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

Background

The Fiber Distributed Data Interface (FDDI) specifies a 100-Mbps token-passing, dual-ring LAN using fiber-optic cable. FDDI is frequently used as high-speed backbone technology because of its support for high bandwidth and greater distances than copper. It should be noted that relatively recently, a related copper specification, called Copper Distributed Data Interface (CDDI) has emerged to provide 100-Mbps service over copper. CDDI is the implementation of FDDI protocols over twisted-pair copper wire. This chapter focuses mainly on FDDI specifications and operations, but it also provides a high-level overview of CDDI.

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 shows the ring attachments of an FDDI SAS, DAS, and concentrator.

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.

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---Identifie s 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.

FDDI Frame Format

FDDI Frame

Frame Control (FC): 8 bits

has bit format CLFFZZZZ

C indicates synchronous or asynchronous frame

L indicates use of 16 or 48 bit addresses

FF indicates whether it is a LLC, MAC control or reserved frame

in a control frame ZZZZ indicates the type of control

Destination Address (DA): 16 or 48 bits

specifies station for which the frame is intended

Source Address (SA): 16 or 48 bits

specifies station that sent the frame

Here is what the FDDI frame format looks like:

FDDI Frame Format

PA - Preamble 16 symbols

SD - Start Delimiter 2 symbols

FC - Frame Control 2 symbols

DA - Destination Address 4 or 12 symbols

SA - Source Address 4 or 12 symbols

FCS - Frame Check Sequence 8 symbols, covers the FC, DA, SA and Information

ED - End Delimiter 1 or 2 symbols

FS - Frame Status 3 symbols

Token is just the PA, SD, FC and ED

Preamble
The Token owner as a minimum of transmits the preamble 16 symbols of Idle. Physical Layers of the subsequent repeating stations can change the length of the Idle pattern according to the Physical Layer requirements. Therefore, each repeating station may see a variable length preamble from the original preamble. Tokens will be recognized as long as its preamble length is greater than zero. If a valid token is received and cannot be processed (repeated), due to expiration of ring timing or latency constraints the station will issue a new token to be put on the ring. A given MAC implementation is not required to be capable of copying frames received with less than 12 symbols of preamble; Nevertheless, with such frames, it cannot be correctly repeated.
Since the preamble cannot be repeated, the rest of the frame will not be repeated as well.

Starting Delimiter

This field of the frame denodes the start of the frame. It can only have symbols 'J' and 'K'. These symbols will not be used anywhere else but in the starting delimiter of a token or a frame.

Frame Control

Frame Control field descibes what type of data it is carrying in the INFO field. Here are the most common values that are allowed in the FC field:

40: Void Frame.

41,4F: Station Management (SMT) Frame.

C2,C3: MAC Frame.

50,51: LLC Frame.

60: Implementor Frame.

70: Reserved Frame.

Please note that the list here are only the most common values that can be formed by a 48 bit addressedynchronous data frames.

Destination Address

Destination Address field contains 12 symbols that identifies the station that is receiving this particular frame. When FDDI is first setup, each station is given a unique address that identifies themselves from the others. When a frame passed by the station, the station will compare its address against the DA field of the frame. If it is a match, station then copies the frame into its buffer area waiting to be processed. There is not restriction on the number of stations that a frame can reach at a time. If the first bit of the DA field is set to '1', then the address is called a group or global address. If the first bit is '0', then the address is called individual address. As the name suggests, a frame with a global address setting can be sent to multiple stations on the network. If the frame is intended for everyone on the network, the address bits will be set to all 1's. Therefore, a global address contains all 'F' symbols. There are also two different ways of administer these addresses. One's called local and the other's called universal. The second bit of the address field determine whether or not the address is locally or universally administered. If the second bit is '1' then it is locally administered address. If the second bit is a '0', then it is universally administered adress.A locally administer address are addresses that have been assigned by the network administrator and a universally administered addresses are pre-assigned by the manufacturer' s OUI.

Source Address

A Source Address identifies the station that created the frame. This field is used for remove frames from the ring. Each time a frame is sent, it travels around the ring, visiting each station, and eventually (hopefully) comes back to the station that originally sent that frame. If the address of a station matches the SA field in the frame, the station will strip the frame off the ring. Each station is responsible for removing its own frame from the ring.

Information Field

INFO field is the heart and soul of the frame. Every components of the frame is designed around this field; Who to send it to, where's this coming from, how it is received and so on.The type of information in the INFO field can be found by looking in the FC field of the frame. For example: '50'(hex) denodes a LLC frame. So, the INFO field will have a LLC header followed by other upper layer headers. For example SNAP, ARP, IP, TCP, SNMP, etc. '41'(hex or '4F'(hex) denode s SMT (Station Management) frame. Therefore, a SMT header will appear in the INFO field.

Frame Check Sequence

Frame Check Sequence field is used to check or verify the traversing frame for any bit errors. FCS information is generated by the station that sends the frame, using the bits in FC, DA, SA, INFO, and FCS fields. To verify if there are any bit errors in the frame, FDDI uses 8 symbols (32 bits) CRC (Cyclic Redundancy Check) to ensure the transmission of a frame on the ring.

End Delimiter

As the name suggests, the end delimiter denodes the end of the frame. The ending delimiter consist of a 'T' symbol. This 'T' symbols indicates that the frame is complete or ended. Any data sequence that does not end with this 'T' symbol is not considered to be a frame.

Frame Status

Frame Status (FS) contains 3 indicators that dictates the condition of the frame. Each indicator can have two values: Set ('S') or Reset ('R'). The indicators could possibly be corrupted. In this case, the indicators is neither 'S' nor 'R'. All frame are initially set to 'R'. Three types of indicators are as follows: Error (E):This indicator is set if a station determines an error for that frame. Might be a CRC failiure or other causes. If a frame has its E indicator set, then, that frame is discarded by the first station that encounters the frame. Acknowledge( A): Sometime this indicator is called 'address recognized'. This indicator is set whenever a frame in properly received; meaning the frame has reached its destination address. Copy (C): This indicator is set whenever a station is able to copy the received frame into its buffer section. Thus, Copy and Acknowledge indicators are usually set at the same time. But, sometimes when a station is receiving too many frames and cannot copy all the incoming frames. If this happens, it would re-transmit the frame with indicator 'A' set indicator 'C' left on reset.

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