Information Systems Theory

Chapter 6: Communications and Networks

Objectives:

This chapter discusses various storage methods. The objectives important to this chapter are:

1. Understanding communications
2. Understanding how communications technology is used
3. Understanding transmission media
4. Understanding line configurations
5. Understanding data transmission
6. Understanding some commonly used equipment
7. Understanding LANs, MANs, and WANs

Concepts:

The chapter begins with a definition of communications that says it is the transmission of data or information between two or more computers across a communications channel.

Many kinds of computer applications use this kind of communications. Several are listed on page 6.2, some of which have been discussed in previous chapters. Some of the new ones are:

• voice mail - this uses a computer to store voice messages for delivery to a user later
• telecommuting - this involves working at one site and accessing data or program files at another site
• videoconferencing - transmission of sound, video and data between locations for users who can't all meet in one place
• electronic data interchange - sharing of database files or records between companies and their suppliers, to enhance the order and supply of parts or products
• global positioning systems - this technology uses communications between satellites and earth-based systems to determine a user's location
• bulletin board systems - like e-mail systems, except that all users can usually read all messages placed in the system
• online services - often Internet Service Providers who add access to their own content as well, like America Online and the Microsoft Network
• Internet - the Internet itself is given as an example of a network of networks, which is an example of computers linked for communication

A communications system contains several elements, each of which may come in several types.

Transmission media are the pathways across which data and information are passed. Several types are listed:

• Twisted pair cable - several insulated copper wires (usually 8) are grouped into pairs. The first graphic on page 6.9 shows four twisted pairs of wires. Each wire is covered with an insulator, and the two wires in each pair complete a circuit. This type of wire suffers from crosstalk, leakage of signal from one circuit to another. The twists help cancel out such leaks. This type of cable comes in several varieties: two pair, three pair and four pair are common. The graphic shows a UTP cable with eight wires in it, making four pairs. This kind of cable would be connected to a computer with an RJ-45 connector. If the cable had only four wires in it (two pairs), it could use an RJ-11 connector, and it would probably be a telephone cable, not a data network cable. Twisted pair cable comes in many grades and two types: unshielded (UTP) and shielded (STP). The shielded type has an extra insulator to shield it from interference. The unshielded type does not.
• Coaxial cable - is less flexible, costs more and is harder to work with than UTP. It can be similar to the cable used in cable television system, or it can be much thicker and even harder to use. A coaxial cable has one central conductor, enclosed by insulation, enclosed by a ground wire (or wire mesh), which is enclosed in the cable insulation.
• Fiber-optic - can be glass or plastic, and is meant to conduct light instead of electricity. The conductor is called a waveguide, and is covered with cladding, a material to reflect the signal back into the center of the conductor.
• Microwave signals are used in two formats: terrestrial (earth-based) and satellite systems. Terrestrial systems are used in line of sight connections where it is not possible to put a wire, such as across several city blocks.
• Satellite systems are used to connect sites that are widely separated. Usually, signals are sent to geosynchronous satellites, orbiting 22,300 miles above the earth. This orbit puts the satellite in the same part of the sky relative to a ground based observer at all times. Of course, the orbit should be around the equator of the earth, not the poles as indicated in the picture in your book. (Best lines in the Pauly Shore film Bio-Dome:
  smart aleck question: "What are you, a rocket scientist?"
  deadpan answer: "Yes."
  no possible response: "...sorry...")
• Wireless transmission media - actually, microwaves are one form of wireless media, which just means there is no cable. Other forms often used are infra-red light and radio waves. A cellular phone actually is a radio which contacts the nearest radio receiver of a land based telephone network.

An actual channel may consist of several parts, called links, which can be of any type. The illustration on page 6.14 is of a computer on the west coast of the US contacting a server on the east coast. The signal passes through phone lines, terrestrial microwaves, satellite microwaves and possibly other media along the way.

Lines in communication channels are available to users in several service categories.

• switched lines - when a person makes a telephone call, the switching network of the phone companies set up a channel for the conversation. This channel will only exist temporarily, for the time this conversation is in effect, and it may vary in quality from the next channel set up for a similar conversation.
• dedicated line - this is a guarantee of a connection of a certain quality through a provider that charges more for this service than for a switched line
• point to point - this kind of connection only connects two devices, such as the connection between a television and its remote control. No other devices share or use the connection.
• multipoint - also called multidrop, because each place where a device can be connected to the line is called a drop. Several devices share the connection medium.

