Black Box Modular Switches Manual De Usuario

Black Box Modular Switches           

        Installation and User Guide (8/99) 

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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 LE14XXA 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. 

The Black Box LE14XXA-Series 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., 

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 very 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 

loads, 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