3 ms·

100base-T1 and 1000base-T1 are indeed the old OPEN, and are indeed what's deployed in production cars. 100 Mb PHYs are available from multiple vendors, and are

by addaon 5y ago

100base-T1 and 1000base-T1 are indeed the old OPEN, and are indeed what's deployed in production cars. 100 Mb PHYs are available from multiple vendors, and are mature and quite pleasant to work with (built in TDM for detecting wiring issues, etc). 1000 Mb PHYs are rarer (I've only seen Broadcom in person), but it's clearly coming. The 100- and 1000- protocols are pretty different; both are really focused on allowing cheap crappy wiring, but this is hard enough for 1000- that things like FEC are added. The equivalent of PoE for -T1 is PoDL (power over data lines), which is much more civilized than PoE (no transformers); and since 1000- runs at a higher lowest frequency than 100-, it's significantly easier to get power over it with smaller inductors. I'm not too familiar with the current state of -T1L, but the little I've seen about it in the past was a focus on taking the advantages of -T1 (low wiring cost, simplicity, robustness, PoDL) and applying it to industrial applications, where MUCH longer cable runs than in automotive are desired. While I was following this was mostly done by adding in a 10base-T1L physical layer, giving up an order of magnitude of speed in exchange for an order of magnitude or two more run length. Don't know if there's any intent to have faster -T1Ls, haven't followed that far. 10base-T1L, 100base-T1, and 1000base-T1 are all pretty different physical layers, although they share some principles. I /think/ the Broadcom 1000base-T1 PHY can run in 100base-T1 mode, and maybe even auto-negotiate (yuck), but they're quite different under the hood.

tinco 5y ago
Thanks for sharing your experience! What is the advantage of using single pair ethernet in for example cars? My company owns a largish drone and it has a regular CAT5 network so surely it can't be weight or size.
- mimentum 5y ago
  Surprised it isn't running a CANBUS system.
  - detaro 5y ago
    CAN is slow compared to Ethernet and really optimized for tiny payload sizes, so for plenty purposes the former isn't that useful. (Which is also why Ethernet is pushing into automotive)
- zbrozek 5y ago
  I've spent ten years designing electronics for cars. It's very often harness reduction under various motivations. The bundle needs to be small to fit through chassis holes (which are kept small to keep structures strong). Keeping the pin count low increases available connector choices and reduces connector costs (especially where they must be waterproof). Off in the distant corners of minds is weight. Your drone would probably do the same if the manufacturer had more engineers to spend on optimizing the design. But it's probably also sold in lower volume than car parts. Alternatively, maybe it's using CAT5 in a nonstandard way. For example, maybe two links or non-PoE power or even not Ethernet at all.
  - sorenjan 5y ago
    How are car communication done in modern cars? Is there a single cable run from the ECU in the front to the cabin and one to the back, and then some kind of switch connects stuff like gauge cluster, AC, buttons etc in the cabin, and lights, camera etc in the back? How about fiber cable, is there any movement towards that? Some cars have fancy 360 camera systems that I imagine can use a lot of bandwidth.
    - tfigment 5y ago
      Cars use CAN for most controller networks. It can go up to 1 Mbps but lots of time maybe 250 kbps. It has advantage of being broadcast so all devices get messages at same time. The wiring tends to have noise reduction properties and floating reference voltages as i recall. For cameras i'd imagine they use something else for bandwidth and ease of use reasons.
      - sgtnoodle 5y ago
        CAN is content addressed rather than sender/receiver addressed. The address is also used to arbitrate transmit priority on every single message frame. This allows for messages to be sent with well controlled latency, which is important for stuff like drive-by-wire steering and braking. CAN bus wiring isn't floating in that it isn't isolated. The transceivers are required to deal with a large common mode voltage range, though, and there are isolated transceivers for specific use cases. The noise reduction properties are simply that the digital signal is encoded as the difference in voltages between two wires, and those wires are supposed to be twisted together. Also, the typical behavior is for a CAN controller peripheral to automatically re-transmit frames that didn't make it on the bus, for example, if there was too much noise that a receiving node got a CRC mismatch. A video feed from a camera would be way too much bandwidth to run over a typical CAN setup. For reverse cameras, it's probably HDMI or something. For more elaborate safety and convenience systems, it's likely either close enough to the computer that it's a parallel bus, or far enough from the computer that it's a differential high speed interconnect that's qualitatively similar to HDMI.
- theincredulousk 5y ago
  Having single-pair vs. cat5 is kind of a bonus, for wiring simplicity and weight. I could also see the power delivery part being useful for things like sensors and other oddly placed/small powered devices. The main benefit is Ethernet. It has already started, but in the next few model years of cars, there is a large transition happening from CAN -> Ethernet as the primary communication bus. The simplest reason for that is software + bandwidth + integration. To implement semantics like an RPC call across two modules in car via CAN, it is kind of ugly and almost always results in awkward software interfaces and extremely inflexible implementations. Whereas with Ethernet now you can do something like use gRPC and protobufs, which is entirely flexible and results in well-defined/formed software interfaces. CAN will still be used for a while for what it is best at: low-level, reliable, low-bandwidth chatty data interfaces that make sense for electro-mechanical parts - especially those that don't have (or need) anything more than a very bare-bones microcontroller. Cost is still a huge factor in automotive design, because a $1 difference x 1 million cars blah blah it makes a difference. Bandwidth is obvious. CAN can't do video, and in practice audio either (although theoretically possible I guess). Cars have lots of audio and video devices now, and it is unnecessarily complicated to always have command/control and media data on separate buses. Imagine if your computer used one physical network interface for HTTP requests and another interface for streaming video. Sure it'd work fine but you'd have a bunch of extra cables, ports, hardware, and software complexity to make it work.
nousermane 5y ago
> I think Broadcom 1000base-T1 PHY can run in 100base-T1 mode Marvel claims their 88Q211x PHYs can do that. No auto-negotiation. [0] https://www.marvell.com/content/dam/marvell/en/public-collateral/automotive-solutions/marvell-automotive-ethernet-88q211x-product-brief.pdf https://www.marvell.com/content/dam/marvell/en/public-collat...

