L-ET4148-50C-DB LSI, L-ET4148-50C-DB Datasheet - Page 288

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

Specifications of L-ET4148-50C-DB

• Lead Free Status / RoHS Status: Supplier Unconfirmed

ET4148-50
Single-Chip 48 x 1 Gbit/s + 2 x 10 Gbits/s Layer 2+ Ethernet Switch
Appendix B: Configuration
Quality of Service
Packet Buffer Congestion Management
The central packet buffer is a limited resource that must be shared among all users (“users” being defined as
receive ports). Various congestion thresholds are used to limit the number of buffers that may be allocated to each
user.
Two forms of ingress flow control are available: policer and MAC. The policers may be configured to discard out-of-
profile traffic at all times or only when the packet buffer is congested. Hence, the policers may be made responsive
to the congestion state of the interface. The policers are configured for the type of action to take during congestion
via the Policer_Flow_Mode_Table register. Setting the discard_out_of_profile_when_congested bit
makes the respective policer responsive to the congestion state.
The MAC flow control utilizes IEEE 802.3x flow control packets to prevent the other end of a full-duplex Ethernet
link from transmitting for a limited period of time. This form of flow control is not flow based and has the effect of
shutting down all receive traffic on a port regardless of its priority or destination.
The congestion thresholds for these two flow control mechanisms are set via the
Packet_Buffer_Global_Congestion_Threshold and Packet_Buffer_Channel_Congestion_Threshold reg-
isters. As the name implies, the global threshold is applied globally (affects all ports or channels) and is sensitive to
the total allocation of buffers. The channel (or port) congestion threshold is applied to only a single group of ports
and is sensitive to the number of buffers allocated to that receive group.
Each type of threshold (global or per-port) has two subtypes: policer or MAC.
Whenever the total number of buffers allocated exceeds a threshold set in
Packet_Buffer_Global_Congestion_Threshold, the appropriate congestion indication (policer or MAC) is pro-
vided to all ports simultaneously.
If the global congestion threshold has not been exceeded, then when a per-port congestion threshold is exceeded,
the appropriate indication (policer or MAC) is provided to only the affected port.
The congestion thresholds are typically set as follows:
■
■
Queue Management (Revision C Only). In revision C, some new registers are added to create maximum queue
sizes. To enable these new registers, hol_mode in the Packet_Buffer_Mode register must be asserted.
To create the maximum queue size, two sets of queue limits are configured. One set limits the number of packets
that may occupy a queue during packet buffer congestion, and the second set limits the number of buffers
(128 bytes) that may be occupied by each queue during packet buffer congestion. Each set of limits has a corre-
sponding congestion threshold used to determine when the packet buffer is considered congested for these limits.
The supervisor_queue_limit[12:0] and queue_limit[12:0] fields in the
Packet_Buffer_Queue_Limit register are global limits used to restrict the number of packets that may occupy
a queue during packet buffer congestion. For these limits, the packet buffer is considered congested if
queue_mask_congestion_threshold[14:4] in the Packet_Buffer_Queue_Management_Thresholds
register is exceeded.
When this global threshold is reached and a queue is over its allocated packet limit, the ET4148-50 prevents fur-
ther enqueues to that queue until the packet buffer is no longer congested or until the number of packets occupying
the queue drops below its allocated limit. The packet buffer is no longer considered congested for these limits when
the total number of queue entries drops below queue_mask_congestion_threshold[14:4] minus
global_congestion_hysteresis[9:4].
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The policer thresholds should be lower than the MAC thresholds.
The sum of the per-channel thresholds should be greater than the global threshold.
(continued)
(continued)
Agere Systems - Proprietary
Preliminary Data Sheet
Agere Systems Inc.
April 2006