3.4. Stream
This module contains the stream commands deal with configuration of the traffic streams transmitted from a test port. The stream command names all have the form PS_<xxx> and require both a module index id and a port index id, as well as a sub-index identifying a particular stream.
General Information
Enabling Traffic
Whether the port is actually transmitting packets is controlled both by the P_TRAFFIC command for the parent port and by the PS_ENABLE command for the stream.
While the parent port is transmitting, the parameters of any enabled stream cannot be changed.
Test Payload Data (TPLD)
Each Xena test packet contains a special proprietary data area called the Test Payload Data (TPLD), which contains various information about the packet and is identified by a Test Payload ID (TID). The TPLD is located just before the Ethernet FCS.
The selection between the default TPLD and the micro-TPLD is done on the parent port. It is thus not possible to use different TPLD types for streams on the same port.
See also
For more information about the TPLD, see the Test Payload.
Disabling TPLD The TPLD function can also be completely disabled for any given stream by setting the Test Payload ID (TID) value for the stream to the value -1 using :PS_TPLDID
Minimum Packet Size Considerations
The stream will generally accept any configuration and attempt to transmit packets according to the configuration. In order for the various Xena stream features to work correctly certain aspects about the minimum packet size used must be observed.
The minimum packet size must obviously be large enough to accommodate the defined protocol headers + the final Ethernet FCS field.
If the TPLD function explained above is enabled then each packet must also be able to contain the TPLD area (20, 22 or 6 bytes depending on the configuration).
If the stream payload type is set to Incrementing, then an additional minimum payload area of 2 bytes is needed. Otherwise excessive payload errors will be reported. This is however not necessary if the P_CHECKSUM offset (Payload Checksum Offset) option is enabled on the parent port as this will override the payload integrity check implied by the Incrementing payload type.