zl50404 Zarlink Semiconductor, zl50404 Datasheet - Page 34

• Manufacturer Part Number: zl50404
• Description: Lightly Managed/unmanaged 5-port 10/100m Ethernet Switch
• Manufacturer: Zarlink Semiconductor

6.2.3
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
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. The dropping decision includes
the head-of-link blocking avoidance if the source port is not flow control enabled. If the decision is not to drop, the
TxQ manager links the unicast frame’s FCB to the correct per-port-per-class TxQ and updates the FCB information.
If multicast, the TxQ manager writes to the multicast queue for that port and class and also update the FCB
information including the duplicate count for this multicast frame. 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. The detail of the QoS decision guideline is described in chapter 5.
6.2.5
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.
6.2.6
The TxDMA multiplexes data and address from port control, and arbitrates among buffer release requests from the
port control modules.
7.0
7.1
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 7 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. MMAC port actually has four total transmission priorities.
Model
Quality of Service and Flow Control
RxDMA
TxQ Manager
Port Control
TxDMA
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Data Sheet