Voice-over-intranet Protocol for Critical Communications

Internet protocol-based technologies and services have taken the telecommunications market by storm. Once just a data-only transfer mechanism, IP is quickly encroaching on the voice-communications market. The public safety marketplace, in particular, is an area with keen interest in this technology. While most of the industry focuses on voice-over-IP services related to low-cost telephony (such as long-distance calls over the Internet), some land mobile radio companies have either fielded products or announced future plans to provide critical-communications-grade IP networks and equipment.

Some misconceptions exist about the application of Voice-over Internet protocol to land mobile radio. Examining and dispelling some of the myths associated with VoIP for critical communications will make it apparent that VoIP technology provides significant customer benefits when properly applied.

The principal requirements of critical communications networks—reliability, fast access, multiple users and agency broadcasts, and transmission of high-quality voice and data—all can be met with a VoIP private intranet tailored for public safety.

Many VoIP myths and misconceptions result from confusion about the application of Internet concepts to critical-communications requirements. As a digital, packet-switching network, the Internet is a powerful communications medium. But because it is a public entity, designed for data communications, it is not suitable for critical voice communications. It cannot support the fast access times, call priorities and security required by public safety. Furthermore, the common VoIP vocoders that are used to insert digitized voice into the packet-switched data stream are not sufficiently robust for the public safety communications environment.

Advantages of an intranet

A well-designed and managed private intranet can leverage the power of an IP-based network while compensating for the deficiencies of VoIP over the Internet. The intranet can be designed to provide the fast response times, excess network capacity, high-quality voice and security that are needed for critical communications. All of these characteristics cannot be guaranteed over the Internet. Table 1 summarizes the fundamental differences between voice over the Internet and voice over a private intranet.

Packet-switched vs. circuit-switched networks

The key to understanding IP-based technology is the concept of packet switching. Traditional voice communications networks (telephony or land mobile radio) are circuit switched. In circuit switching, users have exclusive use of the connection (a circuit or radio channel) until the conversation has concluded and the connection is released.

In packet switching, the messages are divided into packets, and multiple users share access to a circuit or channel by taking turns placing their packets onto the channel. Packet switching became the basis of Internet communications and was extended to telephony with the advent of asynchronous transfer mode switches, which are high-bandwidth devices used to switch conventional telephone voice calls.

The main advantage of packet switching is capacity. Many users can share a telephone line or radio channel. A secondary advantage is the redundancy associated with a packet-switched network. After a message or voice call is divided into packets, the individual packets can take different paths through the network to the final destination, where they are reassembled into the message. This permits a high degree of redundancy and reliability in the network architecture.

Each radio, personal computer or subscriber device in an IP network has a unique IP address. Messages include a header that contains source and destination IP addresses. Each router functions as a small packet switch that, on receiving a message, looks into the header and routes the message based on the destination IP address. Because there are usually many routers in a system, a distributed switch architecture results, which has an inherent ability to compensate for device failures or congestion points. This distributed IP architecture provides an enormous redundancy benefit, one of the key requirements for most public safety communications systems.

Circuit switching, on the other hand, is essentially a centralized switch architecture with natural bottlenecks at the switches or individual circuits.

Maintaining high-quality voice

Typical VoIP applications use the H.323 telecommunications standard, which includes the G.723.1 low-bit-rate vocoder. H.323 is not suitable for critical communications because it does not offer group, priority, preemption or emergency calling—all required for public safety applications. Also, the H.323 suite of vocoders was designed for point-to-point telephony voice applications. It is inappropriate for use in moving vehicles with traffic noise in the background or for communications over a low-bandwidth radio channel that is subject to multipath fading. This is why we designed an intranet system using DVSI’s AMBE vocoder, which has been optimized for superior performance in the high bit-error rate environments typical of public safety mobile communications.

