Good day. In this lecture we are going to discuss the various protection and restoration
mechanisms which are usually employed in optical networks.We have to discuss what protection
is and what restoration is and why we need them. What is protection, what is restoration,
comparison between the two and the different schemes of protection would be our general
outline of presentation.
Network is unreliable; and then so many failures can occur – a node may fail, a link may
fail or a link may get cut, some fiber optic line may cut in-between because somebody cut
it while digging a hole or something. A node might fail, there may be power failures at
nodes and so there are failures in the network. But if you remember, one of the most important
places where we deploy optical networks is in the core of the network. The core of the
network connects so many people to so many other people and it is very vital that we
cannot allow the services to be severely disrupted because that would have very grave consequences
and anyway the service provider attempts to give a high level of service. Although failures
are unavoidable in real life, we have to find some way of combating this; that means if
there is a failure we want to recover from it as soon or as fast as possible. That is
what protection and restoration is all about. Another thing is that when we want to give
some reliability, protection, restoration, etc., in some form or the other we always
have to bring in some redundancy. Without any redundancy, a system cannot be protected,
or it cannot be restored without any replacement, etc. There has to be some redundant capacity
in some form in the network in order to achieve protection and restoration. So how this is
done is what we will discuss. Protection and restoration are the mechanism to recover from
network failure: their difference will be discussed in the following parts.
Why we need protection and restoration is now clear: to recover from network failure,
to prevent the lot of data loss. Now another point is what would we mean by prevent lot
of data loss? The point is that we will be talking about protection at the optical level,
at the lower, transport level, that is, the low physical level. Above this physical level,
there are a whole lot of other layers like data link layer, network layer, transport
layer, etc. They may have their own protection mechanism and they might be able to tolerate.
In some cases, if such a protection is sort of implemented at higher level, such protection
usually would tolerate a small amount of data loss, which they will retransmit or do something
in the protocol to take care of that. We will be talking about this later on. Within that
limit we do not have a 100% tight case, but we have something to play with. Within that
limit, if the physical layer can come back up, then that is nice; then the end user who
is sitting at the top of the application layer would not notice that a failure has actually
occurred. So this is our general goal and, of course, to prevent lot of data loss.
To provide reliable communication service is a reason for having protection and restoration.
Protection is the primary mechanism; this is fast and routes are usually preplanned
– we will come to this part later on – whereas restoration is the secondary mechanism, used
to provide more efficient routes or additional resilience, etc., over and above protection.
We will see both of these later on.
Techniques for protection: we could protect a path, which is called path protection; we
could protect a link; that is called link protection. There are various schemes of protection:
like 1plus1 protection, 1:1 protection, 1: N protection, M: N protection. This depends
on what kind of redundancy we have built into the network. So we will look at some of these
techniques one by one.
So what are the considerations and tradeoffs that we have for protection? For support for
fast protection, time is dictated by the client layer.
This is what I was talking about earlier – in the client layer, whatever the higher layer
we are talking about, all of them are clubbed together, and we are calling them client layer,
let us say. In the client layer we may have some resilience; that means, we can tolerate
some small amount of data loss. So depending on that, and beyond that, it will be taken
that the service will fail. This is dictated by the client layer and that is the constraint
within which we try to do our protection or restoration at the bottom layer, that is,
the physical layer. We require some switching technologies as we will see, and how to implement
protection.
It could be through dedicated hardware or through software. Here we will only be talking
about – since we are talking about the physical layer – hardware protection, where some
hardware has been put in order to take care of this protection. Support for low priority
traffic – that is another consideration we might have. So low priority traffic supported
using the protection bandwidth; traffic dropped in case of a failure. As I mentioned earlier
that in order to give some protection capability into the network we have to build in some
kind of redundancy over there. Now when there is no failure, naturally the redundant link
would be idle. What you could do is that if you have some low priority traffic, which
could be dropped whenever there is a problem, under normal circumstances you can use your
provision in the network to carry the low priority traffic, and as soon as there is
a failure, where naturally the provision is going to be used up for providing protection,
restoration, etc., the low priority traffic would be discontinued. So that is one thing
we could do.
