Modem - How a Computer Works (2015)

How a Computer Works (2015)

24. Modem



The Modem connects your computer to the Internet. Typically connecting to the internet requires you to connect to your internet service providers (ISP) modem. If your modem is external it is connected to the computers serial port.

The serial port sends data to the modem on just one line. Data is transmitted one bit after the other.

Once inside the Modem a digital to analogue converter chip converts the binary data into analogue data ready to be sent along a Telephone line to another modem.

This type of modem is called a dial up modem.

Status Indicators


The external lights on your Modem tell you what’s going on while the Modem is working. The High Speed (HS) light lets you know that the modem is operating at its highest Transmission rate. Auto Answer (AA) automatically answers incoming calls.

Carrier Detect (CD) comes on when a carrier signal from a remote computer is detected. Off Hook (OH) comes on when the modem has control of the phone line. Receive Data (RXD) light comes on when data is received from a remote computer.

Transmit Data


Transmit Data (TXD) light flashes on when data is sent to a remote computer. The Terminal Ready (DTR) light indicates that a communications program on your computer has issued a DTR signal.

This is typically issued when you make a dial up connection to the Internet.


When you connect to the internet you make a dial up connection, although if you have a permanent connection to the internet this is not the case. This description describes a typical dial up connection over a telephone line.

Connecting to the internet involves loading a program that handles all the protocols and communications between the modem and e-mail or web browser.

When you double-click on your ISP logo a dynamic link library called a Winsock is loaded. This invisible program is the interface between your modem and web browser and other programs you use on the internet.

Data Ready Terminal


Data set ready


A DTR signal is sent to the modem

When you run your network connection a DTR (Data Terminal Ready) signal is sent to the modem. The DTR signal tells the modem that the computer is ready to transmit data to it. The computer also detects a DSR (Data Set Ready) signal from the modem. This command signal lets the computer know that the modem is ready to receive commands and data. If these two signal commands are not both present the modem will fail to function.




Data set ready


A DTR signal is sent to the modem

Once these actions are complete the dial up networking program sends out a command that tells the modem to go off hook. The off hook command is like picking up the handset on a telephone.

The next step is to dial the telephone number of your ISP. The dial up networking program sends out a command that tells the modem to dial the number. The modem responds to the command by replying to the PC on the Receive Data line.

Carrier Detect

When the modem at the isp your are calling answers the call, your modem sends out a tone to inform the isp modem that its being called. The ISP modem responds with a tone of its own.

This establishes a communication path between the two modems. Once this happens your modem sends a carrier detect signal to your computer.

This informs the dial up networking program that your modem is receiving a carrier signal that will be used to transmit modulated data.



The two modems then handshake, which is a standard way of sending data to one another.

The handshake contains data such as the data transfer speed, how many bits make up the data packet. This forms the basis for exchanging data between the two modems.

With the handshake the modems could be sending data that neither understand.

Transmission Speed


Transmission Speed is expressed in bits per second. When the modem sends data it sends one frequency to indicate a 0 bit and another to indicate 1 bit. Analogue telephone lines cannot change frequency more than 600 times a second. Group Coding allows different frequencies to be used on the same telephone line signals. These are still sent up 600 times a second but for instance four different frequencies can be used to represent the four pairs of binary bits 0,0-0,1-1,0-1,1, enabling even more data to be sent. Data compression of frequently sent bit patterns allows shorter codes to substitute those bit patterns.

Data Packet


Each data packet has a single binary bit to signal the start of the data packet called the Start bit. One or two binary bits signal the end of the data packet known as the Stop bit. To check for errors in the data packet a parity bit of either 1 or 0 is added. Both systems decide if the parity is to be odd or even parity. The data packet bits are added up by each system either 0 or 1 is added to make the total an even or odd number. Typically the data packet contains either seven or eight bits.

Request to Send

Now the modem is connected to your ISP's modem and ultimately to the internet you can send an e-mail or request a web page. To do this a request to send (RTS) signal is sent to the modem, this interrogates the modem to see if it's free to receive data from the computer.

If the modem is already receiving remote data for the computer, but the computer is busy with another task such as saving a file to disk, the computer will switch off the RTS signal to inform the modem to stop sending it data while the computer finishes it task.

Clear to Send

If the modem is not busy it sends a Clear To Send (CTS) signal to the computer. At this stage the computer sends the data to the modem. This data could be something like an e-mail or web URL. In turn the modem converts this digital data into analogue frequencies that it sends over the telephone line to your ISP's modem.

If the modem cannot transmit data as fast as it receives it from the computer, the modem switches off the CTS signal, which tells the computer to suspend sending any further data until the modem catches up and switches on the signal again.



When the isp's modem hears the analogue data frequencies it coverts these using an analogue to digital convertor which converts these signals back to their original binary state.

When your internet session has finished the dial up networking program sends a command to the modem that causes the modem to disconnect from the phone line.

After this the modem will drop the Carrier Detect signal to the computer. The dial up networking program will understand that communication with the isp’s modem is over.