Date of Award


Document Type

Master Thesis

Degree Name

Master of Engineering (Research)


Electronic Engineering

First Advisor

Dr. Dirk Pesch


Push To Talk (PTT) services have been in existence for several decades providing dispatch services via two-way radio such as walkie-talkies. For a number of years now PTT services have been available for use over mobile phone networks. These mobile PTT services have occupied a niche market with business users and have been provided on proprietary networks that do not enable roaming.

Over the past year there has been an explosion of interest in the concept of PTT provided over mobile networks using Internet Protocol (IP) technology. This service uses cellular access and radio resources more efficiently than circuit-switched cellular services as network resources are reserved only for the duration of talk-spurts instead of for an entire call session. This also allows for the potential of monetary savings for the user.

Currently, a number of major network operators are developing a Push-To-Talk Over Cellular (PoC) specification, based on the All-IP Multimedia Subsystem (IMS), a Universal Mobile Telecommunication System (UMTS) core-network subsystem for the provision of multimedia services. The PoC specification complies with 3"^^ Generation Partnership Project (3GPP) standards and enables roaming between different voice networks. This PTT service is targeted at a youth orientated market and is viewed as being a supplementary service to and possibly as a replacement for SMS text messaging.

This study presents a PTT service architecture for 3G networks and evaluates two possible implementations of this architecture within the 3GPP UMTS Rel.5/6 specification. For the first implementation the PTT service is located within the IMS. For the second implementation the service is located outside of the IMS and connects to the UMTS core-network through the Gateway GPRS Support Node (GGSN). The Session Initiation Protocol (SIP) is used for PTT Call Setup and Teardown and the Real-Time Transmission Protocol (RTP) is used for the transmission of Talker Arbitration messages and voice data.

The IMS implementation conforms to 3GPP standards and therefore minimises the possibility of interoperability problems and also the PTT framework could be easily adapted to support other multimedia services. The non-IMS implementation is built on the already existing 2.5G GSM/GPRS network, thus enabling a fast service rollout.

It was found that for both the IMS and non-IMS implementations the Call Setup and Teardown times as well as the TA message time were very satisfactory considering the type of service, in both cases it was found that the call establishment times were much faster than a SIP based call establishment for VoIP as per 3GPP.

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