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

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

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

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

Push-To-Talk/Squelch Signaling wer sendet, wer empfängt PTT operator positions

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

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

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

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

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

PTT/SQU Signaling event based continuous 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

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 wir haben SIP genommen weil: ……..

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

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/2.0 200 OK SIP Proxy Radio Server 2b. NOTIFY operator@pc12.atc-center.org SIP/2.0 SIP/2.0 200 OK 1. NOTIFY operator@pc12.atc-center.org SIP/2.0 SIP/2.0 200 OK Operator Position Radio Gateway

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

Combined Services define services on top of existing services using existing protocols all services are built at the application layer,

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 listener to the radio channel transmit radio messages INVITE, a=sendrecv every state transition corresponds with a SIP transaction

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

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

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

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

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

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

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

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 The radio server implementation used dissipate, as SIP stack (of KPhone and ran on a Celeron 1.8GHz PC with Linux 2.4.18 ) Erzeugung von neuen SIP Transaktionen (zw. 3a u. 3b) dauert ziemlich lange –> skaliert schlecht. Multicast wäre besser, geht aber mit SIP nicht. und ergibt wieder Probleme wenn die notifications ACK werden sollen.

Measurements: PTT Signaling processing between time messages 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