## 5 Network Layer
### Spectrum Awareness and Routing
Although this algorithm improves the forwarding reliability by incorporating the spectral diversity, and the usage of a common control channel mitigates the broadcasting overhead as in [108], there are still efficiency concerns, _i.e._, the lack of granularity could lead to wasted channel opportunity and overcrowded frequencies in some parts of the network. The SPEAR routing protocol developed by [PERSON] _et al._[110] is based on a similar discovery mechanism as [109] to build a database of channel availability at every node. However, each node broadcasts its local information so eventually the whole map of availability is synchronized within the network. More than one path towards destination can be established by nodes sharing commonly available channels and used for different data flows. The notion of _Route Robustness_ is introduced in the algorithm devised in [111]. Based on spectral data collected at lower layers, each link between two nodes has a set of probabilities associated with all channels, thus indicating the likelihood of uninterrupted transmissions in corresponding frequencies. The degree of robustness of a route is then determined by the product of the maximum achievable values of associated probabilities of each link along the way, which is essentially controlled by the most robust available channel. Routes are extended from the source towards the destination via nodes that could form links meeting a certain threshold of robustness [111]. After the elimination of those resulting in loops or leading to a dead-end, the destination node is eventually reached by a set of one of more possible routes. Using an Integer Linear Programming (ILP) model, the final route and channel used in each links are decided. The authors of [111] argue that routes chosen with this method improve the throughput as it exhibits the lowest probability of encountering spectral disruptive events. Simulation results also demonstrate the advantage of adopting this criterion over transmission-rate based route selection strategies [111]. The scheme proposed in [112], demonstrates a different way of thinking by introducing bi-directional activity between the MAC and the network layers. No longer passively accepting the channel assignments, the network layer actively casts effects on the spectrum decision making in a way that optimizes the performance whilst still satisfying lower layer constraints. The work employs the concepts of flow-segment (FS) and maximum flow-segment (MFS) to facilitate channel selection. An FS is defined as a route consisting of consecutive nodes that can communicate using the same frequency. Sometimes there exist multiple FSes from a node towards the destination and the one incorporating maximum number of nodes is the MFS. Transmissions in [112] take place in an on-demand manner. A source node with data to send initiates the dissemination of spectrum information. Starting from the sender, every node captures the local channel availability and forwards a summary of FSes leading to itself in all channels upstream towards the destination node. If at some node \(X\) the channel used by MFS from downstream is unavailable, for instance due to interference from other secondary users, node \(X\) is then said to be a \"decision node\" which is responsible for choosing the MFS and assigning the channel for its downstream route. The process is repeated until the destination is reached and the entire route is established, consisting of the smallest possible number of MFSes, which reduces frequency-switchings and therefore achieves lower end-to-end delay and higher throughput. The authors claim the algorithm minimizes the amount of channel hopping. It is noted, however, that minimum channel switching is, in fact, only guaranteed for sub-routes between adjacent decision nodes. The resultant channel choice for the entire path is therefore composed of best decisions of sub-routes.