Brief overview of the Shared Call Appearances module in Kamailio

1. Shared Call
Appearances in Kamailio
Andrew Mortensen
University of Pennsylvania

2. PennNet Phone
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Started as part of the sip.edu initiative
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Ran SER 0.9.x for several years
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Gained traction due to cost savings, now nearing conclusion of
migration to entirely VoIP deployment
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14K VoIP handsets now, targeting 18K
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Roughly 100K calls per day, spikes considerably higher
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Uncomplicated architecture for now
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Replicated proxies and registrars, Asterisk, SEMS

3. PennNet Phone Migration:
Requirements
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Usual suspects
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Voicemail, transfers, forwarding, etc.
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Bridged Line Appearances

4. Bridged Line Appearances
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Event package using XML bodies to deliver line
state to subscribers
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Fine for small groups
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Local patches to the existing got us BLA in
deployment
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But…

5. Bridged Line Appearances:
Limitations
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XML bodies quickly increased NOTIFY packet size
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Fragmentation, retransmission
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Fragmentation of almost every packet,
considerably worse for large BLA groups
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Underpowered handsets quickly overwhelmed by
fragmented packets
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Stuck states

6. Bridged Line Appearances:
Alternatives
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Shared Call Appearances
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…in Kamailio!

7. What is SCA?
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Broadsoft spec for shared line events
•
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Widely implemented by handset vendors
SCA group receives notifications when a member does
something interesting
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line seize, ringing, answered, held
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Held calls can be retrieved by any other member of the
group
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Line state delivered in compact Call-Info headers!

8. SCA at Penn
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Agreement that BLA was a dead end
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In-house familiarity with SER/Kamailio code
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3-4 months of development
•
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RFC 3891 support (“Replaces” header) to
retrieve held calls
Rapid rollout following successful testing

9. SCA at Penn: Results?
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Largest supported group size
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BLA: 6
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SCA: 24 (imposed by handset limitations)
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8000+ handsets using SCA, many with multiple
SCA lines
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Stuck states? Almost eliminated.

10. SCA at Penn: Results?
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Opens possibility of using other vendor
handsets
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•
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Tested with Polycom, Aastra, Cisco, Yealink
Likely to work with Snom, Grandstream, others
BYOD?

11. Simple Demo

12. Future Directions
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Failover improvements
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Event publication
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Barge In
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Others?

13. Thank You