Understanding the network.pdf - Back to Home

100VG-AnyLAN operates over Category 3, 4, and 5 UTP. It uses a star hub topology,
like 10Base-T's, but the hubs can cascade three hubs deep. 100VG-AnyLAN hubs
are deployed in a parent/child hierarchical topology, with one hub (the parent) from
which all other hubs cascade down. Each child hub must have one uplink port that
can reach the parent; the other hub ports can be used as downlink ports or for
end-station connections. Hubs and end-stations can be no more than 100m
(Category 3 UTP) or 150m (Category 5 UTP) apart. The maximum cable distance for
a Category 3 UTP network is 600m, or 900m if Category 5 UTP is used.
The reason for the parent/child topology is that, under the demand-priority MAC,
the parent hub controls end-station access to the network. The effect is that all the
connected hubs act as a single large repeater. When a station wants to transmit
data, it queries the hub. If the network is idle, the hub signals the requesting station
that it's clear to transmit. If more than one end-station needs to transmit data, a
round-robin mechanism alternates transmission access to the hub between the
requesting stations. The demand-priority MAC also provides a priority access
mechanism for special data types (such as real-time audio and video). When a
station makes a request to transmit one of the prioritized data types, the normal
access queue mechanism is overridden and they are permitted access to the
medium. Transmission access requests for these special priority types override the
processing of other kinds of traffic.
1000Base-X PHY Implementations
1000Base Ethernet (Gigabit Ethernet), for all intents and purposes, is 100Base
Ethernet (Fast Ethernet) on steroids. Like 10Base and 100Base Ethernet, at the
data link layer (OSI-RM Layer 2), the 1000Base implementation utilizes the
"standard" IEEE 802.3 MAC frame format, making it completely
backwards-compatible with pre-existing Ethernet implementations. To achieve this
"compatibility," however, certain adjustments need to be made to the 1000Base
Ethernet CSMA/CD mechanism in order to support segment lengths that could be
useful, in light of the greatly reduced slot-time.
In terms of the physical layer (OSI-RM Layer 1), architecturally, 100Base and
1000Base Ethernet are similar. With the most obvious being the retention of the RS
sublayer and the development of an updated variation of the Fast Ethernet MII,
called the Gigabit Media Independent Interface (GMII) to interact with the new PHY
data rate requirements. 1000Base Ethernet also maintains 100Base Ethernet's
Auto-Negotiation capabilities in an updated form to accommodate operation over
for fiber optic transmission mediums, Gigabit Ethernet's primary transmission
medium.
Where the difference between Gigabit and its predecessors is readily apparent is in
the 1,000Mbps transmission speed and in the actual PHY implementation. This (like