# Communication Transport Protocol Strategies for Rail Applications
## 3 Modelling the ACS  
In this section, the description of the ACS model is provided and used for the testbed implementation and to evaluate its performance.  
A conceptual representation of the elements related to the new communication system based on ACS can be represented as in Figure 2, which contains the essential components of an ACS-based network architecture. From left to right we schematized the on-board host devices where user agents (UAs) run. Such devices are connected to an on-board ACS Gateway, which exposes interfaces to offer connectivity to the User Agents towards the bearers and through them to services exposed by server User Agents connected to the network ACS GW. The bearers, at the centre of the figure, can be imagined as channels implemented with different technologies for transporting IP network traffic.  
Regardless of any complexity related to how the components of Figure 2 can be implemented, the ACS can be effectively represented by means of a network of hosts and related network interfaces, distinguishing between physical network interfaces (e.g., the ones representing the bearers) and logical network interfaces (i.e., the tunnel interfaces across the bearers and User Agent logical endpoints). Finally, none of unnumbered interfaces need to be represented in the topology since they are transparent in terms of IP addressing.  
Figure 4 represents such a logical model where physical and virtual interfaces are part of the ACS layered network topology. The scheme of ACS topology places the gateways at the centre. The host devices where the User Agents run (by means of client and server applications) are connected to each adjacent gateway. So, the on-board devices are connected to the OG, and Network Devices are connected to the NG. The connectivity between on-board and ground user agents is established by a virtual circuit. A virtual  
Figure 3: ACS Protocol Stack.  
circuit is nothing more than a defined path (through routes and IP tunnels) between two ACS User Agents. The gateway role is to abstract any complexity due the presence of multiple bearers and provide the necessary signaling mechanism for service registration and circuits' establishing.  
A virtual circuit is obtained by IP routing and multi-path tunneling. Multi-path tunnels are created between the two gateways. The number of bearers and transport technologies involved in a multi-path tunnel depend on configuration, which is meant to ensure a required QoS. A tunnel can be associated to multiple aggregated bearers via a redundancy mechanism based on multi-path transmission techniques.  
The second important aspect of virtual circuit is the logical address (and the logical device) used to represent the endpoints of a virtual circuit and to identify the User Agent in the ACS network. Each User Agent, which can represent either a client or a server of a service, is identified by a unique IP address resolved by the Domain Name System (DNS) once an agent is registered.  
From the implementation point of view, each host device can create a virtual network device by using IPVLAN Driver [15] as a result of a control plane signaling based on the SIP [16] and DNS protocols. Each IPVLAN logical interface represents the endpoint of a virtual circuit.