ency reuse IEEE WiMAX configure the networks. Frequency reuse factor 1 (N = 1) is at the center of the cell to maximize the network spectral efficiency, while higher reuse factor is at cell edges to alleviate (co-channel) interference. 4.3.3 Multicast and Broadcast Service (MBS) MBS can be of two types: single-BS access and multi-BS access. Single-BS access is implemented over multicast and broadcast transport connections within one BS, while multi-BS access is implemented by transmitting data from Service Flow(s) over multiple BS. MS may support both single-BS and multi-BS access. ARQ is not applicable to either single-BS-MBS or multi-BS-MBS [3]. 4.3.4 Additional PHY Features WiMAX physical layer also employs techniques like Adaptive Modulation and Coding, Automatic Repeat Request (ARQ) and Hybrid-ARQ [2,3]: Adaptive Modulation and Coding: The adaptive modulation allows adjusting the modulation scheme according to the RF conditions. WiMAX supports BPSK, QPSK, 16-QAM, and 64-QAM4 modulation schemes. The physical layer supports multiple channel coding schemes including Reed-Solomon, Convolutional Coding, Convolutional Turbo Coding and Block Turbo Coding: Automatic Repeat Request: ARQ and Hybrid-ARQ are optional features in the IEEE 802.16 standard. ARQ can be enabled on a per connection basis and it requires negotiation of window sizes, maximum of acknowledgment times and maximum number of retransmissions. HARQ is only defined for OFDMA PHY as an optional item and it allows operation either using "chase combining" or incremental redundancy5. HARQ combined together with CQICH and AMC provides robust link adaptation in mobile environments at higher vehicular speeds. Network Architecture The WiMAX Forum has identified a WiMAX Network Reference Model (NRM), which is a logical representation of the network architecture. The NRM identifies functional entities and reference points over which interoperability is achieved between functional entities. The architecture has been developed to support fixed, nomadic, portable, 