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HUBSBRIDGESGATEWAYS

SWITCHES

All Pages

Computer Network

Computer Network itself means the interconnection of computers. Thus the interconnection oftwo or more computers via transmission media is called computer network. By means of

computer network we can utilize our resources more concise and efficient manner. Nowadays

networking is the most predominant thing in the world. Lots of people in recent era wants

interconnection of computers for resource sharing, to access remote information, for businesspurpose, entertainment etc.

Computer Network has many uses and applications like business application, home applications,

person-to-person communication, entertainment, mobility, resource sharing, security, access tothe remote information etc. Networking involves many things like transmission media,

protocols, topologies, interconnected devices, add-on cards, connectors and many more.

There are various types of networks are in the scenario like LAN (local area network), WAN(Wide area network), MAN (Metropolitan area network), CAN (Campus area network), Wireless

networks and Internet too. Among these LAN are in the restricted area, MAN is covers some

spanning in distance, WAN can take around the world-wide and CAN in campus or building.Nowadays wireless networks are also possible which provides mobility. Today cell phones,

pagers, laptops, notebooks, PDA are all employ wireless network. Wireless networks are finds its

applications in flights (Flying LAN) also.

Different networks can be connected to one another. Like WAN can be forms via connectingdifferent LANs. The Internet, a worldwide network, is the example of the WAN. Internet is

become more popular as people know how to use it and its beneficial effects. Via internet it is

possible to establish person-to-person communication, e-commerce, online shopping, interactiveentertainment and access to remote information.
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To reduce their design complexity, most networks are organized as a stack of layers orlevels. The number of layers, the name of layers, the functions of layers and contents of eachlayer are differs from network to network. The purpose of each layer is to provide some services

to the higher layers.

Layer n on one machine carries conversation on layer n on another machine. The rules and

conventions are used in this conversation are collectively known as layer n protocol. A set of

layers and protocols is called Network Architecture. A list of protocols used by certain system,one protocol per layer, is called protocol stack.

The design issues of the layers are addressing, flow control, error control, multiplexing and

demultiplexing, and routing. Based on this design issues there are various network models are

invented like OSI model, TCP-IP model etc.

The first step towards international standardization of the protocols used in the various layers is

developed by the ISO - International Standards Organization and known as ISO-OSI referencemodel. OSI stands for Open System Interconnection. The model deals with connecting open

system - that is systems that are open for communication with other systems.

The OSI model is consists of 7 layers. A layer should be created when a different abstraction isneeded. Each layer should perform a well defined function. The below fig shows all the layers

contained within the OSI model.

--- ISOOSI MODEL FOR COMPUTER NETWORK----

The functions of each layer are briefly described as below:

PHYSICAL LAYER Connected via physical connection and concerned with transmitting raw


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bits over a communication channel. It accomplishes the surety.

DATA LINK LAYER Makes transmission free from undetected errors. It broken down the

input data into frames and transmit the frames sequentially.

NETWORK LAYER Controls the operation of subnet and determines the packet routing. Thelayer operates on packets.

TRANSPORT LAYER The layer accepts data frame from above, split into smaller units

(packets), and pass these to the network layer and ensure that all pieces are arrives correctly at

the other end. The layer provides error free transmission and it is a true end-to-end layer.

SESSION LAYER Allows users on different machines to establish sessions between them.Sessions offers dialog control, token management and synchronization.

PRESENTATION LAYER It concerned with the syntax and semantics of the information

transmitted. And represent the abstracts data structures.

APPLICATION LAYER The actual form of data is reveal over here. It contains varieties of

protocols.

So this was brief discussion of OSI model.

The networks are built ups with different topologies. To connect the two isolated networks we

need some mechanism called interconnected devices like hubs, routers, repeaters, gateways,bridges and switches. All the network devices are operated at different layers of the OSI modeldiscussed previous. The layers matters because different devices use different piece of

information to decide how to switch.

