Good day! In this lecture we will continue our discussion about Ethernet. In the previous
lectures, we have seen the basic data link protocol used in Ethernet. Ethernet is a very popular and
nowadays it is almost ubiquitous in the LAN area. It is a very widely used system and
it has evolved from whatever was the approach to Ethernet in the earlier days and the basic
Ethernet protocol that we had discussed. People are sort of shifting away from it because
as the technology develops and as things like switches etc., become cheaper as the speed
of network goes up, there is a slight shift in emphasis in modern Ethernet. In this lecture,
what we are going to do is that first, we are going to have a look at the physical layer
of the Ethernet. There is not much to discuss about that, and then we will see how we are
shifting from a shared medium to a switched Ethernet kind of concept, and how speed is
increasing. Then, we will discuss a little bit about Ethernet, LAN as a concept is. So,
we discuss the modern Ethernet that is how the Ethernet is evolved.
Before we go into that, we will look at the topology and some of the limitations of the
earlier versions of Ethernet. The topology of a network refers to the pattern of workstations,
cables and repeaters. In an Ethernet, there are two primary limitations to the topology:
transmission accuracy and end-to-end transmission time. Transmission accuracy means that Ethernet
really under a data link layer is an unreliable protocol in sense that it does not do a lot
of acknowledgement or negative acknowledgement and things like that. Therefore, essentially
the transmission accuracy should be fairly tolerable. What happens is that the maximum
distance for accurate transmission is determined by signal attenuation along the cable, and
it would depend on the on the quality of the cable, the kind of cable we used and the data
rate. The other important point is the end-to-end transmission time as we have seen, because
of this collision and other things this end-to-end transmission time is important. It's not just
for the data rate or things to move faster but for other reasons, also this end-to-end
transmission time is a factor.
These are the Ethernet physical layer standards that we have some of them, they are actually
more. So 10Base5, that is, 10 Mbps base band transmission and 500 m; this 5 here stands
for 500 m cable length, this is called thick Ethernet or has thick coaxial cables and this
has become obsolete. Then 10Base2 is 10 Mbps base band transmission; once again all of
these are base band transmissions; that means we do not modulate them to a higher channel
or frequency channel. So, 10 Mbps and 2 for above 200 m cable line, or lines of 180 to
200 m length. Then, 10BaseT; this again is for 10 Mbps base band transmission, which
uses UTP cable. There are various versions of UTP cable like categories 3, 4, 5, etc.
100Base-TX is 100 Mbps base band transmission, also uses UTP cable.
So, these are the of maximum segment length. As we can see, 10Base5 is 500 m; 10Base2 is
185 m; 10Base-T is 100 m. Fibre optic cable 10Base-Fl goes up to 2 km. actually, 10Base-F
is not in much use these days, because fibre optic components became cheap and available,
and people have moved from this 10 Mbps rate to the 100 Mbps rate. The point is that, the
fibre optic cable goes much longer than copper cables and it is best between buildings; that
means when it goes through some open space, etc., we avoid copper cable most times because
of electric interference and other problems, not to mention the distance. The nodes per
segment in 10Base-F could also be more; we will come back to this topic of nodes per
segment later. As we have seen previously, 10Base5 and 10Base2 have many limitations;
for example, in 10Base2, you see the figure of 30 and a network segment having only 30
nodes is a constraint these days because all kinds of things are being networked. For completeness
sake, I will make a mention of 10Base2 and other kinds of
Technologies, although they have become obsolete now 10Base2 has 10 Mbps, 200 m maximum cable
length, is a thin coaxial cable in a bus topology; this is a classical bus topology. Repeaters
are used to connect up to multiple segments. Repeater repeats bits; it hears on one interface
to the other interface, so this is a physical layer device just for amplification, strengthening
the signal.
This is a figure showing how they are connected. We have this thin cable, which has terminated
on both the ends and these nodes would be connected using some t connectors over here,
and this is the network adapter. When a network adapter pushes some signal on to the cable,
the transmitted packet travels in both the directions.