by addaon 5y ago

tinco 5y ago

Thanks for sharing your experience! What is the advantage of using single pair ethernet in for example cars? My company owns a largish drone and it has a regular CAT5 network so surely it can't be weight or size.

mimentum 5y ago
Surprised it isn't running a CANBUS system.
- detaro 5y ago
  CAN is slow compared to Ethernet and really optimized for tiny payload sizes, so for plenty purposes the former isn't that useful. (Which is also why Ethernet is pushing into automotive)
zbrozek 5y ago
I've spent ten years designing electronics for cars. It's very often harness reduction under various motivations. The bundle needs to be small to fit through chassis holes (which are kept small to keep structures strong). Keeping the pin count low increases available connector choices and reduces connector costs (especially where they must be waterproof). Off in the distant corners of minds is weight. Your drone would probably do the same if the manufacturer had more engineers to spend on optimizing the design. But it's probably also sold in lower volume than car parts. Alternatively, maybe it's using CAT5 in a nonstandard way. For example, maybe two links or non-PoE power or even not Ethernet at all.
- sorenjan 5y ago
  How are car communication done in modern cars? Is there a single cable run from the ECU in the front to the cabin and one to the back, and then some kind of switch connects stuff like gauge cluster, AC, buttons etc in the cabin, and lights, camera etc in the back? How about fiber cable, is there any movement towards that? Some cars have fancy 360 camera systems that I imagine can use a lot of bandwidth.
  - tfigment 5y ago
    Cars use CAN for most controller networks. It can go up to 1 Mbps but lots of time maybe 250 kbps. It has advantage of being broadcast so all devices get messages at same time. The wiring tends to have noise reduction properties and floating reference voltages as i recall. For cameras i'd imagine they use something else for bandwidth and ease of use reasons.
    - sgtnoodle 5y ago
      CAN is content addressed rather than sender/receiver addressed. The address is also used to arbitrate transmit priority on every single message frame. This allows for messages to be sent with well controlled latency, which is important for stuff like drive-by-wire steering and braking. CAN bus wiring isn't floating in that it isn't isolated. The transceivers are required to deal with a large common mode voltage range, though, and there are isolated transceivers for specific use cases. The noise reduction properties are simply that the digital signal is encoded as the difference in voltages between two wires, and those wires are supposed to be twisted together. Also, the typical behavior is for a CAN controller peripheral to automatically re-transmit frames that didn't make it on the bus, for example, if there was too much noise that a receiving node got a CRC mismatch. A video feed from a camera would be way too much bandwidth to run over a typical CAN setup. For reverse cameras, it's probably HDMI or something. For more elaborate safety and convenience systems, it's likely either close enough to the computer that it's a parallel bus, or far enough from the computer that it's a differential high speed interconnect that's qualitatively similar to HDMI.
theincredulousk 5y ago
Having single-pair vs. cat5 is kind of a bonus, for wiring simplicity and weight. I could also see the power delivery part being useful for things like sensors and other oddly placed/small powered devices. The main benefit is Ethernet. It has already started, but in the next few model years of cars, there is a large transition happening from CAN -> Ethernet as the primary communication bus. The simplest reason for that is software + bandwidth + integration. To implement semantics like an RPC call across two modules in car via CAN, it is kind of ugly and almost always results in awkward software interfaces and extremely inflexible implementations. Whereas with Ethernet now you can do something like use gRPC and protobufs, which is entirely flexible and results in well-defined/formed software interfaces. CAN will still be used for a while for what it is best at: low-level, reliable, low-bandwidth chatty data interfaces that make sense for electro-mechanical parts - especially those that don't have (or need) anything more than a very bare-bones microcontroller. Cost is still a huge factor in automotive design, because a $1 difference x 1 million cars blah blah it makes a difference. Bandwidth is obvious. CAN can't do video, and in practice audio either (although theoretically possible I guess). Cars have lots of audio and video devices now, and it is unnecessarily complicated to always have command/control and media data on separate buses. Imagine if your computer used one physical network interface for HTTP requests and another interface for streaming video. Sure it'd work fine but you'd have a bunch of extra cables, ports, hardware, and software complexity to make it work.

nousermane 5y ago

> I think Broadcom 1000base-T1 PHY can run in 100base-T1 mode Marvel claims their 88Q211x PHYs can do that. No auto-negotiation. [0] https://www.marvell.com/content/dam/marvell/en/public-collateral/automotive-solutions/marvell-automotive-ethernet-88q211x-product-brief.pdf https://www.marvell.com/content/dam/marvell/en/public-collat...