Support for mesh topologies – mesh topologies are bandwidth efficient, fast signaling mechanism,
flexibility in choice of routes by preplanned routes, etc. If we have support for mesh topologies,
it is also nice. As a matter of fact, if you remember our discussion about different types
of topologies – we have star, tree, ring, mesh etc., so these are the different possible
topologies. Out of all these, point-to-point connection, ring and mesh are the three topologies,
which dominate most of the WAN. When you want to communicate between two points, you first
set a point-to-point link. Now consider that link to be quite important.
Then what you would like to do is that you would like to put in some kind of reliability
over there, by making it into a ring. For example, we discussed SONET. A SONET gear
quite often is put in the form of ring, rings that would touch, mutually touching rings.
For example, the telecom people put up all these SONET rings through fiber optic networks.
The rings are quite easy to handle and we will discuss the protection mechanism in ring
quite a lot.
But if you go to a wider geographical area ring, it may not to be feasible due to various
reasons. What you have is a mesh. A mesh, if you recall, is just a graph, where the
nodes are connected in some fashion. It has to be a connected graph and actually later
on we will see that for giving proper protection, it has to be not only a connected graph, it
has to be disconnected graph as well. That means between the two nodes, which are communicating,
there have to be two alternatives paths, which are linked somehow. Otherwise, you will not
be able to give protection. Giving protection in mesh networks is also an important consideration.
Other important considerations are maintenance of large distributed routing tables, that
is, precompiled routes or up to date topology maps. So you have to maintain this dynamically,
because a network is a very dynamic thing as links come up or go down or nodes come
up or go down, this may have to be recomputed and stored in a distributed fashion. We would
like to have support for all failure modes: node failure for mesh networks and for ring
networks, so or it may be of course ring failures.
First, let us talk about path protection. It uses more than one path to guarantee that
data be sent successfully. If you look at this graph, it will have a 6-node graph where
1 and 6 are communicating and on the top, through the dashed line, we show the primary
path, which is the primary connection between 1 and 6. Now what might happen is that the
link between 2 and 3 might snap due to some reason. We have already got a backup route,
which is calculated going through 1, 4, 5 and 6. So we will channel our communication
through the backup link. Please note that this backup or secondary path from the source
to the destination does not share any link with the primary path. That is the requirement:
if any link in the primary path fails, assuming there is only one failure, which means that
the backup link is all intact and you can switch to the backup path for this particular
communication.
Now path protection: there are various ways to protect a path, that is, various ways of
provisioning the extra bandwidth capacity in the network so that you can give protection.
You need to build in some kind of redundancy in the network. So there are various ways
you can build in the redundancy and the schemes may be divided as dedicated path protection,
or shared path protection.
Dedicated path protection may be shown as 1plus1 protection and shared path is 1:1 protection
or 1: N protection. So we will look at this one by one.
This is an example of 1plus1 protection. So you have the source on the left, and then,
you have destination on the right. So it is communicating. What you can see is that from
the source, the signal is coming to the splitter.
If you remember, the splitter is going to split it into two halves, may be of equal
power or something, and then the same signal is been carried through two different links
and over there, we a switch. The switch determines which signal is better and may be switches
to that and a communication is going on.
If that particular link, say, this top one fails, it automatically switches to the other
protection link. So this is some kind of hot redundancy we have. That means there is redundant
source of information in the destination side; so if the primary one fails, the secondary
one is already on. So protection would be very fast. The only trouble is that for each
such path, you will have to give an alternate path, which is also being used at the same
time. This is a dedicate path protection – for this particular path there is a dedicated
alternative path; although this is very good, it is costly.
