LLC Sub-layer

The MAC sub-layer supports two types of frame: IEEE 802.3 frames and Ethernet_II frames. In an Ethernet_II frame, the Type field identifies the upper layer protocol. Therefore, only the MAC sub-layer is required on a device, and the LLC sub-layer does not need to be realized.

In an IEEE 802.3 frame, the LLC sub-layer defines useful features in addition to traditional services of the data link layer. All these features are provided by the sub-fields of DSAP, SSAP, and Control.

The following lists three types of point-to-point services:

• Connectionless service

  Currently, the Ethernet implements this service.

• Connection-oriented service

  A connection is set up before data is transmitted. The reliability of data is guaranteed during the transmission.

• Connectionless data transmission with acknowledgement

  A connection is not required before data transmission. The acknowledgement mechanism is used to improve the reliability.

The following is an example that describes the applications of SSAP and DSAP. Assume that terminals A and B use connection-oriented services. Data is transmitted in the following process:

1. A sends a frame to B to require the establishment of a connection with B.

2. If B has enough resources, it returns an acknowledgement message that contains a Service Access Point (SAP). The SAP identifies the connection required by A.

3. After receiving the acknowledgement message, A knows that B has set up a local connection with A. After creating the SAP, A sends a message containing the SAP to B. The connection is set up.

4. The LLC sub-layer of A encapsulates the data into a frame. The DSAP field is filled in with the SAP sent by B; the SSAP field is filled in with the SAP created by A. Then the LLC sub-layer sends the frame to the MAC sub-layer of A.

5. The MAC sub-layer of A adds the MAC address and the Length field into the frame, and then sends the frame to the data link layer.

6. After the frame is received at the MAC sub-layer of B, the frame is transmitted to the LLC sub-layer. The LLC sub-layer figures out the connection to which the frame belongs according to the DSAP field.

7. After checking and acknowledging the frame based on the connection type, the LLC sub-layer of B transmits the frame to the upper layer.

8. After the frame reaches its destination, A instructs B to release the connection by sending a frame. At this time, the communications end.