Lecture 2 the INTERNET & the WORLD WIDE WEB CONNECTING
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Lecture 2 THE INTERNET & the WORLD WIDE WEB CONNECTING TO THE INTERNET: Narrowband, Broadband, & Access Providers In general terms, bandwidth, or channel capacity, is an expression of how much data—text, voice, video, and so on—can be sent through a communications channel in a given amount of time. The type of data transmission that allows only one signal at a time is called baseband transmission. When several signals can be transmitted at once, it’s called broadband transmission. Broadband —very high speed—connections include various kinds of high-speed wired connections (such as coaxial and fiber-optic) as well as DSL, cable, and satellite and other wireless connections. Today more than half (55%) of all adult Americans have broadband internet connections at home. THE PHYSICAL CONNECTION: WIRED OR WIRELESS? What are your choices of a physical connection —the wired or wireless means of connecting to the internet? A lot depends on where you live. As you might expect, urban and many suburban areas offer more broadband connections than rural areas do. Among the principal means of connection are (1) telephone (dial-up) modem; (2) several high-speed phone lines—ISDN, DSL, and T1; (3) cable modem; and (4) wireless—satellite and other through-the-air links. DATA TRANSMISSION SPEEDS Data is transmitted in characters or collections of bits. A bit is the smallest unit of information used by computers. Today’s data transmission speeds are measured in bits, kilobits, megabits, and gigabits per second: • bps: A computer with an older modem might have a speed of 28,800 bps, which is considered the minimum speed for visiting websites with graphics. The bps stands for bits per second. (Eight bits equals one character, such as A, 3, or #.) • Kbps: This is the most frequently used measure; kilobits per second, or Kbps, are 1 thousand bits per second. The speed of a modem that is 28,800 bps might be expressed as 28.8 Kbps. • Mbps: Faster means of connection are measured in megabits per second, or Mbps — 1 million bits per second. • Gbps: At the extreme are gigabits per second, Gbps —1 billion bits per second. UPLOADING & DOWNLOADING Why it is important to know these terms? Because the number of bits affects how fast you can upload and download information from a remote computer. Download is the transmission of data from a remote computer to a local computer, as from a website to your own PC. Upload is the transmission of data from a local computer to a remote computer, as from your PC to a website you are constructing. Narrowband (Dial-Up Modem): Low Speed but Inexpensive & Widely Available The telephone line that you use for voice calls is still the cheapest means of online connection and is available everywhere. Many home users still use what are called narrowband, or low-bandwidth, connections. This mainly consists of dial-up connections —use of telephone modems to connect computers to the internet. CONNECTING THE MODEM A modem is a device that sends and receives data over telephone lines to and from computers. These days, the modem is generally installed inside your computer, but there are also external modems. The modem is attached to the telephone wall outlet. Most standard modems today have a maximum speed of 56 Kbps. That doesn’t mean that you’ll be sending and receiving data at that rate. The modem in your computer must negotiate with the modems used by your internet access provider, the regional, national, or wireless organization or business that connects you to the internet. Your provider may have modems operating at slower speeds, such as 28.8 Kbps. In addition, lower-quality phone lines or heavy traffic during peak hours—such as 5 p.m. to 11 p.m. in residential areas—can slow down your rate of transmission. MODEMS & CALL WAITING One disadvantage of a telephone modem is that while you’re online you can’t use that phone line to make voice calls unless you’ve installed special equipment. In addition, people who try to call you while you’re using the modem will get a busy signal. You generally won’t need to pay long-distance phone rates, since most access providers offer local access numbers. High-Speed Phone Lines: More Expensive but Available in Most Cities Waiting while your computer’s modem takes 25 minutes to transmit a 1- minute low- quality video from a website may have you pummeling the desk in frustration. To get some relief, you could enhance your POTS —“Plain Old Telephone System”— connection with a high-speed adaptation. Among the choices are ISDN, DSL, and T1, available in most major cities, though not in rural and many suburban areas. ISDN LINE ISDN (Integrated Services Digital Network) consists