Flexray to Optical fiber communication

Hey Saravanakumar,

Haha, what are you building a doomsday machine control system?! In all seriousness though, I’m very curious what application would need to use Flexray at 10Mbps across 10 or 20Km; if you can’t say due to IP reasons don’t worry about it, just intriguing is all. Anyway, I have split this post up into three to avoid the brick wall of text effect.

My first question pertains to the application, before going any further I just wanted to check that this method of communication will beat out any other wireless or hardware signal. There are a lot of options these days for carrying that bandwidth. Gigabit Ethernet for example is clocked in at 100x speed boost for you signal. It may not have the rock solid packet dropping record that Flexray does but it’s in the high 90s for successful packet transmission. Installing 20km of single mode fiber is essentially like laying the start of a network backbone fiber. It will be incredibly costly if burying the line and incredibly costly if just 20km of line in some over air or snaking through building walls.

If you’re sure you’d like to proceed down this route, what level of customization are you looking to use. Essentially, would you like to use consumer off the shelve products (COTS) or are you wanting to make your own optical systems for this down to the laser drivers for this project of yours? Do you want to be able to carry other information besides just Flexray? Seems non-optimal to use 20km of high end single mode fiber to carry 10Mbps max throughput, perhaps there is a need to carry an Ethernet signal and we can do some multiplexing for you or something? You can ignore most of this; I just want you to get the most out of such an awesome cable line. Anyway, with all of that out of the way onto the real info for you!

In the information below I will give you an upper level view of what you need to do to get this system setup. I’m going to make two assumptions, the first being that you want to use COTS hardware. If you do want to design your own hardware for it let me know and I can help you out with that too! The second assumption that I’m making is that this specific run will be point–to–point and not a multi-drop connection (for this cable run). If this is not true let me know and we can try to adjust for that.

Two-Wire Transmission:

Luckily, doing a fiber transmission for a two-wire Flexray setup shouldn’t be terribly difficult. I am going to recommend that you use COTS 1000BASE-EX Ethernet hardware. Everything on the market should be built and guaranteed to work at this IEEE spec and meets your distance requirements. This spec can run to ~40Km over single mode fiber using the 1310nm wavelength as the carrier.

The key to understanding optical communications is that the crux of the problem is taking interfaces with many wires and data channels down to one channel for all of them. Traditionally this is down using serializers and deserializers (SERDES) to shift all of this data into one transmission path. This Transmission path is comprised of a Tx and Rx pair. In practice a challenging task because compressing data lines together and maintaining the transfer speeds of the protocol requires a SERDES signal that can switch at a speed equaling the sum of all the aggregated data rates. For example, gigabit Ethernet runs at a 125MHz over four channels, which is actually125Mbaud and each symbol transmitted contains two data bits of data but this is a topic for another day, resulting in a total of 250Mbps over four differential pairs. Anyway, the switching frequency of this system can be relatively slow at the 125MHz rate but if we want to reduce it to one Tx/Rx pair then we need to boost our switching speed to 1.25GHz, a ten fold increase in speed.

Fortunately, the SERDES conversion doesn’t need to be done since the two-wire configuration is of Flexray is already a high speed differential serial link! You will need to break up the signal however into one Tx channel and one Rx channel since you can’t have bidirectional transmission on an optical fiber. This can be done with an analog DPDT switch that goes to the Tx channel when transmitting and switches to the Rx channel when receiving. The rest will be taken care of by the SFP modules form the Ethernet standards, which should provide you the power needed to traverse the 10 to 20km run.

Four-Wire Transmission:

I originally thought that the four-wire version of Flexray was just more signal content. Now, it seems from what little online information I’ve found online that the four wire version is just one differential pair for transmission and a second for receiving. Can you verify this for me, I can’t seem to find any non-two-wire info on the web. If this is accurate then I would recommend going with the four wire variant since it will be easier than the two. You should be able to make a board that routes into an SFP module and don’t even need to bother with switching.

If this answered your question please verify each of the solutions by hitting the green verify button at the bottom of each post! If you have more questions, please feel free to send them along and I will try to answer them or clarify information presented.

Best Regards,
Nick