• Network is a connection of two mor more devices that can exchange information by transmitting information from one device (or node) to another device (or node).The device can be a Personal Computer,Laptop,Printer.For exchanging of information between two devices we need what is called as Medium Access Control(MAC) Address so that the devices can recognize itself as to which device its sending information to.The connection between two devices may involve

1. Point to Point link-Refers to link that connects only one and one device to another device.

2. Multipoint Link-Refers to link that connects one to many devices.

    

PHYSICAL LAYER DEVICES:

Passive Hubs-

• A passive hub is just a connector. It connects the wires coming from different branches.In a star-topology Ethernet Local Area Network (LAN), a passive hub is just a point where the signals coming from different stations collide; the hub is the collision point.

• This type of a hub is part of the media, its location in the Open System Interconnection (OSI) model is below the physical layer.

   

Repeaters: 

• A repeater is a device that acts similar to an amplifier.A repeater amplifies a weak signal to a strong signal.This action is similar to an amplifier.An amplifier amplifies a signal applied to it.
• A repeater is a device that operates only in the physical layer-Physical layer deals with devices that are hardware specific. Signals that carry information within a network can travel a certain distance before getting degraded or lossy due to loss of power in signal or due to interference of nosie.

• A repeater receives a signal and, before it becomes too weak or corrupted, regenerates the original bit pattern. The repeater then sends the refreshed signal. A repeater can extend the physical length of a Local Area Network(LAN).A repeater connects two segments of the same LAN.The segments connected are still part of one single LAN.

   

Active Hubs:

• An active device is one which can run by use of an battery,so is an active hub.It needs power to run.An active hub is actually a partly a repeater or(amplifier). Its normally used for connections between central hub and other devices (stations) in a star topology.

• When the central hub is gone then data transmitting becomes difficult in star topology but fault isolation is an easy process.

   

PHYSICAL AND DATALINK LAYER DEVICES-THESE DEVICES ARE ALSO CALLED TWO LAYER DEVICES AS THEY OPERATE IN TWO LAYERS:

Bridges: 

• A bridge is a device that connects two switches.Switches connect two nodes.A bridge operates in physical and the data link layer. At physical layer , it acts as regenerator device and regenerates or amplifies the signal given to it. At data link , the bridge can check the physical Media Access Control (MAC) addresses in the frame.A bridge can also act as a filter.
• 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 frame must be forwarded through a specific port,the port number is contained in table of bridge which is similar to a routing table in a router.

Switches:

• Switches are devices that connect to nodes.Nodes may be any device like a personal computer,printer. The Switch operates in data link layer and is responsible to connect routers to PC's.It diverts traffic from one network segment to another network segment.

   

PHYSICAL ,DATALINK AND NETWORK LAYER DEVICES-THESE DEVICES ARE ALSO CALLED THREE LAYER DEVICES AS THEY OPERATE IN THREE LAYERS:

Routers:

• A router is a device that operates at network layer and is responsible for forwarding packets from source to destination via intermediate nodes which are called hubs.

   

TRANSPORT,SESSION,PRESENTATION AND APPLICATION LAYER DEVICES:

Gateways:

• A gateway is a device operating at layers transport, session, presentation and application, and is used to interface two different devices running in different protocols,for example to interface a device running in File Transfer Protocol (FTP) and and Device running using AppleTalk protocol.

Repeaters

A repeater is a device that operates only in the physical layer. Signals that carry information within a network can travel a fixed distance before attenuation endangers the integrity of the data. A repeater receives a signal and, before it becomes too weak or corrupted, regenerates the original bit pattern. The repeater then sends the refreshed signal. A repeater can extend the physical length of a LAN.

A repeater does not actually connect two LANs; it connects two segments of the same LAN. The segments connected are still part of one single LAN. A repeater is not a device that can connect two LANs of different protocols.

A repeater connects segments of a LAN.

A repeater can overcome the 10Base5 Ethernet length restriction. In this standard, the length of the cable is limited to 500 m. To extend this length, we divide the cable into segments and install repeaters between segments. Note that the whole network is still considered one LAN, but the portions of the network separated by repeaters are called segments. The repeater acts as a two-port node but operates only in the physical layer. When it receives a frame from any of the ports, it regenerates and forwards it to the other port.

A repeater forwards every frame; it has no filtering capability.

It is tempting to compare a repeater to an amplifier, but the comparison is inaccurate. An amplifier cannot discriminate between the intended signal and noise; it amplifies equally everything fed into it. A repeater does not amplify the signal; it regenerates the signal. When it receives a weakened or corrupted signal, it creates a copy, bit for bit, at the original strength.

A repeater is a regenerator, not an amplifier.

The location of a repeater on a link is vital. A repeater must be placed so that a signal reaches it before any noise changes the meaning of any of its bits. A little noise can alter the precision of a bit's voltage without destroying its identity. If the corrupted bit travels much farther, however, accumulated noise can change its meaning completely. At that point, the original voltage is not recoverable, and the error needs to be corrected. A repeater placed on the line before the legibility of the signal becomes lost can still read the signal well enough to determine the intended voltages and replicate them in their original form.

Bridges

A bridge operates in both the physical and the data link layer. As a physical layer device, it regenerates the signal it receives. As a data link layer device, the bridge can check the physical (MAC) addresses (source and destination) contained in the frame.

A bridge has the filtering capability. 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, the decision must specify the port. A bridge has a table that maps addresses to ports.

A bridge has a table used in filtering decisions.

A bridge does not change the physical (MAC) addresses in a frame.

Transparent Bridges

A transparent bridge is a bridge in which the stations are completely unaware of the bridge's existence. If a bridge is added or deleted from the system, reconfiguration of the stations is unnecessary. According to the IEEE 802.1 d specification, a system equipped with transparent bridges must meet three criteria:

1. Frames must be forwarded from one station to another.

2. The forwarding table is automatically made by learning frame movements in the network.

3. Loops in the system must be prevented.

Switches

Switches are devices that connect to nodes. Nodes may be any device like a personal computer, printer. The Switch operates in the data link layer and is responsible to connect routers to PCs.It diverts traffic from one network segment to another network segment.

Routers

A router is a three-layer device that routes packets based on their logical addresses (host-to-host addressing). A router normally connects LANs and WANs on the Internet and has a routing table that is used for making decisions about the route. The routing tables are normally dynamic and are updated using routing protocols.

A three-layer switch is a router, but faster and more sophisticated. The switching fabric in a three-layer switch allows faster table lookup and forwarding. In this book, we use the terms router and three-layer switch interchangeably.

Gateway

Although some textbooks use the terms gateway and router interchangeably, most ofthe literature distinguishes between the two. A gateway is normally a computer that operates in all five layers of the Internet or seven layers of OSI model. A gateway takes an application message, reads it, and interprets it. This means that it can be used as a connecting device between two internetworks that use different models. For example, a network designed to use the OSI model can be connected to another network using the Internet model. The gateway connecting the two systems can take a frame as it arrives from the first system, move it up to the OSI application layer, and remove the message.