Write short notes on the following:

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

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

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

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  Packet switching combines the advantages of
message and circuit switching.

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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.

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

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.

·        
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.

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