Internet Access Methods for Your SME
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.
1. 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
elephone 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.