Contents Telektronikk - Telenor

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ATM cell
header
2.1.1 Measuring the cell error ratio
using the AAL-3/4 protocol
In order to calculate the CER a test traffic
of ATM cells with AAL3/4 protocol
should be generated. An overview of this
protocol structure is shown in Figure 5.
See [6] for more information on the
AAL3/4 protocol.
The AAL-3/4 protocol includes a cyclic
redundancy code (CRC)-10 field which
should detect single bit errors in the entire
ATM cell payload. The error detection is
performed on a cell basis, therefore lost
or misinserted cells should not influence
the number of CRC errors (i.e. the number
of errored cells) detected.
During this measurement period the following
primary measurement values
were recorded:
- Selected cell bandwidth (used to derive
the total number of user cells)
- Duration of measurement period
- Number of CRC-10 errors.
The following secondary measurement
values were also recorded for verification
of the functionality:
- Number of corrected headers (single
bit header errors)
- Number of bad headers (multiple bit
header errors of multiple consecutive
single bit header errors)
- Number of lost cells.
By calculating the relationship Number
of CRC10 Errors to Total Number of
User Cells we obtain a figure for the cell
error ratio. If a secondary measurement
value was found to be larger than expected,
then the measurement was deemed
invalid.
2.1.2 Measuring the cell error ratio
using the AAL5 protocol
In order to calculate the cell error ratio a
test traffic of ATM cells with AAL5 protocol
should be generated. An overview
of this protocol structure is shown in Figure
5.
ATM cell payload
HEADER SN SNP INFORMATION PAYLOAD
Figure 6 ATM cells supporting the AAL1 protocol
The AAL5 protocol includes a CRC-32
field which should detect single bit errors
in the entire ATM cell payload. The error
detection is performed on a protocol data
unit (PDU) basis, but by knowing the
PDU length (i.e. the number of ATM
cells used to transport each PDU) we can
here too calculate the cell error ratio. In
addition, if we generate PDUs which are
carried by a single cell (i.e. as shown in
the figure above) then the likelihood of
multiple errors occurring in a single PDU
is reduced.
During this measurement period the following
primary measurement values
must be known or recorded:
- Selected Cell bandwidth (used to
derive the total number of cells)
- Duration of measurement period
- Number of CRC-32 errors
- PDU length (number of cells used to
carry each PDU).
The following secondary measurements
values were also recorded for verification
of the functionality:
- Number of corrected headers (single
bit header errors)
- Number of bad headers (multiple bit
header errors of multiple consecutive
single bit header errors)
- Number of lost cells.
By calculating the relationship Number
of CRC32 Errors to Total Number of
Cells and having knowledge about the
length of the segmentation and reassembly
(SAR)-PDU we obtain a figure
for the cell error ratio. If a secondary
measurement value was found to be
larger than expected, then the measurement
was deemed invalid.
2.2 Cell loss ratio
The cell loss ratio (CLR) is defined as
the ratio between the number of cells lost
to the number of user cells transmitted in
a defined period of time.
At least three possible causes of cell loss
are identified. The cell delineation mechanism,
which is based on the verification
of the header error control (HEC) field
may correct single bit errors and detect
multiple bit errors. Double bit errors lead
to a cell discarding, while more severe
errors may lead to even worse conditions
such as cell misinsertion. Due to the statistical
nature of ATM traffic, buffer
overflow may also occur. Finally, the
user parameter control (UPC) function
may discard cells if the agreed traffic
contract parameters are violated.
A fourth condition which will cause cell
loss is the occurrence of severely errored
cell blocks, though this condition should
be filtered from any measurement period.
Due to the difficulty in filtering this
occurrence and the fact that these should
occur relatively seldom, measurements
may be necessary to perform several
times if the results indicate that such an
event has taken place.
Normally, severely errored cell blocks
should cause an Out of Cell Delineation
(OCD) event and result in a Loss Of Cell
Delineation (LOC) alarm. By measuring
the total duration in the OCD state during
the measurement period (if such an event
should occur), we may calculate the
number of cells which should have
passed during the OCD state and exclude
these from the measurement results.
Two methods of measuring cell loss are
possible; cell losses detected using the
sequence number, and cell losses detected
using cell blocks. Because this second
method was not supported by the test
equipment obtained, the first method was
chosen. Two different protocols for testing
the CLR have been looked at and are
described in the following sections.
2.2.1 Measuring the cell loss ratio
using the AAL-1 protocol
In order to calculate the cell loss rate a
test traffic of ATM cells with AAL-1
protocol should be generated. An
overview of this protocol structure is
shown in Figure 6.
The AAL-1 protocol includes a Sequence
Number (SN) field and a Sequence Number
Protection (SNP) field, thereby providing
a very reliable cell surveillance
mechanism. Lost cells should only
remain undetected when a multiple of
eight cells disappear. Though if a multiple
of eight cells are lost the switching
system should also report a Loss Of Cell
Delineation and this may constitute a