WDM-Networks WDM-Networks Notes | EduRev

: WDM-Networks WDM-Networks Notes | EduRev

 Page 1


Module 20 : WDM Networks
Lecture : WDM Networks
 
   Objectives
   In this lecture you will learn the following
Wavelength Routing
Wavelength Management in a Router
 
 Wavelength Routing:
 
Distribution of data received at a central receiving station to their final destination is done
by switches and routers. Though the terms switching and routing are used interchangeably,
there are some differences about the way they work and the level of their operation. A
switch is a dedicated service. For instance, an electro-mechanical switch connected to
household electrical equipment provides a dedicated connectivity between the switch and
the equipment. More advanced electronic switching (e.g. that used in a phone service) does
not provide a dedicated physical connectivity but provides a dedicated time slot. Such
circuits are called virtual circuits. 
In packet switching , a message is divided into small segments called packets . Each
packet is transmitted individually and may follow different paths to the destination. After all
packets have arrived at the destination, they are re-assembled to form the original
message. A packet has a header which identifies destination. Routers read the information
on the header and decide on the best route for a packet based on the network condition. 
Wavelength routing is a process in which arriving optical signals are directed to different
output ports depending on the wavelength of the input. A network which uses this
procedure is called a wavelength routed network . A wavelength router may be looked upon
as a fixed wavelength demultiplexer which directs different wavelengths to different ports. A
wavelength which is used for one destination is not available for another destination. Thus
the device must also have wavelength converters in addition to optical cross connects. By
changing the wavelength of an arriving input signal, one can change the destination port of
the signal. 
A wavelength routing network has edge nodes which provide interface between the optical
layer and other systems such as IP routers, ATM switches etc. All switching inside the
optical layer is done by OXCs which may work in conjunction with wavelength converters. 
Wavelength routing requires that a route must be defined in the network which connects a
source (input) with the destination. Such an optical path is called a light path (also called a
clear path. as the signal does not undergo any conversion from optical to electrical domain
in its passage from the source to destination.)
Page 2


Module 20 : WDM Networks
Lecture : WDM Networks
 
   Objectives
   In this lecture you will learn the following
Wavelength Routing
Wavelength Management in a Router
 
 Wavelength Routing:
 
Distribution of data received at a central receiving station to their final destination is done
by switches and routers. Though the terms switching and routing are used interchangeably,
there are some differences about the way they work and the level of their operation. A
switch is a dedicated service. For instance, an electro-mechanical switch connected to
household electrical equipment provides a dedicated connectivity between the switch and
the equipment. More advanced electronic switching (e.g. that used in a phone service) does
not provide a dedicated physical connectivity but provides a dedicated time slot. Such
circuits are called virtual circuits. 
In packet switching , a message is divided into small segments called packets . Each
packet is transmitted individually and may follow different paths to the destination. After all
packets have arrived at the destination, they are re-assembled to form the original
message. A packet has a header which identifies destination. Routers read the information
on the header and decide on the best route for a packet based on the network condition. 
Wavelength routing is a process in which arriving optical signals are directed to different
output ports depending on the wavelength of the input. A network which uses this
procedure is called a wavelength routed network . A wavelength router may be looked upon
as a fixed wavelength demultiplexer which directs different wavelengths to different ports. A
wavelength which is used for one destination is not available for another destination. Thus
the device must also have wavelength converters in addition to optical cross connects. By
changing the wavelength of an arriving input signal, one can change the destination port of
the signal. 
A wavelength routing network has edge nodes which provide interface between the optical
layer and other systems such as IP routers, ATM switches etc. All switching inside the
optical layer is done by OXCs which may work in conjunction with wavelength converters. 
Wavelength routing requires that a route must be defined in the network which connects a
source (input) with the destination. Such an optical path is called a light path (also called a
clear path. as the signal does not undergo any conversion from optical to electrical domain
in its passage from the source to destination.)
In the figure shown, the edge nodes are marked E-1 to E-5 which are connected by fibers
to all optical portion of the network. Two optical paths, one from E-1 to E-3 and another
from E-2 to E-4 are shown in the figure. A wavelength which is being used for one of the
light paths is not available for another path. In the optical domain there is no conversion
from photon signal to electrical signal.
 
