## 3 O-MAC Protocol Design Goals and Architecture
### Interior Communication  
This process involves the communication of a region's nodes using the flat network topology. In this process, the sender uses the semi-synchronization methodology to synchronize the particular nodes in the 1-hop neighborhood. The semi-synchronization methodology does not require synchronization with all of the nodes but rather selects particular nodes within the 1-hop neighborhood based on the shortest distance, energy and buffer capacity of each node. Our scheme addresses the pair of nodes needed to forward the data in each neighborhood: the principal and backup nodes; however, each sender stores the information of all nodes if the principal and backup nodes run out of energy or cease communicating. Thus, in the situation of handoff or loss of a node (e.g., failure of node, depleted node energy), other nodes are chosen as the next principal and backup nodes. The selection process for the principal and backup nodes is explained in Algorithm 1.  
In line 4, the sender node attempts to determine the principal and backup nodes in the interior region of the network. In line 5, the checking process for identifying the candidate node is performed. In line 6, the initial candidate node is selected based on the shortest distance from the sender node. Subsequently, in lines 8 and 9, a candidate node is selected based on the greatest energy and buffer capacity. In line 11, the weight of the candidate node is computed based on the distance, energy and buffer capacity. Finally, the node with the maximum weight is selected as either the principal node or the backup node in line 12.  
The chosen principal node takes on the responsibility of forwarding the data to the next 1-hop neighbor node. Furthermore, in the case of a loss or handoff of a principal node, a backup node replaces the principal node for forwarding the data. Subsequently, another backup node is selected within the 1-hop neighborhood. When either the principal or backup node hand off or begin to run out of energy, each of the nodes (principal and backup) must incorporate a flag signal in the last packet sent that indicates the current situation of the node. This feature also helps the node to adjust for mobility. To forward the data to the next hop, the anycast addressing methodology is introduced to manage either the principal or backup node. The anycast addressing methodology is not a feature of the IEEE 802.15.4 data format packet, but is introduced in the ODFF model, which helps manage particular nodes in the 1-hop neighborhood to forward the data to the next hop. The interior communication process also involves the randomization function to avoid congestion when the node attempts to access a slot in the channel. For example, when node A wants to access slot 5, if slot 5 is already occupied by node D, then the randomization function redirects node A to another available empty slot to avoid any possible collision in the channel. When node acquires access to a channel, it uses the short preamble prior to sending the original data. The impact of the reduced sized preamble introduced in the ODFF is a tradeoff between the time and energy efficiency in the network. We also prove the tradeoff impact of O-MAC with other MAC protocols that use the preamble-based approach, as explained and analytically validated in the next section. The interior communication involves the following phases:* The ODFF model.
* The preamble tradeoff validation model.