Modems How They Work

Modem A device used by a PC to communicate over a phone line or cable A device used by a PC to communicate over a phone line or cable Can be connected to a USB port, a serial port, a network interface card Can be connected to a USB port, a serial port, a network interface card Can be installed as an internal modem (modem card) Can be installed as an internal modem (modem card)

Modulation Conversion of digital signal (from computer) to analog signal (for phone line use) Conversion of digital signal (from computer) to analog signal (for phone line use) Digital data Digital data One state – on or off One state – on or off 1’s and 0’s 1’s and 0’s Analog data Analog data Varying states Varying states

Demodulation Conversion of analog signal (from telephone line) to digital signal (for computer use) Conversion of analog signal (from telephone line) to digital signal (for computer use) MOdulate/DEModulate MOdulate/DEModulate That’s where the word modem comes from That’s where the word modem comes from

Modems Sound traveling over phone lines has an infinite number of different states just as voices do. Sound traveling over phone lines has an infinite number of different states just as voices do. Computers communicate with all 1’s and 0’s Computers communicate with all 1’s and 0’s In order to communicate over phone lines the simple on/off states of 1’s and 0’s have to be converted In order to communicate over phone lines the simple on/off states of 1’s and 0’s have to be converted Modems use different characteristics of waves to represent the 1’s and 0’s Modems use different characteristics of waves to represent the 1’s and 0’s

Modems Have an RJ-11 connector to connect to the wall outlet (for the phone) Have an RJ-11 connector to connect to the wall outlet (for the phone) Have an additional one to attach to a telephone Have an additional one to attach to a telephone Modems have to transmit and receive data Modems have to transmit and receive data

Modems Half-duplex Half-duplex Modem can communicate in only one direction at a time Modem can communicate in only one direction at a time Like a CB radio or walkie talkie Like a CB radio or walkie talkie Full duplex Full duplex Modem can communicate in both directions at once Modem can communicate in both directions at once Like regular phone voice communications Like regular phone voice communications

Modems When dialup connection first established, noises you hear are the modems handshaking or training When dialup connection first established, noises you hear are the modems handshaking or training The modems are establishing the rules of communication between them The modems are establishing the rules of communication between them They each determine what speeds and protocols of data transmission they can support They each determine what speeds and protocols of data transmission they can support They agree on one that is mutually functional, usually the fastest one that is agreeable to both They agree on one that is mutually functional, usually the fastest one that is agreeable to both

Modem Speeds bps – Bits Per Second bps – Bits Per Second How speed is measured How speed is measured Older modems expressed speed as baud rate Older modems expressed speed as baud rate Number of times a signal changes in one second Number of times a signal changes in one second Bps = baud rate Bps = baud rate One signal represents one bit One signal represents one bit Newer modems Newer modems One signal can represent more than one bit One signal can represent more than one bit Baud rates differ from bps rates Baud rates differ from bps rates

Modem Speeds When speed measured in baud rates, bps rate always a multiple of the baud rate When speed measured in baud rates, bps rate always a multiple of the baud rate Most common bps rate = 56.6 Kbps Most common bps rate = 56.6 Kbps Still a few 28.8 or 33.6 Kbps modems out there Still a few 28.8 or 33.6 Kbps modems out there Maximum speed often included in manufacturer’s name for the modem Maximum speed often included in manufacturer’s name for the modem U.S. Robotics 56K Fax Internal PnP modem U.S. Robotics 56K Fax Internal PnP modem

Ceiling On Modem Speeds Many factors contribute Many factors contribute Analog phone lines designed with sufficient quality to handle voice communications Analog phone lines designed with sufficient quality to handle voice communications Not able to handle high speed transmissions of data Not able to handle high speed transmissions of data Older phone lines (pre-1940’s) were analog from start to finish Older phone lines (pre-1940’s) were analog from start to finish New ones now convert to digital for much of the path New ones now convert to digital for much of the path

Ceiling On Modem Speeds Signal is converted to digital soon after leaving house or office Signal is converted to digital soon after leaving house or office Converted back just before going into the house or office destination Converted back just before going into the house or office destination One limitation results from method used when analog signal converted to digital One limitation results from method used when analog signal converted to digital

Ceiling On Modem Speeds Phone company takes sampling of analog signal (8000/second) and converts to digital 8-bit byte Phone company takes sampling of analog signal (8000/second) and converts to digital 8-bit byte Limits signal to 8000 bytes (64,000 bits) per second Limits signal to 8000 bytes (64,000 bits) per second Additional bits and bytes sent with data to regulate the transmission means that maximum rate over phone line is about 56,000 bps or 56K Additional bits and bytes sent with data to regulate the transmission means that maximum rate over phone line is about 56,000 bps or 56K

