The pad field can vary from 0 to 46 bytes in length. This field is used to ensure that the frame length is at least 64 bytes in length (the preamble and SFD fields are not considered part of the frame for this calculation) which is required for successful Carrier Sense Multiple Access with Collision Detection (CSMA/CD) operation. The values in this field are used in the frame check sequence calculation but are not included in the length field value if it is used. The length of this field and the data field combined must be at least 46 bytes. If the data field contains 0 bytes, the pad field is 46 bytes. If the data field is 46 bytes or more, the pad field has 0 bytes. For transmission, this field is inserted automatically by the AXI Ethernet Lite MAC if required to meet the minimum length requirement. If present in the receive packet, this field is always retained in the receive packet data.