Upgrading And
Migrating From Ethernet To Fast Ethernet
Introduction
Here we are
going to analyse the following aspects of
upgrading/migrating from 10Mbit Ethernet to 100Mbit
Ethernet.
-
Incompatible Implementations
-
Repeaters In Fast Ethernet
-
Replacement Of Illegal Byte
-
Codes Data Translation
-
Error Handling And
Partitioning
Cabling
There are
two methods of running Fast Ethernet over UTP and one
method of running it over fibre.
IMPLEMENTATION
..........CABLE
TYPE...............
NUMBER OF PAIRS
..100BASE-TX
................
Category 5
.........................2
..100BASE-T4..................Category
3 or 5................. ..4
..100BASE-FX..................
Fiber.......................
(Not Applicable)
Category 3
cabling is not rated to carry the fast signaling of
100BASE-TX, so 100BASE-T4 must be used. 100BASE-T4 may
also be used on Category 5 cabling, but 100BASE-TX is
probably a better choice.
Incompatible Implementations
Fast
Ethernet brings a new urgency to an old problem. Many
network technologies use RJ-45 connectors. In the past,
it was usually not difficult to figure out whether a
jack was Ethernet or token ring: even at a site where
both were in use they seldom were found in the same
vicinity, so the network administrator could make an
"educated guess". Today, with Fast and classic Ethernet
interspersed and 10/100 cards common, some mechanism is
needed to allow quick identification of the signal that
is running across the wire.
Autonegotiation works by having each end of the
connection send a series of pulses down the wire to the
other end. These pulses are the same signals used in
10Base-T to test link integrity and cause the link
indicator light to turn on. If a station receives a
single pulse, referred to as a Normal Link Pulse (NLP),
it recognizes that the other end is only capable of
10Base-T.
If
autonegotiation is being used, a station will transmit a
series of these pulses spaced closely together, referred
to as a Fast Link Pulse (FLP). An FLP consists of 17
"clocking" pulses interspersed with up to 16 "signal"
pulses to form a 16-bit code word. If a signal pulse
occurs between two clocking pulses, that bit is a one.
Absence of a signal pulse is a zero.
By
comparing the 16-bit code words received in the FLP, a
station and hub will agree on what implementation of
Ethernet to use. The 16-bit code word describes what
implementations of Ethernet are supported. Both station
and hub will compare what it supports to what the other
end supports, then choose which implementation to use
for that link according to following priorities, defined
by IEEE 802.3 clause 28B.3:
100BASE-TX full duplex
100BASE-T4
100BASE-TX 1
10BASE-T full duplex
10BASE-T
If the
station supports 100BASE-T4, 100BASE-TX, and 10BASE-T
and the hub supports full duplex 100BASE-TX,
single-duplex 100BASE-TX, and 10BASE-T, they will each
discover that the Ethernet implementations they have in
common are 100BASE-TX and 10BASE-T. Since 100BASE-TX is
defined to have a higher priority that 10BASE-T, the
station and hub will use 100BASE-TX. This decision takes
place independently on each side of the link, but since
each side uses the same decision-making process and
priorities, the same decision is reached on each end.
Because each end of the connection agrees on what
implementation of Ethernet is being used, the potential
problem of incompatible signaling is averted.
Repeaters In Fast Ethernet
In Fast
Ethernet the number of repeaters allowed per network
segment is only 1 or 2. Whether one or two repeaters may
be used is determined by what class of repeater will be
used on the segment. Two classes of Fast Ethernet
repeater are defined, Class I and Class II. Only one
Class I repeater can be used in a single collision
domain. Two Class II repeaters are allowed in a single
collision domain, with up to a 5 metre inter-repeater
link between them. The only technical difference between
Class I and Class II repeaters is that Class II
repeaters are faster than Class I repeaters. This allows
Class I repeaters to provide other services besides
simple repeating, such as translating between 100BASE-TX
and 100BASE-T4. Class II repeaters are primarily used to
link two hubs supporting only a single implementation of
Fast Ethernet.
However,
with the trade-off in fewer repeaters comes greater
intelligence in each repeater. In addition to
implementing the functionality of 10Mbps repeaters,
100Mbps repeaters are responsible for the following:
Replacement Of Illegal Byte
Unlike
classic Ethernet, Fast Ethernet does not send a
straightforward representation of the actual bits across
the physical layer. A different representation of the
information is sent instead. As a result, there are
possible patterns on the wire which are not defined for
use in Fast Ethernet. If a repeater detects an illegal
pattern on the wire, it may replace that pattern (and
every remaining pattern in the frame) with a special
symbol identifying that the frame is corrupt.
Codes Data Translation
For
repeaters that implement more than one implementation of
Ethernet, the repeater will change the data encoding to
be appropriate to the outgoing ports. 100BASE-T4 and
100BASE-TX use very different representations when
sending data across a network. A Class I repeater which
implements both 100BASE-TX and 100BASE-T4 needs to
ensure that the signal going across the wire is the
appropriate representation for the Ethernet
implementation.
Error handling and partitioning
A Fast
Ethernet repeater will monitor the state of each port in
order to protect the network from any faults that might
interrupt the flow of information.
If 60
consecutive collisions are detected from any particular
port, the repeater will partition that port: it will
stop forwarding information from that port to the rest
of the network, but will still continue to repeat all
frames from the network to the port. If the station on
that port has broken so that it no longer is obeying the
rules of CSMA/CD, then it needs to be separated from the
network to allow traffic to flow.
However, it
is possible that there could be 60 consecutive
collisions on an extremely busy segment, so the repeater
still forwards information to that port. If the repeater
detects between 450 and 560 bits of information from
that port without a collision occurring, the repeater
will re-activate that port. A legal frame is received
from the partitioned port, so we know that the hardware
is working.
If between
40000 and 75000 consecutive bits are received from a
port, the device at the other end of that cable is
assumed to be "jabbering", sending an endless stream of
bits, so the output from the port is cut off from the
rest of the network. Such a "jabbering" device could
prevent any traffic from flowing on a network, since
there would never be a break for the other stations to
transmit. If the station stops "jabbering", then the
repeater will once again activate the port.
In
100BASE-TX and 100BASE-FX, a repeater will further
monitor traffic to ensure that only frames with a valid
preamble are passed. If two consecutive "false carrier
events" occur, or a "false carrier event" lasts for
450-500 bits, the repeater will declare that link to be
"unstable" and stop sending information to that port. As
a result, faulty links are isolated from the rest of the
network, resulting in improved overall network
reliability. The link will be reactivated if between
24814 and 37586 bit-times have passed without any
information having been received, or if a valid carrier
is received after between 64 and 86 bit-times of idle
have occurred. |
