ctivity. Thus the frequency of collisions and backoffs increases to a point that throughput can drop to zero. Some users argue that efficiency starts to drop off at around 30% capacity, while others argue that that point is nearer 50%. We will discuss ways to mitigate this problem in our coverage of bridges. 11.6 LAN ACCESS PROTOCOLS REFERENCE MODEL LAYERS IEEE-STD-802 LAN APPLICATION HIGHER LAYERS PRESENTATION (LOGICAL LINK CONTROL) SESSION (MEDIA ACCESS CONTROL) TRANSPORT (PHYSICAL SIGNALING) NETWORK DATA LINK PHYSICAL - MDI MEDIUM AUI - ATTACHMENT UNIT INTERFACE MAU - MEDIUM ATTACHMENT UNIT MDI - MEDIUM DEPENDENT INTERFACE PMA - PHYSICAL MEDIUM ATTACHMENT Figure 11.7 CSMA/CD LAN relationship to the OSI model as well as the functional blocks required. [From IEEE 802.3 (Ref. 3); Courtesy of the IEEE, New York.] DESTINATION PREAMBLE SOURCE LENGTH ADDRESS ADDRESS (7 OCTETS) (1 OCTET) (2 OR 6 OCTETS) (2 OCTETS) (4 OCTETS) (2 OR 6 OCTETS) LENGTH - GIVES NUMBER OF OCTETS IN DATA FIELD LLC - LOGICAL LINK CONTROL (OSI LAYER 3 AND ABOVE) FCS - FRAME CHECK SEQUENCE - A 32-BIT CRC PAD ADDS OCTETS TO ACHIEVE MINIMUM FRAME LENGTH WHERE NECESSARY SFD - START FRAME DELIMITER Figure 11.8 MAC frame format (Ref. 3). 11.6.2.2 System Operation - The MAC Frame. The MAC frame is shown in Figure 11.8. There are eight fields in the frame: preamble, SFD, the addresses of the frame's destination(s) and source, a length field to indicate the length of the following field containing the LLC data, a field that contains padding (PAD) if required, 2 and the FCS for error detection. All eight fields are of fixed size except the LLC data and PAD fields, which may contain any integer number of octets (bytes) between the minimum and maximum values determined by a specific implementation. 2 Padding means the adding of dummy octets (bytes) to meet minimum frame length requirements. ENTERPRISE NETWORKS I: LOCAL AREA NETWORKS The minimum and maximum frame size limits refer to that portion of the frame from the destination address field through the f