Mini Tutorial

Internet for You

Katarzyna J. Macura, M.D., Ph.D.

The Russell H. Morgan Department of Radiology and Radiological Science •  Johns Hopkins Medical Institutions

Part 2 — Connecting to the Internet
by Katarzyna J. Macura, M.D., Ph.D.

By connecting local, regional, national and international networks, the Internet forms the world's largest network of networks. Computers connected to the Internet work together to transfer information around the world using servers and clients. A server is a computer that manages the resources on a network and provides a centralized storage for programs and data. The server, also called a host, "serves" files and services out to clients, computers that access the contents of the server. The speed at which data can be transferred between the sender and receiver in a network is called the data transfer rate. Transfer rates are expressed in bits per second (bps). A bit, short for binary digit, is the smallest unit of computerized data. A bit has a value of either 1 or 0 that the computer interprets as "on" or "off" respectively. When calculating Internet-access speeds, it is important to recognize the difference between bits and bytes. There are eight bits in a byte. The small "b" stands for bits, and the big "B" stands for bytes. Transfer speeds are often shown in kilobytes per second (KB/s), and connect speeds are usually quoted in kilobits per second (kbps). For example, if the Web browser is downloading a file at 100 KB/s over a cable modem connection, that is equal to a speed of 800 kbps. Bandwidth is the width of the communication channel, the total volume of traffic that can be transferred across a given transmission line in a given period of time, usually measured in bits per second.

There are four ways of connecting a client computer to the Internet: a dial-up connection using a telephone line or an Integrated Services Digital Network (ISDN), a Digital Subscriber Line (DSL), a cable TV connection and a satellite connection.

Dial-up Connection

A dial-up connection uses the analog telephone line for establishing a temporary communication. Computer's digital signals must be converted to analog signals before they are transmitted over standard telephone lines. This conversion is performed by a modem, a device that modulates (changes into an analog signal) and demodulates (converts an analog signal into a digital signal). Both the sending and receiving ends of a communication channel must have a modem for data transmission to occur. Using a dial-up line to transmit data is similar to using the telephone to make a call. The client computer modem dials the preprogrammed phone number for a user's Internet Service Provider (ISP) and connects to one of the ISP's modems. After the ISP has verified the user's account, a connection is established and data can be transmitted. When either modem hangs up, the communication ends. The advantage of a dial-up line is that it costs no more than a local telephone call. Computers at any two locations can establish a connection using modems and a telephone network, to include wireless modems and cellular telephone connections. The limitation of a connection using the ordinary telephone line is a low speed, 28 kbps. There are dedicated telephone lines that can transmit data at 56 kbps. Most 56 kbps modems connect at a speed less than 46 kbps because of the limitations of analog phone lines and telephone company switches.

ISDN

ISDNs are special digital telephone lines that can be used to dial into the Internet at speeds ranging from 64 to 128 kbps. These types of connections are not available everywhere telephone companies have to install special ISDN digital switching equipment. ISDNs require use of a special "digital modem" that sends and receives digital signals over ISDN lines. With an ISDN, the telephone line is divided into three channels (BRI - Basic Rate Interface), two-64 kbps B (bearer) channels that send data and one 16 kbps D (data) channel that sends routing information. This type of access is commonly referred to as 2B+D. To use the ISDN access to the Internet, an ISP has to offer the ISDN access. ISDN lines cost more than normal phone lines, so the telephone rates are usually higher.

Cable TV Connection

Currently most households with cable TV have the option for cable modem Internet access. The cable modem offers a high-speed link at low cost for unlimited, "always connected" access. The connection speeds range from 128 kbps up to 10 mbps (megabits per second). A cable modem is a device that connects to the existing TV cable feed and to an Ethernet network card in the user's PC (also called an NIC - Network Interface Card). The cable network is designed to support the highest speeds in the "downstream" direction, which is from the Internet to the client computer. This downstream speed affects the performance of downloading Web pages and software. The "upstream" bandwidth for data sent from a user's computer to the Internet is typically less, in the range of 200 kbps to 2 mbps. The benefit of the cable modem for Internet access is that, unlike DSL, its performance doesn't depend on distance from the central cable office. However, with the cable TV network, the computer is put on a Local Area Network (LAN) with other users in the neighborhood and like with any LAN, the performance degrades as usage increases. A more disturbing issue is that of network security. One of the main purposes of a LAN is to allow file sharing among the computers on the LAN. This LAN feature doesn't work well with cable Internet access, as most users do not want neighbors accessing their files. Turning the sharing option off can prevent file sharing. Also, installing the firewall hardware or software may protect from hackers.

