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CHAPTER 5 SAR FUNCTION
148
(3) Unassigned/idle cell transmission
(a) Transmitting unassigned/idle cells for rate adjustment
While at least one transmit VC is active, the
μ
PD98405 transmits cells continuously. (Note, however, that
even when a VC is active, congestion of the host bus may cause the TENBL_B signal to be set to
inactive, such that cells are not transmitted, once the transmit FIFO empties.) Data cells of the active VC
are transmitted at the average and peak rates specified for the relevant shaper. If a shaper has no data
cell to be transmitted, the
μ
PD98405 transmits unassigned/idle cells for rate adjustment. The user can
select either an unassigned or idle cell for rate adjustment and have the selected cell transmitted by the
μ
PD98405. Cell selection is performed using the corresponding bit of the GMR register. The default
setting is to transmit an unassigned cell.
Rate adjustment cell selection
0
An unassigned cell is inserted.
1
An idle cell is inserted.
ICM bit
(Bit 28 of the GMR)
Default setting = 0
Example:
One-VC transmission with a shaper setting of I/M = 1/2, C = 1, and P = 0
VC1 = VC1 data cell; UC = unassigned cell; IC = idle cell
TX
VC1
UC
VC1
UC
UC
UC
UC
UC
UC
VC1
VC1
VC1
VC1
VC1
PHY
If ICM = "0"
TENBLE_B
TX
VC1
IC
VC1
IC
IC
IC
IC
IC
IC
VC1
VC1
VC1
VC1
VC1
PHY
TENBLE_B
If ICM = "1"
Packet 1
Packet 2
Packet 1
Packet 2
Rate adjustment cell format
Unassigned cell
Header = all 00s in hex; payload = all 00s in hex
Header
Payload
Byte 1
Byte 2
Byte 3
Byte 4
Byte 5
Byte 1
Byte 2
...
Byte 47 Byte 48
Content
00
00
00
00
00
00
00
...
00
00