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Black Box Modular Switches
Installation and User Guide (8/99)
10
The RJ-45 ports run at 10/100Mbps with N-way auto-negotiation capability, whereas
the fiber port runs at 100Mbps with half
- or full-duplex capability manually selected. The
default condition is full
-duplex. Internal jumpers settings allow technicians to set the 100Mb
fiber port to half
-duplex mode. (See Section 3.4).
On LE14X
XA Combo 4-port modules, there are three LED’s for each RJ-45 port, which indicate
status as described for the LE1426C in Section 2.2.4 above. The fiber port will run at 100Mbs
speed at all times, and has LEDs that indicate status the same way as described
for the Fiber 4-
port modules in Section 2.2.2 above.
2.2.6
Frame Buffering and Latency
The Black Box LE14XXA-Series are store-and-forward switches. Each frame (or
packet) is loaded into the Switch’s memo
ry 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.,
are good packets. This eliminates the 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 out to the Ethernet segments
connected. The "cut
-through" technique permits collision fragment frames, which are a result of
late collisions, to be forwarded to add to the network traffic. Since there is no way to filter
frames with a bad CRC (the entire frame must be present in order for CRC to be calculated), the
result of
indiscriminate cut-through forwarding is greater traffic congestion, especially at peak
activity. Since collisions and bad packets are more likely when traffic is heavy, the result of
store
-and-forward operation is that more bandwidth is available for good packets when the traffic
load is greatest.
To minimize the possibility of dropping frames on congested ports, each Black-Box
LE14XXA
-Series Switches dynamically allocates buffer space from an 8 MB memory pool,
ensuring that heavily used ports receive ver
y large buffer space for packet storage. (Many other
switches have their packet buffer storage space divided evenly across all ports, resulting in a
small, fixed number of packets to be stored per port. When the port buffer fills up, dropped
packets result
.) The other two LE14XXA-Series Switches LE1408A and LE1416A dynamically
allocates buffer from an 4MB memory pool. This dynamic buffer allocation provides the
capability for the maximum resources of the LE14XXA
-Series unit to be applied to all traffic
load
s, even when the traffic activity is unbalanced across the ports. Since the traffic on an
operating network is constantly varying in packet density per port and in aggregate density, the