A "string can" telephone as an OSI protocol stack

Picture a protocol stack

So since it's communications month on this scada blog, I thought a brief intro relevant for a warm up. Let's start by taking a look at something old and somewhat familiar from childhood and recast it in terms of current work on open standards and usage. The opening graphic shows a sequence of communications between 4 entities.  Real implementations of protocols involve not only message translation and abstraction, but also a variety of state machines to help each entity understand the probable state of the other entities involved in the communications.

Protocols are to communications, what algorithms are to computing.  What is interesting about protocols is that they have always been application specific, and always multi-layered.  However with the advent of the categorization of the Open Systems Interconnection [OSI] stack, they have become a little easier to describe and therefore manage in terms of open access.  This month we will start with the 'string-can' telephone, and move through some of the key protocols used in SCADA up to the newer 'sets' of protocols which operate in concert on high speed ethernet.

I call it the string-can telephone, but it is also known as the 'lovers telephone' or 'tin can' telephone. I know of many children's books had methods of creating such a device for communications.  Other related devices included voice tubes, sometimes used on ships and offices in the latter part of the 19th century.  See figure 2 for what we need here. People speaking two different idioms, a tin can each, a whistle each, and a long piece of red string.

Now I recall trying out the 'string-can' telephone and based on my experience I have created a graphic using the now familiar 7 layer OSI stack.

The left right plane of the figure shows two protocol stacks.  The left hand stack describes the protocol from the point of view of the American named Mary in this case.  The right hand stack describes a similar protocol from the point of view of the Irishman named Gerry in this case.  OSI decided that the best way to categorize 'open' protocols would be to describe 7 distinct layers, where each layer can communicate either up or down to the layer just above or just below, and which corresponds to a similar layer on the other end of the communication.

In our case we have Mary and Gerry discussing something but in order to do that they employ a fairly complex but well understood protocol stack.

The Media Layers

Layer 1 - Physical

The OSI stack contains layers 1-3 which are generally associated with the media over which communications are transmitted.  In the example, Mary and Gerry decided to use a piece of RED string. This is the physical connection to be used for communication.  Now we know the string cannot transmit audio if it is not under tension, and so we say TAUT STRING for the definition of the physical layer.

Layer 2 - Data Link

The second layer uses the lower layer to communicate but acts as the method to link data from the upper layers onto the string.  In this case we need a can of some kind.  A beer can for Gerry and a Soda can for Mary.  The only requirement is that there is some kind of can on the other end of the string to make this work.  The can converts audio signals at one point to compression waves in the string and vice versa at the other end.

Layer 3 - Network

In this case Mary and Gerry have a private point to point communication, so we would describe the network layer as being not applicable.  If there were more than another party on the physical layer which needed to be addressed, then a networking layer would be required.  Many of the modern protocols using Ethernet would include this layer as we will see in later blog posts.

The HOST Layers

The host layers consist of protocols which apply to the host.  In our case the 'host' on each end is a person, and so much of what is described here is within the context of the individual but represents the layering effect of the communication pretty well.

Layer 4 - Transport

The transport layer is representing how the communication takes place.  We have established above that the actual communication between Mary and Gerry consists of sound waves somehow converted to vibration on a red string, however the actual communications is in terms of a language which both parties or both 'hosts' can understand.  The transport layer then demands that the communication be in the audio spectrum and consists of 'words' which when arranged in sequence will convey meaning, and a set of 'flow control' items which allow both hosts to control the conversation and therefore communicate.

We can well imagine the difficulties of flow control, since we assume the 'hosts' have to use the protocol stack to control the flow of conversation. They might choose something simple, like a whistle which when blown could indicate either that they wish to start or end a conversation, or they might use the word 'over' to indicate that he or she has finished talking and now waiting for a response.

Flow control then helps 'host's decide which host gets access to the 'wire' to transmit and when.

Layer 5 - Session

This layer refers to the fact that Mary and Gerry have established a session or time during which they will communicate using say for example the 'English language' over a string-can telephone circuit and both are parties to that conversation at the same time.  From Mary's point of view, it is a conversation between Gerry and I.  From Gerry's point of view it is a conversation between Mary and I.

Layer 6 - Presentation

The presentation layer is probably one of the most interesting and really represents the point where each 'host' can apply it's own personality to the communication.  In this case the presentation layer is taking the idiom of the other speaker and translating it to the local idiom of the listener.  Although Irish and American English is similar, there are enough differences to have to force each 'host' to do some translation of meaning and encoding of communications so that the other can understand.  In digital protocols, particularly where a level of syntax is expected, the Basic Encoding Rules or BER would sit at this layer.

Layer 7 - Application

Now we really don't know what is going on at the application layer inside Mary and Gerry's head other than thoughts and discussion continue to some conclusion.  This conclusion may be determined prior to or during the call, and may depend on topic's covered in the call.  The important point about the application layer is that it represents the point of connection between COMMUNICATION as an activity and other activities which will take place within the host.

In subsequent post's this month, I will talk over some of the protocols which we use in our SCADA systems, their importance, key features, and give an idea of how they work and why use them and where. Our SCADA products would in many cases use one or more protocols as part of a system integration or system solution in remote locations.

Once again if you wish to help improve this posting by meaningful experiences, please feel free to comment.  It's all part of the protocol so to speak.

Chris Smith
[Sydney, Australia - April 2012]





Feature Photo - Attribution - F Decomite [Flickr] creative commons licence.
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