Topic 3: Physical Layer Interoperability

The basic premise and various definitions of interoperability were addressed in the first tech topic along with three objectives and the overarching goal of insuring functional human interaction for an interoperable network. The second topic established a common framework for network interoperability via the Open Systems Interconnection Model and further highlighted the use of the Internet Protocol (IP) as one method of establishing interoperability between disparate network users. It was shown that by using the networking layer of the model, scalable networks could be established among otherwise incompatibly equipped networks by using IP-based protocols and routers within each network to provide an interface mechanism, hence interoperability, with other networks.

As stated in the previous topic, the operation of any network, whether 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. In addition to providing a standard reference, or framework, to build the various components necessary for communications between the elements of the network, it was also shown that the networking layer provides one means to create an interface for interoperable networks. This topic looks at a different aspect of the model and suggests an alternative for creating interoperable networks.

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 (Layer #1) of the OSI model by making sure that the components of the network operate with the same communications characteristics at the transmission interface.

The physical layer provides the mechanical, electrical, functional, and procedural characteristics necessary for the transmission of fundamental bits of information. As long as all of the users in the network operate on a common set of transmission parameters, security features, operational procedures, including agreement on any common messaging formats, etc., then interoperability may be attained and maintained. This is the conventional "same settings" mode of operation where all users share the same network.

This concept of interoperability can be extended to networks that do not have common transmission parameters by use of an interface device at the physical layer that makes the transition or translation of the parameters of one network to the parameters of another network. This concept has been used for a long time and is known simply as radio relay, retransmission, or cross-connection.

The figure below shows a typical radio relay configuration between a UHF radio and a VHF radio². The radio relay occurs when the retransmission unit receives a UHF signal, translates it to the transmission characteristics of the other network (VHF in this case) and transmits the reformatted signal. Of course, retransmission would occur in the opposite direction as well for two-way communications. In this way, interoperability between a UHF radio network and a VHF radio network is attained. This interoperability, or bridging, is accomplished at the physical layer between the two networks.

However, in terms of the OSI model, it is important to note a very important characteristic of the retransmission facility in the diagram above. It should be recognized that in the retransmission process, the transmitted signal from the UHF radio is received and demodulated down in frequency to the original information at baseband level – that is to say all the way down to the original audio information (or perhaps to some common intermediate frequency). Then the audio or data information signal is re-modulated for the transmission characteristics of the other network – in this case the VHF network. Of course the reverse is true of transmissions from the VHF radios to the UHF radios; the retransmission requires demodulation and re-modulation. This process (the process of demodulating from transmitted signal to baseband information and re-modulating to different transmitted signal characteristics) is extremely important because it forms the basis for an entire methodology for interoperability among multiple radio networks.

By extension, if one radio network signal can be broken down and re-modulated, then several radio networks can be interconnected using the same approach. But it is important to recognize that in terms of the OSI model, interoperability is really NOT at the physical layer of transmission. In fact, it occurs at the highest layer, Layer #7 of the model, which is the application layer, or in this case, the original audio (voice) information from the user. In this case though, if the original audio information can be "handled" digitally fast enough, then interoperability is attained simply by providing a relay interconnection between the various radio networks. This concept is the basis for the Raytheon-JPS ACU 1000, ACU-M, and other equipments of this nature.³

Thus, a unique interoperability capability is provided by signal processing fast enough to satisfy any number of interconnections. The accompanying diagram (see below) illustrates this cross-connection capability for various devices and radio networks⁴.

Thus, a unique interoperability capability is provided by signal processing fast enough to satisfy any number of interconnections. The accompanying diagram illustrates this cross-connection capability for various devices and radio networks². It should be noted that conventional telephones, satellite networks, cellular networks, and other trunked radio networks can also be interconnected. Each brings particular technical characteristics though, such as the delay of push-to-talk networks, that complicates the interface processing. All of this "control information" is referred to as the COR, or Carrier Operated Relay, signaling requirements. However, the diagram suggests the ability to interface numerous disparate networks and the only requirement is an interface cable between one of the radios in each network and the interoperability device. While it seems to be a relatively uncomplicated process, in fact there are several issues that make the process non-trivial.

