An FDDI concentrator
(also called a dual-attachment
concentrator [DAC]) is
the building block of an FDDI
network. It attaches directly
to both the primary and secondary
rings and ensures that the failure
or power-down of any SAS does
not bring down the ring. This
is particularly useful when
PCs, or similar devices that
are frequently powered on and
off, connect to the ring. Figure
4 shows the ring attachments
of an FDDI SAS, DAS, and concentrator.
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Figure 4: A concentrator
attaches to both the primary
and secondary rings.
FDDI
Fault Tolerance
FDDI provides a
number of fault-tolerant features.
In particular, FDDI's dual-ring
environment, the implementation
of the optical bypass switch,
and dual-homing support make
FDDI a resilient media technology.
Dual Ring
FDDI's primary fault-tolerant
feature is the dual ring. If
a station on the dual ring fails
or is powered down, or if the
cable is damaged, the dual ring
is automatically wrapped (doubled back onto itself) into
a single ring. When the ring
is wrapped, the dual-ring topology
becomes a single-ring topology.
Data continues to be transmitted
on the FDDI ring without performance
impact during the wrap condition.
Figure 5 and Figure 6 illustrate
the effect of a ring wrapping
in FDDI.
Figure 5:
A ring recovers from a station
failure by wrapping.
Figure 6:
A ring also wraps to withstand
a cable failure.
When a single station
fails, as shown in Figure 5,
devices on either side of the
failed (or powered down) station
wrap, forming a single ring.
Network operation continues
for the remaining stations on
the ring. When a cable failure
occurs, as shown in Figure 6,
devices on either side of the
cable fault wrap. Network operation
continues for all stations.
It should be noted
that FDDI truly provides fault-tolerance
against a single failure only.
When two or more failures occur,
the FDDI ring segments into
two or more independent rings
that are unable to communicate
with each other.
Optical
Bypass Switch
An optical bypass
switch provides continuous dual-ring
operation if a device on the
dual ring fails. This is used
both to prevent ring segmentation
and to eliminate failed stations
from the ring. The optical bypass
switch performs this function
through the use of optical mirrors
that pass light from the ring
directly to the DAS device during
normal operation. In the event
of a failure of the DAS device,
such as a power-off, the optical
bypass switch will pass the
light through itself by using
internal mirrors and thereby
maintain the ring's integrity.
The benefit of this capability
is that the ring will not enter
a wrapped condition in the event
of a device failure. Figure
7 shows the functionality of
an optical bypass switch in
an FDDI network.
Figure 7: The optical
bypass switch uses internal
mirrors to maintain a network.
Dual Homing
Critical devices,
such as routers or mainframe
hosts, can use a fault-tolerant
technique called dual homing to provide additional redundancy
and to help guarantee operation.
In dual-homing situations, the
critical device is attached
to two concentrators. Figure
8 shows a dual-homed configuration
for devices such as file servers
and routers.
Figure 8: A dual-homed
configuration guarantees operation.
One pair of concentrator
links is declared the active
link; the other pair is declared
passive. The passive link stays
in back-up mode until the primary
link (or the concentrator to
which it is attached) is determined
to have failed. When this occurs,
the passive link automatically
activates.
FDDI Frame Format
The FDDI frame format
is similar to the format of
a Token Ring frame. This is
one of the areas where FDDI
borrows heavily from earlier
LAN technologies, such as Token
Ring. FDDI frames can be as
large as 4,500 bytes. Figure
9 shows the frame format of
an FDDI data frame and token.
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Figure 9:
The FDDI frame is similar to
that of a Token Ring frame.
FDDI Frame Fields
The following descriptions
summarize the FDDI data frame
and token fields illustrated
in Figure 9.
Preamble---A unique
sequence that prepares each
station for an upcoming frame.
Start Delimiter---Indicates
the beginning of a frame by
employing a signaling pattern
that differentiates it from
the rest of the frame.
Frame Control---Indicates
the size of the address fields
and whether the frame contains
asynchronous or synchronous
data, among other control information.
Destination Address---Contains
a unicast (singular), multicast
(group), or broadcast (every
station) address. As with Ethernet
and Token Ring addresses, FDDI
destination addresses are 6 bytes
long.
Source Address---Identifies
the single station that sent
the frame. As with Ethernet
and Token Ring addresses, FDDI
source addresses are 6 bytes
long.
Data---Contains
either information destined
for an upper-layer protocol
or control information.
Frame Check Sequence
(FCS)---Filed by the source
station with a calculated cyclic
redundancy check value
dependent on frame contents
(as with Token Ring and Ethernet).
The destination address recalculates
the value to determine whether
the frame was damaged in transit.
If so, the frame is discarded.
End Delimiter---Contains
unique symbols, which cannot
be data symbols, that indicate
the end of the frame.
Frame Status---Allows
the source station to determine
whether an error occurred and
whether the frame was recognized
and copied by a receiving station.
FDDI Frame Format
FDDI Frame
Frame Control
(FC): 8 bits
has bit format
CLFFZZZZ
C indicates
synchronous or asynchronous
frame
L indicates
use of 16 or 48 bit addresses
FF indicates
whether it is a LLC, MAC control
or reserved frame
in a control
frame ZZZZ indicates the type
of control
Destination
Address (DA): 16 or 48 bits
specifies station
for which the frame is intended
Source Address
(SA): 16 or 48 bits
specifies station
that sent the frame
Here
is what the FDDI frame format
looks like:
FDDI Frame
Format
PA - Preamble
16 symbols
SD - Start
Delimiter 2 symbols
FC - Frame
Control 2 symbols
DA - Destination
Address 4 or 12 symbols
SA - Source
Address 4 or 12 symbols
FCS - Frame
Check Sequence 8 symbols, covers
the FC, DA, SA and Information
ED - End Delimiter
1 or 2 symbols
FS - Frame
Status 3 symbols
Token is just
the PA, SD, FC and ED
Preamble
The Token owner as a minimum
of transmits the preamble 16
symbols of Idle. Physical Layers
of the subsequent repeating
stations can change the length
of the Idle pattern according
to the Physical Layer requirements.
