the future of gsm-r

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The future of GSM-R and its possible replacement 18 Apr 2013 by Clive Kessell in Featured rail engineer Rail News, Rail News, S & T 0 Comments GSM-R (Global System for Mobile – Railways) is now well established as the radio system of choice for track- to-train voice communication in Europe and many countries beyond. A comprehensive description of the system was given in issue 48 of The Rail Engineer (October 2008). It is also a constituent part of ERTMS (European Rail Traffic Management System), being the transmission link between the control centre and the ETCS (European Train Control System) equipment on the train. Its use in that role is much less understood, mainly because ERTMS deployment is still somewhat limited. GSM-R is, however, an old technology by current standards and, unlike its public network equivalent (GSM-P or 2G), the railway radio system has not been developed into 3G or 4G offerings. So how long can GSM-R exist and what are the implications for the longer term? This subject was extensively discussed at the recent RailTel conference in Vienna with some illuminating facts emerging. GSM as a standard Whilst newer technologies have emerged, GSM-P or 2G systems continue to be used by a significant proportion of the population worldwide. There can be no escape from this and the European Radio Licensing authorities, under instruction from the EU, have to keep the technology supported until 2025. The mobile radio supply industry must equally continue to supply products for this period, meaning that these radio sets will continue to be made and sold. The same situation should exist for GSM-R where there remain two suppliers of infrastructure (Siemens and Kapsch, the latter acquiring the erstwhile Nortel business) plus sufficient makers of train radio equipment. The UK GSM-R network is now fully operational in the south of the country and has replaced the National Radio Network (NRN) between the Wash and the

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It’s no exaggeration to say that GSM-R is one of the defining railway technical developments of recent decades. Developed in the 1990s to replace disparate national analogue communications, the GSM-R standard brought railway telecommunications into the digital age and crucially delivered true interoperability in transmission networks, providing a basis for train radio applications and the implementation of the European Train Control System.

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Page 1: The Future of GSM-R

The future of GSM-R and its possible replacement

18 Apr 2013 ⋅ by Clive Kessell ⋅ in Featured rail engineer Rail News, Rail News, S

& T ⋅ 0 Comments

GSM-R (Global System for Mobile – Railways) is now well established as

the radio system of choice for track- to-train voice communication in Europe and

many countries beyond. A comprehensive description of the system was given in

issue 48 of The Rail Engineer (October 2008). It is also a constituent part of

ERTMS (European Rail Traffic Management System), being the transmission link

between the control centre and the ETCS (European Train Control System)

equipment on the train. Its use in that role is much less understood, mainly because

ERTMS deployment is still somewhat limited.

GSM-R is, however, an old technology by current standards and, unlike its

public network equivalent (GSM-P or 2G), the railway radio system has not been

developed into 3G or 4G offerings. So how long can GSM-R exist and what are the

implications for the longer term? This subject was extensively discussed at the

recent RailTel conference in Vienna with some illuminating facts emerging.

GSM as a standard

Whilst newer technologies have emerged, GSM-P or 2G systems continue to

be used by a significant proportion of the population worldwide. There can be no

escape from this and the European Radio Licensing authorities, under instruction

from the EU, have to keep the technology supported until 2025. The mobile radio

supply industry must equally continue to supply products for this period, meaning

that these radio sets will continue to be made and sold. The same situation should

exist for GSM-R where there remain two suppliers of infrastructure (Siemens and

Kapsch, the latter acquiring the erstwhile Nortel business) plus sufficient makers of

train radio equipment.

The UK GSM-R network is now fully operational in the south of the country

and has replaced the National Radio Network (NRN) between the Wash and the

Page 2: The Future of GSM-R

Severn. It will be extended to the whole country shortly and, whilst the Cab Secure

Radio systems operating in the South East will remain for the time being,

eventually these too will transfer to GSM-R. Other countries are also moving to

nationwide networks so a lot of capital has been invested. Railway management

would not be impressed if it were to be announced that GSM had to be switched

off within, say, five years!

GPRS and ERTMS

GSM was designed primarily for voice communication, this being foreseen

as the prime need at the time – early 1990s. Its capacity for sending data has

always been somewhat limited and the growth of texting and internet connections

meant that some improved data capacity would be needed. Within the GSM

standard has thus been developed the General Packet Radio Service (GPRS)

which, instead of establishing a connection to a mobile and holding it, sends data

in small packets to different mobiles near-simultaneously. The GPRS service has

been available on the public networks for some time, much use of it being made by

people needing internet connectivity.

The ERTMS programme has similarly encountered data capacity problems

with its GSM-R bearer and only a circuit switched connection (i.e. the connection

to a train is held continuously) is currently approved for ETCS operation. This has

meant that, in busy traffic areas such as terminal stations and complex junctions,

there is insufficient capacity to have all the trains connected to the system. As

such, railways have resorted to conventional lineside signalling in these localities.

So why not use GPRS? Tests done so far might have indicated some evidence of

packet loss but this is being disputed. Signal engineers, being a conservative breed,

thereby declared GPRS as ‘not proven’ and have insisted on keeping circuit

switching, a decision that is unsustainable.

Mindful of the problem, the ERTMS Users Group based in Brussels has

commissioned a new set of trials that have progressed beyond the laboratory

testing stage and are now progressing to field tests. These will be conducted firstly

on a typical suburban line, secondly on a high speed line and finally on a line that

crosses a country border. The trial will include the Hertford Loop section in the

UK, where a Class 313 EMU is being equipped for extensive ERTMS proving

Page 3: The Future of GSM-R

trials before a final commitment to rolling out the technology on the Great Western

main line. Tests will also be done on a TGV Unit in France to prove the high speed

element.

