4. Leased Lines

If you are wiring a business, you can rent "pipes of any diameter", to match your needs. If that's what you are planning, then you should already know best.

4.1 Leased Lines
4.2 Multiplexed throughput
4.3 Probable Demand
4.4 Measured Demand
4.5 Feeding the feeder
4.6 LAN / WAN monitors
4.7 Queue Delays
4.8 Unseen Delays


Leased Lines

A leased line, is a wire from you to the exchange, through a patch panel to another exchange, and a wire to the other end. It could be a single dedicated wire, but it's usually a share of a multiplexed line.

If you have planning permission, and a large site, you might use a phone wire between two buildings with V34 modems at each end.


Multiplexed throughput

Telecom companies, have the technology to do things that make it cheaper for you (or make more profits for themselves).

They can sell you a share of a wire. If the others aren't using their share, you get a high throughput. If everyone is using their share, you get guaranteed minimum throughput.

The laws of averages work to your benefit, but only if you understand that you have agreed to a range of bandwidths, (unless you paid extra for a guaranteed fixed bandwidth). You may even be lucky, and be the first (only) user of a line, or have quiet neighbours.

Rember also, that you are buying at the providers best guess of normal line usage. At any time of national disaster, your share of the percentage might be re-prioritised to nothing.


Probable Demand

Your line is often idle. You don't use it at 100%, all the time. In-between fetching pages, you probably spend ages reading them. Even when you are waiting for a page, there are moments when no traffic is happening, you have to be doing several things concurrently to really fill a line, and then each thing becomes slower!


Measured Demand

With 100 users connecting to an ISP, there are a lot of empty gaps, that make space for active connections, though of course there are peak traffic moments when everything happens at once. To some extent, it all "averages out", so that your ISP doesn't have to have 100 times capacity, and the added delays are "not unacceptable".

If a traffic peak "happens" at some instant, all of the traffic will be added to a queue (on the routing device), and the ISP's pipe will work at full speed to drain that queue.

If that queue gets drained within one second, and is followed by a quiet time, the average usage, for those 2 seconds may be 50%, and the average for the minute 25%.

The ISP monitors their router queues, and when the dials reach a level they consider relevent, they get a bigger pipe. Medium size networks should stay below 50% for 90% of the day-time.


Feeding the feeder

Typically, connections go from leaf, to twig, to branch, to trunk, then back to branch and leaf again (or root tips if you prefer). Even then, there is no clear 'trunk' to the Internet, more of a criss-cross-cloud of interconnecting traffic routes.

Normally your "upsream" access provider, has plenty of capacity to handle whatever traffic you can generate, and ditto for your remote connection(s) provider, but how can you be sure?

When you get a "wide diameter pipe", you need to check that your upstream provider can handle that capacity, and continues to do so! Confirming that isn't so easy. Even if you experience delays, it may not be directly upstream, but way beyond. Conversely, backlogged queues might not cause you much of a delay!

Your provider may have a feed double what you take, but you share that with others! And it isn't easy to measure, unless you have several remote computers assisting you in monitoring.


LAN / WAN monitors

There are tools to monitor line bandwidth availability, eg Net-Medic. These are still experimantal, and error prone. Not least because they do not have realistic TCP services to measure TCP throughputs against, but may use ICMP packets (where ICMP services are generally available) to "guess" the TCP performance. ICMP is different, because the packets are generally smaller, and have a higher (queue jumping) priority.


Queue Delays

Your V34 modem works at 2.8 KPS (ish). Attempt to send more, and a queue build up. That queue is held on the modem, and also on the device that feeds it.

In some ways, a queue is a good thing, as it means you are using your connection to the maximum - with a queue there are no "idle times", and your overall throughput is increased.

For example, if you are downloading one huge file, why not download another (from a different site). Running the two in parallel, is usually faster than running one after the other, one fills in the gaps from the other.

However, queues are always irritating, particularly for connection establishment, when lots of small packets have to be transferred, before the real traffic begins.

Fortunately FTP and other IP protocols, seem to adapt well to the available bandwidth. They send just enough to keep the (bottleneck) device busy, without "drowning" it with too much of a backlog.

That means that if you do some interactive work, when you want small packets (eg key-presses) to get through soon, they do not have too much of a queue, to delay them. And when the data returns, it will get a decent percentage of the throughput.

The balence is a fine one, typically sending two 1K packets, then only sending the next, each time the previous but one is acknoledged. That way there is a maximum of 2 in transit, probably one in the queue.

Of course, if the return trip is also clogged, then the acknoledgement gets delayed, and it's not so beautiful packet, and waiting


Unseen Delays

you will only notice a 1 second delay.

The queue at your V34 device will mean that you never experience any

However, for those who are just interested in the idea ...

Of course, just like water pipes, you have to be sure that the provider can handle that much data on the other end (and ditto as you fan out across the Internet).

Chances are, they know you only use a percentage, preferably 40% peak, and they increase their feeds, as the sum of all their clients increases.

If you are lucky, you don't need it. Yesterday, you managed to run a business without every desktop simultaneously fetching lots of big files, so why is it so essential today?

Maybe you even feel that browsing slows you down - all that waiting for pages to load - must get a bigger pipe. Maybe you feel that you should be working smarter - not faster.

If you use mostly EMAIL, and a few well chosen news groups, a permanent V34 line will take you a long way. Remember 2KPS is 1MB every 9 minutes, and some (non-uk) providers, will put a V34 modem on a 24x7 leased phone line for relatively low cost. Then you can also use some disk space at the ISP, for others to upload files into your area, or spool files (as email).