Now we come to shared mode protection; in the shared mode, the figure looks almost the
same, expect if you note, this splitter on the left has been replaced by a switch. So
what we have is we have a working fiber and we have a protection fiber. Unlike the 1plus1
protection, this is not a hot standby. This is cold in the sense that the protection fiber,
to start with, is not carrying any signal, let us say.
The source is just simply passing through the switch; the signal from the source is
passing through the switch, then down the path through the destination switch to the
final destination. If the working fiber goes down, then this switch will flip and the protection
fiber would be in place. Since this is not a hot standby, meaning that since it is not
carrying any signal under normal circumstances, you could share this path with something else.
You can use this to send some other channel or some other information, etc. So that is
why this is a shared mode of protection.
Generalizing this, we get this 1: N protection, where N line is sharing 1 protection line.
We have the inputs from 1 to N lines, so these are the N sources, and let us say so many
destinations. So they may be going – 1- to 2- and N to N- and so on; this is the normal
mode of operation. This part of the network is for normal mode of operation and each of
them is connected to a switch over here. Another link from the switch comes to one bigger switch,
so this is an N input port over here and then there is a single link from this switch to
this switch, on the destination side, which again feeds to all the switches.
What would happen is that in case there is a failure in any one, let us say, ith link
from 1,2 to N, out of that, the ith source to ith destination, which is going through
the ith link and the ith link fails, what this ith switch could do is that the ith switch
could switch the signal from the ith source to this particular switch at the bottom and
now the protection fiber would be carrying the signal that was flowing down ith channel
over there and then again it will feed it to the ith switch on that side. Of course
these two switches have to communicate that the ith one has failed over here, so you switch
it to your ith mode, or this switch may sense it that this line has gone down, so it will
take signal from this line.
This one protection fiber is been shared by these N working fibers, and as I mentioned
earlier, when everything is fine this protection fiber could be carrying some low priority
data. When everything is fine these N working fibers are actually carrying the most of the
important traffic so some low priority data could be flowing down the protection fiber.
As soon as there is any failure anywhere, the low priority data would be stopped or
it will be dropped and then this protection fiber would switch to give the services between
the nodes that have experienced a link failure. Just as we have a path protection that means
for a path you try to give an alternate path, similarly you could do it at the link level
also. In general, it may be more efficient or more
proper to do that because if you have N nodes and if it is slightly large, potentially you
have very large number of paths through the networks and for each path having an alternate
path may not always be a very good idea. So we concentrate at the link level and for each
link level we may give a protection.
So use an alternate path if the link has failed. This is the primary where the link may have
failed so I have an alternate path to that from 2 to 3, 2 to 4, 4 to 5, and 5 to 3. This
alternative, please note, is for the link from 2 to 3. We will have another diagram
for that.
Dedicated link protection is not always practical although sometimes we may have it; shared
link protection is practical and it is quite often implemented. This link protection may
fail because here you are only provisioning for the failure of a link, but if a node fails,
then it may lead to some complication as we will see.
To compare between path switching and line switching, path switching is a coarser scheme
and line switching is finer scheme; and line switching can again be a span protection.
Span may be several links together; that may be span or a line protection.
In mesh networks of course restoration is possible only if the graph is two edge connected
i.e., biconnected, which means that there are two edge disjoint paths between any pair
of nodes so that no single edge failure can disconnect the network. This is a necessity
and we usually try to keep it that way unless it’s very difficult or very cost it’s
not cost effective, etc.
Protection in a mesh network is more complicated than a ring. Simple minded scheme would be
two edge or node disjoint paths for each connection, 1plus1. As is mentioned, it is not very efficient.
There may be many paths and provisioning double the number of paths, which are pair wise mutually
node or edge disjointed may be very difficult. That may be a lot of extra provisioning in
the network. A better approach would be line protection, which of course has the problem
of coordination. I will show that and protection cycles in mesh net later on.
In the path layer and mesh protection, there is protection of mesh networks to protect
the mesh at the single unit. Pre-computed routes means all possible routes and alternative
routes are pre-computed. 1plus1 protection is protection route per light path, protection
route per failure. We will discuss this later but, as I said, this is a costly alternative.
