Understanding the network.pdf - Back to Home

etween different PHY encoding implementations. This enables manufacturers to
support all the 100Mbps media types (TX, FX, and 4T) on a single repeater. Using
one repeater does add an additional level of delay on what is already a
time-sensitive media platform, so only a single Class 1 repeater can exist within a
segment (collision domain). Some Class 1 repeater implementations provide an
interconnect bus to connect repeaters together so they appear as single repeaters
with a high port density.
Class 2 repeaters only support a single PHY implementation. This enables the
repeater to simply repeat the incoming signals to its ports, without the added delay
of translating different PHY encoding schemes. This limitation to one standard also
enables Class 2 repeaters to use the traditional 10Base-T hub-to-hub
interconnection model (which is, in part, why they were specified). The
interconnection cable is limited to a length of no more than 5m. Class 2 repeater
networks can only run two repeaters deep, which keeps them within the maximum
205m end-station-to-end- station segment distance.
100Base-T's small segment diameter makes the large-scale implementation of
shared 100Base-T impracticable. With the advent of 100Base-TX multiport bridges,
however, 100Base-TX became an extremely viable network technology. Fast
Ethernet switching coupled with 100Base-T's full-duplex operation eliminated the
segment distance limitations required for proper 100Mbps CSMA/CD functionality.
Today, most 100Base-T implementations are based on multiport bridge topologies.
This approach has effectively laid waste to the concept of using FDDI or ATM for LAN
backbone infrastructures.
100VG-AnyLAN
During the Fast Ethernet standards development process, there were two technical
models assessed. The first (by Grand Junction Networks) proposed using the
existing CSMA/CD access protocol with the FDDI PMD sublayer. The second (by
AT&T and Hewlett-Packard) proposed discarding the CSMA/CD access protocol and
replacing it with a new protocol called demand-priority. Demand-priority was
optimized to support the transmission of real-time multimedia applications. It did
this by using network access demand prioritization levels that are assigned to
different application types. The debate between the CSMA/CD-compatible and
demand-priority MAC architectures escalated to a holy war of sorts during the
standardization process. In the end, Demand-Priority did not make it, largely
because it was not compatible with existing Ethernet. It didn't fade away—it became
a new access method and a new IEEE working group. 802.12 was started and
developed as a separate standard, 100VG-AnyLAN. In addition to the replacement
of the CSMA/CD access protocol, the 802.12/100VG-AnyLAN standard can support
any MAC frame type (Token Ring, FDDI, or Ethernet).