This is just a mention about 10Base5, because connecting a 10Base5 is more cumbersome and
in 10Base2, connection would get loose quite often in the earlier days. Anyway, this is
a 10Base5; it has a thick coaxial cable, the transceiver, and transceiver, drop cable and
there would be a network interface card here with this MAC unit and protocol control firmware
etc. As I said, this has become obsolete now.
Let us talk a little about repeaters. Repeater is a physical layer kind of device, which
regenerates the signal, and provides more flexibility in network design because it depends
on where we want to extend your LAN. So if you want to extend your LAN in some direction,
make running into this distance limitation 200 m for 10Base2 etc. You may sort of increase
that by just putting another segment on the other side of repeater so that you can go
another 200 m. Therefore you can extend the distance over which a signal may travel down
a cable; an example of a repeater is Ethernet hub. A hub has two purposes: one is to provide
a collision domain and replace the cable. The other function of the hub is the repeater function; that means it regenerates
the signal. This connects together or a repeater or a hub. It connects together one or more
Ethernet cable segments of any media type. If an internet segment were allowed to exceed
the maximum length or the maximum number of attached systems to the segment, the signal
quality would deteriorate.
You can see that you have one cable segment here. This goes up to its maximum distance
there is a cable segment too again going up to the maximum distance. But we put this repeater
in between so that, we can increase the total distance that may be covered. These are the
different computers connected to the cable. It is used between a pair of segments; this
is a simple repeater to provide signal amplification and regeneration to restore a good signal
level before sending it from one cable segment to another.
Hubs are essentially physical layer repeaters; bits come in one link and go out all other
links. There is no frame buffering or CSMA/CD at the hub. Adapters detect the collisions
and provide net management functionality, which means that this hub, although it is
an active device, in the sense that this has some electronics in it, in its action it is
more passive. Its active component is restricted to the fact that it regenerates the signal;
it amplifies and regenerates it. So far as the other intelligence is concerned, like
detecting collisions, it does nothing like that. If two signals coming at two different
ports of the hub come to the hub at the same point of time, they will collide. So, a hub
is a whole co-axial cable, which has been collapsed into one collision domain inside
the hub; we could look at that way.
But it does provide some net management functionality. That means, if you have a managed hub, there
are two kinds of devices. The point is that a network is usually a geographically distributed,
dispersed kind of entity; but this has to be managed. Therefore if there is a problem
somewhere, may be some user has put in a complaint, what we will do is that there are so many
things which connect from the user to the central network. There are cables, may be
switches, hubs, and other things, so it is important that you should be able to remotely
tell whether some active device is functioning. We cannot do this on passive things like a
cable but there are managed switches and managed hubs etc., whom a central network station
may interrogate and find out whether its health is alright or not. So if you have a managed
hub, you have net management functionality; of course, the managed hubs are costlier than
unmanaged hubs. So this is the picture of hub. This is a co-axial cable coming in here
and this is a t joint; the t joint is feeding into the hub. There may be a number of ports
and a number of segments may be connected together.
Now, we come to 10BaseT; so far as the physical part is concerned, this 10BaseT can go on
to become 100BaseT; that means, from 10 Mbps it can go to 100Mbps. But for this UTP, unshielded
twisted pair cable of various categories like 3, 4, 5, etc., are the dominant physical medium
and the varying technology today for local area networks.
We can have category 5 cable data transmission up to 100 Mbps and we can go a little bit
more on this depending on the distance. The impedance of a UTP cable is about 100 ohm,
give or take another 15 ohm over the useful frequency range of the cable. The transmission
speed is 0.6 to 0.65 into c, which is about 2108 m/s. Time delay is about 5 nanoseconds
/m.
Here, you can see there are some 10BaseT repeaters and then 10Base-Fl segments. This is just
to give you an idea about the various delays which are involved. The first segment delay
is about 0.57 s for this 100-m segment, middle segment delay is about 10's and last segment
delay of about 0.57 s. The transceiver delay is about 7 s, Fl repeater delay is about 1.7
s, and 10BaseT delay is about 21 s and so on. All these delays really add up and you
have some limit on this in order for your CSMA/CD to work properly.
