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The ATM passive optical network (APON) is a cost-effective alternative to the telephone-based and cable-based access networks. An APON uses an optical distribution network that consists of optical fibers and passive splitters. It can be used to provide high-speed Internet connection, voice over IP, voice over ATM, and video on demand services.
In this chapter, we describe ADSL-based access networks, cable-based access networks, and the APON. The ADSL-based access network and the APON have been designed to support ATM and consequently they are connection-oriented networks. The cable-based access network supports the IP network. Although the cable-based access network is not a connection-oriented network, it has been included in this chapter for completeness and because of its importance in the access network market.
Chapter 12: Voice over ATM and MPLS
Voice over packet solutions have been developed for the IP network, ATM, frame relay, and MPLS. In this chapter, we explore the topic of voice over ATM and voice over MPLS. Both ATM and MPLS are suitable technologies for voice over packet, since they can provide QoS, a necessary requirement for real-time traffic such as voice.
The ATM Forum has defined several specifications for transporting voice over ATM. These standards can be organized into two groups. The first group of specifications, referred to as ATM trunking for voice, deals with the transport of voice over ATM between two telephone networks. The second group of specifications deals with how to provide voice over ATM to a user at a desktop or to a user over ADSL. In this chapter, we describe two of the ATM trunking for voice specifications (circuit emulation services [CES] and ATM trunking using AAL 2 for narrowband services ). Circuit emulation services emulate a TDM link, such as a T1 or E1 link, over an ATM network. The ATM trunking using AAL 2 for narrowband services specification is used to transport voice traffic between two distant private or public telephone networks.
The MPLS and Frame Relay Alliance has so far defined two different specifications for voice over MPLS. These two specifications use ATMs AAL 1 and AAL 2 protocols. The first specification deals with circuit emulation services over MPLS, and it makes use of AAL 1. The second specification deals with the transport of voice over MPLS and it uses AAL 2. Both specifications are described in this chapter.
1.4 STANDARDS COMMITTEES
Standards allow vendors to develop equipment to a common set of specifications. Providers and end-users can also influence the standards so that vendor equipment conforms to certain characteristics. Because of the standardization process, one can purchase equipment from different vendors without being bound to the offerings of a single vendor.
There are two types of standards: de facto and de jure. De facto standards are those that were first developed by a vendor or a consortium, and then were accepted by the standards bodies. De jure standards are those generated through consensus within national or international standards bodies. ATM and MPLS, for instance, are the result of the latter type of standardization.
Several national and international standards bodies are involved with the telecommunications standardization process, including:
International Telecommunication Union (ITU)
International Organization for Standardization (ISO)
American National Standards Institute (ANSI)
Institute of Electrical and Electronics Engineering (IEEE)
Internet Engineering Task Force (IETF)
MPLS and Frame Relay Alliance
Optical Internetworking Forum (OIF)
These standards bodies are described below.
1.4.1 The International Telecommunication Union (ITU)
ITU is a United Nations specialized agency whose job is to standardize international telecommunications. ITU consists of the following three main sections: the ITU Radiocommunications Sector (ITU-R), the ITU Telecommunications Standardization Sector (ITU-T), and the ITU Development Sector (ITU-D).
The ITU-Ts objective is the telecommunications standardization on a worldwide basis. This is achieved by studying technical, operating and traffic questions, and adopting recommendations on them. ITU-T was created in March 1993, and it replaced the former well-known standards committee International Telegraph and Telephone Consultative Committee, whose origins are over 100 years old. This committee was commonly referred to as CCITT, which are the initials of its name in French.
ITU-T is formed by representatives from standards organizations, service providers, and more recently by representatives from vendors and end users. Contributions to standards are generated by companies, and are first submitted to national technical coordination groups, resulting to national standards. These national coordinating bodies can also pass on contributions to regional organizations or directly to ITU-T, resulting in regional or world standards. ITU more recently started recommending and referencing standards adopted by the other groups, instead of rewriting them.