Designing Home and SME Networks 09 – Making the Connection

As mentioned in earlier articles, most people access the Internet using a DSL (Digital Subscriber Line) connection using the telephone (POTS) network. More subscribers are beginning to use the cable connections provided for cable TV, either co-ax or optical fiber, and, increasingly, 3G mobile telephone connections. A minority use other forms of connectivity such as dedicated telephone lines or optical fibers (leased lines) or satellite links.

Of these, the ADSL telephone link or cable-provider connection is by far the most common and is, usually, the cheapest.

1. What are the characteristics of the ADSL connection?
   • It is mainly a data-download facility, with download (downlink) bandwidths usually much greater than upload (uplink) bandwidths. For most people, this is fine, as they want to access information that is available on the Internet, such as web sites, movies or music. Hence mostly their uplink is used for issuing requests and their downlink for receiving the content, so a download speed greater than the upload speed is fine. This is why ADSL is known an Asymmetric DSL connection.
     However, if you want to source content from your home network, or you are a business that wants to use servers on your site to provide live content to the Internet, then you might find that the upload speed is inadequate, or that your ISP puts much tighter limits on upload data than on download, in which case you will need to use a connection with greater upload speeds.

     One area often forgotten by businesses with multiple sites is that, if you want to use VPNs to allow users on satellite sites to access data and services on the main site, then the main site’s uplink speed will limit the rate at which data can be sent to all the satellite sites, and the satellite sites uplink speed will limit how fast data can be sent to the main site. Therefore, at least the main site will probably require a much greater uplink speed than a normal domestic or small commercial ADSL link will provide.

   • It is non-resilient. If the device at either and (or the wire in the middle) fails, then you will be completely isolated from the Internet. This might be OK for a domestic service (although you might disagree if you watch a lot of Internet TV) but is usually unacceptable for business connections. Hence, most businesses will want a backup Internet connection. This used to be fairly simple: a dial-up modem was an integral part of most early Internet Access Routers, and the router would automatically dial in to the ISP using a spare telephone line if the main link failed. This is no longer a practical proposition. Users familiar with ADSL download speeds of several megabits per second will not be able to cope with relying on a 56Kbps dial-up line, and nor will most of the software.

     So, if your business has more than one physical telephone line it might be worthwhile running ADSL services on two or more of them. It would be worth investigating commercial broadband packages that support dual connections and load-sharing and/or auto fail-over in the event of loss of the primary link. This will provide resilience and greater available speeds at the cost of a more expensive broadband account, the need for more sophisticated routers and significantly more complex system configuration.

2. Why are commercial ADSL links dearer than domestic ones for the same link speed?

   They are aimed at different markets.

   • One issue is ‘available’ bandwidth. You may be able to, in theory, download at 8Mbps, and so can the other 100,000 subscribers connected to your telephone exchange. If you all tried to download at full speed at the same time, then the exchange would have to provide 800Gbps. Since most exchanges have nothing like this bandwidth available either within its own equipment or across its data links to the Internet, the providers have introduced the concept of ‘contention ratio’. This applies in two ways: The exchange equipment may apply a contention ratio of 20:1 to 50:1 to the subscriber lines. This means that a single (say) 8Mbps channel will be shared by between 20 and 50 subscribers. The other contention ratio is the ratio of total contracted bandwidth (say 100,000 subscribers at 8Mbps each) to the bandwidth of the Internet connection from the exchange (Say 1Gbps). Hence the exchange has a contention ratio of 1Gbps/800Gbps or 800:1.
     Typically, the subscriber line contention ratio applied to business lines is lower than that applied to domestic lines. Also, the Internet access bandwidth available to the exchange may be artificially partitioned so that commercial services get higher priority, and hence a better contention ratio, than domestic services. This is called ‘traffic shaping’ and may vary at different times to, say, give higher business priority during normal working hours and higher domestic priority at evenings and weekends.

     Most of the time this is fine as not all subscribers try to download at full speed all the time.

   • Another issue is that ISPs and carrier services may prioritize different types of traffic, giving, for instance, web browsing a higher priority than email. Again, commercial services are usually more customizable than domestic services in which traffic they will allow through at high priority.
3. How fast do I need?

   How fast can you afford and what will your phone line support? Generally, you need all the speed you can get.

4. Do I need an ‘unlimited’ account?

   There really is no such thing. So far, all the ‘unlimited’ offers are anything but unlimited, although normal users will struggle to hit the limits the ISPs impose. If you try to download the entire Internet to your PC, then I suspect that the PC will give up before the ISP limits your traffic. However, unlimited accounts are more expensive than download-limited ones, and you should try to estimate how much data you think you will download each month. Once you’ve estimated, at last double it: remember that Internet usage almost always increases, you may be tied in to this contract for some time, you may have to pay to uprate your data limit, and that the download limit is a real limit: and some ISPs either cut you off or cripple your downlink speed and others charge a fortune if you exceed your quota.