Section 5.1. LANs and Basic Topologies

5.1. LANs and Basic Topologies

A LAN is used for communications in a small community in which resources, such as printers, software, and servers, are shared. Each device connected to a LAN has a unique address. Two or more LANs of the same type can also be connected to forward data frames among multiple users of other local area networks. In LANs, packets are additional headers appended for local routing. These new-looking packets are known as frames . Users in a local area network can be interconnected in several ways. The fundamental network topology in LANs can be categorized into bus , ring , and star , as shown in Figure 5.1.

Figure 5.1. Three fundamental LAN configurations to connect users

In the bus topology, all users are connected to a common transmission medium referred to as a bus. The users are connected to a common bus via a duplex link that allows both uplink and downlink operations, as seen in the figure. The transmission from a user is propagated on the bus in both directions, and all users in the path receive its frame. However, only the destination user copies the frame into its computer; all other users discard the frame.

The ring topology comprises of layer 2 devices called repeaters (see Section 3.3.1). Repeaters are interconnected to form a closed loop, and each user is connected to one repeater, shown in the figure by a smaller circle. When a user transmits a frame, its associated repeater forwards the frame to the ring. The ring is normally unidirectional, so a frame can flow in one direction. During the circulation of the frame in the ring, the destination user copies the frame onto its buffer. Once copied by the destination, the frame continues its circulation until the sender receives it and removes it from the system. To avoid collision, only one user can transmit at a given time.

In the star topology, all users are directly connected to a central user through two unidirectional links: one for uplink and the other for downlink. The central user functions in either broadcast mode or frame-switch mode. In broadcast mode , the central user is called a hub (see Section 3.3.1). When it receives the frame from a user on the uplink, the hub retransmits the frame to all users on the downlink. Broadcast mode allows only one user to transmit at a time. In frame-switch mode , the central user buffers the received frame and retransmits the frame only to the destination.