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Welcome - FDDI Frequently Asked Questions (FAQ)

This FDDI page is currently under construction and you can expect changes frequently.

Revision History

Your input on changes/corrections is welcome. Please include an explanation of what is correct if you find something incorrect. Comments and corrections to: X3T12 POC
These answers are courtesy of (last known addresses):
Author email
Bob Grow
Scott Hiles ------
Ron Mackey
Mike Yip
and others, who's identity has been lost in the multiple ownership of the FAQ.

Q. What does FDDI stand for?

Fiber Distributed Data Interface

Q. What is FDDI?

It is a 100 Mbps Local Area Network, defined by ANSI and OSI standards. It was originally designed to operate over fiber optic cabling, but now also includes standard copper media for interconnection. FDDI uses a token ring media access control protocol.

Q. Who developed the FDDI standards?

Most of the standardization of FDDI was done in Accredited Standards Committee X3T9.5. In 1995, X3T9.5 became X3T12. FDDI standards are approved by both ANSI (American National Standards Institute) and ISO (International Standards Organization). ISO approval usually occurs after ANSI approval.

Q. What is the name of the standards?

This information may be obsoleted by either the FDDI document overview or the FDDI document summary pages.

Q. Where can I get copies of the standards?

There are no on-line copies of the final standards documents. The standards are available in print, and now also on CD-ROM.

Q. What are other good sources of printed information?

Q. I've heard that FDDI uses a token passing scheme for access arbitration, how does this work?

A token is a special three octet FDDI frame. The station waits until a token comes by, grabs the token, transmits one or more frames and release the token. The amount of frames that can be transmitted is determined by timers in the MAC protocol chips.

Q. How is FDDI's token protocol different from 802.5 Token Ring?

FDDI uses a timed token protocol, while 802.5 uses a priority/reservation token access method. Therefore, there are some differences in frame formats, and significant differences in how a station's traffic is handled (queueing, etc.) Management of the rings is also very different.

Q. I've heard that FDDI is a counter-rotating ring, what does this mean?

FDDI is a dual ring technology. And each ring is running in the opposite direction to improve fault recovery.
      | +----------------------------------------+ |
      | |     _____        _____        _____    | |
      | +--->|  A  |----->|  B  |----->|  C  |---+ |

Q. What is a dual ring of trees?

A concentrator provides for connection of either another concentrator or station into one of the dual rings. This allows a tree to extend from each of the master ports of the concentrator.
      | +----------------------------------------------+ |
      | |     _____        ___________        _____    | |
      | +--->|  A  |----->|     B     |----->|  C  |---+ |
                            |   |   |
                  +---------+   |   +---------+
                __|__      _____|_____      __|__
               |  D  |    |     E     |    |  F  |
               |_____|    |___________|    |_____|
                             |     |
                       +-----+     +-----+
                     __|__             __|__
                    |  H  |           |  I  |
                    |_____|           |_____|

Q. What is dual homing?

When a DAS is connected to two concentrator ports, it is called dual-homing. One port is the active link, where data is transmitted and the other port is a hot standby. The hot standby link is constantly tested and will kick in if the active link fails or is disconnected. The B-port in a DAS is usually the active port and the A-port the hot-standby.

Q. What is a DAS?

DAS (Dual Attachment Station) is a station with two peer ports (A-Port and B-Port). The A-port connects to the B-Port of another DAS, and the B-port is going to connect to the A-Port the yet another DAS. i.e.,:
             _________         _________        _________
       +--->|A       B|------>|A       B|----->|A       B|----+
       | +--|_________|<------|_________|<-----|_________|<-+ |
       | |                                                  | |
       | +--------------------------------------------------+ |

Q. What is a SAS?

SAS (Single Attach Station) is a station with one slave port (S-Port). It is usually connected to a master port (M-Port) of a concentrator.

Q. What is a wrapped ring?

When a link in the dual-ring is broken or not connected, the two adjacent ports connecting to the broken link will be removed from the ring and the both stations enter the wrap state. This joins the two counter-rotating rings into one ring.
              Wrap A             Wrap B
             _________         _________        _________
       +--->|A---+   B|--X    |A   +-->B|----->|A       B|----+
       | +--|<---+    |<------|    +----|<-----|         |<-+ |
       | |  |_________|       |_________|      |_________|  | |
	 | |                                                  | |
	 | +--------------------------------------------------+ |

Q. Do I need a concentrator port for each workstation, or can workstations be chained together?

Usually you will need a concentrator port (M-Port) to connect each SAS. A DAS can be connected in the dual rings or to concentrator port(s). FDDI allows two S ports to be connected (a two station ring), or an S port to be connected to an A or B port of a DAS causing a wrapped dual ring. If more than one DAS is used, ring redundancy is lost. (Not all equipment vendors allow S to A, B, or S connections without special configuration.)

