Quantification of network capacity and throughput is crucial for performance evaluation of integrated space and terrestrial network (ISTN).However, existing studies mainly consider the maximum throughput as the network capacity, but such a definition would make it unreasonable that the value of the network capacity would change with different employed routing algorithms and congestion control policy, instead of being a constant quantity. In this paper, we argue that the capacity of an ISTN is solely dependent on the characteristics of the network infrastructure,and the throughput of an ISTN is the aggregate traffic transported by the network under a given traffic scenario. Then, we present a quantitative approach to assessing network capacity in relation to an unreliable ISL model (cap-uISL), and a Constrained Path Expansion throughput calculation method (THP-CPE) based on a set of known traffic paths. This method allows us to obtain the current throughput value of the network based on any given traffic paths and load demand matrix. As the traffic load increases, the throughput approaches its maximum value, which is notably smaller than the network's capacity. We experimentally determine the network capacity of CAP-uISL under various link parameters and compare our throughput quantization method, THP-CPE, with other state-of-the-art methods under four emerging ISTNs. We find that, compared with the THP-CPE, existing throughput calculation methods tend to be overestimated, while our proposed throughput calculation method maintains reasonable intervals in terms of path utilization ($<1$) under all load cases.

翻译：网络容量与吞吐量的量化对于天地一体化网络（ISTN）的性能评估至关重要。然而，现有研究主要将最大吞吐量视为网络容量，但该定义会导致网络容量值随所采用的路由算法和拥塞控制策略而变化，而非保持恒定，这并不合理。本文认为，ISTN的容量仅取决于网络基础设施的特性，而ISTN的吞吐量是在给定流量场景下网络传输的总流量。随后，我们提出了一种基于不可靠星间链路模型（cap-uISL）的网络容量量化评估方法，以及一种基于已知流量路径集的约束路径扩展吞吐量计算方法（THP-CPE）。该方法允许我们根据任意给定的流量路径和负载需求矩阵获得网络的当前吞吐量值。随着流量负载增加，吞吐量趋近于其最大值，该值显著小于网络容量。我们通过实验测定了不同链路参数下CAP-uISL的网络容量，并在四种新兴ISTN场景下将我们的吞吐量量化方法THP-CPE与其他先进方法进行了比较。研究发现，与THP-CPE相比，现有吞吐量计算方法往往存在高估，而我们所提出的吞吐量计算方法在所有负载情况下均能保持合理的路径利用率区间（$<1$）。