Community Wireless Networks
The key points of a Community ISP (Internet Service Provider) system are:
- The Internet Feed (this page) – commonly a leased line
- The Network Services – email and web servers, bandwidth control, controlling access and security.
- Wireless Distribution – distributing the internet and services to the community.
The Internet Feed
There are many different ways of obtaining an internet connection. However if you are considering a wireless network, it seems likely that you cannot get an ADSL or similar low priced broadband connection. The two remaining options are satellite or leased line. Satellite connections are very cost effective, however they have significant problems if the system is to be used for home-workers: there is a long delay between the user requesting information from the internet and getting a reply back. This delay is sufficient to make several commonly used business systems unworkable.
Leased Lines
This leaves us with the leased line as the most practical and most commonly used method of getting internet connectivity. A leased line is a dedicated feed from your ISP to your chosen location. This is an uncontended service (i.e. one that is not shared with any other users) and is available in typical sizes (or bandwidths) of 64kbs, 256kbs, 512kbs, 1Mbs, 2Mbs all the way up to 56Mbs. As a comparison, most dial up internet users run at a maximum of 56kbs (kilo or thousand bits per second). Leased lines are 'symmetrical', i.e. a 2Mb leased line will give 2Mb in both directions simultaneously.
Ordering a Leased Line
Leased lines can be purchased from most larger ISPs and it is worth getting several quotes as the prices can vary enormously. The price for the leased line comprises of three elements: firstly the ISP's charge for providing you with the connection to the internet as a whole via their network; secondly providing the data via a low cost data carrier where your data is carried with others' using fibre optics to a point nearer you called a POP (point of presence), normally in a large town; finally the most expensive element is for carrying your data from the POP to your premises via dedicated BT lines.
Leased lines are paid for with an installation charge followed by annual payments. The further from the POP you are, the more expensive the annual rental. This can make the final price for a 2mbs line anywhere between £7,000 and £25,000 per year.
A device called a router is used to convert the connection at the end of the leased line to a standard connection that can be connected to normal network equipment. The router is normally purchased or leased from the ISP.
There is normally a lead time of 3-4 months for a leased line install. Leased lines are available as managed or unmanaged. In the case of a managed leased line, the router is configured and monitored by the ISP in case of a fault. For a community system, the improved reliability of a managed service will be worthwhile.
Leased Line IP Addresses
Each device connected to the internet requires a unique number, called an IP address, so you will either need to apply for a unique set of numbers from the Internet number authority (in Europe this is done by filling in a RIPE application form via your ISP) or by using a system called NAT which allows all the users on the network to share one address. Certain applications do not work with NAT so its use is not recommended.
You will need to apply for enough IP addresses for all your users plus one for every network device on your system. The Ripe application can take several weeks to be approved and therefore should be completed at the same time as ordering your leased line.
Leased Line Size
Although it was mentioned earlier that a dial up user obtains a connection of 56Kilobits per second, one hundred users do not require 100 times as much (5.6Mbs). In the case of broadband which is loosely accepted as being a speed of 512Kbs this would equate to a leased line of 56Mbs. This would be very expensive and each user would have to pay around £5000 per year!
Internet usage, especially when using http, or the web as it is commonly known, is characterised by short bursts of activity followed by long pauses. Typically a user clicks on a web link, the page is displayed and the user then reads the page. On a broadband connection the page loading takes approximately a second – the user then spends the next 20 seconds reading the page. Accordingly internet providers introduce a 'contention ratio', where several users share the same bandwidth. For domestic users a typical contention ratio is 50:1 where 50 users share a 512Kbs feed. For business users this is normally reduced to 20:1. Business users tend to want a larger bandwidth of 2Mbs so this should be taken into consideration.
So to size the leased line:
(number of domestic users x 512/50) + (number of business users x 512/20) + (number of business users x 2048/20)
So for a small village with say 50 domestic users, 2 small businesses and 1 larger business this would equate to:
(50x512/50) + (2x512/20) + (2048/20) = 512 + 51 + 100 = 660Kbs so a 1Mbs would appear suitable. However, if you have sold a business a service that is capable of 2Mbs then the originating line must be at least that size.
Navigator are experienced in the design and implementation of large scale wireless systems, and working with our partner companies can offer a complete solution for a community system.
