I've been using the xPL Protocol since 2005, prior to that I was using Jabber (or XMPP as it is known now). As I'm thinking about changing protocol, I thought I'd write a bit about xPL and, in later posts, something about the candidates to replace it.

The xPL Protocol has three types of messages: commands (xpl-cmnd), triggers (xpl-trig) and status (xpl-stat). They have the same simple format consisting of the message type, common "header" fields, the schema type and schema-specific "body" fields. For example:

xpl-cmnd
{
hop=1
source=vendor-device.host
target=*
}
x10.basic
{
command=on
device=c3
}

xPL doesn't use a server as its "message bus" is simply UDP port 3865 on the local network broadcast address. In order to avoid multiple devices having to listen on the same port on the same broadcast address, each host runs a hub that distributes messages to all local xPL clients. I think this is a poor solution to this problem. It introduces an unnecessary single point of failure (albeit one with very simple-to-test behaviour). To avoid this single point of failure, my xPL clients also support a hubless mode where they simply set the SO_REUSEADDR socket option on the listen socket so that all clients can coexist on the same port on the same broadcast address. However, if you need to interoperate with clients from other developers then you'll need to use the standard hub model.

Using UDP broadcasts to avoid any (unnecessary) single points of failure is a good idea. However, I have more than 20 xPL clients on my network and all clients process all messages they receive, discarding most of them and responding to relatively few. This means that all of the clients have to wake up and consume CPU resource for every single message. (This is one of the issues that is prompting me to re-evaluate my choice of protocol.)

I thought about avoiding this problem by combining several clients in to one more complex client but this seems like a bad idea as it will inevitably lead to more complexity and thus more complicated failure cases. (My xPL clients are structured in such a way that this is very easy to do and some users with lower-powered servers do this.)

For me, the xPL Message Schema are what make the protocol work so well. Essentially, the schema define the fields to expect in the body of an xPL message. This means I can write a client for a UDIN USB Relay Controller or a Phaedrus VIOM that supports the control.basic schema and someone else can write a client that uses the same schema to control relays and it will happily interoperate with my clients. (In fact, I realised this advantage when I migrated from using a VIOM to several more compact UDIN controllers. I only needed to change the names of the relays for open/close on my blinds/curtains after each set of wires were migrated to the new hardware.)

The schema, as described in the protocol documentation, support a type of inheritance. A message schema that inherits from another must include all of the mandatory fields of the other, in the same order, so clients can support the basic function even if they don't understand the additional sub-schema fields. However, since optional vendor-specific fields are also allowed without inheritance (giving a simple duck typing approach), the inheritance mechanism doesn't really give much benefit.

Take hbeat messages, there are two types, basic:

xpl-stat
{
hop=1
source=vendor-device.host
target=*
}
hbeat.basic
{
interval=5
}

and, inherited from basic, app:

xpl-stat
{
hop=1
source=vendor-device.host
target=*
}
hbeat.app
{
interval=5
port=43438
remote-ip=198.51.100.2
}

Why do the messages need the .basic or .app? So that a client knows to expect the port and remote-ip fields? Why does a client need to know to expect them? A client can just parse the message and if the fields are present - i.e. no } following the interval=5 line - then it can treat the message like an .app message and like a .basic message if not. It is simply redundant.

The only obviously valid use of the .suffix is .confirm suffix but this would probably be better implemented as a fourth message type such as xpl-ack or xpl-conf.

The wildcard target=* line is also redundant since a lack of explicit target=vendor-device.host could just as easily be used to distinguish the wildcard case.

These superfluous elements mean that the "Lite on the wire, by design" subtitle on the xPL Protocol specification isn't really as "lite" as it could be (and IMHO should be).

Another confusing aspect of the xPL Protocol is the notion of a device. In the protocol spec, for instance in the Device Groups section, it is clear that the target field is addressing an individual device - a controller for a single curtain/drape. This is fine in theory, but in practice, individual devices are general addressed using fields in the body. For instance, control.basic, remote.basic, sensor.basic, x10.basic, and x10.security use device, homeasy.basic uses the combination of address and unit, zwave.basic uses node, etc. This is because xPL clients are typically controllers/gateways for several devices. For instance, I have a 1-wire sensor/relay network with dozens of devices attached to it but one xPL client opens the 1-wire USB interface and manages all of these devices based on the device field in control.basic and sensor.basic messages.

The protocol would be simpler to use if addressing of devices was more consistent. In fact, devices (rather than clients) should be first-class citizens on the "network". That is, they should be discoverable (like clients are today with hbeat requests) and probably should be aliasable (so that they can be given memorable names).

I like the xPL Protocol; it has been my protocol of choice for more than 5 years and most of these issues are not a big deal. The broadcast message bus is simple (if you ignore hubs) and reliable but the inefficiency of waking all clients on every message is starting to become an issue for me. So, in the next few posts, I'll write about some of the alternatives I am considering.

In the previous post, I described two of the three significant X10 problems we had when setting up our home automation system. This post will cover the third problem, latency.

