Fig. 2.2c Hardened PXES6P-2SC
The
switch also includes a hardware operated Alarm terminal block for providing extra reliability
to the unit. The Alarm feature allows the PXES6P users to be aware and to monitor any internal
power failure. See section 4.4 for details.
On switches with the fiber option, the front side of the unit has four/five twisted-pair
10/100Mb switch ports and one/two 100Mb fiber port. All the
RJ-45 ports of the PXES6P Series switches support auto-cross
(MDIX) operation under auto-negotiation mode. The PXES6P
Series provides switching among four 10/100 auto-negotiating
copper ports and two 100Mb fiber ports which are ST multi-
mode.
Two sets of LEDs to indicate the operating status of ports are mounted on the top and
front (for extra viewing advantage while rack-mounted). For each port, there are Link and
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Activity (LK/ACT) LED’s on the top indicating that the
media cables are connected correctly and blink when there
is traffic. The LK/ACT LED’s are repeated on the front as
1 (port 1)…6 (port 6), whereas on the side as LA1…LA6.
There is another set of LEDs on the front for 10 or H and 100 or F, to indicate the data rate as
well as a set to indicate duplex for ports 1 and 2 only. 10/100 indicates the speed for copper
ports, whereas F/H indicates Full and Half duplex for fiber ports only. There is a power (PWR)
LED to indicate that the unit is turned ON.
The external DC power plug connector and/or “jack” and the internal DC input
terminal are provided on the rear of the unit.
Fig 2.2d shows PXES6P-2ST
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2.3
Frame Buffering and Latency
The PXES6P Series Switches are store-and-forward switches. Each frame (or packet)
is loaded into the Switch’s memory and inspected before forwarding can occur. This technique
ensures that all forwarded frames are of a valid length and have the correct CRC, i.e. they are
good packets. This eliminates propagation of bad packets, enabling all of the available
bandwidth to be used for valid information.
While other switching technologies such as "cut-through" or "express" impose
minimal frame latency, they will also permit bad frames to propagate to the Ethernet network.
The "cut-through" technique permits collision fragment frames, which are a result of late
collisions, to be forwarded--which add to the network traffic. There is no way to filter frames
with a bad CRC (the entire frame must be present in order for CRC to be calculated). Since
collisions and bad packets are more likely when traffic is heavy, store-and-forward switch
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