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Hardware of computer Network


Network Interface Card (NIC)
 
A network card (network adapter, network interface card, NIC, Ethernet adapter etc.) is a piece of computer hardware designed to allow computers to communicate over a computer network. It is an OSI model layer 2 item.

Every network card in the world has a unique 48-bit serial number called a MAC address, which is written to ROM carried on the card. Every computer on a network must have a card with a unique MAC address. 

The IEEE is responsible for assigning MAC addresses to the vendors of network interface cards, which means that two cards sharing the same MAC address is impossible.

A network card typically has a twisted pair and BNC sockets where the network cable is connected, and a few LEDs to inform the user of whether the network is active, and whether or not there is data being transmitted on it. 

The Network Cards are typically available in 10/100/1000 Mbit/s. This means they can support a transfer rate of 10 or 100 or 1000 Mbit/s.

Network Cables

Cable is the medium through which information usually moves from one network device to another. There are several types of cable which are commonly used with LANs. In some cases, a network will utilize only one type of cable, other networks will use a variety of cable types. The type of cable chosen for a network is related to the network's topology, protocol, and size.

The types of cables used in networks.
  • Twisted Pair
  • Coaxial cables
  • Fiber Optic Cable
Twisted Pair: A thin-diameter wire commonly used for telephone and network cabling. The wires are twisted around each other to minimize interference from other twisted pairs in the cable. Twisted pairs have less bandwidth than coaxial cable or optical fiber. 

Coaxial cables: Coaxial cables are widely used for transmitting voice, video and data over LANs.

There are two types of cable available based on the thickness of core
  • Thinnet
  • Thiknet
Thinnet: Thinnet is a thin and flexible coaxial cable, which has a thickness of about one-fourth of an inch. Since this type of coaxial cable is flexible, it is easy to work with thinnet cables. They can be used for any kind of installation. Another benefit of using thinnet coaxial cable is the low cable cost. Thinnet cable is inexpensive as compared to thicknet cable.

Thiknet: The other type of coaxial cable, the thicknet cable, is a thicker cable. It is about half an inch in diameter. The copper core is thicker than a thinnet core. Since the thicknet cable has a thicker copper core, it can carry a signal out of than a thinnet cable. It can carry a signal for 500 meters (1,640 feet).

Fiber Optic Cable

A thin glass strand designed for light transmission. A single hair-thin fiber is capable of transmitting trillions of bits per second. In addition to their huge transmission capacity, optical fibers offer many advantages over electricity and copper wire. Light pulses are not affected by random radiation in the environment, and their error rate is significantly lower. 

Fibers allow longer distances to be spanned before the signal has to be regenerated by expensive "repeaters." Fibers are more secure, because taps in the line can be detected, and lastly, fiber installation is streamlined due to their dramatically lower weight and smaller size compared to copper cables. 

There are two primary types of fiber. For intercity cabling and highest speed, singlemode fiber with a core diameter of less than 10 microns is used. Multimode fiber is very common for short distances and has a core diameter from 50 to 100 microns.

Switches

A switch is fundamentally similar to a bridge , but usually supports a larger number of connected LAN segments and has a richer management capability.


A network switch is a computer networking device that connects network segments. It uses the logic of a Network bridge but allows a physical and logical star topology.

It is often used to replace network hubs. A switch is also often referred to as an intelligent hub or switching hub.

As a frame comes into a switch, the switch saves the originating MAC address and the originating port in the switch's MAC address table. The switch then selectively transmits the frame from specific ports based on the frame's destination MAC address and previous entries in the MAC address table. 
  • If the destination MAC address is unknown a broadcast address or (for simpler switches) a multicast address the switch simply transmits the frame out of all of the connected interfaces except the incoming port.
  • If the destination MAC address is known, the frame is forwarded only to the corresponding port in the MAC address table.
  • If the destination port is the same as the originating port, the frame is filtered out and not forwarded.
Routers

A router is an Intermediate System (IS) which operates at the network layer of the OSI reference model. Routers may be used to connect two or more IP networks , or an IP network to an internet connection. 