Q. If I use a concentrator, what are the advantages/disadvantages?

Advantages: Fault tolerance. When a link breaks, the ring can be segmented. A concentrator can just bypass the problem port and avoid most segmentations. It also gives you better physical planning. Usually people prefer tree physical topology. Generally star configuration of a concentrator system is easier to troubleshoot. Stations can be powered off without serious ring effects. Disadvantages: A concentrator represents a single point of failure. A concentrator configuration may also be more costly.

Q. Can I cascade concentrators? Are there limitations as to how many?

Yes. And you can build a tree as deep as you want. Many users have multiple levels of concentrators. The only limit is FDDI's limit of 500 stations.

Q. What is a bypass and what are the issues in having or not having one?

A bypass relay is a device that is used to skip a station on the dual ring if it is turned off, without causing the ring to wrap. One problem with them is that they attenuate the light in the fiber, so you can't have too many of them. (The maximum number in a row depends on the bypass loss, and how the cable plant is constructed. When the bypass joins two fiber links, the number of connectors between the optical transmitter and receiver will usually increase.)

Q. What are the minimum/maximum distances between stations?

Q. What are the types of fiber that are supported?

Multimode (62.5/125 micron graded index multimode fiber) and other fiber like 50/125. 85/125. 100/140 allowed Single mode (8-10 micron)

Q. Can different PMD types be connected together?

All the FDDI optical PMDs use the same wavelength (1300nm), so they can be connected together. For example, PMD can be connected to LCF-PMD as long as you stick to LCF-PMD configuration rules. If you don't understand this optical stuff, don't attempt to mix SMF-PMD with PMD or LCF-PMD devices without advice.

Q. Are there any hazards with the fiber optics?

Too much optical power can permanently damage someone's eyes. This is not generally a problem with PMD and LCF PMD but can be with SMF-PMD especially Category II SMF-PMD. Inspecting the end of any fiber (e.g., with magnification) without knowing what is at the other end is not a smart thing to do.

Q. I've hear of FDDI over Copper, what type of cable does this scheme use?

Q. Is there any advantage to separating the fiber pairs (will the ring work better if only one strand is broken on a DAS connection?)

Not in most applications. Some military applications anticipate physical damage to the cable and therefore separate it to improve survivability.

Q. I have Ethernet, can I bridge/route between the 2 protocols?


Any problems that arise are generally with the transport protocol. Frame fragmentation is standardized for TCP/IP. It should also be noted that frame fragmentation will not work for DECNET, IPX, LAT, Appletalk, NETBEUI etc. IP is the only protocol that has a standard method of fragmenting. Other protocols destined for Ethernet LANs must stay below the 1500 MTU.

Q. I've heard that there is a frame length difference, what are the issues and problems here?

FDDI frames have a max size of 4500 bytes and Ethernet only 1500 bytes. Therefore your bridge or router needs to be smart enough to fragment the packets (e.g., into smaller IP fragments). Or you need to reduce your frame size to 1500 bytes (of data).

Q. What does an FDDI frame look like?


PAPreamble (II) (8 or more Idle symbol pairs at initial transmission)
SDStarting Delimiter (JK) (J followed by K control symbol)
FCFrame Control (nn) (Tell you if it is a token, MAC frame, LLC frame, SMT frame, frame priority, sync or async)
DADestination Address (nn) (6 bytes of MAC Address in MSb first format)
SASource Address (nn) (6 bytes of MAC Address of this station)
INFOInformation field (nn) (Variable Length. Usually starts with LLC header, then SNAP field, then the payload e.g., IP packet)
EDEnding Delimiter (T) (one T control symbol)
FSFrame Status (EAC) (Three symbols of status: Error, Address_match, and Copied. Each symbol is either SET or RESET. e.g., If EAC == RSS, then the frame has no error, some station on the ring matched the DA, and some station on the ring copied the frame into its buffer.

