tailieunhanh - Kỹ thuật truyền số liệu 7

Tham khảo tài liệu 'kỹ thuật truyền số liệu 7', giáo dục - đào tạo, cao đẳng - đại học phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Vs 1APTER7 Transmission Media ii As discussed in Chapter 4 computers and other telecommunication devices use signals to represent data. These signals are transmitted from one device to another in the form of electromagnetic energy. Electromagnetic signals can travel through a vacuum air or other transmission media. Electromagnetic energy a combination of electrical and magnetic fields vibrating in relation to each other includes power voice radio waves infrared light visible light ultraviolet light and X gamma and cosmic rays. Each of these constitutes a portion of the electromagnetic spectrum see Figure . Not all portions of the spectrum are currently usable for telecommunications however and media to harness those that are usable are limited to a few types. Voice-band frequencies are generally transmitted as current over metal cables such as twisted-pair or coaxial cable. Radio frequencies can travel through air or space but require specific transmitting and receiving mechanisms. Visible light the third type of electromagnetic energy currently used for communications is harnessed using fiber-optic cable. Figure Electromagnetic spectrum Transmission media can be divided into two broad categories guided and unguided see Figure . 187 188 CHAPTER 7 TRANSMISSION MEDIA Figure Classes of transmission media Transmission media Guided Unguided GUIDED MEDIA Guided media which are those that provide a conduit from one device to another include twisted-pair cable coaxial cable and fiber-optic cable see Figure . A signal traveling along any of these media is directed and contained by the physical limits of the medium. Twisted-pair and coaxial cable use metallic copper conductors that accept and transport signals in the form of electrical current. Optical fiber is a glass or plastic cable that accepts and transports signals in the form of light. Figure Categories of guided media Guided media Twisted-pair cable Coaxial cable Fiber-optic cable .