Intel System Engineers Documents. DSL General Overview

Transcription

1 Intel System Engineers Documents DSL General Overview Alex Lattanzi SC LAR

2 Whatt IIs Brroadband? Broadband describes a number of different technologies that deliver digital data to homes and businesses at speeds faster than today s narrowband technology, analog modems. Today, analog modems comprise the vast majority of the world s 40 million connections for homes and small businesses. All of these modems use standard telephone lines as their medium to transmit information. The goal of broadband technologies is to overcome the current limitations of analog modems, namely: Bandwidth Limitation. Today s fastest analog modems (V.90 standard) can transmit and receive data at only 56 Kilobits per second (Kbps). Moreover, line quality and line distances typically reduce bandwidth to 33 Kbps or less. Dial-Up Connection. An analog modem establishes a connection to another modem using standard telephone dial tones. This process is lengthy and prone to busy signals if the receiving modem is busy. Dedicated Line. When an analog modem uses a telephone line, the line cannot simultaneously be used for voice communication. For homes or businesses that frequently use the modem, a dedicated phone line for the modem is usually necessary. Technology Overrview Broadband technologies can be classified as either one-way or two-way. One-way technologies are capable of sending information to the end user at very high speeds, but they rely on some other means (usually an analog modem and a phone line) to receive information from the end user. Although lacking a high-speed return path for information, one-way technologies are often wireless and offer much easier deployment over large areas. In contrast, two-way technologies can send and receive information at very high speeds over the same medium but require a wiring infrastructure that supports these technologies.

3 In this document, we focus on the two most prevalent two-way broadband technologies, cable and Digital Subscriber Line (DSL), and the most prevalent one-way broadband technology, satellite. For each of these technologies, the following sections will provide a quick overview, a discussion of the technology s advantages and disadvantages, background on standards development, and trends in the near future. DSL Overrview DSL is a modem technology that uses digital coding techniques to transform ordinary phone lines (also known as "twisted copper pairs") into high-speed data transfer lines. While voice data is transmitted below frequencies of 4 Kilohertz (KHz), Internet data can be transmitted at 25 to 138 KHz upstream (from client to Internet) and 139 to 1100 KHz downstream (from Internet to client), on the same phone line. DSL enables simultaneous high-speed Internet access and analog voice service. As a result, DSL enables telephone companies to use the world's nearly 750 million existing phone lines to deliver affordable, high-speed access to the Internet, corporate networks, and online services. Voice Upstream Downstream KHz

4 DSL IIntterroperrabilitty:: DMT orr CAP There are two technologies competing to be the modulation standard for ADSL, carrierless mplitude and phase modulation (CAP) and discrete multitone modulation (DMT). Both CAP and DMT are coding technologies that allow ADSL to utilize more of copper's available frequency range. CAP and DMT are not interoperable. In 1994, the ANSI working group adopted DMT as the ADSL standard. However, many DSL manufactures and providers continue to offer CAP services. Although DMT is cited as superior to CAP by ANSI, DMT is a relatively new technology, while CAP chipsets have been used for almost twenty years. Consequently, it's believed that CAP technology actually has a larger installed ADSL base than DMT. Until DMT catches up in deployments, there will continue to be uncertainty in modulation standard. Despite the existance of two modulations standards, great steps have been made in DSL interoperability, with many vendors domonstrating that their CPE will work with the Central Office equipment (DSLAMs) of another vendor, or vice versa. Increased interoperability should help encourage retail sales of DSL modems-customers will not haveto be concerned that DSL modem they purchase in a store will not function with the DSL equipment installed at the phone company's Central Office. A number of vendors have demonstrated the interoperability of their ADSL equipments. Types off DSL Technologies A number of DSL technologies have been developed to meet a full range of customer needs. The most common is Asymmetric Digital Subscriber Line (ADSL), which delivers greater downstream bit-rate transmission than upstream. Conversely, Symmetric Digital Subscriber Line (SDSL) delivers symmetric upstream and downstream bit-rate transmission between the subscriber s home and the CO. SDSL is deployed when customers, usually Small-Office/Home-Office (SOHO) users, telecommuters, or small businesses, need larger upstream bandwidth to upload files or handle videoconferencing. Other DSL technologies include High-bit-rate Digital Subscriber Line (HDSL), which uses more than one twisted pair to deliver T1 speeds, and ISDN Digital Subscriber Line (IDSL), which delivers data at 128 Kbps into an IDSL "modem bank" connected to a router. IDSL is generally deployed when ADSL is not viable because of the length of the customer loop. The highest