In a typical scenario, the user generates some data to be sent to a remote machine. Those data


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are passed to different layers. The transport layer adds a header e.g. TCP header and passes theresulting unit down to the network. The network layer adds its own header to form a networklayer packet e.g. IP packet. Then the packet goes to the data link layer which adds its own header

and checksum(CRC) and gives the resulting frame to the physical layer for transmission e.g. over

a LAN.

Thus different interconnected devices are operated at different layers and which device is for

which layer is demonstrated by the following diagram;

_______________________________________________________________

---- Which device is for which layer----

As Shown in diagram, the interconnected devices are repeaters, routers, hubs, bridges, gatewaysand switches. Now we look each devices in detail, one by one.

REPETERS:

In computers in network, one user can send information to other user. While the signal travelsalong transmission line at beyond sufficient distance, the signal becomes weaken out. So it is

necessary to regenerate the signal at desired distance to prevent loss of information because ofweakness.


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Repeater is for this purpose. As signal travel along a cable it will degrade and distorted in aprocess called attenuation will finally make a signal unrecognizable. Repeater regenerates oramplifies the signal so that data can travel along additional length of cable. So a repeater enables

signals to travel further.

Repeater belongs to the physical layer of the OSI model. And it connects like types of networks.

Such as Ethernet LAN to Ethernet LAN.

For a repeater to work, both segments that it joins must have the same media access schemeprotocol, logical link control and transmission techniques.

All token ring cable distance can be increased by using repeaters.

Repeaters do not filter out the data. It sends every bit of data from one cable segment to another

segment even if data contains bad packets or malformed information. Thus it will not use when

data requires filtering.

The repeater just do as explained: It takes weak signal from the one network segment. Thenregenerate or amplifies that weak signal. And finally it gives regenerated signal to the other

desired network.

Repeater takes small amount of times to regenerate a signal. This can cause a propagation delay,

which cause a problem if several repeaters are connected in network. So many network

architectures limit the number of repeater that can be used in a row.

Repeaters can support any kind of transmission media.

Repeater is not an intelligent device because it also amplifies the noise due to attenuation in


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signal. So noise is also amplifies and if corruption in data then it will also sends to the otherdesired network.

Repeater also not handled the traffic problems very well.

The cost of the repeater is very less. The below fig shows the repeater and its working.

USE A REPEATER WHEN YOU WANT TO :

- Connect two segments at the most cost effective manner.

- When you dont want to filter out the data.

- Repeater improves performance by dividing network into segments.

- When there is no intelligence needed and there is less traffic in network.

DO NOT USE REPEATER WHEN :

- There is heavy network traffic.

- Segments are using different access methods

- You need of any kind of data filtering.

ADVANTAGES OF REPEATER:


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- Easy to handle

- It prevents the signal weaken out.

- Supports any kind of transmission media.

- Cost effective.

- There is no protocol conversion so transfer rate is fast.

DISADVANTAGES OF REPEATER:

- Not an intelligent device.

- It cant filter out the data.

- The protocol conversion is not done.

- Requires same network architectures on both sides.

- Traffic problems are not handled by it.

- It cant prevent the noise attenuation.

ROUTERS:

In computer network the packets are traveled over a transmission media needs to find a best path

to its destination.


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Router is a device that can select best path for the incoming packets and forward to the

destination.

Unlike repeaters, routers are used to connect network of different types. Such as those using

different architectures and protocols e.g. token ring and Ethernet.

Routers will work at the network layer of the OSI model. It can switch and routes the packets

among multiple networks. They do this by exchanging protocol specific information between

separate networks.

A network needs a device which not only knows the address of each segment but can also

determine the best path for sending data and filtering broadcast traffic to the network segment.

Such a device is called a router.

So router can filter out the data also and isolates the network traffic.

Router just works as explained: It takes the incoming packet from one network, removes oldprotocol form that packets and assigns new protocol to it (strip off the old protocol and assigns

new protocol to that packets). Then that packet will easily accept by another desired network and

forward to it.

Router finds the destination address by using routing table. The routing table found in router is

contains the network address. Routing table also contains the possible paths for all routers and allnetwork addresses. From that it finds the destination address and best path from available paths

and data based costs. If routing table does not indicate the proper address of a packet, the packetis discarded.