So this is the same data in tabular form. It has various types of cables and these are
all fibre optic kind of cables, which go up to 2 km or 1 km. These are obsolete now these
are the maximum lengths and these are the delays, the receiver delay and the delay at
the repeater and so on. Now, what happened was that the people moved as the demand for
bandwidth grew; so at least in the LAN it moved much faster when LAN bandwidth became
quite cheap and everybody wanted to move from 10 Mbps to the next speed, which is 100 Mbps,
and this was called fast Ethernet? One thing was that, when we come to fast Ethernet, this
co-axial cable really was out. So the thing preferred was the unshielded twisted pair
or UTP. Category 5 is preferred for 100 Mbps operation, although category 4 would also
work.
It is possible to transmit it over category 4 cables also; but category 5 cables are preferred.
Now one thing about this UTP cables is that because they are used for connection from
end to end unlike the coaxial cables in coaxial cables, we have one cable and many nodes may
be connected at intermediate points you put a connector, usually an RJ45 kind of connector on the two ends and then go into
two nodes. It is as simple as that. So if there are no nodes in between, just two end
point connections, where this part of the medium is not shared, at one end may be the
computer, and the other end might go into another machine straight away. These two machines
network together. Usually the other end would go to a network device. Let us say, to start
with, it might go into a 100 Mbps hub. So, all the nodes which are connecting, that means
which are part of the network, all of them are connected by this UTP cable to the central
hub. So the central hub is a shared medium at the collision domain. But so far as the
UTP part is concerned, there is no collision.
Just as we had the 10Base-T, which supported 10 Mbps, may be with the same kind of cable
with this UTP cable, we could support fast Ethernet, that is, 100Base-T. There was 100Base-TX,
But there are some problems with this: one is to increase to 100 Mbps, we have got one
order of magnitude jump in the speed. That is not a mean achievement on the same medium.
Depending on the quality of the medium, as the speed goes up; its performance tends to
decrease. So depending on what kind of medium it is, there is only a certain amount of bandwidth
in a certain range that you can support on that medium. So to increase to 100 Mbps, different
encoding is used to reduce the bandwidth of the transmission.
Encoding data at 100 Mbps using Manchester encoding would create a bit stream of 200
Mbps. So, we cannot straight away use Manchester encoding here, because this is for supporting
200 Mbps, we requires at least 100 MHz of analog bandwidth, which is too much. Category
5 UTP cable is rated up to 100 MHz but for keeping some margin etc., transmitting a signal
of 100 MHz bandwidth would be unreliable. So what we do is that instead of Manchester
encoding, we use a different kind encoding, which is called multiple or multilevel encoding.
To reduce the bandwidth, a different encoding scheme is required. 100Base-TX uses a multiple
level encoding scheme, which is MLT3, Multiple Level Transition 3. The bandwidth required
in this case has come down quite sharply to 31.25 MHz. The encoding is somewhat like this
0: there is no transition and 1: there is a transition. And this transition could be
of various types from low to 0, high to 0, 0 to low or 0 to high, depending on the context.
For example, this is a 10110001110 that is being encoded. Suppose with the 1 there is
a transition from 0 to low, that is, 0 to low level, then for 0 there is no transition,
after 0 there is 1. Now there is a transition; since we go to low now, and the only place
to go to is high, we go to a high and then there is another 1 and since we are going
high, we go high some more, that is, another 1. Should this go from 0 to high, then three
0s, that is, no transition at all. Then from 0 to 1, now from high since 1 has come, you
will have to come to low and again another one has come; we are going in the downward
direction. So we continue from 0 to low and then, another 1 from low to high and then
0; there is no transition. So, this is the way an MLT encodes using different levels
or multiple levels of signals. This reduces the net analog bandwidth requirement
for the cable. So this is how, the ten-fold increase in the speed was achieved on the
same physical infrastructure. So bandwidth required for MLT is half the bandwidth required
for a two-level scheme.Half is a rough figure. Long stream of zeros will cause the line to
hold a constant voltage and lose clocking, synchronization is necessary to read the signal.
This is one problem, which is handled in various ways.
