Topologies and Access Protocols

112 Communication Networks Copyright © 2005 PragSoft Figure 9.95 Token bus arrangement. Bus Logical Ring PIU PIU PIU PIU

9.1.2. Architecture

Figure 9.4 depicts the LAN protocol layers in relation to the OSI model. The role of the physical layer is the same as in the OSI model. It includes the connectors used for connecting the PIU to the LAN and the signaling circuitry provided by the PIU. The next section describes the transmission methods employed by this layer. The OSI data link layer is broken into two sublayers. The Media Access Control MAC layer is responsible for implementing a specific LAN access protocol, like the ones described earlier. This layer is therefore highly dependent on the type of the LAN. Its aim is to hide hardware and access protocol dependencies from the next layer. As we will see shortly, a number of MAC standards have been devised, one for each popular type of access protocol. The Logical Link Control LLC layer provides data link services independent of the specific MAC protocol involved. LLC is a subset of HDLC and is largely compatible with the data link layer of OSI-compatible WANs. LLC is only concerned with providing Link Service Access Points LSAPs. All other normal data link functions i.e., link management, frame management, and error handling are handled by the MAC layer. Figure 9.96 LAN protocol architecture. OSI Layer LAN Layer Purpose higher layers undefined Application dependent. Data Link Logical Link Control Provides generic data link services to higher layers. Media Access Control Implements the protocol for accessing the LAN. Physical Physical Transmission of data bits over the channel. LANs are not provided with a network layer or any other higher layer because such a layer would be largely redundant. Because the stations are directly connected, www.pragsoft.com Chapter 9: Local Area Networks 113 there is no need for switching or routing. In effect, the service provided by the LLC is equivalent to the OSI network layer service.

9.1.3. Transmission

LAN transmission techniques are divided into two categories: baseband and broadband. In the baseband technique, the digital signal from a transmitting device is directly introduced into the transmission medium possibly after some conditioning. In the broadband technique, a modem is used to transform the digital signal from a transmitting device into a high frequency analog signal. This signal is typically frequency multiplexed to provide multiple FDM channels over the same transmission medium. Baseband is a simple and inexpensive digital technique. By comparison, broadband has additional costs: each device requires its own modem; also, because transmission is possible in one direction only, two channels typically need to be provided, one for either direction. Broadband, however, has the advantages of offering a higher channel capacity which can be used for multiplexing data from a variety of sources e.g., video, voice, fax, not just digital data. It is also capable of covering longer distances, typically tens of kilometers compared to up to a kilometer for baseband.

9.2. IEEE 802 Standards

The IEEE 802 series of recommendations provide a widely-accepted set of LAN standards. These recommendations are formulated by nine subcommittees see Figure 9.97. Figure 9.97 IEEE 802 Subcommittees. Subcommittee Title Purpose 802.1 High-level Interface Specification of standards for LAN architecture, interconnection, management 802.2 Logical Link Control Specification of standards for the LLC layer 802.3 CSMACD Specification of standards for CSMACD architectures 802.4 Token Bus Specification of standards for token bus architectures 802.5 Token Ring Specification of standards for token ring architectures 802.6 Metropolitan Area Networks Specification of standards for MANs 802.7 Broadband Technical Advisory Group Provision of guidance to other groups working on broadband LANs 802.8 Fiber Optic Technical Advisory Group Provision of guidance to other groups working on fiber optic-based LANs 114 Communication Networks Copyright © 2005 PragSoft 802.9 Integrated Data and Voice Networks Specification of standards for interfaces to ISDN Figure 9.98 illustrates the relationships between the main widely-used IEEE LAN recommendations. These are separately discussed below. Figure 9.98 Main IEEE LAN Recommendations. LLC IEEE 802.2 ISO 8802.2 CSMACD IEEE 802.3 ISO 8802.3 Token Bus IEEE 802.4 ISO 8802.4 Token Ring IEEE 802.5 ISO 8802.5 Physical

9.2.1. 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.