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CHAPTER 5 SAR FUNCTION
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5.4 TRANSMISSION FUNCTION
5.4.1 Transmission Processing Flow
The transmission of cells by the
μ
PD98405 is divided into several stages as illustrated below.
Figure 5-6. Outline of Transmission Flow
Host processing
μ
PD98405 operation
(1)
Sets scheduler register
(6)
Activates shaper and links VC with shaper
(2)
Sets transmit data
(7)
Scheduling
(3)
Open_Channel command
(8)
Fetches packet descriptor
(4)
Sets VC table
(9)
Fetches data from system memory in VC segment units
(5)
Tx_Ready command
(10)
Creates cells (inserts AAL-5 trailer or CRC-10 as necessary)
:
(11)
Transmits cells to PHY layer device
:
(12)
Stores transmit indication and issues interrupt
(13) Reads transmit indication
(14) Updates mailbox pointer (MTA)
The
μ
PD98405 can support up to 32K channels consisting of any combination of transmit VCs and receive
VCs.
(1)
The host sets the scheduler register to the indirect address register of the
μ
PD98405 to determine the
transmission rate before transmission can be started.
(2)
The transmit data is stored into system memory and is managed by the host using a transmit queue
set for each transmit channel. The transmit queue stores the descriptor of each packet.
(3)
The host opens a channel (VC) by issuing an Open_Channel command to each connection to be set.
The
μ
PD98405 allocates a block to be used as a VC table, from the free block pool of control memory,
and returns the address of the block to the host.
(4)
The host sets an initial value in the VC table. This completes the preparation required prior to starting
transmission.
(5)
The host issues the Tx_Ready command to the
μ
PD98405 so that the
μ
PD98405 can start
transmission.
(6)
Upon receiving this command, when the VC uses the VBR service, the
μ
PD98405 activates the
shaper to which the VC is to be linked and adds that VC to the shaper link list. When the VC uses the
ABR service, the
μ
PD98405 registers that VC in the ABR scheduler.
(7)
Subsequently, the
μ
PD98405 performs following procedure in accordance with the scheduling.
(8)
Reads the transmit descriptor and writes it in the VC table.
(9)
Fetches data in units of segments.
(10) Generates cells internally.