sorry - this explains it much more clearly
http://www.atsr.ie/CANbusTechnology.htm
Introduction to CAN bus Technology
CAN, Controller Area Network, is the name given to a system of control units that communicate and respond to each other on a CAN bus network. Bus is a network in which all control units are attached to a line directly and all signals pass through each control unit. Each unit has its own unique identity and will only respond to those signals intended for it.
Multiplex is a multi function power and communication cable that visits each control unit. The power is distributed by the control units to operate the chosen devices. It is also known as power-line communication (PLC).
Depending on the manufacturer CAN bus will usually consist of two wires twisted together: CAN high and CAN low. CAN high is the high speed signal network and is used to operate systems such as engine control units, safety features and ABS control.
CAN low is the low speed signal network and is used to operate systems such as lighting, seat electronics and electric windows.
If you think of the CAN bus network as a postal system, each data signal would be a message with a postcode and each control unit a mail sorter with its own postcode address. If we use the brake lights as an example, when you press the brake pedal this is registered by a control unit which then creates the message ‘operate brake lights’ with a postcode ‘rear light control unit’. The message is then sent on the CAN bus, it will pass through each control unit and the postcode will be read. If it does not have that control unit’s specific postcode, then the message is passed on. When the postcode is read by the control unit it is addressed to, the rear light control unit, the message is opened and read. The rear light control unit will then operate the brake lights as instructed. All this happens in milliseconds and so to the user it will seem instant.
The CAN protocol was created by the Robert Bosch Corp. in 1984. The first full production application was in 1992 on a Mercedes Benz. These systems are now used in most vehicles because it is cheaper, lighter and reliable. It also makes fault diagnosis easier. Less wire and less connections makes design and installation faster with only one loom (system of wires) required for each model regardless of specification. The weight of a traditional car loom is around 40% heavier than a multiplex/CAN bus loom. Lighter cars have better performance, economy and emissions.
Most electrical faults are due to poor connections, less wire means less connections and so reliability is improved. Any post production improvements can be downloaded into registered vehicles. All faults are registered and stored allowing the technician to interrogate the car and rectify the fault. Also the in-built diagnosis makes it possible for certain functions to work with limited functionality if a fault occurs, instead of failing to function completely.
It is hard to recognise multiplex and CAN bus unless you have the correct Current Flow Diagram (CFD). The actual CAN bus wires are quite thin and twisted together many times. The twisting helps to reduce interference from other power sources. Most cars with these systems will have fewer relays and fuses as the switching is done electronically. If a fault occurs the control unit will shut the power off until rectified. Vehicles that have a comprehensive Driver Information System or a multifunction screen and controller dial will certainly use CAN bus technology. It has been in vehicles since 1992, most vehicles now have it in some form or another and very soon it will be used for most systems in every vehicle.