One way is to encode each 4-bit sequence as a 5-bit pattern 4B5B encoding. In this 4B5B
encoding what is done is that instead of a 4-bit sequence we introduce a 5-bit sequence
so we get some transition and we can hold on to synchronization.
This pushes up the bit rate to 125Mbps for100BASE-TX, where data is only 125Mbps/4 is equal to 31.25
MHz. Category 3 or 4 cable is used for a rate of 100 Mbps by using all four twisted pairs
in the UTP cable; so this is also possible. Three pairs will transmit data while the fourth
pair is used by each station for collisions. As in 10BASE-TX, a three-level signal is transmitted
in 100BASE-T4. T4 is when we are using category 4 cable but the data rate must be minimised
further. Therefore the signal is broken into 8-bit sequence and each 8-bit sequence is
represented as 6 three level signals, or 8/6t. If we are trying to put in a new network today
we are never going to use category 4 UTP, we are always use category 5 or better. You
know about the increasing requirement of speed in the LAN, but these techniques were developed
so that Ethernet could move into some existing physical infrastructure like wiring, where
category 4 cables is used for moving from 10 to 100 Mbps; that is why these were developed.
Fast Ethernet is done with cabling of the following types 100BASE-T4 with category 5
100BASE-TX; and 100BASE-FX is quite common. As I said, 10BASE-Fl has become outdated but
100BASE-FX is still there, which uses fibre optics and goes up to 2 km. Category 5 UTP
goes only up to 100 m or may be some people are conservative and limit it to 75 m. At
full duplex at 100Mbps, fibre is again full duplex at 100 Mbps; these are the advantages.
The advantage of T4 is that, it uses existing category 3 UTP.
These are the repeater delays. 10BASE-T and 100BASE-TX wiring goes like this from the
end station a maximum distance of 100 m to the hub. We can use multiple hubs, a maximum
of four, to increase the distance between any two stations; a hub may go to another
hub; that may go to a station again. Now, there are three segments. We can use a maximum
of four hubs and five segments with 100 m from the node to the hub.
For upgrading from 10BASE-T to 100BASE-TX, we need new hubs or switches. You may have
some 10 Mbps ports to handle 10BASE-T; NICs may have auto sensing 10/100 ports that handle
either. Auto sensing ports are quite common. As a matter of fact, most of the network interface
cards you get today are 10/100; that means they are auto sensing ports. An NIC senses
the network device at the other end. If the other end supports a rate of 100 Mbps, this
NIC will operate at 100 Mbps. If it senses that the opposite end can handle only 10 Mbps,
it will use only 10 Mbps. By the way, if there are problems with the varying connection,
etc., it may send 100 Mbps port as 10 Mbps. So, if you want to upgrade from 10BASE-T to
100BASE-TX, you may need new NICs, and only for stations that need more speed and no need
to rewire that may be a big advantage in some cases. We may have 100 multiple hubs, one
connecting to the other; you can have multiple hubs for connecting etc.
In 100BASE-TX there is a limitation that you can have only maximum two hubs and they must
be within a few meters of each other. So, the maximum span using these hubs is only
200 m, which is shorter than 10BASE-T. It could be a big problem and that is why 100
Mbps hubs are also becoming obsolete. People have migrated from hubs to switches, and that
is what we will be talking about. So if you had used hubs, this would have been a limitation;
but nowadays people do not usually use hubs.
We will come back to the topic about hubs and switches later on. What happened is that
this development went on and now we are moving into the era of Gigabit Ethernet. The Gigabit
Ethernet in the desktop is still not very common, but it is coming and for servers etc.,
nowadays we routinely get Gigabit ports and, may be, soon your desktop computer may also
be connected with Gigabit ports. So, this is a depiction of a two-station Ethernet two
computers connected straight away, this is a switch or a hub which operates at Gigabit
speed. So, these allow point-to-point links and shared broadcast channels through some
hubs if they are there. In shared mode, CSMA/CD is used for the short distance between the
nodes to be efficient; but this is not very common. It uses standard Ethernet frame format
802.3z; the normal mode is full duplex mode with a central switch. This is the most common
one connected to computers. In this configuration all lines are buffered. On the right-hand
side, you see one switch and the nodes are connected straight to the switch. So this
is a switch rather than a hub, where each of the ports has a buffering.
