Atec Agilent-J2300E - Manual (Page 16)

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For more details about policing and shaping, see the ATM QoS White Paper.
Test Cells
The ITU-T Recommendation O.191 specifies a test cell and a methodology for
using it to determine ATM QoS measurements, as defined in ITU-T
Recommendation I.356 and I.357. The ATM Forum references I.356 in its Traffic
Management 4.1 specification.
The O.191 test cell has a standard header for any user channel (any VCI above
31 is allowed). The payload has a number of special fields which allow the QoS
measurements to be made: there is a 32 bit “sequence number” field for
measurements of cell loss (ratio) and cell misinsertion (rate); there is a 32 bit
time stamp (with 10ns granularity in the least significant bit) for cell delay and
cell delay variation measurements, and the entire payload is error checked by a
CRC-16 for measurements of cell error. In addition, the entire payload is
scrambled to ensure that all bits of the payload are exercised so that memory-
based switch fabrics can be tested for “stuck” bits.
Because the O.191 test cell and the methodology for its use are standardized,
O.191 conformant test equipment from different vendors can inter-operate.
Results should be consistent between equipment from different vendors.
However, proprietary extensions are permitted, and Advisors working together
(one at each end of a virtual circuit) can detect when the network has tagged
(market to low priority, i.e. CLP = 1) a cell, which was sent at normal priority
(CLP = 0); these events are counted.
ATM Signaling and Call Placement
While permanent virtual circuits (PVCs) have dominated the ATM world thus
far, switched virtual circuits (SVCs) are increasing in utilization. Due to this
growth, it is necessary to have confidence in SVCs set up and troubleshooting
when the unexpected happens.
The Advisor includes UNI signaling emulation, ILMI address registration and
LANE emulation to provide connectivity testing. The signaling and call
placement feature supports ATM Forum UNI 3.0, 3.1 and 4.0. The Advisor can
emulate the user or network side of the UNI. In addition, the user can specify
and edit the information elements (IEs) placed on Call Connect messages that
specify what type of SVC to bring up. Figure 4 shows the configuration for call
signaling while figure 5 displays the results.
ATM Testing (continued)