The Bits in the LLC Header
In case you are ever laying awake at three in the morning and you are thinking about the fact that the three bytes in the Type 1 LLC header (DSAP, SSAP, and Control) also represent the individual/group and command/response and poll/final indicators, here’s your answer.. (now you’ll be able to sleep at night).
Suffice it to say, this information is presented only as a curiosity; you are NOT expected to know this from memory.
The SAP identifier is actually only 7 bits long. The least significant bit (b0) in the DSAP and SSAP are used for the Individual/Group (I/G) and Command/Response (C/R) indicators.
A DSAP of ‘11111111’ is referred to as the Global DSAP Address which designates a group consisting of all DSAPs actively being serviced by the underlying MAC layer.
A DSAP of ‘00000000’ is predefined to be the ‘Null’ address. Frames identified with DSAP=00 are discarded by the LLC layer (although they are passed up from the underlying MAC layer). You might see diagnostic frames (as from network load generator tools) sending to the Null address.
The poll/final bit is the low-order (b0) bit in the sequence number field of a Type 2 LLC frame. Hence, the NR and NS number is actually 7 bits long (and has a value from 0 to 127).
If you ever have a need to enter into a detailed discussion of the bit structure of an LLC header you must go directly to the 802.5 spec from the IEEE. There is no other source that describes the other somewhat esoteric bits that are defined including a ‘modifier function bit’ and a ‘supervisory function bit’ . Frankly, unless you are a programmer writing device driver code or microcode for an adapter you will never encounter these esoteric bits or any of their behavior.
You should know that the information presented by the LLC header is encoded in a special bit-wise format within the bytes. If you are looking at a hex display of the LLC header, remember that there are some special bit constructions present. Don’t be confused if you do a hex-to-binary conversion on LLC bytes and you find the structure to be unusual.