Or what we might do is that we can do on-the-fly route computation; that means it is not pre-computed.
As there is a failure, centralized route computation and coordination route computation and coordination
are done at end nodes or distributed route computations – all these are possibilities.
This is an example of mesh network, where let us say this is the primary path and this
is an alternate path. This alternate path may be pre-computed or it may be computed
on the fly when there is a failure. Similarly, from here to here this may be the primary
path and this is an alternate path. Please note that these communications as well as
this communication are going through the same fiber. Maybe they are going through different
wavelengths or maybe they are combined together; so there is various way of handling this.
Let us look at some diagrams: this is a mesh network. Naturally once again, it has the
same 6-node network and this is the normal operation, that is, communication from this
node, node 1 to node 6. Now there is link failure over here; what you might do is you
might switch the entire path like this. So this was the pre-computed path for this path.
You switch to that – this is one possibility. Or this particular span, the span could be
live from here to here or from here to here. That may have an alternative path, to which
you switch.
Or this particular line may have this alternative; that means from this node to this node. If
this is the link, the alternative to this link is this section from here to here to
here to here. It is trying to go through the same path, but then it takes a diversion when
there is a link failure through the alternative, which is provided for this particular link.
I have shown only one node coming into this but you can appreciate that this path may
be very long so you do not have to re-compute the path over the entire length; rather locally,
you can re-compute that for this particular link as an alternative. So the alternative
may well be through some local nodes only. So this path just gets a slight diversion;
that is line protection.
We talked about protection cycles in a mesh network, now for protection cycles, what we
do is that for each of the paths we try to form a cycle – cycle of provisioning of
light paths, let us say. The point in the cycle is that suppose we are going through
some arc of the cycle and if some link in-between breaks, you can always go in the other direction.
So you try to form these cycles in the mesh which you keep ready and pre-computed. So
whenever there is a failure you can switch.
You can see here there are pairs of a fibers going in both the directions and there you
can form cycles over there. There are pairs of a fibers going in both the directions and
there you can form cycles over there. One of them would be protection edge, maybe the
inner one; and one of them would be the working edge. If one of them fails, the other will
automatically take over; the other direction will automatically take over.
So this is another example of a network with both working and protection fibers. The working
fibers have been shown in solid lines whereas protection fibers have been shown with dashed
lines. Once again you must realize that you may not provide the same level of protection
to all paths or to all parts of the network; depending on which part you consider more
sensitive or more important, you may put your protection there. A mesh network may be partially
protected. Some of the parts may be protected or some of the parts may not be protected.
So some more cases: line protection in a mesh network. What we have is a unidirectional
light path from node A to node D; so from node A to node D we have an unidirectional
light path going may be like this through nodes B, C, and E. We are talking about this
path A to B to C and then to E. Now after the link BC fails, the light path is rerouted
by nodes B and C along the route A B F E C E D. The unidirectional light path was going from A B C E and then to D; so this was the path
A B C E D. Now BC has failed; so A B F E C E D.
You can see what has happened is that there was no point in coming from E to C and then
back from C to E. What has happened was that we were doing line protection; that means,
for BC my protection was from B to F to E to C because I wanted to go from B to C so
I am going from B to F to E to C and so now BC has failed. So that is what I do and from
that point I continue wherever I was going and actually I was going from C to E to D,
so once again from C, I go back to E and then to D.
Such a thing is possible because we are taking a local decision; that means, for this particular
link what to do in the case of failure that has already been pre- computed? We are not
taking a global picture; here I have shown you a very small graph so you can immediately
see the entire global picture with your own eyes. But for local nodes we may not have
the global pictures where this node is coming from and where this path is finally going
to. So all the intermediate nodes may not have the global pictures because if all of
them had the global pictures maybe they could have computed a better path but this is for
line protection.
