city calculated for either downlink or uplink divided by the effective bandwidth in the respective direction. In other words, the effective bandwidth is the operating bandwidth normalized appropriately considering the uplink/downlink ratio. During each VoIP session, a user will be in the active or inactive state. The duration of time that the user stays in each state is exponentially distributed. In the active or inactive state, packets of fixed sizes will be generated at intervals of iT + T seconds, where T is the voice frame size equal to 20 ms, T is the network delay jitter in the downlink and i is the VoIP frame index. In the uplink direction, T is equal to 0. As the range of the delay jitter is limited to 120 ms, the model may be implemented by generating packets at times iT + T' seconds, where T' T + 80 ms is a positive value. The air interface delay is the time elapsed from the packet arrival time iT + T' to successful reception and decoding of the packet. Statistical distribution and parameters associated with the VoIP traffic model are summarized in Table 12-4. The assumptions that were used for VoIP capacity calculation are shown in Table 12-5. 12.3.2 Full Buffer Traffic Model In the full buffer user traffic model, all the users in the system are assumed to have data to send or receive at any given time. In other words, there is always a constant amount of data that needs to be 12.4 Link-to-system mapping (PHY abstraction) transferred, as opposed to bursts of data with a probabilistic arrival time distribution. This model allows the assessment of the spectral efficiency of the system independent of actual user traffic distribution type. A user is in outage if the residual packet error rate after HARQ re-transmissions exceeds 1%. 12.4 LINK-TO-SYSTEM MAPPING (PHY ABSTRACTION) The objective of physical layer abstraction is to model link-level performance and to simplify the computations in system-level simulations. The motivation for abstraction of the physical layer is that simulating the physical layer b