The Internet and
oversubscribing
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The
backbone of the Internet is connected networks of
high capacity communication lines. A dedicated
connection to the Internet backbone that operates
all the time, at the full capacity of the
connection technology, is extremely expensive.
Since end users rarely require 100% of the
available bandwidth all the time, ISPs will
purchase these expensive dedicated connections and
use them to run multiple connections of various
sizes to their end users. This is called
"over-subscribing", and is a technique used by the
Internet Service Provider Industry to realize a
profit margin when providing dedicated
connectivity to consumer users. Over-subscription
is usually based on a bandwidth ratio and
typically ranges from 4:1 to 20:1 depending on the
service being provided. Over-subscription may
actually occur several times before it reaches the
end user.
Internet providers are measured
in "Tiers". A Tier 1 provider maintains their own
national network and provides their customers a
1:1 bandwidth ratio. The major Tier 1 service
providers are in a "peer" relationship allowing
traffic from users on separate networks to
communicate seamlessly. As you move down in Tier
levels of providers you encounter
over-subscription rates at a higher degree as the
ISP's resell the bandwidth from a higher Tier. The
lower tiered ISP's have to have some sort of limit
in place for their non-committed or consumer
dedicated product or there would be no economical
way they could provide service to thousands of end
users.
Some companies require a guaranteed
minimum bandwidth at pay for that at a premium.
For instance a 2mb/s (Megabits per second) DSL
connection may cost $500 a month while a
Fractional T3 with a committed bandwidth of 2mb/s
will likely cost over $2000 a month. While that
2mb/s DSL line is capable of operating at 2mb/s,
if all the DSL users of that ISP were downloading
at once everyone's connection would slow down.
This is often observed with residential
connections at "peak hours" like the early
evening, also referred to as the "Internet rush
hour".
Your ISP isn't selling you a
connection to the Internet per se, they are
selling you a connection to their network that is
connected to the network of their provider and so
on. This continues up to the top Tier 1 networks
that are connected to each other. All these
networks and servers and end users' computers
together make up the Internet.
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| Technology |
Max
Speed
mbit/s |
 56k
modem |
0.056 |
 DSL |
varies |
 cable |
varies |
 T1 |
1.544 |
 Ethernet |
10.000 |
| T3 |
44.736 |
 OC-3 |
155.520 |
| OC-12 |
622.080 |
| OC-48 |
2,488.320 |
| OC-96 |
4,976.640 |
| OC-192 |
9,953.280 |
| OC-255 |
13,219.200 |
| These speeds are the maximum the
technology allows. Actual speed will be affected
by supplier imposed limits and other
factors |
| T1 and T3 are also called DS-1 and DS-3
respectively | |
