Feasibility study of IP Multimedia Subsystem(IMS) based Push to Talk over Cellular(PoC) for Public Safety and Security

communications

BySanjay Kanti Das

Insrtuctor: Lic.Tech. Michael HallSupervisor: Prof. Sven-Gustav Häggman

Master’s Thesis PresentationEspoo,8th of August,2006

Acknowledgements

Professor Sven-Gustav Häggman Lic.Tech. Michael Hall Parents and friends

This Thesis has been done in Communication Laboratory,Dept.of Electrical

and Communications Engineering,Helsinki University of Technology(HUT)

Agenda

Introduction PSS communication requirements What is PoC and Technology behind PoC PoC over GPRS implementations and Challenges Improvement proposals Emergency capacity increase for packet data traffic in GSM/GPRS

cell Feasibility study summary Case study: analogue PSS network vs possible PoC over

commercial network Conclusions

Introduction Public safety professionals rely on PMR

networks, most which are analogue Motivation to find an alternative is to save

CAPEX and OPEX and benefited from the advanced services

Feasibility is analyzed considering the requirement of PSS communication

Both day-to-day routine work and emergency situation handling capability are considered

Study has done based on technical specification documents and related literature

Technical Information and factual data are collected from technical papers, reports and news articles posted in the internet.

According to a study in June ’01 by IMS Research

PSS communications requirements

With time, PSS usage scenarios as well as the requirements of the PSS professionals have changed a lot.

In addition to the voice service other types of communication are also required to ease the operation of the PSS professionals in different scenarios

Broadly categorized as Interactive voice communication Non-interactive voice

communication Interactive data communication Non-interactive data

communication

A Modern PSS network A Modern PSS network

Tel

epho

ny

Vid

eo P

hone

Vid

eo c

onfe

renc

ing

Push

to T

alk

func

tiona

lity

Gro

up c

all f

unct

ion

Loc

atio

n ba

sed

serv

ices

Inst

ant m

essa

ging

........

PSS network

Intr

anet

/Int

erne

t acc

ess

GPS

PSS communications requirements continued

Resilience of the network Single node failure does not hamper the operation. Power backup &Transmission redundancy

Capacity to handle extral load during emergency situation handling Security Nation-wide coverage irrespective of geographical nature of land. Seamless roaming-minimum service interruption time during cell change. Fast call setup time-in sub-second range. Specialized group call functionality

Dynamic group formation Late entry Rejoin

Priority and pre-emption settings Good voice quality Data communication Interworking with other PLMN, PSTN or ISDN network.

What is PoC?

Half duplex voice communication that provides 1-to-1 and 1-to-N walkie-talkie type communication.

VoIP in cellular network environment RTP, UDP and IP protocols for Media

tranport One person can talk at a time and others

listen Other participants can request for floor to talk

when one’s talk bursts finish Contention of the Right-to-Speak is managed

through Floor Control mechanism Receiver- Auto Answer or Manual Answer

mode IMS in the core network

AMR Payload (Voice Data) RTP UDPIP

RTP media packet

PoC Architecture

UE has the PoC client software, communicates with PoC server over the It interface

PoC server responsible for floor control,tear down of session, media duplication and forwarding.

IMS core is responsible for routing, charging and finding the called user.

IMS gets presence information of a user through Ips interface.

GLMS server maintains group and list of PoC users.

Core network and Access network elements provide the required connectivity services between UE & PoC server. Is

It

PoC

Client

UE

ACCESS

NETWORK

IMS

CORE

PoC Server

RemoteNetwork

GMLS

Presence Server

In

If

Ik

Im

Ips

Ipl

PoC services

One-to-one personal call One-to-many group call

Pre-defined group call Ad-hoc group call Instant group chat

Dynamic group formation Instant messaging Auto answer and manual answer

mode. Personal alert: that tells the called user

to call back to the calling user. Presence information of individual

group members Access/Reject list etc. Works among different operators PoC Roaming across the boarder Ease of use

PoC over GPRS Network

PoC over GPRS Network

User C

User D

User B

PoC over GPRS

Northstream study concludes that the GPRS network, if tuned correctly, meets the PoC technical demands

According to Northstream study PoC over GPRS is 5 times more efficient that PoC over CS GSM network

