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Back to Archives | Back to October 2008 Contents 

Understanding: IP Voice Connectivity Alternatives

By Stuart Overby, Senior Director, Global Spectrum Strategy, Motorola, Inc.; and Member of the IACP Communications and Technology Committee; and Cynthia Wenzel Cole, Director of Standards and Strategy, Business Operations in System Infrastructure Operations Group, Motorola, Inc.

oday’s police agencies face an increasing portfolio of communications options from which to choose to enhance and support prevention and response activities. This increase in options is largely the result of the marriage of one or more technologies originally developed for land mobile radio (LMR), telephone, or computers. This article demystifies these options and provides a basic understanding of the benefits and trade-offs.

As public safety practitioners examine the communications alternatives available to help manage their range of mission-critical, administrative, and/or wireless telephone communications, they are faced with a dizzying array of terms and claims to sort out. Some common questions emerge, such as the following:

  • What are the differences between voice over Internet Protocol (VoIP) and land mobile radio (LMR)?
  • What are their respective advantages and disadvantages?
  • What happens when they are connected together?
  • What is the difference between radio over IP and radio standards such as the P25 ISSI?

By using “apples-to-apples” comparisons, the real differentiators and trade-offs should become more apparent. However, prior to making any comparisons, it will be helpful to review some of the relevant terminology and background. Ultimately, the goal is to help agency information technology (IT) and communications leaders to ask the right questions and objectively evaluate alternatives so they can implement public safety communications systems that meet operational requirements.

Step One: Understand the Terminology

One of the constant challenges is keeping up with the pace of new and changing technologies even though the terminology is not keeping up. Furthermore, many terms in this area of technology are not consistently used nor precisely defined. This can cause confusion as public safety agencies work to understand the subtle but important differences among the many options available.

SAFECOM, a program of the U.S. Department of Homeland Security notes, “One of the barriers to understanding VoIP usage in public safety communications is the lack of a common definition. The phrase ‘VoIP’ is currently being used in several different ways, such as Internet Protocol (IP) Telephony, Radio VoIP, and Private Wireless VoIP. Pre-meeting interviews confirmed that individuals had very different understandings of VoIP based on their own experience and involvement with the technology.”1

A list of terms and definitions used for the basis of this article is provided here to help establish a common base of understanding.

IP Telephony: IP telephony refers to one-to-one, full-duplex, 10-digit dialing service with familiar services and control such as voicemail, call forwarding, call waiting, and caller ID. IP telephony differs from historical telephone service in that the voice uses a packet-switched, IP-based transport rather than circuit-switched services.

LMR IP Trunking Systems: These systems are the core networks that provide trunked call services over wide-area, IP-based radio access networks. Today’s LMR trunked networks utilize an IP-based network core and IP connectivity across the infrastructure elements of the system.

Project 25 Inter RF Subsystem Interface (P25 ISSI): P25 ISSI is an interface specification under joint development by public safety representatives and equipment manufacturers. When implemented, the P25 ISSI enables two or more disparate, trunked P25 radio systems or subsystems to be connected at the network layer in a manner that supports voice and data services, end-to-end encryption, and roaming.

Radio over IP: Radio over IP is similar to VoIP but augments two-way radio communications rather than telephone calls. With radio over IP, at least one device is a radio connected via IP to other devices on the radio network. Radio over IP offers the flexibility to use a variety of form factors and devices. Note that radio over IP is used here as a general technology category and does not refer to any specific company or product offering.

Radio-over-IP Bridging: Radio-over-IP bridging means using IP protocols to bridge between voice system gateways. An example of a protocol interface is the Bridging Systems Interface (BSI) standard profile that was recently established by the VoIP Roundtable.

VoIP: In the broader sense, VoIP is a general term to refer to a category of technologies that use IP to carry voice services. The term is used to refer to the protocol, a network interface, a radio access network, and sometimes even an LMR system.

VoIP Telephony: This term refers to the original VoIP protocol used to transmit telephony voice services through or “over” IP-based, packet-switched networks. VoIP uses telephony signaling to control the service and often includes additional capabilities such as billing.

