PROVIDING INTEROPERABLE SECURE VOICE IN CONVERGING HETEROGENOUS NETWORKS: THE NATO SECURE VOICE STRATEGY

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1 MILITARY COMMUNICATIONS AND INFORMATION SYSTEMS CONFERENCE PROVIDING INTEROPERABLE SECURE VOICE IN CONVERGING HETEROGENOUS NETWORKS: THE NATO SECURE VOICE STRATEGY Michael Street, Brian Bottesini, Rita Russo NATO C3 Agency, Brussels, Belgium and The Hague, The Netherlands Michael.Street, Brian.Bottesini, Patrick DeLaere SHAPE, Mons, Belgium Gerard Elzinga NATO HQ C3 Staff, Brussels, Belgium Ric Murtland Supreme Allied Command Transformation, Norfolk, Virginia, USA ABSTRACT This paper provides an overview to the NATO secure voice strategy which will ensure the continued provision of the essential user service of secure voice. Even in today s era of network-centric capabilities the need for military users to talk securely to each other remains a crucial requirement. Providing a reliable voice service over heterogeneous networks presents an increasingly challenging problem as network technologies proliferate and military users make greater use of networks over which they have limited control via service provision contracts for telecommunication services. The NATO secure voice strategy (SVS) [1] sets out a course through the evolving network technologies, accounting for obsolescent and developing technologies, and reflecting the needs of the operational community for interoperability to support current and future NATO operations. Keywords: Secure voice, heterogeneous networks, SCIP, NINE. INTRODUCTION Even in a network centric environment voice communications are an essential service for military planning and operation execution. Secure voice communication provides a service which is still unrivalled for its responsiveness, interaction and trustworthiness. In addition to the military role, in NATO secure voice is one of the key services used for political consultation and command and control of military forces. Therefore even in the current era of NATO network-enabled capability (NNEC) operations, secure voice remains an essential requirement. The NATO secure voice strategy addresses NATO s transition strategy towards network-enabled end-to-end (person-to-person or person-to-process) secure voice. The strategy leads from the current position and addresses incremental transitions towards the desired end state of omnipresent secure voice.

2 Prague, Czech Republic - September, 29-30, 2009 NEED FOR A SECURE VOICE STRATEGY The NATO secure voice strategy has been developed in response to key drivers, namely: Evolving, diverse communication network technologies, Equipment obsolescence, Availability of new cryptographic technologies, Demand for wider interoperability, Need for simplified key distribution, Perceived inadequacies of many current secure voice solutions. Equipment obsolescence and changing network technologies are endangering the current secure voice capability. Historic key management mechanisms are inefficient in current operations. Plans for the telecommunication networks necessary to support NNEC, coupled with new cryptographic technologies, can offer significant enhancements to available secure voice services and their mobility and interoperability. CURRENT STATUS Today s communications networks and their associated technologies are changing at an ever increasing rate. Traditional real-time voice communication circuits are steadily being replaced by other technologies such as Voice over Internet Protocol (VoIP). The dramatic increase in the use of mobile (e.g. cellular, GSM) phones has changed the way we communicate and users expectations. Individuals are always connected not only with voice communications, but with and web browsing capabilities as well. Many developing countries are even completely by-passing the implementation of terrestrial telephone infrastructures and establishing cellular telephone networks instead. In the tactical environment, voice communications require interoperable and flexible infrastructures to support the rapid reaction capabilities of an operational commander. Current operational needs require these services to be interoperable between wired and wireless networks, between networks deployed by different nations and between military and civilian organisations. The services are required within and between all levels of command. The requirement for secure voice communications adds an additional layer of complexity to this already challenging environment due to the additional stringent timing and reliability requirements for the synchronization of security protocols, cryptographic algorithm exchanges needed to support a realtime encrypted service and the need for a supporting cryptographic key management infrastructure. NATO has several secure voice solutions in use today, shown in figure 1. These solutions have largely evolved independently of one another, resulting in the terminals and their supporting infrastructures being non-interoperable. Figure 1 shows a diverse range of non-interoperable secure voice solutions and only considers secure voice in NATO. Virtually every nation has at least one, and often many, bespoke secure voice equipments, usually developed exclusively for a specific network technology e.g. PSTN, ISDN, GSM, CDMA, personal satellite communications, high-frequency radio etc.

