Write short notes on the following:

(a) Fire Wall

It is the first line of defense for any computer system or network. All packets that enter the network should come though this point. A modern firewall is a system of applications and hardware working together. A sophisticated firewall performs a combination of packet filtering  network address translation and proxy services. FIrewalls have two general methods of implementing security for a network .Firewall have two general methods of implementing security for a network and are explained below

1 Packet filtering:

Packet filering were designed to look at header information of the packet. packet filtering, the first type of firewall used by many organizatin to protect their network. The general method of implementing a packet filter was to use a router

2 proxy servers: 

Proxy servers use software to imtercept network traffic that is destined for  a given application. The proxy server, recognizes the request and behalf of the client makes the request to external server. Instead, the proxy functions as middleman and speaks to both the client and server

(B) packet Switching:

·           Packet switching combines the advantages of message and circuit switching.

·         Packet switching is functionally similar to message switching, in which data is transmitted in block, stored by the first switching node it meets in the network and is forwarded to the next and subsequent downstream nodes until it reaches the destination.

·         The message is fragmented in small fixed size packets in packet switching network. Typical lengh of packets is between 1 kilobyers to 4 kilobytes

·         In virtual circuit approach, a fixed logical path though the network from the sender to the receiver is established befor any packets are sent. This path remains unchanged for the duration of the sesseion.

(c) Unguided  Transmission media

which does not guide along a physical path e.g. propagation through air, water or vacuum.

Twisted pair cable

Physical description:

·         consist of two insulated copper wires arranged in a regular spiral pattern, acts as a single communication link

·         bundled together into a cable by wrapping them in tough protective sheath.

·         The twisting tends to decrease the cross- talk interfarenes between adjacent pair in a cable. the twist lengh varies from 2 to6 per inch.

·         Thickness is 0.016 inch to 0.036 inch.

Application

·         Used for both analog and digital signals.

·         Used in telephone network.

·         Used in local area networks.

·         It is widely used due to easy handling and teast expensive.

(d) Multi threading

Multitherading is a popular technology that is used to improve application performace throgh parallel execution of multiple threads. A thread is alight weight process