IBM PowerPRS Q-64G
Preliminary
Packet Routing Switch
16 to 2048) is programmable using the send grant antistreaming threshold field in the Configuration 0
Register. When the number of packet cycles is exceeded, a violation is reported via the send grant violation
bit in the Status Register (page 120), and an interrupt is raised if the violation is not masked with the send
grant violation bit in the Interrupt Mask Register (page 122). The port with the violation is reported in the Send
Grant Violation Register (page 131).
When the send grant antistreaming function is enabled, the output queue scheduler also considers the send
grant active and sets an internal send grant to avoid congestion inside the PowerPRS Q-64G. The internal
send grant is removed when the attached device resumes the normal send grant.
3.5.3 Credit Table
In the basic PowerPRS Q-64G configuration, the output queue scheduler schedules packet transmission for
each output port in the following order:
1. Service packets
2. Control packets
3. Priority 0 data packets
4. Priority 1 data packets
5. Priority 2 data packets
6. Priority 3 data packets
The credit table provides a weighted cycling mechanism that can be programmed to guarantee minimum
bandwidth for low-priority packets. For each output port, the credit table indicates the packet priority to be
transmitted during each packet cycle. The credit table includes 256 entries, or credits, per port. The credit
table is read once per packet cycle; that is, the priority entry is read for the current credit number, which
generates a credit for that priority. The credit number is incremented by one for every packet cycle. After
credit number 255, the credit number returns to 0.
When a credit is generated for a priority, a packet of that priority is sent on the output port if there is a packet
in the output queue and the send grant is active for that priority. If either the output queue is empty or the
send grant is inactive, the basic algorithm applies.
Use of the credit table is specified by the credit table enable field in the Configuration 1 Register (page 112).
This field also specifies which packet priorities can be preempted by lower-priority packets. Indirect access to
the credit table is provided via the Credit Table Access Register (page 116).
3.6 Subport Flow Control
A single PowerPRS C192 can be attached to four OC-48 protocol engines, or subports. The PowerPRS C192
uses two types of grants to control egress traffic flow to the subports:
• Subport output queue grants
• Subport multicast grants
Although these grants function much like the PowerPRS Q-64G output queue grants and multicast grants
discussed in Sections 3.4.1 and 3.4.3, they reflect egress packet occupancy of the attached
PowerPRS C192s. A PowerPRS C192 inserts the subport grant status into ingress data, control, and idle
packets. The PowerPRS Q-64G extracts the subport grant status and stores it in an internal table, and then it
inserts the subport grant status into egress data, control, service, and idle packet headers to broadcast it to
the ingress side of all the attached PowerPRS C192s. The PowerPRS C192s use the subport grant status to
control ingress traffic to the PowerPRS Q-64G.
prsq-64g.01.fm
December 20, 2001
Functional Description
Page 53 of 199
IBM [ IBM ]
