Chapter 2: Introduction
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2.3
Fiber port, SC or LC Connector for Model LBH18x
The Fast Ethernet fiber port on the LBH18x is set to operate at fixed
100Mbps speed for guaranteed high performance. The LBH18x fiber port is factory-
built as either a multi-mode SC, LC or single-mode SC or LC connector. The
100Mbps fiber port will run at 100Mbps speed at all times. The 100Mbps fiber port is
a switched port and performs as a domain, providing a high bandwidth backbone
connection (no media converter is required!) and supporting long (up to 40km) fiber
cable distances for installation versatility.
On LBH18x units, there are two LED’s for the fiber port. One (LK/ACT) is
steady ON to indicate LINK, blinking indicates the port is transmitting and receiving.
The F/H indicates full-duplex when ON, when it is OFF, operation is half-duplex.
A fiber cable must be connected to the 100Mbps port and a proper link (LK
lit) must be made with the device at the other end of the cable in order for these LEDs
to provide valid indications of operating conditions.
2.4
Frame Buffering and Latency
The LBH08x/LBH18x 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
LBH08x/LBH18x Industrial Switch dynamically allocates buffer space from a 1MB
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.) This dynamic buffer
allocation provides the capability for the maximum resources of the LBH08x/LBH18x
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 Industrial Switches are
constantly adapting internally to provide maximum network performance with the
least dropped packets.
