Data Communication & Networking

Fiber Distributed-Data Interface (FDDI) :

FDDI (Fiber-Distributed Data Interface) is a standard for data transmission on fiber optic lines in that can extend in range up to 200 km (124 miles). The FDDI protocol is based on the token ring protocol. In addition to being large geographically, an FDDI local area network can support thousands of users. An FDDI network contains two token rings, one for possible backup in case the primary ring fails. The primary ring offers up to 100 Mbps capacity. If the secondary ring is not needed for backup, it can also carry data, extending capacity to 200 Mbps. The single ring can extend the maximum distance; a dual ring can extend 100 km (62 miles). FDDI is a product of American National Standards Committee X3-T9 and conforms to the open system interconnect (OSI) model of functional layering. It can be used to interconnect LANs using other protocols. FDDI-II is a version of FDDI that adds the capability to add circuit-switched service to the network so that voice signals can also be handled. Work is underway to connect FDDI networks to the developing Synchronous Optical Network. Function of FDDI FDDI uses a dual-ring architecture with traffic on each ring flowing in opposite directions (called counter-rotating). The dual-rings consist of a primary and a secondary ring. During normal operation, the primary ring is used for data transmission, and the secondary ring remains idle. The primary purpose of the dual rings, as will be discussed in detail later in this chapter, is to provide superior reliability and robustness. Figure 1 shows the counter-rotating primary and secondary FDDI rings.   Figure 1: FDDI uses counter-rotating primary and secondary rings. FDDI Specifications FDDI specifies the physical and media-access portions of the OSI reference model. FDDI is not actually a single specification, but it is a collection of four separate specifications each with a specific function. Combined, these specifications have the capability to provide high-speed connectivity between upper-layer protocols such as TCP/IP and IPX, and media such as fiber-optic cabling. FDDI's four specifications are the Media Access Control (MAC), Physical Layer Protocol (PHY), Physical-Medium Dependent (PMD), and Station Management (SMT). The MAC specification defines how the medium is accessed, including frame format, token handling, addressing, algorithms for calculating cyclic redundancy check (CRC) value, and error-recovery mechanisms. The PHY specification defines data encoding/decoding procedures, clocking requirements, and framing, among other functions. The PMD specification defines the characteristics of the transmission medium, including fiber-optic links, power levels, bit-error rates, optical components, and connectors. The SMT specification defines FDDI station configuration, ring configuration, and ring control features, including station insertion and removal, initialization, fault isolation and recovery, scheduling, and statistics collection. FDDI is similar to IEEE 802.3 Ethernet and IEEE 802.5 Token Ring in its relationship with the OSI model. Its primary purpose is to provide connectivity between upper OSI layers of common protocols and the media used to connect network devices. Figure 3 illustrates the four FDDI specifications and their relationship to each other and to the IEEE-defined Logical-Link Control (LLC) sublayer. The LLC sublayer is a component of Layer 2, the MAC layer, of the OSI reference model.   Figure 2: FDDI specifications map to the OSI hierarchical model. FDDI Station-Attachment Types One of the unique characteristics of FDDI is that multiple ways actually exist by which to connect FDDI devices. FDDI defines three types of devices: single-attachment station (SAS), dual-attachment station (DAS), and a concentrator. An SAS attaches to only one ring (the primary) through a concentrator. One of the primary advantages of connecting devices with SAS attachments is that the devices will not have any effect on the FDDI ring if they are disconnected or powered off. Concentrators will be discussed in more detail in the following discussion. Each FDDI DAS has two ports, designated A and B. These ports connect the DAS to the dual FDDI ring. Therefore, each port provides a connection for both the primary and the secondary ring. As you will see in the next section, devices using DAS connections will affect the ring if they are disconnected or powered off. Figure 3 shows FDDI DAS A and B ports with attachments to the primary and secondary rings. Figure 3: FDDI DAS ports attach to the primary and secondary rings. An FDDI concentrator (also called a dual-attachment concentrator [DAC]) is the building block of an FDDI network. It attaches directly to both the primary and secondary rings and ensures that the failure or power-down of any SAS does not bring down the ring. This is particularly useful when PCs, or similar devices that are frequently powered on and off, connect to the ring.  