[asterisk-users] Wi-SIP vs. SIP-DECT

Gordon Henderson
Fri, 29 Aug 2008 15:54:07 -0000

On Fri, 29 Aug 2008, Karl Fife wrote:

> Anybody care to muse on Wi-SIP vs. SIP-DECT?
>
> My limited research indicates that none of the WiSip phones will ever be
> able to match the performance of DECT phones.  Maybe I'm wrong but a
> Wi-SIP phone seems like a DIESEL sports car.  There is nothing wrong
> with the technology, but it seems like a shoe-horned fit into the
> requirements of a wireless endpoint.  DECT uses a wireless radio layer
> that was engineered from the ground-up with the design priorities of a
> wireless endpoit.
>
> I notice that the standby times of Wi-SIP vs. SIP-DECT are a great
> illustration of this point.  I guess there's no low-power way to
> participate in a WiFi network, hense standby battery life that sucks in
> Wi-SIP.
>
> I've never actually demoed a Wi-SIP phone on premesis, but if the range
> of my WiFi LAPTOP vs. my DECT 6.0 headset is any indication, (DECT more
> than double the range) I'd guess it to be quite hard to make a case for
> Wi-SIP unless you're doing some straight-up network application
> integration right onto the phone.  Can anyone speak to this?

I've used both - with good results.

However, I've also done a *lot* of network building using Wi-Fi and it's
not that good for telephony. Firstly it's half duplex (so's DECT, GSM,
etc. but that's OK, as it's designed that way), and what I've found,
certinly in the consumer and some of the access point aimed at businesses
is that the radio turn-around time sometimes becomes significant. In
tests, I found that most units would degrade horribly when the packet size
was < 140 bytes or so. VoIP packets are 160 bytes, so we're mostly OK
there. The packet size and frequency (50 packets a second, both ways) is
the biggest killer for access points.

They're really optimised for streaming data one way, so big 1500 byte
packet down, tiny ACK packet back. Intersperse that with VoIP and you get
issues. Even with fancy access point that have traffic management, sending
just one big 1500 byte packet can have an impact on a stream on 160 byte
packets that need to be sent at a specific rate.

So you'll get away with it on your home network (or small office) if
you're the only one using Wi-Fi. I make calls with my Nokia E90 + SIP &
Wi-Fi and for the most part, it's fine. But get someone else on the same
access point and have them do some file-bashing to a local server and
you'll get issues.

If you can go to the expense of running a totally separate Wi-Fi network
just for VoIP then you'll probably be fine, but my old AP barely copes
with 2 concurrent calls. 3 calls and you start to get loss.

Then you've got the issues of channel separation. There are only 3 true
clear channels in the spectrum that don't overlap. I can see 5 other
access points from my office now (and the town CCTV system runs in the
2.4GHz band too )-: and I'm in a rural location, so what hope is there in
a busy office complex...

So Wi-Fi works, but only just... DECT with repeaters seems so much
better...

Gordon