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
wired connection
audio + PTT/SQU
loudspeaker
microphone
PTT button
transmitter
receiver

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

6. Push-To-Talk/Squelch Signaling
wer sendet, wer empfängt
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
Operator Position IP
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 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

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
wir haben SIP genommen weil: ……..

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. SIP-based Event Notification
subscription traverses proxies
notifications will be sent directly
2a. NOTIFY SIP/2.0
Operator Position
SIP/ OK
SIP Proxy
Radio Server
2b. NOTIFY SIP/2.0
SIP/ OK
1. NOTIFY SIP/2.0
SIP/ OK
Operator Position
Radio Gateway

16. Squelch Notification NOTIFY sip:operator12@pc44.atc-center.org SIP/2.0
Via: SIP/2.0/UDP :5062
From: “radio34"
To: “operator12"
Contact: “radio34"
Call-ID:
CSeq: 522 NOTIFY
Event: PTT/SQU
Content-Type: text/plain
Content-Length: 76
status=SQU-on

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

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
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 RTP-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

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
The radio server implementation used dissipate, as SIP stack (of KPhone and ran on a Celeron 1.8GHz PC with Linux )
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.

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