sac

SAC

Table of content

• SAC
  • OpenSAND Exploitation
  • OpenSAND Software Design

SAC (Satellite Access Control) messages are implemented in OpenSAND to allow the terminals perform capacity requests to the gateway.

SAC messages are sent periodically (in the Out of Band Request, OBR, slot of each SuperFrame) from the terminals to the gateway; the procedure has two steps:

• The ST sends the SAC message towards the SAT using a dedicated carrier for control messages.
• The SAT broadcasts the SAC message on a dedicated control carrier to the corresponding GW/Spot.

When sending a SAT, the ST includes the capacities to be requested to the gateway, calculated by the Dama Agent entity in the satellite terminal. The capacity request types can be either RBDC or VBDC.

SAC messages are used also to communicate the gateway the C/N measured on the forward link, needed to implement an Adaptive Coding and Modulation (ACM) scheme.

⚠️ In OpenSAND SAC messages are not affected by attenuation, so they are never lost. They carry, however, C/N information that is used by the ACM loop.

OpenSAND Exploitation

The user is capable of specifying the periodicity at which capacity requests (and SACs) are sent. The default value is 1000 ms.

⚠️ The sync_period is not strictly the time between SACs. This value, expressed in milliseconds, is divided by the length of a DVB burst, also expressed in milliseconds, and the result rounded, in order to obtain the periodicity in number of frames.

The maximum volume request that a SAC can hold is 4080 kb.

No probes exist associated to signaling messages, although some logs are emitted when a certain type of signaling message has been received/emitted.

OpenSAND Software Design

SAC messages are implemented as specializations of the DvbFrameTpl (which means that each type of message has different header fields).

The specific header fields to this type of message are:

Header fields:

• tal_id: the terminal ID.
• group_id: the terminal group ID.
• cr_number: the number of capacity requests in the SAC message.
• acm: the emulated ACM fields (described below).
• cr: the emulated capacity requests (described below).

The emulated ACM fields are composed of the following elements:

• cni: the measured C/N on the forward link

The emulated capacity requests are composed of the following elements each:

• prio: the request priority.
• scale: the scale of the value (total allocation will be equal to scale * value).
• type: the request type (VBDC, RBDC or AVBDC).
• value: the request value.

The class Sac implements methods for initializing and working with DvbFrames of this type.

SAC messages are created by the BlockDdvTal periodically, and filled out by the DamaAgent class located on that terminal.