In this paper, we consider the discrete-time Jackson queueing network (which is the discrete-time version of the well-known continuous-time Jackson queueing network), particularly, the one with queues in tandem. Analysis of such a discrete-time network is notoriously difficult except for some special cases. To analyze it in general, we approximate each node of the network with a continuous-time single-arrival M/M/1 queue, a continuous-time batch-arrival MX/M/1 queue, a discrete-time single-arrival Geo/Geo/1 queue, and a discrete-time batch-arrival GeoX/Geo/1 queue, in turn. To evaluate the performance of these approximations, we compare the mean queue lengths obtained from each approximation with those obtained from simulation. As a result, the approximation with GeoX/Geo/1 queue turns out to be the best, among others, and pretty stable in its performance over wide ranges of parameter values. It is because the GeoX/Geo/1 model better describes the discrete nature of the discrete-time tandem queuing network than its continuous-time M/M/1 and MX/M/1 counterparts, and because it better describes arrivals in multiple at a single slot than its single-arrival Geo/Geo/1 counterpart. We conclude that the approximation with GeoX/Geo/1 queue discussed in this paper would be a reasonable way to analyze the discrete-time tandem queueing networks in practice.