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module is equipped with a Media Dependent Interface-Crossover (MDI-X) push-button switch to simplify
cascaded or up-link connections.
ProSwitch Quad-Series Fiber Switches
Installation and User Guide
2.2.5
Combo "3 + 1" Quad Port Modules, 3@RJ45 and 1@fiber
The combo quad-port modules are combination of copper and
fiber media, available as three 10/100-switched RJ45 copper ports and
one 100Mbps switched multimode fiber ST or SC or singlemode SC-type
port.
The RJ45 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 jumper settings allow technicians to set the 100Mbps fiber
port to half-duplex mode. (See Section 3.4).
On ProSwitch Combo Quad-port modules, there are four LED's for each RJ45 port, which
indicate status as described for the QPM-RJ45 in Section 2.2.4 above. The fiber port is fixed at 100Mbps
speed at all times, and has LEDs that indicate status the same way as described for the Fiber Quad-port
modules in Section 2.2.2 above.
2.2.6
Frame Buffering and Latency
The ProSwitch Quad-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 ProSwitch Quad-Series
QS5116 switches dynamically allocates buffer space from an 8MB 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 Q-Series Switches
QS580 and QS5116 dynamically allocates buffer from a 4MB memory pool. This dynamic buffer
allocation provides the capability for the maximum resources of the ProSwitch Quad-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
ProSwitch Quad-Series switches are constantly adapting internally to provide maximum network
performance with the least dropped packets.
When the switch detects that its free buffer queue space is low, the switch sends industry
standard (full-duplex only) PAUSE packets out to the devices sending packets to cause "flow control".
This tells the sending devices to temporarily stop sending traffic, which allows a traffic catch-up to occur
without dropping packets. Then, normal packet buffering and processing resumes. This flow-control
sequence occurs in a small fraction of a second and is transparent to an observer. See Section 4.6 for
additional details.
Another feature implemented in ProSwitch Quad-Series switches is a collision-based flow-control
mechanism (when operating at half-duplex only). When the switch detects that its free buffer queue
space is low, the switch prevents more frames from entering by forcing a collision signal on all receiving
half-duplex ports in order to stop incoming traffic.
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