MVTX2803 查看數據表（PDF） - Zarlink Semiconductor Inc
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MVTX2803 Datasheet PDF : 127 Pages
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MVTX2803
Data Sheet
6.2.3 RxDMA
The RxDMA arbitrates among switch requests from each Rx interface. It also buffers the first 64 bytes of each
frame for use by the search engine when the switch request has been made.
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 (QoS) is an all-encompassing term for which different people have different interpretations. In
this chapter, quality of service assurances means the allocation of chip resources so as to meet the latency and
bandwidth requirements associated with each traffic class. There is nothing presupposed about the offered
traffic pattern. If the traffic load is light, then ensuring quality of service is straightforward. But if the traffic load
is heavy, the MVTX2803AG must intelligently allocate resources so as to assure quality of service for high
priority data.
The network manager must assign importance for the application types, such as voice, file transfer, or web
browsing. 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, though this is not required.
The table below shows examples of QoS applications with eight transmission priorities, including best effort
traffic for which no bandwidth or latency assurances are provided.
Class
Example
Assured Bandwidth
(user defined)
Low Drop Subclass
(If class is oversubscribed,
these packets are the last to
be dropped)
High Drop Subclass
(If class is oversubscribed,
these packets are the first
to be dropped)
Highest transmission
priorities, P7
Latency < 200 µs
300 Mbps
Sample application: control
information
Table 1 - Two-dimensional World Traffic
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