Efficient use of radio resources Always on

GPRS Network

GPRS Network

SGSN

GGSN

Radio Access Network

IMS based PoC core

UE

Chat

Session

Challenges of PoC over GPRS network

Impact of mibilityImpact of mibility• PSS professionals need to move frequently • Cell reselection causes service interruption

time from 1-2 seconds to several seconds depending on the channel availability and traffic load of the target cell

Incident or overload handlingIncident or overload handling• Extra resource are not kept reserved

before hand in commercial network• During any incident traffic increase

drastically• CS voice call pre-empted the PS call• Situation would be worse in rural areas

Setup timeSetup time• PSS communication requires call setup

time in the sub-second range• Still it is not available

RA1 SA1

RA3 SA2

RA2 SA1

Challenges continued

Delay in speechDelay in speech• Best effort GPRS network does not

gurantee the end to end QoS • Reciepient in congested cell will

possibly receive the data packet later and may ask for repetation

• Which certainly hamper the smooth PSS operation

Voice qualityVoice quality• Best effort GPRS network does not

gurantee contanst bit rate• Voice quality can be deteriorated due

congestion in the cell or bad radio condition

Effect of unicastingEffect of unicasting Based on Unicasting. PoC server duplicates the data and

one copy for each recepient is sent over transport and core network.

It will create an extra load to the transport and radio network when PoC user increases.

PoC Server

Core Network

BSC/RNC

BTS

Improvement Proposals

Radio resource allocation schemeRadio resource allocation scheme Radio resource allocation exclusively

to GPRS PS data traffic for the sake of blocking of CS voice calls

To overcome pre-emption by CS voice traffic and reduce delay due to contention for resources.

Shows better performance

Use of early media settings in the Use of early media settings in the networknetwork

Limiting the number of PoC or Limiting the number of PoC or packet switched users per PDCH packet switched users per PDCH helps to maintain a higher bit rate helps to maintain a higher bit rate per userper user

A jitter buffer at the receiving side A jitter buffer at the receiving side can handle small delay variations can handle small delay variations or jitter caused by the GPRS or jitter caused by the GPRS access networkaccess network

Improvement proposals continued

The NACC feature in the PS domain significantly The NACC feature in the PS domain significantly reduces the service interruption time due to cell reduces the service interruption time due to cell reselection. reselection.

• The NACC feature is included in 3GPP Release ‘99 The NACC feature is included in 3GPP Release ‘99 and Release 4and Release 4

Cell Reselection Time(UL)

0

0,5

1

1,5

2

Norm

al C

ell

Resele

ctio

n

NA

CC

(Term

inal)

NA

CC

(Netw

ork

)

Tim

e

Emergency Capacity increase in GSM/GPRS cell

Intermediate solution to handle overload

Assign traffic channel to PS domain by commands from OSS

Increase capacity for packet data traffic as well as for PoC traffic blocking CS traffic for the time being.

BSC

OSS

Summary of Technical Feasibility Analysis

Key findings in favour of PoC

over commercial GSM/GPRS network Radio coverage Data services Interworking with other networks Roaming accross the nation boarder Presence information Easy integration of new IMS based

services etc.

Improvement Areas: Call setup time Prioritization of PoC traffic Cell change time etc.

Resilience of the netw

ork

Radio C

overage

Continuity of operation

Security and privacy

Priority and pre-em

ption

Call setup tim

e

Group call functionality

Voice quality

Integration with control/dispatch room

Ease of use

Availability of end user term

inals

Data/video services

Interworking w

ith PS

TN

, ISD

N or P

LMN

Roam

ing

Acceptable level

Case study: Bangaldesh

PSS communication depends on over-aged analogue technology

Coverage & Capacity: local coverage around police station and wide area coverage only big cities

6 (six) cellular operators and out of them 4 has nation wide coverage

Largest operator GrameenPhone(GP) covered almost 88% of the whole country.

It has been seen that most police officiers use mobile phone for their personal and official communications.

Summary and Conclusions

Finally, it is concluded that, given the demanding requirements for public safety and security users, PoC over commercial GSM/GPRS networks needs proper dimensioning and tuning of the access network and some modification in the core network part.

Taking into account the suggested measures, PoC over a public network can be a good alternative to a legacy analogue PMR network

Thank you!

Any questions?