The performance of Delay Tolerant Mobile Networks (DTMNs) depends on the mobility pattern through the nodes encounter process which constitutes opportunities for exchange of information between nodes. The effective exchange of information depends also on the routing/replication protocol and on several network resources such as buffer capacity, radio propagation conditions and energy. We focus in this work on those two concepts: encounter and exchange, and evaluate them along with resulting network performance for different synthetic mobility patterns and routing protocols, as well as under several constraints on the radio conditions and buffer space. We also propose the use of additional, mobile, dedicated relays so as to enhance the performance of the DTMN in cases of ill-behaved mobility schemes