zl50411 Zarlink Semiconductor, zl50411 Datasheet - Page 45

• Manufacturer Part Number: zl50411
• Description: Managed 9-port Fast Ethernet Switch With Private Vlan
• 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.
A class is capable of offering traffic that exceeds the contracted bandwidth. A well-behaved class offers traffic at a
rate no greater than the agreed-upon rate. By contrast, a misbehaving class offers traffic that exceeds the
agreed-upon rate. A misbehaving class is formed from an aggregation of misbehaving microflows. To achieve high
link utilization, a misbehaving class is allowed to use any idle bandwidth. However, such leniency must not degrade
the quality of service (QoS) received by well-behaved classes.
Each traffic type may each have their own distinct properties and applications. Classes may receive bandwidth
assurances or latency bounds. For example, the highest transmission class may require that all frames receive
50% of the 100 Mbps of bandwidth at that port.
Best-effort (P0) traffic forms a class that only receives bandwidth when none of the other classes have any traffic to
offer. It is also possible to add a class that has strict priority over all others; if this class has even one frame to
transmit, then it goes first. In the ZL50411, each RMAC port will support two total classes, and the MMAC port will
support four classes. We will discuss the various modes of scheduling these classes in the next section.
Model
Quality of Service and Flow Control
RxDMA
TxQ Manager
Port Control
TxDMA
Zarlink Semiconductor Inc.
ZL50411
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Data Sheet