Although all our lights have conventional switches (well, momentary switches in conventional locations), we use a lot of X10 RF Remotes and X10 Motion Sensors to trigger lights and other devices. Initially, we used a TM13U RF Transceiver Module that receives the X10 RF signals and converts them in to X10 powerline signals. If we stopped here and let the powerline signals trigger the X10 devices directly this solution would probably be okay. However, we wanted computer control so we have another level of indirection. The powerline signals are then received by the X10 CM12U Transceiver and sent to the house computer. The computer then figures out how to react to this signal and, if the action involves an X10 device, sends another signal back to the CM12U where it is sent on the powerline.

The problem with this setup is that the X10 powerline is slow, so converting the (fast) RF signal to a slow X10 signal to get it to the computer is not very smart.

Additionally, RF devices repeat their signals which in turn get repeated on the powerline. While the repeats are going on, the computer, responding to the first received signal, is instructing the X10 Transceiver to transmit a command on the powerline. This results in frequent collisions leading to noticeable delays (and X10 powerline signalling isn't fast to begin with).

So, we purchased a W800 X10 RF Receiver and gave up on the TM13U. Using the W800 serial port receiver, the X10 RF could be read directly by the house computer. This avoids the delay and the collisions of the X10 powerline signals for the trigger and any triggered X10 commands.

The W800 was a solid piece of kit and worked very reliably. However, in the end, I replaced it with an RFXCOM RF Receiver as that was able to receive additional protocols. I expect I'll rave about RFXCOM products in future posts.

I've written two posts about some of my favourite home automation features and both happened to use X10. So, I thought I should probably mention the significant problems we had with X10 and how we solved them.

We had three significant problems. The first was noisy devices preventing the X10 powerline signals reaching some parts of our house.

Initially, we attempted to resolve the noise problems by using X10 FM10 Plug in Filter Modules or X10 FD10 DIN-mounted Filter Modules. We still use them on a number of appliances - things like TVs, fridges, laptop power supplies, UPSs, etc. They definitely help but there were still sockets that didn't get any signal. (We know because we used an X10 Signal Tester to check the sockets.)

Eventually, I came across Jeff Volp's excellent explanation of X10 problems and ordered an XTB signal booster. This device takes the X10 signal from the X10 CM12U Transceiver computer interface and boosts it by a factor of 10. This fixed our X10 powerline signal problems completely. We still occasionally plug in a new, exceptionally noisy device that needs a filter but this is very unusual.

The second problem was with X10 RF signals. When the batteries in an X10 Motion Sensor run out, they often end up transmitting excessively. This effectively jams any RF signals (including my car key buttons). Unlike most of the other RF devices we use, these devices don't report battery status so I've solved this problem by using an excellent charger, good quality rechargeable batteries and reminders to change the batteries every 6 months.

The third problem, with latency, is a bit more complicated so I'll cover that in another post.

Lighting is an important aspect of our automated home. Most of our lights are controlled via either X10 DIN-mounted Lamp Modules or X10 DIN-mounted Appliance Modules. (Some are RGB LED lamps but I'll cover them in a later post.)

Although I am a morning person, I still need a little help waking up in the morning. My lovely children help a bit but being able to have the lights gradually brighten to full brightness when it is time to get up helps a lot, particularly in the winter. It also means that the children wake up at the same time every day, which in turn means they tend to go to sleep pretty reliably in the evening. In the event that they wake up too early, they know it's too early as the lights aren't on yet so generally don't wake us.

We also have X10 Motion Sensors so can automatically trigger lights in rooms with no natural light. Our utility room, where we change nappies, is a good example as this behaviour is particularly useful when you have your hands full carrying a reluctant toddler.

The children rather like the low-level stick-on X10 RF Remotes that enable them to turn on their lights (and I like the fact that they don't have to climb on the furniture to reach the "normal" switches). (They also like having the remotes trigger wolf howls, T-Rex roars and Michael Rosen from the ceiling speakers.)

One disadvantage of the dimmable lights is that most dimmers have a minimum rating. The LD11 X10 DIN-mounted Lamp Modules that we use require a minimum load of 60W. This is a problem because it is becoming increasingly difficult to buy bulbs that meet this requirement. At the moment, we typically use Energy Saver GLS Lamps which look great and use just enough power to keep the dimmers happy.

I'd be very interersted to know how others with X10 solve this problem as I'd love a solution that used less power but which was still dimmable and produced a pleasant bright light.

It has been a while since I wrote a blog entry so I thought I'd write a few posts to describe the current state of our house (with respect to home automation, not the washing up or the mess made by my two lovely children). I'll start with a few posts covering the most useful features of our automated home.

The most important device we've automated is the kettle. We "prime" the kettle after making a cup of tea by filling it, switching it on but turning it off at the mains (with an X10 Appliance module). We can then turn it on using either an X10 RF Remote, a web interface, timers or via a number of other mechanisms. This is a real time saver. I can "C-c k" from emacs and only go to the kitchen when the kettle has boiled rather than going to switch it on and wait while it boils.

Tracy says that this is just laziness. I probably used to empty/load the dishwasher while waiting for the kettle, so perhaps she is right. She prefers the fact that you can hit a remote button by the front door when you come home on a cold day, or from upstairs before you come down in the morning, and have the kettle waiting when you get to the kitchen.

The house computer also monitors the mains current and attempts to turn the kettle off for you if it sees a drop in current usage equivalent to the kettle usage. At the moment, it's working off the whole house current usage data so it doesn't always work but it still often manages to recognize the change and switch off the kettle at the main while you are filling the kettle.