Security

Voice communications over the Internet is susceptible to hackers who penetrate the network and listen to conversations. By providing end-to-end encryption and operating over a private intranet system, security is maintained throughout the network. The backbone of M/A-COM’s intranet system, called “OpenSky,” carries voice messages in compressed and encrypted formats. Compression significantly increases the voice traffic capacity of a typical network. End-to-end encryption means that voice communications are encrypted over the air and over the packet-switched network backbone, thereby eliminating all possible hacker entry points.

Cost benefits

The key to savings for a voice-over-intranet system is the use of commercially available off-the-shelf switching equipment and reliance on standard software, as opposed to purpose-built, proprietary, switching hardware and software. This enables competitive procurement of standard IP routers from multiple suppliers, as well as network management systems that operate on Unix workstations. These products provide customers with “economies of scale” cost savings and transparent upgrades. The IP architecture is highly scalable; adding capability is as easy as expanding a LAN.

The key advantage is that the distributed IP network can be upgraded and expanded incrementally and economically, unlike a fixed system that requires a complete “forklift” upgrade once it has reached maximum capacity.

In addition, the IP-based architecture uses established, ubiquitous IP standards that will withstand the test of time and are a much safer long-term investment. In an IP-based architecture, data applications are “plug and play,” and hence, integration costs are drastically reduced. For example, a data application can be integrated on a wired LAN and then easily ported to a wireless application without the need for any middleware.

Voice over IP allows the true integration of voice and data. By using Time Division Multiple Access technology, it is possible to have voice and data capability in “one radio, one infrastructure and one channel,” providing users a two-to-one cost savings over separate voice and data systems.

Creating a network

Table 2 summarizes the comparison between a traditional LMR network and an IP-based voice and data network. M/A-COM’s IP-based network configuration (Figure 1) uses a wide-area intranet backbone to handle multiple switching centers by connecting the Ethernet LANs in each regional operations center. A primary design objective was to use industry-standard and field-proven hardware and software components wherever possible. The Internet protocols, including TCP/IP and UPD/IP, and the CDPD (IS-732 standard) architecture were selected because they have become de facto industry standards. The resulting network enables each radio to handle integrated voice and data on the same RF channel simultaneously, using TDMA to provide two to four simultaneous voice calls for each 25kHz channel. With an “always-on,” end-to-end, IP connection, and with each radio having its own unique IP address, true end-to-end IP connectivity is maintained throughout the network. Over-the-air software provisioning provides automatic downloads for software updates to radios in the field. The air-interface protocol employs advanced channel arbitration and error correction to achieve stable performance. Security is maintained throughout with encryption of the IP backbone and the air interface.

Implementation need not wait

Many manufacturers are endorsing IP and claiming a VoIP system in their “future vision” for public safety communications.

However, “voice over the intranet” is available now. An end-to-end IP network can provide all of these benefits. Three simple questions should be asked when determining whether a system is truly IP-based:

1. Does the system deliver end-to-end IP services/applications?

2. Is the network packet switched?

3. Does every piece of radio, routing and switching equipment have its own IP address?

A truly IP-based wireless private network can be implemented now for private land mobile radio applications. A private intranet radio network, rather than a public IP network, is a cost-effective way to provide the capacity required to ensure high-quality, maximum-clarity voice packets in all communications, especially during peak periods. This end-to-end design also enables the use of data applications without middleware for wireless networks that employ IP as a transport mechanism. Unlike other forms of VoIP, notably those designed to operate over the Internet, a private network can compensate for time-delay latency to provide high-quality voice.

Wireless IP, such as the OpenSky network, provides priority and preemption, mobility tracking and group calling services among a complete suite of services geared to the needs of the government and public safety users.

Herther is director of product integration and Haymond is director of systems technology for M/A-COM Wireless Systems, Lowell, MA. Herther’s email address is [email protected]. Haymond’s email address is [email protected] electronics.com. AMBE is a registered trademark of Digital Voice Systems Inc. (DVSI). For more information: www.opensky.com.