Now line protection in mesh network: here what might happen is that erroneous connection
due to the failure of a node is being treated by its adjacent nodes as link failure. This
is one case of the so-called race condition. What might happen is that node 1 has failed;
so what would 6 do is that it may assume that the link from 6 to 1 has failed, whereas 2
may assume that the link from 2 to 1 has failed.
So both of them perceive as two different failures, so they take some local decisions,
and it might lead to a funny situation where you are going in a cycle. Maybe other kinds
of things may happen after node 1 fails – node 5 gets connected to node 4 after node 6 and
2 invoke line protection independently. If they perceive the same failure, the actual
failure was that of node 1; but if they perceived differently as failure of link 6 1 and failure
of link 2 1 and they have independent actions, it may lead to race conditions.
The advantages and disadvantages of protection: we will also be talking a little bit of restoration.
Protection is simple; it’s quick; does not require much extra process time – and this
is the important part, since this is quick. As I mentioned earlier, there will not be
a lot of data loss. For example, a SONET ring would sort of come back up from a failure
in a less than 15 milliseconds. So that is a benchmark; you are back in action in less
than 15 milliseconds, so whatever little you might have lost during that time would be
taken care of by the higher layer protocols.
But usually they can only recover from single link faults. If there are multiple link faults,
all kinds of funny things may happen. There is inefficient usage of resource, because
protection needs a lot of resources, even if we are sharing them.
Dedicated protection needs even more resources; we talk about path restoration and link restoration.
What we do is that we compute the path after the failure and the resource is reserved and
then used. So in restoration what you try to do that
is that you try to look at the current actual situation. You try to have some protocol for
keeping track of the present situation and then you compute some root and then you reserve
the path and then restore the original service. This has got some software parts, so some
data has to be processed, etc. This usually takes more time, so this has a hardware as
well as software aspect to do it.
The path is discovered at the end nodes of the failed link; but this is more practical
than path restoration. We have both path restoration and link restoration. Path restoration means
the path may be long and to find out an alternative, it may be more difficult, whereas links are
between the adjacent nodes so they may quickly find an alternative.
Advantages and disadvantages of restoration are the following: usually it can recover
from multiplex element faults because you are sort of having some protocol to exchange
information and then find the current situation and form the alternatives, etc. There is more
efficient usage of resources; it is more complex; it is slower; it requires extra process time
to set up path or reserve resources.
So for comparison between protection and restoration, in protection the resources are reserved before
the failure; they may not be used. In restoration the resources are reserved and used after
the failure. So this is the main difference between the
two. Route: in protection it is predetermined; in restoration it can be dynamically computed.
Resource efficiency: in protection it is naturally low and the restoration is comparatively high.
Time used for protection is short; for restoration it is longer. Reliability: protection is mainly
for single fault, whereas restoration can survive under multiple faults. Well, it is
not that it can always survive under multiple faults; it depends on the where the faults
are, but if it is survivable after multiple faults, it will take global view and say that
ok, still I can give the services. So restoration will take care of that, whereas in protection
it may not be possible to handle multiple faults. Implementation: protection is simple
and restoration is naturally more complex.
For optical networks, we not only talk about physical links we talk about virtual wavelength
paths. So in light paths also routing can be centrally controlled or distributed; resource
reservations, forward reservation, as well as backward reservation are done as we do
in optical networks. We will talk more about this later.
Now let us come to this all-important topic of fault management in ring networks. As I
said, ring networks are very ubiquitous in WAN.
All the telecom people will love these things because naturally it allows them to give very
high level of service. So we have so many rings; these SONET rings are very common,
and rings are a very common kind of topology. One of the chief attractions of the ring topology
is its capability to allow some kind of protection and restoration with some redundancy in-built,
as we will see. We will look at two different cases, unidirectional path switch rings and
bidirectional line switch rings or UPSR and BLSR.
This is a diagram of UPSR. We have AB, means A to B and BA means B to A. First of all you
see that we have these two rings – we have this working fiber as well as a protection
fiber. Working fiber is going in only one direction and protection fiber is going in
the other direction, in the counter clockwise direction. So for connection from A to B and
from B to A – this is A, and this is B – my working fiber is going in this direction.