Ceiling On Modem Speeds Other factors that limit speed Other factors that limit speed Noise Noise Lines bumping together in the wind Lines bumping together in the wind Bad wiring Bad wiring Interference Interference Clean line Clean line A line that consistently delivers a high-quality result A line that consistently delivers a high-quality result

DSL – Digital Subscriber Line Phone line is digital from start to finish Phone line is digital from start to finish No analog conversion No analog conversion No limit of 8,000 bytes/second No limit of 8,000 bytes/second

Serial Port Settings Control how serial ports communicate Control how serial ports communicate Five protocols that control these port settings Five protocols that control these port settings Bits per second (port speed) Bits per second (port speed) Data bits Data bits Parity Parity Stop bits Stop bits Flow control Flow control

Bits Per Second (Port Speed) Port speed should usually be about four times the modem speed Port speed should usually be about four times the modem speed Common values Common values 2400; 4800; 9600; 19,200; 2400; 4800; 9600; 19,200; 38,400; 57,600; 115,200; 230,400; 460,800; 38,400; 57,600; 115,200; 230,400; 460,800; 921,600 bps 921,600 bps

Data Bits How many bits needed to send one single character of data? How many bits needed to send one single character of data? Only seven required for ASCII characters Only seven required for ASCII characters In most cases, you should use 8 In most cases, you should use 8 Common values Common values 7 8

Parity Error checking? Error checking? What format? What format? In most cases, let the modem to error checking and use no parity checking on the serial port In most cases, let the modem to error checking and use no parity checking on the serial port Common values Common values Odd Odd Even Even None None

Stop Bits What code to indicate that a string of bits (for a character) is starting or ending? What code to indicate that a string of bits (for a character) is starting or ending? Most cases: 1 bit Most cases: 1 bit Common values Common values 1 bit 1 bit 1.5 bits 1.5 bits 2 bits 2 bits

Flow Control How will flow of data be controlled? How will flow of data be controlled? Stops flow of data if receive port is being overloaded Stops flow of data if receive port is being overloaded Controlled by software (Xon/Xoff) or hardware (RTS/CTS) Controlled by software (Xon/Xoff) or hardware (RTS/CTS) Always use RTS Always use RTS Common values Common values Xon/Xoff (software control) Xon/Xoff (software control) RTS/CTS (hardware control) RTS/CTS (hardware control)

Modem Features Caller ID supported Caller ID supported Can interrupt data connection temporarily Can interrupt data connection temporarily Must have callwaiting Must have callwaiting Display Readout Display Readout Lights or LCD to list what modem is doing Lights or LCD to list what modem is doing Flash ROM Flash ROM Allows upgrade to modem for future standards Allows upgrade to modem for future standards

Modem Features Plug and Play for Windows Plug and Play for Windows Makes installation more automatic Makes installation more automatic Voice/data capability Voice/data capability Allows use of built-in speaker and microphone so modem can be used as telephone Allows use of built-in speaker and microphone so modem can be used as telephone Auto-answer Auto-answer Allows modem to answer an incoming call even if you are away from the PC Allows modem to answer an incoming call even if you are away from the PC

Dial-up Networking Must have TCP/IP installed and bound to the modem (will look at later on) Must have TCP/IP installed and bound to the modem (will look at later on) Under Windows XP, this is found under Network Connections on the Control Panel Under Windows XP, this is found under Network Connections on the Control Panel Uses Point-to-Point Protocol (PPP) to send data packets over a telephone line Uses Point-to-Point Protocol (PPP) to send data packets over a telephone line See fig 13-7 p. 547 See fig 13-7 p. 547 PPP adds its own header and trailer to the packets PPP adds its own header and trailer to the packets

Dial-up Networking Three layers Three layers Data Data Network protocol Network protocol Line protocol Line protocol PPP is the data link layer in the OSI model PPP is the data link layer in the OSI model To send, data is “enclosed’ in TCP/IP protocols (or others, perhaps) To send, data is “enclosed’ in TCP/IP protocols (or others, perhaps) The TCP/IP packet is then enclosed in the PPP and sent to the modem for transmission The TCP/IP packet is then enclosed in the PPP and sent to the modem for transmission

Dial-up Networking Receiving modem receives the packet Receiving modem receives the packet Receiving computer strips off PPP and sends packet to the network still packaged in TCP/IP Receiving computer strips off PPP and sends packet to the network still packaged in TCP/IP Network sends packet to destination computer Network sends packet to destination computer Destination computer removes TCP/IP from data and uses data Destination computer removes TCP/IP from data and uses data

Creating a Dial-up Connection in Windows XP Double click the Network and Internet Connections icon in the Windows XP Control Panel Double click the Network and Internet Connections icon in the Windows XP Control Panel Click Set up or change your Internet connection Click Set up or change your Internet connection Click the Setup button in the top section of the window Click the Setup button in the top section of the window Follow the wizard through the installation process Follow the wizard through the installation process