Serial communications

Data compression

In addition, you usually have a choice of data compression: V 42bis or MNP-5. The choice depends on what the modem at the other end of the line does. You can set most modems to negotiate either protocol. These protocols include related error correction standards, called V 42 or MNP2-4 respectively. If you believe the sales claims, these compression protocols will give you up to 100% increase in data throughput. Consider this the upper limit; a lot of data is binary, and when ftp'ing a typical gzipped tar archive, you will probably get almost no speed improvement.

Data compression has one negative side: it increases the data rate, but it also increases latency, the time it takes for data to get from the local DTE to the remote DTE. The data doesn't take as long on the line, but it spends more time in the modems being compressed and uncompressed. If you're running a protocol like PPP that supplies optional compression in the software, you may find it advantageous to turn off compression. We’ll look at that again in Chapter 20, Configuring PPP.

The link speed

The standard PC serial hardware can run at speeds that are a fraction of 115,200 bps (in other words, 115200 divided by a small integer). This gives the following combinations:

You'll notice that it can’t run at 33600 or 56000 bps. Also, looking at the example above, you'll note that all three links run at different speeds. How can that work? Only afew years ago, it wouldn't, but modern modems can buffer data. For example, the ISP can send data to the modem far faster than the modem can send it to the other modem. It stores the data in internal memory until it can be transmitted. This can also happen at the other end. If you misconfigure your line so that the local link runs at 9600 bps, things will still work, but of course the total speed is the speed of the slowest link, in this case

9600 bps.

This flexibility brings a problem with it: the modem can't know inadvance how fast the connection to the computer is. It needs a way to find out. The modem solves the question of local line speed by a trick: all commands start with AT or at (you're not allowed to mix cases, like aT or At). It can recognize these characters even if they arrive at the wrong speed, and thus it can establish the speed of the connection.

Dialing out

Nowadays, all modems are capable of dialing. That wasn’t always the case, and in some old documentation you may find references to an Auto-Call Unit or ACU, which is simply the dialler part of a modem connected via a separate port. Typically, one ACU could serve multiple modems.

Nearly every modern modem uses a command set designed by Hayes Corporation, which is thus called the Hayes Command Set. We'll look at it in the following section. It is also sometimes called the AT command set, because nearly all the commands start with the sequence AT. The CCITT also created an autodial recommendation, K25, which was occasionally implemented, but now appears to be dead.

Modem commands

Modern modems store their state in a number of registers, called S registers. The register use varies somewhat from manufacturer to manufacturer, but most modems have a number in common. They each store a one-byte value, ranging between 0 and 255. Here's a list of the more important ones for a Rockwell V 34 chip set. The name of the chip set is not the same as the name of the modem. You'll note that one of the commands enables you to find out the chip set version, as we'll see in the example that follows.

The AT command set tells the modem to do something specific. Here are some of the more important ones.