zl50415 Zarlink Semiconductor, zl50415 Datasheet - Page 32

• Manufacturer Part Number: zl50415
• Description: Managed 16-port 10/100 M + 2-port 1 G Ethernet Switch
• Manufacturer: Zarlink Semiconductor

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. 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.
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
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
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 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.
Highest transmission
priority, P3
Port Control
TxDMA
Model
Quality of Service and Flow Control
TxQ Manager
Goals
50 Mbps
Bandwidth (user
Total Assured
Table 7 - Two-Dimensional World Traffic
defined)
Zarlink Semiconductor Inc.
ZL50418
Apps: phone calls, circuit
emulation.
Latency: < 1 ms.
Drop: No drop if P3 not
oversubscribed.
32
Low Drop Probability
(low-drop)
Apps: training video.
Latency: < 1 ms.
Drop: No drop if P3 not
oversubscribed; first P3 to
drop otherwise.
The TxQ manager can also
High Drop Probability
(high-drop)
Data Sheet