Connecting to telephone lines

In this section we want to deal with a situation that is, unfortunately, far too common: how to connect a community network to a set of telephone lines. We assume you are building a network that has for a community that poor or non-existent broadband to somewhere, let’s call it A, in which people are getting “passable” speeds. Perhaps they have connections that are advertised at 8Mb/s advertised but deliver 6-7Mb/s in practice. You want to connect your network to one or more telephone lines at A. How do you do this, and how can you benefit from connecting to several lines? One’s intuition is that the more telephone lines you use, the better your community broadband will be, but the story is a bit subtle.

Incidentally, we say this is unfortunate because, if an 8Mb/s service is offered, it is very likely that there is fibre to the exchange in A, so you could, in principle, get much higher bandwidth. Although the technology of connecting you directly to the fibre is simple, the telephone company that owns the fibre operates in such a way that installing such a connection not perceived as profitable and will probably want to charge you “funny money” for it.

So, suppose we want to connect the ethernet cable that comes from an antenna – or even a fibre to the community – to, say, four telephone lines. You need to install some kind of router or switch. What does this do and from where do you get it?

UK residents shoud know that although the physical copper that connects A to the exchange probably belongs to BT (more accurately BT Openreach) you don’t have to buy your service from BT. Other companies may offer a service that is better suited to your needs.

Static assignment, load balancing and bonding

There are three options open to you: static assingment, load balancing and bonding. In static assignment you split your community into subgroups and assign each group to one. So, if you have a community of 40 residences and 4 incoming lines, you put 10 residences on the first line, 10 on the second and so on. This requires you to program a switch that knows what customer connects to what line. It’s not diffcult to do this, but you need to get the appropriate switch with ports for, say, 4 telephone lines. It’s not quite clear why one would want to do static assignment. Perhaps some customers want less contention (fewer other people on the same line) and are prepared to pay for it.

[Should we say more about contention ratios?]

The two other options are load balancing and bonding. In load balancing, an individual will never see a higher speed than that provided by a single telephone line. In bonding the speed goes up. To understand the difference one needs to know a very little bit about how data gets sent across the internet.

When you download a picture or a Web page, the data for that page is broken up into small packets – internet packets – that are the basic units of transmission. Each packet carries the address of its destination, and this is used to route the packet through the internet. The whole download is called a session. At the destination there is software that assembles the packets for a session in the right order and sends a requests for packets that have gone missing.

[Is the following close enough to correct?]

In {\em load balancing}, each session is assigned to a line, but the router may decide to switch a later session from the same user to another line. This means that the router will try to avoid contention on any one line, and no individual will be cut off if one of the lines fails. However the speed at which any session takes place is still limited by a single line so individuals doing speed tests will never see speeds higher than that of the indvidual telephone lines.

Now your network will probably be capable of transmitting packets at a much higher rate than the individual telephone lines can absorb them, so in {\em bonding}, the router will take one internet packet and send it down one line and the next down another line so that the two packets travel, more or less, side-by-side. This effectively creates a “fatter” pipe through which internet packets flow, and the transmission rate increases.

Bonding is in some sense the nicest and simplest of the solutions, but it requires the company that provides you with the router to have a similar router somewhere else that reassembles the packets in the order in which they are sent. It is simple because all you need to do is to plug your antenna cable (or fibre) straight into the company supplied router. Also do not expect speeds simply to multiply up with the number of telephone lines: there are overheads in bonding and there may be contention at other places in the network. Realistically we have heard of 4 bonded 8Mb/s lines delivering 20-24Mb/s

[Need to add lists of suppliers and equipment]