Ethernet Networks: Design, Implementation, Operation, Management 4th phần 4

Theo tiêu chuẩn IEEE 802, lớp liên kết dữ liệu ban đầu được chia thành hai sublayers: kiểm soát liên kết logic (LLC) và phương tiện truyền thông kiểm soát truy cập (MAC). Lớp LLC là trách nhiệm để tạo ra và giải thích các lệnh điều khiển dòng chảy của dữ liệu và thực hiện các hoạt động phục hồi trong trường hợp lỗi. | 166 chapter four Figure By pressing the F2 key EtherVision will convert the three-byte hex NIC manufacturer ID to the vendor name or an appropriate mnemonic. identify the transport of IPX and SPX protocols. Thus the placement of an appropriate hex value in the Ethernet type field provides a mechanism to support the transport of multiple protocols on the local area network. Under the IEEE standard the type field was replaced by a length field which precludes compatibility between pure Ethernet and frames. Length Field The two-byte length field applicable to the IEEE standard defines the number of bytes contained in the data field. Under both Ethernet and IEEE standards the minimum size frame must be 64 bytes in length from preamble through FCS fields. This minimum size frame ensures that there is sufficient transmission time to enable Ethernet NICs to detect collisions accurately based on the maximum Ethernet cable length specified for a network and the time required for a frame to propagate the length of the cable. frame operations 167 TABLE Representative Ethernet Type Field Assignments Protocol Hex Value Assigned Experimental 0101-DIFF Xerox XNS 0600 IP 0800 Internet 0801 NBS Internet 0802 ECMA Internet 0803 CHAOSmet 0804 Level 3 0805 Address Resolution Protocol 0806 XNS Compatibility 0807 Banyan Systems 0BAD BBN Simnet 5208 DEC MOP Dump Load 6001 DEC MOP Remote Console 6002 DEC DECNET Phase IV Route 6003 DEC LAT 6004 DEC Diagnostic Protocol 6005 3Com Corporation 6010-6014 Proteon 7030 AT T 8008 Excelan 8010 Tymshare 802E DEC LANBridge 8038 DEC Ethernet Encryption 803D AT T 8046 - 8047 AppleTalk 809B IBM SNA Service on Ethernet 80D5 AppleTalk ARP 80F3 Wellfleet 80FF - 8103 NetWare IPX SPX 8137 - 8138 SNMP 814C 168 chapter four Based on the minimum frame length of 64 bytes and the possibility of using two-byte addressing fields this means that each data field must be a minimum of 46 bytes in length. The only exception to .