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FDDI (Fiber Distributed Data Interface)

FDDI is a Fiber LAN network standard composed of two counter-rotating rings, which is how it differs from Token Ring, as described in topologies above. (Token Ring networks usually have only one ring.) As the name implies, FDDI uses fiber optic cable. An unbroken FDDI network can run to 100km with nodes being up to 2km apart on multi-mode fiber, and 10km apart on single-mode fibre. Any single ring can support up to 500 nodes.

The maximum packet size on an FDDI network is 4.5 Kb, which compares well to Ethernet's maximum size of 1.5 Kb. (If FDDI passes through a gateway to join with an Ethernet network, the FDDI packets must be broken up into smaller packets and given new headers.) When an FDDI network is functioning properly, data will move counter-clockwise on the primary ring. If a failure occurs on the primary ring, the working nodes will “wrap” into the secondary ring, which moves the data in a clockwise direction.

The upstream neighbor is the node sending the data. The downstream neighbor is the node receiving data. Nodes on a FDDI network are either Dual-Attached Stations (DAS) or Single-Attached Stations (SAS). DAS are attached to both rings, SAS are attached on to one. Obviously, DAS are much more fault tolerant than SAS.

FDDI standards

Physical Layer Medium Dependent (PMD) - provides link between stations.
Physical Layer Protocol (PHY) - encodes and decodes symbols, the smallest pieces of information between the MAC and PMD standards. The symbol is a 5-bit chunk of data. When data is transferred, one symbol is sent with each “nibble.”
Media Access Control (MAC) - manages data transfers, error recovery, network addressing, token passing, and station access.
Station Management (SMT) - communicates between stations, which is necessary to maintain and monitor the ring. Also collects statistics to detect, isolate and solve

In a normal FDDI data transfer, a token travels around the ring and is “captured” by a node ready to send data. Once the node has the token, it and only it will be able to communicate on the ring. The nodes will send frames toward the downstream neighbor until all of the data has been sent or until the Token Holding Timer (THT) expires. Note the word “toward.” On an FDDI network, data does not bypass other nodes on the ring. Instead, each node checks each frame for errors and retransmits it. The destination node will copy the frame.

The UTP equivalent to FDDI is Copper Distributed Digital Interface (CDDI)

Cisco’s FDDI page
Queen’s University of Belfast’s overview of FDDI
FDDI FAQ

ATM (Asynchronous Transfer Mode)

ATM is a very high-speed communications protocol designed for voice, data, video, and television that combines the best of circuit switching and packet switching. ATM creates a fixed channel between two points before data transfer begins, which makes it like circuit switching, but packets are still sent, instead of an entire message, which makes it like packet switching.

ATM works with very short, fixed-length cells at speeds between 44.7 Mbps to 2.4 Gbps and higher. ATM can support such high speeds because it is designed to be implemented by hardware instead of software. Fiber-based ATMs are being developed that are expected to operate at data rates as high as 10 Gbps.

ATM’s 53-byte cells consist of a 5 byte header and a 48 byte data payload. Cells differs from a packet or frame in that an ATM cell does not always include source or destination addressing information. The ATM cell also doesn’t include higher-level addressing nor packet control information.

ATM is connection-oriented, but the cells are not used to establish and maintain a circuit. Once a circuit is set up, the bandwidth will be used entirely for data transport. After the circuit is set up, ATM associates each cell with a virtual connection, either a channel or a path, between origin and destination. Having both virtual paths and channels makes it easy for a switch to handle multiple connections with the same origin and destination.

The process that segments a longer data into 53 byte cells is called “Segmentation and Reassembly” (SAR). The data in these cells comes from native mode protocols, such as TCP/IP. ATM’s Adaptation Layer (AAL) deals with differences between the various. The AAL uses “classes” to adapt protocols into an ATM intermediate format.

In the United States, the standard for ATM on optical media is SONET.

ATM Tutorial
Another ATM Tutorial
Cisco’s ATM White Paper

Frame Relay

Frame relay is a synchronous HDLC protocol-based WAN technology. Data is sent in variable length HDLC packets, called "frames," set up like the diagram below.

Address

Control

Data

CRC Error Correction

Frame Relay, which is similar to protocols used in X.25 networks, establish contact with destinations via “permanent virtual circuits” or PVCs. Frame Relay circuits are permanent, which is how it differs from X.25. In X.25, circuits can be initiated and ended from users' workstations. Frame relay circuits are set up at the time of installation and are maintained 24x7.

PVC circuits are called “virtual” because the circuits are logical instead of electrical. Frame Relay data moves through an end-to-end logical circuit, not a direct electrical circuit.

Frame relay relies on the customer equipment for end-to-end error correction. Each switch in a frame relay network relays the data (frame) to the next switch. Since the data flows without error correction, data transfer is very fast.

One major advantage of Frame Relay is that it allows you to use speed you don’t pay for. Let’s say you sign up for and pay for a Committee Information Rate (CIR), which offers a minimum bandwidth. If traffic on the network is light at any given moment, Frame Relay will start at the CIR but can reach speeds as high as 1.544 Mbps, the equivalent of a T1 line.

Rad.com’s Frame Relay page
AllianceDatacom’s Frame Relay Tutorial