Many of the different radio systems and networks have distinct capabilities and operating characteristics that would otherwise complicate interoperability. For example, public safety networks typically have a push-to-talk signaling technique that initiates a transmission. On the other hand, other systems use voice activation mechanisms to initiate transmissions. Other major issues involve signaling delays that are inherent in different systems such as either satellite systems or trunked-radio systems. The interoperability devices overcome these technical issues by using signal processing techniques in the interconnection process.

These and other similar systems point out some of the strengths of interoperability achieved by interface devices. These strengths include the following:

• Resiliency. The built-in versatility, reliability and alternative processing capabilities of the physical interconnections provided by the signal processing of these interface devices create a very resilient and durable environment as the basic backbone for interconnection of disparate radio networks.
• Scalability. Since each radio network relies on a single cable connection to the interoperability device, the ability to expand or reduce the neighborhood of connections is simple. As a result, it is a simple interconnection issue to expand or contract the populace of users and/or talk groups based on the interconnection of radios.
• Flexibility. Because the only interface between networks occurs at a common interface – or interoperability - device, the nature of each network is irrelevant to all other users as long as the technical parameters are considered by the interface. In this way, an interface is provided for many different types of networks and differently equipped networks since only an interconnect cable is required. In addition, because the interconnection is essentially via physical interconnection, then all services, functions, and applications can be integrated electronically across all of the networks. This is a generalization, however and the implementation of many potential services is dependent on the capabilities of the interface device.

On the other hand, there are several shortfalls to providing interoperability by using interface devices. They include the following:

• Range and frequency use limitations. The coverage area of the individual radio networks will not change with the use of interface devices. While the overall coverage area may be extended or modified by interconnecting different networks, the range of the individual networks can not change and all of the pre-existing limitations of each network will remain. In addition, while bridging individual networks together, each still requires its own frequency allocation, so there is no gain in spectral efficiency.
• Lack of prioritization. Similar to IP-based interoperability, transmissions via an interface system do not necessarily provide for prioritization of traffic or assured access/transmission. In fact, there may be some negative impact to the overall traffic flow in the conjoined networks due to increased traffic volume. Once again, it is extremely important that the human operational aspects of both networks be reconciled so there is agreement on the use between the two. Furthermore, while there are methods in either hardware of software to obtain prioritization of traffic, it is extremely important to resolve these issues at the operational level.
• Issues of authentication and verification remain. Similar to IP-based network interoperability, it is incumbent on the individual network operators to insure authentication of users and traffic between either users or networks. Therefore, absent the active intervention of the network operators, it is difficult to verify network participants and network traffic. Once again, operational issues must generally be resolved on the human level! This is extremely important because the integration of a new network does not simply follow once an interface cable connection is made. In fact, an operator must establish the new radio interface and any intended talk-group affiliations.
• 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 an 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 radio interfaces, since any addressing or identification may or may not exist a priori.
• Centralized control. With the interconnection of disparate networks via a physical interface, a single focal point for control of the larger integrated network is necessary. As suggested earlier, the operational characteristics of the joined networks must be agreed upon a priori and shared among all users – recall the human interoperability issue!
• Potential security issues. If a closed security network is established, then all elements of the network including those that are interoperable via a physical interconnection must employ the same security measures. These measures and procedures may not be the same among different networks that are brought together by the interconnection.

In summary, interoperability for disparate public safety networks is certainly possible by employing interconnection devices that permit the direct interconnection of various systems.

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

² Handbook of Patchwork Interoperability, A Complete Non-technical Primer, Revision 2.2, JPS Communications Inc., September 2004.

³ See Raytheon-JPS ACU Interoperability Products.

⁴ See Handbook of Patchwork Interoperability, A Complete Non-technical Primer, Revision 2.2, JPS Communications Inc., September 2004.