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IBM3206K0424
IBM Processor for Network Resources
Preliminary
Transmit Buffer (CSKED)
Page 276 of 676
pnr25.chapt05.01
August 14, 2000
Packet Initialization
Packets to be segmented are written to Packet Memory, which has been allocated by POOLS. The address
of the LCD describing the channel that this packet is to be transmitted on must be written to the header of the
packet. Packet segmentation is started by issuing the transmit enqueue primitive to this entity. This entity will
schedule segmentation of the packet according to the parameters set up in the LCD.
Scheduling Options
ABR Scheduling
CSKED has logic to assist in the processing of ABR connections. If the connection is ABR, the LCD will have
a different configuration, as specified in
Transmit Logical Channel Descriptor Data Structures
on page 66.
The following fields need to be initialized before the packets are sent on the connection.
Scheduling type
- This field must be set to the value specifying an ABR connection.
Nrm -
This field should specify the maximum number of cells a source may send for each forward
RM-cell. Number of cells = (2
Nrm)+1.
Trm
- This field provides an upper bound on the time between forward RM-cells for an active source.
Time = 100
(2
-Trm) msec.
ADTF
- The ACR Decrease Time Factor is the time permitted between sending RM-cells before the rate
is decreased to ICR. Time = ADTF
0
.01 msec.
All other ABR fields should be initialized to
’
0
’
.
Frame Scheduling
CSKED has logic to support frame-based scheduling. It is enabled whenever the PHY type is configured for
POS-PHY in LINKC. In frame-based scheduling the packet is sent out at the line rate, but the start time of the
next frame is determined by multiplying the peak interval by the number of 64-byte blocks in the packet. The
average portion of the bandwidth used by a connection will be 1/(peak interval).
For example, if a connection is to use 1/4 of the bandwidth, the peak interval should be set to four. The
frames will be sent out at line rate, but the spacing between the start of each frame will be four timeslots for
each cell sent from the packet, so on average 1/4 of the bandwidth will be used by the connection. The follow-
ing timeline example depicts sending two packets, the first contains four cells and the second contains three
cells. The unfilled slots can be used by other connections.