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CS 635, Lecture 3, Part 4 Logical Link Control

LLC Protocols

The basic LLC protocol is modeled after HDLC and have similar functions and formats.

LLC Types and Classes

There are thre LLC protocols defined in the standard, one for each form of service: It is possible for a single station to support more than one form of service and employ more than one of types of protocols. The combination of service supprted is given by the station class (table 5.2, page 164). Note that all allowable classes support type 1. This ensures that all stations on a LAN will have a common service mode that can be used for management operations.

LLC Protocol Data Units

All three LLCS protocols employ the same PDY format (figure 5.8, page 165)

There are three types of PDU, each with a different control field format:

used to carry user data. The control field includes a 7-bit sequence number , N(S), associated with this PDU; it also include a piggybacking acknowledgement sequence number, N(R)
used for flow and error control. It includes an acknowledgement sequence number and a 2-bit S field to distinquish three different PDUs: receive ready (RR), receive not ready (RNR), and reject (REJ)
Various protocol control PDUs. The 5-bit M field indicates which PDU.

Type 1 Operation

Type 1 operation supports the unacknowledged connectionless service. The UI PDU is used to transfer user data. There is not ack, flow control, or error control. The XID and TEST PDUs support management functions associated with all three types of operation. An LLC entity may issue a command (C/R bit = 0) XID or TEST. The receiving LLC entity issues a correcsponding XID or TEST is response.

The XID PDU is used to exchange two types of operation: types of operation supported and window size.

  1. If the DSAP and SSAP fields are null, the information field indicates which LLC class is provided by the sending LLC entity.
  2. If the XID includes specific DSAP and SSAP addresses, the information field indicates which types of operation may be provided for that particular SSAP. For a SAP that supports type 2 operation, and for a particular connection, the information field also includes the receive window size used in the sliding-window flow control mechanism.
The TEST PDU is used to conduct a loopback test of the transmission. Upon receipt of a TEST command PDU, the addressed LLC entity issues a TEST response PDU as soon as possible.

Type 2 Operation

Type 2operation supports the connection-mode service. Type 2 operation involves three phases: connection establishment, data transfer, and connection termination.

Connection Establishment

A user issues DL-CONNECT.request to the LLC. The LLC issues a SABME PDU (set asynchronous balanced mode extended). If the connection is accepted by the LLC designated in DSAP, the destination LLC returns a UA PDU (unnumbered acknowledgement). The connection from that point is uniquely identified by the pair of user SAPs. If the destination LLC user rejects the connection request, its LLC entity returns a DM PDU (Disconnect mode).

Data Transfer

Once the connection is established both side can start sending data in I PDUs, starting with a sequence number 0. An LLC sending data will number PDUs sequentially, modulo 128, and place the sequence number is N(S). N(R) is the ack for I-PDUs received.

Connection Termination

A connection can be terminated by a user issuing a DL-DISCONNECT.request or LLC terminating the connection itself. In either  case, the LLC issues a DISC PDU to the other LLC on the connection. Ther remote LLC must accept the disconnect by replying with a UA and issuing a DL-DISCONNECT.indication to its user. No assumption can be made about outstanding I-PDUs.

Type 3 Operation

With type 3 operation, each PDU transmitted is acknowledged. A new unnumbered PDU, the Acknowledged Connectiuonless (AC) Information PDU is defined. User data are send in AC command PDUs and must be ack'ed using an AC response PDU. To guard against lost PDUs, a 1-bit sequence number is used.  The sender alternates the use of 0 and 1 in its AC command pDU, and the receiver responds with an AC PDU with the opposite number of the corresponding command. Only on PDU is each direction may be outstanding at any time.

For the DL-DATA-ACK servicem the P/F bit is always set to 0. The AC command PDU contains user data and the AC response PDU does not. For the DL-REPLY service, the P/F/ bit is always set to 1. The AC command may or may not contain user data. The AC response contains user data if they are available; otherwise, it does not, which signals the other side that the reply has failed.

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