The first "Tech Topic" addressed the basic premise and definitions of interoperability and outlined three objectives to attain radio network interoperability: 1.) compatible communications equipment, 2.) adequate signal coverage, and 3.) network scaling. More importantly, it was also postulated that true interoperability was more a function of the human factors involved in establishing the operational parameters of the network than on the technical issues themselves.

This Tech Topic reaches beyond the assumptions of the human factors to address one method of establishing interoperable networks from a technical perspective. If the human elements of network design including the composition, network control, and operational procedures are established, then technical solutions to providing interoperability among and between the various components can be established to allow the full range of operational interoperability.

OSI Model


The operation of any network, whether it is an isolated local area computer network, an over-the-air radio network, or the large-scale interconnection of various networks, such as the Internet, can be described in terms of the International Standards Organization (ISO) Open Systems Interconnection (OSI) model1. This model provides a standard reference or framework to build the various components necessary for communications between the elements of the network. The model consists of seven generalized layers with specific functions that should be accomplished in each layer. The seven-layer model forms the basis for our discussion of interoperability among disparate public safety networks2.

Tech Topic #1 stated that the first objective for interoperability of a radio network was to use compatible equipment operating with the same transmission parameters, which means common radios operating on the same frequency with the same modulation characteristics, etc. This is an example of providing interoperability at the physical layer of the OSI model by ensuring that the components of the network all operate with the same communications characteristics at the transmission interface.

Physical layer interoperability is interesting and will be the basis for attaining interoperability between disparate networks in the next Tech Topic, but for the moment we’ll focus on a different aspect of the model.

The OSI model suggests an alternative method of interoperability that is provided at the Network Layer. The physical layer (Layer 1) provides the mechanical, electrical, functional, and procedural characteristics necessary for the transmission of fundamental bits. The Data Link Layer (Layer 2) provides for the reliable transfer of data units (frames) across the physical connection. The Network Layer (Layer 3) provides for independent routing and switching of the information frames. Hence, at the Network Layer "routers" provide the appropriate addressing and routing over the connected network for the digitized information. As a result of common protocols provided by the Transmission Control Protocol (TCP) and the Internet Protocol (IP), which are the most commonly used Layer 3 protocols, interoperability between equipments of different venders and in fact different networks can be attained.

The methods to do so are based on protocols3, or agreed-upon procedures for the exchange of information, that include the Internet Protocol, or IP4. Specifically, the Internet Protocol "is designed for use in interconnected systems of packet-switched computer communications networks and provides for transmitting blocks of data called datagrams from sources to destinations."5


VOICE OVER INTERNET PROTOCOL (VOIP) MODEL. (click image for larger view)

This concept is also the basis for the development of telephonic devices that use the Internet for the network transport and allows users to connect to the Internet at any number of access points in order to pass telephonic traffic to another phone anywhere in the world. By using Internet Protocol (IP) networking protocols, voice only users are able to digitize the voice traffic by using voice encoders/decoders (vocoders), access the Internet via the network level at an access point, and capitalize on the Internet for Voice Over Internet Protocol (VOIP) voice services as shown in the diagram to the left. The FCC has published an information paper on VOIP along with an information webpage at

Furthermore, this concept is also the foundation for IP-based interoperability for the public safety community. Original information from a network user (such as voice, data, or even video) is converted to a digital format (simply a digital bit stream representation) by an encoder and formatted into an Internet Protocol-based packet format for transmission. Once this is done, the transmission media and methods are irrelevant and can be accomplished by any Internet, or Internet-type, connection. Interoperability is provided by the IP-based formatting and addressing structure. Hence, a router connected to the Internet gives any other local area network or radio network the capability to achieve a certain degree of interoperable communications.

The only other requirement is to ensure that all of the appropriate users are addressed correctly and traffic is allowed to pass through the router to the appropriate end user devices or alternatively in the opposite direction onto the Internet. Unfortunately, this can become a very labor intensive manual process that in a crisis situation may not be as expedient as desired.


Radio over IP Network (click image for larger view)

It should be noted that Wi-Fi, or Wireless Fidelity, technology utilizes the same principles. The wireless, or radio link, simply allows a mobile data user to connect to an access point that in turn performs the conversion of the information to IP-based format for transmission via the Internet. By extension, this same idea is used to provide Radio Over Internet Protocol or ROIP connectivity. This protocol permits the interface between disparate radio systems such as conventional VHF systems and Project 25 systems via the common IP-based routing. A diagram provided by Motorola below shows an example of how this interoperability concept works.

This diagram highlights another important aspect of IP-based routing among different users. The protocols allow various users that have different radio or computer systems to interconnect with each other by interfacing at a common networking level. Each radio network has an Internet router, or gateway, element that translates information to/from the radio network into IP-based traffic for transmission via the Internet. The remainder of the network then performs the overall transport functions without modification.



