From the Foreword to J1939 (Serial Control and Communications Heavy Duty Vehicle Network)…
“The SAE J1939 communications network is a high speed ISO 11898-1 CAN-based communications network that supports real-time closed loop control functions, simple information exchanges, and diagnostic data exchanges between Electronic Control Units (ECUs), physically distributed throughout the vehicle.
Here is a high level overview of the J1939 protocol
An overview of the J1939 family of standards and how they are used
A SAE White Paper http://www.sae.org/misc/pdfs/J1939.pdf
Here is information about custom products

Distance
One of the advantages of J1939 is the distance of transmission over a single wire pair.
Average distance per data rate
500 kbit/sec – 328 Feet
250 kbit/sec – 656 Feet
Custom PGN
A PGN (Parameter Group Number) identifies a message’s function and associated data. We are capable of adding your own custom PGN for your J1939 application or assigning one of the predefined PGNs.
Reliability
The J1939 communications network is suitable for light-duty, medium-duty, and heavy-duty vehicles used on-road or off-road.
Image Gallery
View an image gallery of possible application examples for J1939