Performance of ATM and variable length packet access in broadband HFC and wireless networks

We present an overview of some key features that enable superior performance and efficiency in medium access control (MAC) protocols for broadband wireless and hybrid fiber-coax (HFC) access networks. These features include: (1) minislots for transmission requests, (2) minislots for bandwidth allocation granularity, (3) centralized control for bandwidth allocation and fairness, (4) flexible transport capability for fixed-length (ATM) protocol data units (PDUs) and variable-length (VL) PDUs (e.g., IP, 802.3 frames, IPv6), and (5) piggy-backing for additional transmission requests. We develop performance models for obtaining the mean and high percentile of message delays in both upstream and downstream directions. Assuming certain message delay objectives, we also present the throughput and capacity results for internet data applications for two cases: (1) ATM PDU access mode using AAL-5 adaptation, and (2) variable length (VL) PDU access mode with minimal segmentation. Using the ATM PDU mode does result in less throughput and capacity in comparison to the VL PDU case. However, the MAC protocol feature of providing bandwidth allocation granularity based on minislots is very useful for the following reason: it facilitates a migration path from initial VL PDU based access transport for the dominant TCP/IP applications to ATM PDU based access transport for future ATM end user applications.