Tham khảo tài liệu 'fiber optic telecommunication phần 3', kỹ thuật - công nghệ, kĩ thuật viễn thông phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Fiber Optic Telecommunication I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I i i i i i i i i I i i i i i i i i i i i i i i i i I Figure 8-14 Effects of system rise time for RZ format and NRZ format a Transmitted RZpulse train b Received RZ signal with allowable tr. c Transmitted NRZpulse train d Received NRZpulse train with allowable tr T Bit time Allowed tr 7p X. Multiplexing The purpose of multiplexing is to share the bandwidth of a single transmission channel among several users. Two multiplexing methods are commonly used in fiber optics 1. Time-division multiplexing TDM 2. Wavelength-division multiplexing WDM 2000 University of Connecticut 313 Fundamentals of Photonics A. Time-Division Multiplexing TDM In time-division multiplexing time on the information channel or fiber is shared among the many data sources. The multiplexer MUX can be described as a type of rotary switch which rotates at a very high speed individually connecting each input to the communication channel for a fixed period of time. The process is reversed on the output with a device known as a demultiplexer or DEMUX. After each channel has been sequentially connected the process repeats itself. One complete cycle is known as a frame. To ensure that each channel on the input is connected to its corresponding channel on the output start and stop frames are added to synchronize the input with the output. TDM systems may send information using any of the digital modulation schemes described analog multiplexing systems also exist . This is illustrated in Figure 8-15. Figure 8-15 Time-division multiplexing system The amount of data that can be transmitted using TDM is given by the MUX output rate and is defined by Equation 8-16. MUX output rate N X Maximum input rate 8-16 where N is the number of input channels and the maximum input rate is the highest data rate in bits second of the various inputs. The bandwidth of the communication channel must be at least equal to the MUX output .
