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2–6 MULTILINK ML1600 ETHERNET COMMUNICATIONS SWITCH – INSTRUCTION MANUAL
FEATURES AND BENEFITS CHAPTER 2: PRODUCT DESCRIPTION
2.3 Features and Benefits
2.3.1 Packet Prioritization, 802.1p QoS
Quality of Service (QoS) means providing consistent predictable data delivery to users from
datagram paths that go all across a network. As a LAN device, the ML1600 can do its part
to prevent any QoS degradation while it is handling Ethernet traffic through its ports and
buffers.
The ML1600 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 ML1600 software
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.
2.3.2 Frame Buffering and Flow Control
The ML1600 is a store-and-forward switch. 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 ML1600 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 the ML1600 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, it prevents more frames from entering by forcing a collision signal on all
receiving half-duplex ports in order to stop incoming traffic.
2.3.3 MultiLink Switch Software
The ML1600 includes licensed software, allowing configuration of the ML1600 as a
managed switch.