Voice over Internet Protocol Fact Sheet

In this fact sheet find:

Key Points About Voice over Internet Protocol

  • Voice over Internet Protocol (VoIP) is the digital transmission of voice communications through a data network based on Internet Protocol (IP).
  • VoIP technology allows voice and data communications
    to share networks and thus increase efficiency, reliability, and cost savings.
  • Standards have not been adopted for the use of VoIP by public safety agencies. As a result, data encoded by one vendor’s device will likely
    not be decoded by another vendor’s device.
  • An emerging standard for public safety communications is called Project 25 (P25) InterSubSystem Interface (ISSI).


Internet Protocol is an open, standards-based set of rules that can route data around network failures and persistently transport data with minimal delay and loss of content.[1]

IP also has mechanisms that can automatically discover the best route through a network with multiple paths.

Voice over Internet Protocol is a technology for encoding and routing digitized voice and data traffic over the Internet. Two VoIP categories are particularly relevant to public safety: VoIP telephony and VoIP within public safety communications systems.

Through IP technology, voice communications are digitized and then segmented into standard digital data payloads (i.e., batches or collections of data) that are, in turn, encapsulated within IP packets so they can be transmitted via the IP transport network. This process allows voice and other information—such as video data—to coexist in a single IP data network so it can be transmitted using shared equipment and communications lines. “IP gateway” is a term often used to describe the device that takes voice or video and encodes or decodes it into outgoing or incoming IP data. An IP gateway in this context is not the same as the gateway described in Interoperability Gateways/Interconnects .) A variety of VoIP-based products are available, including personal computers and specialized network appliances.


The potential benefits of IP-based voice technology for public safety include:

  • Reduced communications costs. Voice and data communications may be combined into a single, well-designed network.
  • Increased reliability. A well-designed IP transport network can reroute data around congested network paths so communications integrity is maintained.
  • Enhanced scalability. IP protocols for routing data are scalable to support a large number of users.
  • Interconnectivity. IP-based radio interoperability equipment is often used to interconnect older or different private radio systems if traditional radio techniques cannot be used.

Issues to Consider

Most IP networks, including the Internet and many closed IP networks (such as intranets), will support VoIP traffic. The Internet, however, is relatively non-secure, unpredictable, and uncontrollable, and thus unsuitable for many public safety uses. Agencies need to evaluate their options before deploying VoIP as a solution for communications operations.

An IP network can transmit voice and image data efficiently, but there is also the potential for loss of clarity or delay in transmission. Agencies can implement a quality-of-service protocol to give voice data priority over other data, especially if the overall load of the network is unpredictable.

Although transmitting voice data via IP is common, the methods of encoding and signaling differ from vendor to vendor. Information encoded by one vendor likely will not be decipherable by another vendor’s device or encoding software. Agencies should consider IP interoperability from multiple vendors and across multiple jurisdictions before making final decisions.

Emerging Public Safety Standard

The Project 25 (P25) InterSubSystem Interface (ISSI), which is part of the P25 standards suite, is emerging as the most mature public safety IP standard. The ISSI has been partially approved within the Telecommunications Industry Association and will allow for P25 trunked radio system inter-connectivity over a wide geographic area using IP-based technology. The Telecommunications Industry Association is accredited by the American National Standards Institute.

Contact the Center of Excellence

For more information on communications technologies, contact the Communications Technologies Center of Excellence (Camden, NJ) 866-493-4675.


[1] Although persistent IP transport protocols for preventing loss and delay of data are available, VoIP applications typically do not exclusively employ these IP protocols for various reasons. Persistent mechanisms are used for control signaling and not usually for actual voice data.

Date Created: January 16, 2008