Mitel MiVOICE BUSINESS, Инструкция по инженерии

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Engineering Guidelines
292
STRAIGHT AND CROSSOVER CABLES
Two types of cable connection are used to connect between network equipment devices and
also from the network equipment to the end equipment:
• Straight connection, used to connect end users to the network (for example, an IP phone
to a switch)
• Crossover connection, used to connect between network equipment (for example, between
switches)
The connections between devices contain pairs of wires to transmit data and to receive data.
The transmit and receive pairs must swap over to make a connection work, otherwise, transmit
connects to transmit, and no data passes. The switch ports in the network normally provide this
crossover. This means that the connection between end device and switch can use a straight
connection.
However, when switches within a network connect to each other there are two crossovers, thus
nullifying the effect. A crossover cable is needed for these connections. Alternatively, some
switches provide an additional port with the crossover removed, allowing a straight cable to be
used. Both physical ports on such a connection cannot be used simultaneously, otherwise, data
corruption occurs. In the following figure, the port labeled 5X would be used to connect to an
end device OR the port labelled 5 would be used to connect to an X port of another switch.
Figure 50: Straight and Crossover Port Example
Some switches provide auto crossover detection, so that straight connections can be used for
all connection leads.
IDENTIFICATION OF CONNECTION CABLES
Since a network includes a mix of straight and crossover cables, they need to be identified for
easier maintenance view. Identify each type with an additional marker, or label on the cable,
or use a color code to quickly identify cables (for example, white for connections to users, red
for crossover and inter-switch connections).
Test the cables to identify which cable is which type. Another simpler method is to look at the
color of the wires inside the RJ-45 plug. If the color order is the same in both plugs, then the
cable is a straight connection. If they are different, then it is a crossover cable. Be careful, since
other telecom functions, such as PRI, also use RJ-45 connections. In the following figure, for
the straight cable, the orange and green pairs are in the same position. For the crossover cable,
the orange and green pairs swap positions between the connectors.