MVTX2801
Data Sheet
6.2.2 Rx Interface
The Rx interface is mainly responsible for communicating with the RxMAC. It keeps track of the start and end of
frame and frame status (good or bad). Upon receiving an end of frame that is good, the Rx interface makes a switch
request.
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, by quality of service assurances, we mean the allocation of chip resources so as to meet the latency and
bandwidth requirements associated with each traffic class. We do not presuppose anything 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
MVTX2801 must intelligently allocate resources so as to assure quality of service for high priority data.
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, though this is not required.
The table below shows examples of QoS applications with eight transmission priorities, including best effort traffic
for which we provide no bandwidth or latency assurances.
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