The DFG480XX Modules provide a GBIC opening for insertion of industry-standard GBICs to provide
Gigabit (Gb) media flexibility. GBIC models are available for both multi-mode (550m) and single-
mode 20 and 40 km fiber options, and for Gigabit copper as well, with new models appearing often.
The 1000Mb Gigabit fiber-port modules on the DFG48000s are normally set (factory default) to
operate at AUTO mode for best fiber distance and performance. The 1000Mbps SC fiber-optic module
on the Gigabit -SX and Gigabit-LX 1@ 1000Mb Fiber SC + 4@10/ 100Mb transceivers are compatible
with the IEEE 802.3z Gigabit standards.
There are two LEDs mounted on each Gigabit port module. The Gigabit fiber port is by default
configured at AUTO mode, and can be fix at 1000Mbps speed full duplex mode at any times and has
LEDs that indicate LK (Link status) and ACT (receiving activity) on that port when lit.
2.2.9 Packet Prioritization, 802.1p QOS
Quality of Service means providing consistent predictable data delivery to users from datagram paths
that go all across a network. As a LAN device, the DFG48000 can do its part to prevent any QOS
degradation while it is handling Ethernet traffic through its ports and buffers. DFG48000 switching
hardware supports the IEEE 802.1p standard and fulfills its role in support of QOS, giving packet
processing priority to priority tagged packets according to the 802.1p standard. In addition to
hardware support for QOS, the MNS software (R2) supports two priority queues that can be shared
across the eight levels of defined packet priorities for application-specific priority control by the user
through software configuration settings.
2.2.10 Frame Buffering and Flow Control
DFG48000’s 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
which 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. When the DFG48000 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. Another feature implemented in
DFG48000 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 to stop incoming traffic.