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Institut fürComputertechnik
ICTInstitute ofComputer Technology
Event-based Radio Communication Signaling
using the Session Initiation Protocol
Klaus Darilion
Institut für Computertechnik 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
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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
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Split Audio from Radio Part
transmitter receiver
loudspeaker microphone PTT button
wired connectionaudio + PTT/SQU
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e.g. Air Traffic Control
wired circuit switched network
operator positions
phones gateways
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Push-To-Talk/Squelch Signaling
operator positions
PTT
SQU
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IP based Voice Communication System
ATC Center
gateways
wired network
operator positions
wired IP network LAN: switched Fast Ethernet WAN: depends on the service
provider SIP for session signaling RTP for voice transmission
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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
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Radio Architecture
radio senders and receivers radio gateways radio servers operator positions
Operator Position
Operator Position
4-wire
IPOperator Position
Radio Server
Radio Gateway + Radio
Radio Gateway
Air Traffic ControlCenter
Radio
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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
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Interconnecting
transparent interconnecting via IP WAN connections
sharing of remote resources (radios, data services…)
IP
IP
RadioService
WeatherData
ConferenceService Gateway
Service
Mission DataService
RadioService
GatewayService
PSTN/ATS-Q.SIG
Controller Working Position
Controller Working Position
Radio
Radio
Radio
Controller Working Position
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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
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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
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SIP-based Event Notification
RFC 3265 – SIP-Specific Event Notification framework for event subscription and notification PTT/SQU event package for radio communication
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Operator Position
Operator Position
Radio Server
SIP Proxy
1. SUBSCRIBE sip:[email protected] SIP/2.0
4. SIP/2.0 200 OK
2. SUBSCRIBEsip:[email protected] SIP/2.0
3. SIP/2.0200 OK
SIP-based Event Notification subscription traverses proxies
notifications will be sent directly
Operator Position
Operator Position
2a. NOTIFY [email protected] SIP/2.0
SIP/2.0 200 OK
Radio Server
2b. NOTIFY [email protected] SIP/2.0
SIP/2.0 200 OK
SIP Proxy
Radio Gateway
1. NOTIFY [email protected] SIP/2.0
SIP/2.0 200 OK
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Squelch Notification
NOTIFY sip:[email protected] SIP/2.0
Via: SIP/2.0/UDP 128.131.80.6:5062
From: “radio34" <sip:[email protected]>;tag=F68
To: “operator12" <sip:[email protected]>;tag=1ec23337
Contact: “radio34" <sip:[email protected]:5062;transport=udp>
Call-ID: [email protected]
CSeq: 522 NOTIFY
Event: PTT/SQU
Content-Type: text/plain
Content-Length: 76
radio=sip:[email protected]
sector=sip:[email protected]
status=SQU-on
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Combined Services
define services on top of existing services using existing protocols
SIP presenceevents
SIP PTT/SQUevents
INVITE
RTP/RTCP
ATC phoneservices
ATC radioservices
SIP
TCP/UDP
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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
disconnected
status
RxTx+Rx key-in Tx+Rx granted
key-outkey-out
key-in Rx granted
key-in Rx granted
unsubscribe status
key-in Tx+Rx granted
subscribe status granted
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Push-To-Talk Signaling
radio server grants access to sector distributes PTT signal
RadioServer
ControllerWorking Position
ControllerWorking Position
RadioGateway
Radio
ControllerWorking Position
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Implementation
Linux based prototype
SIP-stack dissipate2 (kphone) 2.5ms for every NOTIFY transaction
RTP-stack jrtplib
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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
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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
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Converged Services
e.g. weather data synthesized radio message
Application and GUI
Convergence layer
Con
figur
atio
n se
rvic
es
Mis
sion
dat
a se
rvic
es
Wea
ther
ser
vice
s
SOAP,etc
AT
C r
adio
ser
vice
sTCP/UDP
Layer 2A
TC
pho
ne s
ervi
ces
Controller working position
IP
RTP + SIP +SIP Events
AT
C r
adio
ser
vice
s
Radio server
Ope
ratin
g sy
stem
Wea
ther
info
rmat
ion
serv
ices
SOAP,etc
TCP/UDP
Layer 2
Weather server
IP
Combined services API
Converged services API
TCP/UDP
Layer 2
IP
RTP+SIP+SIP Events
Ope
ratin
g sy
stem
Ope
ratin
g sy
stem
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Thank You!
slides at: http://www.ict.tuwien.ac.at/darilion/
sip04.zip
email: [email protected]
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Comparison with PoC
AM (real collision
domain)
PoC server
(virtual collision
domain)
GPRS/UMTS
wired
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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
4b. SIP/2.0 200 OK
radio server
3b. NOTIFY PTT
radio gateway 1operatorposition 3
operatorposition 2
PT
T d
elay
1. NOTIFY PTT
2. SIP/2.0 200 OK
3a. NOTIFY PTT
4a. SIP/2.0 200 OK
operatorposition 1
3x.
4x.
PTT indicated tooperator
tgen
treceive
tgen
treceive
operator pressesPTT
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Measurements: PTT Signaling
processing between
time messagestOK – receiving PTT notification 6.4 ms 1 – 2
tforw – PTT forwarding (tgen + treceive) 9.5 ms 1 – 3atsend – repetitive notification sending 2.5 ms3a – 3b
nttt *2* sendforwnPTT,
n = 3 tPTT = 24 ms