 
 
The implementation of wavelength routing is done as follows: 
1. An OXC takes at each of its input ports signals of different wavelengths. If the
signals can be directed to output ports, the OXC acts essentially as independent 
switches. The figure shows equivalent switches for a optical cross connect
with 2 waves per fiber.
 
 
1.
For a network not using wavelength converter, wavelength continuit condition requires that
along any light path, the same
wavelength must be used throughout. This is sometimes a restrictive condition because it
does not allow establishment of a path between nodes even when a wavelength channel is
available. Consider a case shown in the figure where two wavelengths and are
available. There is one light path between N-1 and N-2 and a second light path between N-
2 and N-3, the former using the wavelength and the latter .
Page 3


Module 20 : WDM Networks
Lecture : WDM Networks
 
   Objectives
   In this lecture you will learn the following
Wavelength Routing
Wavelength Management in a Router
 
 Wavelength Routing:
 
Distribution of data received at a central receiving station to their final destination is done
by switches and routers. Though the terms switching and routing are used interchangeably,
there are some differences about the way they work and the level of their operation. A
switch is a dedicated service. For instance, an electro-mechanical switch connected to
household electrical equipment provides a dedicated connectivity between the switch and
the equipment. More advanced electronic switching (e.g. that used in a phone service) does
not provide a dedicated physical connectivity but provides a dedicated time slot. Such
circuits are called virtual circuits. 
In packet switching , a message is divided into small segments called packets . Each
packet is transmitted individually and may follow different paths to the destination. After all
packets have arrived at the destination, they are re-assembled to form the original
message. A packet has a header which identifies destination. Routers read the information
on the header and decide on the best route for a packet based on the network condition. 
Wavelength routing is a process in which arriving optical signals are directed to different
output ports depending on the wavelength of the input. A network which uses this
procedure is called a wavelength routed network . A wavelength router may be looked upon
as a fixed wavelength demultiplexer which directs different wavelengths to different ports. A
wavelength which is used for one destination is not available for another destination. Thus
the device must also have wavelength converters in addition to optical cross connects. By
changing the wavelength of an arriving input signal, one can change the destination port of
the signal. 
A wavelength routing network has edge nodes which provide interface between the optical
layer and other systems such as IP routers, ATM switches etc. All switching inside the
optical layer is done by OXCs which may work in conjunction with wavelength converters. 
Wavelength routing requires that a route must be defined in the network which connects a
source (input) with the destination. Such an optical path is called a light path (also called a
clear path. as the signal does not undergo any conversion from optical to electrical domain
in its passage from the source to destination.)
In the figure shown, the edge nodes are marked E-1 to E-5 which are connected by fibers
to all optical portion of the network. Two optical paths, one from E-1 to E-3 and another
from E-2 to E-4 are shown in the figure. A wavelength which is being used for one of the
light paths is not available for another path. In the optical domain there is no conversion
from photon signal to electrical signal.
 
 
 
The implementation of wavelength routing is done as follows: 
1. An OXC takes at each of its input ports signals of different wavelengths. If the
signals can be directed to output ports, the OXC acts essentially as independent 
switches. The figure shows equivalent switches for a optical cross connect
with 2 waves per fiber.
 
 
1.
For a network not using wavelength converter, wavelength continuit condition requires that
along any light path, the same
wavelength must be used throughout. This is sometimes a restrictive condition because it
does not allow establishment of a path between nodes even when a wavelength channel is
available. Consider a case shown in the figure where two wavelengths and are
available. There is one light path between N-1 and N-2 and a second light path between N-
2 and N-3, the former using the wavelength and the latter .
 