A router consists of a computer with at least two network interface cards + supporting the IP protocol . The router receives packets from each interface via a network interface and forwards the received packets to an appropriate output network interface. Received packets have all link layer protocol headers removed, and transmitted packets have a new link protocol header added prior to transmission.
The router uses the information held in the network layer header (i.e. IP header) to decide whether to forward each received packet, and which network interface to use to send the packet. Most packets are forwarded based on the packet's IP destination address , along with routing information held within the router in a routing table. Before a packet is forwarded, the processor checks the Maximum Transfer Unit (MTU) of the specified interface.

A router forwards packets from one IP network to another IP network. Like other systems, it determines the IP network from the logical AND of an IP address with the associated sub network address mask.

Routers are often used to connect together networks which use different types of links (for instance an HDLC link connecting a WAN to a local Ethernet LAN ). The optimum (and maximum) packet lengths (i.e. the maximum transmission unit (MTU) ) is different for different types of network. A router may therefore uses IP to provide segmentation of packets into a suitable size for transmission on a network.

Repeaters and hubs

A repeater is an electronic device that receives a signal, cleans it of unnecessary noise, regenerates it, and retransmits it at a higher power level, or to the other side of an obstruction, so that the signal can cover longer distances without degradation. In most twisted pair Ethernet configurations, repeaters are required for cable that runs longer than 100 meters. 

A repeater with multiple ports is known as a hub. Repeaters work on the Physical Layer of the OSI model. Repeaters require a small amount of time to regenerate the signal. This can cause a propagation delay which can affect network communication when there are several repeaters in a row. Many network architectures limit the number of repeaters that can be used in a row (e.g. Ethernet's 5-4-3 rule).

Today, repeaters and hubs have been made mostly obsolete by switches.

Bridges

A network bridge connects multiple network segments at the data link layer (layer 2) of the OSI model. Bridges broadcast to all ports except the port on which the broadcast was received. However, bridges do not promiscuously copy traffic to all ports, as hubs do, but learn which MAC addresses are reachable through specific ports. Once the bridge associates a port and an address, it will send traffic for that address to that port only.

Bridges learn the association of ports and addresses by examining the source address of frames that it sees on various ports. Once a frame arrives through a port, its source address is stored and the bridge assumes that MAC address is associated with that port. The first time that a previously unknown destination address is seen, the bridge will forward the frame to all ports other than the one on which the frame arrived.

Bridges come in three basic types:
  • Local bridges: Directly connect LANs
  • Remote bridges: Can be used to create a wide area network (WAN) link between LANs. Remote bridges, where the connecting link is slower than the end networks, largely have been replaced with routers.
  • Wireless bridges: Can be used to join LANs or connect remote stations to LANs.

Firewall

A firewall protects networked computers from intentional hostile intrusion that could compromise confidentiality or result in data corruption or denial of service. 

It may be a hardware device or a software program running on a secure host computer. In either case, it must have at least two network interfaces, one for the network it is intended to protect, and one for the network it is exposed to. 

Hardware Firewall : Hardware firewall providing protection to a Local Network  

Firewall Software : Computer running firewall software to provide protection.

Firewalls can filter packets based on their source and destination addresses and port numbers. This is known as address filtering. Firewalls can also filter specific types of network traffic. This is also known as protocol filtering because the decision to forward or reject traffic is dependant upon the protocol used, for example HTTP, ftp or telnet. Firewalls can also filter traffic by packet attribute or state. 

A firewall cannot prevent individual users with modems from dialling into or out of the network, bypassing the firewall altogether. Employee misconduct or carelessness cannot be controlled by firewalls. Policies involving the use and misuse of passwords and user accounts must be strictly enforced. These are management issues that should be raised during the planning of any security policy but that cannot be solved with firewalls alone


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