TC9208M
Preliminary Data Sheet
5
Classes of Service
TC9208M implements advanced Class of Service (CoS), supporting both traffic priority and delay bound
features. It provides four classes of service: class 0 (low), class 1 (normal), class 2 (high) and class 3
(very high). Each class of service has its dedicated transmission queue for each port. The frames assign
with higher service class will arrive sooner at the destination.
Frames in the class 0 priority queue get the lowest transmission bandwidth percentage, while frames in
the class 3 priority queues get the highest bandwidth percentage. The bandwidth percentage depends
on two elements:
-
-
CoS bandwidth weights;
The corresponding class of all non-empty queues for the respective port.
The CoS weights can be set using PriBndw[1:0] shared configuration pins or the by setting EEPROM
Registers. While the pins provide only four predefined hardwired combinations for the transmission
bandwidth percentage allocation among the queues, the EEPROM gives more flexibility over this
configuration.
When EEPROM is not present, transmission bandwidth percentage distribution among the queues for
the case when all the queues are loaded can be seen in the table below:
EEPROM is not present
Transmission Bandwidth Percentage
PriBndw[1:0]
Class 0 Priority
(lowest)
Class 1 Priority
Class 2 Priority
Class 3 Priority
(highest)
00
7%
13%
27%
53%
01
3%
14%
27%
56%
10
2%
8%
30%
60%
11
3%
5%
10%
82%
The percentage refers to the port’s bandwidth, which is determined by the current operating speed.
Those values are the guaranteed minimum ones and the transmission bandwidth percentage cannot
drop below specified value under any circumstance. If EEPROM is used, the user has more flexible
adjustment of bandwidth weights to choose from the EEPROM register.
A special early packet dropping mechanism is also implemented to offer more protection against
overflow conditions for priority packets. If the global memory load exceeds an overflow threshold, then all
class 0 priority packets will be dropped from the source port(s) in order to save space for the higher
priority packets. This will minimize the probability of packet loss in priority flows for senders that are not
flow control capable.
The CoS mechanism supports multiple prioritization sources: 802.1Q VLAN Tag Header (layer 2), IP
Header TOS bits (layer 3) and/or port based CoS. For IP and VLAN sources a mapping is executed
between the values of the fields extracted from each frame and one of the four CoS provided by
TC9208M. This mapping can be adjusted by using IPTosMap[1:0] and VLANPrMap[1:0] shared
configuration pins or the EEPROM settings. While the pins provide just four predefined hard-wired
mapping schemes, the EEPROM gives a custom explicit mapping.
Under some circumstances, one or more mechanisms can be active (VLAN, IP and/or port based). In
this case there is a resolution function that resolves the CoS for each incoming frame. When EEPROM is
not present and IP and/or VLAN prioritizations are enabled, the corresponding headers of each incoming
frames are parsed. The frame will be assigned the CoS corresponding to the first header parsed that
found valid. When both above prioritizations are enabled the search order is determined by EEPROM
Confidential.
20/53
August 1, 2003
Copyright © 2003, IC Plus Corp.
TC9208M-DS-R06
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