In this paper we present the set of factors in the physical layer that are relevant to the performance evaluation of the routing protocols. Such factors consist of signal reception, path loss, fading and interference considerations. With this in mind we adopt a numerical approach based on Finite State Markov Chain channel model to study the performance of an ad-hoc routing protocol under various radio propagation models. Further this paper presents a new cross-layer algorithm for joint physical and routing layers in wireless ad hoc networks, applying this to the Optimized Link State Routing (OLSR), protocol to demonstrate the effectiveness of the use of Link Lifetime (LLT) and the channel quality measured by Signal to Interference and Noise Ratio (SINR) as metric in the selection of routes. We address the problem of link and route stability, focusing particularly on multipoint relay (MPR) selection method, to find the most optimal routes between any pair of nodes. Our simulation results indicate that the network throughput is greatly improved and the delay is significantly decreased using this cross-layer mechanism, compared to the original OLSR.