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IBM2520L8767
IBM Processor for ATM Resources
atmrm.chapt05.01
08/27/99
Transmit Cell Scheduler (CSKED)
Page 223 of 553
Transmit Data Path Entities
Entity 10: Transmit Cell Scheduler (CSKED)
The transmit cell scheduler entity is responsible for receiving a packet from the processor, determining when
cells from the packets need to be transmitted, and passing this information to the segmentation buffer entity.
The logic consists of timers and counters for determining transmit opportunities, a register for determining
where the timing data is located, a status register, and interfaces to ARBIT (for accessing the timing data and
descriptors), PCINT (for register accesses), RXQUE (for queuing events), POOLS (for returning buffers when
finished transmitting), and SEGBF (which gets the cells from memory to transmit).
Operational Description
A logical channel descriptor (LCD), and optionally a logical path descriptor (LPD), containing scheduling
parameters for the circuit must be initialized before segmentation can be started. The parameters that are
important to the operation of this entity are the average interval, peak interval, transmission priority, and
maximum credits that can be accumulated. See
Transmit Descriptor Data Structures
on page 39 for further
information on these descriptors.
Packets to be segmented are written to packet memory that 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 or VPD.
Scheduling Options
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 Descriptor Data Structures
on page 39. The following fields
need to be initialized before the packets are sent on the connection.
Scheduling parameters
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
.01msec.
All other ABR fields should be initialized to zero.
This section contains descriptions of the registers used by the Cell Scheduler.
.