Chapter 3



Chapter 7

Communication and Networking

7.1 Introduction

From the Analog to the Digital Age

Digital: Digital specifically refers to communications signals or information represented in a two-stat (binary) way.

Analog: Continuously varying in strength and/or quantity.

Purpose of the Modem: Converting Digital Signals to Analog Signals & Back

Modem is short for modulate/demodulate; a sending modem modulates digital signals into analog signals for transmission over phone lines. A receiving modem demodulates the analog signals back into digital signals.

Converting Reality to Digital Form

The analog recording process will produce a near duplicate of the sounds. The way in which music is captured for audio CDs does not provide a duplicate of a musical performance. Rather, the digital process uses representative selections (samples) to record the sounds.

Turning analog reality into digital form provides tremendous opportunities. Now all kinds of multimedia can be changed into digital form and transmitted as data to all kinds of devices.

The Practical Uses of Communications

Videoconferencing, also called teleconferencing, is the use of television video and sound technology as well as computers to enable people in different locations to see, hear, and talk with one another.

Workgroup computing, also called collaborative computing, enables teams of co-workers to use networks of microcomputers to share information and to cooperate on projects.

Telecommuting: Working at home while in telecommunication with the office is called telecommuting. A related term is telework, which includes not only those who work at least part-time from home but also those who work at remote or satellite offices called telework centers.

Virtual offices: The virtual office is an often nonpermanent and mobile office run with computer and communications technology.

7.2 Communications Channels

Wired Communications Channels: Transmitting Data by Physical Means

• Twisted-pair wire (1–128 Mbps): Twisted-pair wire consists of two strands of insulated copper wire, twisted around each other. This twisted-pair somewhat reduces interference from electrical fields. The telephone line that runs from your home to the pole outside, or underground, is probably twisted-pair wire.

• Coaxial cable (up to 200 Mbps): Coaxial cable, commonly called “co-ax,” consists of insulated copper wire wrapped in a solid or braided metal shield, then in an external cover. Co-ax is widely used in cable television. Thanks to the extra insulation, it is much better than twisted-pair wiring at resisting noise. Moreover, it can carry voice and data at a faster rate (up to 200 Mbps).

• Fiber-optic cable (100 Mbps to 2 Gbps): A fiber-optic cable consists of dozens or hundreds of thin strands of glass or plastic that transmit pulsating beams of light rather than electricity. These strands, each as thin as a human hair, can transmit up to 2 billion pulses per second (2 Gbps), each “on” pulse representing one bit.

Wireless Communications Channels: Transmitting Data through the Air

• Infrared transmission (1–4 Mbps): Infrared wireless transmission sends data signals using infrared-light waves. Infrared ports can be found on some laptop computers and printers, as well as wireless mice.

• Broadcast radio (up to 2 Mbps): A wireless transmission medium that sends data over long distances – between regions, states, or countries.

• Microwave radio (45 Mbps): Microwave radio transmits voice and data through the atmosphere as super-high-frequency radio waves called microwaves.

• Communications satellites: Communications satellites are microwave relay stations in orbit around the earth. Transmitting a signal from a ground station to a satellite is called uplinking; the reverse is called downlinking.

Types of Long-Distance Wireless Communications

• One-way communications—the Global Positioning System: The Global Positioning System (GPS) consists of a series of earth-orbiting satellites continuously transmitting timed radio signals that can be used to identify earth locations. A GPS receiver can pick up transmission from any four satellites, interpret the information from each, and pinpoint the receiver’s longitude, latitude, and altitude.

• One-way communications—pagers: Once stereotyped as devices for doctors and drug dealers, pagers are now consumer items. Pagers are simple radio receivers that receive data (but not voice messages) sent from a special radio transmitter.

• Two-way communications—pagers: Recently advances have given us two-way paging or enhanced paging. In one version, users can send a preprogrammed message or acknowledgment that they have received a message. Another version allows consumers to compose and send e-mail to anyone on the Internet and to other pagers.

• Two-way communications—first generation analog cellular services: Analog cellular phones are designed primarily for communicating by voice through a system of ground-area cells.

