Gateways operate over the entire range in all seven layers of the OSI model. Internet routing devices have traditionaly been called gateways. A gateway is a protocol converter which connects two or more heterogeneous systems and translates among them. The gateway thus refers to a device that performs protocol and convert it to a packet formatted for another protocol before forwarding it. The gateway understands the protocol used by each network linked into the router and is therefore able to translate from one to another.



Example :
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1 person from usa calling his/her friend in the uk, both the person uses different phone operator service. So both working on the different protocol. When usa person speaks, the signal will be transmitted to the uk. uk phone operator service dont know the protocol formatting and all that.
At the mean time, both need gatway between them. So gatway converts the signals in each others formate. And two person can talk.

Routers operate in the physical, data link and network layers of the OSI model. The Internet is a combination of networks connected by routers. When a datagram goes from a source to a destination, it will probably pass thougth many routers untill it reaches the router atteched to the destination network. Routers determine the path a packet should take. Routers relay packets among multiple inter connected networks. In particular, an IP router forwards IP datagrams amont the networks to which it connects. A router uses the destination address on a datagram to choose a next-hop to which it forwards the datagram. A packet sent from a station on one network to a station on a neighbouring network goes easiest way to build the Internet is to connect two or more networks with a router. Routers provide connections to many different types of physical networks: Ethernet, token ring, point-to-point links, FDDI and so on.

Example :
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• The routing module receives an IP packet from the processing module. If the packet is to be forwarede, it should be passed to the routing module. It finds the IP address be sent. It then sends the packet with information to the fragmentation module. The fragmentation module consults the MTU table to find the maximum transfer unit (MTU) for the specific interface number.
• The routing table is used by the routing module to determine the next-hop address of network. The every router keeps a routing table that has one entry for each destination network. The every consists of the destination network IP address, the shortest distance to reach the destination in hop count, and the next router (next hop) to which the packet should be deliverd to reach its final destination. The hop count is the number of networks a packet enters to reach its final desination. A router should have a routing table to consult when a packet is ready to be forwarded. The routing table should specify the optimum path for the packet. The table can be either static or dynamic. A static table is one that is not changed frequently, but a dynamic table is one that is updated automatically when there is a change somewhere in the Internet. Today, the internet needs dynamic routing tables.
• A metric is a cost assigned for passing through a network. The total metric of a particular router is equal to the sum of the metrics of networks that comprise the route. A router chooses the route with the shortest (smallest value) metric. The metric assigned to each network depends on the type of protocol. The Routing Information Protocol (RIP) treats each network as one hop count. So if a packet passes through 10 networks to reach the destination, the total cost is 10 hop counts. The Open Shortest Path First Protocol (OSPF) allows the administrator to assign a cost for passing through a network based on the type of service required. A route through a network can have different metrics (costs). OSPF allows each router to have several routing tables based on the required type of service. The Border Gateway Protocol (BGP) defines the metric totally differently. The policy criterion in BGP is set by the administrator. The policy defines the paths that should be chosen.
  

Bridges operate in both the physical and the data link layers of the OSI model. A single bridge connects different types of networks together and promotes interconnectivity between networks together and promotes interconnectivily between networks. Bridges divide a large network into smaller segments. Unlike repeaters, bridges contain logic that allows them to keep separate the traffic for each segment. Bridges are smart enough to relay a frame towards the intended recipient so that traffic can be filtered. In fact, this filtering operation makes bridges useful for controlling congestion, isolating problem links and promoting security through this partitioning of traffic.

A bridge can access the physical addresses of all stations connected to it. When a frame enters a bridge, the bridge not only regenrates the signal but also checks the address belongs. When a bridge ecounters a packet, it reads the address contained in the frame and compares that address with a table of all the stations on both segments. When it finds a match, it discovers to which segment the station belongs and relays the packet to that segment only.


Example :
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When you want to connect the two or more MAN ( metropolytent Area Network), you need bridge. Bridge has decision making logic programs and circuit. It also regenerates the signals and make them strong as the repeater do.

A Repeaters is an electronic device that operates on the physical layer only of the OSI model. A repeater boosts the transmission signal from one segment and continues the signal to another segment. Thus, a repeater allows us to extend the physical length of a network. Signals that carry information can travel a limited distance within a network before degradation of the data integrity due to noise. A repeater receives the signal before attenuation, regenerates the original bit pattern and puts the restored copy back on to the link.

Repeater regenerates the weak signals and make them strong enough so that they can travel its path and reaches to the destination computer or hardware device. Repeater can not make decision, about where to send signals or data. Its work is make weak signals strong.

For example.
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If your cable length is 17 meter between two device. Now your signal strength is only 10 meter. If you want to transmit data between two device without repeater, the signal may become week and receiver will get wrong data. So you need to do is that use the Repeater beteen them.
You can use repeater at 10 meter and 7 meter from both the device. But my point of view you use the repeater at the 8.5 meter from the both device. That will give better perfomance.

A switched network consists of a series of interlinked switches. Switches are hardware/software devices capable of creating temporary connections between two or more devices to the switch but not to each other. Switching mechanisms are generally classified into three methods: circuit switching , packet switching and message switching.





1. Circuit Switching.
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It creates a direct physical connection between two devices such as telephones or computers. Once a connection is made between two systems, circuit switching creates a dedicated path between two end users. The end users can use the path for as long as they want.


2. Packet Switching.
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It is one way to provide a reasonable solution for data transmission. In a packet-switched network, data are transmitted in discrete units of variable-length blocks called packets. Each packet contains not only data, but also a header with control information. The packets are sent over the network node to node. At each node, the packet is stored briefly before being routed according to the information in its header.
In datagram approach to packet switching, each packet is treated independently of all others as though it exists alone. In the virtual circuit approach to packet switching, if a single route is chosen between sender and receiver at the beginning of the session, all packets travel one after another along that route. Although these two approaches seem the same, there exists a fundamental difference between them. In circuit switching, the path between the two end users consists of only one channel. In the virtual circuit, the line is not dedicated to two users. The line is divided into channels and each channel can use one of the channels in a link.


3. Message Switching.
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It is known as the store and forwarding method. In this approach, a computer (or a node) receives a message, stores it until the appropriate route is free, then sends it out. This method has now been phased out.