On the face of it, it seems so simple. It started with stand-alone fax machines. Place the paper in the slot, type in the phone number and press the start button. The paper runs through and a copy spits out on the other end. Simple. On most machines you could hear the dialing and that odd static sound followed by a series of other strange noises. It sounded like the two machines were talking to each other; in fact, they were. Did you ever wonder why your fax machine could call a machine made by another vendor, send it a document ... and it worked?
In the early days of fax machines, you may have had this experience: You were certain you did everything right. You cautiously stuck the papers in the correct way. You dialed that number several times, very carefully. You heard the right noises on the phone line. But for some reason the magic just didn't happen.
You may not have realized it at the time, but you were experiencing the wonderful world of network and communications protocols. The sending fax machine upon connection attempts to "talk" to the receiving machine. If the receiving machine understands what the sending machine is saying, it cooperates and receives the fax.
Let's take a look at some definitions for the word protocol. These are from
1. (Computer science) rules determining the format and transmission of data (synonym: communications protocol).
2. Forms of ceremony and etiquette obs-erved by diplomats and heads of state.
3. Code of correct conduct, e.g. safety protocols or academic protocol.
A protocol is a sort of language by which devices communicate with each other. A device is any machine or component connected to a computer or telecommunication machine (such as a fax machine). In the context of protocols, we're specifically talking about devices communicating with other devices, with the protocol facilitating the exchange of communication.
In the case of fax machines and computers, two common communications devices are modems and network cards. The modem (short for modulation-demodulation, see "A History of Communications Networks," Gover-nment Technology, July 1998) is used to exchange communications across telephone lines. The network card (Network Interface Card or NIC) allows the exchange of data between computers that are linked via network cables into a local area network (LAN) or other kind of network.
Like any communication, a modem or network communication starts with two devices -- the one wishing to send the communication and the one intended to receive it. The sending device (modem) uses a channel (the telephone line and telecommunications network it is connected to) to initiate a communication to the intended receiving device. The sending device follows rules governing how it converts data for transmission down the channel. It expects the receiving device to understand these rules and uses them to accept the data and decode it after reception.
If a channel does not support the protocol used by the sending device, no transmission occurs. As a practical matter, this is rather unlikely. At this level, we're talking about something like buying a modem for your computer and plugging it into your stereo system, assuming you could jam that plug in there somewhere. Devices are generally standardized for the channels on which they are expected to be used.
Phone lines and network cables are designed