This document is the start of discussions on a protocol for communication over "low-performance" wireless links.
Alex raised the issue of a "layout ID" so e.g. portable devices (WiFi throttles) can identify which of several layouts (e.g. at a show) should be joined. See working note. This can't necessarily be handled by e.g. names of access points, because a common wireless infrastructure might be in use at e.g. a convention center hosting multiple layouts.
The following was from several email discussions, not one single sequence. Authors are attributed in the headings.
(Starting a new thread with a better subject) Paul and Alan have raised a bunch of interesting issues associated with wireless connections, see below. It sounds like now is a good time to start thinking about OpenLCB over wireless links, e.g. the characteristics of a "wireless wire protocol" (sorry, couldn't resist). My initial thinking was that this would be handled where the "wwp" connects to the rest of the world. E.g. "wwp nodes" would talk a specific way among themselves, like "CAN nodes" talk a specific way to each other. Then, when information has to go to the wider world, there would be gateways/routers responsible for transforming wwp to other form(s). (As an aside, in 30+ years working with digital communications at the nitty-gritty level, I've _never_ seen anything that's truly "transport independent" across more than one transport. There's always something that's done to adapt to another transport form, whether it be packetization, zero-insertion, reformatting or something else. Two non-identical transport mechanisms are, well, not identical, and the real issue is how hard/easy/automatic it is to make the transformation) But if we want to extend OpenLCB to a transport protocol that's not "reliable" (no loss, no duplicates, no reordering), we should look at the implications of that. Can we handle them at the level of the wire protocol? Or will this force changes at higher levels? For example, datagrams for e.g. configuration have some nice properties. For one, duplicates don't cause trouble for configuration, because each carries address, etc (they cause trouble for the datagram state machine, but that's not the level I'm looking at right now) That requires more overhead in the messages than e.g. send the address, then a bunch of data in separate datagrams, but it enables stateless code at both ends which is very nice. In the presence of duplication, do we need to put more emphasis on doing this kind of thing in all protocols? For another example, streams rely very, very heavily on the "reliable" property. They have no internal structure to detect loss, duplication or reordering, nor to cope with it. I think I start to see how _detection_ can be added with e.g. sequence numbers, but _correction_ has more complex implications for buffering, especially at intermediate nodes. Do gateways have what they need to be able to handle correction of lost/missing/reordered messages in a stream? That needs to be carefully examined, probably simulated, and eventually tested. Or is it possible to take an approach where some other, commonly available protocol can be layered on the wireless to just fix the problem? An analogy would be: when Internet Protocol (IP) doesn't meet the need, one uses TCP/IP instead. Is there an equivalent "TCP for wireless"? There's some really, really interesting stuff here. Sometimes we use "interesting" to mean "insanely hard", though... Bob On Aug 2, 2010, at 8:55 PM, email@example.com wrote: > On 3 Aug, Bob Jacobsen wrote: >> On Aug 2, 2010, at 4:40 PM, firstname.lastname@example.org wrote: >>> Remember here, I'm interested in doing this wirelessly... >>> We cannot consider this an error, because, for all we know, the old >>> next hop neighbor has gone off line (it has gone to sleep, moved out >>> of communication range,etc), and the network topology is adjusting >>> to the new node arrangement... but this leaves open the possibilty >>> of >>> receiving a duplicate message from the network if the next hop >>> neighbor >>> is still active. >>> >> That's going to add some excitement! >> >> A _lot_ of OpenLCB's simplicity comes from assuming reliable >> transport: No messages lost, no messages duplicated. >> >> I'm at a loss for how to deal with nodes that can't be heard at all. >> If e.g. a handheld throttle can't be heard, it can't control things. >> About the only thing one can say is "don't do that". > > Since we're talking about an ad-hoc network here, which is very > beneficial to the modular railroading world, there are two problems > that > do need to be addressed. > > The easy problem is that the node is acting as a relay node. When a > node > like this goes offline, the network needs to re-route traffic around > the > now missing link. > > The second case it is that a node is an endpoint. There are two > possible cases: > case 1) the node moves from one segment of the network to another. > In this case, the network topology needs to be updated to > reflect the new location. > > case 2) A node