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Recent advances in wireless sensor networks have led to many new protocols specially designed for different applications where the energy efficiency is the main consideration. The traffic adaptive medium access protocol (TRAMA) is introduced for energy efficient collision free channel access in wireless sensor networks. TRAMA reduces the energy consumption by avoiding the collisions of transmitted data packets and it allows the nodes to switch low power mode whenever they are not in transmitting and receiving mode. This article is explained about TRAMA operation, related applications developed based on TRAMA protocol and its advantages and disadvantages.
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A wireless sensor network is a wireless network consisting of spatially distributed autonomous devices that use sensors to measure the physical or environmental quantities like temperature, sound, vibration, pressure, motion at different places. The main purpose of wireless sensor network is to collect the information from target domain and transmit the information back to specific task. Such a network usually consisting of a number of wireless sensor nodes arranges in ad-hog fashion and each node consisting of one or more sensors, low power transceiver, processor and an energy source likely a battery. A significant amount of energy is consumed by the sensor nodes radio that degrades the network lifetime. In order to reduce the energy consumption of these sensor nodes research has been done on the design of low power electronic devices. Because of hardware limitations further energy efficiency can be achieved by designing energy efficient communication protocols. Medium access control (MAC) is an important technique that ensures the successful operation of the network by avoiding the collisions and switching the nodes to idle mode which are not participating in the transmission and receptions. The following attributes has to consider while designing a good MAC protocol such as energy efficiency, latency, throughput and fairness.
In a wireless sensor network, the communication part consumes most of the energy which is the main optimization goal for making of MAC protocols. The MAC protocols directly controls the communication module and saves the energy. The major sources of energy consumption in wireless sensor networks are collisions, overhearing, control packet overhead and idle listening. Collisions are generated when a transmitted packet is corrupted due to interference and it has to retransmit again. Collisions also increase the latency. Overhearing is that a node takes up the packets which are intended to other nodes. Control packets used in wireless sensor network include ready-to-send (RTS), clear-to-send (CTS) and acknowledgement (ACK). The transmission of these packets leads to energy consumption, therefore a minimum number of control packets should be used to make a data transmission. Idle listening is one of the major sources of energy consumption. A node really does not know when it will be the receiver of a massage from one of its neighbours. So, it must be wake up all the time for incoming message resulting in idle listening and that can consume 50-100% of the power required for receiving.
The MAC protocols for wireless sensor networks can be classified broadly into two categories i.e. contention based and scheduled based. The schedule based protocol can avoid collisions, overhearing and idle listening by scheduling the transmission and listen periods under the strict time synchronization requirements. The contention based protocols have relax time synchronization requirements and can easily adjust to the topology changes as some new nodes are joining to the network or some of the nodes are expired after deployment. These are based on carrier sense multiple access technique and have higher costs for message collisions, overhearing and idle listening. There are a considerable number of MAC protocols which are implemented for different applications in WSN. Each protocol works on different techniques and at last all are targeted for energy efficiency. TRAMA is one of the scheduled based distributed TDMA (time division multiple access) based protocol that has been designed for energy efficient collision free channel in WSNs. The remainder of this section is explained in next sections.
Traffic adaptive medium access (TRAMA) protocol which aims to achieve the energy efficiency by avoiding the collisions of data packets while receiving and by employing a low power mode for node which are not scheduled in transmission and reception. The usage of low power mode is dynamically determined and adapted according to traffic pattern. TRAMA applies a traffic adaptive distribution election scheme that selects the receivers based on the schedules announced by transmitters. Nodes using TRAMA, exchange their two hop information and the transmission schedules fixing which nodes are the intended receivers of their traffic in chronological order. TRAMA consists of three components which are neighbour protocol(NP), schedule exchange protocol (SEP) which allows to exchange two-hop neighbour information and schedules and adaptive election algorithm (AEA) uses the information of NP,SEP and it selects transmitters and receivers for current time slot and leaving the other nodes in network to switch to the low power mode.
TRAMA assumes a single time slotted channel for both data and signalling transmissions. The above figure shows the overall timeslot organization of the protocol. Time is organized as sections of random and scheduled access periods. The author  assumes that random access periods as signalling slots and scheduled access slots as transmission slots. NP propagates among neighbouring nodes and gathers the data during random access period using signalling slots; this way two-hop topology information is obtained. Collisions may happen when contention based mechanism is used during random access period. Transmission slots make collision free data exchange possible by schedule propagation.
Schedule exchange protocol (SEP) is responsible for maintaining the updated schedules with neighbours periodically. Generally these schedules are holds the traffic based information which is useful for collision free data transmissions. A node announces its schedule before the transmission stage. The length of the transmission slot is fixed based on channel bandwidth and data size. Signalling packets are smaller in size compared to the data packets so the transmission slots are typically longer than signalling slots.
ACCESS MODES AND THE NEIGHBOR PROTOCOL: –
In the sensor networks, often nodes are power drained and some new nodes are may join to the network. To accommodate these dynamic changes in topology, TRAMA alternates between random and scheduled access. TRAMA starts in random access modes where each node transmits the data by selecting a slot randomly. More dynamic networks require more frequent random access periods and it is opposite to static networks. During these random access periods, all the nodes must be in transmit or receive mode. NP gathers the neighbourhood information by exchanging signalling packets during the random access periods. The above figure 2(a) shows the format of the header of a signalling packet. If there is no up dated information, the signalling packets will send”keep-alive” beacons to its neighbours. A node times out its neighbours, if it does not hear from that node for a certain period of time. By the end of random access period all the nodes will have the information about its two hop neighbours with 0.99 probabilities.
