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

PTT

SQU


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

Operator Position

4-wire

IPOperator Position

Radio Server

Radio Gateway + Radio

Radio Gateway

Air Traffic ControlCenter

Radio


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…)

IP

IP

RadioService

WeatherData

ConferenceService Gateway

Service

Mission DataService

RadioService

GatewayService

PSTN/ATS-Q.SIG

Controller Working Position


Radio

Radio

Radio



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

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


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


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


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


Push-To-Talk Signaling

radio server grants access to sector distributes PTT signal

RadioServer

ControllerWorking Position


RadioGateway

Radio



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

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


Thank You!

slides at: http://www.ict.tuwien.ac.at/darilion/

sip04.zip

email: [email protected]


Comparison with PoC

AM (real collision

domain)

PoC server

(virtual collision

domain)

GPRS/UMTS

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

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


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

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