Friday, February 4, 2011


Before the late 1990s, people connected remotely to their offices or the Internet using dial up connections. An always-on remote network connection was not possible for a reasonable price. To connect to the corporate network, the user ran a program that dialed a phone number. Unless the user had a second phone line, being online prohibited incoming or outgoing phone calls. The user entered a user ID and password to gain access to the system. The fastest speed available over phone lines was 56 kbps, which was fine until the web became popular in the 1990s. Downloading large pictures, documents, applications, and audio files took what seemed like forever. Then, along came broadband. Broadband networking offered a reasonable high-speed alternative to traditional dialup networking. Using existing service connections to houses (such as phone wiring, cable TV coaxial cable, or even satellite), service providers offered Internet services at many times the speed of dialup. Downloading large files became palatable with broadband. Broadband technologies allow service providers to offer always-on connectivity similar to what people use in a corporate network. Computers on the broadband network always have access to the network; there is no intermediate dialup step. Sit down, load the browser, and off you go. High-speed Internet access to homes offers new levels of productivity and entertainment not possible before the commercialization of the Internet and the web. Aside from apparent uses such as online shopping and video streaming, corporations can accommodate road warriors and work-from-home folks in a way not previously possible. Using encryption technologies, an employee with a laptop computer can securely access her corporate network from any Internet access point in the world. Additionally, employees can attach IP phones, allowing them to work on their computers and make calls from their office-phone extensions as if they were sitting at their desks.
Broadband Technology Evolution

Integrated Services Digital Network (ISDN) was the first commercially viable broadband option available. Using existing phone lines, home users commonly subscribed to a Basic Rate Interface (BRI), which had a throughput maximum of 128 kbps. ISDN had some significant adoption in Europe, but in the U.S., ISDN was eclipsed by more cost-effective broadband technologies before it had a chance to become commonplace. Cable modem and digital subscriber line (DSL) services became the premier broadband technologies. Although other broadband technologies existed, the primary determination of a technology’s viability was access to “last mile” wiring to houses. Anything that required new wiring probably wouldn’t make it. Other technologies that take advantage of other media exist, such as satellite television dishes, but they did not become widely adopted. For those requiring even higher throughput, some providers now offer fiber links to homes as a premier service. This is already popular throughout major cities in Asia and is being installed in several cities in the U.S. as well.
Cable Modem

Cable modems provide high-speed data communication using existing cable television coaxial cabling. Current implementations of cable-modem technologies offer speeds as fast as Ethernet (greater than 10 Mbps). This means that a file that takes 2 minutes to transfer over ISDN takes 2 seconds over a cable modem. Cable modem can provide higher speeds than traditional leased lines, with lower cost and easier installation. Because a cable-modem connection is permanently established, it cannot dial multiple locations directly. As a result, cable-modem access must be to the Internet. This restriction means that employees can connect to their company’s network only if the company provides access through the Internet. Usually, this is done through a secure VPN connection.

Like cable modems, DSL provides high-speed Internet access for reasonable cost using existing cabling to houses and businesses. DSL carves off a portion of the telephone line to use for data transmission without interfering with existing phone service. Because of the multiple flavors of DSL services, DSL is generically referred to as xDSL. The two popular forms of xDSL service currently available are Asymmetric DSL (ADSL) and Symmetric DSL (SDSL). ADSL provides faster download speeds because traffic toward the user is given more bandwidth than traffic from the user. SDSL assigns equal bandwidth in both directions. ADSL is most often used for residential service, and SDSL is most often used in commercial settings, because of their different usage models.
Which One Is Better?

Both DSL and cable modems provide high-speed Internet access at a relatively low cost. Both provide always-on connectivity. Both have technical advantages and disadvantages. Either technology makes a good to-the-home or small office solution for Internet connectivity. Because both technologies are always on, a firewall must protect the local network from Internet-based attacks. Some practical issues affect how widespread the technologies become. Virtually all businesses and homes have telephone lines, which means that DSL is possible, but fewer homes and businesses have cable TV connections. In general, neither is “better,” and both types of service offer very high-speed connectivity for relatively low cost.
Digital Subscriber Line (DSL)

DSL uses the existing phone wires connected to virtually every home in most countries. The twisted-pair wires that provide phone service are ideal, because the available frequency ranges on the wires far exceed those required to carry a voice conversation. Human speech occupies frequencies of roughly 4000 hertz (4 kHz) or less. The copper wires that provide phone service can carry in the range of 1 to 2 million hertz (1 to 2 MHz). DSL provides more downstream data (from the Internet to you) than upstream data (from you to the Internet) based on user profiles, but this can be changed for businesses or those running web servers.

DSL Equipment

DSL requires some specialized equipment to ensure that the voice and data are kept separate and are routed to the right place. • Low-pass filters (LPF) are placed on all phone jacks not used by a computer to prevent interference from high-frequency data signals. DSL modems are the interface from the phone line to the computer. • DSL access multiplexers (DSLAM) aggregate hundreds of signals from homes and are the access point to the Internet.
Limitations and Advantages

DSL signals are distance-sensitive, which means that the available throughput decreases the farther your house is from the service provider. The maximum distance is about 18,000 feet. DSL signals cannot be amplified, nor can they be converted from one medium to another between the DSL modem and the DSLAM. (For example, opticalfiber extensions are not possible.) Typically the DSL company performs a line test to ensure that the service can be supported at a particular residence. The good news for DSL is that throughput is unaffected by the number of users so long as the phone company continues to add DSLAMs to support
new users.

Cable uses the same basic principle as DSL in that the bandwidth needed to accomplish the primary function is only a fraction of the available bandwidth on the wire or, in this case, cable. Cable is a slightly different concept in how it divides the available frequencies. The cable spectrum is already divided up into several hundred 6-MHz blocks to account for the various cable channels. Your cable-ready TV simply tunes its receiver to the frequency that corresponds
to the channel you have chosen. To add Internet capabilities, each user is assigned one or more blocks for downstream data (each 6-MHz block is good for about 30 Mbps of data). For the upstream piece, the lower end of the spectrum is divided into 2-MHz blocks, because most people download more information than they upload. Each subscriber is assigned one or more 2-MHz blocks.

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