MT90220
The clocking mode and reference link are fixed once
an IMA Group is setup and should remain
unchanged so long as that group is operational. The
reference link should not change unless problems
are reported with the link.
Integration register and the value is indicated in
Table 1.
Preferred Value
PCM Mode
TX IDCR Integration
register
16
T1 ISDN (23 channels)
T1 (24 channels)
0x09 (2 clocks)
2.4.4 Stuff Cell Rate
18
0x0B (2 clocks)
The Stuff event algorithm differs between CTC and
ITC modes. In CTC mode, the Stuff event is typically
fixed and appears in the same IMA frame on all IMA
Group links. In ITC mode, the Stuff event is
determined using an adaptive algorithm that relates
the level of the internal TX Link FIFO to that of the
TX link FIFO of the Reference link.
19
E1(30 channels)
0x0C (2 clocks)
Table 1 - IDCR Integration Register Value
2.4.6 IMA Controller (RoundRobin Scheduler)
The IMA controller produces the cell stream to be
sent to the PCM blocks using the following four cell
types:
The MT90220 implements
2
different stuffing
•
Data cells received from the UTOPIA port (User
cells)
algorithms: a fixed stuffing rate and an adaptive
stuffing rate. The Stuffing events do not happen
more frequently than once every five IMA frames.
•
•
•
Filler cells
IMA ICP cells with Link status information
Stuff cells
TX Group Control Mode register bit 3 selects either
the adaptive or fixed algorithm. Bit 4 determines the
timing mode declared in the ICP cell.
At an IDCR clock tick, the RoundRobin scheduler
inserts either an ICP cell, a User cell or a Filler cell
into the TX Link FIFO of the next link of the IMA
group, based on ascending link ID numbers. An ICP
cell is inserted every M cells and a stuff event is
inserted when indicated by the stuffing algorithm.
There are three possible combinations:
•
•
•
CTC Mode with internal Fixed algorithm
CTC Mode with internal Adaptive algorithm
ITC Mode with internal Adaptive algorithm
If it is not time for an ICP cell and if the traffic is not
enabled for the link (see bit 6 of the TX Link Control
register), then a Filler cell is inserted in the TX Link
FIFO. If the traffic is enabled and there is a User cell
in the TX Utopia FIFO, then the User cell is
transferred from the TX UTOPIA FIFO to the TX Link
FIFO. If there is no User cell in the TX UTOPIA FIFO,
then a Filler cell is inserted in the TX Link FIFO.
In CTC mode, when using the Fixed algorithm, the
Stuff event is periodic and will appear in the same
IMA frame, once every 2048 cells, on each link that
is part of the IMA Group.
In CTC mode, when using the Adaptive algorithm,
the Stuff event will occur at an average rate of once
every 2048 cells on each link and may not occur in
the same IMA Frame on all the links.
2.4.7 ICP Cell Generator
Once per IMA frame, an ICP cell is transmitted on
each link of the IMA Group. The content of the ICP
cell is controlled both by MT90220 and software. The
software content of the ICP cell bytes is stored in
buffer RAM. A copy of the ICP cell for each group is
kept in the internal Transmitter Cell RAM.
In ITC mode, the Stuff event is determined using the
adaptive algorithm that relates the level of the
internal TX Link FIFO with that of the TX Link FIFO
of the Reference link. The reference link has one
Stuff event every 2048 cells.
The ICP cell to be transmitted on each link is
assembled on an as required basis under the control
of the internal RoundRobin scheduler and ICP Cell
Modifier.
The state of bit seven in the TX IMA Control register
determines whether a Stuff indication is generated in
the first or first four frames preceding a Stuff event.
2.4.5 IMA Data Cell Rate
Hardware controls the following bytes of the ICP cell:
The MT90220 computes the internal TX IMA Data
Cell Rate (IDCR) for each IMA Group. The cell rate
for the IMA Group reference link, specified in the TX
Group Control Mode register, is integrated over a
programmable period of time. The preferred
integration period is programmed in the TX IDCR
•
Byte 5 - the HEC is always calculated and
inserted by the MT90220
•
Byte 6 - the TX OAM Label is defined by the
software and the value contained in this
location is transmitted in all ICP cells, Stuff
12
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