• Two-way communications--second generation digital wireless services (2G): Digital wireless services support digital cellphones and personal digital assistants use a network of cell towers to send voice communications and data over the airwaves in digital form.

• Two-way communications planned for the future-third-generation broadband wireless digital services (3G): Broadband wireless digital services, usually referred to as third-generation (3G) technology, are based on the GSM standard, are (like cable modems) “always on”, carry data at high speeds (56 kilobits per second up to about 2 megabits per second eventually), and are able to quickly transmit video, still pictures, and music, along with offering better ways to tap into websites than today’s 2G wireless systems.

• Two-way communication in the near future – 2.5G, a compromise between 2G and 3G.

Short-range wireless communications at 2.4 Gigahertz:

• Bluetooth – up to 30 feet: Bluetooth is a short-range wireless digital standard aimed at linking cellphones, PDAs, computers, and peripherals up to distances of 30 feet.

• WiFi – up to 300 feet: WiFi is a short-range wireless digital standard aimed at helping machines inside offices to communicate at high speeds and share Internet connections at distances up to 300 feet; it connects to a kind of local area network known as the Ethernet.

• HomeRF – up to 150 feet: HomeRF is a separate incompatible standard designed to network up to 10 PCs and peripherals as far as 150 feet apart.

7. 3 Data Transmission

Transmission Rate: Higher Frequency, Wider Bandwidth, More Data

Transmission rate is the function of two variables: frequency and bandwidth.

Frequency: the cycles of waves per second expressed in hertz.

Bandwidth: the difference between the highest and lowest frequencies. The more frequencies it has available and hence the more data that can be sent through the channel.

Line configurations: Point-to-Point & Multipoint

Point-to-Point: line that directly connects the sending and receiving devices.

Multipoint: A multipoint line is a single line that interconnects several communications devices to one computer.

Serial & Parallel Transmission

Serial Data Transmission: bits are transmitted sequentially.

Parallel Data Transmission: bits are transmitted through separate lines simultaneously.

Direction of Transmission: Simplex, Half-Duplex, & Full-Duplex

Simplex transmission: data can travel in only one direction.

Half-duplex transmission: data travels in both directions but only in one direction at a time.

Full-duplex transmission: data is transmitted back and forth at the same time.

Transmission Mode: Asynchronous & Synchronous

Asynchronous transmission: data is sent one byte (or character) at a time. Each string of bits making up the byte is bracketed, or marked off, with special control bits.

Synchronous transmission: sends data in blocks. Start and stop bit patterns, called synch bytes, are transmitted at the beginning and end of the blocks.

Circuit Switching & Packet Switching: For Voice, Data, & Both:

Circuit switching: best for voice: The transmitter has full use of the circuit until all the data has been transmitted and the circuit is terminated.

A packet is a fixed-length block of data for transmission.

Packet switching: electronic messages are divided into packets for transmission over a wide area network to their destination, through the most expedient route.

Multiplexing: Enhancing Communications Efficiencies:

Multiplexers: A device that merges several low-speed transmissions into one high-speed transmission.

Concentrators: Collects data in a temporary storage area.

Front-end processors: A smaller computer that is connected to a larger computer and assists with communications functions.

Protocols: The rules of data transmission:

Protocol: Communications protocol is a set of conventions governing the exchange of data between hardware and/or software components in a communications network.

OSI: Open Systems Interconnection. Backed by the International Standards Organization, OSI is an international standard that defines seven layers of protocols for worldwide computer communications.

7. 4 Networks

A network, or communications network, is a system of interconnected computers, telephones, or other communications devices that can communicate with one another and share applications and data.

The Benefits of Networks

• Sharing of peripheral devices: Expensive devices such as printers, disk drives, and scanners can be shared.

• Sharing of programs and data: People in an organization can use the same software and have access to the same files.

• Better communications: Networks make e-mail possible, keeping everyone posted about important information.

• Security of information: Information on a network is apt to be backed up (duplicated) somewhere. Thus, a fire or flood is less apt to destroy essential information.

• Access to databases: Networks enable users to tap into all kinds of databases.