Routers are intelligent devices because they prevent the amplification of noise attenuation.


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Router requires specific addressing. They only understand the network numbers which allow

them to talk with other routers.

Routers do not talk to remote computers. When routers receives packet destined for a remote

network, they send them to the other router which manages the destination network.

Routers segment large networks into smaller ones, acts as a safely barrier between segments and

prohibits broadcast storms, because broadcast one is not forwarded.

Because routers must perform the complex functions on each packets and protocol conversion isalso done by them they are somewhat slower.

Routers only read addressed network packets. So bad data packets or broadcast data storms arenot passed on to the network. So routers are filter outs the data. Routers do not look at the

destination address they only look at the network address. They will only pass the information if

the network address is known.

The ability to control and data passing through the router reduces the amount of traffic between

networks and also minimize the wait time experienced by user. The below fig shows routers and

its working:

---- ROUTER ----

The collections of communication lines and routers form the subnet.


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TYPES OF ROUTER:

There are basic two types of routers 1) Static 2) Dynamic

Static router:

It requires an administration to manually set up configure the routing table and to specify eachroute. It always uses the same route which is determined by the routing table entry. The routeused in hard code and they are not necessarily shortest route. They are more secure because the

administration specifies each route.

Dynamic router :

It requires manually configuration of the first route and automatically discovery of additional

networks and routers. Can choose best route based on the factors such as cost and amount of link

traffic. It can decide to send packets over alternate routers. Not as much secure as static routers.

ADVANTAGES OF ROUTER:

- It expands the network.

- Router works if the two networks are different.

- Protocol conversion is done.

- Can filter out the data

- It is an intelligent device. Prevent the noise attenuation.


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- It also selects the best path.

- It also handles the traffic problems.

DISADVANTAGES OF ROUTER:

- The cost is high

- It takes a time in protocol conversion so little bit slower.

HUBS:

A special type of network device called Hub can be found in many home and small business

networks. Hub belongs to physical layer of the OSI model.

A hub is a small rectangular box often made up of plastics and used to join multiple computers

or other network devices to form a single network segment. On this network segment allcomputers can directly communicate with each other. Ethernet hubs are of most common types

but there also other hubs such as USB exists in the market.

Hub is most frequently used in a star topology. And it is acts as a multiport repeater.

Hub contains several ports that each port accepts a network cable. There is an LED below each

port. LED glows means connection is OK and that port is enable.

Small hubs network for four computer they contains four or sometimes five ports where one port

is for uplink connection to other hub in the network. Larger hub contains 12, 16 or also 24 ports.


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A 4-port hub is called a Hubby.

Generally hub is directly connected to the server.

There must be a limited number of hubs in network. If the number of hub is beyond limit thanwe get less speed.

Hub is a centralized device used to connect server and multiple clients. It is multiport repeater

where one port is connected with the backbone (server) and other ports are connected with theclients.

Hub works as a centralized device in star topology.

With the help of hub we can expand the network without breaking single network but as the

number of hubs are increasing than we get less speed.

Using hub as a centralized device we can easily managed the cabling problems.

Hubs do not read any of the data passing through them and are not aware of this source ordestination. But hub simply receives an incoming packet, possibly amplifies the electrical signal

and broadcasts this packet out to all devices on the network, including the one that originally sent

the packet.

A hub has a number of input lines that it joins electrically. Frames arriving on any of the line are

sent out to all other. If two frames are arrives at the same time, they will collide just as the co-axial cable. So the entire hub is formed a single collision domain. All the lines coming into a hub

must operate at the same speed.

Below fig shows the hub and its working.


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--- HUB AS CENTRAL CONTROL DEVICE--

There are many types of hubs are available in the market. These are ;

PASSIVE HUB:

- Also known as concentrator. Passive hubs simply pass the signal from one node to another. It

does not regenerate or amplify the electrical signal of incoming packets before broadcastingthem out to the network.

ACTIVE HUB:

- Active hubs are performs the amplification of regenerate electrical signal of the incoming

packets before broadcasting to other nodes. It simply acts like multiport repeater. But it cannot

regenerate the noise level.

