## 3 Proposed EP-MAC Protocol Design
### Frame Structure  
The mechanism of EP-MAC is based on dividing the communication time into variable length frames. The frame structure is shown in Figure 2. Each frame begins with a SYNC period. The purpose of the SYNC packet is to maintain synchronization between the nodes within the same virtual cluster. The next part of the active period of the frame is reservation slot which is used for the data slot reservation. And the last part composed with fixed number of data slots which are used for data and ACK transmission by sensor nodes. The number of data slots in a frame is not fixed. It depends upon the system requirements for a particular application. For example delay bound for the message is not the same for all applications. Moreover, the clock drift and synchronization error are deciding factors for number of data slots in a frame. Within the limit, without violating the synchronization condition, if a frame contains more slots then the nodes will save more energy, because the nodes having no data can sleep longer. On the other hand delay may be increased. So, the duty cycle is chosen very carefully.  
In Figure 3, we have shown a sample frame scenario of EP-MAC with comparison to Sensor MAC (S-MAC). We take four data slots in a frame. It is noticeable that to handle the effect of clock drift and to minimize the synchronization error we fix the data slot length a little longer than the required period for sending data and receiving acknowledgement.  
Identifying the Maximal Set of Communication Links Supporting Collision Free Transmission Simultaneously  
Within two hop neighbors simultaneously more than one transmission is possible without collision. The idea can be explained by the following scenario as explained through Figure 4. We use the similar approach of parallel transmission used in [15]. But we incorporate the concept of transmission power adjustment of sensor nodes in order to find the superset of mutually collision free transmission link which further support more transmission simultaneously. In Figure 4, we take four sensor nodes which are within three hops neighbors and belong to the same virtual cluster to explain the situation. We use the IEEE 802.11 [16] scheme as well.  
Figure 4: The idea of Parallel transmission using four nodes.  
Figure 3: Comparison of Frame structure of EP-MAC with S-MAC.  
Figure 2: Frame structure for the proposed Efficient MAC with Parallel Transmission (EP-MAC) protocol.  
Let us consider that node A wants to send a message to node B. First of all A will send a RTS packet to the sensor node B. But node C will overhear the RTS packet because node C is within the transmission range of node A. By overhearing the RTS the node C concludes that the transmission medium is busy. Now let us consider at the same time node C has a data to send to D. In the conventional method, since node C knows that the transmission medium is busy it will not send any message to initiate any transmission. But we can see from the figure that the communication (RTS) from node C to node D will not affect the previous communication _i.e._, communication (RTS) from node A to node B. Since collision happens at the receiving end, and node B and node C are not within the same transmission range, transmission of C by no means will interfere with the reception of node B. Similarly if node B sends a RTS to node A, node A will reply with a Clear-To-Send (CTS). This CTS will be overheard by C. But C need not stop its transmission and can send or receive during this time without any collision. By using the transmission power adjustment as described in Section 3.4 of this paper we can maximize the set of communication links which can transmit data simultaneously.