3 MILITARY COMMUNICATIONS AND INFORMATION SYSTEMS CONFERENCE Figure 1. Disparate secure voice solutions Secure voice interoperability between each of the systems shown in figure 1 where possible is only possible through bespoke secure voice gateways. These have to be developed specifically for particular pairs of secure voice equipment. Use of bespoke gateways has a number of problems. Voice is decrypted and decoded back to analogue, therefore the gateway must be located in a secure area and voice quality is degraded [2]. Bespoke nature often relies on use of end terminal secure telephones in the gateway, requiring manual operators or inelegant solutions such as mechanical fingers for the gateways to control the secure telephone. Equipment limitations restrict the number of secure calls between different secure voice cryptographic families to the number of channels supported by the gateway. Inability to support secondary dialling e.g. the user must dial the number of the gateway but has no mechanism to provide the desired destination number. This can be resolved by using a manual operator or through more sophisticated methods using a programmable gateway [3]. User Perception and Voice Encoding Secure voice services depend on voice coders 1 to digitise the voice before it is encrypted. The voice coder is one of the closest functions to the user, therefore the behaviour and performance of the voice coder has a major impact in the user s perception of particular secure voice equipment. 1 The term voice coder is used generically in this paper where distinction may be made between voice coders and waveform coders.

4 Prague, Czech Republic - September, 29-30, 2009 Voice coder performance is heavily influenced by the data rate available between the terminals 2 and by the signal processing power available to provide real-time analysis and synthesis of the voice signal. The continuing increase in available signal processing technology results in improvements in voice quality as more sophisticated voice coders become realisable. The widespread, but aging Narrow-Band Secure Voice II (NBSV-II) equipment uses STANAG 4198, a linear predictive coder (LPC-10e). This voice coder is limited by the technology available thirty years ago when it was developed. It was also limited by the need to operate on strategic networks of the time which could only offer 2.4 kbps links. As a consequence, voice quality and intelligibility of LPC-10e is considered poor by many users today. In contrast STANAG 4209, a continuous slope variable delta (CVSD) waveform coder was developed for tactical networks. On these networks typically covering shorter distances and with fewer network switches - higher data rates (16 kbps) were available, allowing a higher rate, simpler coder to be used. Using a range of voice coders allows for the optimum compromise between network limitations and quality of the voice service. However, the more voice coder standards that are used to provide secure voice the greater the problem to provide interoperability. As a result, the NATO secure voice strategy is restricting the number of voice coders. In order to support secure voice service on constrained networks, the mixed excitation linear prediction-enhanced (MELPe) coder is used. This coder, standardised as STANAG 4591, provides a reasonable voice quality on 2.4kbps channels, making it suitable for narrowband channels such as radio and satellite links. Where appropriate, commercial voice coder standards are also encouraged, primarily G.729D and G.711. These three voice coders can cover a variety of scenarios, ranging from toll-quality voice on high-capacity networks (G.711 at 64kbps); STANAG 4591 offering near toll-quality voice on very narrowband channels; and G.729D covering the middle ground between these two extremes. Figure 2. Speech quality of voice coders in diverse noise environments [4] 2 For this reason 2G cellular systems use a variable bit rate (VBR) codec, so users can be given a high quality voice service possible, but quality and bit rate can be reduced when the network is congested in order to increase network capability.

