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Back to Archives | Back to April 2005 Contents 

Communications:Voice over Internet Protocol:The Implications for Public Safety

By Harlin R. McEwen, Chief of Police (Retired), Ithaca, New York, and Chairman, IACP Communications and Technology Committee, and Vice Chairman, National Public Safety Telecommunications Council

oice conversations using data networks (also known as IP, or Internet Protocol, networks) have received much attention from consumers, businesses, and regulators as an alternative to the traditional calling methods over telephone circuits. The name typically given to these calls is Voice over Internet Protocol (VoIP) because it uses the networks and protocols that are in common use by businesses and consumers on the Internet and intranets.

VoIP is a developing technology that has several ramifications for the law enforcement and public safety community. This will require vigilance in addressing both the negative and positive aspects of this technology for law enforcement and public safety, and as strategies develop to deal with this new technology.

VoIP Background
Voice conversations over the Internet started as a way for individuals to communicate while accessing their network-connected computers. The early applications were unreliable and the voice quality was poor to acceptable. As interest increased in the application, the Internet Engineering Task Force (IETF) developed requests for comments that described protocols to be used on data networks that would be appropriate for VoIP and would improve the voice service performance on these networks. The telephone industry, through the International Telecommunication Union (ITU), began to adapt or adopt protocols that would allow voice conversations to be sent as data packets.

VoIP and Emergency 911 Calls
VoIP technology presents challenges to the ability of public safety to obtain caller number, identification, and location information at 911 public safety answering points (PSAPs). Competing vendor services are emerging without legal requirement that this information be provided to public safety. Many of the new VoIP services also have no mechanism for proper routing of 911 calls to the proper PSAP. An example of the problem is a recent incident where a 911 call was commenced through a computer in Boston, Massachusetts, and ended up at a PSAP in Houston, Texas. The IACP is working with the National Emergency Number Association (NENA) and the Association of Public-Safety Officials International to address these issues.

VoIP Intercepts and Electronic Surveillance
VoIP presents legal and technical challenges to traditional lawfully authorized intercepts of telephone calls. The Communications Assistance for Law Enforcement Act (CALEA) provides some help but is clearly in need of revisions to address changing technologies. The FBI's CALEA Implementation Unit (CIU) has established the Law Enforcement Executive Forum (LEEF) to bring together local, state, and federal law enforcement leaders to address law enforcement approaches to dealing with these issues.

The Role of VoIP in Public Safety Wireless Communications
VoIP is moving into the wireless communications service area. The public safety community is seeing products and applications that support VoIP for communications between dispatch centers of one or more jurisdictions, between radio base station and repeater sites to other sites or dispatch centers, and between practitioners' mobile and portable radios with other radios or dispatch centers on the network infrastructure. These products and applications use different technical approaches and are, for the most part, incompatible.

With the increasing commercialization of VoIP for public safety wireless communications, the National Public Safety Telecommunications Council (NPSTC) is currently discussing the value of standardizing Voice over Internet Protocol (VoIP) use in public safety communications situations. NPSTC is considering standardized public safety VoIP applications that can provide interoperability between public safety agency networks and a migration path to future services. NPSTC realizes that the public safety community must develop a common voice on public safety's concerns as well as requirements for VoIP standards and interoperability.

Various commercial wireless service providers are adapting many of the same protocols used by the data and telephone networks into their cellular networks that support push-to-talk (PTT) and conventional telephone services. Following the lead of the commercial sector, PW (Private Wireless, or private land mobile radio) communications equipment vendors are now incorporating VoIP technologies into infrastructure (base radios) and subscriber (mobile and portable radio) equipment.

In order to differentiate between the two types of VoIP applications in this article, the first will be called "IP Telephony," and the second will be referred to as "PW IP." IP Telephony has the following features:

  • Technology allows telephone calls over a data (IP) network from PC (personal computer) to PC, from PC to Public Switched Telephone Network (PSTN) telephone, and PSTN telephone to telephone.

  • Industry is attempting to provide standardization and interoperability.

  • Standards are developed primarily by the IETF and the ITU.
    PW IP has corresponding features:

  • Technology allows voice calls (PTT) between subscriber sets in the field and to a fixed site such as an office-based supervisor or dispatch operator, and with PSTN telephone users.

  • Industry is developing proprietary solutions.

  • Open standards are not being developed, other than a specific purpose standard that is in development for PW by one standards organization described below.

The bottom line: for IP telephony networks, VoIP applications are converging on standards to allow exchange of voice packets whereas, for PW networks, VoIP solutions are proprietary with no movement towards standardization and achievement of multivendor equipment interoperability.

Project 25 Radio Standards Background
The Project 25 Standards process is an effort by the public safety community to work with industry through a standards development organization - the Telecommunications Industry Association (TIA) -- accredited by the American National Standards Institute (ANSI). The standards process is to provide a suite of documents that describe and specify the interfaces of narrowband digital land mobile radio systems developed to meet the requirements of public safety agencies and practitioners.

While the Project 25 radio system uses voice coders (vocoders) to digitize the users' voice communications and inserts the voice frames into data packets for transmission, reception, and network distribution, Project 25 is not based on Internet Protocol (IP) technology from end to end or between subscriber radios. However, the Project 25 system designers were forward-thinking in planning the infrastructure backbone data networks for Project 25 systems. TIA's TR-8 Committee, charged with developing the Project 25 Interface Standards, selected IP for connecting the data networks of Project 25 radio systems through the Inter-RF Subsystem Interface (ISSI). They chose the Session Initiation Protocol (SIP) to set up voice calls between networks and the Real-Time Transport Protocol (RTP) to distribute the voice packets to the intended P25 networks. While the protocols are standardized by the IETF, the actual population of voice information into the protocol messages is defined by the TR-8 Committee and is specific to Project 25-defined voice and control parameters. The completed RTP message specifications will be submitted to the IETF as an RFC (request for comments) that multiple vendors can then use to develop Project 25 products. Additional Project 25 interfaces, such as the Fixed Station Interface and the Console Subsystem Interface, will employ common or complementary P25-defined SIP and RTP message formats.

