Cabletron Systems 100BASE-FX, Руководство по кабельной продукции

Страница: 49 / 192

Connector Types 4-25
Ethernet Media
The EIA/TIA 568B specification reverses the arrangement of Pair 1 and Pair 2, but
does not change the association of pairs within the cable. The Universal Service
Order Code, or USOC, a standard used for Token Ring network installations or
some telephone wiring, uses a different pair association than EIA/TIA 568A. The
USOC standard will cause a split pair condition in an IEEE 10BASE-T
environment, causing a loss of network functionality. For further information on
the differences between the standards, refer to the USOC and EIA/TIA
specifications.
RJ21 (Telco)
The RJ21 or “Telco” connector is another standard 10BASE-T connector type. The
RJ21 connector is a D-shaped metal or plastic housing that is wired and crimped
to a UTP cable made up of 50 wires, a 25-pair cable. The RJ21 connector can only
be plugged into an RJ21 port. The connector itself is sizable, and the cables that it
connects to are often quite heavy, so the RJ21 relies on a tight fit and good cable
management practices to keep itself in the port. Some devices may also
incorporate a securing strap that wraps over the back of the connector and holds
it tight to the port.
Figure 4-17. The RJ21 Connector
The RJ21 is used in locations where 25-pair cable is being run either to stations or
to an intermediary cable management device such as a patch panel or
punchdown block. Due to the bulk of the 25-pair cable and the desirability of
keeping the wires within the insulating jacket as much as possible, 25-pair cable is
rarely run directly to Ethernet stations.
The RJ21 connector, when used in a 10BASE-T environment, must use the
EIA/TIA 568A pinout scheme. The numbers of the RJ21 connector’s pins are
detailed in Figure 4-18, below. The actual association of the wire colors into pairs
and the organization that these pairs may use to connect to a punchdown block
are discussed in the Cabling Types portion of this chapter.
25-Pair Cable
Contact Pins
1845n17