| 单词 | ATM (Asynchronous Transfer Mode) |
| 释义 | ATM (Asynchronous Transfer Mode) A fast-packet, connection-oriented, cell-switching technology for broadband signals.ATM was an outgrowth of the ITU-T development efforts towards broadband integrated services digital network (B-ISDN). Although B-ISDN faltered, ATM became the switching technology of choice in the broadband backbone of the public telephone network, at least for a time. ATM is designed to accommodate any form of data, including voice, facsimile, computer data, video, image, and multimedia, whether compressed or uncompressed, whether real-time or non-real-time in nature, and with guaranteed quality of service (QoS).ATM generally operates at minimum access speeds of DS-1 (e.g., T1 at 1.544 Mbps and E-1 at 2.048 Mbps) and DS-3 (e.g., E-3 at 34.368 Mbps and T1 at 44.736 Mbps). Designed to operate at very high speeds, ATM benefits from fiber optic transmission systems (FOTS) and commonly is provisioned over SDH/SONET networks.Access circuits operating at OC-3 (155 Mbps) are not unusual and backbone transmission rates generally are OC-3, at a minimum. ATM traffic consists of three basic types. • Constant Bit Rate (CBR) traffic requires access to time slots at regular and precise intervals. Realtime, uncompressed voice and video, and circuit emulation are examples of CBR traffic. • Variable Bit Rate (VBR) traffic, such as compressed voice and video and bursty data traffic, requires access to time slots at a rate that can vary dramatically from time to time but each logical connection is guaranteed a level of service defined by burst size, average bandwidth, etc. • Available Bit Rate (ABR) traffic, also known as best-effort ATM, supports bursty LAN traffic and other traffic that can deal with time slot access on an as-available basis. ATM organizes data into cells, as illustrated in Figure A-8. Each cell comprises a header of 5 octets and payload of 48 octets, with the payload including some amount of overhead attributable to Convergence Sublayer and Data Link Layer and Network Layer headers. Although the total overhead is in the range of 10 percent, the small cell size offers the advantage of effectively supporting any type of data.The fixed cell size offers the advantage of predictability, very much unlike the variable-length frames associated with services such as X.25, frame relay, and Ethernet, or the variable-length packets associated with the Internet Protocol (IP). This level of predictability yields much improved access control and congestion control. ATM multiplexes the cells, which contend for access to a broadband facility that ideally is SDH or SONET in nature. ATM also is used in some passive optical network (PON) local loops. 74570c01.qxd 9/12/07 12:34 AM Page 44 |
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