DSL (Digital Subscriber Line)

DSL service is a high-speed data service that works over POTS (Plain Old Telephone Service) copper telephone lines and is typically offered by telephone companies without costly installation of a higher-grade cable. DSL uses a different part of the frequency spectrum than analog voice signals, so it can work in conjunction with a standard analog telephone service, providing separate voice and data "channels" on the same line. ADSL (Asymmetric DSL) is the type of DSL that provides different bandwidths in the upstream and downstream directions, giving the user a much bigger "pipe" in the downstream direction. ADSL can support downstream bandwidths of up to 8 mbps and upstream bandwidths of 1.5 mbps. For comparison, a T-1 connection also provides 1.5 mbps. This scheme works well for the typical Internet user; upstream communication is usually small (link requests) compared to downstream communication (Web pages with graphics).

SDSL (Symmetric DSL) offers the same bandwidth capability in both directions. Besides higher bandwidth, some of the advantages of ADSL access from telephone companies are that there are no per-minute charges and the user gets an "always-on" connection for a monthly fee. Most modern computers can be easily equipped to connect to a DSL service. This is accomplished by connecting an ADSL modem to an Ethernet network card in the PC. The downside of DSL includes strict distance limitation that DSL circuits can operate within. As the connection's length increases, the signal quality decreases and the connection speed goes down. DSL services that provide greater that 1.5 mbps require shorter distances to the central office compared to a cable modem that can be located far away from the service provider.

The limit for ADSL service is 18,000 feet (5,460 meters), though for speed and quality of service reasons many ADSL providers place a lower limit on the distances for the service. At the extremes of the distance limits, ADSL customers may see speeds far below the promised maximums, while customers near the central office have the potential for very high speeds. Unlike cable modem technology, DSL provides a point-to-point connection to ISP. DSL proponents claim this technology is both more secure and less prone to local traffic fluctuations than its cable rival. By not sharing a LAN segment with other users, the systems are not as open to intrusion or susceptible to performance degradations related to local traffic.

Satellite Connection

Getting the Internet feed from a satellite is really not all that different from getting TV signals from one. In both cases data is being sent from the satellite to a user's equipment and then translated and decoded. One major limitation of satellite technology is that it can only send data from the satellite to a user's receiver—not the other way. To get around this problem, a separate ISP connection is needed to send data to the Internet, typically over an analog modem. This connection works in conjunction with the satellite feed. As information is requested via the modem line, data are sent back via the satellite. Since most Internet users need high bandwidth from the Web, downstream (typically Web pages and file downloads), and less bandwidth going to the Web, upstream (typically link requests), this scenario of sending upstream data over a standard modem line and downstream data over the high-bandwidth satellite feed has been effective. The newest satellite technology allows for two-way communications and higher upstream bandwidths. A satellite return channel can be added for traffic bound for the Internet. The upload speeds through this satellite return channel may peak at 128 kbps. Download speeds with this system are up to 400 kbps. Satellite technology has one strong advantage over cable modems and DSL: accessibility. For many it is today's only high-speed option. It can reach areas that are otherwise difficult to establish contact with. The infrastructure exists to provide 400 kbps downstream bandwidth to almost anyone with a 21" satellite dish. It is eight times faster than fastest analog telephone modems and three times faster than ISDN. However, it is not as fast as cable modems or DSL services, which both can provide more than megabits of bandwidth. Also, cable and DSL access methods are cheaper. Equipment required for satellite connection includes installation of a mini-dish satellite receiver and a satellite modem. Like cable modem systems, satellite provides a "shared bandwidth" pipe. This means that download performance may vary depending upon other users of the satellite transponder. Another potential problem can be associated with severe weather. In severe snowstorms and heavy rain, users may experience signal fade.

The general rule about the Internet connection is the faster, the better. The bandwidth and transfer rate determine how quickly pictures, sounds, animation and video clips will be downloaded. Since multimedia and interactivity make the Internet such an exciting tool for information sharing, the speed is the key. Dial-up access provides an easy and inexpensive way for users to connect to the Internet, however, it is a slow-speed technology and most users are no longer satisfied with dial-up or ISDN connections. Fortunately, the broadband access, we once dreamed of, is now possible with TV cable, DSL and satellite links.

To learn more about the cable modem information network and to test the speed of the Internet connection online visit www.cable-modem.net. To learn more about DSL connection visit www.dsl.net. To learn about satellite connection visit optistreams.com.

www.aawr.org Editor's Note: The original Mini-Tutorial on the Internet by Katarzyna J. Macura, M.D., Ph.D., was published in the AAWR Newsletter Focus. Dr. Macura updated her series for RSNA News.