A to B is really going in this direction; B to A is coming in this direction.
So A to B, B to A: this is the protection fiber part, A to B the working fiber is going
in the other direction. A to B is going like this and B to A is coming all the way like
this. Let us say the outer one is a working fiber, it is through the outer ring. For A
to B, this is the path; and B to A, this is the path, whereas for protection purpose – and
please note that this is a 1+1 scheme; that means for this the alternative is already
provisioned and maybe it is something like a hot standby. So A to B is through the protection
fiber and now this path from A to B is going via D and C, so A D C B: that is A to B and
B to A. The protection path is from this through the protection fiber. So if there is a failure
anywhere, this can still continue, so this is a unidirectional path switched ring.
There are some limitations of UPSR. It does not spatially reuse the fiber capacity; so
what is happening is that since this is unidirectional, what is happening is that even if two nodes
are side by side, if they are in wrong direction then it has to come all the way through and
naturally all those links – even if we are talking about particular ?s I mean WDM systems
– in the ring are entirely sort of covered by this. Otherwise even if we are talking
about a WDM system what is happening is that at least one ? around the whole ring is getting
occupied because two side by side things want to communicate. So it does not spatially reuse
fiber capacity and if there is some, you could use it for some other purpose. That is not
possible in a unidirectional ring. Each bidirectional SONET SDH connection uses up capacity on every
link.
That is the thing; if you look at the previous picture we are just talking about a connection
from A to B and B to A; that’s all, so this entire ring now has been used up assuming
only one wavelength. The entire ring now has been used up; these ADMs of course are the
ADMs of SONET.
So it is efficient for lower-speed access networks, one to multipoint only.
And the other point, which may become a problem, is that delays are different for the two paths
because one of them is small and the other one is quite large, all the way around the
link. The relays in the two paths are quite different. A remedy could be bidirectional
lines; that means, bidirectional line switched rings or BLSR. So we were in UPSR, now we
will be talking about BLSR, bidirectional line switch rings. This provides spatial reuse
capabilities and additional protection mechanisms and adopts span as well as line protection.
We will have a look at both of these.
This is a four-fiber bidirectional line switched ring; so once again we have two working fibers,
say, the outer two ones and two protection fibers or the inner ones. So we actually have
four fibers. This is a four-fiber system and it is going, so we are using two such working
fibers. One of them is going in one direction the other is going in the other direction,
so this is bidirectional. We can communicate both ways, so if A to B and B to A both can
communicate on the working fiber in this part at all, the rest of the space can be used
for other communication. So spatial reuse is much better for bidirectional line switched
rings, and the protection fibers are there because if one of them fails then the protection
fiber may take up.
This is a span protection so we are talking about bidirectional traffic supports, maybe
16 nodes or some distances, etc., from A to B. You may pre-compute a span that if this
span goes down what is the alternative span. If this is the case of line protection, if
this line fails what is the alternative line? Now the alternative line of course may go
in the other direction, depending on what the failure is. So there are many different
possibilities with this BLSR.
There is also two-fiber version of BLSR, namely BLSR by 2. Here both the fibers are working
as well as protection fibers. That means if one of them fails the other
one will give the protection in the other direction of the ring. If you note, the rings
are going in the opposite directions and there is a line failure in both of these working
as well as protection fiber. That means A to B is communicating in this direction, B
to A is communicating in this direction. But if one of them fails, the other one will give
the protection by going in the other direction. In that case we have to go all the way round,
then you will have to reserve the resource and if there is something already going on
here, you may be blocked or this may have to be dropped and so on. So naturally you
have two fibers, we have less flexibility. But with two fibers we can get this bidirectional
thing going on.
