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ENTERPRISE NETWORKS I: LOCAL AREA NETWORKS
The LLC is another derivative of HDLC, which was discussed in Section 10.10.3. It is
based on the balanced mode of that link-layer protocol with similar formats and functions.
This is particularly true when operating in the connection mode.
11.5.3.1
LLC PDU Structure
. As shown in Figure 11.3, the LLC appends a header
forming the LLC PDU. The PDU frame format is illustrated in Figure 11.4. The header
consists of address and control information; the information field contains the user data.
The control field is identical with HDLC control field, illustrated in Figure 10.29 and
described in Section 10.10.3.1. However, the LLC control field is two octets long and
there is no provision to extend it to 3 or 4 octets in length as there is in HDLC.
As mentioned previously, the LLC destination address is the user address at an SAP
inside the destination LAN station. It is called the destination service access point (DSAP).
The SSAP is the source service address point and it indicates the message originator inside
a particular LAN station. Each has fields of 8 bits, as shown in Figure 11.5. However,
only the last seven of these bits are used for the actual address. The first bit in the DSAP
indicates whether the address is an individual address or a group address (i.e., addressed
to more than one SAP). The first bit in the SSAP is the C/R bit, which indicates whether
a frame is a command frame or a response frame. The control field is briefly described
in Section 11.5.3.2.
11.5.3.2
LLC Control Field and Its Operation
. The LLC control field is illustrated
in Figure 11.6. It is 16 bits long for formats that 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
Figure 11.4
LLC PDU frame format.
Summary :
The control field is identical with HDLC control field, illustrated in Figure 10.29 and described in Section 10.10.3.1. The first bit in the DSAP indicates whether the address is an individual address or a group address (i.e., addressed to more than one SAP). 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.
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llc,control,field,address,figure,access,frame,section,lan,described,information,bits,pdu