Push-to-Talk (PTT) offers instant communication among groups. Unlike a telephone conversation, which requires a phone number to be dialed and is often limited to a two or three way conversation, PTT allows a speaker to start talking to a large group at the push of a button.

1xEV-DO Rev A enables broadband wireless service providers to introduce a high-quality PTT service to support consumer and business communications requirements.

Drivers for Push-to-Talk over EV-DO Rev. A
PTT over standard 3G Air Interface
Nextel's DirectConnect(R) service sets the performance benchmark for commercial push-to-talk but requires a proprietary wireless technology called iDEN(R) - a proprietary technology that does not offer an upgrade path to 3G and severely limits the choice a service provider has in infrastructure and handset vendors. 1xEV-DO Rev A can provide PTT performance comparable to iDEN(R) without forcing an operator to build a special, proprietary network.
PTT with fast call setup and low latency
PTT calls are often used for short communications - to ask a quick question, to verify information, etc. A PTT conversation may be as long as 30 seconds, or as short as five to ten seconds. Each PTT call is made up of many short "sub-calls" (the periods of time in which any given party is speaking) that are only a second or two in duration. Therefore, a high-quality PTT service is expected to set up calls in approximately one second. Once the call is set up, the time delay between the time a speaker speaks and the group can listen (the mouth-to-ear latency) should be as short as possible. As the table below shows, 1xEV-DO Rev A can enable PTT performance comparable or better than iDEN(R).

PTT over iDEN (Nextel) PTT over 1xRTT PTT over GPRS (PoC Standard) PTT over EV-DO Rev. A
Average call set-up 0.92 seconds 4.3-6.3 seconds 1-2 seconds less 1 second
Mouth-to-ear Latency 0.58 seconds 1.1 - 1.5 seconds 1-3 seconds less 0.5 seconds
Table 1: Comparison of Push-to-talk performance on multiple air-interface technologies.
Source: Publicly available articles and white papers from The MITRE Corporation, Wireless Week, Sonim Technologies
Network Efficiency
Though certain PTT solutions that use the circuit-switched network provide good performance, they do not offer high capacity and can be expensive to scale. Packet switched PTT solutions are better in comparison because:
  • A service provider does not have to buy expensive circuit switching gear to scale
  • Packet-switched solutions utilize the half-duplex nature of PTT to deliver more capacity than can be provided by full-duplex circuit-switched solutions
  • Packet-switched solutions enable enhanced push-to-talk applications in which subscribers can instantly share other media (like pictures or video) with the group
  • Packet-switched solutions are aligned with the future direction of service providers towards IP Multimedia Services (IMS). As operators offer more applications, from VoIP to streaming video, over the same channel, they gain trunking efficiencies.
PTT solutions over 1xRTT and GPRS data networks offer the benefits stated above but not with industry-leading performance. 1xEV-DO Rev A will provide the benefits of packet-switched PTT solutions along with industry-leading performance.
In the following section we will discuss some of the specific technology enhancements that make EV-DO Rev. A an ideal platform for the deployment of push-to-talk services.
EV-DO Rev A Technology Enablers
Three features in 1xEV-DO Rev-A combine to deliver the performance described above:
  1. Data over Signaling
  2. Enhanced Idle State Protocol
  3. Quality of Service
Data over Signaling
Data over Signaling makes use of a low rate access channel that exists in the network prior to traffic channel creation to speed up PTT call initiation. Call set-up messages (e.g. SIP INVITE) can be sent over the access channel, allowing for fast initiation of PTT calls. Without this mechanism, each PTT call would have to begin with the establishment of a traffic channel, and this would lead to long set-up times.
Enhanced Idle State Protocol
Typical cellular handsets enter a sleep mode that may last as long as five seconds. This negatively affects the call set-up time for PTT calls. EV-DO Rev A handsets address this problem with the introduction of an Enhanced Idle State Protocol (EISP) that allows much shorter sleep periods in order to speed up call establishment. The EISP allows the network to page devices differently, depending on their application and stated needs. So PTT devices would wake up, and be paged by the network, more often, for example, than a traditional voice or data device would be.
Quality of Service
Finally, EV-DO Rev. A enhances the EV-DO Quality of Service capabilities. EV-DO Rev. A supports multiple traffic classes, which can be handled differently by the packet scheduler on the forward link. The scheduler recognizes, for example, VoIP and PTT packets, places them in a separate queue, and delivers them in a timely fashion to meet their delay and jitter requirements. The packets on the reverse link are also treated differently in an EV-DO Rev. A network. EV-DO Rev. A allows the handset to transmit delay sensitive packets at higher power to increase the probability that the packets will not have to be retransmitted. Both of these mechanisms are aimed at minimizing the delay experienced by these sensitive services.
The next generation of EV-DO networks creates the ideal infrastructure to launch a high quality PTT service. EV-DO Rev. A is the foundation for a next-generation, IP-based broadband wireless voice and data network. Airvana is embracing EV-DO Rev. A technology to deliver end-to-end application-level QoS via the wireless network, enabling service providers to deliver high-quality voice services alongside data and multimedia applications. In 2002, Airvana was the first 1xEV-DO infrastructure vendor to demonstrate a push-to-talk application running on a 1xEV-DO Rev 0 network. Since then, Airvana has worked closely with other industry leaders to advance the 1xEV-DO standard to enable best-in-class push-to-talk.[source]


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