  
 
As only two wavelength are available, it is not possible to establish a light path between N-1
and N-3 (without violating continuity condition) though there is a free wavelength between
each of the segments. However, if a wavelength converter is located at the node N-2, we
may relax the continuity condition, as shown in the right hand figure.
The degree of wavelength conversion used in a given network depends on the traffic
pattern in the network. The figure below illustrates different situations for a single input port
and a single output port.
 
 
  
A network may allow no conversion at all in which case all the wavelengths are routed as
such to the output port.
Fixed wavelength conversion: A signal entering a particular node with a wavelength 
always leaves the node with a
 
pre-determined wavelength .
Limited wavelength conversion: In this case, an input wavelength can be converted
to any of a limited number of
 
wavelengths. For instance, in the figure, can be converted to either or but not to 
. Similarly, can be converted to and but cannot leave the node without
Page 4


Module 20 : WDM Networks
Lecture : WDM Networks
 
   Objectives
   In this lecture you will learn the following
Wavelength Routing
Wavelength Management in a Router
 
 Wavelength Routing:
 
Distribution of data received at a central receiving station to their final destination is done
by switches and routers. Though the terms switching and routing are used interchangeably,
there are some differences about the way they work and the level of their operation. A
switch is a dedicated service. For instance, an electro-mechanical switch connected to
household electrical equipment provides a dedicated connectivity between the switch and
the equipment. More advanced electronic switching (e.g. that used in a phone service) does
not provide a dedicated physical connectivity but provides a dedicated time slot. Such
circuits are called virtual circuits. 
In packet switching , a message is divided into small segments called packets . Each
packet is transmitted individually and may follow different paths to the destination. After all
packets have arrived at the destination, they are re-assembled to form the original
message. A packet has a header which identifies destination. Routers read the information
on the header and decide on the best route for a packet based on the network condition. 
Wavelength routing is a process in which arriving optical signals are directed to different
output ports depending on the wavelength of the input. A network which uses this
procedure is called a wavelength routed network . A wavelength router may be looked upon
as a fixed wavelength demultiplexer which directs different wavelengths to different ports. A
wavelength which is used for one destination is not available for another destination. Thus
the device must also have wavelength converters in addition to optical cross connects. By
changing the wavelength of an arriving input signal, one can change the destination port of
the signal. 
A wavelength routing network has edge nodes which provide interface between the optical
layer and other systems such as IP routers, ATM switches etc. All switching inside the
optical layer is done by OXCs which may work in conjunction with wavelength converters. 
Wavelength routing requires that a route must be defined in the network which connects a
source (input) with the destination. Such an optical path is called a light path (also called a
clear path. as the signal does not undergo any conversion from optical to electrical domain
in its passage from the source to destination.)
In the figure shown, the edge nodes are marked E-1 to E-5 which are connected by fibers
to all optical portion of the network. Two optical paths, one from E-1 to E-3 and another
from E-2 to E-4 are shown in the figure. A wavelength which is being used for one of the
light paths is not available for another path. In the optical domain there is no conversion
from photon signal to electrical signal.
 
 
 
The implementation of wavelength routing is done as follows: 
1. An OXC takes at each of its input ports signals of different wavelengths. If the
signals can be directed to output ports, the OXC acts essentially as independent 
switches. The figure shows equivalent switches for a optical cross connect
with 2 waves per fiber.
 
 
1.
For a network not using wavelength converter, wavelength continuit condition requires that
along any light path, the same
wavelength must be used throughout. This is sometimes a restrictive condition because it
does not allow establishment of a path between nodes even when a wavelength channel is
available. Consider a case shown in the figure where two wavelengths and are
available. There is one light path between N-1 and N-2 and a second light path between N-
2 and N-3, the former using the wavelength and the latter .
 