INTELLIGENT HUB:

- It adds an extra feature in active hubs which are important for business application. An

intelligent hub is a stackable hub which is built in such a way that multiple units can be placedone top of the other to conserve space. In stackable hub, whole network is depends on all the

hubs instead of any single hub.


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STANDALONE HUB:

- Standalone hub is totally isolated from the entire network and is not directly connected withserver.

- The speed is less.

- Suitable for small computer network, in room or in buildings.

So this was all about hubs.

BRIDGES:

The network bridge provides an inexpensive and easy way to connect network segments. A

network segment is a single section of network media that connects computers.

Traditionally, if you want to have a network that has more than one segment, you have two

options: routing or bridging. However, to set up for IP routing you need either to buy hardwarerouters or set up the computers at the junctions between segments to operate as routers. IP

routing requires difficult configurations for IP addressing for each computer on each network

segment, and each network segment needs to be configured as a separate subnet.

The network bridge, in contrast, allows you to connect network segments by selecting theappropriate network connection icons and clicking Bridge Connections. Similar buttons allow

you to enable the bridge and add connections to it.

The network bridge manages your network segments, and creates a single IP subnet for the

entire network. There is no configuration required.


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The network bridge can create connections between different types of network media. NetworkBridge automates the configuration that is required in order to forward information from onetype of media to another. The network bridge is provide an easy and affordable way to connect

Bridge belongs to the data link layer of the OSI model. A bridge reads the outermost section of

data on the data packet, to tell where the message is going.

It reduces the traffic on other network segments, since it does not send all packets. Bridges can

be programmed to reject packets from particular networks.

Bridging occurs at the data link layer of the OSI model, which means the bridge cannot read IP

addresses, but only the outermost hardware address of the packet.

In our case the bridge can read the Ethernet data which gives the hardware address of the

destination address, not the IP address. Bridges forward all broadcast messages.

By using the bridge we can join two dissimilar or different transmission media. And we alsoconnect two different types of network architectures.

Only a special bridge called a translation bridge will allow two networks of differentarchitectures to be connected. Bridges do not normally allow connection of networks with

different architectures.

The hardware address is also called the MAC (media access control) address. To determine thenetwork segment a MAC address is necessary.

Bridges work on the media access sub layer of the data link layer of the OSI model. Because

they work on this layer, all information contains in higher level is unavailable to them. So they

dont distinguish between protocols.

If the protocols are different of the two networks than we cant connect them by bridge. So

protocol conversion is not done through bridges.

Bridge operates by sensing the MAC address of the transmitting nodes on the network and then

automatically generates an internal routing table. This table is used to determine which

connected segments to route packets and also provides the filtering capabilities.

If the bridges know that to which segment that packet is intended for? Then it simply broadcaststhat packet to whole network and at that time all the terminals on the network have to accept that

packet.


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If the destination address is at the same segment as the source address than bridge simply dropsthis packet.

Bridges also forwarded broadcasts packets to all the nodes except the originated one.

The bridge can filter out the data. Any bad packet is received than bridge is simply ignore thatpacket or sends the acknowledgement.

Bridge can handles the traffic problems very well.

The below fig shows the bridge and its architecture.

--- BRIDGE CONNECTING TWO LANs ---

CLASSIFICATION OF BRIDGES:

INTERNAL BRIDGE:

- It is the modification in the existing network system or the file server to transmit signals

between networks.

- Here file server itself be have as a bridge.

EXTERNAL BRIDGE:

- When a bridge is established on a separate system then is called an external bridge.

- A computer other than file server acts as a bridge.

TYPES OF BRIDGE:

LOCAL BRIDGE:

- It directly connects the local area networks

- So it is for restricted network which does not spanning geographical distances.

REMOTE BRIDGE:


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- can be used to create WAN

- Powerful tool for expanding and segmenting networks which spanning large geographical area

- Used in large networks.

WIRELESS BRIDGE:

- Can be used to connected LANs or connect remote stations to LAN.

- Provides an easy way to link buildings without using cables.

- The examples are flying LAN bridge.

