Internet Telephony
Department of Computer Science
University College London, jon@cs.ucl.ac.uk
http://www.cs.ucl.ac.uk/staff/jon
+44 171 387 1397 or maybe
phone://44.171.380.7296 or
sip://jon@waffle.cs.ucl.ac.uk
7296@sip-replay.cs.ucl.ac.uk :-)
Introduction
- Internet Telephony is a “hot topic” today, due to emergence of new business (Internet) as potentially underlying mechanism and demand for capacity compared with POTS
- We (in Internet Architecture Board, and elsewhere) would like to provide ubiquitous services…how?
Topics of Discussion
- Internet Support for Audio (+ A/V)
- Internet Call Control Protocols
- Internet/POTS Voice Gateways
- POTS/Internet/POTS Signaling and Call Routing
- Internet Carriage for voice multiplex
- Internet Signaling for POTS trunk traffic
Topic One
- Internet Audio - requires premium service or over-engineered (un-congested) net
- Adaptive Playout and FEC/Layered Coding (RAT/FreePhone) only so good.
- Once we have service differentiation (or int-serv+rsvp), then IP voice business possible
PPT Slide
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Topic Two
- Internet has its own user-user signaling protocol (SIP) as well as user-network signaling (RSVP, if needed, or IP TOS P bit)
- Can (have) extended SIP to provide IN type services too…
- No additional work needed
- H.323 is probably a red herring...
Session Invitation Protocol
PA=128.16.64.19 128.16.5.31 239.128.16.254/16
FR=M.Handley@cs.ucl.ac.uk
TO=J.Crowcroft@cs.ucl.ac.uk
o=van 2353655765 2353687637 IN IP4 128.3.4.5
i=Discussion of Mbone Engineering Issues
e=van@ee.lbl.gov (Van Jacobsen)
PA=128.16.64.19 128.16.5.31 239.128.16.254/16
FR=M.Handley@cs.ucl.ac.uk
TO=J.Crowcroft@cs.ucl.ac.uk
Topic Three
- Internet<->POTS needs server location protocol (IP host needs to find gateway which does POTS call. Phone needs to be given phone number of gateway+call forward/selection (DTMF) to call IP host.
- To scale, we need multiple gateways => need call routing!
Topic Four
- Voice Multiplex - to really scale, a POTS IP relay should operate with trunk as well as individual calls
- Needs SS7 (and other NNI) POTS Signaling gateways.
- Needs RTP payload for multiplex.
- Needs de-multiplexors in IP<->IP voice concentrators.
Topic Five
- Signaling for trunk calls (e.g. one RTP/UDP/IP packet flow carries n calls, need to build process in trunk interface processor inside router/phone exchange to pull out each “TDM” RTP slot, and map into right call )
- May need less stringent QoS support for this than individual calls!
Real Life
- Have prototype in java on PC with dialogics cards (same as used in some exchanges) - uses JTAPI and RTP/UDP/IP with RTP header compression on IP side….
- Works, but needs security (PINs) since IP has no auth/priv right now!
- Denial of service attack on gateways possible!
What This Means
- Need to resolve conflicts between PINT (IN in IP), SIP (MMUSIC group in IETF), H.323 and so on.
- Need better (but NOT perfect QoS)
- Need mux spec (Henning Sculzrinne at Columbia has first pass at this design)
- Need public trunk signaling….!
Next Steps
- Eventually, lower cost of service creation in Internet will lead to it replacing POTS and TV networks.
- Main difficulties are getting the dinosaur companies out of their “potential wells” and into the 21st (or 20th) century