No one realizes how long a second can be until they become immersed in e-life. Although telephone companies and Internet service providers are installing most high-speed Internet access, electrical contractors wishing to tackle the vast number of opportunities must first learn the technology behind these high-speed access options.
Fat pipe Internet access
Fattening of the pipes brings faster and more efficient ways to connect to the Internet. New technologies are becoming increasingly affordable.
ISDN as an option
According to Dan Keelan, director of business development for Earth Station Net Access, Conshohocken, Pa., “(Integrated Services Digital Network) ISDN is really obsolete in the high-speed Internet access arena because users are charged by the minute,” he said. However, he did admit that ISDN is still a viable option for the many consumers without DSL or local cable access.
In converting to an ISDN-compatible configuration, the telephone company’s central office is connected to the user through a two-wire loop. This connection supports either voice or data in a single transmission channel. With ISDN, the same two-wire loop can support multiple channels, logically divided to support voice and data.
When a regular telephone line is connected to ISDN, the ISDN-capable digital switch at the telephone company’s central office generates two types of channels.
n The “B” or “bearer” channel transmits at 64kbps and is designed to carry the full range of voice, circuit-switched data, and packet-switched data transmissions. This is “the information the user wants to send over the network.” Two “B” channels are established per ISDN line.
n The “D” or “delta” channel controls the flow of information from the network, providing call-signaling and setup information and other overhead functions, and transmitting lower-speed packs.
According to Claus E. Schmidt, senior partner of Alpine Data Systems, a Newark, Del.-based IT consulting company, there are two basic types of ISDN service: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). BRI consists of two 64kbps “B” channels and one 16 Kbps “D” channel for a total of 144kbps and is intended to meet most individual users’ needs. PRI is designed for users with greater capacity requirements. Typically, the channel structure is 24 “B” channels plus one 64kbps “D” channel for a total of 1,536kbps.
DSL: quickly becoming the high-speed access of choice
Digital Subscriber Line (DSL) is a way to access the Internet over regular telephone lines. DSL is an “always on” connection and at 1.5Mbps, it is up to 30 times faster than a conventional modem.
In addition, Schmidt said, “DSL can be a tremendous advantage if a consumer is considering paying for a second phone line to handle Internet access.”
According to Schmidt, two flavors of DSL are worth considering. The more popular is Asymmetric DSL (ASDL), which is quicker at getting information from the Internet than it is at sending it. “If a user is primarily interested in surfing the Web, this is the way to go,” said Schmidt.
The other flavor is Symmetric DSL or SDSL.
SDSL downloads information equally fast as it uploads it. “If a user is operating a Web site or some other type of business that requires uploading massive amounts of data, then they should consider SDSL,” said Schmidt.
DSL can be ordered through local telephone companies or through an Independent Service Provider (ISP) that has access to the local telephone lines. Either the user or a qualified technician must install a DSL modem and splitters to every telephone connected to the DSL line. The waiting period can last two weeks to three months, and it’s costly. Many companies offer a “do-it-yourself” option or reduced rates if the user signs a long-term contract.
The ADSL technology uses bandwidth from a part of the telephone line that isn’t used during voice communications. But ADSL technology is distance-sensitive—so users must be within approximately three miles of the main telephone switching station. In addition, not all DSL providers allow consumers to connect more than one computer to a DSL line.
This can be resolved if the user knows someone who can network the computers with only one on at a time.
DSL uses existing copper telephone lines and works well once it is up, but the setups can be maddening.
Getting connected with the T-carrier system
The T-carrier system’s original transmission rate (1.544Mbps) in the T-1 line is still used in ISP connections. The T-3 line, providing 44.736Mbps, is also commonly used by ISPs. Another common service is a fractional T-1, which is the rental of some portion of the 24 channels in a T-1 line, with the other channels going unused.
The T-carrier system is entirely digital, using pulse-code modulation and Time-Division Multiplexing. The system uses four wires and provides duplex capability (two wires for receiving and two for sending at the same time). The T-1 digital stream consists of 24 64kbps channels that are multiplexing. The four wires were originally a pair of twisted pair copper wires, but can now also include coaxial cable, optical fiber, digital microwave, and other media.
In the T-1 system, voice signals are sampled 8,000 times per second and each sample is digitized into an eight-bit word. With 24 channels being digitized at once, a 192-bit frame (24 channels each with an eight-bit word) is thus being transmitted 8,000 times per second.
Each frame is separated from the next by a single bit, making a 193-bit block. The 192-bit frame multiplied by 8,000, and the additional 8,000 framing bits make up the T-1’s 1.544 Mbps data rate. The signaling bits are the least significant bits per frame.
Unlike DSL, the distance from the customer to the telephone switch does not limit a T1 circuit, so all customers are eligible for 1.5Mbps at any distance.
Beam me up, Scotty
A satellite-based system basically takes the Internet, tosses it 22,200 miles into space to a satellite, bounces it back to a 21-inch dish on a consumer’s house or place of business, and shoots it straight to their PC. It does this in 400kbps—or 400 times faster than the average 28.8K modem. Satellite access requires an analog modem and telephone line to send data, as the dish can only receive data.
I want my cable TV (and Internet connection)
A cable modem enables a user to hook up his or her PC to a local cable TV line and receive data at about 1.5Mbps. A cable modem can be added to or integrated with a set-top box that provides a TV set with channels for Internet access.
Cable modems are usually furnished as part of the cable access service and subscribers usually complete installation.
A cable modem has two connections: one to the cable wall outlet and the other to a PC or to a set-top box on a TV set.
Although a cable modem does modulation between analog and digital signals, it is a much more complex device than a telephone modem. It can be an external device or it can be integrated within a computer or set-top box. Typically, the cable modem attaches to a standard 10Base-T Ethernet card in the user’s computer.
All of the cable modems attached to a cable TV company coaxial cable line communicate with a Cable Modem Termination System (CMTS) at the local cable TV company office. Cable modems can receive from and send signals only to the CMTS, but not to other cable modems on the line. When the upstream signals are returned by telephone rather than cable, the cable modem is known as a telco-return cable modem.
The actual bandwidth for Internet service over a cable TV line is up to 27Mbps on the download path to the subscriber with about 2.5Mbps of bandwidth for interactive responses. However, since the local provider may not be connected to the Internet on a line faster than a T-carrier system at 1.5Mbps, a more likely data rate will be close to 1.5Mbps.
With cable access, users connect to a common node that feeds into the central system. According to Keelan, cable systems are more vulnerable to breeches, and shared bandwidth can greatly reduce speed.
Obstacles to high-speed access
A major obstacle to high-speed Internet access has been the high cost of high-bandwidth-dedicated links. In addition, even if you have a Porsche bandwidth, you might as well get the Volkswagen out of the garage if you are trying to access a sluggish site, because the Porsche won’t get you there any faster.
WOOD is a freelance writer based in Philadelphia. She can be reached at (215) 563-3071 or firstname.lastname@example.org.