- Depending on the conditions it can be up to 3 miles. Large range wireless bridges are alsoavailable which uses spread spectrum technology.

ADVANTAGES OF BRIDGE:

- Used to expand the network.

- We can join two dissimilar transmission media.

- Translation bridges will allow two different network architectures to connect together.

- Bridges can filter out the data.

- And can handle the traffic problems very well.

- Less expensive too.

- Faster because protocol conversion is not done.

-

DISADVANTAGES OF BRIDGES:

- Not an intelligent device.

- Protocol conversion is not done.


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GATEWAYS:

A gateway is a network element that acts as an entrance point to another network. For example

an access gateway is a gateway between telephony network and other network such as internet.

LANs may have component called gateways, which assists in transferring from one LAN toanother LAN.

A gateway is generally a work station or server. It is a two-way path between networks. It is

used to connect different types of networks.

Gateway is a work station by which we can make out connection between external network and

internal network.

Gateway belongs to transport layer and application layer of the OSI model.

Gateways also connect the two networks even if the protocols are different. So protocolconversion is also done by gateways. It simply stripped off the old protocol and assigns new

protocol to the packets, so it is also called protocol translator. Because it takes it time for

protocol conversion, it is little bit slower.

Gateways are the simple junctions between two network architectures. They repackage andconvert data going from one environment to another so that one environment can understand the

other environments data. So a gateway links two systems which have different communicationprotocols, different language and different architecture. So by that way gateways interconnects

the heterogeneous networks e.g. Microsoft windows NT server to SNA (IBMs system networkarchitecture)

Gateways never filter out the data. So it passes bad packets too.

Gateways are task specific. So they are dedicated to a particular type of transfer. They oftenreferred to by their particular task name e.g. Windows NT server to SNA gateway.

Basically there are two types of gateways (1) Transport gateway and (2) application gateway.The former is for transport layer and the later is for application layer.

Transport gateways connect two computers that use different connection oriented transport

protocol. It can copy the packets to one connection to other, reformatting them as need be.

Application gateways understand the format and contents of the data and translate messagesfrom one form to another e.g. an e-mail gateway could translate internet messages to an SMS

messages on mobile phone. So it identifies the actual meaning of data.

The below fig. shows the gateways and its architectures.


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--- GATEWAY CONNECTING TWO NETWORKS ---

ADVANTAGES OF GATEWAYS:

- Used to expand the network.

- Gateway is a server so it provides some security.

- We can connect two different types of networks.

- Protocol conversion is done.

- Effectively handles the traffic problems.

- And also establishes connections between internal network and external network.

-

DISADVANTAGES OF GATEWAY:

- Not an intelligent device. So noise prevention is not done.

- Never filter out the data

- Some what costly

- Protocol conversion is done so transmission rate is slower.

- Some what hard to handle.

SWITCHES:

Switching is a term in todays industry for any high speed data forwarding technology.

Switches can connect the computers so it is an interconnected device for network.


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Switches are similar to bridges in that both route on frame addresses. Many people use the termbridges and switches are interchangeably.

The main difference is that the switch is most useful to connect the individual computers instead

of whole network as shown in fig. below. As a consequence, when A wants to send a frame to

host B, the bridge gets the frame but just discarded. In contrast to switch, it must activelyforward the frame from A to B because there is no other way for the frame to get there.

--- SWITCH CONNECTING INDIVIDUAL NODES ---

Since each switch port is usually goes to a single computer, switches must have space for manymore line cards. Each line card provides buffer space for frames arriving on its port. So here each

port is its own collision domain. Thus switches never lose frames to collision. However if framescomes faster than they can be retransmitted. But faster approach of frames may run out the buffer

space and switch starts to discard frames.

The solution of this problem is cut-through switches.

Switch works on data link layer of the OSI model. Packets comes on switch, source and

destination address are specified and then make it out connection and then forward the packet to

destination.

Switch can useful for make internet connection and the transfer rate is very fast.

With the help of switches only the destination node will get the packet so this thing improves the

performance and transfer rate too.

The cost of the switch is some what high.

So these are all about inter-connected devices of the computer networks.


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