PRESENTATION ON

MODEM WORKING PROCEDURES

WHAT IS MODEM

A MODEM MODULATES OUTGOING DIGITAL SIGNALS FROM A COMPUTER OR OTHER DIGITAL DEVICE TO ANALOG SIGNALS FOR A CONVENTIONAL COPPER TWISTED PAIR TELEPHONE LINE AND DEMODULATES THE INCOMING ANALOG SIGNAL AND CONVERTS IT TO A DIGITAL SIGNAL FOR THE DIGITAL DEVICE.

A modem is a device or program that enables computer to transmitt data over telephone lines.

WHAT IS MODEMS PURPOSE?

The word modem is an acronym for modulator-demodulator. Basically, a modem is used for transmitting and receiving data over a communication channel, such as twisted-pair telephone lines, coaxial cables, and optical fibers. Currently the purpose of a modem is to convert a computer’s data stream to analog format so that it can be transmitted over the analog telephone line.

At the source, modulation techniques are used to convert digital Data (0’s and 1’s) into analog form for transmission across the channel. At the destination, the

received analog signal is converted to digital data via demodulation. This is a simplified explanation of how a modem works, and there are other issues that require attention; such as channel impairments, encryption, error detection/correction, data compression, modulation, handshake negotiation, and echo cancellation. These features will be discussed a bit later.

SPEED OF MODEM

Modem speed is rated in bps (bits per second).  So a 56k modem transmits at 56,000 bits per second in theory. While 8 bits make a byte, and 1024 bits equal one kilobits, a 56k modem will transmit a maximum of approximately 7 kilobytes of information every second.   A modem will never connect higher than what it is rated for.

TYPES OF MODEM

1.INTERNAL MODEM

2.EXTERNAL MODEM

3.PC CARD MODEM

EXTERNAL MODEM

External modems are the simplest type of the modem to install this kind of modem you didn’topen the computer. The telephone line plugs into a socket on the rear panel of the modem. As external modems have their own power supply you can turn off the modem quickly to break the connection. The examples of these modems are the DSL modems which are used in the broadband connections.

INTERNAL MODEM Internal modem is the device installed in the desktop or laptop computer to communicate over a network with other connected computers. These are cheaper than external modems as they do not require power supply or a chassis. There are two types of internal modems: dial-up and wifi® (wireless). Dial up works on the telephone cables and requires a network access phone number and log on credentials to make a connection and wifi modem comments to the network without filling these credentials.

PC CARD MODEMThese modems, designed for portable computers, are the size of a credit card and fit into the PC card slot on notebook and handheld computers. These modems are removed when the modem is not needed. Except for their size, PC card modems are like a combination of external and internal modems. These devices are plugged directly into an external slot in the portablecomputer. So no cable is required other than the telephone line connection. The cards are powered by the computer, which is fine unless the computer is battery-operated. Running a PC card modem while the portable computer is operating on battery power drastically decreases the life of your batteries.

HOW MODEM WORKS ?

• Modems are computer hardware typically used to transmit digital data over a phone line. The working of modems can be easily understood from this. Modems always work in pair. The sending modem converts computer’s digital information into specific frequencies compatible with the phone line, the process is called modulation. The receiving modem decodes the signal back into the digital information, the process is known as demodulation. In wireless modems,digital data is converted into radio signals and vice versa.

Modem Technical Details

1.Modulation2.demodulation 

Modulation

1.Frequency Shift Keying (FSK)2.Amplitude Shift Keying (ASK)3.Phase Shift Keying (PSK)3.Quadrature Amplitude Modulation (QAM)

FREQUENCY SHIFT KEYING (FSK)FSK modems modulate data by varying the frequency of a signal (frequency modulation).  FSK modems can transmit binary information by using different frequencies to represent bit patterns. In a binary system, one frequency represents one binary value and a different frequency represents the other. These frequencies lie within the bandwidth of the transmission channel.The figure below is an example of a modulated frequency using fsk. The signal is represented in binary as 0010110

Low speed modem that transmits data at 300bps (bits per second) uses frequency shift keying. For rates over 1200bps other modulation methods are used.

AMPLITUDE SHIFT KEYING (ASK)This varies the amplitude of a signal by modulating a higher frequency. Binary information can be transmitted by assigning discrete amplitudes to bit patterns.If we look at the graph below, we can see that the strength of the signal, or the amplitude, is 1. ('P' stands for 'period', which is the amount of time before a wave repeats itself). The signal with amplitude 1 could be represented by a binary 0.

If we look at the next graph, this is the same signal as above as it has the same period 'p'. However, this signal has a higher amplitude of 2, which could be represented as a binary 

The graph below shows what Amplitude Shift Keying looks like. The period is the same for the entire signal and it is only the amplitude that varies. As we have said that amplitude 1 = binary 0 and amplitude 2 = binary 1, this signal can be represented as 0011010

PHASE SHIFT KEYING (PSK)• Phase modulation (PM) varies the phase of a signal. Information is contained within the

change of phase. Phase shift keying is a technique which shifts the period of a signal. As with FM, binary information can be transmitted by assigning phases to bit patterns.