  
 
As only two wavelength are available, it is not possible to establish a light path between N-1
and N-3 (without violating continuity condition) though there is a free wavelength between
each of the segments. However, if a wavelength converter is located at the node N-2, we
may relax the continuity condition, as shown in the right hand figure.
The degree of wavelength conversion used in a given network depends on the traffic
pattern in the network. The figure below illustrates different situations for a single input port
and a single output port.
 
 
  
A network may allow no conversion at all in which case all the wavelengths are routed as
such to the output port.
Fixed wavelength conversion: A signal entering a particular node with a wavelength 
always leaves the node with a
 
pre-determined wavelength .
Limited wavelength conversion: In this case, an input wavelength can be converted
to any of a limited number of
 
wavelengths. For instance, in the figure, can be converted to either or but not to 
. Similarly, can be converted to and but cannot leave the node without
conversion while can be converted to any of the three wavelengths.
  
Full conversion: This is the case where any of the input wavelengths can be converted
to any of the permitted
 wavelengths.
 
 Wavelength Management in a Router:
 
If the pattern of traffic in the network is known in advance and the variations take place
over a long period of time, one can optimize network resources and arrive at a virtual
network. Such a router is called a static router. The continuity condition is to be satisfied
and only one wavelength may be used per route. 
The problem of wavelength management incorporating continuity condition is same as
working out a Latin square . A Latin square is an matrix in which distinct
objects are placed such that no two elements of a given row (or of a column) have identical
objects placed in them. (Students familiar with the game of Sudoku will notice the
resemblance.) Mathematically, a Latin square is a matrix defined as follows:
such that
 
 
The number of distinct Latin squares grows exponentially with increasing . For instance,
there are only 12 squares of order 3, 576 of order 4 but over squares of order 9.
Examples of two Latin square of order 4 are given below.
 
We may think of the matrix as one whose columns represent input ports and the rows the
output ports. Exactly one wavelength (as indicated by different colour in the table) from
each input port will be routed to each output port. The wavelength used by the router from
Page 5


Module 20 : WDM Networks
Lecture : WDM Networks
 
   Objectives
   In this lecture you will learn the following
Wavelength Routing
Wavelength Management in a Router
 
 Wavelength Routing:
 
Distribution of data received at a central receiving station to their final destination is done
by switches and routers. Though the terms switching and routing are used interchangeably,
there are some differences about the way they work and the level of their operation. A
switch is a dedicated service. For instance, an electro-mechanical switch connected to
household electrical equipment provides a dedicated connectivity between the switch and
the equipment. More advanced electronic switching (e.g. that used in a phone service) does
not provide a dedicated physical connectivity but provides a dedicated time slot. Such
circuits are called virtual circuits. 
In packet switching , a message is divided into small segments called packets . Each
packet is transmitted individually and may follow different paths to the destination. After all
packets have arrived at the destination, they are re-assembled to form the original
message. A packet has a header which identifies destination. Routers read the information
on the header and decide on the best route for a packet based on the network condition. 
Wavelength routing is a process in which arriving optical signals are directed to different
output ports depending on the wavelength of the input. A network which uses this
procedure is called a wavelength routed network . A wavelength router may be looked upon
as a fixed wavelength demultiplexer which directs different wavelengths to different ports. A
wavelength which is used for one destination is not available for another destination. Thus
the device must also have wavelength converters in addition to optical cross connects. By
changing the wavelength of an arriving input signal, one can change the destination port of
the signal. 
A wavelength routing network has edge nodes which provide interface between the optical
layer and other systems such as IP routers, ATM switches etc. All switching inside the
optical layer is done by OXCs which may work in conjunction with wavelength converters. 
Wavelength routing requires that a route must be defined in the network which connects a
source (input) with the destination. Such an optical path is called a light path (also called a
clear path. as the signal does not undergo any conversion from optical to electrical domain
in its passage from the source to destination.)
In the figure shown, the edge nodes are marked E-1 to E-5 which are connected by fibers
to all optical portion of the network. Two optical paths, one from E-1 to E-3 and another
from E-2 to E-4 are shown in the figure. A wavelength which is being used for one of the
light paths is not available for another path. In the optical domain there is no conversion
from photon signal to electrical signal.
 