Step Two: Remember Some Basic Guidelines on Protocols

Here are some architectural insights to keep in mind while comparing various protocol alternatives, sort of like understanding the existing framing on a building before beginning a new addition.

Definition of Protocol: A communications protocol is the technical language used to communicate information. It includes a set of standard rules for data representation, signaling, authentication, and error detection required to send information over a communications channel so that the system works properly.

Only Part of the System: Although protocols are critically important, they remain just one of many key elements of a system. Protocols constitute a relatively small portion of the technical capabilities of any system, gateway, or device. For example, the protocol does not change the basic underlying radio physics that results in some frequency bands providing better coverage per site than others.

Least Common Denominator: Services between two different systems are supported in a “least common denominator” fashion. In other words, only the services that every device, interface, and system can support are supported end-to-end across the entire communications path. That does not necessarily mean that current equipment cannot be reused as part of an updated system; it just means that agencies will need a crystal-clear view of what services flow where, what features are or are not supported, and what changes will need to be made to accommodate them.

Optimized Versions of IP: Several of the LMR trunking architectures use a variation of IP called multicast IP. This IP variation has the critical advantage of sending a single call out to the network while the network makes the “copies” of that call only where required, avoiding the need to connect each talkgroup member in a separate IP transaction. This enables the IP-based trunked systems to deliver messages with minimal delay, a requirement for reliable public safety communications, especially in shoot/don’t shoot situations.

Translating Only When Necessary: Translating, often called “transcoding” in the radio and computer world, provides interoperability between different elements of a system that do not speak the same language. Transcoding is the primary function of a “gateway” device. For example, some type of transcoding is normally required when a cellular device is connected to an LMR radio system. In general, although such gateways serve a useful function, the less transcoding required, the better. It is usually much easier for two people to communicate if they speak the same native language; the same is true of radio systems.

When two devices speak the same native language, protocol, or code, they communicate more effectively, in less time, with more depth, and with fewer communication errors. To take the analogy a step further, if one person speaks only French and the other only English, a separate translator (or “transcoder”) is needed. Imagine, then, if the message had to go from French to Spanish to English. Multiple protocol transcoding functions in the same way. It adds delay, reduces functionality, is less secure, and results in a greater potential for errors. Generally speaking, systems will be more complex to manage if more translations are required.

Step Three: Comparing the Various Options

With a basic understanding of the pertinent definitions and guidelines, it is possible to make some “apples-to-apples” comparisons of the options.

Table 1. Comparing wireless VoIP technology with two-way LMR trunking
Table 2: Radio over IP versus LMR trunked IP connectivity
Telephony versus Two-Way LMR Services

The first comparison shows the general attributes of a commercial wireless telephony service compared to a public safety LMR trunked radio system.

Table 1 shows the contrasts between VoIP telephony and LMR services from a public safety perspective. At the most basic level, VoIP telephony services are designed to allow a user to dial a 10-digit number, wait, and talk to one person. LMR services are designed to support groups instantly, at the single push of a button. The vast majority of law enforcement officers carry both a radio and a cell phone, which underscores that both types of services are needed. Although there are ways to make VoIP telephony work in LMR networks, a more relevant focus is the comparison of group-based push-to-talk (PTT) services.

Radio over IP versus LMR IP Trunking

Table 2 provides a high-level view of which types of applications and requirements are more suited to radio over IP and which are generally more suited to LMR IP network connectivity alternatives such as the P25 ISSI. Although these are general capabilities that will hold true across products using the protocols, how the protocols are optimized can vary dramatically when they are implemented in actual products and solutions.

Radio-over-IP services use IP protocols to connect a single unit or single talkpath over an IP network, providing low-cost, versatile, and easy connectivity among multiple types of end user devices, such as LMR and cell phones. Radio-over-IP architectures are optimized to connect smaller clusters of users together and can become clogged in supporting high volumes of calls on a backbone network. Conversely, if a single VoIP network gateway connection is designed to support hundreds of users, it may not scale down in a cost-effective manner for smaller deployments.

