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36f. MAC - Medium Access Control (cont'd) | Next |
The Length field indicates the Length of the Information Field. It allows for variable length frames. The minimum Information Field size is 46 octets and the maximum size is 1500 octets. When the Information Field size is less than 46 octets, the Pad field is used. Due to the 802.3 MAC Frame having a Length field, there is no End Delimiter in the MAC Frame. The Length of the field is known and the receiving station counts the number of octets.
Information Field (Data)
The Information Field contains the Data from the next upper layer : Logical Link Control Layer. It is commonly referred to as the LLC Data. The minimum Information Field size is 46 octets and the maximum size is 1500 octets.
Pad
The Pad is used to add octets to bring the Information Field up to the minimum size of 46 octets if the Info Field is less than the minimum.
Frame Check Sequence (FCS)
The Frame Check Sequence is used for error-checking at the bit level. It is based on 32 bit CRC (Cyclic Redundancy Checking) and consists of 4 octets (4 x 8 = 32 bits). The FCS is calculated according to the contents of the DA, SA, L, Data and Pad fields.
36g. Total Length of a MAC Frame| Min Size (octets) | Max Size (octets) | |||
| Preamble | 7 | 7 | ||
| Start Frame Delimiter | 1 | 1 | ||
| Destination Address | 6 | 6 | ||
| Source Address | 6 | 6 | ||
| Length | 2 | 2 | ||
| Information Field | 46 | 1500 | ||
| Frame Check Sequence | 4 | 4 | ||
| TOTAL: | 72 | 1526 Octets |
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