3.3 Layer 2 Protocol Overview - WAN Protocols
3.3.2 SDLC frame format
After developing SDLC, IBM submitted it to various standards committees. The International Organization for Standardization (ISO) modified SDLC to create the High-Level Data Link Control (HDLC) protocol. The International Telecommunication Union-Telecommunication Standardization Sector (ITU-T), formerly CCITT, subsequently modified HDLC to create Link Access Procedure (LAP), and then Link Access Procedure, Balanced (LAPB). The Institute of Electrical and Electronics Engineers (IEEE) modified High-level Data Link Control (HDLC) to create IEEE 802.2. Each of these protocols has become important in its own right, but SDLC remains the primary SNA link-layer protocol for WAN links. 

A sample SDLC frame is shown in the figure to the left with each field described below: 

  • Flag—This field initiates and terminates error checking. 
  • Address—This field contains the SDLC address of the secondary station, which indicates whether the frame comes from the primary or secondary. This address can contain a specific address, a group address, or a broadcast address. A primary is either a communication source or a destination, eliminating the need to include the address of the primary. 
  • Control—This field employs three different formats, depending on the type of SDLC frame used: 
    • The Information (I) frame carries upper-layer information and some control information. This frame sends and receives sequence numbers, and the poll final (P/F) bit performs flow and error control. The send-sequence number refers to the number of the frame to be sent next. The receive-sequence number provides the number of the frame to be received next. Both sender and receiver maintain send- and receive-sequence numbers. 

A primary station uses the P/F bit to tell the secondary whether it requires an immediate response. A secondary station uses the P/F bit to tell the primary whether the current frame is the last in its current response. 

  • The Supervisory (S) frame provides control information. An S frame can request and suspend transmission, report on status, and acknowledge receipt of I frames. S frames do not have an Information field. 
  • The Unnumbered (U) frame is used for control purposes and is not sequenced. A U frame can be used to initialize secondaries. Depending on the function of the U unnumbered frame, its control field is 1 or 2 bytes. Some U unnumbered frames have an Information field. 
  • Data—This field contains a path information unit (PIU) or exchange identification (XID) information. 
  • FCS—This field precedes the ending flag delimiter and usually requires a CRC calculation. The CRC is recalculated by the receiver. If the result differs from the value in the original frame, an error is detected. 

As you can see, SDLC framing is fairly complex, but not much more so than its successors. SDLC and its derivative protocols have had a long history and it's still going strong, as evidenced by the continued widespread use of SNA (commonly used by banks). SDLC is the first WAN of five data link layer protocols to be discussed in this chapter. Next you'll learn about X.25, Frame Relay, ISDN, and ATM.