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ZL50418
Data Sheet
32
Zarlink Semiconductor Inc.
6.2.4 TxQ Manager
First, the TxQ manager checks the per-class queue status and global reserved resource situation and using this
information makes the frame dropping decision after receiving a switch response. If the decision is not to drop, the
TxQ manager requests that the FCB manager link the unicast frame’s FCB to the correct per-port-per-class TxQ. If
multicast, the TxQ manager writes to the multicast queue for that port and class. The TxQ manager can also
trigger source port flow control for the incoming frame’s source if that port is flow control enabled. Second, the TxQ
manager handles transmission scheduling; it schedules transmission among the queues representing different
classes for a port. Once a frame has been scheduled, the TxQ manager reads the FCB information and writes to
the correct port control module.
6.3 Port Control
The port control module calculates the SRAM read address for the frame currently being transmitted. It also writes
start of frame information and an end of frame flag to the MAC TxFIFO. When transmission is done, the port control
module requests that the buffer be released.
6.4 TxDMA
The TxDMA multiplexes data and address from port control and arbitrates among buffer release requests from the
port control modules.
7.0 Quality of Service and Flow Control
7.1 Model
Quality of service is an all-encompassing term for which different people have different interpretations. In general,
the approach to quality of service described here assumes that we do not know the offered traffic pattern. We also
assume that the incoming traffic is not policed or shaped. Furthermore, we assume that the network manager
knows his applications, such as voice, file transfer or web browsing and their relative importance. The manager can
then subdivide the applications into classes and set up a service contract with each. The contract may consist of
bandwidth or latency assurances per class. Sometimes it may even reflect an estimate of the traffic mix offered to
the switch. As an added bonus, although we do not assume anything about the arrival pattern, if the incoming traffic
is policed or shaped we may be able to provide additional assurances about our switch’s performance.
Table 8 shows examples of QoS applications with three transmission priorities, but best effort (P0) traffic may form
a fourth class with no bandwidth or latency assurances. Gigabit ports actually have eight total transmission
priorities.
Goals
Total Assured
Bandwidth (user
defined)
Low Drop Probability
(low-drop)
High Drop Probability
(high-drop)
Highest transmission
priority, P3
50 Mbps
Apps: phone calls, circuit
emulation.
Latency: < 1 ms.
Drop: No drop if P3 not
oversubscribed.
Apps: training video.
Latency: < 1 ms.
Drop: No drop if P3 not
oversubscribed; first P3 to
drop otherwise.
Table 7 - Two-Dimensional World Traffic