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Text File | 1994-11-10 | 21KB | 415 lines

Newsgroups: comp.dcom.lans.fddi From: barr@spot.Colorado.EDU (BARR DOUG) Subject: FAQ Sender: usenet@cnsnews.Colorado.EDU (Net News Administrator) Organization: A Date: Mon, 3 Jan 1994 23:30:40 GMT Jan. 3, 1994 Here is the 2nd version of the comp.dcom.lans.fddi FAQ. I appreciate your input on changes/corrections. Please include a explanation of what is correct if you find something incorrect. doug barr@spot.colorado.edu Most of these answers are courtesy of: -- Mike Yip my@berlioz.nsc.com Vernon Schryver vjs@rhyolite.wpd.sgi.com Matt Holdrege holdrege@dcv4kd.phs.com and others (please let me know if you others want credit here) Appreciate all your help: Q. What does FDDI stand for? Fiber Distributed Data Interface Q. What is the difference between FDDI and FDDI-II? Both FDDI and FDDI-II runs at 100 M bits/sec on the fiber. FDDI can transport both async and sync types of frames. FDDI-II has a new mode of operation called Hybrid Mode. Hybrid mode uses a 125usec cycle structure to transport isochronus traffic, in addition to sync/async frames. FDDI and FDDI-II stations can be operated in the same ring only in Basic mode. Q. What is the name of the standards and where can I get them? ANSI X3T9.5 standards American National Standards Institute 1430 Broadway, New York, NY 10018, USA Attention: Sales Dept. - IEEE Standards IEEE Service Center 445 Hoes Lane, Piscataway, NJ 08855, USA - X3T9.5 Documents Global Engineering Documents (USA) 1-800-854-7179 Q. What are other good sources of printed information? - FDDI Technology and Applications: Edited Mirchandani and Khanna - Handbook of Computer Communications Standards Vol 2: By Stallings - Call up DEC to ask for the free FDDI tutorial book - Dig up 1986-1992 issue of IEE Local Computer Network Conference Q. I've heard that FDDI uses a token passing scheme for access arbitration, how does this work? A token is a normal FDDI frame with a fixed format. The station waits until a token comes by, grabs the token, transmits the the frames and release the token. The amount of frames that can be transmitted is determined by timers in the MAC protocol chips. [You really need a diagram for the station and/or topology.] 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. Q. What is a dual ring of trees? See the diagram. 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 will constantly testing the link and will kick in if the active link failed or disconnected. The B-port in a DAS is the active port and the A-port is the hot-standby. Q. What is a DAS? DAS (Dual Attach Station) is a station with two peer ports (A-Port and B-Port). The A-port is going to the B-Port of another DAS, and the B-port is going to connect to the A-Port the yet another DAS. ie: --------- --------- --------- +--->|A B|------>|A B|----->|A B|----+ | +--|---------|<------|---------|<-----|---------|<-+ | | | | | | +--------------------------------------------------+ | +------------------------------------------------------+ Q. What is a SAS? SAS (Single Attach Station) is a station with one peer port (S-Port). It is usually connected to the 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 adj ports connecting to the borken link will be disconnected and the both stations enter the wrap state. Wrap Wrap --------- --------- --------- +--->|A B|-X 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. DAS can be hooked up to the main rings or concentrator port(s). 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. Disadvatages: A concentrator represents a single point of failure. There 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. We have a dual-rings of concentrator here connecting machine rooms and wiring rooms. And from the there we connect to other concentrators to different offices. Then we have a concentrator in the lab to different machines. There is a maximum of 500 stations on an FDDI LAN. Q. What is a bypass and what are the issues in having or not having one? Bypass is a ($600-$1200) device that is used to skip a station on the ring if it is turned off. Therefore, you don't need to use concentrator to avoid the segmentation problems. One problem with them is that they increase the db loss of the fiber, so you can't have too many of them (3 activated in a row maximum, I believe). Q. What are the minimum/maximum distances on fiber runs? no min, 2 km max for multimode fiber. 