Another direction in manipulating the behaviour of wireless interface is through the usage of routing information. The main idea behind this is to enable wireless hosts ob- tain their position in the network or obtain the whole topology especially their parent and child hosts in order to wake up for data transmission with neighbours. At the same time, routing topology is used to study potential interference among wireless hosts and the in- formation can be wisely used to achieve high utilization of bandwidth resource through simultaneously transmission of non-interference hosts.
In some protocols, routing information is learned at a central host to perform schedul- ing. Efficient and Delay Aware Medium Access Control (PEDAMAC) [205] is a TDMA based protocol with an access point determining which mobile host should occupy what time slot. Figure 3.18 illustrates how PEDAMACS learns the topology from the network layer, and schedules the sleep time slots for nodes to construct an energy efficient route. The first phase of this protocol is to construct a routing tree at the access point by broad- casting topology learning packets. Knowing the topology of hosts attached to it, an access point determines when a host will be able to use which slot, and the schedule will be broad- casted to other hosts.
scheduling. A TDMA based protocol [220] assumes a small number of wireless hosts rooted to the sink and topology awareness could be acquired and degree of interference from neighbouring hosts could be known and controlled. In this scheme, time is divided into epochs, and each host has k slots in an epoch for the purpose of retransmission. A packet is retransmitted for k times at most if not successfully acknowledged by the receiver, thus the delay is controlled below the duration of an epoch. Energy efficiency is achieved by assigning different duty cycles for each host according to their position in predetermined data gathering tree. Routing information is required by each host to get the data gathering tree. D-MAC [206] includes a mechanism that builds a data gathering tree which describes the depth of a host in multi-hop paths. An offset is specified as u, and any hosts that reside at the depth of n in the tree will wake up du ahead of the destination, which is the top host of the three, of the path. In this case, every host is assigned its own time slot, and the node at the next hop of a path will be able to wake up after the previous hop host wakes up and sends out the packet. This protocol efficiently saves energy by letting only the hosts on the path wake up. It also helps reduce packet delay by letting hosts waking up sequentially. However, collisions may still occur when different branches of the same host try to send packets at the same time, although a back off scheme is included in this protocol. In [221], each host wakes up when it is time for it to sense the environment and when it expects packets from neighbouring hosts, which means it has to route the packets to the next host. To be specific, if there is a route starting from host a to host c via host b. host a samples and transmits a packet, both of which process take 5ms, then host b has to wake up at 10ms when host a starts to transmit. Wu et al. [222] proposes to organize a tree of hosts, which allows data aggregation to perform along the tree structure in energy efficient way than normal tree structure. Furthermore, from the viewpoint of MAC layer, wireless nodes consume different energy in different radio states and state transition. It makes sure every host has to wake up only two times in one scheduling period for receiving data from the children and sending data to the parent in order to reduce the frequency of state transition which causes energy waste. This enhanced wake-up scheduling in MAC with the energy efficient data aggregation tree achieves better energy efficiency.
Some joint protocols utilize information from network layer in MAC protocols, while others are wise routing protocols which adjust routes dynamically according to the sched- ules of hosts to avoid sleeping hosts and decrease end-to-end delay. Bernardos et al. [223] introduces a TDMA communication scheme allowing neighbour hosts cooperatively find required communication time slots and avoid redundant sending of messages. In this manner, mobile hosts will schedule their wake up procedure according to predetermined timetable and transmit packets to the neighbour only if both parties are awake allowing self to sleep for the rest of time. With the information of hosts wake up schedule, this routing solution carefully selects the path from the source to the destination that includes as many hops as possible in one time frame so mobile hosts do not have to wait too long before it forwards it to the next hop in order to achieve short delays.