disappears and possibly re-appears later. You can > think of this as a throttle being turned off for a while (in the > sensor network world, we would say the node is sleeping - which > means at least the transceiver is turned off). While the node is > off, any packets addressed to it either needs to be stored for > later delivery (if the message must be delivered) or simply > discarded, if the application requires the nodes to be online > when the messages are sent. > > In an ad-hoc network, there's nothing to say a node can't be both a > relay and an endpoint. > >> Duplicated messages, particularly intermitted duplicated messages, >> seem somewhat different. Certain ones won't make much difference (A >> truly duplicated PCER event report isn't necessarily a disaster), but >> other ones are going to seriously mess with communications state >> machines and cause much trouble. >> >> What are the usual ways of dealing with this when connecting wireless >> networks? > > When this matters, we allow duplicate packets to arrive, and We use > sequence numbers to filter out duplicates in the networking stack. > This > is true even in the embedded systems I work with (Which are mostly > based > on IEEE 802.15.4 hardware... so the frames are limited to a maximum of > 128bytes.) > > These sequence numbers can be simple sequence numbers... i.e. the > sequence number for the next message received must be larger than the > one for the last message received.... > > Though generally they are tied in with the reliable delivery > mechanisms > somehow... in this case, acknowledgments are sent for single > packets... > possibly on a hop by hop basis (It's cheaper from both a bandwidth and > energy perspective to retransmit over a single hop rather than over > the > entire path...). > On Aug 3, 2010, at 1:13 AM, alanheathrobinson wrote: > > On Paul's comments, my only thoughts so far are:- > > 1. Radio networks are a mystery. > > 2. They can't possibly be ignored. > > 3. Dealing with sequence numbers can't be done at CAN end points > because the packets are too small. Sequence numbers would have to be > added and stripped in the gateways. > > 4. There didn't ought to be any problem in passing sequence numbers > with messages on the backbone. So perhaps the way out would be to > include them in the generic message definitions and make special > arrangements (again!) for CAN. >
On Aug 3, 2010, at 11:16 AM, Mike Johnson wrote: > Is this another problem or just the OpenLCB over IP problem again ? > > If there are OpenLCB CAN to Ethernet gateways, and cheap wireless > routers, > and cheap wireless internet devices (Indian goverment $35 ipad) then > will > anybody want to design and build something completly different ? > > "Wireless" means a lot of different things (1kW CW at 500 KHz anyone?), but I think Paul was referring to Really Low End stuff, such as are used in wireless sensor networks: http://en.wikipedia.org/wiki/Wireless_sensor_network The idea is that each itty-bitty-radio doesn't have a lot of communications capability, hence is cheap/light/small, but the network of them still provides useful communications. It's a reasonable question as to whether e.g. wireless throttles as we know them will survive, as mass-produced devices may replace them. Perhaps people will still want dedicated throttles with knobs, perhaps some other solution will be found. But I think it's a good bet that _other_ uses for itty-bitty-radio nodes will be found as the cost decreases. E.g., how about direct communications to trains via ibr nodes in place of decoders? Or reducing the wire burden under a layout by having ibr for control point boards?
> Or is it possible to take an approach where some other, commonly > available protocol can be layered on the wireless to just fix the > problem? An analogy would be: when Internet Protocol (IP) doesn't > meet the need, one uses TCP/IP instead. Is there an equivalent "TCP > for wireless"? This was my thought, too. We package our packets inside another wireless protocol that handles these problems. This brings to network topology. Should we be setting some restrictions on this, to simply things? Eg should there be 'leaf' segments? CAN segments perhaps should be leaf segments, ie only one connection to the 'backbone'. This would not prevent joining two CAN segments with a bridge/repeaters, though, as long as the other CAN segment was not also connected to the backbone separately. Similarly, wireless segments perhaps should only connect at one place ( although this might be distributed but would appear as one connection).
Suppose a large layout with a number of CAN segments. A modular meet maybe, and there's a problem to fix. Clever geezer wants to diagnose it, so he pulls his laptop, a crossover ethernet cable, and a CAN/ethernet gateway out of his bag and hooks up to his own CAN segment. Just like he always does back home, well within his comfort zone. That CAN segment now has two gateways. Violation of the 'CAN segments must be leaf nodes' rule?
Site hosted by
This is SVN $Revision: 924 $