IP-based interoperable systems are gaining acceptance in many quarters and more venders including CISCO, MA/COM and others are building systems based on this technology. As an example, CISCO has marketed a system under the name IPICS, CISCO IP Interoperability and Collaboration System. A diagram of their notional interoperable system is shown at the left. 6

This and other similar systems point out the strengths of IP-based interoperability. These strengths include the following:

  • Resiliency. The built-in versatility, reliability and alternative routing capabilities of the Internet create a very resilient and durable interconnection environment as the basic backbone interconnection for interoperable networks.
  • Scalability. Because each local network relies on a single Internet connection, the ability to expand or reduce the neighborhood of connections is simple. As a result, it is a simple addressing issue to either expand or contract the populace of users or talk groups.
  • Graceful Development and Evolution. The open access architecture of the Internet allows the interconnection of any number of networks via the common IP framework. This allows for a graceful evolution of devices from legacy networks to next generation networks without obvious impact. This eliminates interoperability issues based on funding, equipment inventory, or version.
  • Flexibility. Because the only interface between networks occurs at a common network level, the nature of each network is irrelevant to all other users. In this way, an interface is provided for different types of networks and differently equipped networks. In addition, because the interconnection is essentially via the Internet, then all "Internet-capable" services, functions, and applications can be integrated across all of the networks.

On the other hand, there are several shortfalls to IP-based interoperability that may arise through the use of the open architecture of the Internet. It is important to note, however, that many of these "open architecture" problems may be overcome through the use of specialized/private networks and pre-planning. They include the following:

  • Lack of prioritization. IP-based interoperability and transmission via the Internet do not necessarily provide for prioritization of public safety traffic or assured access/transmission, unless public safety traffic is transmitted over specialized, private IP-based networks. Military systems, for example, provide for pre-emption and levels of priority of service.
  • No inferred "common language" for operations. Even though there is a "common language" at the technical level for interoperability, i.e. the IP protocol, there is still no common operating language between network users. Hence, the 10-code of one network will not be consistent with the open language of another network. Once again, here is example of the mandate for common "operating principles" before inter-operability, such as those that can be created in a specialized public safety network.
  • Issues of Authentication and Verification remain. There is no inherent authentication of users and traffic between either users or networks. Therefore, it is difficult to verify network participants and network traffic. Once again, operational issues must generally be resolved on the human level! Specialized public safety networks can also be designed to enable authentication parameters.
  • Addressing and Location identification. Although each network has its own inherent control and addressing scheme that includes the establishment of desired talk groups, the admission or deletion of elements to the expanded network must be done by a network administrator who is able to update or revise the address elements of the interoperable network. This is particularly evident in the case of Federal entities entering into agreements with non-Federal users to utilize any established state or local networks. Similarly, although network elements may be interoperable between their networks, there is no inherent geographical location representation capability. The lack of inherent location representation, however, can be mitigated through specialized networks that are designed to accommodate network element interoperability across broad, or nationwide, geographic areas.
  • Coverage areas are still limited. Although each radio network will be able to interface with another network via the IP-based traffic exchange, each network is still limited to its radio coverage area. Other communications platforms, such as satellite or landline, can mitigate this limitation when integrated into the communications network.
  • Lack of control. With the interconnection of disparate networks via a technical level language, there may not be a single focal point for control of the larger integrated network. Systems can be designed to provide for an integrated control, but this is not ubiquitous.
  • Potential security issues. If a closed security network is established, then all elements of the network including those that are interoperable via an Internet connection must employ the same security measures. In addition, the Internet is an open access network; therefore use of the Internet for interoperability is vulnerable to undesired intrusions via the Internet unless specialized, private networks employing security measures are utilized.

In summary, interoperability for disparate public safety networks is possible by employing gateway devices in each network that permit the use of IP-based transmission systems.

1 See ISO/IEC 7498-1:1994, Information technology -- Open Systems Interconnection -- Basic Reference Model: The Basic Model.

2 Courtesy Novell, Inc. at Novell's Networking Primer.

3 A protocol as defined by the McGraw-Hill Telecom Fact Book, Second Edition, is "a strict procedure for the initiation, maintenance, and termination of data communications. Protocols define the syntax (arrangements, formats, and patterns of bits and bytes) and the semantics (system control, information context or meaning of patterns of bits or bytes) of exchanged data, as well as numerous other characteristics (data rates, timing, etc.)."

4 The Internet Protocol is the result of the efforts of the Internet Engineering Task Force (IETF) and the technical aspects are designated in Requests for Comments (RFC). The Internet Protocol is defined in RFC 791.

5 See RFC 791.

6 See the Cisco IPICS Brochure.