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
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
Intranet /Internet access
Instant messaging
GPS
Location based services
Group call function
Video conferencing
Push to Talk functionality

A Modern PSS network
Video Phone

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
Telephony

........
PSS network
PSS communications requirements continued

Resilience of the network








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







Single node failure does not hamper the operation.
Power backup &Transmission redundancy
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
IP
UDP
RTP
RTP media packet
AMR Payload (Voice Data)
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.
Ips
PoC
Client
A
C
C
E
S
S
N
E
T
W
O
R
K
Presenc
e
Server
IMS
Ipl
Im
GMLS
CORE
Ik
If
It
PoC
Server
UE
Is
In
Rem
ote
Netw
ork
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
User B
PoC over
GPRS
Network
User C
User D
PoC over GPRS
SG
SN




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
GG
SN
GPRS
Network
U
E
Radio
Access
Network
Chat
Session
IMS based
PoC core
Challenges of PoC over GPRS network
Impact of mibility
•
•
Incident or overload handling
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
•
•
•
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
RA1
SA1
•
RA3
SA2
RA2
SA1
•
•
PSS communication requires call setup
time in the sub-second range
Still it is not available
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
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.
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
Po
C
Ser
ver
Core
Netw
ork
BSC/R
NC
B
T
S
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 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
Ce ll Re se le ction Time (UL)
2
1,5
1
NACC(Network)
0
NACC(Terminal)
0,5
Normal Cell
Reselection
Time
•
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.
BS
C
OS
S
Summary of Technical Feasibility Analysis
Acceptable level
Roaming
Interworking with PSTN, ISDN or PLMN
Data/video services
Availability of end user terminals
Ease of use
Integration with control/dispatch room
Voice quality
Group call functionality
Call setup time
Priority and pre-emption
Security and privacy
Continuity of operation
Radio Coverage
Resilience of the network
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.
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?