tiny IAX/SIP switch?

Discussion in 'VOIP' started by Kyler Laird, Sep 3, 2004.

  1. Kyler Laird

    Kyler Laird Guest

    I'm hoping to give a demonstration of a mesh network between
    farm vehicles soon. I'd like to use VoIP as the application.
    I have lots of VoIP stuff already and I'm thinking of buying
    some Cisco 7940 phones.

    However...for most of the nodes I want to use Linksys WRT54G
    wireless routers running OpenWRT (and OLSR for the mesh
    routing). The problem is that I want the nodes to be able to
    communicate with each other even when there is no path to the
    wired network. (A situation would be two tractors working a
    distant field with no trucks, etc. to relay back to the
    barn.)

    To do this I just realized that I'm going to want something
    on the order of Asterisk running on each node, able to route
    calls to any other node independently. The Linksys box has a
    tiny amount of memory. I don't think Asterisk can begin to
    fit in it.

    So is there a very simple VoIP switch that can fit on a small
    system like this? Do I need to roll my own with something
    like libiax? Normally I'd want to use IAX but the only IAX
    device I have is an IAXy (which didn't work well) so I'd
    rather have something that can at least speak SIP to the
    devices. I'm thinking IAX is probably better for being able
    to move calls efficiently though.

    Suggestions? Even reasons why it's not likely to work well
    will be appreciated. I can still punt and use more powerful
    systems than the WRT54G.

    Thank you.

    --kyler
     
    Kyler Laird, Sep 3, 2004
    #1
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  2. Kyler Laird <> writes:
    > I'm hoping to give a demonstration of a mesh network between
    > farm vehicles soon. I'd like to use VoIP as the application.


    This sounds like a very cool hack.

    I don't believe you need a sip proxy at all to call between VOIP
    phones. I can call between my Grandstreams by dialing the IP address
    directly. It is a bit of a long "number" to dial, but for a quick
    proof-of-concept demo it might be ok.

    That would leave you with only having to do the code for the mesh
    routing. I understand that isn't as easy as it first appears. The
    MIT rooftop folks have some very uncharitable things to say about the
    802.11b signaling protocol and how it is basically impossible to keep
    their ad-hoc mesh from forming isolated islands.

    www.pdos.lcs.mit.edu/papers/grid:bac-meng.pdf
    www.pdos.lcs.mit.edu/papers/grid:losstr02/paper.pdf

    -wolfgang
    --
    Wolfgang S. Rupprecht http://www.wsrcc.com/wolfgang/
     
    Wolfgang S. Rupprecht, Sep 3, 2004
    #2
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  3. Kyler Laird

    Kyler Laird Guest

    "Wolfgang S. Rupprecht" <> writes:

    >Kyler Laird <> writes:
    >> I'm hoping to give a demonstration of a mesh network between
    >> farm vehicles soon. I'd like to use VoIP as the application.


    >This sounds like a very cool hack.


    We'll see. I'm just starting to work on it and it's only 11 days
    away.
    http://www.agry.purdue.edu/FieldDay/
    I'm trying to bite off enough to be interesting but not so much
    that I end up having to deal with lots of extra equipment, writing
    software from scratch, bailing at the last minute, ...

    >I don't believe you need a sip proxy at all to call between VOIP
    >phones.


    I had found a page on voip-info.org about being able to configure
    Asterisk to set up a call between SIP devices and then get out
    of the way, leaving them to speak directly to each other. I was
    thinking that would be a start, but...

    >I can call between my Grandstreams by dialing the IP address
    >directly.


    Ah ha! That would be *wonderful*. Any idea if that's supported
    on Cisco 7940s?

    >It is a bit of a long "number" to dial, but for a quick
    >proof-of-concept demo it might be ok.


    Oh, it's a perfect solution! I assume I can even use speed dials
    to make it transparent.

    >That would leave you with only having to do the code for the mesh
    >routing. I understand that isn't as easy as it first appears.


    It *is* easy. Well, for me, after playing with it for a long time
    it's easy. I'm using OLSR. There's even an OpenWRT package for
    it. I set it up last night with five devices (including my WRT in
    my car in the driveway) and walked around the neighborhood with my
    pen tablet. Geeky fun!

    >The
    >MIT rooftop folks have some very uncharitable things to say about the
    >802.11b signaling protocol and how it is basically impossible to keep
    >their ad-hoc mesh from forming isolated islands.