20 km max (may be as high as 60km, we're not sure) for single mode fiber. 500 m for the new Low Cost Fiber. 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) The new Low Cost Fiber (plastics?) standard. Q. I've hear of FDDI over Copper, what type of cable does this scheme use? Type 1 STP - distance between connections must be less than 100 m Category 5 UTP - distance between connections must be less than 100 m (The ANSI standard for STP and UTP is incomplete, but a number of companies are already shipping proprietary twisted-pair solutions until the standard is completed, which is expected later this year.) ?Q. Is there any advantage to seperating the fiber pairs (will the ring work better if only one strand is broken on a DAS connection?) Q. I have ethernet, can I bridge/route between the 2 topologies? Yes. But from what we are hearing some protocols are having problems. Only TCP/IP is handling frame fragmentation correctly. (See below). 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 has a max size of 4500 bytes and Enet only 1500 bytes. Therefore your bridge or router needs to be smart enough to fragment the packets (eg 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? PA Preamble (II) (8 or more Idle symbol pairs) SD Starting Delimiter (JK) (J followed by K control symbol) FC Frame Control (nn) (Tell you if it is a token, MAC frame, LLC frame, SMT frame, frame priority, sync or async) DA Destination Address (nn) (6 bytes of MAC Address in MSb first format) SA Source Addrewss (nn) (6 bytes of MAC Address of this station) INFO Information field (nn) (Varibale Length. Usually starts with LLC header, then SNAP field, then the payload eg IP packet) ED Ending Delimiter (T) (one T control symbol) FS Frame Status (EAC) (Three symbols of status of Error, Address_match, and Copied. Each symbol is either SET or RESET. eg If EAC == RSS, then then 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 is 100 Megbits per second, what is the practical maximum bps? Depends. You can get aggregate usage up to 95Mbit/s with no problem. But 75Mbps is pretty good. Actually, this question depends so much on how you construct your test, what equipment you use, etc, that the best idea is to let the user decide. Q. What happens when I bridge between a 100 Mbps FDDI and a 10Mbps 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. Q. What is the latency across a bridge/router? (Yes I know that different vendors are different, but what is a the window?) No idea. Q. Are there FDDI repeaters? Yes. But it is not a standard yet. A group in the ANSI committee is looking into making FDDI repeater a standard. Other companies like ODS has something like simgle mode to multimode converter. Q. What type of test and trouble shooting equipment is available for FDDI? Digital Technology Inc (DTI), W&D, HP, and Tekelec all sell FDDI analyzers. The Sniffer from Network General also has a module that works with the NPI FDDI Cards. SGI has a nice looking ringmap program. IBM has a product called DatagLANce. Most Ethernet tools will also work with FDDI in the protocol level. Also a optical time domain reflectometer (TDR) is recommended for db loss checking and distance measurements, though it has been reported that an FDDI link tester is less expensive and will do the job. Q. What about network station management? Does FDDI support SNMP? Yes. There is a FDDI-SNMP MIB translation from the SNMP working group. Q. What is a beaconing ring? Does FDDI beacon? Beacon is a special frame that FDDI MAC sends when something is very wrong. When Beaconing for a while, SMT will kick in trying to detect and solve the problem. Q. How about interoperability, does one manufacture'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 at InterOp, UNH or ANTC, are this is the equipment that will meet the minimum interoperability requirements. Ask the vendor what type of testing they did and ask them to ship you a system for field trial before you pay big bucks for it. 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? Yes. 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 ist the negotiated target token rotation time) Usually this won't happened. It is only a very very heavily loaded ring but the station be waiting for that long. If this is the case, then change the T_request of the station to some lower value (eg 8 msec). Q. Can I bridge/route TCPIP, SNA, Novell, Sun protocols, DecNet, Banyan Vines, Appletalk, X windows, LAT? Yes for IP, Novell, DecNet, X windows. Don't know about the others. 