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Event-based Radio Communication Signaling using the Session Initiation Protocol

Event-based Radio Communication Signaling using the Session Initiation Protocol. Klaus Darilion. Radio Communication. like walkie talkies transmit a radio message press the “push-to-talk” button (PTT) incoming radio message squelch indicator (SQU)  2 signals PTT SQU.

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Event-based Radio Communication Signaling using the Session Initiation Protocol

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  1. Event-based Radio Communication Signaling using the Session Initiation Protocol Klaus Darilion

  2. Radio Communication • like walkie talkies • transmit a radio message • press the “push-to-talk” button (PTT) • incoming radio message • squelch indicator (SQU) •  2 signals • PTT • SQU

  3. Radio Communication Properties • radio devices • usually amplitude modulated radios • shared medium (collisions) • listen before talk • half-duplex • “push-to-talk” powers up the transmitter • squelch indicates incoming radio messages • area of application • air traffic control • public safety

  4. Split Audio from Radio Part • transmitter • receiver wired connection audio + PTT/SQU • loudspeaker • microphone • PTT button

  5. e.g. Air Traffic Control wired circuit switched network phones gateways operator positions

  6. Push-To-Talk/Squelch Signaling SQU PTT operator positions

  7. IP based Voice Communication System ATC Center wired network • wired IP network • LAN: switched Fast Ethernet • WAN: depends on the service provider • SIP for session signaling • RTP for voice transmission gateways operator positions

  8. What is it not this work does not … • … change the radio communication • radio link is still based on existing analogue radios •  no IP/SIP over the radio link this work is similar to ... • … push-to-talk over cellular (PoC) for 3GPP • … but different requirements

  9. Radio Architecture • radio senders and receivers • radio gateways • radio servers • operator positions Air Traffic Control Center Radio Gateway + Radio IP Operator Position Operator Position 4-wire Radio Gateway Operator Position Radio Radio Server

  10. Radio Architecture Components • radio senders and receivers • typically on remote sites • one voice channel per radio • the bandwidth of the connection between the remote site and the ATC centre is dimensioned for one voice stream per radio. • radio gateway • translates the SIP signaling to radio control lines • translates RTP audio packets to analogue audio • radio server in the LAN • manages the access to the radios (shared resource) • distributes the received radio messages

  11. Interconnecting • transparent interconnecting via IP WAN connections • sharing of remote resources (radios, data services…)

  12. PTT/SQU Signaling • event based • send signal on status changes • signals the beginning and the end of a radio message • delivery must be reliable • continuous • current status will be signaled continuous in small periodic intervals • reliable transport not required

  13. PTT/SQU Signaling • event based – out-band signaling • SIP • independent from audio stream • reliable • RTCP • unreliable  retransmission mechanism • APP packets (like PoC) • continuous – in-band signaling • RTP • extension header • increased traffic (WAN) • setting up an audio stream is necessary

  14. SIP-based Event Notification • RFC 3265 – SIP-Specific Event Notification • framework for event subscription and notification • PTT/SQU event package for radio communication

  15. 2b. NOTIFY operator@pc12.atc-center.org SIP/2.0 SIP/2.0 200 OK Radio Server SIP-based Event Notification • subscription traverses proxies • notifications will be sent directly 2a. NOTIFY operator@pc11.atc-center.org SIP/2.0 Operator Position SIP Proxy SIP/2.0 200 OK 1. NOTIFY operator@pc12.atc-center.org SIP/2.0 SIP/2.0 200 OK Operator Position Radio Gateway

  16. Squelch Notification NOTIFY sip:operator12@pc44.atc-center.org SIP/2.0 Via: SIP/2.0/UDP 128.131.80.6:5062 From: “radio34" <sip:radio34@atc-center.org>;tag=F68 To: “operator12" <sip:operator12@atc-center.org>;tag=1ec23337 Contact: “radio34" <sip:radio34@128.131.80.6:5062;transport=udp> Call-ID: 3c9196445d5240bfb7b660a16ee6d390@128.131.80.223 CSeq: 522 NOTIFY Event: PTT/SQU Content-Type: text/plain Content-Length: 76 radio=sip:radio34@atc-center.org sector=sip:f100@atc-center.org status=SQU-on

  17. Combined Services • define services on top of existing services • using existing protocols

  18. Sector Subscription Model • Status state • only status information about this channel without any voice • SUBSCRIBE • Rx state • status information • passive listener to the channel • INVITE, a=recvonly • Tx+Rx state • status information • listener to the radio channel • transmit radio messages • INVITE, a=sendrecv • every state transition corresponds with a SIP transaction

  19. Push-To-Talk Signaling • radio server • grants access to sector • distributes PTT signal

  20. Implementation • Linux based prototype • SIP-stack • dissipate2 (kphone) • 2.5ms for every NOTIFY transaction • RTP-stack • jrtplib

  21. Conclusion • maximum 25 ms for PTT signaling • easy to achieve for operator – gateway signaling • hard to achieve for operator – operator signaling • hard to compare – existing requirements are trimmed for circuit switched systems • SIP: one protocol for phone and radio communication • compatibility with devices which do not support all SIP extensions (e.g. SIP phones) • open standard guarantees interoperability between different vendors and extensibility of ATC services • transparent interconnecting of ATC centers via IP WAN connection

  22. Future Work • WAN measurements • additional PTT signaling delay • fast and reliable transport of the voice packets • especially bandwidth reservation for WANs • interaction of QoS mechanisms with SIP • high priority calls may disrupt normal calls • enhanced radio services • coupling, coverage

  23. Converged Services • e.g. weather data  synthesized radio message

  24. Thank You! • slides at: • http://www.ict.tuwien.ac.at/darilion/sip04.zip • email: • darilion@ict.tuwien.ac.at

  25. Comparison with PoC AM (real collision domain) GPRS/UMTS PoC server (virtual collision domain) wired

  26. Measurements: PTT Signaling • operator presses PTT • operator position signals PTT to radio server • NOTIFY, body: status=PTT-on • radio server forwards event to • the radio gateway • all subscribed operators

  27. Measurements: PTT Signaling processing between timemessages tOK – receiving PTT notification 6.4 ms 1 – 2 tforw – PTT forwarding (tgen + treceive) 9.5 ms 1 – 3a tsend – repetitive notification sending 2.5 ms 3a – 3b n = 3  tPTT = 24 ms

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