IBM PowerPRS Q-64G
Packet Routing Switch
Preliminary
3.1.2 Control Packets
Control packets carry the communications between the local processor and the protocol engine. They do not
have a specific priority. Control packet transmission is relatively infrequent and does not affect the perfor-
mance of high-priority traffic because the local processor access is slow compared to the data packet traffic
rate.
Ingress control packets originate at the protocol engine. The PowerPRS Q-64G can receive control packets
on either the high channel or the low channel of any input port. An input controller identifies an ingress packet
as a control packet when the destination bitmap value is all zeros. Ingress control packets are stored in the
shared memory by the input controller and then transferred to the local processor using registers (see Section
3.8.2 Control and Service Packet Reception on page 55).
Egress control packets originate at the local processor. The local processor can transmit control packets on
any output port. Control packets are always transmitted on an output before any other packets stored in the
shared memory destined for that output. Egress control packets are transferred from the local processor to
output controllers using registers (see Section 3.9.2 Control and Service Packet Transmission on page 56).
3.1.3 Service Packets
Service packets carry the communications between the local processor and the attached devices. They are
used to test the continuity of the links between the PowerPRS Q-64G and the attached devices and to gain
access to attached device internal resources.
There are three types of service packets: event-1, event-2, and command. Only command service packets
contain payload. Service packets are received and transmitted in the same manner as control packets (that
is, ingress packets are stored by an input controller and exchanged with the local processor using registers).
Note: When the PowerPRS Q-64G is attached to the PowerPRS C192, event-1 service packets are used for
link liveness, event-2 service packets are used for write acknowledgement and switchover, and command
service packets are used for gaining read access to PowerPRS C192 resources.
3.1.4 Idle Packets
Idle packets do not carry user data. They are transmitted on a port only when there are no data, control, or
service packets available for transmission or when these packets cannot be transmitted (for example, during
switchover). Idle packets are also used to perform link synchronization. The attached devices generate
ingress idle packets; the PowerPRS Q-64G generates egress idle packets. Idle packet color coding is used
for switchover support.
3.2 Physical Interface and Packet Processing
As discussed in Section 2.2.1, the physical links between the PowerPRS Q-64G and attached devices are
high-speed serial links called Unilinks. There is one Unilink per PowerPRS Q-64G device port, or 32 Unilinks
per device. Each Unilink is comprised of two pairs of differential lines; one differential pair carries ingress flow
and the other differential pair carries egress flow. Each pair of differential lines (that is, each device port)
carries two packets at a time.
Functional Description
Page 26 of 199
prsq-64g.01.fm
December 20, 2001
IBM [ IBM ]