Radio-over-IP solutions use very basic control functions, which can be an advantage in many public safety applications, such as interoperability and deployable radio systems used to supplement coverage for a specific incident or service in a disaster area. In contrast, LMR trunked IP connectivity provides more advanced critical control capabilities vital to many day-to-day public safety operational situations. In general, these advanced control capabilities need to be integrated into the LMR network infrastructure as a permanent, preplanned installation.

Radio-over-IP Gateways: The illustration in figure 1 shows one manner in which radio-over-IP gateway devices can be used to connect to LMR radios. For example, radio-over-IP gateways can be used to connect private broadband networks with VoIP-enabled systems and devices such as portable radio systems, personal computers, and desk phones. Although basic radio-over-IP gateway connections are relatively easy to deploy and install, public safety agencies are urged to pay attention to the areas of technology that require special enhancements to support public safety cases and services. Without these enhancements, radio-over-IP connections might not meet public safety agency expectations in actual use.

Figure 1. Connecting radio-over-IP gateways to LMR radios

To provide the same level of control and advanced capabilities as modern LMR systems, radio-over-IP networks would need similarly sophisticated mobility applications. Until such time as these can be developed by radio-over-IP providers, agencies need to understand what operational trade-offs remain for public safety agencies when deploying radio-over-IP gateway technologies.

Why LMR Trunking Requires Specialized Technologies: LMR trunked systems incorporate solutions to a number of technical challenges that result from specialized public safety operational requirements, including the following:

  • One-to-many communications in a mobile and portable environment

  • Supporting many talkgroups roaming throughout the operational area

  • Peak loading that can drastically exceed that of more routine operations, among others

Connecting one device to another while those devices are standing still is a relatively simple task. This is the situation when connecting a base station and a dispatch point. Supporting thousands of public safety talkgroups that are constantly roaming across vast areas while controlled by a single operator is an entirely different level of challenge, requiring an entirely different type of solution to help ensure reliability.

LMR networks must also work under peak loads, with better than 99.9 percent reliability during both day-to-day operations and disasters that significantly multiply communications traffic, such as the bridge collapse in Minneapolis, Minnesota, in 2007.

On a day-to-day basis, LMR networks must be able to deliver one or more highly specialized services, such as preemptive emergency call, dispatch priority and takeover, busy carryovers, adjacent-site seamless roaming, audio logging, end-to-end encryption, strict user access control, granular call-by-call priority, data/messaging, over-the-air rekeying and programming, location and text messaging, radio inhibit, fallback modes after failure, fast recovery, detailed call tracking, and call attempt logging.

Public safety services and features such as these have required specialized trunked call processing applications and solutions whose designs are based on many years of operational deployments and experience. These services are being standardized in the public safety P25 standards initiative. The standard already addresses such features and services within a given radio system; it is now being expanded to address how many of these services and features can be provided when separate networks are connected to one another.

Considerations for Decision Making

Most public safety radio network decision makers are faced with operability and interoperability requirements, both of which need to be supported. In addition, these requirements include communications among the officers within an agency, across multiple agencies in a jurisdiction, and, increasingly, across multiple jurisdictions and levels of government. Communications requirements also include lifeline voice dispatch; voice connections with the public; and even data, images, and video.

Ideally, meeting all these needs would be a simple decision. However, given that different agencies have their own unique requirements, may operate on bands of differing frequencies, and/or be at different points in the system replacement cycle, it is likely that decisions will be more complex. In addition, solutions are increasingly likely to involve connections across two or more systems. Fortunately, technology is advancing to assist in making these connections.

IP-based IT infrastructures, which are becoming ever more versatile, standards based, and cost-effective, will support convergence among many different protocols and connectivity capabilities among different networks and devices. In most cases the “layers” of the network are largely independent from one another, so that multiple connectivity solutions can be utilized by the same network. The solutions described do not need to be considered alternatives to one another but should be able to exist side by side in the same network ecosystem.

By moving the protocols “up into the network,” the interoperability protocols can stay isolated from the radio access network (RAN) layer. They could then be more “agnostic” and theoretically could operate over any compatible RAN, be it VHF, 700 MHz, LTE, or CDMA–Rev A. As described, the resulting levels of performance, capabilities, reliability, and value come from how well all the technology works together.

