Network coding was found to be useful for ad hoc wireless multicast in disruptive channel and connectivity conditions. In heterogeneous networks, comprising teams with different technical preparedness, it is possible that only the radios of the most advanced teams have sufficient resources to network encode/decode. In this case, an interesting solution is partial (hybrid) network coding - only a fraction of the nodes encodes, the balance simply forwarding the packets. Partial coding poses interesting problems and opportunities that require novel solutions. The first contribution of this paper is the control of redundant transmissions. Network coding detects duplicates using the ?innovative packet? check; it drops non-innovative packets. Likewise, we require a new duplicate detection scheme for non-network coding (non-NC) nodes using encoding vectors since packet ID may be obliterated during packet mixing. The second contribution is the study of performance loss caused by partial coding. Through the analysis of representative topologies and scenarios, we identify densities and distributions (of non-coding nodes) that render network coding inefficient. The results are of practical importance because they help determine when one should switch from network coding to other forms of protection (e.g., erasure codes or fountain codes). The third contribution is in the protection from malicious packet corruption, i.e., pollution. There is the risk that nodes belonging to an untrusted team (a likely situation in tactical coalitions) can inject polluted packets into the network. Pollution is critical in network coding. If it goes unchecked, pollution makes it easy for an attacker to spoil an entire generation. An opportunity offered by hybrid network coding is to force the untrusted nodes to perform simple forwarding, without coding. We show that their behavior can be more easily and efficiently checked (with hash signatures instead of homomorphic hashes) while still profiting- from their forwarding.
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