Since each of the port is buffered, contention is impossible; because data can always and
gets it on the buffer. So, it makes no sense to the channel to see if it is idle or not
because if there is no contention, CSMA/CD protocol is not used at all. And this is the
mode, in which Gigabit is usually used, although there are buffered distributors also.
Now people are talking about moving to 10 gbps, at least in the metropolitan area network.
They use copper and fibre with 850 nm or 1300 nm lasers and some kind of encoding. The Ethernet
is moving really fast, because it is upgrading quite fast it went from 10 mbps to 100 mbps;
from 100 mbps we are moving to 1gbps; and from 1gbps, we can see the possibility of
moving to 10 gbps, and 10 gbps is already in operation today. From 10 mbps to 10 gbps
is a 1000-fold increase in a very short span of time, which is really amazing.
For Gigabit Ethernet, we have 1000Base-SX; this SX shows the multimode fibre for 1000Base,
so the maximum segment size would be about 550 m. 1000Base-LX uses single or multimode,
so this goes up to 5000 m. Shielded twisted pair would only take to 25 m and 1000BaseT
can go up to 100 m using category 5, category 5b or, for gigabit Ethernet, category 6 is
preferred. We will talk more in detail about the switching, which is very important. So,
Ethernet is usually considered as a shared media LAN.
Only one station can be transmitted at a time even when you have multiple hubs, these hubs
are all shared domain, which means that only one can transmit. All other stations must
wait while one station sends. So this is the big limitation of latency and congestion with
hubs
To improve this, people moved over from hubs to switches. Packet switches are very useful.
They give you improved security. Users are less able to tap into other users data. In
a shared medium, we know everything is being broadcast so all packets are going to every
other node. That is poor security; this gives you much better security; better management
we can control who receives what information. That means, even using a switch, even within
a LAN, we can create a work group and make users members of different virtual LANs. These
virtual LANs may be packet filtering between the virtual LANs, etc., to give a better management
and limit the impact of network problems. This switch will also give you full duplex
communication, rather than half duplex. One side effect of having a shared medium is that
only one sends, and everybody else is in the receiving mode. Obviously that channel cannot
work as a full duplex mode. Because two of them cannot send simultaneously at the same
time, only one may be sending and the other may send. At most, this works as a half duplex
channel, whereas over here this can work as full duplex channel. A hub simulates a single
shared medium, whereas switch simulates a bridged LAN with one computer per segment.
For example, we have only one computer and many segments, and there is only one computer
for each segment. So it does not have any collision and from two different computers
you can send data at the same time to the switch. The switch will buffer it and there
will be no collision. So collision detection may be eliminated. Some of these may connect
to a hub instead of going to a computer. So a switch may be connected to another switch,
that switch may connect to hub, and that hub may connect to so many computers. So there
are many possibilities.
Ethernet switches are highly scalable. We had 10Base originally; we went from 10BaseT
hubs to 10BaseT switches. 10BaseT switches are increasingly being replaced by 100Base-TX
switches with auto sensing codes. 100Base-TX switches are very common these days, they
have higher performance and their costs have come down. These days we get 100Base-TX switch
at the cost of what a 10BaseT hub would cost a few years ago. We can see how the technology
and the economy have improved. Gigabit Ethernet switches are still expensive; it will take
some more time in order for them to move to the desktop.
Other advantages of switches are that there are no limits on the number of Ethernet switches
between the farthest stations and no distance limit on size of switched networks. You remember
when we were talking about 100Base-TX and when we were using 100 base TX hubs. You can
only have a maximum of two hubs and these two hubs have to be close together; they cannot
go far long. So, the maximum span of a local area network could almost be 200 m, which
was really a limitation, but then people moved to100Base-TX. They also moved from hubs to
switches. With these switches, now there is no problem because the distance limitation
had to do with the round-trip propagation delay and things like that. Since there is
no contention and the switches have buffers, there is no distance limitation at all. You
can have any number of switches connected to each other; there is no distance limit
on size of switched networks. So often individual hosts, are connected to switches and may be
different switches are also connected. This is Ethernet without collisions or CSMA/CD.
