The compression step of the Cloud Radio Access Networks (C-RAN) is studied in this paper. In order to maximize the achievable sum rate in the uplink of C-RAN, constrained by the finite-capacity fronthaul links to the Central Processor (CP) in the cloud, an optimization algorithm of quantizers used for compression is proposed. In C-RAN, there are multiple Radio Units (RUs) compressing and forwarding the correlated received signals simultaneously. Due to the correlations, Wyner-Ziv coding is used. Hence a joint optimization of the quantizers is required at the CP. In this paper we extend the Information Bottleneck (IB) method for this joint optimization. It is a combination of the Alternating Information Bottleneck method (AIB) and the Alternating Bi-Section method, which are both proposed in this paper. Then we use the proposed algorithms to study some key features of C-RAN.