1. Scope - This SAE Information Report studies the present transmission media axioms and takes a fresh look at the Class C transmission medium requirements and also the possibilities and limitations of using a twisted pair as the transmission medium.

The choice of transmission medium is a large determining factor in choosing a Class C scheme.

1.1 Background - The Vehicle Network for Multiplexing and Data Communications (Multiplex) Committee has defined three classes of vehicle data communication Networks:

a. Class A - Low-Speed Body Wiring and Control Functions, i. e., Control of Exterior Lamps

b. Class B - Data Communications, i. e., Sharing of Vehicle Parametric Data

c. Class C - High-Speed Real-Time Control, i.e., High-Speed Link for Distributed Processing

1.2 Interrelationship of Classes A, B, and C - The Class B Network is intended to be a functional superset of the Class A Network. That is, the Class B Bus must be capable of communications that would perform all of the functions of a Class A Bus. This feature protects the use of the same bus for all Class A and Class B functions or an alternate configuration of both buses with a "gateway" device. In a similar manner, the Class C Bus is intended as a functional superset of the Class B Bus.

1.3 Electromagnetic Susceptibility (EMS) Considerations - Inherent with the high data rates of a Class C Bus is a higher probability of electromagnetic interference (EMI) corrupting data. There has been a lot of research on Class B Networks that use twisted pair operating at data rates below 50 kbps and methods have been found to overcome the communication problems (SAE J1850). But, it is commonly agreed that the corruption of serial data by EMI will be an issue if a twisted pair or any other kind of conventional wiring and connector design is used at the higher data rates. Also, if data communication requirements dictate transmission rates above 50kbps, another technique may be required because 50 kbps is the practical upper limit of these Class B Networks (SAE J1850) that use twisted-pairs and conventional bus drivers.

1.4 Electromagnetic Interference (EMI) Considerations - A key concern is the generation of EMI when the Class C Vehicle Multiplexing Network is utilizing twisted pair for the transmission medium operating at data transmission rates above 50 kbps. It is because of this EMI concern that most automotive RF engineers commonly accept that either a fiber optic or a shielded cable will be required for the transmission medium at data rates above 100 kbps.

It is expected that the growth of data communications on vehicles, the issue of shielding cost requirements, and electromagnetic compatibility of copper-based systems, will drive future development. These factors and other, as yet undefined, needs for Class C communication will eventually drive the implementation of automotive fiber optic systems for higher data transfer rates.