Comparison of different types: these are all self healing rings. What we mean by self healing
is that the nodes, these ADMs and the SONET, etc., are programmed in such a fashion that
as soon as they sense a failure they know what local action to take and how to adjust
the switch internally, so that it automatically switches from the working fiber or from the
working span or working fiber or whatever through the protection side. That is why these
are self healing rings, so they heal automatically. And as I said these SONET rings really do
this in less than 15 milliseconds; the entire thing will again be up. So we can compare
the three: UPSR, BLSR by 4, and BLSR by 2. For example, fiber pairs 1, 2 and transmission
receiver pair 2 4 and 2 spatial reuse in UPSR it is none, in BLSR it is there, and in BLSRby4
it is there. Production capacity is equal to the working capacity. Link failure is path
protection in the case of UPSR or span or line protection in the case of BLSRby4. In
case of BLSRby2, it is only line protection because it will have to go in the other direction.
Node failure: it is path protection, line protection and line protection restoration
is faster in UPSR; somewhat slower in BLSRs and restoration speed is this. Node complexity
is low, high and high. Another thing we talked about is the dual homing.
By dual homing we mean that suppose we want to deploy your network in such a way that
it is very mission critical and no failure is acceptable and we want a hot standby. So
what you might to do is that you may get connected through two different hubs and what you want
to do is that you want dual home to these two hubs and these two hubs take independent
paths to your destination. This shows a dual homing to handle hub on node failure; so we
have these four ADMs: once again four nodes, let’s say A B C D.
So the end node is A and these are the two hubs, B and C. What we want is that not only
some link failure but even if one of the hubs fails, I should still be able to communicate.
So what you do is on this ring you communicate with hub 1, let us say you are communicating
with hub 1 through this A D C B and you are communicating with hub 2 through A B C. As
we will note, even if one of the hubs fails, you can still communicate through other one.
So this is another kind of protection.
And finally I have mentioned this point before – just a reminder that a network consists
of many layers and each layer may have its own protection mechanism built in, independent
of other layers. So there are both advantages and disadvantages to this. We have already
talked about the advantages.
This is an example of a WDM link carrying SONET traffic. So there is a WDM link, so
there is a SONET ADM. This is a working fiber pair and protection fiber pair.
Please note that the pair has been shown as one line over here because usually as you
know that fiber optic line is a simple line; that means it goes in only one direction;
there is a source in one side and the detector on the other side. Usually these fibers always
come in pairs. The other side is for the communication in other direction so we have a working fiber
pair over there and protection fiber pair through this WDM link. Look at this 1:2 protected
scheme.
What is happening is that there is one protection fiber pair, there may be protection fiber
pair and through the working fiber there may be multiple virtual links going through the
working fiber using different wavelengths. A is a normal operation and B link is cut
and the traffic is restored by the optical layer. That means you automatically assign
new wavelengths and new paths through the fibers etc., to bring it up. So this you may
do at the optical layer rather than at the electronic layer.
But the case we are talking about here is that the SONET is riding on the optical layer.
So we have the optical layer at the bottom then you have a SONET on top of it. On top
of SONET also there will be the data link layer and then so on all the other links.
What we are saying is that each layer may have its own protection mechanism. So for
example I mentioned that the SONET will also have a protection mechanism of its own. So
if a SONET LTE, i.e., the SONET line terminal equipment, senses that it cannot communicate
to the next LTE, this will automatically reroute the traffic and try to reroute the traffic
in the other direction. Sometimes it is good to have multiple protections at multiple layers
but what might also happen is that they might sort of cancel each other or they might go
into a race condition. So these are the disadvantages of having protection at various layers.
There could be some disadvantage also if they are not very well coordinated, which usually
would not be because these layers are sort of independent of each other. They talk only
to their peers and go through their own protocol and give protection. Apart from these of course
the other disadvantage is that you may redo some part of the protection unnecessarily.
You may be unnecessarily duplicating the work at various places. So these are the disadvantages
of having protection at multiple layers.
What is the advantage of optical layer protection – speed and efficiency? Limitation would
be detection of all faults may not be possible; protects traffic in units of light paths.
