Page 126 - DCAP516_COMPUTER_SECURITY

Computer Security

Notes 10.5.2 Metropolitan Area Networks

MAN is a bigger version of a LAN and uses similar technology. It uses one or two cables but
does not contain switching elements. It covers an entire city and may be related to the local cable
TV network.
A MAN standard - DQDB (Distributed Queue Dual Bus) IEEE 802.6.
(a) Two unidirectional buses.
(b) Each bus has a head-end, which initiates transmission activity.
(c) Traffic to the right uses the upper bus.
(d) Traffic to the left uses the lower bus.

10.5.3 Wide Area Networks

A WAN spans a large area, often a country or continent. A WAN consists of two parts:
(a) Application part: Machines for running user programs are called hosts.

(b) Communication part: The hosts are connected by the communication subnet, or just subnet,
whose job is to carry messages from host to host.
The subnet consists of two components:

Transmission lines (circuits, channels or trunks) move bits between machines.
Switching elements (routers) are specialized computers used to connect two or more
transmission lines.

Main Characters

(i) A WAM contains numerous cables or telephone lines, each one connecting a pair of routers.
(ii) For those without direct connection, communication takes place indirectly via other routers.
(iii) When a message (a packet) is sent from one router to another, it is received at each
intermediate router in its entirety, stored there until the required output line is free, and
then forwarded.

Did u know? A subnet using this principle is called point-to-point, store-and-forward or
packet-switched subnet.
WANs may also use broadcast channels, such as satellites or ground radio systems.

10.5.4 Wireless Networks

Mobile computers, such notebook computers and personal digital assistants (PDAs), are the
fastest-growing segment of the computer industry.

Applications using wireless networks:
(a) Portable offices which allow people to send and receive phone calls, faxes and emails, to
read remote files or login remote machines, etc., and to do this from land, sea or air.
(b) Of great value to fleets of trucks, taxis and repair-persons for keeping in contact with
home.
(c) Important to rescue workers at disaster sites and to the military.

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