Mar. 21, 2006 - Welcome to the fourth article in BWE’s series of "How-To" articles that
focus on how to build broadband wireless networks in areas that desperately
need high-speed Internet connections.
This week's article is focused on how to select the amount of Internet
backbone connectivity you’ll need to lease in order to transport your wireless
network's traffic to and from the Internet.
The most common type of telecommunications circuit that a Wireless ISP (WISP)
begins with is a T1 circuit, which carries full duplex traffic at 1.5 Megabits
per second (Mbps).
A T1 circuit usually can support a network of around 100-200 users depending
on their bandwidth requirements. This doesn’t mean that all 200 subscribers can
get a full 1.5 Mbps connection at the same time.
Most WISPs over subscribe their network on a 6:1 ratio. This means that for
every six customers, only one of them will be online at a single point in time.
So if you have 200 customers that would mean that normally only 33 would be
online at one time.
Now divide 1.5 Mbps by 33 and you’ll get around 45 Kilobits per second (Kbps),
which isn't much better than a dial-up connection. The good news is that
all users are not sending email or requesting web pages at exactly the same time.
The best way to know how much bandwidth you'll need is to use network
monitoring tools that let
you know when your users are approaching too much network congestion. Most
network managers will set an alarm to go off when the network surpasses 70%
congestion. This allows managers time to order more bandwidth before users
begin to experience 100% congestion.
Recently a good price for a single T1 was around $500 per month. However,
the FCC just made a ruling that could cause the price of T1’s to jump back
into the $1,000 per month range.
Many WISPs start with a one T1 circuit to support a small wireless cell.
Then they add a second, third and forth cell. About the time they add a
fourth cell they are paying around $3,000 per month in bandwidth costs for
four T1 circuits.
The next step is to move up to a DS3 circuit, which provides 45 Mbps of
bandwidth and costs the same per month as 3-4 T1 circuits. Instead of feeding
each cell with a single point-to-point T1 circuit, a WISP can bring the DS3
circuit to a centralized tower and beam the bandwidth wirelessly to as many
cell sites as needed, which allows them to significantly
expand their WISP’s coverage area.
One problem that exists in many rural areas is no accessibility
of T1 circuits. In this case the only choice available will be a
commercial grade satellite downlink. The monthly costs range in price,
but are usually a much better deal than buying a T1 in a rural area.
While fairly expensive, satellite uplinks from
Loral Skynet or Broadsky Networks can provide a backbone Internet
connection with enough bandwidth to make building a broadband
wireless network in remote areas possible.
Please join us next week to learn, "How to Select the Right WISP Equipment
for Your Town."
If you would like to read other articles in this series or an
expanded version of this article you can read them online at:
www.bbwexchange.com/howto/
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