Computer Networks

Data Communications: How It All Began
Mail, telephone, TV and radio, books, newspapers, and periodicals-these are the principal ways we send and receive information, and they have not changed appreciably in a generation. However, data communications systems-computer systems that transmit data over communications lines such as telephone lines or cables-have been gradually evolving since the mid-1960s. Let us take a look at how they came about.

In the early days of computing, centralized data processing placed everything - all processing, hardware, and software - in one central location. Computer manufacturers responded to this trend by building even larger, general-purpose computers so that all departments within an organization could be serviced. Eventually, however, total centralization proved inconvenient. All input data had to be physically transported to the computer, and all processed material had to be picked up and delivered to the users. Insisting on centralized data processing was like insisting that all conversations between people occur face-to-face in one designated room.

The next logical step was teleprocessing systems-terminals connected to the central computer via communications lines. Teleprocessing systems permitted users to have remote access to the central computer from their terminals in other buildings and even other cities. However, even though access to the computer system was decentralized, all processing was still centralized-that is, performed by a company's one central computer.

In the 1970s businesses began to use minicomputers, which were often at a distance from the central computer. These were clearly decentralized systems because the smaller computers could do some processing on their own, yet some also had access to the central computer. This new setup was labeled distributed data processing (DDP). It is similar to teleprocessing, except that it accommodates not only remote access but also remote processing. A typical application of a distributed data processing system is a business or organization with many locations-perhaps branch offices or retail outlets.

The whole picture of distributed data processing has changed dramatically with the advent of networks of personal computers. By network we mean a computer system that uses communications equipment to connect two or more computers and their resources. DDP systems are networks. But of particular interest in today's business world are local area networks (LANs), which are designed to share data and resources among several individual computer users in an office or building. We will examine networking in more detail in later sections of the chapter.

In the next section we will preview the components of a communications system to give you an overview of how these components work together.

Putting Together a Network: A First Look
Even though the components needed to transmit data from one computer to another seem quite basic, the business of putting together a network can be extremely complex. We begin with the initial components and then move to the list of factors that a network designer would have to consider.

Getting Started
The basic configuration-how the computers are put together-is pretty straightforward, but there is a great variety of components to choose from, and technology is ever changing. Assume that you have some data-a message-to transmit from one place to another. The basic components of a data communications system used to transmit that message are (1) a sending device, (2) a communications link, and (3) a receiving device. Suppose, for example that you work at a sports store. You might want to send a message to the warehouse to inquire about a Wilson tennis racquet, an item you need for a customer. In this case the sending device is your computer terminal at the store, the communications channel is the phone line, and the receiving device is the computer at the warehouse. As you will see later, however, there are many other possibilities.

Data Transmission
A terminal or computer produces digital signals, which are simply the presence or absence of an electric pulse. The state of being on or off represents the binary number 1 or 0, respectively. Some communications lines accept digital transmission directly, and the trend in the communications industry is toward digital signals. However, most telephone lines through which these digital signals are sent were originally built for voice transmission, and voice transmission requires analog signals. We will look at these two types of transmission and then study modems, which translate between them.

Digital and Analog Transmission
Digital transmission sends data as distinct pulses, either on or off, in much the same way that data travels through the computer. However, most communications media are not digital. Communications devices such as telephone lines, coaxial cables, and microwave circuits are already in place for voice transmission. The easiest choice for most users is to piggyback on one of these. Thus, the most common communications devices all use analog transmission, a continuous electric signal in the form of a wave.

To be sent over analog lines, a digital signal must first be converted to an analog form. It is converted by altering an analog signal, called a carrier wave, which has alterable characteristics. One such characteristic is the amplitude, or height of a wave, which can be increased to represent the binary number 1. Another characteristic that can be altered is frequency, or number of times a wave repeats during a specific time interval; frequency can be increased to represent a 1.

Conversion from digital to analog signals is called modulation, and the reverse process-reconstructing the original digital message at the other end of the transmission-is called demodulation. (You probably know amplitude and frequency modulation by their abbreviations, AM and FM, the methods used for radio transmission.) An extra device is needed to make the conversions: a modem.

