institut für computertechnik ict institute of computer technology event-based radio communication...

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

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

transmitter receiver

loudspeaker microphone PTT button

wired connectionaudio + PTT/SQU

e.g. Air Traffic Control

wired circuit switched network

operator positions

phones gateways

Push-To-Talk/Squelch Signaling

operator positions

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

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

Operator Position

4-wire

IPOperator Position

Radio Server

Radio Gateway + Radio

Radio Gateway

Air Traffic ControlCenter

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…)

RadioService

WeatherData

ConferenceService Gateway

Service

Mission DataService

RadioService

GatewayService

PSTN/ATS-Q.SIG

Controller Working Position

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

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

SIP-based Event Notification

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

Operator Position

Radio Server

SIP Proxy

1. SUBSCRIBE sip:f100@atc-center.org SIP/2.0

4. SIP/2.0 200 OK

2. SUBSCRIBEsip:f100@pc01.atc-center.org SIP/2.0

3. SIP/2.0200 OK

SIP-based Event Notification subscription traverses proxies

notifications will be sent directly

Operator Position

2a. NOTIFY operator@pc11.atc-center.org SIP/2.0

SIP/2.0 200 OK

Radio Server

2b. NOTIFY operator@pc12.atc-center.org SIP/2.0

SIP/2.0 200 OK

SIP Proxy

Radio Gateway

1. NOTIFY operator@pc12.atc-center.org SIP/2.0

SIP/2.0 200 OK

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

SIP presenceevents

SIP PTT/SQUevents

INVITE

RTP/RTCP

ATC phoneservices

ATC radioservices

TCP/UDP

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

unsubscribe status

key-in Tx+Rx granted

subscribe status granted

Push-To-Talk Signaling

radio server grants access to sector distributes PTT signal

RadioServer

ControllerWorking Position

RadioGateway

ControllerWorking Position

Implementation

Linux based prototype

SIP-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

Application and GUI

Convergence layer

SOAP,etc

sTCP/UDP

Layer 2A

Controller working position

RTP + SIP +SIP Events

Radio server

SOAP,etc

TCP/UDP

Layer 2

Weather server

Combined services API

Converged services API

TCP/UDP

Layer 2

RTP+SIP+SIP Events

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)

PoC server

(virtual collision

domain)

GPRS/UMTS

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

1. NOTIFY PTT

2. SIP/2.0 200 OK

3a. NOTIFY PTT

4a. SIP/2.0 200 OK

operatorposition 1

PTT indicated tooperator

treceive

operator pressesPTT

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

institut für computertechnik ict institute of computer technology event-based radio communication...

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