L-ET4148-50C-DB LSI, L-ET4148-50C-DB Datasheet - Page 286

• Manufacturer Part Number: L-ET4148-50C-DB
• Manufacturer: LSI

Specifications of L-ET4148-50C-DB

• Lead Free Status / RoHS Status: Supplier Unconfirmed

ET4148-50
Preliminary Data Sheet
Single-Chip 48 x 1 Gbit/s + 2 x 10 Gbits/s Layer 2+ Ethernet Switch
April 2006
Appendix B: Configuration
(continued)
Quality of Service
(continued)
Establishing Flow Bandwidth Limits (continued). Each received packet causes its associated policer accumula-
tor to be incremented by a value that equals the length of the receive packet (as measured in bytes). If the receive
byte rate exceeds the rate at which the deltas are being applied, then the value in the accumulator will tend to
increase over time. Hence, an increasing accumulator value equates to an out-of-profile flow of traffic.
The limit established for a policer defines the maximum duration of out-of-profile traffic that is tolerated without pen-
alty. In other words, a flow may be allowed to operate in a bursty manner as long as the aggregate data rate for the
flow is below the data rate implied by delta value. The value established in Policer_Limit_Table defines the
maximum tolerated burst or, to put it another way, the promptness with which the policer responds to out-of-profile
traffic. A higher value in Policer_Limit_Table equates to longer bursts being tolerated or a somewhat slower
response by the policer.
Specifying Actions for Out-of-Profile Traffic. Once a flow has been identified as being out of profile, the policer
may take action on the packets associated with the flow. These actions are defined by the
Policer_Flow_Mode_Table registers.
Each policer can be configured to do one of the following:
1. Ignore out-of-profile traffic
2. Discard all out-of-profile traffic
3. Discard all out-of-profile traffic only when the packet buffer is congested
Additionally, each policer can be configured to demote the priority of all out-of-profile traffic that is not discarded.
Egress Traffic Shapers
The egress traffic shapers are conceptually similar to the ingress policers in that they serve to limit the bandwidth
allowed by each port. They differ, though, in that they don’t discard excess traffic but, instead, merely retard it. This
is possible because the packet buffer is upstream of the traffic shapers and can absorb sizable bursts of traffic
whereas the policers have no such upstream buffering.
There is one traffic shaper associated with each transmit queue (recall that there are four such queues for every
transmit port).
The traffic shapers are credit-based mechanisms. On a periodic basis, the delta value programmed into
Packet_Buffer_Shaper_Delta is added to a queue’s shaper accumulator
(Packet_Buffer_Shaper_Accumulator). The period for this accumulation is dependent upon the speed of the
associated port (Packet_Buffer_Port_Speed). Higher port speeds result in proportionally higher delta accumula-
tion rates.
The accumulation of these credits occurs continuously but ceases once the limit established in
Packet_Buffer_Shaper_Limit is reached. The higher the limit, the longer the transmit burst that may occur.
Whenever the value of a traffic shaper’s accumulator is greater than zero, the associated queue is enabled to arbi-
trate for transmission. Each successful transmission arbitration causes the byte count of the transmitted packet to
be subtracted from the shaper’s accumulator. If the accumulator’s value goes negative, the queue becomes dis-
abled from arbitrating until sufficient further credit has been accumulated.
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