到目前为止，世界范围内的PSTN供应商已经投入了巨大的成本，包括建设、维护骨干网和用户环路。
Up to now, PSTN providers have invested a lot of costs in building and maintaining their bone networks and subscribers cycle circuit.
到目前为止，世界范围内的PSTN供应商已经投入了巨大的成本，包括建设、维护骨干网和用户环路。
Up to now, PSTN providers have invested a lot of costs in building and maintaining their bone networks and subscribers cycle circuit.
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