modem

Modem is designed for long-distance transmission of digital signals for data communication over analog channels with limited bandwidth. It is generally composed of several parts such as baseband processing, modulation and demodulation, signal amplification and filtering, and equalization. Modulation is the combination of a digital signal and an audio carrier to produce an audio signal (analog signal) suitable for transmission over a telephone line; demodulation recovers the digital signal from the audio signal.

Modems are generally divided into three types: external, built-in, and PC card. Through the telephone line or the special network cable, the external modem and the computer serial interface; The built-in type modem inserts directly in the computer expansion slot; The PC card type uses the notebook computer, inserts directly in the standard PCMCIA slot.

The performance and speed of the modem are directly related to the speed of transmitting information after networking. Modem rates are 14.4K, 19.2K, 28.8K, 33.6K, and 56K, etc. Currently, the use of 56K is more common. The CCITT recommends that the V.34 standard for modems has the biggest feature of "adaptive rate transmission", that is, in the transmission process, the product has the function of automatically adjusting the transmission rate according to the quality of the local subscriber line, which can make the area Poorly connected network users can also enjoy high-speed connectivity. The V.37 standard has 9600 ~ 128000bps signal rate, four-wire full-duplex communication, synchronization, single-sideband modulation and 60 ~ 108kHz base group circuit and other functions; v.42 standard with 56000bps signal rate, full-duplex communication Ways, synchronization, and data compression and error control technologies.

DWDM - Dense WaveLength Division Multiplex

DWDM (Dense Wavelength Division Multiplexing) is the ability to combine a group of optical wavelengths with a single optical fiber for transmission. This is a laser technology used to increase bandwidth on existing fiber optic backbone networks. More precisely, this technology multiplexes the tight spectral spacing of a single fiber carrier in a specified optical fiber in order to take advantage of the achievable transmission performance (for example, to minimize dispersion or attenuation). In this way, under a given information transmission capacity, the total number of optical fibers required can be reduced.

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The relationship between the communication transmission network and the business has become increasingly complex under the situation of a substantial increase in business volume. The original TDM (optical fiber single-wave transmission and time division multiplexing) cannot meet the needs of new technologies. The highest rate of optical fiber single-wave transmission for commercial applications is 40Gbits/s, and it is expensive. TDM technology is difficult to adapt to complex networks and business relationships. The optical fiber multiwave transmission technology that uses pure optical devices for long-wave dispatch has broken the limit of the processing speed of electronic devices. Based on the SDH technology, the optical fiber transmission capacity has been greatly improved. The current commercial application rate of Dense Lightwave Multiplexing (DWDM) technology (DWDM) technology (also known as OTN technology) has reached 3.2 Tbits/s, which means that the communication network can be smoothly upgraded and evolved.
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A key advantage of DWDM is that its protocol and transmission speed are irrelevant. The DWDM-based network can use IP, ATM, SONET/SDH, and Ethernet protocols to transmit data, and the processed data flow is between 100Mb/s and 2.5Gb/s. In this way, DWDM-based networks can transmit different types of data traffic at different speeds on a laser channel. From the point of view of QoS (Quality of Service), DWDM-based networks quickly respond to customer bandwidth requirements and protocol changes in a low-cost manner.

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