t corresponds to STM-1 of SDH. These modules have a bit rate of 51.840 Mbps and they are multiplexed synchronously into higher-order signals STS-N. Each STS-N signal has a corresponding optical signal called an opti- cal carrier (OC-N) for optical transmission. Table 4.3 presents data rates for SONET and corresponding signal levels for European SDH. An STS-1 signal consists of frames and the frame duration is 125 us (8,000 times a second, that is, equal to the PCM sampling rate) just as in Introduction to Telecommunications Network Engineering SDH. Each frame contains 810 bytes that makes up a bit rate of 51.840 Mbps. Transport overhead information such as frame synchronization and pointers uses 27 bytes in each frame and the rest of it is used for payload; for example, for 1.544-Mbps signals that contain PCM speech channels. The detailed multiplexing scheme of either SONET or SDH is not presented here; for more detailed information the reader may refer to, for example, [5]. SONET and SDH were originally designed for transmission of 64- Kbps PCM channels. In Chapter 6 we will see how they are used when data consist of IP packets or ATM cells. 4.7 Transmission Media Transmission systems may use copper cable, optical cable, or radio channels to interconnect far-end and near-end equipment. These channels and their characteristics are introduced next. 4.7.1 Copper Cables Copper cable is the oldest and most common transmission media. Its main disadvantages are high attenuation and sensibility to electrical interference. Attenuation in copper cable increases with frequency approximately accord- ing to the following formula: (4.13) where is attenuation in decibels, fis the frequency, and k is a constant spe- cific for each cable. This formula gives us approximate attenuation at other frequencies if the attenuation at one frequency is known. For example, if we measure that attenuation of a certain cable is 6 dB at 250 kHz, then at the four times higher frequency of 1 MHz it is approximately 12 dB. The speed of signal pro