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CRC-based framing is a kind of frame synchronization used in Asynchronous Transfer Mode (ATM) and other similar protocols. The concept of CRC-based framing was developed by StrataCom, Inc. in order to improve the efficiency of a pre-standard Asynchronous Transfer Mode (ATM) link protocol. This technology was ultimately used in the principal link protocols of ATM itself and was one of the most significant developments of StrataCom. An advanced version of CRC-based framing was used in the ITU-T SG15 G.7041 Generic Framing Procedure (GFP), which itself is used in several packet link protocols. ==Overview of CRC-based framing== The method of CRC-Based framing re-uses the header cyclic redundancy check (CRC), which is present in ATM and other similar protocols, to provide framing on the link with no additional overhead. In ATM, this field is known as the Header Error Control/Check (HEC) field. It consists of the remainder of the division of the 32 bits of the header (taken as the coefficients of a polynomial over the field with two elements) by the polynomial . The pattern 01010101 is XORed with the 8-bit remainder before being inserted in the last octet of the header.〔(【引用サイトリンク】title=ATM User-Network Interface Specification V3.0 )〕 Constantly checked as data is transmitted, this scheme is able to correct single-bit errors and detect many multiple-bit errors. For a tutorial and an example of computing the CRC see mathematics of cyclic redundancy checks. The header CRC/HEC is needed for another purpose within an ATM system, to improve the robustness in cell delivery. Using this same CRC/HEC field for the second purpose of link framing provided a significant improvement in link efficiency over what other methods of framing, because no additional bits were required for this second purpose. A receiver utilizing CRC-based framing bit-shifts along the received bit stream until it finds a bit position where the header CRC is correct for a number of times. The receiver then declares that it has found the frame. A hysteresis function is applied to keep the receiver in lock in the presence of a moderate error rate. In links where there is already a byte lock mechanism present such as within an E-carrier or SDH frame, the receiver need only byte-shift (rather than of bit-shifting) along the receive data stream to find lock. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「CRC-based framing」の詳細全文を読む スポンサード リンク
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