WAVELENGTH SELECTION BASED ON WAVELENGTH AVAILABILITY IN MULTI-FIBER WDM NETWORKS
Presented by
Hrudya.B.Kurup
TOPICS COVERED
WDM and AONs Data Transfer Mechanism in WDM AONs Wavelength Selection based on
Wavelength Availability in Multi-Fiber WDM Networks
WDM and AONs
SINGLE AND MULTI FIBER WDM NETWORKS
Single fiber WDM Networks Each link consists of a single fiber. Two or more light paths with the same
wavelength cannot be established in the same link
Multi fiber WDM Networks Each link consists of multiple fibers. The same number of light paths as fibers can be
established with the same wavelength on each link
WAVELENGTH DIVISION MULTIPLEXING
Wavelength division multiplexing (WDM) multiple wavelengths to transmit different data
streams. optical spectrum, is used more efficiently
Enormous bandwidth is available on fiber WDM can provide an optical transmission
system with an extremely large data rate.
BENEFITS OF WDM Increase Capacity
Same fiber employed for multiple data streams.
Transparency Supports multiple protocols . Supports different bit rates.
Wavelength Reuse Same wavelength can be used at different fiber
links
Reliability very reliable and safe. very low crosstalk
WAVELENGTH ROUTED NETWORK
A wavelength routing network consist of
optical cross connects (OXCs) - serves for switching and routing
Data can be sent through light paths from source to destination.
Each light path is assigned a dedicated wavelength
OPTICAL CROSS-CONNECT (OXC)
Optical switch
Can connect optical signal on input ports to output ports
OXC can Switch to same wavelength
Switch to the different wavelength In such cases OCX should be equipped with
wavelength converters.
ALL-OPTICAL NETWORKS (AONS)
Special kind of optical networks
Path between communicating nodes remains entirely optical.
These paths are called light paths, which use the same wavelength on all the links along a path.
Data Transfer Mechanism in WDM AONs
DATA TRANSFER
IN WDM AONs, to send data from source node to a destination node, three phases are to be considered Light path establishment and set-up (routing
and wavelength assignment) Data transfer Light path takedown (wavelength release)
This process requires the exchange of control messages.
So the phases 1 and 3 requires a control protocol
ROUTING AND WAVELENGTH ASSIGNMENT (RWA)
For successful data transmission
A route has to be found
An appropriate wavelength has to be assigned between transmitter and receiver
This is called the routing and wavelength assignment (RWA)
RWA PROBLEMS
There are 2 problems in the RWA
Wavelength continuity constraint
Distinct wavelength constraint
WAVELENGTH CONTINUITY CONSTRAIN When the routing nodes are not capable for
wavelength conversion Then the light path must use the same
wavelength in all the optical segments it uses. In the absence of a free wavelength along the
entire route, the connection cannot be established and it is blocked
When wavelength conversion is present The only limiting factor is the bandwidth of
every link. In such network, a connection is blocked only
when no wavelength is available at some segment of an optical path.
DISTINCT WAVELENGTH CONSTRAIN
If all light paths using the same link (fiber), then
the light path should be allocated to different wavelengths.
RWA problem can be classified into two traffic assumptions:
Static RWA problem Static Light path Establishment (SLE) traffic requirements are known in advance
and
Dynamic RWA Dynamic Light path Establishment (DLE) The order of light path requests arrive
randomly .
DYNAMIC LIGHT PATH ESTABLISHMENT (DLE)
Objective is to choose a route and a wavelength which
maximizes the probability of setting up a given connection, while at the same time attempting to minimize the blocking for future connections.
Dynamic RWA problem
routing sub problem
wavelength assignment sub problem
ROUTING ASSIGNMENT Fixed Routing
a single fixed route is predetermined for each source-destination pair.
Adaptive Routing Alternate-Path Routing.
Relies on a set of predetermined fixed routes between a source node and a destination node
When a connection request arrives, a single route is chosen from the set of predetermined routes, and a light path is established on this route.
The criteria for route selection are typically based on either path length or path congestion.
WAVELENGTH ASSIGNMENT
A light path is required to be established before data is transferred between two communicating nodes.
No two light paths can share a common link using the same wavelength, known as wavelength continuity constraint
Blocking probability increases
WAVELENGTH ASSIGNMENT CONT..
A possible alternative to reduce blocking probability is the use of opto-electronic wavelength converters
But these converters add substantially to the cost of the network.
So we need some form of network control or signalling mechanism if we do not use wave length converters
THE NETWORK CONTROL (OR SIGNALLING)
Required for managing a light path
Can be Centralised Distributed
CENTRALISED CONTROL
A single control centre maintains the complete network topology including wavelength usage on each link.
Not feasible and reliable in large networks because
A change in network topology and/or wavelength usage should be informed Immediately.
if the control centre crashes, all network information is lost
DISTRIBUTED CONTROL
Every node acts as a controller and maintains its own local database.
In the event of a node crash, other nodes work as usual in the network.
If there is a change in the network topology or wavelength usage, the concerned database is updated immediately.