5 performance DSL technology available is Very high data rate Digital Subscriber Line (VDSL), delivering bit-rate transmission from 12.9 to 52.8 Mbps with corresponding maximum reach ranging from 4,500 feet to 1,000 feet of 24-gauge twisted pair. VDSL may be deployed when extremely high-bandwidth applications are necessary and may eventually be used for Internet video services. The following is a summary of these technologies. Upstream Bit Rate* Downstream Bit Rate* (Kbps, Mbps) (Kbps, Mbps) IDSL 56, 64, 128, 144 Kbps 56, 64, 128, 144 Kbps ADSL (Full 1.5 Mbps 8 Mbps Up to 2 Mbps Rate) ADSL Up to 1.5 Mbps Up to 512 Kbps (G.Lite) SDSL 160 Kbps Mbps 160 Kbps Mbps HDSL 2 Mbps 2 Mbps VDSL 1.6, 2.3, 19.2 Mbps 12.96, 25.82, Mbps Distance is limited. The shorter the distance, the greater the speed DSL Advanttages Point-to-point dedicated connection: the DSL connection is point-to-point between the consumer and the telco and thus provides guaranteed bandwidth and more inherent security than the cable architecture. Always-on connection: instead of having to dial in to their ISP to establish a connection (such as with standard analog modems), consumers have a standby, ready-to-use, high-speed connection, much like a telephone line. Low-cost modems. Dual-mode modems. High-speed access: access from 256 Kbps to 8 Mbps downstream and from 90 Kbps to 512 Kbps upstream. Over 100 times faster than 56 Kbps modems. 70 times faster than 128 Kbps ISDN. Simultaneous voice and data service: consumers can have a high-speed data connection while using voice/fax at the same time.

6 DSL Disadvanttages Cost: DSL service is currently more expensive than cable modem service (approximately $10/month more). Slower rollout of services. Availability beyond 18,000 feet from the CO is problematic. Installations can be difficult. Futturre Trrends Some of the anticipated DSL product developments include modems based on the G.Lite standards and the integration of various capabilities. G.Lite was made an ITU standard in October 1998 and is projected to be available in the second half of As more manufacturers adopt the G.Lite standard, it is expected that DSL modems will become more cost effective. Another expected improvement to the DSL modem is the integration of voice-over- DSL and home networking capabilities. Other anticipated improvements include USB-enabled DSL modems, making it easier for users to install a DSL modem, and the development of soft modems, which leverage the power of the PC s processor to execute functions. Changes in the DSL industry are also anticipated. DSL providers and content providers are forming partnerships to offer customers a full range of compelling broadband service. As performance levels increase and new equipment is introduced, we expect that the availability of DSL services will rapidly expand. Noise//EMII With all broadband technologies, the modem sends and receives data from specific frequency bands within the transmission medium (cable, copper, or airwaves). This requires components and circuitry that can tune into that frequency band. Some of these components can be fairly noisy meaning they generate EMI. In an external modem, the manufacturer can design the

7 modem housing such that EMI leakage is minimized. However, with an integrated modem, the PC OEM must ensure that the modem, in concert with the rest of the PC s internal components, does not exceed EMI regulations. Capabilitti ies An ADSL circuit connects An ADSL modem on each end of a twisted-pair telephone line, creating three information channels: a high speed downstream channel ranges from 1.5 to 8 Mbps, a medium speed duplex channel ranges from 16 to 640 Kbps and a POTS channel. The POTS channel is split off from the digital modem by filters, thus guaranteeing uninterrupted POTS (the old voice calls), even if ADSL fails. ADSL modems can be purchased with various speed range and capabilities. The minimum configuration provides 1.5 or 2 Mbps downstream and 16 Kbps duplex channel; other provides rates of 6.1 Mbps and 64 Kbps duplex. Products with downstream rates up to 9 Mbps and duplex rates up to 640 Kbps are available too. Downstream data rates depend on number of factors, including the length of the copper line, it s wire gauge, presence of bridged tapes and cross-coupled interference. Line attenuation increases with line length and frequency, and decreases as wire diameter increases. Ignoring bridged taps, ADSL will perform as follows: Data Rate Wire Gauge Distance Wire Size Distance 1.5 or 2 Mbps 24 AWG 18,000 ft 0.5 mm 5.5 Km 1.5 or 2 Mbps 26 AWG 15,000 ft 0.4 mm 4.6 Km 6.1 Mbps 24 AWG 12,000 ft 0.5 mm 3.7 Km 6.1 Mbps 26 AWG 9,000 ft 0.4 mm 2.7 Km Premises beyond these distances can be reached with fiber based digital loop carrier systems.

8 Worrl ldwide DSL Lines Deployed by Geogrraphy Worldwide DSL Lines Deployed (Cumulative in Millions) IDC 12/99 14,000 12,000 10,000 8,000 6,000 4,000 2, NAMO ,870 4,110 7,780 12,570 EMEA ,056 3,485 9,344 APAC ,060 2,125 3,750 ROW ,670