Cambio climático en la microcuenca del Pita

The example scheduler architectures considered in clauses 6.1.1 and 6.1.2 are summarized in figures 20 and 22. Both of them, in principle, assume FIFO buffering (of un-specified depth) at any queuing entity, and both allow for any type of encapsulation. Figure 20 refers to a single input stream plus the ACM command, thus avoiding DVB-S2 modulator scheduling functionality. Figure 22 refers to multiple Input Streams (one per each of the K protection levels), where scheduling functionality is included in the DVB-S2 modulator (merger/slicer function). The baseline scheduling policy (so-called "merging policy") is a cyclic polling of input buffers (Round Robin, RR) plus a timeout, as explained in clauses 6.1.1 and 6.1.2. The use of Normal and Short FECFRAME is allowed. The scheduling relevant to service level (N buffers, one per each QoS) is performed by the ACM router, outside the DVB-S2 system.

In order to perform a preliminary analysis, the following simplistic design decisions were taken according to figure 22:

• Single QoS services (best effort) and fixed user packets length.

• Total number of physical layers and FIFO buffers K = 24, out of the 28 allowed by DVB-S2.

• Depth of the K buffers: equal to two times DFL (see note 1) (DFL depends on modulation and coding scheme).

• Slicing/merging policy: simple round robin plus timeout, no padding, only Normal FECFRAME.

NOTE 1: To be noted that the K FIFO buffers are included in the ACM control loop, therefore their depth should be minimized.

In order to set up a simulation framework, a jointly event and time driven simulator has been developed in OPNET, based on the block diagram of figure 26. A multi-beam system has been implemented in order to simulate the

location-and-time dependant SNIR. The RF interference from the neighboring beams is assumed to be constant in time but location-dependent. The model allows arbitrary number of beams and arbitrary antenna pattern and frequency reuse pattern. Each beam is split in a number of uncorrelated fading regions [typically 4 for a ~0,67° beam aperture].

Traffic Source user # 1 of Beam Traffic Source user # 2 of Beam Traffic Source user # N of Beam SYSTEM MODEL (channel and system model is time-driven)

TRAFFIC FIFO (inifinite length)

ACM DVB-S2 Subsystem

Beam # i

REALISTIC

SYSTEM

PARAMETERS

Figure 26: Block diagram of the jointly time and event driven simulator

Assuming a total number of U users within the beam, the corresponding SNIR levels at location of user u_i (including time and location variations) are computed, and the spectral efficiencies η_i (useful bits per unit transmission bandwidth by satellite) are derived from the DVB-S2 characteristics (according to the DVB-S2 specification [2], clause H.1). A "SNIR report" is sent back to the GW any time a change in the physical layer is detected. One of the key parameters under analysis is the end-to-end delay experienced by user data packets in the forward link path.This can be derived as

q prop

loop T T

T = + , where T_prop, propagation time, is around 240 ms to 250 ms, depending on the latitude and longitude of the user location, and T_q is the waiting time at the queues encountered across the system.

Traffic injected into the system is event-driven and exponential inter arrival times are considered. Also the whole packet scheduling procedure is event-driven.

In [15] a study-case simulation is illustrated, verifying the behavior of the ACM subsystem in case the unique FIFO architecture is implemented. The load of the simulated DVB-S2 system should be either limited below 90 % or controlled by some admission control mechanism. FIFO queuing places an extremely low computational load and implementation complexity compared with more elaborate policies. Their behavior is easily predictable, since maximum delay is determined by the maximum depth of the queue. However, a unique FIFO queuing also poses the following limitations:

• routers may not organize/access buffered packets, e.g. according to a particular physical layer;

• the queuing delay equally impacts all flows without discrimination. In the ACM case, "congestion" due to the bit rate reduction of "bad" users (low SNIR) will affect "good" users (high SNIR);

• a very bursty flow may consume the entire buffer space causing all others not to be serviced for a while. A worst-case would occur if a bursty flow occurs on a faded channel.In the case of ACM using multiple TS or generic streams, assuming a per-physical layer buffering, the simple Round Robin polling strategy adopted as baseline in the specification can dynamically distribute the capacity between the various protection levels when the average traffic load is below the maximum capacity. Instead, during overload periods, it implies a pre-defined (overall and per protection level) capacity allocation profile (see note 2), and predefined delay characteristics penalizing both "good" and "bad" users. It should be noted that "bad" users consume a significant amount of the overall system "transmission time" resources, while achieving a reduced throughput (due to the low code rate). Therefore the baseline round robin policy is a sort of "simple solution" not specifically designed to facilitate good-users or to maximize the overall system throughput during channel fades. In order to allow optimizations according to specific service requirements, the DVB- S2 specification opens the door to additional merging/slicing policies, for example weighted Round Robin strategies (see note 3) profiled by the service operator.

NOTE 2: One data field contains KBCH bits and is transmitted in a time interval inversely proportional to the modulation capacity: for example rate ¼ users transmit at half bit rate than rate ½ users assuming the same modulation, but QPSK users are allocated double "transmission time resources" than 16APSK users.

NOTE 3: For example, the merger may read k_i consecutive data fields from the i-th protection branch, where k_i may be profiled by the service operator.