A seminal facet of protocol is that of establishing contact. Humans do it by looking each other in the eye, touching, picking up a phone and calling, flashing a signal, sending a messenger, sending a letter and so on. In each case, the existence of a potential physical conduit of communication is established. A second facet is that of establishing readiness to engage in an interaction. Humans do it by shaking hands, embracing, saying “Hi,” flashing a signal back, answering e-mail and more. A third facet is that of conversing, which follows rules, such as how to talk in turn, what subjects are approached, how they are developed; in short, all the conventions that have to be followed for the interaction to proceed. This leads to the concept of a protocol stack. A stack is a set of protocols related to each other and dependent upon one another. For example, in France when two persons want to talk, they may first establish eye contact, then say “Bonjour” and then converse. Here we have three protocols, each dependent on the previous one. We say that the three protocols are stacked, as they each lay upon the foundation created by the other.

Staying in France, another way for two persons to talk is for them to pick up a phone and first dial a number, then say “Allo” and then converse. In this case, contact is established through the phone line connection instead of through the eyes, readiness is signaled with “allo” instead of “bonjour” and conversation on the phone follows slightly different rules than face-to-face conversation. Actually, the pattern contact-readiness-conversation is a general pattern of interactions, a fact that has not escaped the attention of the pioneers that were tasked with making computers talk to each other. Accordingly, in the early stages of the evolution of the computer (within the modern era), the International Standards Organization, based in Switzerland, undertook an effort to better understand the mechanics of computer-to-computer interactions within an infrastructure of widespread connectivity among computer systems; that is, interactions through networks of computers. This effort resulted in the development of a computer interaction model known as the Open Systems Interconnection (OSI) Reference Model.

The Open Systems Interconnection reference model decomposes the general problem of two entities communicating with each other into seven layers, stacked on top of each other; specifically:

The physical layer is the way computers establish a conduit for communication. Such conduits may be a wire, or may be radio waves. In the later case, the communication is said to be wireless. Wired communication can use, for example twisted pairs, coaxial cable or optical fibers. Wireless communication can be done through various radio technologies like cellular telephony, Wi-Fi, or via other means such as optical lasers.

The next four layers are about establishing readiness to talk. The link layer allows two computers directly connected physically to recognize each other. The network layer allows two computers not directly linked physically to still recognize each other, using intermediate computers to which they are physically linked, thus establishing a chain of physical connections yielding the desired result. The transport layer allows two computers linked either directly or in a network fashion to

Book available at Midori Press (regular)
Book available at Midori Press (signed)
Book available at Amazon (regular)