Push-to-talk is coming of age


Digital public access mobile radio combines cellular phone capability with group calling, fast press-to-talk call setup and data messaging. Here, Tony Greaves, explains how the development of PAMR systems has revolutionised group communication

The development and smooth running of a national mobile network is a challenge in any environment. Technology, finance and the demands of the market make interesting bed-fellows. Although much focus these days is on cellular mobile telephony, PAMR (public access mobile radio), and latterly Digital PAMR, has established itself as the technology for group communications. The Dolphin network is currently Europe’s only public offering of this technology and provides an interesting example of how one sector of the communications market may evolve.

Ironically, the original opportunity for a mobile communication network using Digital PAMR developed in part because of the success of mobile phones and the GSM system. That GSM success changed the PAMR market’s view of mobile communications and exposed the shortcomings in existing PMR/PAMR networks. For instance, the two-way radio technology standard (MPT1327) had effectively stood still for 20 years. It lacks many of the features that we now take for granted on cellular systems: widespread hand portability, high call and service quality and call handover, to name but a few. No longer do users accept outdated systems. Competition from consumer services – which are quite frankly far better suited to the communications needs of individuals – is driving up service specifications in all communication sectors.

Communication now

In today’s marketplace, imagine being responsible for the communications for a company in the transport, distribution or construction industry. As a bare minimum, you need the functions of traditional mobile radio combined with the messaging that has been so successful for GSM networks, namely:

  • group call – one person speaking to many; keeping everyone informed quickly and cost effectively is a critical part of many businesses

  • fast press-to-talk call set-up; in public safety or security applications there is not the time to dial the number and wait for connection

  • some form of data messaging to ease work flow; simple logging of activities or transactions keeps the paperwork to a minimum and helps businesses retain control.

Typical users rely on speed and simplicity and as businesses become more integrated and spread over wider geographical areas even this basic list is not that easy to fulfil.

To get these services, the choice is between analogue PAMR, which has the basics but is limited by location and equipment; digital cellular mobile telephony, which is better in many respects but lacks PAMR features; data networks and the newer mobile data access networks, which have their own restrictions (such as total lack of voice functions); and/or pagers, which have even more restrictions. The choice is not quite bewildering, but for any business the technology needs to be carefully matched to the requirement. Not surprisingly perhaps, many companies find that more than one system is required.

The market that digital PAMR now addresses has been poorly served. For example, it is likely that a taxi driver has a PMR radio, a cellular phone and possibly a pager as well. A delivery truck probably also has a mobile data terminal fitted, but the driver will be issued with a mobile phone to maintain a voice link and for personal security and safety reasons. By recognising the high cost of running multiple services like these, newer technologies such as TETRA provided the opportunity to upgrade PAMR and add features. For instance, the range of despatch radio services today is limited to a site or a city – at most the coverage of a small group of base stations. A TETRA network will provide despatch services internationally across the entire network. It provides full cellular telephone capability, so the taxi driver has no need for a separate cellular phone. Instead of supplying despatch phones to the service fleet and cellular phones to the management, a company can run the whole organisation on a single communications system.

The virtual private networks used by Digital PAMR subscribers will also cross regional boundaries. A fleet of trucks spread throughout France and the UK can make and take phone calls and have despatch and press-to-talk functions operating across the whole team, wherever they are located.

TETRA installed in Britain

To take advantage of this opportunity, the British government licensed the building of a new mobile network based on TETRA in 1996. The intended customer base was typically using more than one mobile system to meet its communications needs.

TETRA is an ETSI standard for digital mobile radio which combines the benefits of mobile phones with PAMR radio and IP protocol packet-switched data transfer. (ETSI is the same standards body that specified GSM, the technology standard of most cellular mobile phones.) It is an accepted European-wide standard. This means that as other national networks are completed, the same handsets and network equipment can be used and the radio spectrum is allocated to Digital PAMR use everywhere.

From the operator’s point of view there is a variety of benefits derived from using a standard technology. The standard defines an open interface and handover algorithms allowing for multiple vendors, which ensures there is competition and attractive pricing. For example, Dolphin in Britain has included handsets and in-vehicle terminals from major manufacturers such as Motorola, Nokia and Cleartone. One of the newest second generation TETRA handsets not only maintains this compatibility but also borrows much from Nokia’s leading GSM user interface – making the handsets ‘familiar’ and easier to use too.

At the network level, the intersystem interface in the specification enables network components from different vendors to be connected together easily and still maintain functions, such as press-to-talk, across different networks. TETRA therefore has the prerequisites of a national or pan- European network, in that it can be rolled out at a comparatively low cost. This made it a very attractive system for a late entrant into the mobile wireless market. The lower cost resulted from TETRA's requirement for fewer base stations, due to its coverage attributes.