Modems
A modem is a device that converts a digital signal to an analog signal and vice versa. Modem is short for modulate/demodulate.

Modem Data Speeds
Users who connect their computers via communications services may pay charges based on the time the computers are connected. Thus, there is a strong incentive to transmit as quickly as possible. The old standard modem speeds of 1200, 2400, and 9600 bits per second (bps) have now been superseded by modems that transmit at 33,300 or now 56,000 bps. If you buy a modem today, you will probably find only modems with the 56K bps. However, transmission from one modem to another can be no faster than the speed of the slower modem, and many services to which you may be connected still receive and transmit at the lower rates. Still, there is no harm having the faster rate in anticipation of the day when all services will also transmit at that rate.

Cable modems, sold by your cable TV company, are more expensive than traditional phone line modems (often over $200) and some require an external modem and an internal ethernet card (LAN) (discussed below). A cable modem essentially puts the home on a Local Area Network (discussed below). These modems can deliver over 1Mbps (million bits per second) - substantially faster than phone modems.

An alternative to cable for fast internet access in the home is the Digital Subscriber Line (DSL) usually offered by phone companies. DSL does not require cable, but instead uses the existing phone lines coming to the home. It also differs in that once connected, you have a point-to-point direct connection to the Internet, like you do with a normal modem. This differs from being on a LAN as happens with cable modem connections. DSL too can achieve speeds of 1Mbps and allow simultaneous phone and computer connections. It was less widely available in Rhode Island in 2001, but is growing. DSL has a stronger base in small businesses which are choosing it as their means of connecting to the Internet.

Another technology, called Integrated Services Digital Network (ISDN) is popular in Europe and has a small clientele in the US. ISDN uses existing phone lines, although some older ones need to be upgraded. ISDN allows data transfer at 64,000 bps and a phone call on the same line. Without the phone call being there too, it can achieve up to 128,000 bps speeds. ISDN is provided by phone companies in some areas of the US.

Communications Links
The cost for linking widely scattered computers is substantial, so it is worthwhile to examine the communications options. Telephone lines are the most convenient communications channel because an extensive system is already in place, but there are many other options. A communications link is the physical medium used for transmission.

Types of Communications Links
There are several kinds of communications links. Some may be familiar to you already.

Wire pairs
One of the most common communications media is the wire pair, also known as the twisted pair. Wire pairs are wires twisted together to form a cable, which is then insulated. Wire pairs are inexpensive. Further, they are often used because they had already been installed in a building for other purposes or because they are already in use in telephone systems. However, they are susceptible to electrical interference, or noise. Noise is anything that causes distortion in the signal when it is received. High-voltage equipment and even the sun can be sources of noise.

Coaxial Cables
Known for sending a strong signal, a coaxial cable is a single conductor wire within a shielded enclosure. Bundles of cables can be laid underground or undersea. These cables can transmit data much faster than wire pairs and are less prone to noise. These are the kind of cables that carry Cable TV into homes. Cable companies are now coming out with Internet Service to the homes (Cox@Home is an example in Rhode Island) that use cable lines and provide much faster access than phone lines can provide.

Fiber Optics
Traditionally, most phone lines transmitted data electrically over wires made of metal, usually copper. These metal wires had to be protected from water and other corrosive substances. Fiber optics technology eliminates this requirement. Instead of using electricity to send data, fiber optics uses light. The cables are made of glass fibers, each thinner than a human hair, that can guide light beams for miles. Fiber optics transmits data faster than some technologies, yet the materials are substantially lighter and less expensive than wire cables. It can also send and receive a wider assortment of data frequencies at one time. The range of frequencies that a device can handle is known as its bandwidth; bandwidth is a measure of the capacity of the link. The broad bandwidth of fiber optics translates into promising multimedia possibilities, since fiber optics is well suited for handling all types of data-voice, Pictures, music, and video-at the same time.

Microwave Transmission
Another popular medium is microwave transmission, which uses what is called line-of-sight transmission of data signals through the atmosphere. Since these signals cannot bend around the curvature of the earth, relay stations-often antennas in high places such as the tops of mountains and buildings-are positioned at points approximately 30 miles apart to continue the transmission. Microwave transmission offers speed, cost-effectiveness, and ease of implementation. Unfortunately, in major metropolitan areas tall buildings may interfere with microwave transmission.