Page 6.16 brings up the concepts of digital and analog again. This time, it is explained that the signals on a channel could be either digital or analog. A digital signal might be a series of pulses, where a pulse might stand for one kind of bit and the absence of a pulse might stand for the other kind. An analog signal might consist of one or more continuous electrical waves. We might send one kind of wave (one frequency) to represent a one and another kind of wave (another frequency) to represent a zero.

Digital data lines exist, and they are available in several grades of service. Some types are:

• T1 lines - can carry up to 1.544 Megabits per second
• T3 lines - can carry 45 Megabits per second
• ISDN lines - can have multiple data channels, each of which can carry 128 Kilobits per second

Data transmissions can be classified into three types:

• simplex - data flows one way from one device to anther, but does not flow the other direction. This is like a remote control.
• half-duplex - data can flow both directions between devices, but only one way at a time. Devices must take turns. This is like CB radio, ham radio, and walkie-talkies.
• full-duplex - data can flow both directions between devices, at the same time. This is like a telephone.

Several kinds of communication equipment are described in the chapter:

• modem - stands for modulator/demodulator, often used with computers connected to phone lines to convert between digital computer signals and analog phone signals. Available as internal (installed inside the computer) and external (attached to a port) models.
• multiplexer - sometimes called a MUX, a multiplexer combines several signals from different devices, passes them along a communications channel to another MUX, which separates the signals and passes them to their intended destination.
• front end processor - this is a computer that does the communications work for another computer
• network interface card - a NIC (don't call it a "NIC card", that's redundant) is used to connect a computer, printer, or other device to a network
• wiring hubs - a hub can act like a telephone switchboard in a network, passing signals from one device to another
• gateway - the word "gateway" can mean several different things. In this chapter, a gateway is a device that connects networks that are different from each other, and acts as a translator between them
• bridge - a bridge connects networks that are similar to each other, so translation is not necessary
• routers - a router provides connections to several networks. Routers can act as bridges or as gateways.

Three size designations are used for networks:

• LAN - a Local Area Network is often inside one building, may cover several buildings, but is never as large as a city.
• MAN -  a Metropolitan Area Network spans an area the size of a city. It is larger than a LAN, smaller than a WAN.
• WAN - a Wide Area Network spans an area larger than a city, often covering many states or countries. The Internet, which covers the world, is a WAN.

Two major types of networks are:

• Client-Server - clients are machines that request services on a network, servers are machines that provide services. In a client-server network, most machines are only one or the other.
• Peer-to-Peer - peers are machines that can both request and provide services on a network.

A network operating system (NOS) is the set of programs that make it possible to provide and share resources on a network. Several software publishers offer NOSs, such as Novell (NetWare), Microsoft (Windows NT, Windows 2000), and IBM (LAN Server).

Three network configurations are described in the text. Before you consider them, a word of advice: things do not always work the way they look. Another phrase that describes this topic is topology, with is the science of shapes. Physical topology describes how network is actually shaped, but logical topology describes how it acts. The three physical topologies described here are:

• star - all the workstations, printers, etc. on a star network are wired to one or more central point. Imagine the wire running from the central point to each device, and you can see the star.
• bus - a single continuous line (wire, fiber optic, etc.) is run from device to device. Each device is said to tap in to the line (bus). The ends of the bus are not connected to anything.
• ring - like a bus, in that the line used runs from one device to the next, but different in that the line eventually returns and connects to the first device, making a circle (ring)

Protocols are sets of rules that describe how networks function. Many sets of protocols exist. The text summarizes two types of protocols:

• Ethernet - an Ethernet is a contention based system. Contention systems work by letting each device try to send a message on the net as needed, contending or competing with all the other devices for the bandwidth. An example of a protocols that supports such a systems is CSMA/CD (Carrier Sense, Multiple Access, with Collision Detection). A collision occurs when two signals collide on the medium, causing signal loss. In a CSMA/CD system, like Ethernet, the collision is detected and the devices each wait a random number of seconds before sending again. This usually results in one device going ahead of the other.
• Token Ring - a token ring system uses a protocol that is referred to as token passing. Token-Passing involves passing a token, a few bytes, from workstation to workstation. When a station has the token, it is that station's turn to access the medium. This type of media access is predictable and consistent, allowing large or small transmissions. It is not the best for time sensitive data since waits are built in, but it will support more devices than contention.

An Ethernet acts like a logical bus, a Token Ring acts like a logical ring, but both kinds of network can actually be wired like a star (and still act the way they are supposed to act). If that doesn't confuse you, you may have a future in networking.