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 36 of 45 _________________                                                                                  __________

                                             FDDI Architecture ?

                                        Inner and Outer Rings

                                        Wiring Concentrator

                                        Channel Reconfiguration on FDDI

                                        FDDI Token Protocol

_________________                                                                                  __________

The wiring concentrator allows reconfiguration. Stations A and B are connected to the Inner Ring while  stations A, C and D are connected to the Outer Ring. The Inner Ring in this architecture is the primary ring and the Outer ring is the secondary ring. When a channel is lost between stations A and B, the network is re-configured to maintain its survivability. 

FDDI Token Protocol - The principle is the same as the Token Ring LAN. FDDI permits multiple users to occupy the ring at one time. When a station captures the token it transmits its frame on to the ring. The station also transmits a new token behind the frame. The token circulates on the ring and is copied by the stations that are identified in the frame address field. If another station wishes to use the ring it captures the token, and places its data behind previous stations' frame. The later station issues a new token behind its frame. The frame circulates on the ring. The originating stations absorb their frames as they circulate back around the ring. The station checks the source address to determine if it should absorb the frame.