Therefore, each repeating station
may see a variable length preamble
from the original preamble.
Tokens will be recognized as
long as its preamble length
is greater than zero.
If a valid token is received
and cannot be processed (repeated),
due to expiration of ring timing
or latency constraints the station
will issue a new token to be
put on the ring.
A given MAC implementation
is not required to be capable
of copying frames received with
less than 12 symbols of preamble;
Nevertheless, with such frames,
it cannot be correctly repeated.
Since the preamble cannot be
repeated, the rest of the frame
will not be repeated as well.
Starting
Delimiter
This field of the frame denodes
the start of the frame.
It can only have symbols 'J' and
'K'. These symbols will
not be used
anywhere
else but in the starting delimiter
of a token or a frame.
Frame Control
Frame Control field descibes what
type of data it is carrying inthe INFO field. Here
are the most common values that
are allowed in the FC field:
| 40: |
Void Frame. |
| 41,4F: |
Station Management (SMT) Frame. |
| C2,C3: |
MAC Frame. |
| 50,51: |
LLC Frame. |
| 60: |
Implementor Frame. |
| 70: |
Reserved Frame. |
Please
note that the list here are
only the most common values
that
can be formed
by a 48 bit addressedynchronous
data frames.
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Destination
Address
Destination Address field contains
12 symbols that identifies the station that is receiving
this particular frame. When
FDDI is first setup, each station
is given a unique address that
identifies themselves
from the others.
When a frame passed by the station,
the station will
compare its address against
the DA field of the frame.
If it is a match,
station then copies the
frame into its buffer area waiting
to be processed. There
is not restriction on the number
of stations that a frame can reach
at a time. If the
first bit of the DA field is set
to '1', then the address
is called a group or global address.
If the first bit is '0', then
the address
is called individual address. As the name
suggests, a frame with a global address setting
can be sent to multiple stations
on the network. If the frame
is intended for everyone on the network, the address bits
will be set to all
1's. Therefore, a global
address contains all 'F' symbols. There are also two different
ways of administer these addresses.
One's
called local and the other's called universal.
The second bit of the
address
field determine whether or not
the address is locally or
universally
administered. If the second
bit is '1' then it is locally
administered address. If
the second bit is a '0', then
it is universally
administered adress.A locally
administer address are addresses
that havebeen assigned
by the network administrator and
a universally administered
addresses
are pre-assigned by the manufacturer's
OUI.
Source Address<>
A Source Address identifies the
station that created the frame.
This field is used for remove
frames from the ring. Each
time
a frame is sent,
it travels around the ring, visiting
each station,and
eventually (hopefully) comes back
to the station that originally
sent that frame.
If the address of a station matches
the SA field inthe
frame, the station will strip
the frame off the ring.
Each stationis responsible
for removing its own frame from
the ring.
Information
Field
INFO field is the heart and soul
of the frame. Every components
of the
frame is designed around this
field; Who to send it to,
where's this
coming from, how it is received
and so on.The type of information
in the INFO field can be found
by looking in
the FC
field of the frame. For
example:'50'(hex)
denodes a LLC frame. So,
the INFO field will have a LLC header followed by
other upper layer headers.
For example SNAP,
ARP, IP, TCP, SNMP, etc.'41'(hex or '4F'(hex) denode
s SMT (Station Management) frame. Therefore,
a SMT header will appear in the
INFO field.
Frame
Check Sequence
Frame Check Sequence field is
used to check or verify the traversingframe for any bit errors. FCS information is generated by
the stationthat
sends the frame, using the bits
in FC, DA, SA, INFO, and FCSfields. To verify
if there are any bit errors in
the frame, FDDI uses 8 symbols (32 bits) CRC
(Cyclic Redundancy Check) to ensure
the
transmission of
a frame on the ring.
End Delimiter
As the name suggests, the
end delimiter denodes the end
of the frame. The ending delimiter
consist of a 'T' symbol.
This 'T' symbols indicates that
the frame is complete or ended.
Any data sequence that does
not end with this 'T' symbol
is not considered to be a frame.
Frame Status
Frame Status (FS) contains
3 indicators that dictates the
condition
of the frame.
Each indicator can have two values: Set ('S') or Reset ('R').
The indicators could possibly
be corrupted. In this case,
the
indicators
is neither 'S' nor 'R'.
All frame are initially set to
'R'.Three types
of indicators are as follows:Error (E):This indicator
is set if a station determines
an error for that frame.
Might be a CRC
failiure
or other causes. If a frame
has its
E indicator
set, then, that frame is discarded
by
the first station that encounters
the frame.Acknowledge(A):
Sometime this indicator is called
'address
recognized'.
This indicator is set whenever
a frame in properly received;
meaning the
frame has
reached its destination address.
Copy (C): This indicator is set
whenever a station is able to
copy the received frame into its
buffer section. Thus, Copy
and Acknowledge
indicators
are usually set at the same time.
But, sometimes when a station
is receiving
too many
frames and cannot copy all the
incoming frames.
If this happens, it would
re-transmit the frame with
indicator 'A' set indicator 'C'
left on reset. |