    Oh, there are certainly non-trivial problems to overcome with mesh
    networks but for a small example like this I think I'll be o.k. I
    will probably try using CTS/RTS to deal with hidden nodes. It'll
    be interesting to see how a SIP session survives.

    Thank you for the info!

    --kyler
     
    Kyler Laird, Sep 4, 2004
    #3
  4. Kyler Laird

    Kyler Laird Guest

    "Wolfgang S. Rupprecht" <> writes:

    >I don't believe you need a sip proxy at all to call between VOIP
    >phones. I can call between my Grandstreams by dialing the IP address
    >directly.


    It looks like Sipura devices can do this too.
    http://www.broadbandreports.com/forum/remark,9737256~mode=flat

    I'll try to hook up a couple and give it a shot.

    --kyler
     
    Kyler Laird, Sep 4, 2004
    #4
  5. Kyler Laird <> writes:
    > "Wolfgang S. Rupprecht" <> writes:
    > We'll see. I'm just starting to work on it and it's only 11 days
    > away.
    > http://www.agry.purdue.edu/FieldDay/
    > I'm trying to bite off enough to be interesting but not so much
    > that I end up having to deal with lots of extra equipment, writing
    > software from scratch, bailing at the last minute, ...


    There is nothing like a bit of pressure for getting the creative
    juices flowing. ;-)

    > >I can call between my Grandstreams by dialing the IP address
    > >directly.

    >
    > Ah ha! That would be *wonderful*. Any idea if that's supported
    > on Cisco 7940s?


    Sorry, I don't know much about the Ciscos. They were always way too
    rich for my blood. (The ones I looked at were $350 w/o SIP firmware.)

    > It *is* easy. Well, for me, after playing with it for a long time
    > it's easy. I'm using OLSR. There's even an OpenWRT package for
    > it. I set it up last night with five devices (including my WRT in
    > my car in the driveway) and walked around the neighborhood with my
    > pen tablet. Geeky fun!


    If the connectivity between nodes is generally good, you should be ok.

    What the MIT folks found is that the nodes in their northern Cambridge
    mesh had a tendency to form different BSSID networks even though their
    SSID's were set to the same value.

    from: grid:bac-meng.pdf p.48

    BSSID partitioning

    When 802.11 interfaces are put into Ad Hoc mode, they go through
    a somewhat complicated pro cedure to join what is known as a Basic
    Service Set, or BSS [11]. A BSS is a kind of virtual network; even
    if two different no des are op erating on the same channel, they
    will not "see" each other's packets unless they are in the same
    BSS. Basic Service Sets are identified by a 6-byte numb er known
    as the Basic Service Set ID, or BSSID, which is transmitted in the
    header of every 802.11 frame. When an 802.11b adapter enters Ad
    Ho c mo de, it is configured with a Service Set ID (SSID), which
    is a string that is intended to identify the network it wishes to
    join. The adapter then scans through all of the 802.11b channels,
    listening for b eacons sent by other no des. Beacons contain,
    among other things, the SSID and BSSID b eing used by the sending
    no de. If the adapter hears a beacon containing the SSID that
    matches its own, then it joins this existing Basic Service Set by
    setting its own BSSID to the one received in the beacon.

    If the adapter does not receive any b eacons with a matching SSID
    within a certain period of time, it decides that no Basic Service
    Set currently exists with its SSID. In this case, it sets up
    itself as a new Basic Service Set by selecting a random BSSID and
    using that for all of its traffic. Any other no des using the same
    SSID which start up in range of this no de will hear its beacons
    and configure themselves to use its BSSID.

    This creates a serious problem for an Ad Ho c network. Namely, if
    two no des start up at different times and are not within radio
    range of each other, they will start with different BSSIDs and b e
    on different logical networks, despite b oth having the same
    SSID. Because 802.11 adapters ignore all frames whose BSSID do es
    not match their own, this creates a partition in the network and
    prevents all of the nodes from being able to communicate
    properly. Ideally, it would be possible to set the BSSID
    directly to ensure that a given set of nodes would all be able
    to communicate. Unfortunately, this is not possible using 802.11b
    hardware.

    > Thank you for the info!


    My pleasure!

    -wolfgang
    --
    Wolfgang S. Rupprecht http://www.wsrcc.com/wolfgang/
     
    Wolfgang S. Rupprecht, Sep 5, 2004
    #5
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