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CHAPTER 5 SAR FUNCTION
146
5.4.4 Traffic Control
This section mainly describes VBR and CBR traffic control. It focuses on the behavior of the VBR shaper and
scheduler. See
Section 5.8
for a description of ABR traffic control.
(1) Cell transmission
The ATM layer of the
μ
PD98405 transmits cells to an external PHY layer device via the UTOPIA interface.
It also sends them to the built-in PHY layer.
Cells are transmitted continuously if at least one transmit VC is active or if a shaper is specified as an
unassigned/idle cell generator. In all other cases, the TENBL_B signal is deactivated, preventing cells from
being transmitted (from the ATM layer). See
Chapter 6
for an explanation of cell transmission from the
built-in PHY layer to a PMD device. The cell transmission rate is controlled in cell units. Up to 16 different
rates can be specified for VBR. For those channels that use the ABR service, the ABR scheduler performs
this control.
Example
: One-VC transmission with I/M = 1/2, P = 1, C = 1, and ICM = "0"
TX
VC
UC
VC
UC
UC
UC
UC
UC
UC
VC
VC
VC
VC
VC
PHY
TENBLE_B
Packet 1
Packet 2
VC: Data cell
UC: Unassigned cell
(2) Scheduling
The term "scheduling" refers to determining the channel (transmit VC) from which cells will next be
transmitted and the transmission sequence of those cells. The term "scheduler" refers to that module that
performs scheduling. Scheduling is realized using 16 VBR traffic shapers, ABR schedulers, and the
scheduler that supervises them.
Each VBR shaper has its own scheduler register. The host loads the scheduler register of the shaper to be
used with parameters for determining the peak and average rates at which cells are transmitted. Each
shaper performs a dual leaky bucket algorithm operation according to given parameters to generate the
cell transmission timing. The host selects a shaper whose transmission rate is to be used for an open
transmit VC and sets it in the transmit VC table. When an effective packet has been prepared for a
transmit VC, and the transmit VC is activated by the Tx_Ready command, the transmit VC is linked to the
specified shaper under the control of a shaper link list and shaper pointer entry. Each VBR shaper has a
priority. The VBR shaper with highest priority can be used for CBR.
The scheduler checks the parameters of all the enabled shapers once every 36 system clock pulses to
identify those shapers which are at the transmit timing. It then selects the shaper having the highest
priority. The scheduler also transmits a cell in the transmit VC linked with that shaper. Traffic control
consists of repeating this cycle.