IBM PowerPRS Q-64G
Packet Routing Switch
Preliminary
The particular set of subport grants carried by the current packet cycle is determined by this flywheel in
combination with the subport grant port flywheel. This flywheel is incremented each time the subport grant
port flywheel returns to ‘000’, and reset to ‘100’ after it reaches the number of priorities defined in the number
of priorities field in the Configuration 1 Register. After reaching ‘100’, this flywheel returns to ‘000’ after only
one packet cycle. In effect, the combined value of the subport grant type/priority flywheel and the subport
grant port flywheel is a six-bit field that counts to ‘100000’ and then returns to ‘000000’.
Transmission of a complete set of subport grants requires 8, 16, 24, or 32 packet cycles, depending on the
number of enabled priorities. Every 61 packet cycles, PowerPRS Q-64G multicast grants (not subport multi-
cast grants) are inserted in place of the subport grants. (Packet cycle 61 was chosen so that the multicast
grants would not preempt subport grants of the same priority every time the multicast grants were sent.) The
insertion of multicast grants is indicated by the grant type bit in the packet qualifier byte (H0) of egress
packets on the high channel. Multicast grants are also sent immediately when the multicast grant status
changes. The subport grant type/priority flywheel and subport grant port flywheel are incremented even when
multicast grants, rather than subport grants, are transmitted.
To synchronize the PowerPRS Q-64G flywheels with those in the attached devices, the subport grant/type
priority flywheel status and subport grant port flywheel status are inserted into byte 6 (that is, the FC byte) of
egress idle packets. The subport grants are inserted into header bytes H1 and, for the 512-Gbps configura-
tion, H2 of egress data, control, service, and idle packets carried on the high channel. See the following
packet format descriptions for more information.
3.3.6.4 Subport Grant Port Flywheel
The subport grant port flywheel determines the ports for the subport output queue grants carried by the cur-
rent egress packet cycle. Each packet cycle carries grants for two ports in the 256-Gbps configuration and for
four ports in the 512-Gbps configuration. For the 256-Gbps configuration, this flywheel cycles as follows:
000
001
010
011
100
101
110
111
Ports 0 and 1
Ports 2 and 3
Ports 4 and 5
Ports 6 and 7
Ports 8 and 9
Ports 10 and 11
Ports 12 and 13
Ports 14 and 15
For the 512-Gbps configuration, it cycles as follows:
000
001
010
011
100
101
110
111
Ports 0, 1, 2, and 3
Ports 4, 5, 6, and 7
Ports 8, 9, 10, and 11
Ports 12, 13, 14, and 15
Ports 16, 17, 18, and 19
Ports 20, 21, 22, and 23
Ports 24, 25, 26, and 27
Ports 28, 29, 30, and 31
This flywheel is incremented every packet cycle.
Functional Description
Page 38 of 199
prsq-64g.01.fm
December 20, 2001
IBM [ IBM ]
