iso 7-layer dip

The ISO 7-layer model for telecoms is alive and well 30 years on… in the connected car

James MacTavish Blog

Everyone has heard of the 7-layer dip shown in the picture above but only die-hards in the telecoms industry will have heard of the mysterious “ISO 7-layer model” for telecommunications networking. As an apprentice in 1985 I was one of the first students to be taught the 7-layer model (in the days when there was no internet, no TCP/IP, no WiFi, and no digital communications), now here I am 30 years later reflecting on how relevant the model is in new industries like the connected car.

I amazed myself the other day when I easily recalled the 7 layers and what it all means. I must have had a good lecturer in those days.

Prior to the mid 1980s and the arrival of the ISO 7-layer model, there was no common framework or architecture for how to view communications networks. Honeywell Information Services were the pioneers of the ISO model and their work lives on to this day. How so?

To answer that, let me show you just how relevant the ISO model still is today by taking a real example … the connected car. Please note this isn’t a strict interpretation of the purist 7-layer model but it’s a great way to understand the layers in any architecture and how one technology always layers on top of (and depends on) the one below it. Just like the 7-layer dip I suppose. Get those chips out, here we go…

1) Physical layer: in cars that have lots of electronics and ECUs, the CAN bus is the way they are all connected in a quasi-industry-standard manner.

2) Data Link layer: interestingly in “connected car”, the CAN bus handles both the physical connectivity in the car and the data link protocol that runs over the CAN bus itself. It is rare to see a standard or protocol like CAN bus apply to 2 of the 7 layers of a typical system; for example if we look at a typical connected device like an Xbox, the physical layer is the WiFi / Ethernet card, and the data link layer is how data gets passed over the LAN.

3) Network layer: in today’s connected car, the wide area connection is normally cellular. Many cars are now being fitted with mobile network “modems” and others rely on the synched driver’s smartphone to act as the wide area connection.

4) Transport layer: 30 years ago, TCP/IP was barely a twinkle in anyone’s eye, now TCP is the standard transport layer, i.e. the protocol that safely transports IP packets from the car to the cloud. Almost all of the connected device projects we work on at AppCarousel use TCP as the transport layer.

OK, 4 layers gone, 3 to go. So far we have TCP/IP running over cellular, sending and receiving data to and from the CAN bus protocol in the car and being distributed around the car via the bus itself. When you think back 10 years ago cars weren’t really connected at all, so what I described is already 4 layers of technological magic working together. But there are 3 more upper layers to make the connected car a reality, the first of which is …

5) Session layer: the session is when the car, or the driver, logs on / authenticates / connects to a service somewhere in the cloud. This could be a navigation session, a streaming radio session, or looking up flight arrival times.

6) Presentation layer: the presentation layer is technically the way the session is rendered to the user, e.g. HTML5. Interestingly some companies like Jaguar Land Rover are using exactly that; HTML5, via specially written apps that run on the head unit of the car and are presented to the driver as the interface to a world of infotainment.

7) Application layer: think back a few years; there were no “applications” running in cars, apps only ran on smartphones. Now the connected car can download, update and run apps and services. Apps are adding more functionality than ever to the connected driving experience, including diagnostics and telematics. Those apps have a UI (presentation layer), they create sessions back to their server, TCP/IP is used to send and receive data reliably over patchy cellular networks, and the CAN bus connects all of that to the car’s electronics so that value-added end-to-end services like advanced telematics, vehicle diagnostics and SOTA (software over-the-air updating) can take place.

If you remember the halcyon days of the ISO 7-layer model;

Don’t shoot me for liberally twisting its exact definition because as I said I am not a purist – I just enjoyed sharing the example of the car
Tell me, how did you remember the 7 layers; what mnemonic did you use? Answers below please. I used “Presidential Democrats Never Tease Senators Presenting Awkwardly” and my friend Rick used “Please Do Not Throw Sushi (and) Pizza Away”.

7 layers are still around and relevant after 30 years. Food for thought.

Terry Hughes writes for AppCarousel, they work in the top 3 layers of the 7 layer model delivering sessions, user experiences and applications to industries like connected car, smart devices and IoT.