Connecting to LMR IP Trunking Systems

Figure 2. Comparison of methods of connection to LMR IP trunking systems
When systems are connected, the ability to carry these services across the interface is determined by the capabilities of the interface and at what “layer” the interface connects into the network. The illustration below shows a simplified version of an LMR trunked system architecture and summary of the primary connection methods. In general, the “higher” into the network the interface connects, the “fatter” or more capable the interface becomes (see figure 2). The trade-off is that the systems have to be more similar, in some cases the exact type and vintage. The “lower” the interface connects, the “thinner” the interface becomes. That means more different types of devices can be connected, but the trade-off is in control, performance, and functionality.

The primary methods of connection to LMR IP trunking systems follow:

  • Network to network: A network-to-network connection works just as it sounds: it connects two networks together. This connection provides the highest-capacity options and the highest level of functionality but requires the networks to be similar.
  • Console operator patch: Point-to-point, console-to-console audio patches are commonly used, but they provide only basic audio and must be manually set up and torn down; furthermore, encryption is not possible with these connections.
  • Site links: Site links provide medium-level capacity and functionality and enable trunked signaling and encryption, but they do not provide full call control.
  • Air interface: P25 Common Air Interface (CAI) is used to enable radios of different manufacturers to roam on compatible P25 networks.
  • Radio wireline gateway: A radio wireline gateway provides an IP “bridged” connection; its basic functionality enables a broadband radio-over-IP device to be connected. This gateway connects on a unit-by-unit basis, has minimal call control options, and offers no end-to-end encryption.

With such a wide variety of needs, applications, situations, equipment types, and, most importantly, user requirements, multiple alternatives are needed to provide effective solutions. By developing a clear understanding of how systems can be connected, it should become easier to navigate the available options and select the most appropriate solutions to meet agency requirements. Following are some questions that agencies should ask solution providers; the answers will help agencies make good decisions:

  • What services are supported end to end?
  • How will the network, connections, and devices need to change?
  • What limitations and differences are introduced by the connectivity?
  • How will those limitations affect the office in various situations (such as emergency, nonemergency, and roaming)?
  • How are the calls controlled?
  • From end to end, how many times is audio transcoded?
  • How integrated are the management features?
  • What happens when a link or device fails?

As the communications industry marches toward IT networks, higher penetration of IP-based devices, and ever-increasing needs for interoperability, some specific practices can help public safety agencies move more purposely forward as well. Some of the recommendations discussed to help ensure that the solutions deployed meet expectations are as follows:

  • Clarify terminology and definitions before starting discussion with equipment suppliers or integrators

  • Decide exactly what services are needed and where

  • Understand what protocols do—and what they do not do

  • Become educated enough about the underlying technologies to ask the right questions
  • Fully understand the inevitable trade-offs when connecting systems
  • Determine the best interface “fit” for connecting to LMR networks based on operational and interoperability requirements

Taking these steps, understanding some of the underlying architectural issues, and becoming familiar with the various IP alternatives available when using and connecting LMR networks across IP should be helpful for public safety practitioners working to meet their operational requirements. ■

Stuart Overby has 34 years of experience in spectrum management and communications. In addition to his membership on the IACP Communications and Technology Committee, he currently serves as vice chair, Spectrum Management Committee of the National Public Safety Telecommunications Council (NPSTC); chair, 700 MHz Advocacy Working Group, NPSTC; chair, In-Building Communications Working Group, NPSTC; and an adviser to the In-Building Wireless Alliance. Prior to joining Motorola, he was with the U.S. Federal Communications Commission for 12 years.

Cynthia Wenzel Cole and her team manage a variety of public safety technology areas related to Project 25 standards interoperability and IT enterprise management. She received a bachelor of science degree in electrical engineering–RF design from the University of Michigan.


1Roundtable on Public Safety Interoperability and Voice over Internet Protocol, National Inistitute of Standards and Technology, Office of Law Enforcement Standards and SAFECOM, August 22, 2006, (accessed August 19, 2008).



From The Police Chief, vol. LXXV, no. 10, October 2008. Copyright held by the International Association of Chiefs of Police, 515 North Washington Street, Alexandria, VA 22314 USA.

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