Friday, 30 January 2015

RADAR over wifi: Not always a pretty picture

OpenCPN plugins support overlay of imaging from some Garmin and Navico RADAR units.  Unfortunately some users attempting to use these plugins over wireless networks on their boats have encountered issues. Not only does it not work very well, it can make the whole wireless network unusable.  Moreover where people bridge a wireless segment to their wired network (e.g. with a simple domestic access point/switch/router) the wireless network can become unusable when the radar is switched on even if nothing on the wireless side is using it.  What's going on and what can be done to resolve it?

Why RADAR imaging might not mix well with wireless

The problem is with the way images are transmitted by the types of RADAR which OpenCPN plugins support (and probably others).  These RADAR images are not transmitted using standard NMEA protocols: The data rate is too high even for NMEA 2000. Instead manufacturers transmit RADAR images using proprietary methods over Internet Protocol running on 100Mb/s copper ethernet. Crucially manufacturers tend to use UDP multicast, sending data to multiple receivers at once (as opposed to "unicast", or sending to a single receiver).  In a previous post I extolled the virtues of multicast for transmission of boat data.  It allows all computers who have subscribed to a given multicast group to see the same data without them being replicated to each client.  Sending RADAR data over multicast on an ethernet network as the manufacturer intends works well.  Sending it over wifi is another matter.

The issue is not simply that the data rates are too high to be handled by wireless networks.  Navico state that their RADAR transmits at "up to 8 Megabits per second" which seems to concur with what users have observed.  Surely this is within the capabilities of most modern wireless networks?

Unfortunately a wireless access point has a number of "known unknowns" to deal with when transmitting to associated stations on a wireless network including the distance of the station from the transmitter, objects in between and interference in the environment which makes the task more complex than it is with copper wires.  The IEEE 802.11 standards on which common wireless networks are based each define a number of data rates.  Transmissions at lower data rates tends to be more robust than those at higher rates. Typically an access point will dynamically adjust the rate at which it sends data to each associated station based on packet loss (where transmitted frames are unacknowledged by the receiver at the link layer).  The connection between a "150 Mb/s" access point and a "150 Mb/s" laptop may be considerably lower than 150 Mb/s if the laptop is far away from the access point with intervening obstacles and/or there is interference from other devices in the environment.

Multicast (and broadcast) data must be sent at a rate which all listeners can be assumed to reliably receive.  Standards dictate that multicast should generally be sent using a rate in the "basic rate set" (the set of rates which all clients are assumed to support). Although not universally the case,  many retail access points will use the lowest of these rates, often 1Mb/s.  This of course is the physical data rate.  Various overheads mean that the rate of transmission of useful information will be be even less than that.

On top of this, multicast over wireless is more prone to packet loss than unicast.  Unlike ethernet, unicast 802.11 frames are normally acknowledged by receivers allowing them to be re-sent if they are lost in what is an inherently less reliable medium than copper wires.  Multicast frames are not usually acknowledged so aren't retransmitted if a receiver loses them.  Another potential source of packet loss  with high data rate multicast is the buffering that occurs on an access point when some stations on the network are in power saving mode, although without information on the sizes of buffers involved in a given access point it is difficult to assess the likely impact of this.

Note that these issues don't mean that multicast is a bad choice for "normal" NMEA-0183-style data. In those cases data rates are far lower, should not significantly impact the wireless network, and by the repeated nature of transducer data, occasional dropped packets are not necessarily a problem.  Remember also that broadcast data suffers from the same problems on wireless as multicast.

Why does RADAR stuff up all your other wireless data too?

If it's only the multicast which is constrained to be transmitted at 1Mb/s, why is your other wireless traffic badly impacted?

When transmitting multicast data the access point is operating at the multicast rate (e.g. 1Mb/s). Not only is there likely much more RADAR data than is involved in the average browsing session, the rules of contention for the radio give the multicast data an unfair advantage.  The radio in the access point is likely spending most of its time at the lower data rate.

Why does wired RADAR stuff up your wireless data when only wired devices are listening to it?

Your boat network may use an access point with an integrated switch for wired devices or you may have a wireless access point connected to switch to which your wired devices are also connected.

