Wavelength selection based on wavelength availability in multi -fiber WDM networks

1. WAVELENGTH SELECTION BASED ON
WAVELENGTH AVAILABILITY IN MULTI-FIBER
WDM NETWORKS
Presented by
Hrudya.B.Kurup

2. TOPICS COVERED



WDM and AONs
Data Transfer Mechanism in WDM AONs
Wavelength Selection based on Wavelength Availability in MultiFiber WDM Networks

3. WDM and AONs

4. 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


7. 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.

8. 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


9. 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

11. 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.

12. 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.

13. Data Transfer Mechanism in WDM
AONs

14. 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

15. 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)

16. RWA PROBLEMS

There are 2 problems in the RWA

Wavelength continuity constraint

Distinct wavelength constraint

17. 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.


18. DISTINCT WAVELENGTH CONSTRAIN

If all light paths using the same link (fiber), then
the light path should be allocated to different wavelengths.

19. 
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 .

20. 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.

21. 
Dynamic RWA problem

routing sub problem

wavelength assignment sub problem

22. ROUTING ASSIGNMENT

Fixed Routing


a single fixed route is predetermined for each sourcedestination 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.


23. 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

24. 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

25. THE NETWORK CONTROL (OR SIGNALLING)

Required for managing a light path

Can be
Centralised
 Distributed


26. 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


27. 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.

28. 
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.

29. WAVELENGTH SELECTION BASED ON
WAVELENGTH AVAILABILITY IN MULTI-FIBER
WDM NETWORKS

30. 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

31. Assumptions
 The route between source and destination is previously
known.
 We consider the class of optical networks without
wavelength conversion facility .

32. 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


33. 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.

34. 
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


35. 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

36. 
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.

37. 
In general, the forward reservation has high blocking
probability because the sender nodes cannot get the
wavelength information along routes.

Temporary locking of wavelength.

39. 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.

40. 
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.

41. 
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.

44. AN EXAMPLE OF THE BACKWARD RESERVATION IN
MULTI FIBER WDM NETWORKS

45. 
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.

46. 
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.

47. 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.

48. 
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

49. 
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.

51. 
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.