Types of Networks: WANs, MANs, & LANs

Networks may be classified into three categories according to their geographical range:

• Wide area network: A wide area network (WAN) is a communications network that covers a wide geographical area, such as a country or the world.

• Metropolitan area network: A metropolitan area network (MAN) is a communications network covering a city or a suburb.

• Local area network: A local area network (LAN) connects computers and devices in a limited geographical area, such as one office, one building, or a group of buildings close together.

Types of LANs: Client-Server & Peer-to-Peer

• Client-server LANs: A client-server LAN consists of clients, which are microcomputers that request data, and servers, which are computers used to supply data.

A file server is a computer that acts like a disk drive, storing the programs and data files shared by users on a LAN.

• Peer-to-peer LANs: In a peer-to-peer LAN, all microcomputers on the network communicate directly with one another without relying on a server. These are less expensive than client-server networks and work effectively for up to 25 computers; beyond that, they slow under heavy use.

Components of a LAN

• Connection or cabling system: LANs may use a wired or wireless connection system. Wired connections may be twisted-pair wiring, coaxial cable, or fiber-optic cable. Wireless connections may be infrared or radio-wave transmission.

• Microcomputers with network interface cards: Two or more microcomputers are required, along with network interface cards. A network interface card (inserted into an expansion slot in a PC) enables the computer to send and receive messages over a cable network.

• Network operating system: The network operating system (NOS), such as Novell NetWare, Microsoft Windows NT/2000, Unix, and Linux, is the system software that manages the activity of a network.

• Other shared devices: Printers, scanners, storage devices, and other peripherals may be added to the network as necessary and shared by all users.

• Routers, bridges, and gateways:

A router is a special computer that directs communicating messages when several networks are connected together.

A bridge is an interface used to connect the same types of networks. An example is the Ethernet.

A gateway is an interface permitting communication between dissimilar networks—for example, between a LAN and a WAN.

Topology of LANs

• Star network: one in which all microcomputers and other communications devices are connected to a central server. The advantage of a star network is that the hub prevents collisions between messages.

• Ring network: one in which all microcomputers and other communications devices are connected in a continuous loop. Electronic messages are passed around the ring until they reach the right destination. There is no central server.

• Bus network: all communications devices are connected to a common channel. Each communications device transmits electronic messages to other devices. If some of those messages collide, the device waits and tries to transmit again.

7. 5 Intranets, Extranets, & Firewalls: Private Internet Networks

• Intranets—for internal use only: An intranet is an organization’s internal private network that uses the infrastructure and standards of the Internet and the World Wide Web. Access to an intranet is not available to people outside the organization.

• Extranets—for certain outsiders: Extranets are private intranets that connect not only internal personnel but also selected suppliers and other strategic parties.

• Firewalls: A firewall is a system of hardware and software that blocks unauthorized users inside and outside the organization from entering the intranet.

7.6 The Future of Communications

Satellite-Based Systems

More than half the people in the world, mostly in underdeveloped countries, live more than 2 hours from the nearest telephone. These people, as well as business travelers and corporations needing speedy data transmission, are demanding more than wire-line or cellular service can deliver.

In the next few years, four kinds of satellite systems will dot the skies to provide a variety of consumer services. The first is the TV direct-broadcast system, and the second is the GPS system, both of which we described earlier. The third type is designed to handle cellular-phone and paging services, using satellite transmissions in place of tower-to-tower microwave transmissions.

Probably most interesting is the fourth, which consists of global high-speed satellite networks that will let users exchange a much broader range of data, including Internet pages and videophone calls, anywhere in the world.

Beyond 3G to 4G

3G Cellphones: send and receive voice and data at speeds of up to 2 megabits per second. The 3G radio spectrum is not yet available in the U.S.

4G wireless technology is started to be developed.

Photonics: Optical Technologies at Warp Speed

Photonics is the science of sending data bits by means of light pulses carried on hair-thin glass fibers. Photonics has achieved breakthroughs that enable glass fibers to carry more light signals than ever before.

Older fiber-optic technologies were limited to only a few dozen miles. An optical amplifier boosts light signals without converting them first to electrical signals.

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