Fiber Distributed Data Interface (FDDI)

FDDI usually finds placement as a high-speed backbone for mission-critical or high traffic LANs, MANs or WANs. Operating at a data rate of 100 Mb/s, FDDI was originally designed for optical fiber transmission. An unbroken FDDI network can run to 100 km with nodes up to 2 km apart on multimode fiber, and 10 km apart on single-mode fiber. However, a copper standard exists, known as a copper distributed data interface, or CDDI, although it is restricted to distances of only 100 m. Any one ring, copper or fiber, may contain as many as 500 nodes.

FDDI’s niche is high reliability, the result of its counter-rotating ring topology illustrated in Figure 3. A dual-attached station connects the two paths via Port A, the primary path, and Port B, the secondary path. Port A may also have a number of M ports which attach to single-attached stations such as computer workstations.

Information is passed around the FDDI ring via a token generated by the main station. The token moves around the ring until a requires access to the network. When a station needs to transmit information, it takes control of the token, and transmits in an FDDI frame, after which it releases the token, signaling that it has completed its transmission. Each FDDI frame contains the address of the station or stations that need to receive this frame. All nodes read the frame, but only to verify this address. If the node address and the FDDI frame address match, the station extracts the data from the frame and then retransmits it to the next node on the ring. When the frame returns to the originating station, that station strips the frame, and the network remains quiet until a node captures the token.

A second generation network, FDDI-2 currently under development, supports the transmission of voice and video information as well as data. It uses a circuit-switched configuration in which a physical path is obtained for and dedicated to a single connection between two end-points in the network for the duration of the connection. In addition, another variation of FDDI, called FDDI full duplex technology (FFDT) uses the same network infrastructure but can potentially double data rates. If the secondary ring is not needed for backup, it can also carry data, extending the network’s capacity to 200 Mb/s. Work is underway to connect FDDI networks to the developing synchronous optical network (SONET).

Fibre Channel
Fibre channel, originally developed in the United Kingdom, was designed to provide high bandwidth (100 Mb/s), long distance connectivity (over several kilometers), and flexible topologies that allow the use of the same physical interface and media as existing channel and networking protocols. In fact, fibre channel was an attempt to combine the benefits of both channel and network topologies.

Channels are closed, direct, structured and predictable mechanisms for data transmission. No decision making is required, allowing for a high speed, hardware intensive environment. Channels connect peripheral devices such as disk drives and printers, to a work station using protocols such as HIPPI or SCSI. By contrast, networks are unstructured and unpredictable in that much decision making is required to correctly route the data from one point to another.

Fibre channel’s biggest impact has been made on storage devices, using an upper layer SCSI protocol. This gives fiber channel the ability to access mass storage devices more quickly and from a greater distance. Three main topologies include a point-to-point configurations, an arbitrated loop topology or a fabric topology. As a result, the most common of these three is the arbitrated loop, illustrated in Figure 4.

In a fibre channel arbitrated loop (FC-AL), when a device is ready to transmit to the rest of the network, it first arbitrate for control of the loop. This is done via an arbitrate primitive signal (APBx), and each device in the network has its own APBx. It submits the signal to the network control, and the signal is looped around until the originating device receives its APBx, its signal that it has control and may begin transmitting. An open primitive signal allows the device to communicate with other devices in the loop by creating, essentially, a point-to-point connection between the two devices. All other devices in the loop simply repeat the data.

A fabric topology represents the costliest configuration, because it requires a cross-point switch to connect multiple devices in a switched configuration. The benefit of this topology is that many devices can communicate at the same time; the media is not shared.