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CHAPTER 4 ATM CELL PROCESSOR
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Preliminary User’s Manual S14767EJ1V0UM00
4.8.5 ABR function
ATM Cell Processor supports ABR service which is defined in ATM Forum “Traffic Management Specification
Ver.4.0.”.
4.8.5.1 Source / destination behavior
(1) Source behavior
(a) The value of ACR never exceeds PCR, nor be less than MCR. ATM Cell Processor never sends in-rate cells at
a rate exceeding ACR. It always sends in-rate cells at a rate less than or equal to ACR.
(b)Before ATM Cell Processor sends the first cell after connection setup, it sets ACR to ICR. The first in-rate cell
to be sent is a forward RM-cell.
(c) After the first in-rate forward RM-cell, ATM Cell Processor sends in-rate in the following order.
<1> The next in-rate cell is a forward RM-cell if and only if, since the last in-rate forward RM-cell was sent,
either: at least Mrm in-rate cells have been sent and at least Trm time has elapsed, or Nrm-1 in-rate cells
have been sent.
<2> The next in-rate cell is a backward RM-cell if condition <1> above is not met, if a backward RM-cell is
waiting for transmission, and if either; no in-rate backward RM-cell has been sent since the last in-rate
forward RM-cell, or no data cell is waiting for transmission.
<3> The next in-rate cell to be sent is a data cell if neither condition <1> nor condition <2> above is met, and if
a data cell is waiting for transmission.
(d) Cells sent in accordance with procedure #1, #2, and #3 have CLP=0.
(e) Before sending a in-rate forward RM-cell, if ACR>ICR and the time T that has elapsed since the last in-rate
forward RM-cell was sent is greater than ADTF, then ATM Cell Processor reduces ACR to ICR.
(f) Before sending an in-rate forward RM-cell and after following procedure #5 above, ATM Cell Processor adjusts
ACR again. If at least CRM in-rate forward RM-cells have been sent since the last backward RM-cell with
BN=0 was received, then ATM Cell Processor reduces ACR by at least ACR*CDF. If this reduction makes
ACR below MCR, it sets ACR to MCR.
(g) After following procedure #5 and #6 above, ATM Cell Processor places the new ACR value in the CCR field of
the outgoing forward RM-cell.
(h) When a backward RM-cell is received with CI=1, then ATM Cell Processor reduces ACR by ACR*RDF. If this
reduction makes ACR below MCR, ATM Cell Processor sets ACR to MCR. If the backward RM-cell has both
CI=0 and NI=0, then ATM Cell Processor increases ACR by RIF*PCR. ATM Cell Processor sets ACR to PCR if
the result of the calculation above exceeds PCR.
(i) When a backward RM-cell is received (in-rate or out-of-rate), and after ACR is adjusted according to procedure
#8, ATM Cell Processor sets ACR to the minimum of ACR as computed in procedure #8, and the ER field. But
ATM Cell Processor doesn’t sets ACR less than MCR. If the backward RM-cell has NI=1, the ATM Cell
Processor doesn’t increase ACR.
(j) When ATM Cell Processor generates a forward RM-cell (in-rate or out-of-rate), it assigns values to the various
RM-cell fields as specified for source-generated cells in Section. 4.8.4.2
(k) Forward RM-cells may be sent out-of-rate (i.e. not conforming to the current ACR). Out-of-rate forward RM-
cells shall not be sent at a rate greater than TCR.
(l) ATM Cell Processor resets EFCI on every data cell it sends.