IPT - A Virtual Approach IPT A Virtual Approach by Peter Whitehouse
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Networks and Systems

Networks

Briefly, a Network is at least 2 computers (or associated pieces of technology) connected together for the purposes of sharing resources that might otherwise be under-utilized. Networks are also used for connecting people to other people for collaborative tasks.

Resources made available via a network may include:

  • Secondary Storage: hard disks, tape drives etc.
  • Attached Peripherals: printers, scanners, modems, CD-ROMS....
  • Administration Personnel: many networks allow access to diagnostic routines and procedures, help systems and operator controlled technical assistance.
  • Communications: most networks allow machine-machine communication, including e-mail, messaging, video links, teleconferencing etc.

Networks exist in the most unlikely places. In the era of the 'no computer' computer, we see networks extending to control the family car (the 1998 BMW had a 6 processor network controlling the many electronic features of the car - mechinics log on to the network to ask each of the systems how they are), Busses and Taxis, and even the fridge (a japanese invention sees the Refrigerator connected to the Internet, monitoring and ordering goods used).

Terminology

A Network physically comprises, usually, a collection of WORKSTATIONS, network cabling, may include a SERVER and attached DEVICES. Controlling the interaction of these components is a NETWORK OPERATING SYSTEM (a collection of specialised programs).

Each workstation usually has a NETWORK ADAPTER which manages signal propagation and signal reception for the workstation.

Client Server/Star network

CLIENT SERVER networks usually involve a main computer that acts as the main storage area for data, and usually stores much of the software used on the workstations. Workstations are connected to this server (via a number of different topologies), and under the supervision of the network operating system are able to access the resources stored there.

peer to peer network

PEER TO PEER networks usually involve each of the workstations cooperatively sharing data, peripherals and software, with no central server. Each peer shares responsibility for maintaining the network, and as such acts like both a workstation and a server in it's own right.

Client-server networks rely on the server's presence - should it crash, much of the resources are no longer available to the client workstations. In a Peer-to-Peer network, since no one workstation is centrally responsible, the loss of a workstation (due to a crash) results in only the loss of resources directly connected to it, the network usually is still able to function.

Media and Signalling

For successful network communication, you need a means of transporting signals (digital or analogue) between workstations, and some device that can propagate (or send and recieve) the signals.

Some networks use UNGUIDED media - broadcast TV, microwaves, infared and radio waves are considered unguided - the signals are BROADCAST in the general direction they need to travel (and may use atmospheric or other conditions to 'bounce' signals to roughly where they need to go).

GUIDED MEDIA networks include those that use UTP (Unshielded Twisted Pair copper wiring), Fibre Optic cables, coaxial cables etc. These are point to point connections, and are often more reliable than unguided media (but more expensive to install)

Signal propagation is usually handled by the NETWORK ADAPTER (or NIC = Network Interface Card).

Different networks use different TRANSPORT PROTOCOLS, with many relying on successful transport of PACKETS of data. A typical packet might contain some or all of the following information (although not necessarily in this order):

  • Header Bits (signalling the beginning of a packet)
  • Address information
    • Senders address
    • Receivers address
  • Integrity checking information (to allow the detection of errors in the packet caused by transport)
  • A Sequence number (so the receiving device can reconstruct the order of arriving packets)
  • The data (usually in manageble 'chumks', like 1024 bits)
  • Encryption information (If the data is sensitive, it may be sent encoded, and will need to be decoded on receipt)
  • End bits (to signal the end of a packet)

In large networks (like the telephone network), packets may not all travel the same route (ie. they may use different paths to get to the same place), so the sequence number can help the receiver re-assemble the data

ATTENUATION refers to deterioration of signal quality due to interferance or distance. Should the data need to travel over large distances, to prevent signal loss REPEATERS are used - these boost the signal strength making the transport distances potentially limitless. Nearly all network adapters have inbuilt repeaters to ensure the highest quality signal propagation.

Client-Server Networks typically use NETWORK HUBS to provide central points of connection to the server. The hubs usually have one fast connection to the server, with workstation network cabling connecting into the hubs.

