Computer networks and telecommunication systems are often described by a conceptual model called layers. Every layer uses the services of the lower layers to fulfil its own services. This abstraction allows to discuss each layer separately, evolving and changing them without breaking the functionality of the upper layers. It also allows to create heterogenous networks, where each node might be connected using different physical connections. The same conceptual model can be used to describe the transportation system. It also forms a heterogenous network of many nodes all over the world. Instead of information, it transports humans and physical goods. This article introduces the use of the layering analogy to the transportation system, analyzes its structure and identifies the different roles that comprise this network.
One of the most important layering model is the OSI Model. It describes communication systems using seven different layers. Each layer uses the services of the layers beneath it.
A communication network can contain two or many more nodes. The connection between each pair of nodes can have different physical and link layers. The responsibility of the nodes is to transfer the data from the physical layer of one connection to the physical layer of the other connection. This is why these nodes are also called routers.
The physical layer is the physical connection between two or more nodes and the representation of information on this connection. This could be a network cable between two PCs or even the radio frequency used between a cell phone and the base station.
The link layer regulates how the physical layer can be used. It makes sure that data can be transmitted over that medium. For instance, it regulates how two nodes should behave if they want to transmit at the same time.
The task of the network layer is to find a route between two nodes in the network.
The transport layer is responsible for reliably transmitting data along this route. Its tasks include checking if the data reached its destination and trying again in case of data loss.
The session, presentation and application layer use the services of the transport layer to implement an application. The distinction between these services is hard to draw. This is why they are combined into a single layer in the Internet Protocol Suite.
Every time one layer uses the service of a lower layer, it adds additional information to the original payload. The additional data might contain the source and destination address, a checksum or the information about all the nodes that were already visited. The payload plus the layer specific data is called a protocol data unit (PDU).
Layers of the Transportation Systen
A transportation network has a structure very similar to a computer network. It also comprises of nodes, that are connected via different kinds of modes, like roads, rails or water. Nodes in the network are either the destinations for the payloads of the transportation system – people and freight, or the places where they can change the mode of transport. Nodes can be houses, shops, hotels, airports, factories, mountain tops or any other place of interest.
These modes form the physical layer of the transportation network.
Each of these transport modes has its own set of rules that govern the access to the physical layer. Traffic rules regulate how cars and trucks can access roads. The rules for air traffic are different. We can consider these rules the link layer of the transporation system. As in a computer network, there can be package loss on the physical layer. We call this an accident.
The routers in the transportation system are the hubs. There a payload can change the physical layer. An airport performs the transfer from road to air and back again. A harbour connects water transport, rail and road. These hubs make it possible that a payload can reach any place in the world.
Finding a route along all these hubs towards the destination is also part of the network layer. This kind of service is often provided by a travel agency, or the passenger does it himself. A similar service for freight is performed by the logistic planners in companies that want to ship freight.
Finding a route along the network is often not enough. There can be changes in the itinerary of a transport mode. Or a container might get stuck in a port due to missing documents. A service that takes care of the payload and ensures it reaches its destination can be called the transport layer. For logistics, this is done by provides of door-to-door or even just-in-time delivery. Passengers can have this service performed by a travel agency.
All these layers provide the basic for the applications of the transportation network. Each trip to work, each holiday trip and each supply chain are applications of the network.
The transportation network also has the concept of protocol data units. The payload can be a person or a unit of freight that needs to travel from A to B. Every layer adds more information. A person that starts a business trip receives a travel itinerary from the travel agency at the transport level. At the network level a travel plan and a ticket for every leg of the trip is added. The link layer puts the person into a vehicle, which can be considered the PDU of the corresponding mode of transport. At the next hub where a change of transport modes takes place, the person is “unpacked” from the PDU of the current link layer – e.g., he leaves the car. Then he is put into the PDU of the next link layer, for instance a plane. This goes on and on until the destination is reached.
A Trip to Work
Let’s start with a trip to work by car. In this case, the journey contains only two hubs: the house as source and the office as the destination. The physical layer connecting these hubs is the road, and the PDU is the car.
Because this example does not use multiple modes of transport and no transport layer service is involved, we only examine the link and the physical layer.
When the person starts the trip at the house, he enters the car as the PDU of the road network. Then the car transport the passenger along the road network towards the office. At the destination, the person leaves the car and thus leaves the road network.
A Business Trip
Now the same person has to make a business trip to another continent. His company hires the services of a travel agency to plan the itinerary and assist in case of problems during the trip. The travel agency plans all parts of the trip: Which modes of transport to use, where to change the modes and organises the tickets.
The person travels by car to the airport, which is the first hub of the journey. He leaves the car at the parking lot and changes to the airplane. This is the PDU of the air network. The airline company provides the service of transporting him to the next airport. There he leaves the plane and switches to a taxi. Thereby he again uses a road network, but a different one this time as it is on another continent. Thus the air network provides a link between two separate networks of the same kind. This airport “routes” our passenger to the road network again, this time using a taxi as a different implementation of the link layer. In this implementation, the passenger does not have to steer the car himself. After arrival at the destination company, the passenger leaves the taxi and has completed the application “business trip”.
Each layer can have its own kind of addressing system. A layer that can connect only a few nodes only needs a small address space, where a worldwide network needs a rather huge number of addresses that need to be globally unique. Thus the Internet Protocol on the network layer of the internet has to provide addresses for billions of connected computers. And the Transmission Control Protocol of the transport layer allows to address different services on each computer with a number system called ports. A port can contain services such as a web server, a file server or an internet radio.
The addressing system of the transportation system spans the whole globe. Every node has a unique location that can be specified with latitude and longitude. Of course, humans rarely use these coordinates. They have addresses on a higher level, using street names, house numbers and city names. But at the end, each address needs to be resolved to a global coordinate. This resolution can be done using a map, or nowadays much more comfortable with a navigation system. The concept of ports is also present in this addressing system. A house has a unique location, but it can contain multiple flats that are addressed using the names of the inhabitants. Or an office building can contain different companies.
Standardised containers provide end-to-end protection of the goods inside them. This reduces many problems that occured with the breakbulk handling that was used before containers were introduced. The use of container greatly reduces the time that is needed at each hub, because they don’t need to be unpacked manually. And they reduce pilferage, because the longshoremen don’t have direct access to the cargo. These aspects greatly improved the speed and reliability of cargo transport.
As in the internet, authorities are eager to break the end-to-end protection between the nodes to peak inside, because containers make it easy for criminals to smuggle contraband accross borders.
As this article showed, the conceptual model of layers that originated in the telecommunications world also works for the transportation system. In contrast to computer network protocols that were specifically designed to use the advantages of the layered approach, the transportation system has evolved to use a similar structure. But in the end, both types of networks can be analysed with a similar mindset.