5 MILITARY COMMUNICATIONS AND INFORMATION SYSTEMS CONFERENCE Figure 2 shows the relative performance of three voice coders routinely used in secure voice equipment: the historic LPC-10e and CVSD coders and the more modern MELPe. Figure 2 shows the speech intelligibility performance which is measured as a %. This is shown in four different noise environments; quiet - with an infinite signal to noise ratio (SNR) is shown at the back of the graph, a modern office environment typical SNR 20dB, a noisier environment 12dB SNR, and finally a very noisy environment is shown at the first of the graph with a 6 db SNR. Figure 2 shows that MELPe offers significant improvement in performance in all environments over the other 2.4kbps coder, LPC-10e. Compared to CVSD, MELPe offers improved performance in all but the noisiest of environments at a fraction of the data rate. MELPe is therefore the natural choice of voice coder for low throughput networks and should lead to an improvement in performance for the users. NEW TECHNOLOGIES Two recent technologies provide significant portions of the solution to the complex problem of providing secure voice services on NATO networks. These are the use of voice over secure IP networks (VoSIP) and the NATO secure communication interoperability protocol (SCIP). Secure communication interoperability protocol (SCIP) SCIP has been developed to provide end to end encrypted voice and data communication between terminals operating on heterogeneous networks. As such it meets many of the requirements for NATOs secure voice strategy and provides a fundamental piece of the solution. STANAG 5068 (Draft) covers the suite of SCIP specifications that define SCIP. SCIP is the product of a multi-national group involving NATO, national governments and industry. The SCIP specifications are available to nations and industry in order to develop interoperable secure voice and data terminals. As a result a SCIP terminal from vendor A in nation X will be interoperable with a SCIP terminal from vendor B in nation Y. SCIP uses the STANAG 4591 voice coder which offers state of the art voice coding, giving comparatively high voice quality and intelligibility from a 2.4 kbps channel. SCIP terminals place some demands on the intervening networks, although in general these are only that the networks conform to the characteristics and interfaces defined in their respective standards e.g. PSTN (V.32), ISDN (V.110), GSM (bearer service 26) etc. In providing SCIP over IP networks SCIP relies on standard VoIP services and protocols rather than rely on a proprietary VoIP solution. Session initiation protocol (SIP) and real-time protocol (RTP) are both used by SCIP terminals to establish an IP connection between SCIP terminals which is capable of transporting the real-time encrypted traffic needed to support SCIP [5]. SCIP over IP may be referred to as secure voice over IP (SVoIP). SCIP was developed to operate over heterogeneous networks therefore the network interfaces must also support SCIP. This is of particular concern at the boundary between circuit-switched and packet-transfer networks. Here, SCIP uses the ITU-T standard v [6]. V is commonly referred to as modem over IP. Although this is an agreed international standard, it is not widely available and steps must be taken to ensure that military and commercial networks will be able to support SCIP across these interfaces. The SVS recommends use of SCIP to provide secure voice through SCIP enabled terminals operating on a wide range of military and commercial networks e.g. PSTN, ISDN GSM, CDMA, Iridium SATCOM, IP etc. Obsolescent secure voice terminals should be replaced by SCIP equipment as necessary.

