Traditional narrowband critical communications are beginning the process of migrating to broadband technologies. The international standards body 3GPP is gradually writing the key functionalities developed for narrowband two-way radio systems into the mission critical 4G LTE standard through its SA6 Mission Critical Applications Working Group.
Mission critical push-to-talk (MCPTT) for voice over LTE just made it into LTE Release 13 standard before work was ‘frozen’ in March this year. However, one aspect that did not make into in Release 13 was interworking between legacy narrowband mission critical systems like TETRA and P25 and the new 4G LTE radio networks.
But as these narrowband and broadband systems are likely to co-exist, possibly for a considerable period of time in some countries, it is vitally important an interworking standard is designed and written into the evolving mission critical LTE standard.
Interworking of legacy radio systems was put into the 3GPP SA1 Services Working Group some time ago, but as it did make it into Release 13 the US standards body ATIS set up an ad hoc group in the US to look at the issue.
Phil Kidner, CEO of TETRA + Critical Communications Association (TCCA) became aware of this after attending an international standards meetings last year and knowing the importance of interworking, moved quickly to get European standards body ETSI involved as well.
A group of largely industry people was put together to raise awareness that interworking was not in the standard and to submit a Work Study Item on it to SA6. ‘The (TCCA) has played a crucial role here to ensure interworking is pushed back up the 3GPP priority agenda,’ says Malcolm Quelch, chair of the ETSI Working Group responsible for TETRA and critical communications evolution (TC TCCE) and a TCCA board member.
Quelch got involved in ETSI Working Group 1 after a work item was put in for review looking at interworking in particular in relation to TETRA and LTE. A parallel working methodology has been established with Quelch joining ATIS calls every other week and in the other week working with ETSI Working Group 1.
This enabled a common set of requirements to be established between the US and Europe in particular, but also with input from South Korea, which has an interest in interworking too. The fruit of this effort so far is a Work Study submission to SA6.
‘We put together a terminology document to explain what terms mean in P25, TETRA and in MCPTT,’ says Quelch. ‘It is quite useful to see what terms are used and how they are used. There is a lot of similarity in the way P25 and TETRA deal with the user requirements.’
At the same time as the ATIS and ETSI work, Quelch consulted nine TETRA operators in Europe to get their views on whether or not they might want to change their requirements. The responses have been published on the ATIS and ETSI websites and the results will also be an input to SA6.
End user views
Quelch reports that the operators provided a range of views on the requirements. Some operators see no need for interworking as they aim is to make a ‘big bang’ switchover to LTE. Others believe interworking will be needed for a relatively short period, while a number anticipate quite a long period of co-existence.
‘In the UK, for example, the new Emergency Services Network will have to use a pre-standard interworking solution between TETRA and LTE,’ points out Quelch. ‘I understand that, but it is not the position that everyone takes.
‘Those who don’t think they need it for long say the standard doesn’t need to be that functional, while those who foresee a long period of interoperability think the standard needs to be a pretty functional interface to facilitate interworking between TETRA/P25 and LTE. It is a balance of timing versus functionality. If you make it too complex it will take longer to complete.’
Quelch has submitted a top level report to ATIS and ETSI on these varied operator requirements. He adds that other studies have also been undertaken. One was led by Harris, which Quelch has contributed to, concerning identity translation.
‘The key thing is for one radio system to be able to call another,’ explains Quelch, ‘so you need a common phone book and the good thing about TETRA is that it has globally unique identities. It is a long dialling code, but you can dial directly. However, it is not the same address that will be used in MCPTT, so we need to be able to translate between the two identities.’
The same is true of trunked P25 systems (but not non-trunked ones). ‘Harris has proposed some scenarios for call routing, which we have contributed to as well,’ adds Quelch.
Harris has also initiated a gap analysis of what the industry thinks is needed now and what SA1 originally specified. ‘We are addressing the SA1 issues and some additional ones; there are not many of them, but they are needed for successful interworking,’ insists Quelch.
Items include things like managing emergency calls between the two systems. ‘We need some way of trying to mimic SDS (short data services) in TETRA and a way for them to interoperate with MCPTT systems. We also need to deal with end-to-end encryption, as well as aliases and the like. In TETRA, quite a few users have an alias, so we need to transfer that feature over to MCPTT systems in some way.’
TETRA also contains patch and regrouping features where, for example, at nights when staffing levels may be lower, operators may need to patch and regroup different police forces in a command centre. This is a very helpful function and would be useful as an interwork in the new systems.
‘This list of the new requirements is coming together and will go to SA6,’ says Quelch. ‘These are the kinds of features that were not captured in the original SA1 input. But with 3GPP if things don’t get in to a particular release, you have to put them back on the table for the next release.’
All of these work items are being turned into papers to be submitted to SA6 to help move interworking up the priority list. The hope is that interworking will make it into Release 14, or if not, then 15, but Quelch says it is hard to know which one.
‘The study item has to run until the end of this year and then be fed into the specifications, so it is difficult to predict hard and fast time lines for it to get into any particular release,’ he notes. ‘The key thing we have achieved is getting this flagged up and on the agenda again and the TCCA has been keen to help that happen.’
Quelch says that many narrowband mission critical operators had not expected as much mission critical functionality to be standardised by 3GPP and that has given them confidence. ‘It has been a pleasant surprise to see how much the 3GPP community has been willing to listen to inputs on critical communications.
‘The SA6 Working Group has been very helpful. Things have moved on quite rapidly through that. I believe progress is good and although there is still some way to go, I think we can take a lot of satisfaction in how things have proceeded.’
As a result, Quelch thinks many operators are hoping they may be able to migrate to broadband LTE systems more quickly than originally anticipated. But there is still quite a lot of concern about getting everything standardised and then rolled-out, so co-existence between legacy systems and LTE is expected for some time to come.
‘That is partly because it is not just about the technology; it is about the working practices and procedures. Those need to change too. End users need time to experiment with the new technology and build up all the new procedures, so there is quite a lot of work to be done in that area too,’ observes Quelch.
He adds that a lot of the TETRA functionality will go into LTE and that LTE in turn has a lot of its own additional functionalities. But he warns that some TETRA functionality won’t get in, or will happen in a different way, so that all needs to be thought about and worked through. Mission critical video over LTE and mission critical data over LTE are work items that still have to be considered for Release 14, for example.
‘The ETSI work was entirely catalysed by the TCCA,’ he says. ‘The US work was going to go ahead, so the meeting Phil Kidner attended did enable our parallel working with ATIS and the TCCA has had a very influential role here.
‘But I am happy that something has started in SA6,’ emphasises Quelch, ‘and with continued pressure from ATIS, ETSI and the TCCA I hope we will see this moved up agenda and delivered as a global specification.’