Figure 4: A concentrator attaches to both the primary and secondary rings. FDDI Fault Tolerance FDDI provides a number of fault-tolerant features. In particular, FDDI's dual-ring environment, the implementation of the optical bypass switch, and dual-homing support make FDDI a resilient media technology. Dual Ring FDDI's primary fault-tolerant feature is the dual ring. If a station on the dual ring fails or is powered down, or if the cable is damaged, the dual ring is automatically wrapped (doubled back onto itself) into a single ring. When the ring is wrapped, the dual-ring topology becomes a single-ring topology. Data continues to be transmitted on the FDDI ring without performance impact during the wrap condition. Figure 5 and Figure 6 illustrate the effect of a ring wrapping in FDDI. Figure 5: A ring recovers from a station failure by wrapping. Figure 6: A ring also wraps to withstand a cable failure. When a single station fails, as shown in Figure 5, devices on either side of the failed (or powered down) station wrap, forming a single ring. Network operation continues for the remaining stations on the ring. When a cable failure occurs, as shown in Figure 6, devices on either side of the cable fault wrap. Network operation continues for all stations. It should be noted that FDDI truly provides fault-tolerance against a single failure only. When two or more failures occur, the FDDI ring segments into two or more independent rings that are unable to communicate with each other. Optical Bypass Switch An optical bypass switch provides continuous dual-ring operation if a device on the dual ring fails. This is used both to prevent ring segmentation and to eliminate failed stations from the ring. The optical bypass switch performs this function through the use of optical mirrors that pass light from the ring directly to the DAS device during normal operation. In the event of a failure of the DAS device, such as a power-off, the optical bypass switch will pass the light through itself by using internal mirrors and thereby maintain the ring's integrity. The benefit of this capability is that the ring will not enter a wrapped condition in the event of a device failure. Figure 7 shows the functionality of an optical bypass switch in an FDDI network. Figure 7: The optical bypass switch uses internal mirrors to maintain a network. Dual Homing Critical devices, such as routers or mainframe hosts, can use a fault-tolerant technique called dual homing to provide additional redundancy and to help guarantee operation. In dual-homing situations, the critical device is attached to two concentrators. Figure 8 shows a dual-homed configuration for devices such as file servers and routers. Figure 8: A dual-homed configuration guarantees operation. One pair of concentrator links is declared the active link; the other pair is declared passive. The passive link stays in back-up mode until the primary link (or the concentrator to which it is attached) is determined to have failed. When this occurs, the passive link automatically activates. FDDI Frame Format The FDDI frame format is similar to the format of a Token Ring frame. This is one of the areas where FDDI borrows heavily from earlier LAN technologies, such as Token Ring. FDDI frames can be as large as 4,500 bytes. Figure 9 shows the frame format of an FDDI data frame and token. TOP Figure 9: The FDDI frame is similar to that of a Token Ring frame. FDDI Frame Fields The following descriptions summarize the FDDI data frame and token fields illustrated in Figure 9. Preamble---A unique sequence that prepares each station for an upcoming frame. Start Delimiter---Indicates the beginning of a frame by employing a signaling pattern that differentiates it from the rest of the frame. Frame Control---Indicates the size of the address fields and whether the frame contains asynchronous or synchronous data, among other control information. Destination Address---Contains a unicast (singular), multicast (group), or broadcast (every station) address. As with Ethernet and Token Ring addresses, FDDI destination addresses are 6 bytes long. Source Address---Identifies the single station that sent the frame. As with Ethernet and Token Ring addresses, FDDI source addresses are 6 bytes long. Data---Contains either information destined for an upper-layer protocol or control information. Frame Check Sequence (FCS)---Filed by the source station with a calculated cyclic redundancy check value dependent on frame contents (as with Token Ring and Ethernet). The destination address recalculates the value to determine whether the frame was damaged in transit. If so, the frame is discarded. End Delimiter---Contains unique symbols, which cannot be data symbols, that indicate the end of the frame. Frame Status---Allows the source station to determine whether an error occurred and whether the frame was recognized and copied by a receiving station.