Here, we have A, B, C, and so on. Suppose A wants to communicate with A' and B wants
to communicate with B', both the communications may take place simultaneously. This is a switch.
If this had been a hub that would not be possible but since this is a switch, B can communicate
to B' and A can communicate to A', just as in a telephone switch, two pairs or distinct
pairs of people can communicate at the same time. Similarly, two distinct pairs of machines,
through this packet switch, can communicate at the same time.
This is essentially a multi-interface bridge; it has a layer 2 frame forwarding, filtering
using LAN addresses; LAN addresses mean MAC addresses. We will talk more about MAC addresses
presently. It allows switching A to A' and B to B' simultaneously, but there are no collisions.
This can have a large number of interfaces. A modern switch may have hundreds of ports,
so hundreds of machines can be connected to the same switch. Similarly, a number of such
switches can connect to each other.
Ethernet switches must be arranged in a hierarchy or daisy chain with only one possible path
between any two stations. Suppose, from 4 to 3 there is only a single
path that is 4, 2, 1, 3; you do not make a loop connecting two ports of a switch because
that would really create a lot of problem. Therefore, Ethernet switches must be arranged
in a hierarchy or daisy chain. There is only one possible path between any two stations
or switches.
A station is an Ethernet frame switch plug-in card. It checks to see if it is destined to
the same card. If so, the frame is copied there. We mentioned that in a packet switch,
usually the ports would be grouped into small groups and each of these groups will go into
a particular line card. This line card is similar to the Ethernet. A switch is nothing
but a packet switch; there will be line cards over there and may be 4 lines coming to one
particular line card and maybe there are 8 line cards for a 32-port switch. Now, for
those that are coming into the line card, there are variations of how they are handled
by the line card. It could be that a frame which has arrived in a switch plug in card
is destined to the same card; that means it is destined to some other line in the same
card. If so, the frame is simply copied there; if not, the frame is sent. Now these line
cards should get connected to each other through the backplane.
If the frame is sent over from the high-speed backbone to the destinations card, it will
travel to the other distinct destination through the switching fabric. This backplane may be
active or passive; they are of various types. Anyway, the backbone typically runs at many
gigabits per second. As a matter of fact, nowadays it is possible to have a high-end
switch with something of the order of 100 gbps backplane switch. It is a very high-speed
switch that can support a lot of machines at the same time. If two machines attached
to the same plug in card transmit frames at the same time, what would happen? There are
two machines attached to the same plug in a card and they transmit frames at the same
time what would happen would really depend on the way the card handles it; here there
are some variations. For example, let us say, there are 4 line cards 1, 2, 3, 4 coming into
this one particular line card. Line 1 and line 2 frames arrived simultaneously and both
of them are destined to line 3; what is going to happen? It depends on how it is handled.
One possibility could be that all the ports on the card are wired together to form a local,
on-card LAN. This LAN card itself is a local on-card LAN, and on this on-card LAN, there
will be a CSMA/CD network. CSMA/CD may operate on that particular card itself; in that case,
there will be a collision that will be detected. Only one transmission per card is possible
at any instant of time, so if you are doing that, you have made that line card to be a
shared medium. All the cards can be transmitting in parallel; the different cards may be transmitting
in parallel. Each card forms its own CSMA/CD network independent of the others; that is
one possibility.
The other possibility is that the plug-in card of each input port is buffered; so incoming
frames are stored in the cards on board. This allows all input ports to receive or transmit
frames at the same time for a parallel, full duplex operation. This is not possible with
CSMA/CD on a single channel. Previously we had talked about buffers in packet switches.
We mentioned how buffers could be on input side, how buffers could be on output side
and distributed throughout the switch fabric. That was from the point of view of handling
head of line, blocking, switching, speed etc. Here, it is slightly different in the sense
that if there is some small buffer at each of the ports and on input side, full duplex
transmission at the same time is possible on all the lines. Instead of making a local
CSMA/CD and allowing some kind of collision, this naturally is a better kind of switch.
