Upgrading and Migrating from 10MB Ethernet to Fast Ethernet

There are three important issues to consider when upgrading a network to Fast Ethernet.


Cabling

Cabling has been mentioned before in this series of essays, but it bears restating. There are two methods of running Fast Ethernet over UTP, and one method of running it over fiber.

Implementation Cable Type Number of Pairs
100BASE-TX Category 5 2
100BASE-T4 Category 3 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 Ethernet cards common, some mechanism is needed to allow quick identification of what signal is running across the wire. That mechanism is provided by the IEEE specified in 802.3u section 28.

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
  • 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 repeats 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 meter 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:

  • Illegal code replacement.
    • 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.

  • 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 either 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.

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