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| 型號(hào)： | IBM3288H2848 |
| 英文描述： | Telecommunication IC |
| 中文描述： | 通信集成電路 |
| 文件頁(yè)數(shù)： | 18/131頁(yè) |
| 文件大小： | 1679K |
| 代理商： | IBM3288H2848 |
PRS28.4G
IBM Packet Routing Switch
Architecture
Page 18 of 131
prs28.03.fm
August 31, 2000
priority) is below the output priority threshold. It is removed whenever this count exceeds the threshold value
(no hysteresis). Grants are also provided for the shared memory for each priority. The shared memory grant
(memory grant) for a given priority is provided whenever the total packet count in shared memory is below the
shared memory priority threshold. It is removed whenever this count exceeds the threshold value (no hyster-
esis).
An adapter is only allowed to transmit a packet when it has the memory grant for the packet priority as well as
the output queue grants for the destination logical outputs. For multicast packets, the adapter is allowed to
transmit a packet if it has the memory grant for the packet priority, regardless of output queue grants.
Incoming packets also carry a drop flag. This flag allows packet discard at the input of the island whenever
the memory grant and/or output queue grants are removed before the packet is received from the attached
adapter (due to the latency in grant update). The grant information used by the input controllers to decide to
drop a packet is updated once every sequencer cycle, providing fairness among all inputs.
Finally, an anti-streaming function is provided at the input controllers to detect badly behaving adapters.
When an adapter sends a packet to an output priority for which the output queue grant or memory grant have
not been given in the past eight packet cycles, the packet is discarded and an interrupt is raised. For multicast
packets, the same mechanism is applied, but it takes only the memory grants into consideration.
2.1.8 Outgoing Flow Process
In each of the four logical output queues, control packets and Data Packets are transmitted in the following
order:
1. Control Packets
2. Priority 0 packets
3. Priority 1 packets
4. Priority 2 packets
5. Priority 3 packets
This order cannot be changed. Packets of higher priority overtake packets of lower priority.
Control Packets are constructed by the local processor and can be transmitted one at a time to multiple
outputs The control packets take precedence over any other packet present in the shared memory.
2.1.9 Outgoing Flow Control
Flow control at the output is also provided by send grants (one per output), regardless of priority. A packet on
a given output can only be transmitted if the send grant is provided for that output.
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