Bookshelf Apps

Cara 7 update released, good news for SongBook users

June 22nd, 2010 2

Linn have just released a firmware update called Cara 7. This is an important one for SongBook as it has a change that makes the network eventing much more reliable. If you look at the release notes you may think that this is not a very exciting release, but if you use SongBook it is well worth the update.

We worked with Linn on making this change as we saw a lot of problems occurring due to network latency in the wireless network and specifically with the iPhone. We wrote a test application to record the latency of network messages and recorded the results over a relatively standard but busy network. The results were very interesting and showed that a longer timeout for messages was required. This will help the reliability of all control points that use network eventing (not all of them) so will be useful to most users.

For those interested in the details, the issue is to do with how UPNP sends network events. Events are things such as a track change, or play or pause being pressed, or the volume changing. In general SongBook needs to know about these things, so it tells the DS that it wants to know about these things through a process called subscription. SongBook subscribes to certain type of event messages on the DS. This is all standard UPNP by the way, theres nothing particular to Linn or SongBook here.

Now with the subscription in place every time something interesting happens the DS sends a HTTP message to SongBook to let it know. Normally this is fine and everything updates as it should. But if the message is delayed for any reason the DS detects this and cancels the subscription. SongBook has no idea that it missed an event message, or that it’s subscription has been cancelled and just thinks nothing is happening. Before Cara 7 the timeout was set to 100 milliseconds, reasonable on a normal network, but not so good on many networks out there. We regularly saw round trip times in our testing of much more than this, on a busy network maybe 5% of messages would take longer than this time, and that would mean a lot of cancelled subscriptions.

In Cara 7 this timeout has been increased (to 1 second I am told, but I have not confirmed this). In our testing of Cara 7 this has meant a complete elimination of dropped subscriptions. I can’t speak for all networks out there, but I think if you were to still see dropped subscriptions then there is something very wrong with your network and it requires some fixing up.

We deliberately try and make our test network a bit unreliable. It’s been very useful for catching bugs like this so while it’s tempting to fix it we do plan on leaving it as it is. All the flaws in our network are related to wireless, if you have a wired network with a decent switch then there is no chance of these kinds of issues arising. It’s worth making sure your wireless network is as fast as possible too. It’s not always possible to have a 802.11n only network as many devices such as iPhones and iPod touch’s still require 802.11b or g, but the day will soon come when everything is 802.11n enabled.

This also explains why some users were finding that SongBook worked more reliably when the beaconing interval on the wireless access point was decreased. Actually I’m not sure if they decreased it or increased it. If you decrease the beaconing interval then devices will associate with the network faster, and that will make waking from sleep on an iPhone work faster. If you increase the beaconing interval you reduce the network overhead and make the actual data transmission faster, which could also help out. Either way it did seem to have some effect on decreasing the latency in the network and would have reduced the number of cancelled subscriptions. The good news is that this sort of low-level network setup should not be required any more, which is how it should be.

2 Responses

Matt
June 25th, 2010

This seems to be beta (Cara 7)? Do you advise we upgrade from Cara 6 to the beta of Cara 7, or is it best to wait until it’s full release?

Jason
June 25th, 2010

Linn released the final version 3 days ago, so it should not be showing as beta now.

This is from their release announcement:

Updates are available for all current Linn DS products, including Klimax DS, Akurate DS, Majik DS, Majik DS-I, Sneaky Music DS, and Sekrit DS-I.

Added 24-bit ALAC support for up to 96kHz
Enabled AAC+ decoding for radio stations broadcasting in AAC
Improved DS discovery over wireless

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