 
 
The implementation of wavelength routing is done as follows: 
1. An OXC takes at each of its input ports signals of different wavelengths. If the
signals can be directed to output ports, the OXC acts essentially as independent 
switches. The figure shows equivalent switches for a optical cross connect
with 2 waves per fiber.
 
 
1.
For a network not using wavelength converter, wavelength continuit condition requires that
along any light path, the same
wavelength must be used throughout. This is sometimes a restrictive condition because it
does not allow establishment of a path between nodes even when a wavelength channel is
available. Consider a case shown in the figure where two wavelengths and are
available. There is one light path between N-1 and N-2 and a second light path between N-
2 and N-3, the former using the wavelength and the latter .
 
  
 
As only two wavelength are available, it is not possible to establish a light path between N-1
and N-3 (without violating continuity condition) though there is a free wavelength between
each of the segments. However, if a wavelength converter is located at the node N-2, we
may relax the continuity condition, as shown in the right hand figure.
The degree of wavelength conversion used in a given network depends on the traffic
pattern in the network. The figure below illustrates different situations for a single input port
and a single output port.
 
 
  
A network may allow no conversion at all in which case all the wavelengths are routed as
such to the output port.
Fixed wavelength conversion: A signal entering a particular node with a wavelength 
always leaves the node with a
 
pre-determined wavelength .
Limited wavelength conversion: In this case, an input wavelength can be converted
to any of a limited number of
 
wavelengths. For instance, in the figure, can be converted to either or but not to 
. Similarly, can be converted to and but cannot leave the node without
conversion while can be converted to any of the three wavelengths.
  
Full conversion: This is the case where any of the input wavelengths can be converted
to any of the permitted
 wavelengths.
 
 Wavelength Management in a Router:
 
If the pattern of traffic in the network is known in advance and the variations take place
over a long period of time, one can optimize network resources and arrive at a virtual
network. Such a router is called a static router. The continuity condition is to be satisfied
and only one wavelength may be used per route. 
The problem of wavelength management incorporating continuity condition is same as
working out a Latin square . A Latin square is an matrix in which distinct
objects are placed such that no two elements of a given row (or of a column) have identical
objects placed in them. (Students familiar with the game of Sudoku will notice the
resemblance.) Mathematically, a Latin square is a matrix defined as follows:
such that
 
 
The number of distinct Latin squares grows exponentially with increasing . For instance,
there are only 12 squares of order 3, 576 of order 4 but over squares of order 9.
Examples of two Latin square of order 4 are given below.
 
We may think of the matrix as one whose columns represent input ports and the rows the
output ports. Exactly one wavelength (as indicated by different colour in the table) from
each input port will be routed to each output port. The wavelength used by the router from
 
port to port is given by the colour scheme of the element of the table. Note that if
two signals of the same wavelength arrive at two different ports, they will be directed to two
distinct output ports. For instance, if the scheduling is done by the left hand side figure in
which the same wavelength (blue) arrive at both input port 1 as well as 2, the former will
be routed to output port 1 and the latter to 2. The same physical wavelength being used for
different I-O connection is known as wavelength re-use . The figure shows a typical
static wavelength router.
 
 
If the network condition changes frequently, a static routing is not helpful. One needs to
then use dynamic switching of wavelength. In the following figure, two WDM rings are
connected to a wide area network. Each ring can transmit in two wavelengths and .
Problem arises if the node A of the left LAN wishes to communicate with the node B of the
right LAN. As the wavelength at which node A transmits is in active use in another
channel of the right LAN, a dynamic
 
 
switching of wavelength has to take place.
 References:
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