Logical Link Control

LLC is specified by the IEEE 802.2 and ISO 8802.2 standards. It provides link services to LAN users, independent of the MAC protocol involved. LLC offers three types of service:

• Unacknowledged connectionless service. This service must be provided by all 802.2 implementations. It is based on data being transferred in independent data units, the delivery of which is neither guaranteed, nor acknowledged. Furthermore, there are no provisions for ordered delivery of data units or for flow control. Obviously, a higher-level protocol is needed to make this service reliable.

• Connection-oriented service. This service is based on the use of logical connections. Data is transferred using ordered, acknowledged, and flow controlled data units. Transmission errors are detected and reported.

• Acknowledged connectionless service. Same as the unacknowledged connectionless service, except that the delivery of each data unit is acknowledged before the next data unit is sent.

The LLC service is provided through a set of service primitives. Figure 9.99 summarizes these primitives.

Figure 9.99 LLC service primitives.

Primitive Types Parameters Purpose

DL-CONNECT request indicate response confirm (addresses, priority)

Used for the connection-oriented service. Establishes a connection between two LSAPs.

DL-DISCONNECT request indicate

(addresses) Disconnects a connection between two LSAPs.

DL-DATA request

indicate

(addresses, data)

Used for the connection-oriented transfer of data units. DL-CONNECTION- FLOW-CONTROL request indicate (addresses, data-quantity)

Used to flow control data transfers by specifying how much data can be accepted.

DL-RESET request indicate response confirm (addresses, reason)

Used to reset a (possibly in error) connection. All data units in transit will be lost.

DL-UNIT-DATA request indicate

(addresses, data, priority)

Used for the connectionless

unacknowledged transfer of data units. DL-DATA-ACK request indicate response confirm (addresses, data, priority, status)

Used for the connectionless acknowledged transfer of data units. Status returns the delivery status of the data unit.

LLC also uses two primitives to communicate with the MAC layer. MA-UNIT- DATA is used to transfer data units between LLC and MAC. MA-DATA-UNIT- STATUS is used for acknowledgments.

The general structure of an LLC protocol data unit is shown in Figure 9.100. The header (Source and Destination addresses and Control field) are appended to user data by LLC before the PDU is passed onto the MAC layer. The addresses identify peer LLC users. The Control field is very similar in format and content to the control field of HDLC frames discussed in Chapter 3.

Figure 9.100 LLC PDU structure.

Field Description

DSAP Address Destination Service Access Point Address. SSAP Address Source Service Access Point Address.

Control Control field (similar to HDLC control field). Data Actual user data.

9.2.2. CSMA/CD

The Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol is specified by the IEEE 802.3 and ISO 8802.3 standards. CSMA/CD is based on the widely-publicized and well-adopted Ethernet specification, and offers data rates in order of 10 mbps using the baseband or the broadband technique. The overall behavior of the protocol is as described in Section 9.1.1.

The general structure of a CSMA/CD MAC frame is shown in Figure 9.101. It consists of a Data field (which is an LLC PDU) with a header and a trailer added at either end. The header provides synchronization, addressing, and length information.

The trailer provides a CRC-style Frame Check Sequence (FCS). CSMA/CD imposes a minimum frame size which in turn translates to a minimum data field size. Should the data field be shorter than required, it is padded with enough octets to achieve the required minimum.

Figure 9.101 CSMA/CD MAC frame structure.

Field Description

Preamble Special bit pattern for synchronization purposes. Start Delimiter Marks the beginning of frame.

Addresses Source and destination addresses.

Length Denotes the length of the LLC data unit in octets. Data Actual user data (i.e., LLC PDU).

Padding Appended to the LLC data unit to ensure minimum length. FCS Frame Check Sequence.

9.2.3. Token Bus

The token bus protocol is specified by the IEEE 802.4 and ISO 8802.4 standards. The logical ring is determined by the descending numeric order of the station addresses. When a station takes possession of the token, it is given exclusive network access for a limited period of time. The station either transmits during this time window or hands over the token to its successor. The token holder can also poll other stations in order to learn about their status.

The protocol provides for token-related fault handling. When a station passes the token to its successor, it listens for transmissions on the bus for a period of time. A transmission would be a sign that the successor has successfully received the token and is either passing it on or is transmitting data. If the token sender establishes that the token has not been received after two attempts, it will attempt to bypass the presumably faulty station. To do this, it polls other stations to find out who is the next logical successor and will attempt to pass the token to that station.

The general structure of a token bus frame is shown in Figure 9.102. The beginning and end of the frame are marked by two delimiting octets. The Frame Control field is used for building different types of frames (e.g., token frame, polling frame). For some frames (e.g., the token frame) the Data field may be completely empty. The Preamble, Addresses, and FCS fields are as in CSMA/CD.

Figure 9.102 Token bus MAC frame structure.

Field Description

Preamble Special bit pattern for synchronization purposes. Start Delimiter Marks the beginning of frame.

Frame Control field.

Addresses Source and destination addresses. Data Actual user data (e.g., LLC PDU). FCS Frame Check Sequence. End Delimiter Marks the end of frame.