MIL-STD-188-176
4. GENERAL REQUIREMENTS
4.1 General. ATM is an integrating concept that enables all types of information, from voice to data to video, to be handled by common transmission and switching facilities. ATM bearer service supports connection-oriented, sequence-preserving, cell-based communications service between two or more ATM end systems. ATM has been widely adopted for use in broadband- integrated services digital networks (B-ISDN). This MIL-STD, which is a profile of ATM standards, addresses the
standardization of digital hierarchies, multiplexing structures, switching, and interfaces ATM subnetworks. The term B-ISDN is used for convenience, to refer to and emphasize the broadband aspects of ISDN. The intent, however, is that there be one comprehensive notion of an ISDN that provides broadband and narrowband ISDN services. B-ISDN is a telecommunications concept defined by ITU recommendations and ANSI standards to provide integrated services, two of which are interactive services and distribution services. These two main service categories are defined in ITU-I.211, sections 1.2 and 1.3.
4.2 Asynchronous transfer mode . ATM networks use fixed- size packets transmitted across ATM networks with the use of fast packet-switching technology at each node. The ATM function is divided into two layers: the ATM layer, which is common to all services and provides fast packet-transfer capabilities; and the ATM adaptation layer (AAL), which maps higher-layer information into ATM cells to be transported over the network. Commercial standards for ATM are based on the availability of highly
reliable (for example BER = 10
-11) transmission facilities (fiber,
cable, SONET). The use of ATM over transmission media exhibiting significantly higher BERs (e.g., satellite, wireless, and
tactical radios) will require forward error correction (FEC) tailored to each specific transmission media. This is especially true for satellite links that also introduce significantly higher delay. Application of FEC is part of the transmission equipment and is outside the scope of this MIL-STD. The upper layer protocols needed to accommodate the satellite delays have not
been standardized. Use of ATM over limited-bandwidth and noisy channels, in general, is an area for future study.
4.2.1 Cell format . All user traffic appears at ATM network interfaces as fixed-length cells. Each cell consists of
53 octets. The cell header consists of 5 octets, and the cell payload consists of 48 octets.
4.2.2 User-to-network interface (UNI) . The UNI is a shared boundary between the network and user terminal equipment. A
private (or base) network switch is viewed as a terminal by the public network. Thus, the UNI is also the interface between base
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