include sequence numbering and 8 bits long for formats that do not. The three formats described for the control field are used to perform numbered information transfer (I-frame), unnumbered control (S-frames), and unnumbered information transfer (U-frames) functions. These functions are described in Section 10.10.3. 11.6 LAN ACCESS PROTOCOLS 11.6.1 Introduction In our context here an access protocol is a means of permitting all users to access a LAN fairly and equitably. Access can be random or controlled. For the random access schemes Control Information Address Address 8 bits 8 bits 8 or 16 bits M * 8 bits DSAP Address = destination service access point address field SSAP Address = source service access point address field Control = control field (16 bits for formats that include sequence numbering, and 8 bits for formats that do not ) Information = information field = multiplication = an integer value equal to or greater than 0. (Upper bound of M is a function of the medium access control methodology used.) Figure 11.4 LLC PDU frame format. 11.6 LAN ACCESS PROTOCOLS Address field Address field LSB of address Least signifigant bit First bit delivered to/received from the MAC sublayer I/G 0 Individual DSAP I/G = 1 Group DSAP C/R = 0 Command C/R = 1 Response XODDDDDD DSAP Address XOSSSSSS SSAP Address X1DDDDDD Reserved for ISO definition 1SSSSSSS Reserved for ISO definition Figure 11.5 DSAP and SSAP address field formats (Ref. 2). LLC PDU Control field bits 10-16 Information transfer command/response (I-format PDU) Supervisory command/response (S-format PDUs) Unnumbered command/response (U-format PDUs) N(S) = Transmitter send sequence number (Bit 2= low-order bit) Transmitter receive sequence number (Bit 10 = low-order bit) Supervisory function bit Modifier function bit Reserved and set to zero Poll bit-command LLC PDU transmissions Final bit-response LLC PDU transmissions (1 Poll/final) Figure 11.6 LLC PDU control field formats (Ref. 2). we will review CSMA (carrier sense multiple access) and CSMA/CD, where CD s