In parallel, the ETCS specifications are being modified in anticipation of

GPRS operation which will impact on the delay performance for timely packet

delivery, modifications to the coding criteria and a network assisted cell change.

Every indication is that GPRS will be successful. No evidence of packet loss

has so far occurred. The final outcome is crucial as, without GPRS, the whole

ERTMS program is at risk since circuit switching will not enable any national roll

out right to the ‘buffer stops’ and a mixture of signalling technology will continue

to be necessary. The data demands of ETCS are not great but they need to be

continually available otherwise movement authority data will be lost. Calculations

indicate that a GPRS based service will be adequate for the task within the present

dedicated frequency allocation of 4MHz uplink and downlink of the GSM-R band.

Another twist in the situation is whether GSM-R networks should be

migrated to IP (Internet Protocol). Existing GSM-R networks rely on TDM (Time-

Division Multiplexing) infrastructure that is not IP compatible. Jochen Nowotny

from Kapsch suggested that preparation for a transition to IP needs to happen since

the telecom industry will progressively standardise around secured mobile IP

access, a process that was conceived more than ten years ago. Above all, safety of

connection must be maintained since both voice calls and the ETCS bearer have

safety implications.

IP interfaces should follow the adoption of GPRS. A sensible plan will be to

prepare the infrastructure first, leaving the mobile radios as the ‘last mile’ of the

transition. If achieved, this will converge with the evolution of other standards

used for secure communication that potentially could make GSM-R a workable

system for long after the 2025 date proposed.

Beyond GSM-R

There has been considerable speculation as to what will eventually replace

GSM-R with much talk, principally by some radio equipment suppliers, that LTE

(Long Term Evolution) within a 4G service is the answer. Detecon, a specialist

Page 4: The Future of GSM-R

consultative firm based in Germany which has studied and advised on railway

radio technology for many years, has investigated the possibilities. Certainly, LTE

has a much higher data handling capability than GSM. A high peak data rate,

greater than 100Mbit/sec with 150Mbit/sec maximum, will be possible. Using an

IP based flat architecture, flexible bandwidth allocations of 1.25MHz, 2.5MHz and

up to 20MHz will be available. High spectrum efficiency for both uplink and

downlink ‘round trip’ times will be much better than GSM – typically 10msec as

against 150msec.

The word ‘evolution’ is important as LTE is a mixture of well proven

techniques combined with other standards and technology. Duplex operation can

be either in separate frequency bands or a single time-shared band. LTE spectrum

in Europe will initially be in the 0.8GHz, 1.8GHz and 2.6 GHz bands. As other

mobile services are closed, additional spectrum in the 0.85GHz, 0.9GHz and

1.9GHz bands is likely. Applications such as on-board CCTV surveillance,

passenger entertainment and information, and maintenance diagnostics will all be

possible. Products are not yet readily available but, as the service takes off, so the

price will fall. Backward compatibility with existing networks and enhanced

interoperability will be a requirement.

There is a downside: LTE has been conceived as a data highway and voice services

may be difficult. Although voice over IP (VoIP) is reasonably well established, it is

not readily useable on LTE at the present level of maturity. Voice calls remain

currently on the 2G and 3G networks. Since GSM-R was conceived for track to

train speech, this situation is a show stopper in the short term. Group call and call

priority requirements, also back to back radio operation and local call routing, are

all under consideration but it will be some time before they become available. The

specifications for voice services in LTE, even in the public sector, are not due to be

finalised until mid-2014 with roll out unlikely before 2016.

So, whilst LTE will offer many advantages over GSM-R, migration is

probably not going to be a practical proposition until 2022. This will give some

comfort to railway management in terms of getting a payback on the considerable

investment already spent.

Page 5: The Future of GSM-R

So where are we?

GSM-R has taken a long time to get established. With GPRS it can do the

job it was designed for, including supporting ETCS operation, and should remain

in service until at least 2025. By sensitive negotiation with the licensing

authorities, it may well be able to continue longer than that. As such, the lifecycle

should be much more aligned to what railways expect for technology.

When change does come, there are a number of questions to answer:

» Will there need to be special features built into LTE to satisfy railway

requirements if that is the eventual chosen system, such as an LTE-R? The

consensus seems to be ‘No’, since deviating from standard products means higher

cost and long development times.

» Will there need to be a dedicated frequency allocation for railway use? Opinion

is divided on this, although some moves are already being made to reserve

spectrum for ‘public services’ operation including fire, police and ambulance. If

this transpires, the railways could well be part of it.

» Will there need to be a dedicated railway radio infrastructure – masts, towers,

base stations, etc? Again, opinion is divided with one vision being to build

networks on a shared basis with other operators. Calculations indicate this would

lead to a 30% cost reduction.

» How would a future LTE network operate for the railway? If a shared network

proposition were to be established could this create an ‘IP cloud’ that would

connect a network control centre to the relevant group of base stations.

» It is likely that Gateways will be designed to link LTE to earlier radio systems.

How will this work with GSM-R and will this be the solution for a migratory path?

So, there is some comfort in the short term but time marches on. It only

seems like yesterday that GSM-R was first thought about but it is in fact 20 years

ago. In less time than that, it will be necessary to decide what will replace it, so

resting on one’s laurels would be foolish. Railway telecom and radio engineers

Page 6: The Future of GSM-R

need to continue collaboration at a European (actually worldwide) level to keep

abreast of what is happening and actively plan for the future.