CRC-4 (Cyclic Redundancy Check 4) is a form of cyclic redundancy checking (a method of checking for errors in transmitted data) that is used on E-1 trunk lines. CRC-4 is structured in a multiframe consisting of 16 frames numbered 0 to 15. The CRC-4 multiframe is then divided into two eight frame Sub-Multiframes (SMF) called SMF I and SMF II. Each SMF contains four CRC-4 bits designated C1, C2, C3, and C4. Through multiplication and division processes, the CRC-4 bits sent in each SMF are compared to each other and if they match, the frame is considered error free. Bit 1 is used for the CRC-4 bits.
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CRC-4 support is required for all network switches in Europe. However, some older switches including private branch exchanges (PBXs) do not support CRC-4.