No national network roll-out, however economic the technology, is a minor undertaking – the total cost of the Dolphin network, including the British national service and regional services elsewhere in mainland Europe, was considerable and the pioneers of this plan were unable to maintain the funding needed to see it through. Following a period of administration the network was purchased and is now under new ownership. The aim remains the same – to ensure that this as yet only digital PAMR network in the world continues to grow and ultimately succeed as a business.

Simple is often best

Dolphin is a seamless mobile communications network. In 1999 it operated on a nationwide basis from day one and incorporated a combination of cellular telephony, mobile radio and interactive messaging in a single handset or in-vehicle terminal. The network integration did not go exactly as planned, partially because of the ‘public’ feature of Dolphin’s PAMR network. This was the first time TETRA had been implemented for a public network, which needed to be paid for by its users. Billing systems, for example, which in theory could be applied to any network, had to be put to the test! It took some thousands of hours of engineering effort, but the network now works, and works well.

Users and why they are loyal

In spite of the administration process, users remained loyal to the service. Recent technical enhancements have made the network operate more efficiently, which improves the service and means that the network becomes economic with fewer users. This in turn allows focus on the people who really need the key features of push-to-talk and group calls with virtually instant connection time.

These key users include, as expected, mobile workers such as logistics and security companies for whom a push-to-talk system has always been ideal. However, the construction industry has adopted digital PAMR very easily, possibly because the options that preceded it were so far from what was needed. For example, it is very easy to swamp the capacity of a local cellular network if a dozen site workers all try to make a mobile call at the same time.

The re-evaluation of security issues by major organisations has led to digital PAMR’s adoption by service and security departments of financial institutions – in fixed locations. Although not truly mobile, these businesses are attracted to a service that is provided independently of any local power or communication link.

Where does digital PAMR go next?

Now that there is ready adoption of the basic voice and messaging services, it is important to understand the future needs of customers, and this is where the technology must follow.

Location-based services will not provide consumer marketing opportunities, or ‘where is the nearest chemist’ features such as those offered by mainstream operators. Instead, facilities such as asset tracking, asset management, fleet management and uncomplicated navigation will be required. Currently these can be served only by bespoke GPS-based services. The cost of these services has to be relatively low, which is where the clever use of technology, applied to a relatively uncomplicated network, provides exciting possibilities.

Data and data speeds are an interesting area where customer needs are driving development, particularly where the newer security applications are involved. Fast, simple images, transport information systems, industrial telemetry and the growing use of SDS (short data service) to facilitate job tracking (stock and progress reports) are examples. These will all build to make the data proportion of network traffic on the digital PAMR network higher than that typical on a regular mobile service. Because we have the ability to adapt the network to these changing needs, application development should not be hindered but indeed welcomed.

The total European market for push-to- talk mobile communications for businesses and organisations is estimated to be in the region of 20 million users. This sounds a lot – and it is – but in the USA, digital push-to- talk mobile communication has been adopted by nearly 10 million users, over 4% of the population. In Europe we have only scratched the surface of what is proving to be an exciting market where technology is being driven by the requirements of users, not the other way around.

TETRA’s operating frequency is around half that of GSM, which means the propagation distance of the signal is a lot greater. However, an increased radio range does not just depend on the strength of the signal; with any form of digital communications it is limited by the tolerances allowed to take account of the signal travelling between the base station and handset. The adjustment in the alignment of the time slots limits GSM to a range of 35 km. TETRA can achieve over twice this figure: some 75 km. Furthermore, TETRA signals specify a 25 kHz bandwidth radio and require a receiver capable of dealing with smaller signals than a GSM receiver. This sensitivity, which is more important on the return path between handset and network, together with twice the range and half the signal frequency, means that TETRA requires considerably fewer sites than GSM. To quantify this, the entire British TETRA network requires 1000 sites to provide well over 90% of the population with hand-portable coverage.


PAMR – public access mobile radio

GSM – global system for mobile communication

PMR – private mobile radio

TETRA – terrestrial trunked radio

ETSI – European Telecommunications Standards Institute

IP – Internet protocol

PTT – press to talk

GPS – global positioning system

SDS – short data service

Tony Greaves

CEO, Dolphin

Tony Greaves has over 15 years’ experience in the telecommunications industry. He joined InquamTelecom and Dolphin from O2 (formerly BT Wireless), where he was a Senior Vice President and Executive board member of both O2 and BTCellnet. He launched the ‘Blackberry’ product into Europe, and led the Home Office sponsored Airwave TETRA business. Prior to O2, Tony Greaves held a number of senior positions in BT: he led BT’s 3G activity for several years, and achievements included the UK 3G licence acquisition, and initiation of the 3G rollout. Tony also has sound experience of the satellite sector: he was responsible for implementing Europe’s first direct-to-home broadcast satellite TV services. Later, he was appointed Director of Intelsat, the world’s largest satellite operator.

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