1. MODEMS

2. Introduction Modem is a device that converts the digital signal produced by computer into analog signal. The term Modem is a combination of 2 terms “Modulation” and “Demodulation

3. Analog Signal

4. Digital Signal

5. Performance of Modem Most of the modem manufacturer use a parameter called Baud Rate to measure modem performance. Although baud rate is still an important parameter, recent advances in compression technology have made it less meaningful. Some modem now provide a data transfer rate in bits/sec. this is better measure of network performance than baud rate.

6. Types of Modem The modems are classified on the basis of data transfer rates provided by them. The older modem series are: 103/113 Series 202 Series 212 Series 208 Series 201 Series 209 Series

7. . The modern modems are based on ITU-I standards(International Telecommunication Union) ensures the standardization of telephones and information exchange all over the world. The popular modem series are: V.22 bis V.29 bis V.32 bis V.34 V.42

8. Working of Traditional Modems Transmission of data from site A to site B takes following steps: Digital data is modulated at site A. Analog data is sent from modem to switching station at A by using local loop. At switching station analog data is converted to digital by PCM( Pulse Code Modulation). Digital data travels through the digital network and reaches the switching station B. Analog data is sent to site B modem. At B area switching office, inverse PCM is used to convert the digital signal to analog. The Modem demodulates the signal.

9. 56K Modems These are modern modems with a data transfer rate of 56000bits/sec. These modems can be used on if one party is using digital signaling. The downloading rate can be upto 56kbps but uploading speed is limited to 33.6kbps. In case of 56k modems, the one side is internet so there is no need of PCM

10. Working of 56k Modems Data transmission from ISP to user follows these steps: Digital data are sent by the computer of ISP through digital phone network. At switching station of A, digital data are converted back to analog using inverse PCM. Analog data are demodulated by modem at site A.

11. . Transmission of data from user to ISP: Digital data are modulated Analog data are sent from modem to switching office At switching office, data are converted to digital by using PCM. Digital data travels through the telephone network and received by ISP.