6 Prague, Czech Republic - September, 29-30, 2009 The NATO SVS identifies use of SCIP over heterogeneous networks - military and commercial, wired and wireless, circuit-switched and packet-transport. The strategy also identifies a number of steps to ensure future NATO telecommunication networks and network interfaces will support SCIP. This can be achieved by procuring future voice switches which will support V Voice over Secure IP In many areas, such as static headquarters or major deployed bases, NATO has established high capacity, resilient IP networks. These networks typically rely on a combination of physical security of the network infrastructure coupled with IP network encryption to protect all network traffic and signalling. Conventional VoIP telephones can be used on such a network to provide secure voice through voice over secure IP (VoSIP). This can be considered as a system high voice and data network where the voice is provided by insecure VoIP on a secure IP network. The NATO SVS recommends use of VoSIP to provide secure voice in headquarters and locations where a secure IP network is available to large numbers of users on a secure local area network (LAN) and where the majority of secure voice traffic is within the LAN. VoSIP service can be provided between secure LANs where there is a sufficient capacity, quality of service and security between the LANs. In future secure IP networks encryption of these inter-lan connections will be provided by the NATO IP network encryptor (NINE) which provides another common element to the solution. Secure voice requirements beyond NATO The two technical solutions, SCIP and VoSIP, between them are expected to accommodate virtually every current and future NATO requirement for secure voice. National secure voice requirements are expected to be similar. SCIP is capable of supporting NATO, coalition and national modes. Similarly, VoSIP is suitable for secure IP networks operating in a range of security domains. As a result, the SVS is capable of being extended to support the secure voice needs of nations as well as of the NATO Alliance. SCIP can also support operational modes with reduced security levels which are suitable for sharing with user communities outside the military. This capability allows military and civil actors to communicate in a trusted manner to facilitate information sharing, cooperation and collaboration. Secure voice interoperability to facilitate such interaction is necessary to enable a comprehensive approach, involving military and civil organisations in order to achieve the desired outcomes of many current and future NATO operations. FUTURE ARCHITECTURE Having proposed two different solutions to provide secure voice in order to meet the future needs, it is clear that gateways between SCIP and VoSIP will be necessary. These are likely to be located at the edge of VoSIP enclaves to allow secure voice users within the enclave to communicate securely with other users equipped with SCIP devices and vice versa. Figure 3 shows the envisaged future secure voice architecture based on SCIP terminals operating on a variety of networks. These networks support the requisite interfaces to allow SCIP calls to transit the full range of networks. SCIP-VoSIP gateways are located at the edge of VoSIP enclaves to provide secure voice interoperability between the two solutions. This reduces the gateway requirements to a single gateway family to operate between SCIP and VoSIP although there will be numerous locations where these gateways will be required.

7 MILITARY COMMUNICATIONS AND INFORMATION SYSTEMS CONFERENCE Figure3. Future secure voice architecture Gateways to national secure voice systems Other gateways may be required to legacy NATO and national secure voice equipment families. The SCIP signalling and gateway specifications are capable of supporting these services as well as those required above. However as legacy systems will phase out in the future, less and less different gateways will be required CONCLUSIONS Secure voice, like many military requirements, is not an area where one size fits all. However by making sensible use of recent and developing technologies it is possible that two sizes fit virtually all. By adopting this approach NATO and Nations are able to benefit from cost-effective solutions to provide their secure voice requirements. This allows NATO to plan for the major steps needed to deliver these capabilities. The NATO secure voice strategy identifies over 40 specific recommendations which should be carried out within the short (0-3 years), medium (3-5 years) and long-term (5-10 years) timeframes. These recommendations are providing specific guidance to strategic commands and organisations within the NATO C3 structure. The recommendations are reflected in a number of technical initiatives and procurement plans.

8 Prague, Czech Republic - September, 29-30, 2009 The general guidance can be summarised in four high level recommendations of the secure voice strategy, which are to: halt procurement of any non-essential secure voice capabilities which are not consistent with the NATO secure voice strategy, to facilitate the deployment of NATO SCIP terminals and appropriate key management system to provide end to end secure voice; to enforce support for SCIP on the black communications infrastructure, to enable VoSIP for local secure voice services, to encourage the development and deployment of gateways from SCIP to VoSIP and national systems as required. ACKNOWLEDGEMENT The authors wish to thank the many colleagues within NATO and the Alliance who have contributed to the NATO secure voice strategy and without whom this paper would not have been possible. BIBLIOGRAPHY [1] NATO C3 Board, NATO Secure Voice Strategy v.1.0, AC/322-D(2009)0020-AS1, May 2009 [2] Blašková, L, Holub, J, Street, M, Szczucki, F and Tomíška O., Objective and subjective degradations of transcoded voice for heterogeneous radio network interoperability, RTO Tactical Communications symposium, Prague, Czech Republic, April [3] Street, M, Secure voice conferencing facility, Technical Note 1067, NATO C3 Agency, Netherlands, July [4] Street, M, Future NATO narrowband voice coder (STANAG 4591): selection process (phase two), Technical Note 913, NATO C3 Agency, Netherlands, July [5] Velasco, L. and Street, M, Performance of SCIP devices on public and classified heterogeneous networks, Military CIS Conference, Krakow, Poland, September [6] SCIP-216 Rev 2.0 Minimum Essential Requirements (MER) for V Gateways, SCIP International Interoperability Control Working Group, November 2007.