MT90220
complete cell read, a read pointer is set to the buffer
corresponding to the next LID. At the following IDCR
clock cycle, the next available cell is read. ICP cells
are skipped and Filler cells are discarded. This
operation is done in a RoundRobin fashion based on
the LID value for each IMA Group link. Faulty
conditions (i.e., buffer overflow, excessive delay) are
reported through the IRQ Link Status and IRQ
UTOPIA Status registers.
The minimum recombiner delay would be the current
worst case differential delay. In the example above,
the recombiner delay would be set to 12 msec. In
this case, a link with larger transport delay than the
current worst value cannot be added to an existing
IMA group: the cells from this slower link have not
arrived when the cells sequence is rebuilt, as the
read pointer was set using the previous worst case
link. If this slower link is to be added, then the
recombiner process has to stop for the time required
to receive the cells on the slower link and then the
recombiner process can resume. This causes
disruption in the operation of the recombiner and will
affect the Cell Delay Variation (CDV).
3.3.10 Delay Between Links
The delay values between links are reflecting the
various transit delays though the network. In order to
rebuild the original ATM cell sequence, the link that
exhibits less transport delay has to be stored until
the data from the slowest link (link having the largest
transport delay) has arrived. The link that exhibits
the largest transport delay will be the link that
requires the least cells to be stored. Conversely, the
line that exhibits the least transport delay is the link
that requires the largest number of cells to be stored.
To provide an optimal recombiner delay, the
MT90220 adds a guardband delay to the current
worst case recombination delay when the IMA Group
is first started up. Guardband delay is programmable
and minimizes the number of disruptions that would
otherwise occur in accommodating link delays
exceeding the current worst case. The guardband
delay is added to the minimum recombiner delay,
when the recombiner process is enabled for the first
link of an IMA group. The operational delay
corresponds to the guardband delay added to the
current worst case delay value.
Indirect access is provided to internal registers which
hold the various link delay values. The link number
and delay type are first selected by writing to the RX
Delay Select register. After 2 system clock cycles,
the 14-bit value in the RX Delay MSB and LSB and
the RX Delay Link Number registers are updated
and can be read. The valid delay types are: the
Maximum Delay over Time, the Current Maximum
Delay and the Current Minimum Delay for an IMA
group and the Current Delay values for any links.
The guardband delay value is specified for each IMA
group by writing to the Guardband/Delta Delay
register. It should be the smallest value possible
consistent with minimizing the disruptions and the
smallest allowed value is 4. When operational, the
value of the guardband delay corresponds to the
delay value of the link having the greater transport
delay (the link where the data is the last to arrive to
the MT90220).
The delay values can be converted to time values by
multiplying the number of cells by the conversion
factor listed in the Table 5.
Time per cell
Link Type
(msec)
3.3.10.2 RX Maximum Operational Delay Value
The various delays on links of the same IMA Group
are measured and compared to the programmed
’maximum allowable value’ stored in the RX
Maximum Operational Delay register for the IMA
Group. This value corresponds to the worst delay
value that is expected. This value cannot be larger
than the number of cells that can be stored in the
external memory. The smallest ’maximum allowable
value’ is four cells. These values are independently
established for each of the four IMA Groups.
T1 ISDN (23 ch. per frame)
T1 (24 ch. per frame)
E1 (30 ch. per frame)
0.288
0.276
0.221
Table 5 - Conversion Factors Time/Cell
(msec)
3.3.10.1 RX Recombiner Delay Value
The ICP Cell from each link of the same IMA Group
is used to determine the external SRAM read and
write pointers. The distance between the read and
write pointers is referred to as the recombiner delay.
Setting the recombiner delay to the maximum
acceptable delay results in a fixed recombiner delay
that is not optimum. For example, setting recombiner
delay to 25 msec when the worst case delay is 12
msec results in an additional, unnecessary delay of
13 msec.
3.3.10.3 Link Out of Delay Synchronization (LODS)
If a link to be added is slower and cannot be
accommodated by the present guardband, an LODS
signal is generated and the link delay value is
reported negative. A delay is negative when the 2
most significant bits are set to "1". The value
reported is with respect to the read pointer and
20
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