2. External ModemDifference between Internal and External Modem<br />Internal ModemExternal Modem This is circuit board that is plugged into one of the computer’s expansions.Separate device has to be attached externallyIt cannot be detached from the computer.Can be easily detached from computer Tedious to configure & installVery easy to installIf internal modem gets stuck, you have to switch off the computer and reinstall.If the gets stuck, just switch off the modem and switch it on. No additional cable required.Additional cable is required<br />How it works:<br />A modulator converts a digital signal into analog signal. A demodulator converts an analog signal to digital one. It doesn’t sample a signal to create digital facsimile. It is merely a process of modulation. The two pc at ends are the DTE, the modem are DCE’s. The DTE create digital signal and relay it to modem via interface. Modulated signal is received by the demodulation function of the second modem. The demodulator takes the ASK, FSK, PSK, or QAM signal and decodes it into a format. Its computer can accept. If the relays the resulting digital signal to receiving computer via an interface. Each DCE is compatible with its own DTE and with Other DCE’s.<br />Today, no one uses dumb terminals or terminal emulators to connect to an individual computer. Instead, we use our modems to connect to an Internet service provider (ISP), and the ISP connects us into the Internet. The Internet lets us connect to any machine in the world (see How Web Servers and the Internet Work for details). Because of the relationship between your computer, the ISP and the Internet, it is no longer appropriate to send individual characters. Instead, your modem is routing TCP/IP packets between you and your ISP.<br />The standard technique for routing these packets through your modem is called the Point-to-Point Protocol (PPP). The basic idea is simple -- your computer's TCP/IP stack forms its TCP/IP datagrams normally, but then the datagrams are handed to the modem for transmission. The ISP receives each datagram and routes it appropriately onto the Internet. The same process occurs to get data from the ISP to your computer.<br />Modulation Techniques<br />* AM - amplitude modulation<br />This technique changes the amplitude of the sine wave. In the earliest modems, digital signals were converted to analog by transmitting a large amplitude sine wave for a quot;
1quot;
and zero amplitude for a quot;
0quot;
, as shown in figure Modem-3. The main advantage of this technique is that it is easy to produce such signals and also to detect them. This technique has two major disadvantages. The first is that the speed of the changing amplitude is limited by the bandwidth of the line. The second is that the small amplitude changes suffer from unreliable detection. Telephone lines limit amplitude changes to some 3000 changes per second. The disadvantages of amplitude modulation causes this technique to no longer be used by modems, however, it is used in conjunction with other techniques <br />. <br />* QAM - quadrature amplitude modulation<br />This technique is based on the basic amplitude modulation . This technique improves the performance of the basic amplitude modulation. In this technique two carrier signals are transmitted simultaneously. The two carrier signals are at the same frequency with a 90 degrees phase shift. The mathematical form of the transmitted signal will be as follows:S(t)=A*SIN(Wc*t)+B*COS(Wc*t)A, B, are the amplitude of the two carrier signals. Each of them can get a value from a known set of values. In this way a few bits can be transmitted in the period of one symbol time. For example consider the set of values {1 , 2 , 3 , 4 }. In this example 4 different values can represent 2 bits. During one symbol time 4 bits will be transmitted, quot;
Aquot;
will represent 2 bits and another 2 bits will be represented by quot;
Bquot;
.<br />* FM - frequency modulation<br />In this technique the frequency of the carrier signal is changed according to the data. The transmitter sends different frequencies for a quot;
1quot;
than for a quot;
0quot;
as shown in figure Modem-4.This technique is also called FSK - frequency shift keying. The disadvantages of this technique are that again (as it was with amplitude modulation) the rate of frequency changes is limited by the bandwidth of the line, and that distortion caused by the lines makes the detection even harder than amplitude modulation. Today this technique is used in law rate asynchronous modems up to 1200 baud only. <br />* CPM - continuous phase modulation<br />A modern technique which derives from basic frequency modulation. The only difference is that in the transition from one symbol to another the phase is continuously changed, there are no phase steps. Continuous phase means that the transmitted signal bandwidth is limited and faster data rates can be achieved for the same bandwidth.<br />* PM - phase modulation<br />In this modulation method a sine wave is transmitted and the phase of the sine carries the digital data. For a quot;
0quot;
, a 0 degrees phase sine wave is transmitted ( PHI = 0 ). For a quot;
1quot;
, a 180 degrees sine wave is transmitted ( PHI = 180 ) as shown in figure Modem-5. This technique, in order to detect the phase of each symbol, requires phase synchronization between the receiver's and transmitter's phase. This complicates the receiver's design. <br />Speed variation from 1960 to till now!<br />Future Trend<br />Today in addition external modems, there are internal modems which are included as an additional board within the computer. There are advantages to each type.<br />Actual transfer rates are limited due to type of phone lines. Using slower phone trunks, international circuits where half the normal bandwidth is used, and the slow cellular connections where it might run at only 14.4 kbps (without compression) although the modem itself enable 28.8kbps.<br />More and more users are accessing the Internet and on-line services such as Compuserve, so, use of modems has increased dramatically. The more powerful processors such as Pentium and PowerPC in workstations and PC's, enable the modem h/w to be less complicated. Part of the functions done in the DSP or microcontroller might be performed by the host. So, modems might drop in price.<br />The advent of semiconductor modems will enable a wide range of applications to be implemented: Vending machines will call up when they need more goods to vend, or elevators will call when they require service, and so on.<br />The last approved standard of V.34 with 28.8 kbits/s speed will enable the Digital Simultaneous Voice and Data (DSDV) applications. DSVD is a modem specification that lets voice and data to be shared over a single dial-up connection. The data is multiplexed into packets, much like an ATM stream.<br />V.34 is approaching the theoretical speed limit of an analog line - estimated to be in the low 30kbps range. This may be the last of the new modem protocols which doubled the previous speeds.<br />