Modems and Other Communications Devices Guide to Operating Systems Second Edition

Analog Modem Architecture Computers handle information in a digital format A modem is a piece of hardware and associated software that connects these two incompatible systems in a way that lets them communicate with each other The analog modems that you are most likely to use work over regular copper telephone lines Typically these are called plain old telephone service (POTS) lines or publicly switched telephone network (PSTN) lines,

Modem Hardware Basics A modem consists of three basic electronic hardware or software components: the data pump, the controller, and the UART The component that performs basic modulation/demodulation is sometimes called a data pump

Modem Hardware Basics The controller provides the modem’s identity The controller also interprets Attention (AT) commands A protocol is an established guideline that specifies how networked data, including data sent over a telephone network, is formatted into a transmission unit; how it is transmitted; and how it is interpreted at the receiving end

Modem Hardware Basics Modem protocol standards are established by the International Telecommunications Union (ITU) and compose the ITU-T standards The latest standards, V.90 and V.92, define a 56,000 bits per second (56 Kbps) communications protocol

Modem Standards and Protocols

Modem Hardware Basics A desktop computer with a modem is an example of a data Terminal equipment (DTE) because it prepares data to be transmitted The modem is called the data communications equipment (DCE) and its speed is the DCE communications rate The UART is an electronic chip, the Universal Asynchronous Receiver-Transmitter The UART converts data from the computer into data that can be sent to serial ports

Modem Hardware Basics The UART reads in one byte of data at the computer’s bus speed, adds a start bit at the beginning, a stop bit at the end, generates an interrupt, and feeds the bits to the serial port at a slow speed that won’t overwhelm the peripheral (a modem in this case, but the same system applies to other serial devices)

Software-Based Modems All modems need the functions of the data pump, controller, and UART Software-based modems (often referred to as Winmodems, after the trademarked name of 3Com/U.S. Robotics popular models) replace one or more of these components with software The 3Com Winmodem is a controllerless modem that retains a hardware data pump Implementing the functions of the controller with software

Software-Based Modems One disadvantage of implementing modem functions with software rather than hardware is that such software takes up memory and processor cycles, although these cycles are now quite inexpensive The biggest disadvantage of software-based modems is their dependence on particular operating systems The advantages of using a software modem are cost savings and upgradeability

Hayes AT Command Set Dennis Hayes devised a way to create a general-purpose modem that could be configured using a command language he invented, the Hayes command language, also called Attention (AT) commands Hayes commands begin with the letters AT, which tells the modem to interpret the next character string as a command A Hayes-compatible modem is equipped with software that acts as a command interpreter The command interpreter ignores spaces and dashes

Hayes AT Command Set Summary

Digital Modems Digital modems are digital devices that use digital transmission media Digital modems can be network devices or serial devices When connecting to TCP/IP networks, they connect via Point-to-Point (PPP), which is a popular communications protocol for Internet communications

Digital Modems Today there are three popular telecommunications or cable networks, which each use different types of digital communications and digital modems: ISDN Cable DSL

ISDN Integrated Services Digital Network (ISDN) uses a digital telephone line for high-speed computer communications, videoconferencing, Internet connections, and so on ISDN uses standard copper telephone line pairs with digital equipment on either end of the connection to encode and transmit the information—an ISDN router (to route the transmission to the right place) and terminal adapter (TA), a type of digital modem Two interfaces are supported in ISDN: basic rate interface and primary rate interface

ISDN The basic rate interface (BRI) has an aggregate data rate of 144 Kbps The BRI consists of three channels: two are 64 Kbps Bearer (B) channels for data, voice, and graphics transmissions; and the third is a 16 Kbps Delta (D, sometimes called Demand) channel used for communications signaling, packet switching, and credit card verification The primary rate interface (PRI) supports faster data rates, with an aggregate of switched bandwidth equal to 1.544 Mbps

ISDN In the United States and Japan, PRI consists of twenty-three 64 Kbps channels and one 64 Kbps D channel for signaling communications and packet switching (plus 8 Kbps for maintenance) European PRI ISDN is thirty 64 Kbps channels and one 64 Kbps signaling or packet switching channel The PRI is used for LAN-to-LAN connectivity, ISP sites, videoconferencing, and corporate sites that support telecommuters who use ISDN