Q. So FDDI operates at 100 Megabits per second (Mbps), what is the practical maximum bps?

Depends. You can get aggregate usage greater than 95 Mbps in most installations. As with any LAN, high utilization corresponds to longer queuing times. Many systems run fine at 75 Mbps. The maximum for a station is implementation dependent. Some can't do more than 20 Mbps, while others can sustain more than 90 Mbps.

Q. What happens when I bridge between a 100 Mbps FDDI and a 10 Mbps Ethernet if the FDDI traffic destined for the Ethernet gets above 8 Mbps? 10 Mbps?

After the buffer fills Frames start dropping. This is not a problem unique to FDDI however. Consider Ethernet to T1, or multiple Ethernets to a single Ethernet, or a lightly loaded Ethernet to a heavily loaded Ethernet.

Q. What is the latency across a bridge/router? (Yes I know that different vendors are different, but what is the window?)

Ask the vendors.

Q. Are there FDDI repeaters?

Yes. The FDDI Physical Layer Repeater Protocol (PHY-REP) standard is in the final stages of approval. Repeaters are sometimes used to convert between single mode and multimode.

Q. What type of test and trouble shooting equipment is available for FDDI?

FDDI protocol analyzers of varying capability are available from multiple manufacturers. Many FDDI equipment vendors also have management software that implements ring mapping, inspection of other stations operational parameters, general ring state monitoring, etc.

An optical time domain reflectometer (OTDR) and optical power meters are used for testing optical fiber. There are also FDDI link testers that measure power and low level FDDI protocol response.

Q. What about network station management? Does FDDI support SNMP?

Yes. The IETF has an FDDI SNMP MIB. The new MIB is called RFC1512. The previous FDDI SNMP MIB (RFC1285) was based on X3T9.5 SMT 6.2.

Q. Is there RMON for FDDI?

The IETF has not standardized an RMON MIB for FDDI, but many RMON vendors have done their own.

Q. What is a beaconing ring? Does FDDI beacon?

Beacon is a special frame that FDDI MAC sends when something is wrong. When Beaconing persists, SMT will kick in to detect and try to solve the problem. A few FDDI implementations will beacon on initial entry into the ring, but this is a short term condition.

Q. How about interoperability, does one manufacturer's equipment work with others?

Just like any networking products, Ethernet, Token, FDDI, ATM, there is a possibility that one vendor does not work with another. But most of the equipment shipping today is tested for interoperability. There are test labs like UNH and ANTC. Ask the vendor what type of testing they did.

Q. Can I interface FDDI to a PC (ISA Bus), PC (EISA Bus), PC (Micro channel Bus), Macintosh, Sun workstation, DECstation 5000, NEXT computer, Silicon Graphics, Cisco router, WellFleet router, SNA gateway (McData), other?


I am not sure if NeXT has any FDDI adaptor software, but there are ~5 different NuBus FDDI cards in the market. But FDDI adaptors are available for all other buses or vendors.

Q. What is the maximum time a station has to wait for media access. What type of applications care?

MaxTime = ~(#of stations * T_neg)
(T_neg is the negotiated target token rotation time)

Even at maximum load, this is unlikely. To get this worst case the ring basically must go from zero offered load to maximum offered load (all stations enqueueing ~T-neg of frames) virtually instantenously. If this doesn't happen, the ~T_neg of available bandwidth is split between multiple stations with each fraction of the bandwidth being passed along the ring with each token rotation. This results in a given station having multiple transmit opportunities within MaxTime.

To reduce MaxTime change the T_req of a station to some lower value (e.g., 8 msec). This is done through the MIB parameter fddiPATHMaxT-Req.

Q. Can I bridge/route TCP/IP, SNA, Novell's IPX, Sun protocols, DecNet, Banyan Vines, Appletalk, X windows, LAT?

Yes, but some equipment may not support bridging of all of these protocols. Check with the bridge vendor for your required protocols.

Q. What are the applications that would use FDDI's bandwidth?

Basically anything will be at least a bit faster. From NFS to images transmission. Even if a single station cannot take advantage of the 100 Mbps, the aggregate bandwidth will help a lot if your Ethernet is saturated. However, note that though FDDI has higher bandwidth than Ethernet, the signals travel at the same speed. The propagation of a signal on the transmission line is the same for Ethernet, token ring, and FDDI.