SCHEDULE EXCHANGE PROTOCOL: –
The name itself tells that SEP maintains the traffic based schedules across the neighbours and it periodically updates the schedules. A node has responsible to announce its schedule using SEP before starting the actual transmission. Schedule generation process as follows that each node computes SCHEDULE_INTERVALL based on the packets are generated by the higher layers. The SCHEDULE_INTERVALL of a node represents the number of transmission slots for which the node announce to its schedule to its neighbours according to its current state of MAC layer queue. The node then pre-computes its number of slots in the interval for which it selects the highest priority among its two hop neighbours which we called as “winning slots”. These winning slots are selected as transmitter and as well node announces these slots to intended receivers. If the node does not have data to send in its slots then it gives up the corresponding slots to other nodes who want to use these “vacant slots”. A node lost winning slot is this interval is reserved for broadcasting the schedule in next interval.
Nodes announce their schedule through schedule packets. In these criteria there is no need of receiver address which has obtained through NP. Nodes convey the information to intended receiver using a bitmap whose length is equal to the number of one hop neighbours. Each bit in the bitmap corresponds to one of the receiver order by their identities. An advantage of using these bitmaps is simple with broadcast and multicast communication can be supported. To broadcast a packet to all its neighbours then all the bitmap bits are set to one. If the packet need to multicast with the neighbouring nodes then only that bits are set in the bitmap. For vacant slots the node announces zero bitmap. The slots which are mapped as zero can be used by other nodes in the two-hop neighbourhood. The format of scheduled packet is shown below. In that format the last winning slot in the bitmap is always reserved for announcing the next scheduling.
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Additionally, the data packets can carry the summary information of a node schedule to its neighbours. These schedule summaries help to minimize the effects of packet loss. As shown in the figure 2(b) the data packets contain the three blocks of summary information of scheduled data in that bitmap corresponds to the number of winning slots in the current interval. The size of the bitmap is number slots and is used to indicate whether it in transmit mode or giving up the corresponding slot. Nodes will maintain synchronization with their one hop neighbours by this schedule information. The slot after which all the winning slots go unused is called change over slot. All nodes have to listen during the change over slot to synchronize their schedules.
ADAPTIVE ELECTION ALGORITHEM(AEA): –
A node is selected to transmit when it has the highest priority among its contending set. At any given timeslot T, the state of a given node U is determined by U’s two-hop neighbourhood information and the schedules announced by U’s one-hop neighbours. The node can be in three states at given timeslot T, that are transmit, receive and sleep. (i) If a node U at T in transmits, then it has the highest priority and it has the data to send then the slot is used for transmission. (ii) If a node U in receive mode, when it is the receiver of the node which is in transmit mode in the current slot. (iii) Otherwise it will go to sleep mode.
Whenever a node has absolute winner for a particular slot and it has non zero bitmap for this slot, it knows that no other node in its two hop neighbourhood will be transmission in this slot. Then collision free transmission is happened to intended receivers. When a node is not absolute winner, then it is not sure who the actual transmitter for a particular slot is. For example, consider a topology in figure (4), it shows that node D is the highest priority in B’s two-hop neighbourhood. In a given time slot and node A is the highest priority in its two-hop neighbours. So, both the A&D transmit in the timeslot because they are absolute winners. The absolute winner to node B is node D. The node B is looks at it schedule for D and finds that B is not intended receiver of D, then it goes to sleep mode by missing the A’s transmission. So, before going to sleep mode a node must also account for the alternate winner. This inconsistency will occur when the alternate winner is hidden from the absolute winner i.e. they are three hops away.
ADVANTAGES AND DISADVANTAGES: –
Higher percentage of sleep schedules and collision free transmissions are achieved compared to CSMA based protocols. TRAMA performance can get adapted dynamically to the network traffic conditions and adapts the topology changes throughout the network.
Moreover, without considering the transmission and reception, the duty cycle is at least 12.5%; the percentage is excessive for the kind of networks. The latency of TRAMA is more when compared with other contention based protocols such as S-MAC and IEEE 802.11. The delay performance is obtained by analytical mode which notes that TRAMA has higher delay when compared with NAMA.
TRAMA protocol is suitable for applications like not delay sensitive but require high energy efficiency and throughput. A typical example is sensor network used for periodic data collection and monitoring applications.
TRAMA is energy- aware channel access protocol for wireless sensor networks. TRAMA uses the traffic based schedules and avoids the wasting slots and switch the nodes to low power mode when there is no data to send and they are intended receivers of traffic. Through extensive simulations, TRAMA shows significant energy savings i.e. nodes can sleep 87% of its time when compared with scheduled and contention based MAC protocols depending on the offered load. TRAMA also have higher throughputs around 40% over S-MAC and CSMA and 20% over 802.11. It avoids the collisions due to hidden terminals. TRAMA protocol is more suitable for applications like with not delay sensitive and produce high energy efficiency.
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