• 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.