Monday, August 22, 2011

Multiple Handsets

Having multiple handsets registered to one base is somewhat a strange situation. Usually in snom, one product has one handset and with the m9, we do have up to nine handsets. This raises a couple of questions in provisioning, calling into the base and calling out from the base station.

Provisioning becomes pretty much completely different and I know that many PBX devices have problems coming up with configurations that register multiple SIP identities. One MAC address is suddenly associated with multiple accounts. We were thinking to “virtualize” the configuration, so that you can essentially provision the device with multiple MAC addresses. However after some initial tests and thoughts we came to the conclusion that this would be even more complicated and we stick to the good old XML provisioning style that most users know from the 300-series. We also had the snom m3 product before, and there the configuration was also in one file with one MAC.

Calling into the device was also causing some problems. If you have multiple handsets, the question is which handset should ring. Essentially, we can up with two answers: the first one uses the home-based model (most of the traditional DECT phones are still sold in the residential area), where the incoming call rings all handsets that are registered to the base station. This is what you pretty much get when you have only one identity, multiple handsets and you don’t configure anything else on the device. The other answer is to use the PBX extension model, where the base is more or less just a router and routes the call to the handsets. Even internal calls are routed through the external PBX in this model; for example for call recording that is necessary anyway. In this model the base has multiple identities and each identity has one handset assigned. Each identity also has its own address book, own codec settings and so on. It is almost like the handsets are registered to different base stations.

The outbound model is similar to the inbound model. In the “home” model, every handset uses the same identity (not much choice here, there is only one identity). In the extension model, each handset uses its own identity and uses that for the outbound call.

There are some cracks in the extension model. For example, when it comes to the PIN, the handset cannot deny that it is registered to the same base like other handsets. If one knows the PIN for the base, essentially that handset has control over the base. In the beginning the DND feature required the entry of the PIN, which was obviously not very practical; also because it did mean that everyone who has the right to put him/herself on DND, also had the right to nuke the base configuration. Our recommendation is to keep the PIN an administrator secret and don’t give it out to the user.

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