Once a frame has been completely received, the card will then check to see the frame
is destined for another port on the same card, or for a distant port. In the A case, it can
be directed to the destination; in the B case it must be transmitted over the backbone to
the proper card.
Each port has a separate CSMA/CD so collisions do not occur. There is now no question of
any CSMA/CD because each port has only one machine. Then there is another term called
cut through switching: frame forwarded from input to output port without waiting for assembly
of the entire frame. That means even when the frame has not fully come in, the bits
are transmitted and there is a slight reduction in latency. So, combinations of shared dedicated
10/100/1000 mbps interfaces are possible.On the same switch you might get some 10 mbps
ports, some 100 mbps ports, also some 1000 mbps or Giga bit ports for up linking to the
main backbone.
This is a typical LAN of an IP network. Let us look at this figure some hubs are remaining,
but these hubs will give way to switches from 10BaseT to 100BaseT very soon, but let us
say, there is a legacy system, through which some 10BaseT hubs and some computers are connected
to this. These hubs may be connected to a switch. This switch could be connected to
another switch and that switch might connect to some other nodes, etc.
This constitutes the LAN maybe there is a 100 mbps connection from this switch to the
mail server, because mail server may be something which everybody is using. This is a server,
and you might want to have a higher speed of this one also these are 10 mbps ports.
You might want to make it 100 mbps. There is a www server, that is, a web server, which
is also connected through a 100 mbps port. So from the same switch, you may connect to
some hubs, you may connect to some servers, you may connect to some individual desktops,
PCs or machines. So this is what the LAN part looks like. And of course, nowadays nobody
would want a LAN which is standing by itself. People want to connect their LANs to the internet,
which is the great network of networks. There are probably lot of networks, may be millions
of networks, in other places; we want to connect to other networks also. So we have to go through
what is known as a router. We go through a router to the external internet. We will be
talking about this part when we talk about the network layer in more detail; so this
is a typical picture of a LAN.
If we abstract this a little bit, we have a LAN, and different nodes or the different
machines are connected to this LAN. And then there is a router, which connects you to the
internet. This internet may be a connection between a lot of different networks, may be
a lot of different LANs to their own routers etc. This is a slightly more abstract view.
And then looking into it in detail in the LAN part of it, if we go to the previous picture,
this is the LAN part of it and these nodes are connected.
These nodes are usually connected through adapters. That means the network interface
card, NIC, or the adapter. These nodes may communicate with each other in the LAN if
it is connected to the internet. Naturally it may communicate with the other machines
in the outside world also; but let us for the time being just focus our attention on
the nodes which are in the LAN, and they are sort of trying to talk to each other. If they
want to talk to each other using either a hub or a switch, or in this case, the Ethernet
frame format, what is going to happen is that the frame format has certain fields and those
fields have to be filled up. That means the destination address has to be there, and the
destination address is the LAN address or the MAC address or the hardware address. Sometimes
this is also called an Ethernet address.
And this Ethernet address is distributed by IEEE; the manufacturers of these adapters
buy blocks of addresses from IEEE. And these are all 6-byte addresses, something like this:
1a23f9, each of them is 1 byte. And if you look at this figure, say 1a23f9cd069b and
88b22a do not have any relationship with each other. They are just 6 bytes, may be 6 arbitrary
bytes. This card may have come from some manufacturer and got entirely different blocks from IEEE,
the other may have an entirely different address, which has no relationship with the other;
but they are all sharing this LAN. There is a small problem how does the source node get
to know the destination address? One thing is sure since one central agency, namely IEEE,
gives this Ethernet addresses, each adapter on LAN has a unique LAN address; no two adapters
will ever have the same address Sometimes, there may be some nodes with a number of LAN
addresses, with a number of adapters etc., but one particular adapter will have one unique
address, two addresses will never be the same. The point is that, how do get to know this
is the source? This is a destination how does the source know the destination, LAN address
or the MAC address?
There are at least two kinds of addresses, which are used in networks. One is the IP
address, which is used by the network layer to decide on the total route from the source
to the destination. What will happen is that it will know where it will have to go; in
order to reach its destination address; it must have some kind of relationship with geography.
