CHAPTER 2: INTRODUCTION
PRODUCT DESCRIPTION
MULTILINK ML810 MANAGED EDGE SWITCH – INSTRUCTION MANUAL
2–3
2.2.1
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 Multilink ML810 can do
its part to prevent any QOS degradation while it is handling Ethernet traffic through its
ports and buffers.
The Multilink ML810 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.
2.2.2
Frame Buffering and Flow Control
Multilink ML810’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 Multilink ML810 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 it
packets to cause “flow control”. This tells the sending devices to temporarily stop sending
traffic, which allows the traffic to catch-up 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 the Multilink ML810 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.
