# On the Feasibility of Wireless Multimedia Sensor Networks over IEEE 802.15.5 Mesh Topologies
## 3 Technological Background
### IEEE 802.15.5 Low Rate
#### 3.2.3 Synchronous Energy Saving (SES)  
Unlike ASES, the Synchronous Energy Saving (SES) mechanism is intended for static networks. It is based on a precise synchronization mechanism of the _active_ and _inactive durations_ of the nodes, which potentially decreases delays in packet transmissions. It is well-suited for delay-sensitive applications, ensuring quick reporting of events of monitoring interest.  
SES defines two transmission methods, namely the _contention-based method_ and the _reservation-based method_. In the _contention-based method_, devices can only carry out data transmission within the _active duration_, competing with other nodes for medium access. In the _reservation-based method_, nodes can transmit data within the _inactive duration_ when other nodes are in sleep mode. In this way, it is possible to take advantage of the available transmission bandwidth to relay data, making it potentially suitable for reliable transmission of multimedia data.  
In the _reservation-based method_, the _inactive duration_ is divided into slots of a fixed length. Every time a node wants to transmit, it sends a _reservation request_ message during the _active duration_, competing with other nodes for accessing the medium through the CSMA-CA mechanism. If the reservation succeeds, a slot in the _inactive period_ is assigned to transmit to the next hop in the source-destination route. The reservation process is repeated for each hop in the route during the _active duration_. Once the _inactive duration_ starts, all the successfully reserved slots are used to transmit packets from the source to the following nodes in the route. If the _active duration_ has ended and there are no slots reserved for all the hops in the route, the reservation for the remaining hops of the route will continue in the next _active duration_.  
The format of the _reservation request_ message can be observed in Figure 4. As the names of the fields suggest, the _end address_ is the address of the destination or sink node, the _next address_ is the address of the next hop of the route, selected by the TDLS routing protocol, and the _reservation slot number_ identifies the requested slot. The purpose of the _previous address_ field is to save bandwidth, since the same message used to request the reservation of a slot can also be used to inform the previous node that its reservation was accepted.  
Figure 4: Format of the reservation request message.  
As an example of the _reservation-based_ method, Figure 5 illustrates a scenario where node/device A transmits to device D through a route consisting of devices B, C and D. First, A sends a _reservation request_ message (identified in the figure by label 1) setting the _end address_ to D, the _next address_ to B and the _reservation slot_ to zero (first slot). The message is sent to the broadcast address and received by all neighbor nodes but only B processes it. In a second step, B sends a _reservation request_ (label 2) for the second slot to C (_next address_ of the message) and acknowledges A that it has successfully reserved the first slot by means of the _previous address_ field. Next, device C sends a reservation request (label 3) for the third slot to D. Finally, D sends a _reservation reply_ message (label 4) to C confirming the allocation of the third slot. After the reservation in the _active duration_, the data transmission starts in the _inactive duration_, each device using the slot that was assigned (labels 5-7) and receiving acknowledgement frames from the receivers during the same slot. This example shows, as mentioned previously, how the _reservation-based method_ can forward a data frame from the source to the destination, using a single _inactive duration_.