Cable Modems In some areas, cable TV providers also offer data services to businesses and homes A cable modem is used to attach to cable data services This type of modem is usually an external device that plugs into a USB port or network interface card in your computer, and is connected to the coaxial cable used for the cable TV system

Cable Modems The cable modem communicates using upstream and downstream frequencies that are already allocated by the cable service The upstream frequency is used to transmit the outgoing signal over a spectrum (contiguous range) of frequencies that carry data, sound, and TV signals The downstream frequency is used to receive signals, and is also blended with other data, sound, and TV downstream signals The advantage of cable modem communications is that currently unallocated bandwidth can be allocated to you, even for a millisecond or two, when you are downloading a large file

DSL Modems Another high-speed digital data communications service that is challenging ISDN and cable modems is Digital Subscriber Line (DSL) DSL is a digital technology that works over copper wire that already goes into most residences and businesses for telephone services (newer forms of DSL can be used over fiber-optic telephone lines)

DSL Modems A DSL line can be dedicated to a single user, which means that there is less likelihood that the signal can be tapped without the telco being altered DSL can also be shared via a DSL Router Also, the DSL user employs the full bandwidth of his or her line, in contrast to the cable modem user who shares bandwidth with others On networks, DSL is connected by means of a combined DSL adapter and router

DSL Modems A router is a device that can be used to direct network traffic and create a firewall, so that only authorized users can access network services

DSL Modems There are five types of DSL services: Asymmetric Digital Subscriber Line (ADSL) Rate Adaptive Digital Subscriber Line (RADSL) High Bit-Rate Digital Subscriber Line (HDSL) Very High Bit-Rate Digital Subscriber Line (VDSL) Symmetric Digital Subscriber Line (SDSL) ADSL is the most commonly used version of DSL

DSL Modems Aside from traditional data and multimedia applications, ADSL also is well suited for interactive multimedia and distance learning Before transmitting data, ADSL checks the telephone line for noise and error conditions in a process called forward error correction ADSL employs different upstream and downstream speeds (i.e. 768k up and 384k down)

DSL Modems SDSL is similar to ADSL, but it allocates the same bandwidth for both upstream and downstream transmissions SDSL is particularly useful for videoconferencing and interactive learning because of the symmetrical bandwidth transmissions Sometimes it is possible to get SDSL where ADSL is not available due to distance limitations

DSL Technologies

Error Correction Modems transfer bits over the phone line Three possible errors can occur: a bit can be lost, an extraneous bit can be introduced, or a bit can be flipped (changed from zero to one or from one to zero) The most basic form of error correction involves the start and stop bits Each eight-bit byte is framed by a start bit at the beginning and a stop bit at the end

Error Correction The start and stop bits always have the same value Parity checking is a data verification process that ensures data integrity through a system of data bit comparisons between the sending and receiving computer

Error Correction Parity can be either even or odd (or none, if parity checking is turned off) Most modems use the ITU’s V.42 standard to provide error checking V.42 employs an error-checking protocol called Link Access Protocol for Modems (LAPM), which is used to construct data into discrete frame-like units for transmission over communications lines

Data Compression Modems usually compress the data they send A data compression routine can study this picture and see that there’s a lot of repetitive blue in the picture The “compressed” representation of the screen shows a blue dot and a number that represents the number of times the blue dot is repeated If you are familiar with compression utilities, such as PKZIP or WINZIP for the PC, or StuffIt for the Mac, then you have seen the results of data compression

Data Compression Modem data compression uses a similar technique to reduce the total number of data bytes that must be transferred over the connection, but it does it “on the fly,” compressing the data while you send it Compression is typically accomplished by using the V.42bis standard, which employs the Lempel-Ziv-Welch (LZW) compression method

Data Compression This method works “on the fly” in two important respects One is that it compresses data as it is sent, rather than waiting for all of the data to be prepared in a buffer, compressing it, and then sending it The second is that it can detect when the data is already compressed, such as a file that is compressed using PKZIP—and it does not attempt to compress this type of file