Week 2A

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Computer Networks

A network connects digital devices, like computer, together allowing them to send binary data (1s and 0s) to each other. There are many different kinds of networks: The Internet, various cell phone networks, computers networked together using a wireless router in a home, processors networked together in a car to control its operation, are a few examples. Many networks, like the Internet, are actually networks of networks. Networks may use many different types of communication links to connect the devices on it. For instance, when sending an email message on the Internet, binary data that goes from the sender's computer to receiver's computer may travel over copper wire, radio waves, fiber optics, and other media during its journey. Here are some of the most common communication media used in networks:

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. Ethernet cables that have connectors at the end that look like telephone line connectors, but slightly bigger, are a common way to connect computers to their network. Another example is Digital Subscriber Lines (DSL) that uses the twisted pair wires of telephone lines to bring high speed Internet connectivity to homes over the existing phone lines. A DSL modem is necessary to translate the binary data to data that can be carried on the phone line.

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. Within the past few years cable companies have been providing homes with Internet Service (Cox@Home is an example in Rhode Island) by using cable lines that carry a cable television signal into a house. A cable modem is required in the house to translate the binary data of computer communication to a signal that can travel on the coaxial cable TV wire.

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. Cell phone networks use microwave communication via cell phone towers that re-transmit signals.

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.

Standards 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 ways of sending the 1's and 0s. 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. We discuss the protocols of the OSI model below.

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, which 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 standards so that devices connected to them agree how to communicate. In addition to standards for communication links, computers have a set of rules, called protocols, to transmit data in an orderly fashion that will be understood by other computers. A protocol is embedded in the network software in the operating system of the devices on the network. 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 DSL providers, like Verizon in Rhode Island, get their customers on the Internet with a PPP protocol implemented in the DSL modem they provide to the customer.

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 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 are 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 send 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. Firefox) 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 Firefox. In fact, the most important thing a program like Firefox 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 adds 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 are Limewire or Kazaa sharing applications. With Limewire and Kazaa millions of personal computers act as both clients and servers to each other requesting and serving up music and other 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 as 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.


Google Mail, also known as Gmail, is a free e-mail service that is run by Google. It can be accessed on the web, by POP3, or by IMAP. Some of the competitors to Gmail are Yahoo! Mail, Hotmail/Windows Live Mail, and Inbox.Com. The space given to any GMail member is increased a small amount every second, and as of July 26, 2012, Google provides each account with about 10272 MB of space.

Google Apps Google Apps is a service from Google that was created in February 2006 as Gmail for your domain. It is to allow system administrators of a company or organization to create email accounts for their own domain.

Trademark issues In the United Kingdom (UK), the trademark "Gmail" was owned by another company before Gmail by Google was started. Thus, the United Kingdom uses a domain of "googlemail.com" for their users, and the logo has the words of "Google Mail" instead of the normal "Gmail".

In September 2009 Google began to change the branding of UK accounts back to Gmail following the resolution of the trademark dispute.[1]


Parts of this page are based on information from: Wikipedia: The Free Encyclopedia