Satellite Transmission
The basic components of satellite transmission are earth stations, which send and receive signals, and a satellite component called a transponder. The transponder receives the transmission from an earth station, amplifies the signal, changes the frequency, and retransmits the data to a receiving earth station. (The frequency is changed so that the weaker incoming signals will not be impaired by the stronger outgoing signals.) This entire process takes a matter of a few seconds.

If a signal must travel thousands of miles, satellites are usually part of the link. A message being sent around the world probably travels by cable or some other physical link only as far as the nearest satellite earth transmission station. From there it is beamed to a satellite, which sends it back to earth to another transmission station near the data destination. Communications satellites are launched into space where they are suspended about 22,300 miles above the earth. Why 22,300 miles? That is where satellites reach geosynchronous orbit-the orbit that allows them to remain positioned over the same spot on the earth.

Mixing and Matching
A network system is not limited to one kind of link and, in fact, often works in various combinations, especially over long distances. An office worker who needs data from a company computer on the opposite coast will most likely use wire pairs in the phone lines, followed by microwave and satellite transmission. Astonishingly, the trip across the country and back, with a brief stop to pick up the data, may take only seconds.

Protocols
A protocol is a set of rules for the exchange of data between a terminal and a computer or between two computers. A protocol is embedded in the network software. Think of protocol as a sort of pre-communication to make sure everything is in order before a message or data is sent. Protocols are handled by software related to the network, so that users need only worry about their own data.

Protocol Communications
Two devices must be able to ask each other questions (Are you ready to receive a message? Did you get my last message? Is there trouble at your end?) and to keep each other informed (I am sending data now). In addition, the two devices must agree on how data is to be transferred, including data transmission speed and duplex setting. But this must be done in a formal way. When communication is desired among computers from different vendors (or even different models from the same vendor), the software development can be a nightmare because different vendors use different protocols. Standards would help.

Setting Standards
Standards are important in the computer industry; it saves money if we can all coordinate effectively. Nowhere is this more obvious than in data communications systems, where many components must "come together." But it is hard to get people to agree to a standard.

Communications standards exist, however, and are constantly evolving and being updated for new communications forms. Standards provide a framework for how data is transmitted. The International Standards Organization (ISO), based in Geneva, Switzerland, has defined a set of communications protocols called the Open Systems Interconnection (OSI) model. (Yes, that is ISO giving us OSI.) The OSI model has been endorsed by the United Nations. As we will discuss shortly, particular types of protocols are used for local area networks.

Network Topologies
As we have noted, a network is a computer system that uses communications equipment to connect computers. They can be connected in different ways. The physical layout of a network is called a topology. There are three common topologies: star, ring, and bus networks. In describing a network topology, we often refer to a node, which is a computer on a network.

A star network has a hub computer that is responsible for managing the Network. All messages are routed through the central computer, which acts as a traffic cop to prevent collisions. Any connection failure between a node and the hub will not affect the overall system. However, if the hub computer fails, the network fails.

A ring network links all nodes together in a circular chain. Data messages travel in only one direction around the ring. Any data that passes by is examined by the node to see if it is the addressee; if not, the data is passed on to the next node in the ring. Since data travels in only one direction, there is no danger of data collision. However, if one node falls, then the entire network falls.

A bus network has a single line to which all the network nodes are attached. Computers on the network transmit data in the hope that it will not collide with data transmitted by other nodes; if this happens, the sending node simply tries again. Nodes can be attached to or detached from the network without affecting the network. Furthermore, if one node fails, it does not affect the rest of the network.

Wide Area Networks
There are different kinds of networks. We begin with the geographically largest, a wide area network.

A wide area network (WAN) is a network of geographically distant computers and terminals. In business, a personal computer sending data any significant distance is probably sending it to a minicomputer or mainframe computer. Since these larger computers are designed to be accessed by terminals, a personal computer can communicate with a minicomputer or mainframe only if the personal computer emulates, or imitates, a terminal. This is accomplished by using terminal emulation software on the personal computer. The larger computer then considers the personal computer or workstation as just another user input/output communications device- a terminal.

