## 3 Design of Isl Network for IoT
### P2P Duplexing Scheme  
A data link layer (OSI layer 2) must incorporate rules or procedures for link establishment, for lossless data transfer, and for terminating communication. A P2P protocol and TDD seem to be a suitable solution for nanosatellite networks and perform well; particularly in a meshed topology where multi-hopping is unavoidable and no central controller exists. The two nodes of a P2P link are referred to as the _caller_ and _responder_. A caller satellite is the initiator of the session establishment process. A responder satellite receives session establishment  
Figure 5: Simplified diagram of random channel access. Boxes indicate short bi-directional sessions with flexible or fixed lengths. Shadowed boxes indicate session frames which have collided. parameters from the caller. A session is a continuous dialog between the caller and responder and consists of three distinct phases: Session establishment, data services, and session termination. Figure 6 illustrates the TDD frame structure for: (a) Link establishment and (b) data transfer. The P2P bi-directional TDD data stream is divided into frames with constant length \(t_{\text{W}}\). Each frame contains two subframes: One caller frame Caller frame (Xmt) and one responder frame Responder frame (Rcv). The data transfer is asymmetrical: Xmts are longer than Rcvs, which typically consist only of an ARQ or control information. Each Xmt consists of a Preambel (synchronisation field) (Pr) for one-shot packet frame synchronization, one or two headers (\(L_{0}/L_{1}\)), and one data block Data packet (information field from network layer) (Pkt); Rcvs have a similar structure. The time \(t_{1}\) between Xmt and Rcv is clearly equal to the two-way propagation time \(2\times t_{p}\). To keep the frame length constant, \(t_{1}+t_{2}\) is chosen to be slightly longer than the maximum time required to communicate over the greatest distance. The so-called zero layer header \(L_{0}\) contains 12 BPSK modulated and convolutional coded data bits with a code rate of \(r=1/2\). The \(L_{0}\) header provides a frame counter for the ARQ procedure and data direction information. The \(L_{1}\) header is used for: (i) Session establishment; and (ii) adaption of the modulation and coding scheme during data transfer. Thus, the modulation and coding scheme can be altered quickly within the session, depending on the channel condition, by \(L_{1}\). Parameters such as S/C caller and responder IDs, as well as modulation and coding options are also provided by \(L_{1}\). The caller and responder IDs are especially relevant for routing in the network layer. \(L_{1}\) includes 31 bits, where the modulation and coding are the same as in the case of \(L_{0}\) to guarantee a low bit-error probability in bad channels. Once a session is established and the channel is stable, no \(L_{1}\) header is necessary, and more bandwidth can be provided for data packets by the data transfer frame, as illustrated in Figure 6b. The responder answers during the data transfer phase with \(L_{0}\), which is detected by the caller as an Acknowledge (ACK). If no ACK can be correctly decoded by the caller within the time \(t_{1}+t_{2}\) (Negative Acknowledge (NAK)), Pkt will be retransmitted in a new Xmt frame. The ARQ protocol follows the classic stop-and-wait procedure. A terminating rule is implemented in the case that only NAKs are detected and the SNR is not good enough to guarantee an orderly data transfer on the lowest modulation/FEC combination. The modulation and coding scheme can be quickly changed within a few TDD frames, depending on the current SNR. The SNR detected by the responder is transmitted in each Rcv frame. After receiving a few Rcv frames and building an average SNR value, the caller makes a  
Figure 6: TDD frame structure: (a) Link establishment and adjustment phase; and (b) data transfer phase (with ARQ possible in one direction only). decision, based on responders and his own \(\mathrm{SNR}\), about the necessity of changing the modulation and coding scheme.