But, in this kind of control, a connection request may be unnecessarily blocked due to the wavelength-continuity constraint.
So an efficient distributed wavelength reservation protocol is needed for dynamic WDM networks with rapidly changing wavelength availability.
WAVELENGTH SELECTION BASED ONWAVELENGTH AVAILABILITY IN MULTI-FIBER WDM NETWORKS
Objective To establish wavelength-continuous light paths
dynamically and efficiently so as to minimize the overall blocking probability at the cost of a nominal increase in control overhead
Assumptions The route between source and destination is
previously known. We consider the class of optical networks
without wavelength conversion facility .
WAVELENGTH RESERVATION PROTOCOL
Before transmission data in optical networks, a light path have to establish by reserving a wavelength in all links along a route between a sender and a receiver.
There are two types of wavelength reservation protocols which are forward reservation backward reservation
CONTROL MECHANISMS
In order to support distributed wavelength reservation protocols wdm networks are equipped with a shadow network in addition to the optical data network
The shadow network Used to exchange control information. Has same physical topology as data network. Operates in packet switching mode . Traffic on shadow network consist of small
control packets. Lighter traffic compared to data network.
Routers and intermediate nodes examine these control packets and updates accordingly.
Can be implemented as electronic network a virtual channel on data network can be
reserved exclusively for exchanging control information
FORWARD RESERVATION
Source initiated
When a transmission request arrives,
The source node sends a reservation (RESV ) packet to the destination node along the decided route
Each node along the path processes the RESV packet and temporarily locks one or more appropriate wavelengths on the next link for connection
If no suitable wavelength is found on the next link the intermediate node sends a failure (FAIL) message back to source node.
Fail packet unlocks all the wavelengths reserved so far.
Otherwise at the destination one of the available
wavelengths is picked up and as acknowledgement packet is send back from destination to source.
On its way back to source this ACK packet permanently locks the selected wavelengths and unlocks the other wavelengths at the intermediate nodes.
In general, the forward reservation has high blocking probability because the sender nodes cannot get the wavelength information along routes.
Temporary locking of wavelength.
BACKWARD RESERVATION
Destination initiated when a transmission request arrives,
The sender node sends a PROB packet
PROB packet collects information on available wavelengths in each link along a route. It will not lock any wavelength
When the PROB message reaches the receiver node, the receiver node selects a wavelength from a set of available wavelengths along the entire route based on certain criteria.
The RESV packet locks the wavelength along the reverse path towards the source node
If the wavelength is not found available at some intermediate node the node generate a FAIL packet to the destination and NAK packet to source
The FAIL packet releases the wavelength locked so far .
NAK packet informs the source about connection failure.
The backward reservation can reduce blocking probability more efficiently than the forward reservation because wavelength usage in all links along a route is
known before selection.
Furthermore, duration of reservation in the backward reservation is smaller than that in the forward reservation.
AN EXAMPLE OF THE BACKWARD RESERVATION IN MULTI FIBER WDM NETWORKS
Each link consists of 3 fibers.
Firstly, the source node sends a PROB message.
In this example, wavelengths {ω1, ω2, ω3 and ω4} are available on fiber1 between the source node and the intermediate node.
Similarly, wavelengths {ω1, ω2} and {ω2, ω4} are available on fiber 2 and fiber 3, respectively.
Therfore wavelengths available on fiber 1, fiber 2, and fiber 3 between the intermediate node and the destination node are {ω1, ω3}, { ω3, ω4}, and { ω1}, respectively.
The PROB message collects information on wavelength availability.
After receiving the PROB message, the destination node knows that ω1, ω3 are available along the entire route.
Thus, the destination node selects a wavelength from {ω1, ω3}.
Then, the destination node sends RESV message to the source node in order to reserve the selected wavelength.
WAVELENGTH SELECTION SCHEME The receiver selects a wavelength based on
wavelength availability in fibers of each link along a route between a sender node and a receiver node, which is collected by a PROB message.
Specifically, the proposed scheme selects the least used wavelength along the route.
By doing so, wavelength usage in each link is smoothed and thus the generation of bottleneck links is suppressed.
As a result, blocking probability of lightpath establishments is expected to be reduced.
To do so, we define a cost C ω of wavelength ω along route p as follows:
where x l,f,w = 0; if wavelength ω is available in
fiber f of link l along route p between a sender node and a receiver node
x l,f,w = 1 otherwise
In the proposed scheme, when a receiver node receives a PROB message, it selects a wavelength ω with the smallest cost Cω.
Then the receiver node sends a RESV message to reserve the selected wavelength.
The cost of ω1 is 2 because ω 1 is not available on fiber 3 between the sender node and the intermediate node and on fiber 2 between the intermediate node and the receiver node.
Similarly, the costs of ω 2, ω 3 and ω 4 are 3, 3 and
3, respectively. ω 2 is not available along the entire route because
ω 2 is not available on all fibers in the link between the intermediate node and the receiver node.
Therefore, the receiver node selects a wavelength from ω 1, ω 3 .
In this case, ω 1 is selected because it has the smallest cost.
