These are used to connect different devices in the network or to connect two or more different networks. Following devices are used for interconnection:
- Modulation/ Modem
A repeater is a device which operates only in the physical layer.
- All transmission media weaken the electromagnetic waves that travel through them.
- Attenuation of signals limits the distance any medium can earn, data. Devices that amplifies signals to ensure la transmission are called repeaters.
- A repeater receives a signal and before it becomes too weak or corrupted, regenerates the original bit pattern.
- Hence a repeater can extend the physical length of LAN.
- Repeater is not an amplifier because amplifiers simply amplify the entire incoming signal along with noise.
- Signal- regenerating repeaters create an exact duplicate of incoming data by identifying it amidst the noise,
reconstructing it and re-transmitting only the desired information.
- The original signal is duplicated, boosted to its original strength.
- A repeater does not actually connect two LANs. It connects only two segments of the same LAN.
- It cannot connect two LANs of a different protocols.
- A repeater should be placed at a precise point on the link. Such that the signal reaches it before the noise has
induced an error in any of the transmitted bits.
- A repeater forwards every frame, it has no filtering capability.
- Repeaters operate at the physical layer of the OSI model and they deal with the actual physical signals.
- In general the word hub can refer to any connecting device. But its specific meaning is multiport repeater.
- It is normally used for connecting stations in a physical star topology.
- All networks require a central location to bring media segments together.
- These central locations are called Hubs. A hub organises the cables and relays signals to the other media segments.
- There are three main types of hubs:
- A passive hub simply combines the signals of a network segments. There is no signal processing or regeneration.
- A passive hub reduces the cabiling, distance by half because it does not boost the signals and in fact absorbs
some of the signal.
- With a passive hub, each computer receives the signals sent from all the other computers connected to the hub.
- They are like passive hubs but have electronic components for regeneration and amplification of signals.
By using active hubs the distance between devices can be increased.
- The main drawback of active hubs is that they amplify noise along with the signals. They are also much
expensive than passive hubs.
- In addition to signal regeneration, intelligent hubs perform some network management and intelligent path
- A switching hub chooses only the port of the device where the signal needs to go, rather than sending the
signal along all paths.
- One advantage to this is that all transmission media segment can be connected permanently because each se
meat will be used only when a signal is sent to a device using that segment.
- Hubs can also be used to create multiple levels of hierarchy.
- The number of hubs that can be connected to each is typically four, but the maximum number of hubs
depends on the type of network topology used.
- When possible each hub should be connected directly to a server network card rather than to another hub.
- The more data hub passes through, the slower the connection.
- A bridge operates in the physical layer as well as in the data link layer.
- It can regenerate the signal tht it receives and as a data link layer device, it can check the physical (MAC) addresses of source and destination contained in the frame.
- The major difference between the bridge and repeater is that the bridge has a filtering capability. That means it can check the destination address of a frame and decide if the frame should be forwarded or dropped.
- If the frame is to be forwarded, then the bridge should specify the port over which it should be forwarded.
- So a bridge has a table relating the addresses and ports.
- It is important to note that the bridges do not change the physical address contained in the frame.
Types of bridges
The bridges are of two types:
- Transparent bridges and
- Routing bridges.
Transparent bridge is a bridge in which the stations are not at all aware of the existence of the bridge. Transparent bridges keep a table of addresses in memory to determine where to send data.
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The duties of a transparent bridge are as follows
- Filtering frames
- Forwarding and
In source routing a sending station defines the bridges that should be visited by the frames. The addresses of these bridges are included in the frame. So a frame contains not only the source and destination address but also the bridge addresses.
Source routing bridges are used to avoid a problem called looping. These bridges were designed for the token ring LANs. But these LANs are not very common now a days.
- Routers are devices that connect two or more networks as shown in Figs. They consist of a combination of hardware and software.
- The hardware can be network server, a separate computer or a special device. The hardware includes the physical interfaces to the various networks in the internetwork.
- These interfaces can be Token Ring. Ethernet, TI, Frame Relay, ATM or any other technology.
- The software in a router are the operating system and the routing protocol. Management software can also be used.
- Routers use logical and physical addressing to connect two or more logically separate networks.
- They accomplish this connection by organizing the large network into logical network segments called as subsets.
- Each of the subset is given a logical address. This allows the networks to be separate but still access each other and exchange data.
- Data is grouped into packets, or blocks of data. Each packet has a physical device address as well grouped as logical network address.
- The network address allows routers to calculate the optimal path to a workstation or computer.
- Route discovery is the process of finding the possible routes through the internetwork and then building
routing tables to store that information.
- The two methods of route discovery are:
- Distance vector routing
- Link state routing
- Routers work at the network layer of the OSI model.
- The cost of a route can be defined in several terms: a time estimate, a distance estimate or an estimate that includes monetary terms.
- In distance vector routing, each router advertises its presence periodically to other routers on the network
- Link state routers broadcast their complete routing tables only at startup and at certain intervals.
- Dynamic route selection permits routers to constantly adjust to changing network conditions.
- With static route selection, packets always follow a predetermined path.
When the networks that must be connected are using completely different protocol from each other, a powerful and
intelligent device called a gateway is used. A gateway is a device that can interpret and translate the different protocols that are used on two distinct networks.
- Gateway comprise of software, dedicated hardware or combination of both. Gateway operate through all the seven layers of the OSI model.
- A gateway can actually convert data so that it works with an application on a computer on the other side of the gateway. For e.g. a gateway can receive e-mail message in one format and convert them into another format.
- Gateways can connect systems with different communication protocols, languages and architecture. For e.g. IBM networks using Systems Network Architecture (SNA) can be connected to LANs using a gateway.
Note: Gateways are slow because they need to perform intensive conversions.
It works at the Data Link Layer. It is used for dividing a network into segments called subnets. It provides filtering of data packets and prevents network traffic also.
Modulation is used to send an information bearing signal over long distance. It usually involves varying one wave form relation to another waveform. Modulation is a process to change the analog signal to digital signal to analog
signal. A device that perform these operations is called a modem.
There are three types of modulation
- Amplitude modulation: It is a process to change the amplitude of carrier signal according to digital signal having information.
- Frequency modulation: It is a process to change the frequency of carrier signal according to digital signal having information.
- Phase modulation: It is a process to change the phase of carrier signal according to digital signal having information.