ll the devices attached baseband transmission, only one channel is available to all network to the transmission medium to share that medium in a controlled users, therefore only one signal can survive on the transmission fashion. medium at any one time. The two most common access methods are Carrier Sense Multiple Broadband transmission utilises radio frequency modems to con- Access with Collision Detection (CSMA/CD) and Token Passing. vey data signals over the transmission medium. Because the avail- (See Figure 56.11 and Chapter 13.) CSMA/CD Station A listens to the medium and sensing that it is Station B is busy does not transmitting transmit Station B has TOKEN PASSING completed its transmission and has released Station A is awaiting receipt the Token of the Token so it can transmit. Token Figure 56.11 LAN access methods Development of Local Area Networks (LANs) 56/9 STAR-SHAPED RING Figure 56.12 Network topology CSMA/CD permits any device attached to the medium to transmit 4. Star-Shaped Ring networks use dedicated cabling to each on that medium (multiple access), if it senses that the medium is free workstation, which converges together in a wiring closet. (carrier sense). Occasionally two or more devices simultaneously sense that the medium is free and begin to transmit. This creates a collision which is detected by the sending device and the data is 56.4.4.5 LAN standards retransmitted after a set random time. Token Passing permits each device on the LAN to control the Because the first LAN systems were based on vendor specific medium for a predetermined maximum time while it is in possession technology, customers were forced to deal with single suppliers. In of a message packet (token). The token is passed from workstation order to allow users the freedom to purchase and interconnect to workstation on the LAN until the one wishing to transmit receives equipment from multiple vendors, it was clear that equipment it. When the workstation has finished transmitting, the token is standardisation was required. 