N-GEN Communications Web-based Seminars ©NGCS

Seminar - An Overview of Communications Technologies

Module 7 - LAN, WAN, VPN, WLAN, MAN, FDDI and Optical MANS                                                      Slide 13 of 45 _________________                                                                                  __________

                                                Media Access Methodology

                                                   CSMA / CD

                                                Token Passing

_________________                                                                                  __________

Two major media access methodologies used in LANs are Collision Sense Multiple Access with Collision Detect (CSMA / CD) in Ethernet LANs and Token Passing in IBM's Token Ring LANs.

CSMA/CD: A station before transmitting data on the cable checks for the presence of a signal. If no signal is present then the station begins transmission. If a signal is detected than it will delay transmission until the media is not in use. If two signals detect an idle cable and transmit data at the same time a collision occurs. The two stations involved with the collision stop transmitting data for an interval of time which is selected by an algorithm.

Token Passing: A token concept was developed by IBM for it's LAN technology in 1970s. A token passing network moves a token (a frame) around the network. A station ready to transmit the data seizes the token and has the right to transmit. A station, with no data to send, passes the token to the next station.