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Feasibility study of IP Multimedia Subsystem(IMS) based Push to Talk over Cellular(PoC) for Public Safety and Security communications By Sanjay Kanti Das. Insrtuctor: Lic.Tech. Michael Hall Supervisor: Prof. Sven-Gustav Häggman Master’s Thesis Presentation Espoo,8th of August,2006.

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

Insrtuctor: Lic.Tech. Michael Hall

Supervisor: Prof. Sven-Gustav Häggman

Master’s Thesis Presentation

Espoo,8th of August,2006

  • 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)

  • 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
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


Video Phone

Video conferencing

Push to Talk functionality

Group call function

Location based services

Instant messaging

Intranet /Internet access



PSS network

PSS communications requirements
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


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


Priority and pre-emption settings

Good voice quality

Data communication

Interworking with other PLMN, PSTN or ISDN network.

PSS communications requirements continued
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

RTP media packet




AMR Payload (Voice Data)

What is PoC?
poc architecture





























PoC Server


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.



poc services

User B

PoC over GPRS Network

User C

User D

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
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





IMS based PoC core


GPRS Network

Radio Access Network

PoC over GPRS
challenges of poc over gprs network
Impact 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 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 time

PSS communication requires call setup time in the sub-second range

Still it is not available




Challenges of PoC over GPRS network
challenges continued
Delay 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 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 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.




Core Network


Challenges continued
improvement proposals
Radio 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 network

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

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

Improvement Proposals
improvement proposals continued
The NACC feature in the PS domain significantly reduces the service interruption time due to cell reselection.

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

Improvement proposals continued
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.



Emergency Capacity increase in GSM/GPRS cell
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.

Acceptable level

Summary of Technical Feasibility Analysis
case study bangaldesh
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
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?