Local Networks and the Internet: From Protocols to Interconnection

Chapter 4

The LLC and SNAP Sublayers

4.1. Definition

The LLC (logical link control) sublayer lies on the MAC sublayer. The MAC sublayer contains mechanisms to obtain a mutual exclusion between stations sharing the same support (bus, ring or radio frequencies). When the station has gained its right to talk, the LLC sublayer controls data transmission.

Three types of transmission services are offered:

— Type 1 LLC or datagram mode: no error control function is performed on the frames. The LLC layer branches data towards the different protocols of the level 3 layer. The vast majority of protocols in local networks use this encapsulation.

— Type 2 LLC or connected mode: in addition to the switching functions of type 1, data and flow sequencing error control is performed. The protocol is identical to HDLC. For example, this type of encapsulation is used to carry off X.25 packets on a local network.

— Type 3 LLC or acknowledged datagram mode: this functional mode has been added to the original specifications to address industrial networks’ needs. It allows, among others, the acknowledgement of datagrams and automatic response.

The functioning principle of the LLC sublayer is described in specifications IEEE 802.2 and ISO 8802-2.

Type 1 is most frequently used in local networks for its switching function. In fact, not all networks need this function. Ethernet networks do not use it, switching being done by reading the type field in the MAC frame. On the other hand, for IEEEE 802.3 networks, ...