IBM PowerPRS Q-64G
Packet Routing Switch
Preliminary
3.9 Packet Transmission
3.9.1 Output Port Servicing
PowerPRS Q-64G output ports are designed for continuous packet transmission. Packet transmission starts
at a fixed point in time, which is synchronized by the internal sequencer (see Table 3-26). If neither a data
packet nor a control packet is available for transmission on an output port and the send grant is removed, the
port transmits an idle packet. Control packets are always transmitted on an output port before any other
packets in the shared memory destined for that port, but they do not affect the performance of high-priority
traffic. Control packet transmission is relatively infrequent because the local processor access is slow
compared to the data packet traffic rate. In the external speed expansion configuration, packet transmission
starts at the same time on the master and slave output controllers.
Table 3-26. Packet Transmission Time
Internal Sequencer Cycle (8-ns)
Output Port Number
0
1
2
3
0, 16
2, 18
4, 20
6, 22
1, 17
3, 19
5, 21
7, 23
8, 24
10, 26
12, 28
14, 30
9, 25
11, 27
13, 29
15, 31
4 to 7 for an 8-byte LU
4 to 8 for a 9-byte LU
4 to 9 for a 10-byte LU
–
3.9.2 Control and Service Packet Transmission
The local processor prepares control packet and service packet payload using the Egress Control Packet and
Service Packet Payload Registers (page 134) and specifies the destination bitmap with the Egress Control
Packet and Service Packet Destination Register (page 135). Control packet or service packet transmission is
requested when the Egress Control Packet and Service Packet Destination Register is written.
3.9.3 Idle Packet Transmission
An output port transmits idle packets only when there are no data, control, or service packets available, or
when the send grant is removed. Idle packets are also transmitted to support switchover (see Section
3.11 Switchover Support on page 57).
3.9.4 Look-Up Table
The look-up table allows the byte transmission sequence of egress data, control, and service packets to be
rearranged. It identifies if and when, and how many times, a data byte will be sent. Table 3-27 presents an
example of how the bytes can be rearranged. Note that any byte can be exchanged with or overwritten by
another byte.
All output ports use the same look-up table, and the byte transmission sequence in the look-up table applies
to all data streams. The Look-Up Table Registers (page 146) provide indirect access to the look-up table.
Functional Description
Page 56 of 199
prsq-64g.01.fm
December 20, 2001
IBM [ IBM ]