Q. What are the effects of powering off a workstation on a DAS or SAS connection?

Depends. Let's do SAS first, it is easier. If a SAS is connected to a concentrator, then the concentrator will bypass the SAS connection using an internal data path. If a DAS is connected to a concentrator, then the concentrator will also bypass the DAS. If a DAS is connected to the trunk rings without using an optical bypass switch, then the trunk ring will wrap. If multiple stations power off on the trunk rings, then the ring will be segmented. Now if a DAS is using an optical bypass switch, the switch will kick in and prevent the ring from wrapping.

Q. What are the effects of disconnecting a link on a DAS or SAS connection?

If either conductor (fiber or wire) in a link is broken or disconnected or loses a transmitter or receiver the link is removed from the ring.

Q. What is a common topology?

Connect concentrators and other equipment which is powered on all the time (e.g., bridges, routers, ring monitors, etc.) into the trunk ring. In a large enterprise, workgroup concentrators and users stations are then connected to the backbone concentrators. Some connect bridges and routers and critical servers to backbone concentrators using dual- homing.

Q. What is Graceful Insertion? Should I demand it from my vendors?

Graceful Insertion is a method to insert a station (or a tree) into a ring minimizing disruption. Graceful Insertion is not standard, and therefore different by vendor. Some implementations are both "frame friendly" (don't corrupt frames) and "token friendly" (don't destroy the token). The theory goes that Graceful Insertion can minimize ring non_op and lost frames, therefore saving transmission timeout in upper layer protocols (e.g., TCP).

The following is the counter argument: Graceful Insertion can use more ring bandwidth (holding the token) than is consumed by a ring recovery. And upper layer protocols are designed to perform frame recovery and retransmission anyway. Also, no vendor can guarantee 100% Graceful Insertion anyway.

Q. Is there a Graceful De-insertion?

Not really, but on some concentrators, if you know a station is to be removed from the ring, it could be removed gracefully by sending a command to the concentrator.

Q. Can I run FDDI on coax cable?

There is no standard for FDDI on coax, but check with DEC.

Q. What does SMT stand for? What does it do? Do I need it?

Station Management (SMT). It is part of the ANSI FDDI Standards that provides link-level management for FDDI. SMT is a low-level protocol that addresses the management of FDDI functions provided by the MAC, PHY, and PMD. It performs functions like ring recovery, frame level management, link control, etc. Every stations on FDDI needs to have SMT.

Q. What is SMT 6.2 or SMT 7.3?

A lot of FDDI equipment was shipped before the SMT standard was completed. Most of this equipment was shipped with SMT software based on SMT Revision 6.2, and earlier shipments were even made with SMT 5.1. SMT 7.3 was the final working document in the standards committee. It is functionally identical to SMT 7.2 and the approved SMT standard. All of these versions of SMT work together on a ring, but they will look different to a management station. (The SMT 6.2 and SMT 7.2 MIBs are very different, as are the frame protocols related to the MIB.)

Q. Can I connect two Single attach stations together and form a two station ring without a concentrator?

Yes. You can do that if both stations support the S-S port connection. Most vendors support the S-S connections.

Q. What are ports? What are the different type of ports?

A port is the connector and supporting logic for one end of an FDDI link. Each port has a transmitter and a receiver. Ports are given names descriptive of their position in FDDI topologies. SMT defines four types of ports (A, B, M, S). A dual-attachment station has two ports, one A-port and one B-port. A single attach station has only one port (S-port). A concentrator will have many M-port for connecting to other stations' A, B or S-ports.

Q. What are the port connection rules?

When connecting DASs, one should connect the A-port of one station to the B-port of another. S-port on the SAS is to connect to the M-port on a concentrator. A and B-port on DASs can also connect to the M-port of a concentrator. M-ports of concentrators will not connect to each other. In more detail, SMT suggests the following rules:

Other Port
This Port


Q. What is the difference between FDDI and FDDI-II?

Both FDDI and FDDI-II run at 100 Mbps on the fiber. FDDI can transport both asynchronous and synchronous types of frames.

FDDI-II has a new mode of operation called Hybrid Mode. Hybrid mode uses a 125 usec cycle structure to transport isochronous traffic, in addition to sync/async frames.

FDDI and FDDI-II stations can be operated in the same ring only in Basic (FDDI) mode.

There is very little FDDI-II product available on the market.

Q. Where can I get vendor information for FDDI products

Previous versions of the FAQ contained vendor information embedded in the questions. This information has been consolidated and categorized in HTML format.
Page Administrator: Bob Hott
Last Modified: 21 May 1997
Comments: Tim Plunkett
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