The world is full of information; its inhabitants never stop sending and receiving data. From the dogs barking at night to the blackbirds singing in the garden, both are transmitting signals. And, over time, we, as human beings, have developed more and more sophisticated ways of moving information.

It’s easy enough to pass information to the person in front of you. All you have to do is just speak the same language. Over the years, our culture has evolved a way of passing information as sounds that have a mutually agreed meaning. But that is not all; this very same culture of ours has developed a range of other cues to help in this communication. A look of puzzlement, a shrug or a nod – they all mean something.

Suddenly, this simple example starts to look quite complicated, so what about the more complex cases?

Well, how do you get information from A to B?

One part of the answer is by using ‘protocols’, but what is a ‘protocol’?

The online Oxford English Dictionary has four definitions for the word ‘protocol’.

Top of the list is “The official procedure or system of rules governing affairs of state or diplomatic occasions”. At the bottom of the list is “A set of rules governing the exchange or transmission of data between devices”.

In truth, the two meanings are very similar and this can be demonstrated with the following example. The prime minister of country A calls in the ambassador for country B and hands her a formal letter expressing displeasure at the behaviour of her country’s snooker fans.

The ambassador passes the letter to her secretary, together with her personal notes on the meeting. The secretary writes up the notes and gives the letter and the notes to the diplomatic courier who flies to country B. Once there, they hand them to a deputy under-secretary at the foreign office.

Eventually, the foreign minister receives them, replaces the ambassador’s observations with his own and hands both to the prime minister of country B.

A key element in this system is localised ignorance. The prime ministers are communicating, but do not care which secretary or under-secretary was involved. They deal respectively with the ambassador and the foreign minister, who in turn deal with their secretaries, who deal with the under-secretaries and so on and so forth.

The diplomatic courier does not know or care what is in the bag.

This breaks up the problem of exchanging messages across continents into steps. Each step can then be handled by those who specialise in solving that small part of the whole problem.

The designers of electronic communication systems have adopted this well-tried technique of simplifying a big problem by breaking it up into smaller tasks. A common model used to guide the design is the Open Systems Interconnection model (OSI model), also known as the 7 Layer model.

Layer 7 of the model is the “Application Layer”, the highest layer that interfaces to a user’s application. By stretching my earlier example further, this can be compared to the ambassador and the foreign secretary.

Layer 1 is the “Physical Layer”, the lowest layer that transfers the data across the physical medium (e.g. the wire of an Ethernet cable or the plane carrying the diplomatic courier).

Each of the layers in the model adds some distinct type of functionality to the communication system. Layer 2 is concerned with node-to-node data transfer; whereas, Layer 3 is concerned with communication within a network of multiple nodes. Layer 4 deals with communicating across multiple networks.

A key principle of the model is that each layer communicates with a peer at the other end of a communication link and each layer does this by using the services of the layer below it. It is also important to understand that not all the layers are used in every communications link.

For instance, messages for a printer in a router that have arrived from a PC over a WiFi interface will only reach Layer 3. This is because Layer 3 has the knowledge to route them to the Ethernet interface connected to the printer. In other words, when the courier arrives at the airport in country B, he does not ask the foreign minister how to get to the foreign office – he asks the information desk.

The OSI model is useful for understanding existing protocols and designing new ones, but it must be used with care. There are many different interpretations of how it should be applied because, essentially, there are many different communications scenarios.

Here is one such scenario. A few years ago, Plextek designed a Formula 1 telemetry system that transferred data from the cars to the pit in real time. At the time, the regulations forbad any form of communication from the pit to the car because the authorities were concerned that it would be used to re-configure the car during the race. As a result, the regulation was strictly enforced and a protocol had to be implemented that provided a one-way communication path with no feedback to indicate if data had been successfully transferred to the pit. Frustratingly, a few years later the regulation was relaxed to allow more conventional protocol designs to be used.

Another example of an unconventional protocol is that used in the Telensa street-lighting system. Although bi-directional communications are permitted, the throughput available in each direction differs by a factor of 10. The design for that protocol was also heavily influenced by the regulation governing the ISM bands that are being used.

Despite the different interpretations of the model that exist, particularly in the wireless domain, it remains a useful starting point. When I start analysing an unfamiliar protocol, I invariably look for the similarities between its structure and that of the OSI model.

Of course that’s not the end of it, but perhaps that is a blog for another time…

Image credits: Blackbird: Malene Thyssen

Ambassador: “John Adams 1st American Ambassador to English Court, Presented to King George III”

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