The larger computer to which the terminal or personal computer is attached is called the host computer. If a personal computer is being used as a terminal, file transfer software permits users to download data files from the host or upload data files to the host. To download a file means to retrieve it from another computer and to send it to the computer of the user who requested the file. To upload a file, a user sends a file to another computer.

Local Area Networks
A local area network (LAN) is a collection of computers, usually personal computers, that share hardware, software, and data. In simple terms, LANs hook personal computers together through communications media so that each personal computer can share the resources of the others. As the name implies, LANs cover short distances, usually one office or building or a group of buildings that are close together.

Local Area Network Components
LANs do not use the telephone network. Networks that are LANs are made up of a standard set of components.

All networks need some system for interconnection. In some LANs the nodes are connected by a shared network cable. Low-cost LANs are connected with twisted wire pairs, but many LANs use coaxial cable or fiber optic cable, which are both more expensive and faster. Some local area networks, however, are wireless, using infrared or radio wave transmissions instead of cables. Wireless networks are easy to set up and reconfigure, since there are no cables to connect or disconnect, but they have slower transmission rates and limit the distance between nodes.
A network-interface card, sometimes called a NIC, connects each computer to the wiring to the network. A NIC is a circuit board that fits in one of the computer's internal expansion slots.
Similar networks can be connected by a bridge, which recognizes the messages on a network and passes on those addressed to nodes in other networks. For example, a fabric designer whose computer is part of a department LAN for a textile manufacturer could send cost data, via a bridge, to someone in the accounting department whose computer is part of another company LAN, one used for financial matters.
A gateway is a collection of hardware and software resources that lets a node communicate with a computer on another dissimilar network. A gateway, for example, could connect an attorney on a local area network to a legal service offered through a wide area network.

Network Protocols
We have already noted that networks must have a set of rules, called protocols, to transmit data in an orderly fashion that will be understood by other computers. Recall that a protocol is embedded in the network software. There are four layers of protocols that are widely used in the ISO model:

