Image Processing Reference
In-Depth Information
19.5.1
Nailed-Up Connections
A nailed-up connection is when your router connects via broadband and doesn't hang up.
It just holds the line up all the time. This is good for browsing because you don't have to
incur the delay of a dialup when you link to another page.
ISPs often bulk-purchase lines from a telecoms provider and bundle the call charges
with your monthly subscription. You may still need to pay the line-rental fee to the tele-
coms provider.
If your desktop PC is directly connected via the broadband line, then it has to han-
dle the routing for any other machines that you have networked at home. Keep in mind
that the network is “down” when your PC is shut off. Intrusion protection takes place in
that directly connected PC, and it is a weak point in your security whenever the broad-
band link is connected and you are online. This is especially true if you have a nailed-up
(always-on) broadband line and you leave your PC on all the time. Unprotected machines
can be detected and compromised with malware within minutes of being connected to the
Internet.
An ideal solution is to place a router on that broadband line and turn off any responses
to incoming connections. This does mean your ISP cannot PING your router for diagnostic
purposes, but if they need to do that you could allow them access for a short while and then
switch the router back to a silent mode. That would be a very rare occurrence, though.
Boundary protection like this is a good policy for broadband users, but you should still turn
off any unnecessary ports and services on your machines inside your network.
19.5.2
Contention Ratios
You may encounter this terminology when discussing bandwidth available via your ISP.
It most commonly relates to consumer connections. The contention ratio describes the
number of available input connections versus the number of requesting connections.
In terms of broadband-modem connections you often find that the number of users
to available modems at the ISP is perhaps 2:1 or even 4:1. Therefore, you might have a
1 in 4 chance of achieving a successful connection.
Contention is also used to describe the available bit rate. When you purchase 2Mbps
of bandwidth and share that between all the members of your household, as long as you
only use the Internet one at a time, you all have 2Mbps of capacity. If four members of the
household begin to watch streamed movies on separate computers, you have a contention
ratio of 4:1 and all of you on average get about 500Kbps. Your client player should do some
content negotiation with the server to select a lower bit rate if necessary.
19.5.3
Bit-Rate Aggregation
You might design your connectivity to the outside world using multiple redundant con-
nections, each delivering a proportion of your total bandwidth. By connecting sophisti-
cated routers across these multiple links, their bandwidth is aggregated together so that
they behave like a single virtual link.
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