Emulators

ACE emulator

The aim of the ACE system is to emulate a network environment composed by a set of earth stations that communicate through a satellite platform. Every station may act as router and consequently may interconnect different networks.
Furthermore, in order to achieve the data communication through the satellite network, a satellite modem is included within each station. In more the modem may be an independent hardware entity connected to other units by means of a cable or also a network adapter card plugged into a unit (e.g. the router itself or a PC). In practice, it can be though as a data link layer of an overall protocol stack.
It is possible to identify, in a real satellite system, the following main parts:
  • a modem with an interface towards the upper layers (namely the network layer);
  • a channel characterized by its own peculiarities;
  • a data link protocol over the satellite channel and a satellite with its on-board switching capabilities.

Fig. 1
Fig. 1 Overall Emulator Architecture and Real System.

The reference architecture of the emulator is shown in Fig. 1, along with one possible system to be emulated enclosed in the cloud (a GEO satellite system has been depicted in this case). Different units called Gateways (GTW) operate as interface among the emulator and the external PCs.
Each GTW is composed of a PC with two network interfaces: one towards the external world (a 10/100 Mbit/s Ethernet card), the other towards the emulator. An Elaboration Unit (EU), which has a powerful elaboration capacity, carries out most of the emulation, as the decision about the "destiny" of each PDU.
The interface towards the external world concerns the GTWs; the loss, delay and any statistics of each PDU regards the EU; the real transport of the information PDU through the network concerns the input GTW and the output GTW.

The various components are connected via a 100 Mbits/s network, completely isolated, by a full-duplex switch. In such way, the emulator has an available bandwidth much wider than the real system to be emulated, which should not overcome a maximum overall bandwidth of 10/20 Mbits/s.

Fig. 2
Fig. 2 Emulator vs. Real System

In more detail, Fig. 2 shows how the different parts of the real system (modem, data link protocol, channel and switching system, as mentioned in the previous sub-section) are mapped onto the different components of the emulator.
It is clear in Fig. 2 that, the architecture of the emulator is not exactly correspondent to the real system. The earth station, identified by the grey rectangle, is divided, in the emulator, into two parts (GTW and EU). The network layer, the network interface towards the external world and the interface between the network layer and the satellite modem are contained in the Gateway (GTW).
The other parts of the modem (i.e. the data link layer, protocol and encapsulation), the overall transmission characteristics (e.g. bit error ratio, channel fading, lost and delayed packets), the on-board switching architecture as well as the queuing strategies are contained in the Elaboration Unit (EU).
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