Schedule-based MAC Protocols

TRaffic Adaptive Medium Access (TRAMA)

TRAMA [94] is designed for periodic data collection and monitoring applications. It organises time into random access and scheduled access slots. Nodes send signal- ing packets for neighbour discovery and time synchronisation during the random access slots and data packets during the scheduled access slots. TRAMA uses a dis- tributed hash function to schedule collision-free slots for data transmissions. Every data packet contains a summary of a node’s schedule. Therefore, each node has to listen to the last data messages from its one-hop neighbours in order to synchronise its schedule with theirs. With TRAMA, nodes are allowed to exchange information about network topology and traffic conditions regularly in their two-hop neighbour- hood. Based on this information, TRAMA uses a distributed election scheme to

determine the state of a node, i.e. transmit, receive, or sleep. Although TRAMA adapts to topology changes by utilising CSMA periods to allow new nodes to join the network, it only provides limited capabilities to adjust to traffic fluctuations. This is achieved by allowing a node to release its slot to be used by other nodes if it has no data to send.

Compared to S-MAC with 10% duty cycle, TRAMA is more energy-efficient. S- MAC has a fixed duty cycle, so it has a constant percentage of sleep time, i.e. around 80% during both light and heavy traffic. Nodes with TRAMA sleep 6% more than S-MAC when the traffic is light, but wake up 18% more when the traffic load is high to achieve high delivery ratio. The simulation results in Figure 13 have shown that TRAMA achieves higher delivery ratio, which is around 40% to 60% over S-MAC when the traffic load increases. However, as other schedule-based protocols, TRAMA suffers higher latency, which is around 10 times higher than S-MAC.

FLow-Aware Medium Access (FLAMA)

FLAMA [93] is a schedule-based MAC protocol that extends TRAMA in an attempt to reduce idle listening overhead from neighbourhood traffic information exchange. TRAMA requires nodes to exchange traffic information regularly to maintain sched- ules during the scheduled access periods. Unlike TRAMA, FLAMA exchanges traf- fic information implicitly only during the random access periods. FLAMA uses a data gathering tree, where a node has incoming traffic flows from all its children and it has only one outgoing flow to its parent. Because of this predictable traffic pattern, FLAMA uses flows to represent one-hop traffic information and to specify the senders, the receivers and the packet’s rate. The traffic information of a node is determined based on a function of incoming flow rates from its children. These flows are used to set up transmit, receive and sleep schedules of nodes in the network using a distributed election algorithm. In FLAMA, nodes that produce or forward more traffic are assigned more slots.

Figure 13: Delivery ratio and average delay of FLAMA versus TRAMA and S-MAC with 10% duty cycle [93]

Simulations conducted in [93] have shown that FLAMA outperforms TRAMA and S-MAC with 10% duty cycle for better delivery ratio and energy consumption. Nodes with FLAMA sleep around 85% regardless of the traffic loads, but achieve 5% higher delivery ratio than TRAMA. However, S-MAC outperforms these two protocols in terms of average latency as depicted in Figure 13.

Virtual TDMA for Sensors (VTS) MAC Protocol

VTS [38] is designed for soft real-time applications, where a packet has a bounded la- tency. This protocol adaptively adjusts a virtual TDMA superframe (set of frames) length according to the number of nodes in range. Virtual means that nodes know neither superframe limits nor their relative position in the superframe. They only know that they can transmit packets every superframe length cycle. Using the flexible superframe length, VTS allows nodes to join or leave the network easily. When new nodes join the superframe, latency increases. Thus, VTS tries to keep the latency below a given threshold value by reducing the sleep interval (which corresponds to increasing the duty cycle).

This protocol assumes a network with a single-hop cluster. Therefore, there is a sink to start the setup by broadcasting a control (CTL) packet with an initial predefined value of the duty cycle. The sink then dynamically adjusts the superframe length

by recomputing the new duty cycle value based on the number of nodes belonging to the superframe and informs it to the nodes with its CTL packet. VTS uses the CSMA/CA mechanism for data delivery, where a unicast transmission follows the RTS/CTS/DATA/ACK sequence. The TDMA frame of VTS is illustrated in Figure 14. A CTL packet can be used as a SYNC, an RTS or a keep-alive beacon.

A1 A2 A3 A4 Cluster A/Cycle _{1 2 3 4 1 2} A1 A2 A3 A4 A1 A2 Time Frame Superframe CTL Active Sleep

Figure 14: VTS TDMA frame

VTS sacrifices average latency for energy efficiency at low loads, but guarantees latency at high loads. VTS is compared to 10% duty-cycled S-MAC with and without adaptive listening. With adaptive listening, nodes which overhear an RTS or CTS packet wake up at the end of the transmission, instead of waiting for their next schedules. This scheme achieves higher throughput and lower average latency at the cost of higher energy consumption. The results in [38] have shown that VTS has the lowest maximum latency compared to the two types of S-MAC at high loads, but S-MAC with adaptive listening has the lowest average latency. S-MAC with and without adaptive listening suffer 8 and 15 times the maximum latency of VTS, respectively, because the latency of VTS never exceeds the superframe length. VTS’s throughput is slightly better than S-MAC without adaptive listening. The throughput of S-MAC with adaptive listening is three times better than VTS at high loads, but its power consumption is twice as much as VTS. In addition, VTS consumes less energy than the two types of S-MAC at both high and low loads.