A simple switch has no knowledge of what devices on the network need to receive multicast data, so it will send multicast (and broadcast) data out of all ports.  This will include an attached wireless network, even if there are no wireless devices interested in receiving the multicast data.

Preventing multicast getting onto your wireless segment

Some switches do offer a method of ensuring that they only send multicast data out of ports to which a device which wants to receive it is attached: "IGMP snooping".  Whenever a program tells a computer that it wants to receive a particular group of multicast data, the computer will send an Internet Group Management Protocol (IGMP) "report" message on the local network.  Such messages are intended for routers which forward Internet Protocol (IP) packets between networks (e.g. your local network and your Internet connection) but some switches (which are generally link layer devices so know little of the higher level protocols carried in their data link frames) can listen for these reports and use the information to decide which switch ports to send multicast data out of.

Many high-end switches will have this functionality. Many low-end devices won't.

Making RADAR work over wireless

Wireless networks are also a problem for those wanting to deliver IPTV (Television over Internet Protocol)  using multicast so the problem has received extensive attention.  A number of solutions have been proposed and implemented in various pieces of hardware.  Retail access points are less likely to offer a solution than "Enterprise Class" (i.e. unaffordable) devices.  Two relevant solutions which require only support from the access point are:
  • Multicast-to-unicast translation.  Access points send multicast packets to clients requesting them via link layer unicast.  In other words, IP packets to multicast groups are delivered in unicast 802.11 frames to the stations interested in receiving them and as such these frames are delivered reliably and at maximum reliable speed.  This technique is variously implemented in some enterprise class access points using various methods.  It is one of the subjects in the (currently not commonly implemented) IEEE 802.11aa and related 802.11v standards where it is referred to as "Directed Multicast Service" (DMS).  Some implementations use IGMP snooping to determine which clients are interested in which multicast data.  With 802.11v the client can request multicast to be delivered over unicast with DMS: a cleaner solution as the OSI layers aren't so mixed up but I haven't been able to find any hint of an API for this.  DMS does not scale well which can be an issue for IPTV delivery, but on board where no more than two clients are likely to require RADAR data it is ideal.
  • Setting the multicast rate. Some devices will allow you to explicitly set the "multicast rate".  Setting this to a high value might make using RADAR over wifi viable.  The downside is that as this is the minimum rate at which clients will be allowed to connect to the access point, the range at which the access point is usable will be reduced.  In the average GRP yacht this will be a non-issue. Once again, this is more likely to be a feature on higher end equipment but is more likely to feature in consumer units than multicast-to-unicast translation.  It may also be possible to implicitly alter the multicast rate by disabling certain modulation types.

Which access points will work?

Without a RADAR unit to test or a multiplicity of access points to test I can only report what might work based on Internet research and reader feedback.  Please do let me know if you have any relevant information.  This section will be updated as new information is received.

"Enterprise Class" products capable of multicast-to-unicast are not generally found on boats so high end devices from the likes of Ruckus and Cisco are not considered here.
The popular open source alternative firmware for a number of routers implemented multicast-to-unicast in the recent "Barrier Breaker 14.07" release.

Apple Airport Extreme

This apparently allows the user to set the multicast rate.  I will endeavour to test the effectiveness of this using simulated data.

Navico wifi-1

The manual for Navico's wifi-1 module describes an option of interest:

IGMP snooping. Convert Multicast data to Unicast one.The default setting is "Enable". 
I contacted Navico to clarify this.  It seems that the "Multicast to Unicast" statement is a mistake.  The wifi-1 does do IGMP snooping but only to keep unwanted multicast data off the wireless segment. According to Navico's product manager for the wifi-1 it does not do multicast to unicast translation at layer 2 and Navico do not support RADAR data over wireless.

1 comment:

  1. Hey there -- this is a great article -- thanks for writing it. I have a Simrad MFD on my boat network connected to a Gig Ethernet switch and a router that's running OpenWRT and a laptop with OpenCPN. I find that I can get the radar working over WiFi and OpenCPN can see the radar image -- however, it seems like the AP gets overwhelmed when the MFD is transmitting radar and after about 5-10 minutes it locks up and needs to be rebooted.

    Any thoughts on improving? Should I disable the 802.11b speeds on the WiFi router? Ensure IGMP snooping is enabled or disable the multicast altogether (radar on OpenCPN is just a nice-to-have for me)?