IBM PowerPRS Q-64G
Packet Routing Switch
Preliminary
These two flywheels do not apply to ingress idle packets, each of which has the capacity to carry all 20 sub-
port grants and up to 4 send grants for the port. When an idle packet is transmitted, these flywheels continue
to increment. If a port receives only data packets and control packets and all four priorities are enabled, 20
packet cycles are required to update the complete set of subport grants and 4 packet cycles are required to
update the complete set of send grants.
3.3.2.1 Subport Grant Type/Subport Flywheel
The subport grant type/subport flywheel determines the type of subport grant carried by the current ingress
packet cycle. This flywheel cycles as follows:
000
001
010
011
100
Subport output queue grants, subport 0
Subport output queue grants, subport 1
Subport output queue grants, subport 2
Subport output queue grants, subport 3
Subport multicast grants
For each of the above subport grant types, there is one grant per priority (20 subport grants total per port,
when all four priorities are enabled). The subport grant carried by the current packet cycle is determined by
the subport grant type/subport flywheel value in combination with the grant priority value. The subport grant
type/subport flywheel is incremented each time the grant priority flywheel returns to ‘000’. The grant priority
flywheel returns to ‘000’ after reaching ‘100’.
To synchronize the PowerPRS Q-64G flywheels with those in the attached devices, the subport grant
type/subport flywheel status and grant priority flywheel status are inserted into the packet qualifier byte (H0)
of ingress idle packets. The subport grants are inserted into header bytes H1 and H2 and byte 6 of ingress
idle packets and into byte H0 of ingress data and control packets. See the following packet format descrip-
tions for more information.
3.3.2.2 Grant Priority Flywheel
The grant priority flywheel determines the priority of the grant carried by the current ingress packet cycle:
00
01
10
11
Priority 0
Priority 1
Priority 2
Priority 3
This priority is used for both the send grant and the subport grant. There is only one send grant per priority.
The grant priority flywheel is incremented every packet cycle, and returns to ‘00’ after reaching the number
of priorities defined in the number of priorities field in the Configuration 1 Register (page 112).
3.3.3 Ingress Idle Packet Format
The ingress idle packet format for the PowerPRS Q-64G 16-Gbps configuration is presented in Figure 3-1.
Bytes 0:2 and bytes 5:6 of the master LU carry control information, as described in Tables 3-1 through 3-3.
Ingress and egress idle packets both carry SCC information. See Section 3.10 Side Communication Channel
on page 57 for more information.
Functional Description
Page 30 of 199
prsq-64g.01.fm
December 20, 2001
IBM [ IBM ]
