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
This Thesis has been done in Communication Laboratory,Dept.of Electrical
and Communications Engineering,Helsinki University of Technology(HUT)
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
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
Push to Talk functionality
Group call function
Location based services
Intranet /Internet access
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
Priority and pre-emption settings
Good voice quality
Interworking with other PLMN, PSTN or ISDN network.
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)
PoC over GPRS Network
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
IMS based PoC core
Radio Access 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
PSS communication requires call setup time in the sub-second range
Still it is not available
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
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.
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
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
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.
Key findings in favour of PoC
over commercial GSM/GPRS network
Interworking with other networks
Roaming accross the nation boarder
Easy integration of new IMS based services etc.
Call setup time
Prioritization of PoC traffic
Cell change time etc.