  This paper focuses on the routing algorithm for the communications between
vehicles and places in urban VANET. As one of the basic transportation
facilities in an urban setting, buses periodically run along their fixed routes
and widely cover city streets. The trajectory of bus lines can be seen as a sub
map of a city. Based on the characters of bus networks, we propose a bus
trajectory-based street-centric routing algorithm (BTSC), which uses bus as
main relay to deliver message. In BTSC, we build a routing graph based on the
trajectories of bus lines by analyzing the probability of bus appearing on
every street. We propose two novel concepts, i.e. the probability of street
consistency (PSC) and the probability of path consistency (PPC) which is used
as metrics to determine routing paths for message delivery. This aims to choose
the best path with higher density of busses and lower probability of
transmission direction deviating from the routing path. In order to improve the
bus forwarding opportunity, we design a bus-based forwarding strategy with ant
colony optimization (FACO) to find a reliable and steady multi-hop link between
two relay buses in order to decrease end-to-end delay. BTSC makes the
improvements in the selection of routing path and the strategy of message
forwarding. Simulation results show that our proposed routing algorithm has a
better performance in transmission ratio, transmission delay and adaptability
to different networks.
