Initially mainframe computers were employed at the centre of a network. Today networks are characterized
by decentralization and downsizing facilitated by the increased performance of PCs and the
decrease in the price of workstations.
It is common for an individual computer with its periphery, e.g. printer, to facilitate
data processing, and, only when there is insufficient capacity, for it to have access to the resources
of a special system via a network. Mainframe computers are employed only when required.
Independent of mainframe computers networks are independent of manufacturer i.e. different brands
of computer can communicate with one another in a network.
In addition when part of a decentralized network is down the the rest
of the network can continue in operation.
A network simplifies access to programmes, data, and information for different platforms and computer systems
All computers in a network are connected with one another via cable or satellite. For a computer to function in a network it requires a network board connected by cable, a driver with which the network board establishes a connection to other computers, and communication software. Apart from the physical connection i.e. cable connection, before a computer can establish a connection with another computer an agreement is required as to how information is to be sent and received. This agreement is called Network Protocol e.g. TCP/IP (Transmission Control Protocol/Internet Protocol) for the Internet, IPX (Internet Protocol Exchange) for Novell Netware Network, Appletalk or Localtalk for the Apple Networks. These protocols regulate the building of a connection, supervise correct data transfer, and make for an orderly disconnection. A communication between two computers consists of a multitude of packages sent back and forth between the two.
Computer networks are computers that are physically connected to one another and which communicate with one another via protocols.
Networks are differentiated by size:
1. LAN: Local Area Networks are small, local networks e.g. a firm or university institute.
Diagram 1 LAN : Local Area Network
2. MAN: Metropolitan Area Networks are a combination of several LANs.
3. WAN: Wide Area Networks are very large or world wide networks e.g. the Internet.
It is possible to number computers consecutively within a LAN. This is not possible within a MAN or WAN as the location and jurisdiction of a computer must be known. This is why the allocation of an Internet Address is decentralized i.e. number strings are allocated to institutions which may then add computer addresses independently to this number string. These number strings allocated to institutions are called Domains, or for the sake of simplicity networks.
Computers are also assigned names as remembering numbers is difficult. This name consists of
a computer name and domain name, and must be unique, as is the number.
Unfortunately domain address lists rarely exist, and a complete worldwide "telephone book"
does not exist.
Individual computers within individual networks are unique and likewise their name and/or address is also unique.
Diagram 2 Heidelberg Hamburg
LAN 2 LAN 3
LAN 1 LAN 4
It would be undesirable for a package to be sent round the world when simply one computer wished to communicate with another in a neighbouring institute. If this were the case the network would very quickly breakdown due to overloading. A Router prevents this by functioning as a connecting agent. It sends the package in the direction of the target computer. The Router reads the address of the target computer contained in the package and decides, with reference to its routing table, where to send the package.
All LANs and sub-networks are connected to Routers and have their own domain addresses.
Routers are additionally connected to other Routers.
See diagram 2.
Router 2 knows to which sub-network it belongs and the address of its neighbouring Router,
(Router 3). Router 2 generates a routing table which it sends to Router 3, and vice versa. The
routing tables of the respective Routers consist of their own routing table plus that of the
neighbouring Router. When a package is sent from LAN 3 to LAN 4, Router 3 knows from its routing
table that this network is connected to Router 2 and sends it there.
When a package is sent from LAN 3 to LAN 1, Router 3 quickly registers that LAN 1 is not listed
in its routing table. However, it has its "gateway of last resort". This is the Router to which
it sends all packages it does not know where to send i.e. Router 2.
The "gateway of last resort" for Router 3 is Router 2, and so the package for LAN 1 is sent to
Router 2. Router 2 knows either that Router 1 is connected to LAN 1, or that Router 2 has a
"gateway of last resort" to which it will send the package destined for LAN 1. Finally Router 1
sends the package to LAN 1 and the target computer collects the package.
Routers or Gateways are connecting agents that direct packages in the correct direction.
The packages that computers exchange with one another are electrical signals that consist of
"ones" and "naughts", comparable with Morse code. Hubs are responsible for maintaining the
package in a stable electrical state.
The electrical voltage of a signal determines whether it is a "one" or a "naught". Without an
amplifier a signal flowing over the network would become weaker. Hubs are amplifiers that
maintain the original voltage.
They additionally control whether the transmission
instructions are maintained. e.g. if the stipulated interval between two packages is
maintained. This transmission protocol is determined by network topology (Star, Bus, Ring,
Tree) e.g. Token Ring networks have a ring structure; Ethernet networks have a bus or star
structure. This transmission protocol is independent of network protocol i.e. Novell Netware
operates equally on Token Ring as on Ethernet Bus.
A computer or Segment (several computers connected by cable to a Hub), that does not maintain
the transmission protocol is switched out until it maintains it again.
A Hub connects several computers and transmits a package, in a stable electrical state, from a sender to all addressees.
Diagram 3.
A Bridge registers which computer is attached to which of its Ports and only sends a package to another Segment when the target computer is to be found there. It keeps a routing table of all connected computers and searches this table for every package received. Unlike the Hub which sends packages to all computers in the network, the Bridge selectively forwards packages to the targeted computer. And unlike the Router which administers the entire network, each network containing numerous computers, the Bridge is practical only for a limited number of computers, otherwise the search through its routing table would be too time consuming.
Bridges connect Segments of a LAN.
The sender must know the address of the addressee to send a package.
With particular Network Protocol the target computer must
announce its services or existence before it is possible to establish contact.
The Novell Netware system for example is a Client-Server system in which the Client (PC)
must first contact a Server (a larger PC) when it wishes to use the network. The nearest
Server registers itself and initially relates its name and address. At this point the
Client and Server know each others address and that they both exist. They can now build
a connection. This established, the Server offers the Client services
such as the use of printers, programmes, or memory, and the Client is able to make use
of them.
Within the Apple Network each computer can be simultaneously a Server and a Client. A
computer exists only after it has announced to the network its name and what services
(programmes, memory, etc.) it has at its disposal.
Apple Network and Novell Netware network are also called Broadcast Networks.
Within a Broadcast Network a computer may only be communicated with when it announces its existence at regular intervals.
The first services available were the transfer of files and mailing. Today the
Internet offers a wide range of services. One can make literature enquiries via
the OPAC Server (Online Public Access Catalogue), one can download Share Ware
(inexpensive programmes), or Public Domain programmes (free programmes) from
FTP Servers. One can carry on discussions with others, or ask questions of experts
on every subject immaginable via the News Server (Usenet). One can surf the Internet
with the WWW (World Wide Web). The WWW is a connection of servers. Each WWW site
consists of graphically prepared pages which contain Links on which one may
click and acquire the respective page. These pages can be located anywhere in
the world. This process is called "surfing the Internet". All imaginable information
is to be found in the WWW from academic publications to the local cinema programme.
Whereas originally, principally universities and research institutes were connected
to the Internet, today there are increasingly more commercial users and suppliers.
The Internet is a network for the exchange of all imaginable information.