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Article 2342 of comp.dcom.lans.fddi:
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From: [email protected] (BARR DOUG)
Subject: FAQ
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Date: Thu, 3 Mar 1994 21:56:32 GMT
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March. 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 [email protected]
Most of these answers are courtesy of:
-- Mike Yip [email protected]
Matt Holdrege [email protected]
Larry Stefani [email protected]
Ron Mackey [email protected]
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
- DEC's FDDI primer is now available through Digital Press. It is
called "FDDI: An introduction to Fiber Distributed Data Interfce"
Its ISBN # is 1-55558-093-9.
- 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 running FDDI over STP and UTP, "FDDI Twisted
Pair Physical Medium Dependent (TP-PMD)", is still a working draft,
but a number of companies started shipping TP-PMD products last year
using modular PMD's so that they can be updated if changes occur in
the standard prior to completion.)
?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 has 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.The IETF FDDI SNMP MIB working group completed work last year
on a new FDDI SNMP MIB based on ANSI SMT 7.3. The new MIB is called
RFC1512. The previous FDDI SNMP MIB (RFC1285) was based on ANSI
SMT 6.2.
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's IPX, Sun protocols, DecNet,
Banyan Vines, Appletalk, X windows, LAT?
Yes for IP, Novell, DECnet, X windows, also NetBEUI (LAN Manager).
DEC reports they can bridge Appletalk.
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,
Distributed Systems etc ...
(vendors feel free to email me to be included here)
Q. Can I run FDDI on electrical cable?
DEC already sells an 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.
Q. Who supports FDDI-II?
National Semiconductor Corp, and possibly some others?
Q. Who is working on Synchronous frame type utilitization?
Alfa, IBM, and many more companies. Try to contact
[email protected] and [email protected]. 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 an illegal connection and will be rejected
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