6.4
Further network fundamentals

Local Area Network - LAN
The connection is physical, such as a cable, and the network extends over a small geographical area, such as a single building or site.

Wide Area Network - WAN
The connection is via some sort of communication system, such as the telephone system or via satellite and usually extends over large distances.

(see this page for further details)

The internet itself might be described as a series of interconnected WAN"s or one huge WAN depending on your point of view. Information you find on web pages on the internet are from different types of organisation or individuals - information providers .

Much, much more in:

Stallings, Data and Computer Communications, Prentice Hall, ISBN 0130863882, p472

The term datagram is often used synonymously with packets .

There is a detailed description of an IP Datagram at:

http://www.tcpipguide.com/

The datagrams themselves may even be split on their journey - for example if one of the nodes can't handle packets of a given size.

You don't need this much detail but it is probably useful to have the idea that computer communications are not simple.

Also we want to avoid confusion between these OSI layers and the "layered access" model that the Computer Science Guide uses in 6.4.5 - discussed in the next section.

You couldn't steal a copy of the crown jewels from the Tower of London if you were located in New York.

Spyware usually attempts to collect information on other users of the system, such as what websites they visit, what files they use, which adverts they view.

on this page: [ terminology | communications | packet switching | security ]

Providers, servers and clients

The simplest form of computer system is a stand-alone pc. In order to share information, communicate and collaborate, a pc can be connected to another computer, or a network of computers.

Once connected to other computers we need terminology to distinguish the different types of computers and tio describe their roles on the network.

Earlier we distinguished between LAN's and WAN"s. In the simple case of a LAN, 2 or more computers may be connected together.

One or more of the connected computers may be designated a special task, such as holding data files for users, running the network operating system or handling print queues.

These computers are known as servers . The other pc's or workstatins that are connected and make use of these servers are the clients .

Servers are usually maintained and operated by specialist staff.

One advantage of putting files or services on a server is that they can usually be accessed from any connected workstation. So users don't always have to use the same computer workstation giving them increased flexibility.

Typically, a workstation or stand alone pc has access to the internet. At home this might be via a dedicated modem, cable modem or even a satellite dish. On a network at school or work, a communications server - probably a gateway - provides an internet connection.

A gateway is a computer that connects different types of network together; it handles any neccessary format/protocol conversions required.

As an individual, you typically use an ISP or Internet Service Provider to look after the technology for you and provide you with the software to upload your web pages or handle your email.

A large organisation might be effectively its own ISP, operating a WAN for the benefit of its employees, members or customers - sometimes known as an intranet (typically uses equipment wholly owned by the organisation) or VPN (Virtual Private Network - uses some part of the internet to carry traffic - which is probably encrypted ).

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Communications in networks

Ethernet is the most common type of LAN; it is a broadcast network - only 1 station transmits at a time and all stations receive the broadcast.

Because the medium is shared by all stations, collisions (two stations decide to trasmit at the same time) have to be detected and dealt with. Carrier sense is used to detect of one station is already transmitting so the technology is known as CMSA/CD (Carrier Sense Mutliple Access/Collision Detection).

1. If the medium is idle - transmit
2. If it is busy listen until idle then transmit immediately
3. If a collision is detected transmit brief jamming signal to indicate collision.
4. Wait a random time and repeat from step 1.

POTS - the Plain Old Telephone System uses copper wire (increasingly supplemented by fibre optic cable) to transmit a range of signal types. Telephone lines can be rented or leased - usually shared with other users, or for a fee, a dedicated line can be leased for private use.

While POTS typically (used to) use analogue data transmission techniques today's phones are almost always digital. Two common digital technology standards are ISDN and ADSL.

A modem ( mo dulator/ dem odulator) converts a computer's digital data (stream of bits) into analogue (continuous wave) data which can then be transmitted over the telephone line.

Speeds are measured in bps (bits per second ) and the current limit is around 56K .

Other forms of connection using the telephone system are ISDN (International Standard Digital Network with transfer rates of up to 128K) and ADSL (Asynchronous Digital Subscriber Line) with incoming rates of about 2.5 Mb and outgoing rates of 512K). None of these rates are achieved in practice. You do not need to know the technical details of these systems for your IB examinations.

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Packet Switching

The telephone system uses a technique known as circuit switching; a direct circuit is established between callers. However, this would be too slow for today's broadband requirements and does not allow for a range of hardware to be utlised.

The alternative to this is called packet switching and the packets can be sent via the same route (a virtual circuit ) or by different routes ( datagrams ).

In the TCP/IP protocol, the most commonly used, a stream of data to be transferred is first split into datagrams/ packets which are numbered so as the correct sequence can be re-assembled at the destination.

Each packet may travel by a different route (this makes the technique more robust in case of network node failures) and contain a " hop count " more formally known as a Time To Live (TTL) which is decremented every time the packet is forwarded . If this count reaches zero the packet is discarded . When the receiver finds out it is missing it will send a message back to the sender.

The OSI model includes 7 different layers that fall into 3 main groups:

When you use a communications application like a browser, requests from your computer are initially handled by the application layer software and then by successive layers down through your operating system until they are converted into digital signals to be sent by the hardware .

Each layer has its own associated set of protocols.

Network Security

Networks need to be secured to protect data from corruption and also because some data may be confidential .

In the physical environment this is achieved by locks , for example. However, because networks allow remote access to data and a copy of the data can be removed without trace, the problems of network security are different.

In fact, in many systems, all the actions of a particular user can be traced, logged and accounted for. In Clifford Stoll's book The Cuckoo's Egg , a hacker was caught because of a tiny discrepancy in the accounting software which tracked and billed for each user's activity on the system.

User's can be put into groups and each group given a different set of privileges to access certain areas or to use certain items of hardware. Users can set permissions on the files they create which details who can read, alter or delete them.

For people who access the system from outside the organisation's own network, via a web interface for example, extra checks are made.

There is usually a firewall in place - software which identifies all attempts to access the system from the internet and filters out unwanted users. These " users " may be applications which automatically scan for open systems so they can send on viruses or spyware programs.

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related: [ Topic 6 home | previous: operating systems | next: peripherals ]

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