Published In November 2001
Today, the Internet is an information superhighway with bottlenecks at every exit. These congested exits call for the deployment of broadband access to homes and businesses. Broadband technology allows you to watch television (video), browse the Internet via a high-speed connection (data), and use the telephone (voice) simultaneously. Broadband access means support for voice, data, and video services over a single cable with no dial-up required. Broadband access is primarily deployed via hybrid fiber coax (HFC) or digital subscriber line (DSL) installations. HFC is a residential technology and is brought into the home as a 75-ohm coaxial cable carrying cable television, telephone, and Internet access services. Internet access via HFC is accomplished using cable modems. DSL access provides data, voice, and video services over the familiar telephone installations. Within local premises, Ethernet is the data network technology of choice used to interconnect workstations, servers, and other network resources on a local area network (LAN). The demand for broadband access is definitely ahead of its deployment. Most of us still connect to the Internet through dial-up lines. We yearn for more bandwidth and the “always-on” connectivity of broadband access. These services are now being deployed, but will take some time to reach a significant portion of the population. Meanwhile, installers would do well to become educated on the new broadband technologies such as DSL and HFC and the installation and testing issues associated with their deployment. DSL DSL is the solution for providing broadband services over existing telephone connections. [FM1] DSL data flows between the central office (CO) and user premises. Data flows “upstream” from the user premises to the CO. Data flows “downstream” from the CO to the user. Different DSL technologies offer a range of data rates and modes of operation. Symmetric technologies integrated services data network (ISDN), high-data-rate digital subscriber line (HDSL), and single-line digital subscriber line (SHDSL) operate at the same data rate in the upstream and down stream directions. The primary application used by businesses for symmetric DSL is T1. Asymmetric technologies— asymmetrical digital subscriber line (ADSL)—and very high data rate digital subscriber line (VDSL) operate at different upstream and downstream data rates and are used primarily by residential customers for Internet access. Asymmetric transmission maximizes the bandwidth to the user to speed up common download and streaming audio and video operations. DSL installation and testing Major DSL issues include: • Load coils. • Excessive bridged tap length (unterminated stubs of twisted pair). • Too much attenuation. • Too much noise or external interference (e.g., AM radio stations). • Crosstalk from other services in the same cable bundle. • Interference between plain old telephone service (POTS) and data signals. • Data rate performance limits. To qualify the copper plant, a Transmission Impairment Measurement Set (TIMS) is used to measure line loss, noise, balance, return loss, and impedance. A Time Domain Reflectometer (TDR) is used to measure cable length and to locate faults, such as bridged taps. A test set that contains a built-in ATU-R “golden modem” transmits and receives data over the circuit under test and determines the link’s transmission capacity. To verify the end-to-end throughput performance of the system, an Internet Protocol (IP) test set connected to the Ethernet interface should be used. Hybrid fiber coax (HFC) HFC is the broadband distribution network used by cable television operators. Signals are carried from the head end to the customer premise over a combination of fiber optic and coaxial cable. Cable TV operators use the wide bandwidth of 5 to 750 MHz to carry analog and digital TV, high-speed Internet access via cable modems, and telephony. The frequency range from 50 to 750 MHz is used for analog and digital TV and is organized into 6 MHz channels. Traditional analog TV requires the entire 6 MHz channel, while the emerging digital TV can support four to six programs per channel. The frequency range from 5 to 50 MHz, called the reverse or return path, is used for the transmission of signals from user to the head end. Reverse-path applications include upstream transmission from cable modems and commands from interactive TV set top boxes. Bi-directional telephony service also resides in this band. HFC installation and test The cable plant must be aligned so that the signal levels are uniform across the band. Cable sweep is performed to measure attenuation and to find faults in the passive components, splitters, and taps. The signal level of each analog channel and the power level of each digital channel must also be measured. Ambient, ingress noise and spurious signals must be measured to ensure the performance of HFC networks. Ingress is greater in the reverse path because noise accumulates from multiple user taps and because more sources of interference are present in the frequency band of 5 to 50 MHz. Analyzers can perform these loss, signal level, power, and noise measurements. Losses in the optical portion of the network are measured using an optical power meter. To locate major cable faults, such as breaks and water infiltration, optical and metallic TDRs are used. A set of analog TV measurements known as “proof-of-performance” ensures the quality of TV signals. These measurements focus on signal distortion at the levels of AM and FM modulation. To qualify digital signals such as data and digital TV, cable operators need to characterize the digital modulation. The type of digital modulation used in HFC networks is Quadrature Amplitude Modulation (QAM). The important measurements of a QAM signal include Modulation Error Ratio (MER) and Bit Error Ratio (BER). MER is a type of signal-to-noise ratio. BER is the measure of error rate in the data stream. The QAM signal may be viewed graphically in a “constellation” display. End-to-end protocol testing Broadband data access performance is of great importance to users with critical Internet applications such as e-commerce or financial services. Typically, throughput and responsiveness of broadband access is measured between Ethernet networks end-to-end. This can be done using a hand-held Ethernet analyzer optimized to verify the throughput and responsiveness of common network devices such as servers. DSL and HFC are expected to provide VDV to homes and businesses over a single broadband connection—DSL over telephone wires and HFC over cable TV coax. Installation and testing issues range from cable-quality to protocol-level performance. The right test tools can guide even inexperienced installers through testing and troubleshooting at each phase of the broadband access qualification. MLINARSKY is the general manager of the WireScope Operation at Agilent Technologies. She can be reached at email@example.com. MILLER is a product marketing manager at the Service Test Division of Agilent Technologies. He can be reached at firstname.lastname@example.org.