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 Acredited
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.
TOP
Q.
What is the name of the standards?
- FDDI STANDARDS (if year
is listed as 199x it is
still in approval)
- Media Access Control
(MAC), ANSI X3.139-1987,
ISO 9314-2:1989
- Physical Layer Protocol
(PHY), ANSI X3.148-1988,
ISO 9314-1:1989
- Physical Layer, Medium
Dependent (PMD), ANSI
X3.166-1990, ISO 9314-3:1990
- Station Management
(SMT), X3.229-1994,
ISO 9314-6:199x
- Single Mode Fiber
PMD (SMF-PMD), ANSI
X3.184-1993, ISO 9314-4:199x
- Low Cost Fiber PMD
(LCF-PMD), X3.237-1995,
ISO 9314-9:199x
- Twisted Pair PMD (TP-PMD),
X3.263.1995, ISO 9314-10:199x
- Physical Layer Repeater
(PHY-REP), X3.278-199x
- Conformance Test PICS
Proforma for FDDI (CT-PICS),
X3.262-199x, ISO 9314-13:199x
- Abstract Test Suite
for MAC (MAC-ATS), X3.245-199x,
ISO 9314-26:199x
- Abstract Test Suite
for PHY (PHY-ATS), X3.248-199x,
ISO 9314-21:199x
- Abstract Test Suite
for PMD (PMD-ATS), X3.255-199x,
ISO 9314-20
- Abstract Test Suite
for SMT (SMT-ATS), X3T9.5/92-102,
Rev 1.4
- FDDI-II STANDARDS
- Hybrid Ring Control
(HRC), ANSI X3.186-1993,
ISO.IEC 9314-5:199x
- Enhanced Media Access
Control (MAC-2), X3.239-1994,
ISO 9314-8:199x
- Enhanced Physical
Layer Protocol (PHY-2),
X3.231-1994, ISO 9314-7:199x
- Enhanced SMT Common
Services (SMT-2-CS),
X3.257-199x
- Enhanced SMT Packet
Services (SMT-2-PS),
X3.259-199x
- Enhanced SMT Isochronous
Services (SMT-2-IS),
X3.258-199x
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.
- ANSI standards (published
FDDI standards)
- American National
Standards Institute
- 1430 Broadway, New
York, NY 10018, USA
- Attention: Sales Dept.
- Standards and X3T9.5 &
X3T12 Documents
- Global Engineering
Documents
- 800-854-7179 (USA)
- 303-792-2181
- 303-792-2192 (Fax)
- FDDI also references 802.1
and 802.2 standards
- IEEE Standards
- IEEE Service Center
- 445 Hoes Lane, Piscataway,
NJ 08855, USA
Q.
What are other good sources
of printed information?
- FDDI Handbook By Raj Jain
Its ISBN # is 0-201-56376-2.
- 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.
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.
TOP
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.
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.
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. ie:
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.
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.)
TOP
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.
Disadvatages: 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
reciver will usually increase.)
Q.
What are the minimum/maximum
distances between stations?
- No min, 2 km max for stations
with the original multimode
interface (PMD).
- 40-60 km max is possible
for single mode fiber (SMF-PMD).
An optical attenuator will
be necessary if high power
(Category II lasers are
used on a short link).
- No min, 500 m for the
new Low Cost Fiber on multimode
(LCF-PMD).
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.
TOP
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.
- There is no minimum distance
for either of these TP-PMD
links.
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?
Yes.
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 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 |
SD |
FC |
DA |
SA |
INFO |
ED |
FS |
Symbol |
Meaning |
Definition |
| PA |
Preamble (II) |
(8 or more Idle symbol
pairs at initial transmission) |
| 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 Address (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: Error, Address_match,
and Copied. Each symbol
is either SET or RESET.
eg 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 Megbits
per second, what is the practical
maximum bps?
Depends. You can get aggregate
usage greater than 95Mbit/s
in most installations. As with
any LAN, high utilization corresponds
to longer queuing times. Many
systems run fine at 75Mbps.
The maximum for a station is
implementation dependent. Some
can't do more than 20 Mpbs,
while others can sustain more
than 90Mbps.
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, or a lightly loaded
ethernet to a heavly 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.
TOP
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 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 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?
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.
TOP
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 (eg 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 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 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.
- SAS connecting to concentrator:
- DAS dual-homed to concentrator(s):
- If A-port link breaks,
usually no effect since
A port is generally
the backup port. (And
SMT will NOT send out
alert msg.)
- If B-port fiber breaks,
A-port will be joined
into the ring after
a short disruption.
- DAS on trunk rings, with
no bypass:
- If one link breaks,
then the ring will wrap.
- If both links break,
ring will wrap, station
won't be able to communicate.
- DAS on trunk rings, with
bypass:
- When a bypass is switched
it breaks its connections
before making the new
ones. This causes a
temporary wrap when
either inserting or
removing a station from
the ring. The A and
B ports of the inserting
station will usually
be added serially, though
it may appear to be
simultaneous.
- If any one of the
four link entering the
bypass breaks, the ring
will wrap, living the
bypass station in the
ring.
- If both links between
the bypass and its neighboring
stations break, or both
links between the bypass
and its station break,
the ring will wrap,
and the station connected
to the bypass won't
be able to communicate.
TOP
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 (eg 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
gaurantee 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?
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 topoligies.
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 |
| A |
B |
S |
M |
| This Port |
| A |
V,U |
V |
V,U |
V,W |
| B |
V |
V,U |
V,U |
V,W |
| S |
V,U |
V,U |
V |
V |
| M |
V |
V |
V |
I,U |
where:
- V indicates a valid connection
- I indicates an illegal
connection
- U indicates an undesirable
connection with notification
to SMT required
- W indicates, if Active,
prevent THRU in CFM and
Port B takes precedence
TOP
Q.
What is the difference between
FDDI and FDDI-II?
Both FDDI and FDDI-II run at
100 M bits/sec 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 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 (FDDI)
mode.
There is very little FDDI-II
product available on the market. |
