SYSTEM ARCHITECTURE

This page allows you (in minutes) to become familiar (as a user) with what are the main elements in your PC and the Internet as a whole. Refer to the picture below, which is a very much simplified overview of the internet system in relation to a PC. It shows your PC being connected by your Internet Service Provider (ISP) with the main parts of the worldwide web (www) in both directions, via a modem (short for modulator / de-modulator) for a dial-up connection (see Ref 23 for details when you want them) or a cable modem (see Ref 24 for details when you want them) for a continuous broadband connection. A modem provides the necessary interface between the signals whizzing around in those coaxial, fibre-optic or old style telephone cables and the cable connected to the port on your PC (COM, USB or Ethernet). The ISP, eg your telephone company, cable TV supplier, AOL or Freeserve, etc, connects your PC to where you tell it to go to on the www very much like your telephone company provides its telephony service really. Besides being much more complex (and not fully controlled!), the main difference is that your ISP doesn’t have a Directory of everyone’s internet addresses; that’s where the Search Engines and Search Directories come in, one of the biggest being Yahoo for example. They register and trawl the internet for websites allowing us to then perform searches in their resulting indexes and directories for what we want. At this point, it’s worth mentioning three more important things:

E-mail on your desktop, eg using MS Outlook Express, is enabled via a POP3 server provided by your ISP, but separate ‘internet e-mail’ facilities are also made available (via your browser) by all ISPs and Search Directories such as Lycos.

Nowadays it’s essential that internet security is in place on your PC, including good virus checking software and a properly configured (software and/or hardware) firewall, otherwise your PC will most surely be invaded by viruss, trojans and hackers on a large scale. The invaders (mafia trying to make money out of you of course, but also all the sad bed-wetting geeks who can’t ‘get it up’ and have to get their thrills in some other way) are numerous and worldwide. The mob elements are international, organised and very clever. To put it into perspective, for whatever reason my software firewall has blocked several trojan horse and other intrusion / hacking attempts every day and I recently had to re-format / re-install the system on a friend’s unprotected PC after one of the afforementioned geeks had trashed it (it’s now protected before you ask). Now that I’m using the BT (British Telecom) ADSL service hosted by a third-party company called PlusNet, I have a Solwise SAR705 ADSL router (with an integral ADSL cable modem), which includes a hardware firewall (for incoming communications only) the use of which I will advise on to you in due course!

Each PC has an Internet Protocol (IP) address to identify it. This is a number comprising four octets (numbers with 256, or 28, possibilities). IP addresses therefore all lie between 0.0.0.0 and 255.255.255.255. Numbers are not memorable, so we use a Uniform Resource Locator (URL) for each PC as well. A URL includes the domain name you yourself chose with your ISP and looks something like ‘http://ispname.com/yourname’ (in free webspace provided by your ISP), or ‘http://www.yourname.com’ (if you buy a domain name from an approved domain name provider). A Domain Name Server (DNS) system, comprising many databases covering the whole planet (not shown on Fig 1 to avoid clutter), is interrogated by your browser when you select the URL of a website you want it to pull down. The DNS system then returns the corresponding IP address needed by the browser. For a more in-depth description of the (amazingly efficient and transparent to you) DNS system, see Ref 15.

When you get to the stage where you’re designing your own websites, you will upload these from your PC to a Web Server, using the File Transfer Protocol (FTP). The Web Server may be provided by your own ISP, in which case a limited amount of space on this Server may be free. If not, there are many other enterprises providing this service, at various levels.

When you address an established website on a Web Server using your browser, wherever this is worldwide, your browser downloads this website, using the HyperText Transfer Protocol (HTTP), to your PC.

Other entities connected to the www include the organisations concerned with regulating our use of the internet (performing important functions and internationally respected by the major players):

WorldWide Web Consortium (W3C), which is the main body responsible for all internet communication standards, such as HyperText Markup Language (HTML) and Extended Markup Language (XML) and which appears to have an international ‘flavour’ (but isn’t actually controlled by the United Nations as we might have expected).

Internet Corporation for Assigned Names and Numbers (ICANN), which is a private body based in the US that took over from a US government-sponsored body called the Internet Assigned Numbers Authority (IANA). Amongst other things, the ICANN assigns domain names, IP addresses and port numbers, eg so you can’t use someone else’s URL. Interestingly but regretably at the present time folks, the ICANN is not controlled by the United Nations, as we might have expected.

The Domain Name Servers mentioned above.

etc.

More details on the infrastructure of the Internet are available at Ref 39.

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