Station Management Protocol

7/28/2019 Station Management Protocol

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Station Management Protocol

The station management protocol (SMT) monitors and controls all FDDIactivity on its station. SMT manages processes in the various FDDIlayers (PMD, PHY, and MAC) at the station level and ensures thecorrect operation of the station on the ring. (See FDDI Ring for adescription of the FDDI ring.) SMT's responsibilities include overseeingstation insertion and removal from the ring, initializing the station toconform with the current ring status, and identifying, isolating, and

recovering from faults on the ring.An FDDIXPress station's SMT functionality is distributed. Some of it iscontained within a software module that includes the SMT daemon(smtd) and a special database file called the management informationbase (MIB); some functionality is located within chips on the FDDIboard.The MIB resides in the local memory on each FDDI station. Thisdatabase maintains statistical and operational information used tomanage the ring.Control within an FDDI ring is distributed among the SMT entities of allthe stations on that ring; control is not handled by a master station.

SMT entities communicate with each other to manage theadministration of addressing, allocation of network bandwidth, andconfiguration and control of the ring. Some of these SMT parametersare site-configurable. For FDDIXPress, the SMT configuration file is/etc/fddi/smtd.conf.For more information about the SMT daemon, see the smtd(1M) man page.

FDDI Ring

An FDDI ring is a length of cable laid out in a closed loop. Current

standards require that the ring cable be fiber optic cable. An opticalsignal (light) passes through the cable (around the ring) and returns toits point of origin. Whenever a station is connected to the ring, it isphysically inserted into the ring so that the optical signal passesthrough the station (illustrated in Figure 1-3). Stations on the ring arereferred to as upstream or downstream in relation to each other. Thedownstream neighbor station is the first station to see a transmittingstation's transmission. In Figure 1-3, station A is station C's downstreamneighbor and station B's upstream neighbor.Figure 1-3. Simple Token Ring
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The FDDI dual ring (or trunk ring) has two separate loops (rings). Onering is called the primary ring and the other is the secondary ring, asillustrated in Figure 1-4. Most sites use the secondary ring as a backupring. The light signal within each loop of a dual ring travels in theopposite direction from the signal in the other ring; in FDDI jargon thisis referred to as counter-rotating. Because the signal travels indifferent directions, upstream and downstream neighbors are oppositeon each ring. In Figure 1-5, where station 2 is station 1's downstreamneighbor on the primary ring, station 2 is the upstream neighbor onthe secondary ring.Figure 1-4. A Basic FDDI Ring

The cabling for FDDI is available in a number of forms. Multimode (62.5micron) fiber optic cable was the first transmission medium (cable)defined for FDDI. Recently, the use of single-mode (50-micron) fiberoptic cable was approved. Copper cable has also been approved, foruse only between concentrators and stations.

FDDI Devices
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In addition to the FDDI components, the FDDI standard defines thetypes of devices that can be connected to the ring. These devicesinclude (but are not limited to) the following:

stations DAS: dual attach station (usually attaches directly to FDDI

dual ring)

SAS: single attach station (attaches to the FDDI ring

through a concentrator)

concentrators

DAC: dual attach concentrator (usually attaches directly tothe FDDI dual ring)

SAC: single attach concentrator (attaches to the FDDI ringthrough another concentrator)

optical bypass switch

Dual Attach Station

A dual attach station (DAS) has two ports (A and B). A DAS can beconnected to the dual ring or to a concentrator.When connected to the dual ring, each port connects to both theprimary ring and the secondary ring (as shown in Figure 1-5). This dualconnection is known in FDDI jargon as connecting to the dual ring.The station's SMT ensures that the station can continue to transmitand receive data even when the primary ring experiences a break. (Abreak in the ring occurs when the signal cannot make a complete trip

around the ring; this can be caused by a station failing or by a faultycable.)When connected to a concentrator, the two ports can each beconnected to one of the concentrator's M ports. A DAS station canbehave as a single attach station (SAS) if configured to do so, in whichcase only one of its ports is connected to the concentrator and theother port is not used.

Note:As illustrated in Figure 1-5, for DAS connections to the dual ring, port A must always beconnected to port B of the downstream station, while port B connects to port A of the upstream station.

Figure 1-5. Connection of DAS Ports to Primary and SecondaryRings


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Single Attach Station

A single attach station (SAS) has a single slave (S) port that attaches tothe ring through a master (M) port on a concentrator. The concentratorroutes the signal from the functioning ring through every SASconnected to that concentrator.

Concentrators

A concentrator allows many single-attachment FDDI devices to obtaintheir connection to the FDDI ring through one devicetheconcentrator. Concentrators have one or more master ports (M), eachof which accepts a connection from one single-attachment device.

The FDDI standard defines two types of concentrators: dual-attachment and single-attachment. A dual attach concentrator (DAC)has two ports (A and B), each of which connects to both the primaryand secondary rings, just like the DAS. A single attach concentrator(SAC) connects to an FDDI ring through another concentrator, in thesame manner as an SAS. Figure 1-6 illustrates the use of concentratorson an FDDI ring.Figure 1-6. FDDI Ring With Concentrators
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Optical Bypass Switch

An optical bypass switch (OBS) is an optional device that can beattached between a dual ring and a DAS or DAC. The OBS allows adual-attachment device to become dysfunctional without wrapping the

ring.

