In
conclusion, the Safe OSNCP protection is very fast compared with the
Transponder sharing protection; we pay this speed with more bandwidth to
reserve for a specific circuit, because the protection bandwidth cannot be shared
with other protection LSPs of other circuits and with an additional transponder
that must be present.
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Thank you for taking the time to listen to this ASON-WSON Fundamentals
course.
For more technical information regarding WDM and OTN, please view therelevant Fundamentals courses.