MT90220
•
•
one common Filler Cell used in IMA mode
one Idle Cell used in UNI mode
2.3 Parallel to Serial PCM Interface
ATM cell octet byte alignment conforms to ITU G.804
recommendations for T1 or E1 framer parallel to
serial format conversion.
The remaining 58 cells can be assigned to any of the
20 TX FIFOs. The TX FIFOs are divided in 12 TX
UTOPIA FIFOs and 8 TX Link FIFOs.The MT90220
implements one TX UTOPIA FIFO for each link when
used in UNI and one for each IMA Group for a total of
12 TX UTOPIA FIFOs. Each TX UTOPIA FIFO is
associated with one TX UTOPIA Address. Please
refer to the paragraph 5.0 UTOPIA Interface
Operation for more details.
The TX PCM Control and RX PCM Control
registers are used to select the T1 or E1 mode of
operation. Refer to Section 4, Description of the
PCM Interface, for more details.
2.4 ATM Transmit Path in IMA Mode
The MT90220 supports up to four independent IMA
Groups. Each of the eight T1/E1 trunks can be
assigned to any one of these IMA Groups. A T1/E1
trunk cannot be assigned to more than one IMA
Group. Refer to Figure 3 for a functional block
diagram of the transmitter.
In addition, for each link to be used in IMA mode, an
internal TX Link FIFO is utilized. These TX Link
FIFOs are holding the cell streams that are to be
sent on each TX serial port. There is a total of 8 TX
Link FIFOs and their size is programmed on a per
group basis using the TX IMA Control register.
When a link is used in UNI mode, its corresponding
TX Link FIFO is disabled and the TX UTOPIA FIFO
is used.
The IMA transmitter splits the incoming stream into N
sub-streams, where 1 ≤ N ≤ 8. Each sub-stream is
passed to a separate line interface device that
transmits the cells on a physical link.
TX UTOPIA FIFO Length Definition registers are
used to set the TX UTOPIA FIFO size. A maximum of
15 cells can be assigned to any single FIFO. The
size of unused TX UTOPIA FIFOs should be set to
zero. The recommended size for the IMA Group TX
UTOPIA FIFO is 1.
The physical line rate is either 1.544 Mbps (T1) or
2.048 Mbps (E1). The transmitter inserts so-called
ICP cells in the various outgoing streams according
to the IMA specification. The ICP cells are inserted
every M ATM cells on each link. This is the task of
the scheduler.
In IMA Mode, the ATM User Cells are first placed in
the IMA TX UTOPIA FIFO and then transferred, by
the internal round robin scheduler, to the proper TX
Link FIFO. The TX UTOPIA FIFO length for each link
configured in IMA mode should be set to zero.
2.4.1 IMA Frame Length (M)
The IMA frame length (value of M) can be 256, 128,
64, or 32. The value of M for each IMA Group is set
by the TX Group Control Mode registers. M is fixed
once an IMA Group is setup and should remain
unchanged so long as that group is operational.
The TX IMA Control registers are used to set the
size of the internal TX Link FIFO. An upper and lower
level limit must be set for the internal TX Link FIFO.
2.4.2 Position of the ICP Cell in the IMA Frame
The TX ICP Cell Offset registers control the position
of the ICP cell in the IMA frame for each link. This
parameter should remain unchanged so long as that
group is operational.
In UNI Mode, the ATM User Cells are queued in the
TX UTOPIA FIFO until sent over the T1/E1 link.
The recommended upper limit value for the internal
TX Link FIFO is five and the recommended lower
limit is one when operating in ITC clocking mode.
When operating in CTC mode, the recommended
upper limit value for the internal TX Link FIFO is six
and the recommended lower limit is one. In the case
where CTC mode is used and when the ICP cells on
all the links are sent with the same ICP cell offset
and when carrying a CBR-type traffic, an upper value
of 7 may be required.
2.4.3 Transmit Clock Operation
The MT90220 supports both the Common Transmit
Clock (CTC) and Independent Transmit Clock (ITC)
modes of operation. The desired mode is specified
by writing to the TX Group Control Mode register. A
reference link must be specified in the TX Group
Control Mode register. The MT90220 introduces a
Stuff cell on the reference link every 2048 cells and
determines the appropriate time to insert a Stuff cell
on the remaining group links. See paragraph 2.4.4,
Stuff Cell Rate, for more details.
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MITEL [ MITEL NETWORKS CORPORATION ]
