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The STM-4 (Synchronous Transport Module) is a SDH ITU-T fiber optic network transmission standard. It has a bit rate of 622.080 Mbit/s. The STM-4 specification is designed to carry 7,680 8-bit "voice" frames every 125 micro-seconds for a total payload bit rate of 491.520 Mbit/s. The other levels defined by the SDH standard are STM-1, STM-16 and STM-64. Beyond this we have wavelength-division multiplexing (WDM) commonly used in submarine cabling. Although STM-4 is comparable to OC-12 the SDH frame structure allocates more space to overhead than that of SONET. Because of this, STM-4's payload bandwidth differs from that of OC-12. ==SDH Rates== SDH is a transport hierarchy based on multiples of 155.52 Mbit/s. The basic unit of SDH is STM-1. Different SDH rates are given below: * STM-1 = 155.520 Mbit/s * STM-4 = 622.080 Mbit/s * STM-16 = 2,488.320 Mbit/s (~2.5 Gbit/s) * STM-64 = 9,953.280 Mbit/s (~10 Gbit/s) Each rate is an exact multiple of the lower rate, therefore the hierarchy is synchronous. !!! 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「STM-4」の詳細全文を読む スポンサード リンク
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