Topology

The TOPOLOGY of a network refers to the LOGICAL layout of the Workstations (and server if there is one). Ther topology can often differ from the visible wiring pattern.

Each workstation on a network typically has a unique ID - this allows packets to be addressed to individual machines on the network. It is usually possible to send to individual workstations, as well as groups of workstations using these ID's.

A network

Star Topology

Client Server/Star network

In a STAR network, each workstation is connected to a common switch (say, a bank of hubs). When one workstation sends a message to another, the message is first sent to the common switch, and then routed from there to the appropriate destination workstation. The switch decides who recieves the packets of data depending on the addressing information attached to that packet.

Should one workstation crash, the rest of the network is largely unaffected, unless someone attempts to signal the 'missing' workstation or access resources attached to it. Clearly, if the Server or switch fails, the network also grinds to a halt.

It is possible for more than one message to be routed through a star network at one time - thus these styles of networks tend to be faster than those that follow.

Bus or Ring Topology

token ring network

Both the BUS and RING topologies are BROADCAST NETWORKS - that is, when one machine wishes to send packets to another machine, everyone gets a copy of the packets, and all but the TARGET machine discards them

Use of the network is often controlled by a scheme similar to a central DATABUS - ie. the workstations take turns of being allowed to access the network, when they have had their 'turn', they pass a TOKEN code to the next machine in line. Ownership of the token means it is your turn to use the network.

With RING topology, the network wiring forms a LOOP, so that signals travel around the loop in one particular direction, as does passing the token.

With BUS topology, the network is layed out in one long line. When signal generation occurs, signals are sent from the sending machine two ways - towards both ends of the network where they are absorbed.

Workstations using BUS topology listen to the network line until there is no traffic, then begin signalling. Occasionally, workstations will begin signalling at the same time - a collision or contention then occurs, is detected, and then each of the workstations wait a random time delay before trying again to signal.

Thin Ethernet is a common application of a BUS topology that uses a contention scheme


The Telephone Network - PTSN
(Public Telephone Switched Network)

The telephone network is one of the few trully ubiquitous networks (ie. it connects to nearly every place on the globe). Originally an analogue network (transmitting sound as waves of varying intensity), these days most of the telephone network is digital.

Your handset collects analogue information (your speech) and transmits it to your local exchange via UTP cabling (copper wires). At the exchange, analogue id converted to digital, wrapped up into packets (containing about 1024 bits of data each + control information) and then routed via FIBRE OPTIC CABLE to the destination exchange.

The paths that packets may take can be incredibly varied and scenic. At the destination exchange, the control information is stripped off the packets, they are then re-joined in the correct sequence, converted into analogue, and pumped to a handset through copper wire at the other end - emerging once again as sound.

The telephone network is incredibly flexible and robust - allowing the transmission of all sorts of data (from FAX streams, ATM transactions to primitive VIDEO CONFERENCING)

The PTSN has a maximum BANDWIDTH of about 5Mb/s

The Telephone Network - ISDN
(Integrated Services Digital Network)

The IDSN is a high speed DIGITAL service that avoids the need to convert analogue to digital and back again. Using dedicated phone lines, it is possible to plug digital devices directly into ISDN lines, which provide a much greater bandwidth (100Mb/s and beyond)

Whilst still in it's infancy, ISDN services will play an important part in the replacement of the PTSN, as user demand for more and more services reequiring highr and higher bandwidths emerge.


The Internet

The INTERNET is a collection of COMPUTER NETWORKS from AROUND THE WORLD connected together and able to exchange information of various types. This data exchange is possible because all connected computers on the 'Net use the same method of data transfer (TCP/IP - Transport Control Protocol/Internet Protocol) allowing relatively fast platform independent communications.

The Internet is everywhere and nowhere. It arose from a number of educational and military networks, as was modelled on a distributed computing model - no one computer on the Internet is centrally responsible for it. With so many computers and network paths for data to travel through, if one computer or network ceases to function, the data can be automatically routed via another path.