of hardware and software that allow voice, video, and data to be communicated over traditional copper-wire telephone lines. Capable of transmitting 64 to 128 Kbps, ISDN is able to send digital signals over POTS lines. If you were trying to download an approximately 6-minute- long music video from the World Wide Web, it would take you about 4 hours and 45 minutes with a 28.8-Kbps modem. A 128-Kbps ISDN connection would reduce this to an hour. ISDN is a viable solution for single users of small business networks when other high- speed options are not available. ISDN is not as fast as DSL, cable, or T1 and is expensive. DSL LINE DSL (digital subscriber line) uses regular phone lines, a DSL modem, and special technology to transmit data in megabits per second. The data bit rate of consumer DSL services typically ranges from 256 kbit/s to 40 Mbit/s in the direction to the customer (downstream), depending on DSL technology, line conditions, and service-level implementation. In symmetric digital subscriber line (SDSL) services, the downstream and upstream data rates are equal. In ADSL (Asymmetric DSL), the data throughput in the upstream direction, (the direction to the service provider) is lower, hence the designation of asymmetric service. Most homes and small business users are connected to an ADSL line. ADSL divides up the available frequencies in a line on the assumption that most Internet users look at, or download, much more information than they send, or upload. Under this assumption, if the connection speed from the Internet to the user is three to four times faster than the connection from the user back to the Internet, then the user will see the most benefit most of the time. A big advantage of DSL is that it is always on (so you don’t have to make a dial-up connection) and, unlike cable, its transmission rate is relatively consistent. Also, you can talk on the phone and send data at the same time. There is one big drawback to DSL: You have to live within 3.3 miles of a phone company central switching office, because the access speed and reliability degrade with distance. However, DSL is becoming more popular, and phone companies are building thousands of remote switching facilities to enhance service throughout their regions. Another drawback is that you have to choose from a list of internet service providers that are under contract to the phone company you use, although other DSL providers exist. A new competitor to DSL, is a service (fiber optic) called FiOS, which is faster than DSL. However, FiOS is not available in all areas. Still, the real problem for DSL—or any other kind of broadband—is that it is not available to most rural residents, who thus lack the sort of high-speed services and opportunities, such as distance learning and web- based commerce (E-commerce), that urban dwellers take for granted. DSL Equipment ADSL uses two pieces of equipment, one on the customer end and one at the ISP (Internet service provider), Telephone Company or other provider of DSL services. At the customer's location there is a DSL transceiver, which may also provide other services. The DSL service provider has a DSL Access Multiplexer (DSLAM) to receive customer connections. 1. The Transceiver Most residential customers call their DSL transceiver a "DSL modem." It's the point where data from the user's computer or network is connected to the DSL line. The transceiver can connect to a customer's equipment in several ways, though most residential installation uses USB or 10 base-T Ethernet connections. 2. The DSLAM The DSLAM (DSL Access Multiplexer) at the access provider is the equipment that really allows DSL to happen. A DSLAM takes connections from many customers and aggregates them onto a single, high-capacity connection to the Internet. DSLAMs are generally flexible and able to support multiple types of DSL in a single central office, and different varieties of protocol and modulation in the same type of DSL. In addition, the DSLAM may provide additional functions including routing or dynamic IP address assignment for the customers (DHCP: Dynamic Host Configuration Protocol). The DSLAM provides one of the main differences between user service through ADSL and through cable modems. Because cable-modem users generally share a network loop that runs through a neighborhood, adding users means lowering performance in many instances. ADSL provides a dedicated connection from each user back to the DSLAM, meaning that users won't see a performance decrease as new users are added - - until the total number of users begins to saturate the single, high-speed connection to the Internet. At that point, an upgrade by the service provider can provide additional performance for all the users connected to the DSLAM.