Stream
This module contains the L23 stream classes deal with configuration of the traffic streams transmitted from a L23 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.
Stream 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 and consists of the following sections:
Field |
Length |
Explanation |
|---|---|---|
Checksum (optional) |
2 bytes |
See the note. |
Sequence Number |
3 bytes |
Packet sequence number used for loss and misordering detection. |
Timestamp |
4 bytes |
Timestamp value used for latency measurements. |
Test Payload ID (TID) |
2 bytes |
Test payload identifier used to identify the sending stream. |
Payload Integrity Offset |
1 byte |
Offset in packet from where to calculate payload integrity. |
First Packet Flag |
1 bit |
Set if this is the first packet after traffic is started. |
Checksum Enabled |
1 bit |
Set if payload integrity checksum is used. |
<reserved> |
7 bits |
|
Payload Integrity Offset (MSB) |
3 bits |
Offset in packet from where to calculate payload integrity, MSB (bits 10:9:8) |
Timestamp Decimals |
4 bits |
Additional decimals for the timestamp. |
Checksum |
8 bytes |
TPLD integrity checksum. |
Total TPLD Size |
20 or 22 bytes |
Note
If the P_CHECKSUM offset (Payload Checksum Offset) is enabled on the parent port, then an additional 2-byte checksum field is inserted in the TPLD, just before the Sequence Number. This increases the total size of the TPLD to 22 bytes.
Field |
Length |
Explanation |
|---|---|---|
First Packet Flag |
1 bit |
Packet sequence number used for loss and misordering detection. |
<reserved> |
1 bit |
|
Test Payload ID (TID) |
10 bits |
Test payload identifier used to identify the sending stream. |
Timestamp |
28 bits |
Timestamp value used for latency measurements. |
Checksum |
8 bits |
TPLD integrity checksum (CRC-8) |
Total Micro-TPLD Size |
6 bytes |
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.
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.
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.