4.4.1.1 SNMP Parameter Translations

SNMP treats parameters (objects) in a different way as the sat-nms software does. The SNMP agent translates the internal representation of M&C parameters to the SNMP representation and vice versa. The default translation rules are listed in the table below:

Default Translation

• sat-nms data type --- SNMP data type**comment
• INTEGER --- INTEGER : No translation necessary.
• FLOAT --- INTEGERSNMPv1 : cannot handle floating point numbers. FLOAT variables therefore are translated to integer values when exported as a SNMP object. The value is scaled by 10^ n where n is the number of precision digits set for this variable in the device driver definition file. Example: The azimuth value of an antenna controller appears with three precision digits as 180.000 in the sat-nms user interface. The SNMP representation of this angle will be 180000 (floating point value multiplied by 10^3). When the SNMP agent receives values to set at the M&C, the translation happens the other way round.
• TEXT --- OCTET-STRING : Text type variables translate as OCTET-STRING SNMP objects. In the MIB, text variables should be declared as 'DisplayString' (defined in RFC1213-MIB). This ensures, that the strings are shown as expected by the MIB browser. (Some browsers assume OCTET-STRINGs to be binary and display a hex-dump instead of the textual contents.)
• CHOICE --- INTEGER : Internally the sat-nms software treats CHOICE parameters as text. SNMP uses enumerated integers for this. The value of a sat-nms CHOICE parameter translates to the position of the value in the list of choices as defined in device driver file. If - for some reason - the value does not match any of the selections given by the device driver, it translates to 0. The MIB should define memnotics for all selections.
• BOOLEAN --- INTEGER : A boolean value translates to an enumerated integer with the values 0=FALSE and 1=TRUE.

The SNMP agent knows from the sat-nms variable definition if a parameter may be accessed read-write or read-only. Each individual parameter may be limited to read-only access when exported to SNMP (see below).

'force to text' Translation

The SNMP agent may be configured (individually for each parameter) to force a text-translation to OCTET-STRING, regardless of the type of the parameter. In this case, INTEGER or FLOAT values translate to a readable text, formatted with the precision defined in the device driver file and the unit string appended. BOOLEAN value translate to the keywords TRUE or FALSE and any other parameter translate the text the sat-nms user interface shows. The MIB should contain a 'DisplayString' entry for each parameter which is configured this way. If the agent receives a SET request for a parameter which ist translated in 'force to text' mode, it parses the text and sets the value as the user interface would do.

The 'force to text' option specially is valuable together with the 'force to readonly' option described below. It provides a simple method to create easy to read status reports which can be displayed in a MIB browser.

'force to readonly' Translation

The 'force to readonly' option permits to limit the access to read-only for each individual parameter. This may be used to prevent SNMP users from changing sensible settings in the M&C.

'suppress range limits' Translation

The 'suppress range limits' option suppresses the output of the min/max range of a numeric parameter. This may be useful if this range dynamically changes and shall not be fixed to the limits valid at the time when the MIB is created.

Limitations

Not all M&C parameters may be translated to SNMP objects in a way that makes sense. First of all, there are a couple of M&C variables which carry complex information, binary coded as Java objects. Translating these objects to SNMP is not possible as the information contained in these objects is for internal use in the sat-nms software only.

Another problem of translating values between their sat-nms representation and SNMP objects arises if the M&C dynamically changes the definition of this variable during runtime. This is a special feature of the sat-nms software which makes it's internal message concept superior to SNMP for this application.

Example: A satellite modem is capable to encode with FEC rates 1/2, 3/4 and 7/8 with BPSK modulation. If switched to QPSK, also the rates 4/5 and 5/6 are available. The device driver will change the definition of the 'fec' parameter as the 'modulation-type' parameter changes.

It is easy to understand that the translation of the 'fec' along the enumeration list given in the device driver will give wrong results after the device driver changed the parameter definition. It is strongly recommended to configure such parameter as 'force to text' and 'force to readonly'. This way at least a correct display via SNMP can be achieved.

A similar situation arises if a device dynamically changes the limits for a numeric parameter. An example for this are the pointing angles of an antenna: The soft limits set by the operator will be used as range limits for this parameter. If they are changed after the MIB has been created, they will no longer match the limits stated in the mib. Such parameters should be marked with the 'suppress range limits' translation option in the configuration file.