In a Vehicular Ad-hoc Network (VANET), the performance of the communication protocol is influenced heavily by the vehicular density dynamics. However, most of the previous works on VANET performance modeling paid little attention to vehicle distribution, or simply assumed homogeneous car distribution. It is obvious that vehicles are distributed non-homogeneously along a road segment due to traffic signals and speed limits at different portions of the road, as well as vehicle interactions that are significant on busy streets. In light of the inadequacy, we present in this paper an original methodology to study the broadcasting performance of 802.11p VANETs with practical vehicle distribution in urban environments. Firstly, we adopt the empirically verified stochastic traffic models, which incorporates the effect of urban settings (such as traffic lights and vehicle interactions) on car distribution and generates practical vehicular density profiles. Corresponding 802.11p protocol and performance models are then developed. When coupled with the traffic models, they can predict broadcasting efficiency, delay, as well as throughput performance of 802.11p VANETs based on the knowledge of car density at each location on the road. Extensive simulation is conducted to verify the accuracy of the developed mathematical models with the consideration of vehicle interaction. In general, our results demonstrate the applicability of the proposed methodology on modeling protocol performance in practical signalized road networks, and shed insights into the design and development of future communication protocols and networking functions for VANETs.

The safety and commercial benefits of Intelligent Transportation System (ITS) raised interests towards inter-vehicle networking technologies such as Vehicular Ad-hoc Network (VANET). Being an approved standard for wireless access in vehicular environments, IEEE 802.11p attracts a lot of research attentions, especially on its broadcasting performance. However, most of the previous network performance models paid little attention to vehicle distribution, or simply assumed homogeneous car distribution. It is obvious that vehicles are distributed non-homogeneously along a road segment due to traffic controls and speed limits at different portions of the road. In light of the inadequacy, we present in this paper an original methodology to study the performance of 802.11p VANETs with practical vehicle distribution in urban environment. An empirically verified stochastic traffic model is adopted, which incorporates the effect of urban settings (such as traffic lights) on car distribution and generates practical car density profiles. Based on the knowledge of car density at each location from the traffic model, the 802.11p broadcasting model is developed and a new metric, Broadcasting Performance Index (BPI), is introduced to better characterize the broadcasting performance and packet collision probability in VANETs. Furthermore, the analytical closed form for BPI is derived and its accuracy is confirmed with extensive simulation. In general, our results demonstrate the applicability of the proposed methodology on modeling protocol performance, and shed insights into the performance analysis of other communication protocols and network configurations in urban vehicular networks.Index Terms-Vehicular Ad-hoc Network (VANET), IEEE 802.11p, Stochastic Traffic Model, Markov Chain, Broadcasting Performance Index (BPI).

Broadband CDMA packet access is one of the most promising air interfaces for future 4G mobile communication systems. In this study, a novel reservation-based access protocol with prioritized resource allocation (PRA) is proposed as a reverse channel MAC protocol, which enables various rate realtime communications on such a system. In this protocol, a lowrate Temporal Dedicated Control CHannel (TDCCH) is momentarily assigned between a base station (BS) and mobile station (MS). A TDCCH is used to transmit not only access control signals including a reservation signal, but also transmission control signals such as Transmission Power Control (TPC) commands. This protocol prevents a packet-by-packet setup delay and provides contention-free packet reservation so long as a TDCCH is established. The duration of the TDCCH can be controlled according to the Quality of Service (QoS) requirement. PRA is based on such a reservation scheme, and two different resource allocation areas are defined in time slots that correspond to real-time and best-effort classes. The realtime packets have a wider allocation area so that they can acquire radio resources prior to best-effort packets. Thus, resources are assigned to real-time packets first, and the remaining resources can be used for best-effort packet transmission. By combining admission control and tlow-control with PRA, real-time packet communication can be achieved in the reverse channel. The performance of the proposed protocol is evaluated by computer simulation and through an experimental system.