[Asterisk-Users] iax2 wireless and Multicast

[tim panton]

On 4 Jan 2006, at 13:28, Francisco Pérez Botella wrote:

> El Miércoles, 4 de Enero de 2006 12:28, tim panton escribió:
>> On 3 Jan 2006, at 19:10, Francisco Pérez Botella wrote:
>>> Hi.
>>>
>>>
>>> I will have to manage From asterisk to clients IP-phones, so biefly
>>> the idea
>>> is to multiplex voip flows in large packets and multicast them from
>>> asterisk/AP to client stations. flows from client stations to  
>>> asterisk
>>> gateway go unicast. I wonder how iax2 protocol will be good for
>>> multiplex
>>> (trunk) and multicast ??
>>
>> Hmm, it won't be easy.
>> The IAX protocol is not multicast aware, so it is expecting a single
>> ack to each
>> full frame.  You will have to do quite a bit of work on the IAX
>> implementation
>> for it to do the right thing in that area.
> I see, maybe I could redirect at network layer unicast-->multicast
> addresses/group and give back a "false" single ack at that point.
> On the other side (client side). I need some like a "virtual trunk"  
> where each
> station recieves the full frame and "stealth" the payload it needs  
> for the
> user/phone(s) it serves. I could at client station redirect traffic  
> from
> multicast to unicast interface address and serve the full frame to  
> iax2 at
> client station, silently dropping the acks they give back.

yes, but you need to ensure that only one client station sends an  
ack, or
that the server station can cope with multiple acks.

>
>>
>> I'm also not sure I see the advantage of multicast, given that  
>> normally
>> phone calls are 1 to 1 connections, (except conferences I suppose).
>
> That maybe true for wired but wireless in infraestructure mode  
> there's a point
> of distribution (the AP) that even can police and pool in a pseudo  
> TDM, I
> mean all the traffic in the subnet is going to pass trought that point
>

Sure, but that isn't any different from any asterisk server connected
to an ethernet (except in speed) (Wasn't the pre-cursor of ethernet
a radio based net in Hawaii ?). You aren't saving very much capacity,
as IAX miniframes have a low overhead. You would probably do better to
run IAX over the lowest level protocol you can get at (i.e. lose IP  
and UDP
headers and go straight to the packet radio level).

>> Is it a packet size problem ?
>
> It's a capacity problem first, it's an avoidance of collisions too.
> wireless is a shared medium (radio) and minimizing overhead without  
> latency
> penalty will be important. I think that in a radio system broadcast  
> is for
> free capacity and overhead is not.

Yep, but apart from the headers you won't be saving any actual  
payload bytes,
  unless more than client is listening to the same stream at the same  
time.

As for collisions, I see a (nasty) problem that trunking might cause:
Many IAX clients use the incoming audio stream as a timing source to the
outgoing one. In a Trunked/multicast situation you'd have all your
clients replying in sync - which would cause collisions, since they
would all reply at once. You would have to impose some delay on the
client side to ensure they didn't. Easier not to trunk I'd say.


T.


http://www.westhawk.co.uk/

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