# A Novel Self-Forming Virtual Sub-Nets Based Cross-Layer MAC Protocol for Multihop Tactical Network
## 3 Proposed VSCL-MAC Architecture
### Control Frames Transmission Unit  
It is the foremost part of VSCL-MAC protocol and encompasses major design utilities. Therefore, before sending data, SDRs go through a control unit. In this unit, SDRs exchange intended transmissions using REQ and REQ_ACK packets, send in their respective time slots, and also establish a route for data transmissions. It enables single as well as multihop data transmission for which packets are sent to the next-hop node selected from the NBR_table, further transmitted to the next node until they are received by the destination. The format of REQ and REQ_ACK packets is shown in Figure 4; they contain source, destination, and the next-hop node's MAC ID. The NBR_MAP is the same as that of the NBR_DSCV packet, and the transmission table (TRANS_table) contains intended transmissions of the TX node as well as other nodes that help in forming sub-nets and time and frequency allocation in distributed manner.  
The addition of NBR_MAP in control packets benefits when a node does not receive information on any of its \(k\)-hop neighbors during the neighbor discovery process and also when the neighborhood changes due to mobility or link failures. The concerned nodes can update and maintain their NBR_table by extracting the hop information of its \(k\)-hop neighbor(s) from received control packets.  
When sending the REQ packet, the source node adds the intended transmission in TRANS_table, which contains all the route paths of the transmission perceived through NBR_table. The format of TRANS_table is shown in Table 2 in which the first and second columns contain the transmitter's and receiver's MAC ID of the transmission, respectively.  
Upon receipt of the REQ packet, all overhearing neighbor(s) add the shared transmissions in their TRANS_table. The receiving next-hop node of the REQ message can be the destination itself or a relay node. In the case of destination, the next-hop ID is the same as of destination ID, and the node responds with the REQ_ACK packet in its time slot, whereas, in another case, the next-hop relays REQ in its corresponding slot.  
The proposed design can send combined REQ and REQ_ACK packets, or nodes can have multiple requests from different transmitters [29]. This reduces the number of control packets and puts a marginal effect on control overhead, call setup delays, and network  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Transmitter** & **Receiver(s)** \\ \hline Source/Relay node & Relay/Destination node - - \\ Transmission of all 1- and \(k\)-hop neighbors - & - \\ - & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Transmission table of node \(N_{i}\).  
Figure 4: Format of request and acknowledgment control packets.  
convergence. Furthermore, in SDRs communication, not all radios have messages all the time; therefore, if a node does not have any REQ or REQ_ACK message to send, it sends a broadcast (BRDCST) message in its time slot. The BRDCST packet contains NBR_MAP and TRANS_table to maintain similar neighbor and transmission information on every node when not received from REQ or REQ_ACK packets.  
The TRANS_table alone helps form autonomous sub-nets, frequency selection, and time slot vectors for collision-free simultaneous data transmissions. For \(k\)-hop communication, the protocol runs multiple control frame cycles; it is essentially used to establish a route and maintain similar information on all nodes, as each frame can be heard within its two-hop vicinity.