Voice over Internet Protocol (VoIP) networks combine the best of voice and data communications networking technologies. But that combination also creates some challenges, as the industry attempts to meld the best of circuit switching (from the voice side) and packet switching (from the data side) into single technology.
Perhaps the biggest challenge for network managers comes in the area of multivendor interoperability—the concept that allows hardware and software from different vendors to be integrated into a cohesive system. But since vendors typically approach each other from a competitive, rather than collaborative point of view, some neutral parties are required to referee these interactions. Enter the standards bodies, internationally recognized groups whose purpose is to define and document implementation rules, called standards. Networking standards are typically developed by a committee, which is made up of interested parties, including inventors, developers, and vendors, that have an interest in a specific technology. Most committees are international in scope, and meet in person on a rather infrequent basis—from every few months to every few years—to hash out major issues, but rely heavily on online collaboration for most of their research.
Two key groups produce standards that influence VoIP technologies. The first is the International Telecommunications Union, or ITU, which is headquartered in Geneva, Switzerland. The ITU’s work dates back to the 1860s when agreements were developed to support connections between individual country’s telegraph facilities.
As new technologies—radio, television, satellite, digital telephony, and now VoIP—have emerged, the ITU has expanded and grown. At the present time, the ITU’s work is divided into three sectors: the Radiocommunication Sector (called ITU-R), which manages the available wireless spectrum; the Telecommunication Standardization Sector (ITU-T), which develops internationally-agreed upon networking standards; plus the Telecommunications Development Sector (ITU-D), which endeavors to make modern telecommunications services available to people in developing countries. ITU-T efforts have produced many international networking standards, including Integrated Services Digital Network (ISDN) and Asynchronous Transfer Mode (ATM), with a focus on wide area networking technologies (harkening back to their early days in international telegraph interconnections.). ITU-T standards are designated by a letter, which identifies a specific area of technology, followed by a series of numbers which identify the particular standard. For example, standards beginning with the letter H deal with audiovisual and multimedia systems, including VoIP. One of the often-quoted VoIP standards in this area is H.323, titled Packet-based Multimedia Communications Systems. ITU-T standards are available online from the ITU-T.
The other key player in the VoIP standards world is the worldwide Internet Society. The Internet Society has served as the global clearinghouse for Internet-related technologies since 1992, and as such is substantially younger than the ITU. This age difference causes a difference in focus as well—where the ITU has a rich history in circuit switched communications, such as voice, the more youthful ISOC concentrates more on packet switching and data transmission.
Like the ITU, however, the ISOC parcels its work into smaller groups, including the Internet Architecture Board (IAB), the Internet Research Task Force (IRTF), the Internet Engineering Steering Group, and the Internet Engineering Task Force (IETF). The IETF is responsible for developing and publishing Internet Standards, which are called Request for Comments, or RFC documents. RFCs begin as draft documents from a specific Working Group, and after extensive review and approvals are assigned a number, and then made available online by the RFC Editor. Example RFCs would include the Internet Protocol (IP), RFC 791; Transmission Control Protocol (TCP), RFC 793, the Hypertext Transmission Protocol (HTTP), RFC 2616, and the Session Initiation Protocol (SIP), RFC 3261.
Other organizations may also influence VoIP standards, but with a more regional or technology-specific focus. These include: the American National Standards Institute (ANSI); the European Telecommunications Standards Institute (ETSI); the World Wide Web Consortium (W3C); and the International Multimedia Teleconferencing Consortium (IMTC).
In summary, an understanding of the underlying standards should help network managers sort through the various systems and products that they are considering for VoIP deployment on their network. Products that adhere to ITU-T standards, such as H.323, are most likely to have originated from a telephony and circuit switching perspective. Conversely, products that adhere to IETF standards, such as SIP, are most likely to have originated from the data and packet switching side of the house. Both are quite workable, but approach technical issues such as connection setup/disconnect in different ways. Adopting an architecture that leans in one standards direction or the other, however, can help focus all product decisions down the same road, and thus bypass some of the interoperability challenges that you would prefer to read about, rather than experience first hand.
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The next article in this ongoing series, Fundamentals of Voice over IP, will deal with some technical challenges relating to TCP/IP as a transport platform for voice. Subsequent articles will examine properties of specific protocols and deployment issues.
Mark A. Miller, P.E. is President of DigiNet® Corporation, a Denver-based consulting engineering firm. He is the author of many books on networking technologies, including Voice over IP Technologies, and Internet Technologies Handbook, both published by John Wiley & Sons.