link capacity

Link capacity

Table of content

• Link capacity
  • OpenSAND Exploitation
    • Probes
  • OpenSAND Software Design

The capacity available on each link (Forward and Return in transparent mode, or Uplink and Downlink on regenerative mode) is determined in OpenSAND by two factors, the carrier bandwidth and the MODCOD used: the larger the carrier bandwidth in that link is, and/or the higher the MODCODs (more performing) used are, the larger the available capacity will be (and vice-versa).

Since the MODCOD used on each link may change dynamically (if VCM or ACM coding and modulation schemes are used), the capacity will not always be the same. However, the maximum and minimum values can be calculated.

Given a carrier symbol rate ($R$), the total carrier capacity ($C$) for a certain MODCOD ($m$) can be obtained as:

$$C = R \times modulated\_bits\_per\_symbol\left(m\right) \times coding\_rate\left(m\right)$$

The maximum carrier capacity is determined by the most performing MODCOD, whereas the minimum is determined by the most robust MODCOD, giving thus the range of the capacity of a given link.

In reality, however, the attainable data rate is smaller, because of different reasons (protocol overhead, pilot symbols, etc.).

NOTE: Coding rates in MODCOD names (for example QPSK 1/4) are not exact, since they only specify the coding rate of the outer code for simplicity purposes. Real coding rate is slightly smaller, when the inner code is considered.

NOTE: Another way of determining the capacity $C$, considering the real coding rate, is using the following expression: $C = BW_{effective} \times spectral\_efficiency\left(m\right)$ where $BW_{effective} = R \times 1 sym^{-1}$ is the effective bandwidth (without considering the roll-off), and the spectral efficiency is a parameter of the MODCOD $m$.

OpenSAND Exploitation

Since the $modulated\_bits\_per\_symbol$ and $coding\_rate$ values are determined by the MODCOD (fixed by the standard), the user only can change the carrier symbol rate to adjust the capacity (other than enabling/disabling different MODCODs). Refer to the carrier page for more information.

The maximum and minimum capacities available are displayed on the Resource Configuration tab on the OpenSAND Manager, under each link configuration. The capacities are obtained for each category, adding the capacities of all carriers of that category.

Bear in mind that the data rate on the return link will never exceed the allocations for each terminal. On the forward link, there is no such limitation, so the total capacity should be attained without additional configuration.

Probes

There are several probes that concern the link capacities in OpenSAND, albeit with different units depending on the link, and only available on the gateways.

On the forward link, capacity probes are available for each spot and category (Standard and Premium). For each category, two probes are available for each carrier (and two for the sum of all carriers) inside the Down_Forward_capacity section:

• Available: The available capacity per frame (expressed in symbols per frame).
• Remaining: The remaining (not used) capacity per frame (expressed in symbols per frame).

On the return link, capacity probes are available for each spot and category (Standard and Premium). For each category, two probes are available for each carrier (and two for the sum of all carriers) inside the Up_Return_capacity section. In the section Up_Return_total_capacity the same two probes are also available for the sum of all categories:

• Available: The available capacity (expressed in symbols per second) in one Superframe.
• Remaining: The remaining (not used) capacity (expressed in symbols per second) in one Superframe.

OpenSAND Software Design

Link capacities are calculated and used when scheduling the different protocols, and their implementation is detailed on each page. Please refer to the DVB-S2 and DVB-RCS2 for more information.