How FDDI Works

The FDDI local area network consists of two or more stations or nodesconnected serially by fiber optic cables to form a closed loop, the ring.Each FDDI local area network has two rings: a primary ring and asecondary ring. Figure 1-6 and Figure 1-10 show common FDDI ringconfigurations. The secondary ring is usually configured as a backupring.

Operational Ring

An optical signal (light), encoded to represent data, is beamed into thecable by a transmitting station. The signal travels through the cableand is read by each station on the ring, until it returns to the originalsender. As long as the signal can make a complete trip around theloop, the ring is operational. When a break or fault occurs in the ring,the signal cannot complete the loop. Situations that break the ringinclude, among other things, a missing or damaged cable, a looseconnection, and a dysfunctional station.


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Fixing a Broken Ring

The optical signal travels in opposite directions in each ring. Thisdesign makes closure of a broken primary ring feasible. When the SMTmodule within a station notices that the primary ring is broken, itconnects the secondary ring to the primary one to complete the loop.This action bypasses (cuts out) the faulty section, as illustrated inFigure 1-7. In FDDI jargon, fixing a broken primary ring in this manner iscalled wrapping the ring. The original two rings are joined to form a

single loop (ring). Notice that the ring must wrap in two locations tocomplete the loop. In this condition, transmission proceeds withoutinterruption for all the stations on the functioning portion of the ring.Figure 1-7. Wrapping the Ring

When a ring wraps, two stations change their internal optical signalpaths. Instead of the signal passing through both port A and port B (asillustrated in Figure 1-5), it is received and transmitted through a singleport (either A or B). Figure 1-8 illustrates the altered optical signalpaths. The two stations that make this change are located at the endsof the functional portion of the primary ring.Figure 1-8. Connection of DAS Ports at Points Where Ring IsWrapped

If more than one fault occurs on the FDDI ring, the ring may becomefragmented, as shown in Figure 1-9. In this condition, communicationcontinues among the stations within each fragment, butcommunication is not possible with stations located on a differentfragment.
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FDDI management tools such as smtstat and smtring (or the graphicalproduct, FDDIVisualyzer) can be used to identify problems with thering.Figure 1-9. A Fragmented Ring

Optical Bypass Switch

FDDI defines an optional device that allows a DAS to becomedysfunctional without wrapping the ring. This device is called an opticalbypass switch (OBS). The optical bypass switch is connected between astation's two ports and the dual ring.Without an optical bypass switch, when a DAS becomes dysfunctional,the signal going around the ring cannot continue past thedysfunctional station; stations downstream from this station do notreceive any signal. The ring is broken, which causes an automaticwrap.When an optical bypass switch is present in this situation, it maintainsan intact loop by simply routing the signal through the switch,bypassing the dysfunctional station as if it were not attached to thering. The SMT modules of neighboring stations will notice that theyhave acquired different neighbors, but they will continue tocommunicate without the disruption caused by a wrapped ring.

Transmitting and Receiving on the Ring

A station on a ring gains access to transmit information onto that ringby capturing the ring's token. Only one token is allowed on each ring.Various controls are built into FDDI to limit or specify the length of timethe token can be held. Once a station captures the token, it cantransmit data onto the network. When the station finishes transmitting,or its time expires, it places the token back onto the ring, thus allowingthe next station the opportunity to capture it. When a station does not

have anything to transmit, it does not capture the token.Once a frame is transmitted onto the ring, it moves around the ring inthe following manner: Each station reads the frame and transmits itback onto the ring. If a station makes a local copy of the frame, itindicates this action by altering various bits in the copy that itretransmits onto the ring. As frames pass around the ring, thetransmitting station recognizes the return of its own data anddetermines if reception has been successful and error free by checkingthe changed bits in the frame. Each station is responsible for removing(stripping) all the data that it placed on the ring.

Multiprotocol Networking With FDDI


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You can use FDDI as a standalone network, or you can incorporate itinto an existing internetwork. When incorporating FDDI with an existingnetwork, it is standard practice to use FDDI as the backbone and theslower networks (Ethernet or token ring) as subnetworks. This involvesusing a router (for example, an FDDI-to-Ethernet router) that isconnected to both the non-FDDI network and the FDDI ring. The routerallows information (packets) to flow between the two networks eventhough they use different protocols. Figure 1-10 shows FDDI with anEthernet network; the ring illustrated has five dual-attachment nodes,

one of which is a concentrator. A Silicon Graphics workstation or serverthat has two network interfaces automatically and by default performsas a router.Figure 1-10. FDDI With an Ethernet Network
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Prev Table of Contents Next

IntroductionChapter 2. Configuring FDDIXPress

Software

FDDIXPress Administration Guide

(document number: 007-0813-060 / published: 1997-11-25) table ofcontents | additional info | download

Front Matter

Introduction

Chapter 1. Introducing FDDI

Chapter 2. Configuring FDDIXPress Software Chapter 3. Managing Your FDDI Station and Ring

Chapter 4. Troubleshooting

Appendix A. Error Messages

Appendix B. smtstat Reports

Appendix C. Configuring the SMT Daemon and the FDDIXPress Driver

Appendix D. Man Pages

Glossary

Index

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Station Management Protocol