Talk:Logical Link Control

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If the LLC sub layer is responsible for sequencing, then why is it that Ethernet frames created by MAC sublayer also has sequence fields and error checksum fields? How exactly are the MAC frames and HDLC frames related? What exactly does multiplexing protocols mean? —The preceding unsigned comment was added by 220.226.77.222 (talk • contribs) 13:05, 18 May 2007 (UTC).

Ethernet frames don't have sequence fields unless some protocol running above the MAC layer puts them there, e.g. the N(R) and N(S) fields in 802.2 LLC I-frames, or the offset field in IPv4 packets, or the sequence number fields in TCP segments. What Ethernet frames have at the MAC layer is a 6-octet destination address, a 6-octet source address, and a 2-byte type/length field, followed by the payload, followed by a 4-byte checksum.

The checksum is to detect errors; it's up to a protocol running above the MAC layer to do something about the error. An Ethernet adapter will (normally) drop frames if the checksum is bad; there's no acknowledgment of Ethernet frames at the MAC layer, so there won't be any transmission at the MAC layer - a protocol running above that layer will have to detect the loss of the frame and retransmit it.

"Multiplexing protocols" means allowing, for example, ARP packets, IPv4 packets, IPv6 packets, IPX packets, DECNET packets, etc. to be transmitted from host A to host B, and allowing host B to know which type of frame it's received. That can be done by using the 2-byte type/length field as a type field; the type field indicates the type of packet. If the type/length field is used as a length field, it is instead done with the 802.2 LLC header; either the SAP fields indicate the type of packet or, if they indicate that it's a SNAP packet, the OUI and protocol ID fields in the SNAP header indicate the type of packet. Guy Harris 22:07, 18 May 2007 (UTC)