## 3 Architecture of the Platform and Related Software and Hardware Aspects
### Core Facilities for Nodal Interactions and Ad Hoc Networking
#### 3.1.4 Traffic Endpoints and Support for Calculating Performance Metrics  
As currently implemented, the platform directly supports MANET experimentation scenarios that employ fixedtraffic source and destination endpoints (as in the scenario of Figure 1). While it is readily possible to implement the source and destination nodes by the same infrastructure used for the mobile nodes, simply by keeping the source and destination stationary at their fixed locations, the platform provides specific stripped-down  
Figure 4: Format of the Routing Message.  
Figure 5: Format of the Acknowledgment Message.  
facilities for them, reflecting their special nature. In more detail, neither the source nor the destination node need the localization module, as they remain stationary at locations known in advance. Similarly, the source node does not need to transmit beacons advertising its status; it just needs to listen to beacons from mobile nodes in its neighborhood so that it can exercise the routing protocol to select the next hops for the messages originating from itself. Analogously, the destination node must send beacons to advertise its presence but does not need to maintain a beacon table for its neighborhood or to exercise the routing protocol functionality (except from returning an ACK for each message delivered to it).  
Unlike the mobile nodes, the source node includes a message generation facility for periodically creating messages to be forwarded to the destination node. As already discussed in the previous subsection, the platform supports experiments employing multiple routing protocols in parallel, so each generated message is marked with the ID of the particular protocol pertaining to it. Parameters control the number of different routing protocols that will be employed and the number of messages that will be generated for each protocol.  
It is straight-forward to upgrade the platform so that it can support traffic originating from mobile source nodessimply by incorporating the message generation functionality in the mobile node infrastructure. Implementing mobile destination nodes is more difficult, as many routing protocols require knowledge of the destination node's location. However, the platform can directly support the delivery of traffic to multiple alternative fixed destination nodes. All that is required is to mark each message generated at a source (fixed or stationary) with the destination node applying to this message (according to a pattern determined by the experiment).  
The platform also provides facilities for calculating performance metrics: Once a message is generated at its source, a time-stamp of the message generation instant is recorded and relayed all the way to the destination through the corresponding routing messages (see the respective field in Figure 4). When the destination node takes delivery of the message, it subtracts the time-stamp from the current time to determine the end-to-end delay. To enable accurate delay calculations, the source and destination nodes are equipped with real-time clocks (RTC) to maintain their synchronization. Alternatively, this synchronization can be achieved without RTCs, using a Network Time Protocol (NTP) server instead, provided that all MANET nodes are equipped with Internet connectivity through an additional network interfacedifferent than that used for the ad hoc connectivity. Such a dual connectivity feature is also useful for monitoring the operational status of the platform, as discussed further in Section 3.2.  
In addition to delays, the platform provides support for determining the number of hops along the path from source to destination. This metric is calculated incrementally as the routing message propagates from hop to hop, and the updated value is stored in the last but one field of the routing message (see Figure 4). The final value of the metric becomes available when the message is delivered to its destination node.