The internet is comprised of Mainframes, Mini computers, humble PC's and Mac's on people's desktops. It is largely uncensored and open, pervasive and growing. Built to use existing network infrastructure (the global telephone network), the protocols in place are sufficiently robust to ensure error free transmission across the planet in under 1/2 a second.

Originally only ACADEMICS and MILITARY used the Internet - they established a need to communicate between networks. Now, all sorts of people from Educators, Business People, Students, Politicians, and Librarians (Cybarians) use the 'Net

The Internet of today makes available a VAST collection of resources and services make the 'Net an attractive place to graze. More importantly, it provides a global medium for people to communicate with other people.

As a rich resource base, means of communication and recreational vehicle, cyberspace is a fun place to visit (so long as you don't live there) - the 'CDROM' in the sky is a poor analogy - as anyone can publish and share their ideas, the amount of content is growing at an unprecedented rate. The quality and veracity of that content is more of a problem.

How to Get Connected:

dialup net connection
A 'dial-up' connection

Most home users can buy time from an Internet Service Provider (ISP). ISP's own computers that are (usually) connected to other computers that have fast (high bandwidth) connections to the major Internet backbones that criss-cross most countries these days. The range of deals is wide, and the charging structures vary from ISP to ISP.

Some accounts are time/volume charged, some have small amounts of connection time per week, others are charged Annually, others Monthly

Terminal Connection

Shell accounts and other TEXT based services were the most comon form of Internet connection as recent as 6 years ago. Graphs were available, but had to be downloaded separately using a command line command.

PINE interface
E-mail using PINE

Electronic Mail (or E-Mail) remains one of the most used services available throught the Internet. E-Mail, like normal (Snail) mail involves people composing messages to other people, addressing them (an email address like pwhitehouse@optusnet.com.au follows a general pattern or username@hostname) and sending them off to find their own way through the network. E-Mail can be read, replied to, forwarded and redirected.

A relatively recent addition to the services available through the Internet is the World Wide Web. First prototyped by Tim Berners-Lee, the www uses a special protocol to transmit web pages (HTTP = Hypertext Transport Protocol) that can include text, pictures, links and other interactive media.

LYNX interface
World Wide Web browsing using LYNX

The World Wide Web is accessed through a Browser (reading this text book means you are already using a browser - either Netscape, Internet Explorer or one of the less common browsers). Originally, a browser could only view web pages. These days, most of the Internet's services can be acessed through your browser using plugins and helper programs

Internet Services

net services

The World Wide Web

Usually 1 Way Communication - you browse collections of grapically displayed resources (including Text, Pictures, Video, animations, Sound FX), navigating with the use of LINKS

Typically, WWW browsers are converging many services available on the 'Net into the one interface - they can be colorful, intuitive and fun to use.

Typically the WWW is navigated one of three ways:

  1. Browsing (not sure what you want nor where it is located) = wandering around, following interesting links until you arrive at a suitable destination
  2. Searching (know what you want but not where it is) = using search engines to locate resource
  3. Bookmarking (know what you want and where it is to be found) = jumping straight there with the aid of a bookmark placed during a previous visit

E-Mail

2 Way Communication 'delayed' where you send and recieve electronic mail, forward it, store it, reply to it and so on.

E-mail relies on knowledge of e-mail addresses, it is a less formal way of communicating with other people than say a physical letter to someone. E-Mail communication is effective if it is checked often and replied to as soon as possible.

FTP (file transfer protocol)

Usually 1 Way Communication - you LOG IN to remote computers and transfer files from there to you or from you to them.

You typically need to know the name (or IP address) of the computer you wish to get files from (dingo.cc.uq.oz.au or 130.12.128.43 for example), as well as the directory and file name of that which you want (e.g. /a/long/and/complicated/path/file.doc)

Gopher

1 Way Communication - you navigate MENUs searching for topics of interest, then either read or e-mail yourself the results of the search

Newsgroups (Usenet)

2 Way Communication 'delayed' - you browse posted E-Mail notes on world wide notice boards, post your own messages and reply as you see fit.

IRC (Internet Relay Chat)

2 Way Communication 'live'- you join discussion groups and chat online (via keyboard).

 

 

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