Although the ISSI and other interface standards for Project 25 are not advertised as using VoIP, in truth, they do. With the exception of the P25 common air interface, the P25 interfaces will all employ common VoIP protocols and standards but in a manner specific to Project 25. While today's Project 25 radio system is a narrowband solution with its emphasis on voice communications and some low-bit rate data communications, the Project 25 internetwork backbone incorporates features potentially capable of migrating to future user services such as multimedia applications. The Project 25 ISSI development (as well as the development of the other interfaces) follows the ANSI-accredited process for standards development. The process facilitates competitive offerings by multiple vendors of products that are interoperable for data exchange across a standardized interface. P25 standards are also being developed to (1) enable acceptable performance to be realized in basic and complex networking configurations and (2) define user-oriented tests to evaluate multivendor product interoperability. This suggests that the Project 25 ISSI Standard should be the model or platform on which to develop future user services inter- and intra-network interface standards for public safety agencies.

VoIP and Project 25 PW Standards Summary
Voice over Internet Protocol (VoIP) has received much discussion and excitement as an efficient technology to route and distribute voice communications over data networks. The telephone industry is actively promoting standardization of the technology and many vendors now sell products such as routers that will support VoIP services over diverse types of data networks (i.e., for different types of networking infrastructures). However, there is no equivalent standardization effort on the Private Wireless front, including those serving public safety. There, vendors are developing proprietary VoIP solutions, which require that proprietary products be used in each of the interconnected networks supporting public safety voice services (such as PTT). Because equipment vendors are implementing various vocoders and various protocols (populated with their own parameters and information), there seems to be little chance that a Private Wireless VoIP internetworking standard based upon the vendors' current applications will naturally emerge.

Public Safety's Position
The public safety community, through organizations like NPSTC and federal public safety programs such as CommTech at the National Institute of Justice and Safecom at the Department of Homeland Security, is developing a consensus position on VoIP. To date, the partnership is considering the following points to arrive at a structured approach of ensuring that the public safety community will be able to advance with the available technology (VoIP) while ensuring that the various agencies that use the technology will be interoperable:

  • Public safety's Statement of Requirements for Public Safety Wireless Communications and Interoperability (SOR), jointly developed by CommTech and Safecom, promotes standardization to achieve interoperability among public safety communications systems and suggests the use of commercial off-the-shelf (COTS) solutions, such as consumer-offered products and widely used network protocols, as the means to lower costs to public safety with robust and timely products.

  • The standards committees are developing widely used network protocols, such as IP, SIP, and others, that will be readily adaptable as network interface standards for public safety communications networks. These protocols will accommodate current as well as future public safety services and applications.

  • The Project 25 narrowband digital land mobile radio standard for public safety contains a suite of interface standards that are partially complete. The Project 25 interface standards that address voice call setup and voice-frame packet transport (VoIP functions) among Project 25 IP-based networks, such as those for the Inter-RF Subsystem Interface (ISSI), the Fixed Station Interface (FSI), and the Console Subsystem Interface (CSSI), need to be completed, balloted, and adopted by the Telecommunications Industry Association (TIA) TR-8 Committee as soon as possible. Standardized PW VoIP implementation can then occur on public safety agencies' Project 25 radio systems.

  • The Project 25 ISSI Standard, while developed for narrowband services, can and should be used as the basis for the interconnection of networking infrastructures that support future user services such as those described in the public safety SOR.

  • The Project 25 ISSI Standard should be adopted for use in interconnecting Project 25 narrowband and the new broadband networking infrastructures with other PW and commercial or public wireless networking infrastructures to meet the evolving needs of the public safety communications community.

  • The Project 25 VoIP message sets (developed for the Project 25 ISSI Standard) should be used by local area networks to connect an agency's base or repeater stations with its consoles and system administration hardware, as well as to interconnect audio gateways and other devices that interconnect voice communications among agencies.

  • The Project 25 VoIP message sets should be used to provide interoperability between conventional and trunked public safety systems, among agencies, and as the common interoperability protocol among Project 25 and non-Project 25 Private Wireless radio systems for public safety.

A Police Chief's Perspective
While VoIP has taken a strong role in communications using data networks and the public telephone network, the movement within IP telephony is to use standards that allow greater interoperability of VoIP subscriber equipment. This is not to say that all VoIP equipment is interoperable but that there actually is a good chance of obtaining equipment that is. Prior to acquiring the equipment, it needs to be checked out for the intended situations. While the manufacturers are moving towards standardization, due care is necessary now.

The same movement toward standardization is not present on the PW IP side of communications. For that reason, police chiefs are best advised to move slowly in adapting PW IP solutions that are being marketed to their agencies. The chief may end up having the ability to communicate with other agencies but may find that the department's interoperability is very limited. Instead, chiefs should seek standardized PW solutions. NPSTC, CommTech, and Project Safecom are attempting to define that solution. ■



From The Police Chief, vol. 72, no. 3, April 2005. Copyright held by the International Association of Chiefs of Police, 515 North Washington Street, Alexandria, VA 22314 USA.

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