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 100M bit/sec, 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 propogation 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 the DAS is connected to a concentractor, then the concentrator will also bypass the DAS. If the 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 rign will be badly segmented. Now if the 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 the fiber on a DAS or SAS connection? SAS connecting to concentrator: Same as above. DAS dual-home to a concentrator: If A-port fiber breaks, no effect on B port since A port is a backup port. (And SMT will NOT send out alert msg.) If B-port fiber breaks, A-port will kick in, complete PCM and be used as the primary connection. DAS on trunk rings, with no optical bypass: If one fiber breaks, then the ring will wrap. If both fibers break, ring will wrap, station won't be communicate. DAS on trunk rings using optical bypass: If one fiber between bypass and the next station breaks, then the ring will wrap. If both fibers between bypass and the next station break, ring will wrap, station won't be able to communicate. If one fiber between bypass and the host station breaks, then the ring will wrap. If two fiber between bypass and the host station breaks, then the ring will wrap. Q. What is one recommended topolpgy? Connect backbone concentrators and ring monitors to the trunk rings, and connect all the workgroup concentrators and users stations to the backbone concentrators. Connect bridges and routers 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) in a concentrator without losing any data frames (and not going into Ring_Non_Op mode). The theory goes as Graceful Insertion can minimize ring non_op and losing frame, therefore it saves you transmission timeout of lost frame in upper layer protocol (eg TCP) and retransmission effort. The following is the counter argument: Graceful Insertion can hold up the ring for more time that the FDDI ring non-op recovery time. And Upper layer protocol is designed to perform frame recovery and retransmission anyway. And no vendor can gaurantee 100% Graceful Insertion anyway. Should I get Graceful Insertion in my concentrators? If it is free, take it. You are going to get ring_op no matter what (eg insertion in the trunk ring and station power down). Q. Is there a Graceful De-insertion? No. Q. Can you name a few FDDI Concentrator vendors? IBM, Optical Data System, SynOptics, Cabletron, DEC, Chipcom, NPI, Synernetics, 3Com, Interphase, Ungermann-Bass, Timeplex, Crescendo/Cisco, Sumitomo etc ... (vendors feel free to email me to be included here) Q. Can I run FDDI on electrical cable? DEC is already sell a FDDI link that runs on coax. ANSI is currently finishing up the TP-FDDI Standard for running FDDI on twisted-pair media (Category 5 Cable). ANSI is also working on a standard (long term TP working group) to run FDDI on telephone cable. [Please comment.] IBM and a group of vendors (SynOptics, National Semiconductor ...) promote SDDI that runs FDDI on Shielded Twisted-Pair cable. (this is incomplete), there is much work being done on FDDI over various types of electrical cable, most notably twisted pair. 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 need to have SMT. The latest version of the SMT standard is version 7.3, but most vendors ship products with SMT version 6.2. Q. Who supports FDDI-II? National Semiconductor Corp, IBM, Apple Computer, XDI, Alpha Inc, etc Q. Who is working on Synchronous frame type utilitization? Alpha, IBM, and many more companies. Try to contact scoop4@aol.com and warren@lgevm2.vnet.ibm.com. They are working with a group of companies to define the usage of SYNC frame in FDDI-I rings. Q. Can I connect two Single attach stations together and form a two stations 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 basically the fiber optic connector on the card. FDDI SMT defines 4 types of ports (A, B, M, S). A dual-attach 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 the concentrators. A and B-port on DASs can also connect to the M-port of concentrator. But M-ports of the concentrator will not connect to each other. In more detail, SMT suggested tthe following rules: A B M S A - + + - B + - + - M + + X + S - - + - ==> '+' is the preferred connection ==> '-' connection has possible problems, and a vendor can choose to disable that connection in the default configuration ==> 'X' indicates a legal connection and will be rejected .