The Data Link Layer - The Data Link Layer determines how digital data is pulsed over the communication like: how many bits at one time, when to pulse, how often to pulse, etc. There two most prevalent Data Link Layer protocols are ethernet and Point-To-Point Protocol (PPP). Ethernet is used on local area networks and cable modems, and PPP is used for phone modem connections and DSL.
Ethernet uses a bus topology and is inexpensive and relatively simple. Since all the nodes (computers) in a LAN use the same cable to transmit and receive data, the nodes must follow a set of rules about when to communicate; otherwise, two or more nodes could transmit at the same time, causing garbled or lost messages. Operating much like a party line, before transmitting data a node "listens" to find out if the cable is in use. If the cable is in use, the node must wait. When the cable is free from other transmissions, the node can begin transmitting immediately. This transmission method is called by the fancy name of carrier sense multiple access with collision detection, or CSMVCD.
If, by chance, two nodes transmit data at the same time, the messages collide. When a collision occurs a special message, lasting a fraction of a second, is sent out over the network to indicate that it is jammed. Each node stops transmitting, waits a random period of time, and then transmits again. Since the wait period for each node is random, it is unlikely that they will begin transmitting at the same time again
Unlike ethernet, PPP is a direct connection from one modem to another modem over a phone line. There are no collisions when data is transmitted. Most Internet Service Providers (ISPs), like America On-line or Edgenet (in Rhode Island) will expect to interact with their customers using PPP. Windows 95/98/ME/2000 have PPP built in to their dial-up adapter software that most people use to call into their ISP.
The Internet Layer - The Internet Layer allows computers of different networks to talk to each other - essentially forming the large multi-faceted network that we know as the Internet. The key to the Internet Layer is that each computer that participates is assigned a unique 32 bit address called an IP Address (Internet Protocol Address). IP addresses are usually shown in four three digit numbers. For instance, the Web server for this text, homepage.cs.uri.edu has IP address 131.128.81.37 . Every computer that does anything on the Internet (send and receive email, serve a Web page, browse a Web page etc) must have an IP address. If your computer is on a LAN, the IP address is probably fixed. For instance, students in URI's dorm rooms with their computers attached to the campus network were given an IP Address by URI for their room. They had to enter this address into their computer using the Network Control Panel on either Windows 95, 98, or a Macintosh. If your computer dials an ISP to gain access, then the IP address is assigned to your computer by the ISP for the duration of your connection to the ISP's modem. ISPs have a large pool of IP Addresses that they temporarily assign to customers while they are connected. You could have a different IP Address every time you use America Online, for instance. With 32 bits, there about 4 billion possible IP Addresses - and the world is running out! A new IP address format is being designed to allow many more IP addresses.
The IP protocol software adds bits to all messages that the computers sends indicating the IP address for the destination of the message.
The Transport Layer - The Transport Layer Protocol checks messages that are sent and received to make sure that they are error free and received in the right order. If the Transport Layer software of the receiving computer detects errors, it sends a message to the original sending computer asking it to re-transmit the message. The Transport Layer protocol used on the Internet is called TCP (Transmission Control Protocol). On the Internet IP and TCP are used together so you often see the protocol referred to as TCP/IP. TCP/IP software is usually part of the operating system. Other than occasionally setting an IP address via the Network Control Panel, you probably will not directly notice or interact with the TCP/IP software.
The Application Layer. The Application Layer provides protocols for specific tasks like sending email or obtaining a Web page. For instance the Simple Mail Transfer Protocol (SMTP) is used on the Internet to format email messages. It is the SMTP that requires the fields we are familiar with: to, from, subject, etc, as well the date of the message and all of the other information we see in email headers. Here are some common Application Layer Protocols in the Internet:
- SMTP - protocol for transmitting email messages;
- POP - protocol for retrieving email message from a server to a local disk (Eudora uses this protocol).
- IMAP - protocol for viewing email via a Web browser where the email is stored on the server (URI's WebMail uses this protocol).
- HTTP - protocol for a client (e.g. Netscape) to ask for a Web page from a Web server (e.g. from einstein.cs.uri.edu).
- FTP - protocol for a remote computer to ask for any file to be transferred to or from it.
- Telnet - protocol to allow one computer to act as a terminal for remotely logging into another computer. This is how you can access URI's or Brown's library catalog from a remote computer.
- SSL - protocol to allow secure transmission of data. This protocol scrambles messages on the sending end and de-scrambles them on the receiving end.
There are other protocols too. Application Layer protocols are usually hidden in an application program like Netscape or Eudora. In fact, the most important thing programs like Netscape does is to be able to "talk" these protocols to other computers on the Internet.

Each protocol layer adds bits to a message. For instance, say you wanted to send "HI" in email to a friend. The 'H' and 'I' each take eight bits for their ASCII representation, so you want to transmit 16 bits. However when the email leaves your email program, the SMTP protocol requires that to, from, subject, etc fields be added - all of which add on a few hundred more bits. The IP protocol software in the operating system then adds 32 bits for the IP address of the destination computer and 32 bits for the IP address of the sending computer (and some other Bits) - for another 100 or so bits added. The TCP protocol software adds bits to allow error checking and sequencing. Ethernet or PPP protocol software also add bits to control the pulsing on the communication link. Thus, a simple 16 bit "HI" message gets transmitted as several hundred bits! This seems wasteful, but is necessary to get computers from all over the world to understand each other.

Organizing Computers On a Network
Two ways to organize the resources of a network are client/server and peer-to-peer.

Client/Server
A client/server arrangement involves a server, which is a computer that controls the network. In particular, a server has the hard disks holding shared files and often has the highest-quality printer, which can be used by all nodes. The clients are all the other computers on the network. Under the client/server arrangement, processing is usually done by the server, and only the results are sent to the node. Sometimes the server and the node share processing. For example, a server, upon request from the node, could search a database of cars in the state of Maryland and come up with a list of all jeep Cherokees. This data could be passed on to the node computer, which could process the data further, perhaps looking for certain equipment or license plate letters. This method can be contrasted with a file server relationship, in which the server transmits the entire file to the node, which does all its own processing. Using the jeep example, the entire car file would be sent to the node, instead of just the extracted jeep Cherokee records.

