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34c. Control Field |
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34c. Control Field
The Control Field is used to indicate the Type of Information being sent as Data. The Type of Information can be Control information used when establishing a connection (handshaking) or it can be Data such as file transfers between clients and servers. The purpose of the Control Field is to identify what the purpose of the packet or frame is: Control or Data. It can also be used to indicate the size of the packet and Data.
The Data Field or Message is the actual information that is being transmitted. It can contain Control Information for handshaking or actual Data used by applications. The Control Field would indicate the Data Field size. The Data field is also called the Info field by some protocols.
The optional Pad is used to pad the data field when the protocol has a fixed Data Field size. If the Data Field size is fixed at 1200 octets and only 300 octets of information is available then the Pad will fill in the remaining 900 octets with characters (e.g. 900 octets of 00h). The protocol may also use the Pad to ensure a minimum Data field size.
The CRC / Frame Check Sequence (FCS) contains an error checking number that the Destination can use to verify that the packet is okay and error-free. CRC is an abbreviation for Cyclic Redundancy Checking. The Frame Check Sequence typically incorporates a 32 Bit CRC check. Checksums work similarly but use a different algorithm.
As each packet is sent, the Source calculates a check number from the data using a predetermined algorithm (formula). The result of this calculation is appended to the packet in the Frame Check Sequence (FCS) field. At the Destination, the same calculation is performed and the result is compared to the transmitted Frame Check Sequence. If the result generated at the Destination is identical to the FCS, then it is assumed that the packet is error free at the bit level.
The End Frame Delimiter is a series of octets that have a specific bit pattern that identifies the end of the packet to the Destination. Not all protocols have End Frame Delimiters fields, protocols with fixed packet size may not need the End Frame Delimiter field as the Destination may simply count the number of octets it has received.