• If we look at the graph below, we can see that the start of this signal's period 'p' is at 0.

However, if we look at the graph below, we can see that it's phase has been shifted. The period now starts at the wave's highest point which is '1'.

 It has shifted by a quarter of the signal's full period. This could be shifted another quarter and another quarter so that it would be shifted three quarters of its original period.

We can see from the above graph that by having 4 separate signals, four different binary values could be applied to each, with each binary value signifying 2 bits (00,01,10,11). The applying of bit values to each signal is the principle of phase shift keying.

QUADRATURE AMPLITUDE MODULATIONThis method of modulation combines amplitude shift keying (ASK) and phase shift keying (PSK) and is the technique most often used for modern modems. The idea is to 'pack' as many bits as possible into a baud. 16-QAM uses four amplitudes and four phase shifts, allowing the binary value to be represented four bits at a time. In this case, the baud rate will be a quarter of the bit rate.The example below shows a signal that is transmitting at 3600bps, or 3 bits per baud.

DEMODULATION

Demodulation is the act of removing the modulation from an analog signal to get the original baseband signal back. Demodulating is necessary because the receiver system receives a modulated signal with specific characteristics, which must be returned to base-band.

• Link access procedure for modems (LAPM) is part of the V.42 error correction protocol for modems

• LAPM is an error control protocol defined in ITU-T recommendations V.42. Like many data link layer protocols, it is a variant of HDLC Like the microcosm networking protocols that preceded it, LAPM uses cyclic redundancy checking (CRC) and retransmission of corrupted data (ARQ) to ensure data reliability.

• Error control protocols such as lapm use frames of variable lengths, each beginning with a header and ending with a frame check sequence trailer (a cyclic redundancy check) Correctly received packets are acknowledged, and unacknowledged packets are automatically retransmitted (ARQ).

LINK ACCESS PROCEDURE FOR MODEMS

 PARTS INSIDE A MODEM WORKING PROCEDURE

• ON/OFF BUTTON: SPRINGLOADED SWITCH TURNS THE POWER ON AND OFF.

• CAPACITORS: HAVE A VARIETY OF JOBS TO DO IN A MODEM, INCLUDING SMOOTH OUT CURRENT PEAKS. (SEE OUR ARTICLE ON CAPACITORS FOR MORE ON HOW THEY WORK.)

• VOLUME CONTROL: CONTROLS THE LOUDSPEAKER VOLUME.

• LOUDSPEAKER: RELAYS WHAT'S HAPPENING ON THE PHONE LINE AS YOUR MODEM DIALS. READ MORE ABOUT LOUDSPEAKERS.

• MODEM CHIP: MODULATES (ADD DIGITAL INFORMATION TO THE OUTGOING TELEPHONE SIGNAL) AND  DEMODULATES (SEPARATE THE DIGITAL INFORMATION FROM THE INCOMING SIGNAL).

• OTHER CHIPS: CONTROL MODEM CHIP AND OTHER COMPONENTS.

• MICROPHONE: ALLOWS YOU TO SEND YOUR OWN VOICE DOWN THE PHONE LINE. DISCOVER HOW MICROPHONES WORK.

• SERIAL CONNECTION: CONNECTS THE MODEM TO YOUR COMPUTER'S SERIAL (RS-232) PORT. NEWER MODEMS CONNECT TO THE USB PORT INSTEAD.

• MICROPHONE SOCKET: CONNECTS AN EXTERNAL MICROPHONE SO YOU CAN RECORD MESSAGES IN HIGHER QUALITY THAN IF YOU USE THE BUILT-IN MICROPHONE.

• TELEPHONE SOCKETS: CONNECT YOUR MODEM TO A PHONE SOCKET WITH A STANDARD (RJ11) TELEPHONE CABLE. THERE'S A SECOND SOCKET WHERE YOU CAN PLUG A TELEPHONE HANDSET INTO YOUR MODEM. THIS LETS YOU TO USE YOUR PHONE THROUGH THE MODEM WHEN YOUR COMPUTER'S NOT ALREADY USING THE LINE.

• POWER INPUT: CONNECTS THE MODEM TO AN EXTERNAL POWER SUPPLY UNIT (ELECTRICITY TRANSFORMER TO YOUR MODEM

THE ORIGIN OF MODEMS

• The word "modem" is a contraction of the words modulator-demodulator. A modem is typically used to send digital data over a phone line.

• The sending modem modulates the data into a signal that is compatible with the phone line, and the receiving modem demodulates the signal back into digital data. Wireless modems convert digital data into radio signals and back.

• Modems came into existence in the 1960s as a way to allow terminals to connect to computers over the phone lines. A typical arrangement is shown below:

MODEM SECURITY

Modem security can be an issue for some people, especially if they leave their modems on for a continuous connection to the internet. However, many modems have built-in security software to protect your home computer from invasion. Using a router will enhance your security, as will shifting to a less popular but highly secure operating system like linux.

QUERIES