Many protocols designed for wireless sensor networks require the existence of special purpose nodes distributed throughout the network to perform such services as aggregation, access coordination, etc. To avoid inter-node interference, it is desirable to schedule beforehand the nodes that will interact with the servers in a given time interval. Previous solutions either assume a fully connected network with a single server or clustered architecture with one server per cluster and no inter-cluster interference. In this paper, we propose a contention-based protocol for scheduling service requests by nodes in environments with multiple servers without any assumption about clustering. We describe two versions of our protocol: one based on random backoff and the other on truncated binary exponential backoff. Results show that a high number of requests can be fulfilled within a reasonable response delay and its performance is better than TDMA-like clustering-based schemes when the interference between clusters is taken into account.