And then, there are these hardware or MAC addresses, which are at a lower level and
are just as unique; they do not have any relationship with each other. So we have this 32-bit IP
address, network layer addresses used to get datagram to the destination IP.
We have LAN, MAC, physical, Ethernet or hardware address used to get datagram from one interface
to another physically connected interface in the same network. So this is a 48-bit MAC
address for most LANs burned in the adapter ROM.
Analogy of MAC address is like a social security number; IP address is like postal address.
The question is how do you get to know the other person's address? Address translation:
how does a host know the MAC address of the other host in its local area network? And
secondly, how does a router determine the MAC address from the IP address of a host?
These are two interrelated questions.
We need some kind of a translator for IP addresses from MAC addresses. Address translation can
be static or dynamic; static translation means you keep some static table. Static translation
requires a lot of book-keeping work from system administration and this is not feasible.
We have this address resolution protocol or automatic resolution protocol, the so-called
ARP protocol, which is a dynamic address translation scheme. If the destination IP address is not
in the local ARP cache ARP cache is a table, which is stored in the local machine and which
gives this IP address to MAC address mapping (the broadcast query for that address) at
the most one host will reply because the IP address will match with only one of the host
addresses. So, the ARP table entries are cached for up to 15 minutes, unless if refreshed.
It is firmly at 15 minutes so that a machine can go from one LAN and can also be shifted
to another LAN. What will happen is that these old entries must automatically wash out. And
if the new entries communicate to their communicating machine, their MAC addresses are going to
come into the cache. So this is how it is always kept fresh. ARP uses layer 2 broadcasting
Each IP node on LAN has an ARP table. ARP table is IP, MAC address mapping for some
LAN nodes. And there is a time to leave, something like 15 minutes or so, it depends on the time
after which the address mapping will be forgotten it is typically around 15, 20 or 25 minutes.
So starting at A, we want to send a datagram addressed to B. We look up the net address
of B or link layer. What will happen is that we want we do not need B's MAC address? That
means this 2 23.1.1.1 is an IP address we know it is B's IP address, but we want to
know its MAC address. So A's IP address, B's IP address and some IP pay load form a frame
like this, which is broadcast.
A wants to send datagram to B and A knows B's IP address. Suppose B's MAC address is
not in the ARP table, A broadcasts an ARP query packet containing B's IP address. All
machines on LAN receive the ARP query. B receives the ARP packet; replies to A with B's MAC
address; the frame is sent to As MAC address, which is unicast. It is now specifically given
as MAC address is known, because A sent the query with a source address.
A cache saves IP to MAC address pair in its ARP table until information becomes old, that
is, it times out. But if it is being used, then it will not become older. So in a soft
state, information that time out goes away unless refreshed. ARP is plug_and_play, otherwise
it will get washed out and new entries will come in automatically. Nodes create their
ARP table without intervention from the node administrator.
All hosts receive the request but only B responds with the IP address; however, other nodes
may update their own cache from the transaction. The cache is required since broadcast is a
very expensive proposition and more than one packet transfer is very likely between any
two stations.
This is an important question, what is a network? We always talk about networks; it is the interconnected
nodes in the same broadcast domain. We are doing the broadcast this broadcast is limited
to that domain. Networks are connected through routers. When we want to route to another
LAN, it is more involved. We will talk about going through a router and then we will specifically discuss bridges. Thank
you.
Today we will be talking about local internetworking. Our topic of today is local internetworking,
what is internetworking? Internetworking is connection of different networks, everybody
is aware of the term internet today why are comes to this term internetworking and by
internetworking we mean connecting different networks. Just to remind you, what is a network?
We discussed this in the last lecture that the interconnected nodes in the same broadcast
domains.
And networks are usually connected through routers but as we will see for local internetworking
we may not need a router we may need something called the bridge we will come to that. This
is the set of nodes which are in the same broadcast domain; this broadcast may be a
good thing to have for some applications. But for the operation of the network there
is one very crucial reason, why we require the broadcast that is to discover the MAC
addresses of the different computers. 474