  With the development of mobile communication and industrial internet
technologies, the demand for robust absolute time synchronization based on
network for diverse scenarios is significantly growing. TAP is a novel network
timing method that aims to achieve sub-microsecond synchronization over air
interface. This paper investigates the improvement and end-to-end realization
of TAP. This paper first analyzes the effectiveness and deficiencies of TAP by
establishing an equivalent clock model which evaluates TAP from timing error
composition and allan variance. Second, this paper proposes a detailed base
station and terminal design and the corresponding improvement of TAP. Both
hardware compensation and protocol software design are taken into account so as
to minimize timing error and system cost while maximizing compatibility with
3GPP. Finally, this paper presents a TAP end-to-end 5G prototype system
developed based on software defined radio base station and COTS baseband
module. The field test results show that the proposed scheme effectively solves
the problems of TAP in application and robustly achieves 200ns level timing
accuracy in various situations. The average accuracy with long observations can
reach 1 nanosecond. It is 2$\sim$3 orders of magnitude better than common
network timing methods, including NTP, PTP and the original TAP.
