Below is a logical representation of token ring network

Fiber Distributed Data Interface (FDDI)  is a 100 Mbps Local Area Network (LAN) technology built using fiber-optic cables. It is standardized by American National Standards Institute (ANSI) and also conforms to the Open Systems Interconnection (OSI) model. Like token ring FDDI uses a token passing mechanism but it has two rings and since it uses fiber-optic cables it supports high bandwidth connectivity over long distances.

The FDDI standard defines two types of devices

  •  Stations: These are end user stations and devices such as computers and printers
  •  Concentrators:  They make it possible for Single Attached Systems (SAS) to connect to the FDDI ring

The FDDI standard also defines three ways of connecting devices to the ring

  •  Single Attached Systems (SAS): These devices cannot connect directly to FDDI dual ring since they have a single port, to connect theses device to the FDDI network a concentrator is used.
  •  Dual Attached Systems (DAS): All stations connected to the FDDI dual ring must be dual attached and always be up and running. For stations that cannot be connected using DAS methodology can use a SAS which uses a concentrator.
  •  Dual Homing: Some devices may connect to two concentrators using a DAS card for redundancy purposes. One link will be active and the other link will take the active role if the active links fails.

Shown below is an example of FDDI LAN.



Ethernet standardized as IEEE 802.3 was developed by Xerox, DEC and Intel. It uses physical bus and star topologies, the original Ethernet ran at 10 Mbps but over the year never version were introduced such as fast and gigabit Ethernet which ran at 100 Mbps and 1Gbps respectively.

Initially Ethernet was based on the bus topology, shared infrastructure using coaxial cable, but as network requirements grew this design was not scalable and performance issues were faced, this led to the replacement of coaxial cables by twisted pair cables and switched Ethernet which uses switches as central point of connectivity replaced shared Ethernet.

Ethernet specification also defined a mechanism known as Carrier Sense Multiple Access/Collision Detection (CSMA/CD) which is use to govern communications over a shared Ethernet segment. On a shared segment collisions occur when two or more systems simultaneously send frames.  With CSMA/CD algorithm any system that needs to communicate over the network will first listen to the medium to check if any other systems is transmitting, if not it will transmit the frame. If any collisions occur the attached systems will detect it and cease any transmission. After a random time interval the systems will attempt to transmit if they detect silence on the medium.

Shown below are examples of shared and switched Ethernet LANs

In today’s lesson we covered three important data link protocols. Token Ring and FDDI LANs are very rare today and almost all LANs are built using Ethernet technologies. As we move further we will learn in detail about Ethernet and LAN technologies.