Setting Up A Reliable Data Link Connection

Type 2 LLC is connection-oriented. This implies that a connection setup process must precede the transmission of data. It also implies that a connection tear-down process will occur at the end of an LLC connection.

The session setup process always includes the following sequence:

  • #1 Requests: LLC C D=F0 S=F0 SABME P Set Asynchronous Balance Mode Extended
  • #2 Replies: LLC R D=F0 S=F0 UA F Unnumbered Acknowledgment
  • #1 Requests: LLC C D=F0 S=F0 RR NR=0 P Receiver Ready; Next Expect To Receive Zero
  • #2 Replies: LLC R D=F0 S=F0 RR NR=0 F Receiver Ready, Next Expect To Receive Zero

The Asynchronous Balance Mode Extended comes from the IBM world of SNA (Systems Network Architecture) and SDLC (Synchronous Data Link Control protocol). In the IBM mainframe world the normal way that a device communicates is when it is polled by the central control unit. This mode of interaction is referred to by IBM as the Normal Response Mode. When an IBM controller wants to set up this type of SDLC communications it asks to ‘Set Normal Response Mode’ (SNRM). IBM also had the SDLC Asynchronous Balanced Mode. It was considered ‘asynchronous and balanced’ because either side could initiate a conversation and either side could send data without the explicit permission of the other side. This is the opposite of a ‘polled’ conversation mode. The original SDLC Asynchronous Balanced Mode allowed the transmission of no more than 8 frames without receiving an acknowledgment.

With the advent of local and wide area networking IBM extended the original Asynchronous Balanced Mode to allow it to send up to 128 frames without requiring a specific acknowledgment. This was referred to as the Asynchronous Balanced Mode Extended. This one SDLC communications mode was introduced to the IEEE 802 specs and is called Type 2 LLC. Consequently, the SABME frame, while being conceived in the SNA/SDLC mainframe world comes alone into the LAN world. No other SDLC modes are implemented in the 802 specs.

When Station #1 makes the SABME request it also sets the poll bit (that’s the letter ‘P’ at the end of the line). The poll bit is saying, “Answer me immediately; don’t wait until I’ve sent the full 128 frames that are possible”.

Station #2 replies to the SABME with a simple Unnumbered Acknowledgment. The final bit is set to indicate that this response is complying with the request to answer immediately. The presence of the final bit is expected but irrelevant. The poll bit, on the other hand, does indicate an expected immediate acknowledgment from the station to which it is sent.

Next, Station #1 says that his Receiver is Ready (RR) and that the next expects to receive frame #0 from the Station #2 (NR=0).

Finally, Station #2 says that its Receiver is also ready (RR) and that he next expects to receive frame #0 from Station #1