So this is another problem. As I mentioned, in light path the granularity is very coarse.
It may be 2.5 Gbps; so you are really giving the protection at a level of granularity,
which may be quite high as I just now mentioned; it could lead to race conditions when optical
and client layers both try to protect against the same failure.
Of course, on the optical layer you have one more dimension to play with, which is in the
case of a WDM. That means you have different wavelengths, so instead of 1plus1 link, you
can talk about 1plus1 wavelength path selection. You can sort of try to select two independent
light paths and the signal is bridged on both protection and working fibers if you are doing
1plus1 protection kind of thing; the receiver chooses the better signal. In case of a failure,
the destination switches to the operational link that is operational light path; there
is revertive or non revertive switching; that means, if the original link comes back, it
may revert or it may not revert; and no signaling is required.
So that is the unidirectional light path. I have just shown you some of the schemes
which are used for protection and restoration. And as I mentioned, the schemes may be partially
deployed and some of the parts may be protected and some of the parts may not be protected
and so on. But the essential idea is the same – that you build in some kind of redundancy
and the redundancy may be in the form of entire fibers or the redundancy may be in the form
of light paths or wavelengths. It may be pre-computed and pre-reserved like 1plus1; it may be pre-computed
but not pre-reserved like in a shared one or it may be computed as and when the failure
occurs; that is, in the case of restoration. So there are various approaches to it. Depending
on how critical the problem is or how critical the application is, what the cost is and how
much extra provisioning you can do, you can choose your own way of protection and restoration.
Thank you.
Good day, so today we will talk about multiple accesses. Ok, now what is multiple access.
If you remember that we had seven layers in the so called OSI stack the top on being application
then we have presentation session transport network link and physical ok. We had been
mostly talking about the physical layer till now although for optical networks we sort
of ventured into some of the hired layers but from this lecture onwards we want to concentrate
on the link layer ok. Now what medium access does is it coordinates completing request
for what for medium that means that there is a medium. Which may be an object of contention
meaning that I mean several nodes may want to use it. A and this medium access control
protocol has to do with how to handle that. So sharing of link and transport of data over
the link that is an general the description of what the data link layer does. So when
we share a link there is a question of committing request and we have to have some way of reserving
that and of course there is an also question of transport of data. Over a link if you remember
when we say a link I mean that two nodes which are connected the nodes may be or computers
routers switches etc. They are two networking nodes they are directly connected now the
when I do like this it might mean cable copper cable or a fiber or it might mean a shared
medium like the free space. ok so there is a but there is some way of
communicating directly between these two nodes that is what we mean by link that means that
is just one hop in the network that is what we are talking about in the data link layer.
So there is a question of reliable transfer of data over these link and if this link like.
When you have a free space transmission with so many nodes in the network now so many people
would like to transmit. So there is a question of sharing this medium and there is a question
of who would access it when and just as I said free space could be a shared medium similarly.
If you remember that if you have some kind of a bus if you remember our discussion about
topology of networks when we have some kind of a bus from which the number of nodes are
hanging and that bus may also be an object of contention. So that bus is the medium through
which communication is taking place and there is and there is some kind of competition or
sharing between the of this shared medium between the nodes. So we have to handle that
that is the other thing so examples of contention based or ALOHA and slotted ALOHA hese refers
to some protocols which we are used in satellite communication.
So we will discuss these and we talk about satellite communication. We have CSMA it stands
for carrier sense multiple access or CSMA CD which is carrier sense multiple access
with collision detection. There are other variances of these like carrier science multiple
access with collision avoidance and things like that so these ALOHA slotted ALOHA for
satellite CSMA CD or specific specifically. CSMA CD is used by Ethernet in many situations
and then we have CSMA CA may be for cellular communication etc. So these are contention
based MAC and Round Robin there are these are token based protocols and so two very
common. Ones are token bus and token ring so actually we will discuss these in the next
in this lecture as well as the next we will be discussing these. 505