3Com 4007, Руководство по внедрению, Страница: 149 / 684

Key Concepts 149
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PACE Interactive Access — An algorithm that controls traffic flow on
a point-to-point link with an end station. In a typical half-duplex
Ethernet connection, you can never achieve high rates of utilization
because of the randomness of collisions. If a switch and end station
both try to send data, a collision occurs, forces retransmission, and
lowers link utilization.
PACE Interactive Access enables higher link utilization by altering the
switch’s
back-off behavior. Instead of continuing to send data after
winning a collision, the switch waits, allows the end station to send a
packet, and then retransmits. The result is an interleaving of
transmissions between the end station and the switch.
This feature avoids repetitive collisions and prevents an end station
from “capturing” the link. (With conventional Ethernet, a packet
collision can cause the last station that transmitted successfully to
monopolize Ethernet access and cause delays.)
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Network areas — 3Com uses a three-tiered framework to describe
the different functional areas in a LAN:
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Wiring closet — This area provides connections to user
workstations. It also includes downlinks into the data center or
campus interconnect.
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Data center — This area receives connections from wiring closets
and campus interconnect areas. Most local server farms reside
here.
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Campus interconnect — This area appears as a separate location
only in larger networks; smaller networks usually have only wiring
closets and data centers. The campus interconnect links campus
data centers to each other. It may also include an enterprise server
farm and connections to a wide area network.