# Improvements of the xAAL Home Automation System
## 2 The xAAL Architecture
### Communication Channel  
The xAAL architecture is based on the many-to-many communication principles, concretely a communication bus. Any component can post messages (e.g., sensor data) to another, several, or all components without preliminary connection setup. This has many advantages over the usual client-server communication principles. Point-to-point communications are avoided, which saves device memory resources. Indeed, a home automation component is usually linked to many others for things to work. Maintaining many permanent connections in parallel may be extremely heavy for constrained devices. A bus also brings some functional benefits. This allows the discovery: when a new component appears in the installation, it announces itself. All other entities can then take it into account. Similarly, when a new component enters, it can query the bus to discover the other components already present. This greatly facilitates the configuration, allows dynamicity as well as the evolution capacities of the system. For these reasons, the xAAL system has been designed on an IP _multicast bus_ (IPv4 or IPv6). WiFi and Multicast  
WiFi weaknesses are out of the scope of the home automation study. However, this may have an impact on the xAAL architecture. This is why a discussion is welcome. WiFi (IEEE 802.11) is a common use wireless technology for IP networks, also at home. It is widely used to provide Internet access to mobile devices (smartphones, tablets, laptops, etc.). It is also used for non-mobile devices (connected TV, video game consoles, IP CCTV cameras, etc.) when users are refractory to the installation of Ethernet cables. Unfortunately, IEEE 802.11 protocols do suffer from design choices regarding the management of broadcast and multicast packets. Simply speaking, it has been chosen not to acknowledge broadcast and multicast packets at the radio level, contrary to unicast packets. Indeed, avoiding managing a list of nodes having not acknowledged packets greatly simplify the protocol implementation. Broadcast and multicast packets are sent only once, and at a lower rate to get a better chance to be received without error. This point has been studied by research in wireless networks since the very first years of WiFi. Several proposals have been made, by modifying the radio MAC layer, by introducing a central coordinator, by replacing multicast by unicast either at the MAC layer or at the application layer, etc. [15; 16; 17; 18]. For twenty years, the Mboned IETF working group has aimed to facilitate the deployment of multicast technologies for the Internet at large. This working group also considers the issues of deploying multicast on WiFi networks [19]. Nowadays, several mitigation solutions are listed, but none are perfect. The conclusion of this study is that \"IEEE 802.1, 802.11, and 802.15 should be encouraged to revisit L2 multicast issues.\"  
Indeed, this situation contributes to the \"Ossification of the Internet\" [20]: due to (technical and administrative) limitations of several types of infrastructure equipment, innovations tend to restrict themselves to a small subset of Internet services. To get a chance to get clients, one only uses TCP and Web communication technologies to implement innovations. On the other hand, equipment manufacturers take time to enhance their product to fully support Internet principles with the argument there are a few consumer requests. Note that the situation is probably similar when diffusing multicast packets over HomePlug-AV PowerLine Adapters [21; 22]. Is this severe? Well, this greatly depends on the quality of the WiFi Access Point in use. Let us consider a managed mode 802.11 network: several WiFi Stations (STA) connected to a WiFi Access Point (AP). If multicast packets are transmitted from an STA to the AP, packets are well received almost as good as unicast packets. However, if packets are transmitted from the AP to STAs, there are risks packets are lost, depending on the quality of the AP and the global situation.