Client/server has attracted a lot of attention because a well-designed system reduces the volume of data traffic on the network and allows faster response at each node. Also, since the server does most of the heavy work, less expensive computers can be used as nodes.

Peer-to-Peer
All computers in a peer-to-peer arrangement have equal status; no one computer is in control. With all files and peripheral devices distributed across several computers, users share each other's data and devices as needed. An example might involve a corporate building in which marketing wants its files kept on its own computer, public relations wants its files kept on its own computer, personnel wants its files kept on its own computer, and so on; all can still gain access to the other's files when needed. The main disadvantage is lack of speed-most peer-to-peer networks slow down under heavy use.

A prime example of peer-to-peer computing is Napster. With Napster millions of personal computers act as both clients and servers to each other requesting and serving up music files.

Many networks are hybrids, containing elements of both client/server and peer-to-peer arrangements.

The Internet
Although the Internet could fall under the previous section on the work of networking, we choose to give it its own section because it is unique and important. The Internet, sometimes called simply the Net, is the largest and most far-flung network system of them all. Surprisingly, the Internet is not really a network at all but a loosely organized collection of hundreds of thousands of networks accessed by computers worldwide. Many people are astonished to discover that no one owns the Internet; it is run by volunteers. It has no central headquarters, no centrally offered services, and no comprehensive online index to tell you what information is available.

How can all the different types of computers talk to each other? They use a standardized protocol called Transmission Control Protocol/Internet Protocol (TCP/IP). A user must access the Internet through a computer called a server, which has special software that uses the Internet protocol.

Originally developed and still subsidized by the United States government, the Internet connects libraries, college campuses, research labs, and businesses. The great attraction of Internet for these users is that, once the sign-up fees are paid, there are no extra charges. Therefore, and this is a key drawing card, electronic mail is free, regardless of the amount of use. In contrast, individuals using the Internet on their own personal computers must pay ongoing monthly fees to whoever is their service provider.

The Internet consists of many applications such email, web browsing, instant messaging, video conferencing, IP telephony, and many others. All of them are based on standard protocols using the ISO model we discussed previously.

The complexity of Networks
Networks can be designed in an amazing variety of ways, from a simple in-office group of three personal computers connected to a shared printer to a global spread including thousands of personal computers, minicomputers, and mainframes. The latter, of course, would not be a single network but, instead, a collection of connected networks.

You have already glimpsed the complexity of networks. Now let us consider a set of networks for a toy manufacturer.

The toy company has a bus local area network for the marketing department, consisting of six personal computers, a modem used by outside field representatives to call in for price data, a shared laser printer, shared marketing program and data files, and a server. The LAN for the design department, also a bus network, consists of three personal computers, a shared printer, shared files, and a server. Both LANs use the Ethernet protocol and have client/server relationships. The design department sometimes sends its in-progress work to the marketing representatives for their evaluation; similarly, the marketing department sends new ideas from the field to the design department. The two departments communicate, one LAN to another, via a bridge. It makes sense to have two separate LANs, rather than one big LAN, because the two departments need to communicate with each other only occasionally.

In addition to communicating with each other, users on each LAN, both marketing and design, occasionally need to communicate with the mainframe computer, which can be accessed through a gateway. All communications for the mainframe are handled by the front-end processor. Users in the purchasing, administrative, and personnel departments have terminals attached directly to the mainframe computer. The mainframe also has a modem that connects to telephone lines and then, via satellite, to the mainframe computer at corporate headquarters in another state.

Network factors that add to complexity but are not specifically addressed in our example include the electronic data interchange setups between the toy manufacturer's purchasing department and seven of its major customers, the availability of electronic mail throughout the networks, and the fact that-via a modem to an outside line-individual employees can access the Internet.

The near future in data communications is not difficult to see. The demand for services is just beginning to swell. Electronic mail already pervades the office and the campus and homes. Expect instant access to all manner of information from a variety of convenient locations (e.g. hand held wireless devices, your car, your TV). Prepare to become blas� about communications services available in your own home and everywhere you go.