DSL vs. Cable : A Synopsis of Competing Broadband Technologies

Olufemi Anthony
EMTM 615 - Introduction to Networking

1. Introduction

Broadband or high-speed Internet access is provided by a series of technologies that give users the ability to send and receive data at volumes and speeds far greater than current Internet access over traditional telephone lines. In addition to offering speed, broadband access provides a continuous, "always on" connection (no need to dial-up) and a "two-way" capability, that is, the ability to both receive (download) and transmit (upload) data at high speeds. Broadband access, along with the content and services it might enable, has the potential to transform the Internet: both what it offers and how it is used. It is likely that many of the future applications that will best exploit the technological capabilities of broadband have yet to be developed.

There are multiple transmission media or technologies that can be used to provide broadband access. These include cable, an enhanced telephone service called digital subscriber line (DSL), satellite, fixed wireless, and others. While many (though not all) offices and businesses now have Internet broadband access, a remaining challenge is providing broadband over "the last mile" to consumers in their homes. Currently, a number of competing telecommunications companies are developing, deploying, and marketing specific technologies and services that provide residential broadband access.

2. The Technology Explained

Both DSL and cable Internet access are more similar to a local area network (LAN) connection than a dialup connection.

2.1 Digital Subscriber Line (DSL)

2.1.2 What is it?

DSL is a technology for providing high speed net access to businesses and residences over ordinary copper telephone wires. There are multiple flavors of DSL, such as ADSL, SDSL, HDSL, VDSL, IDSL¹. For ADSL (the A is for asymmetric), in which upload speeds are slower than downloads; SDSL (the S is for symmetric), where upstream and downstream speeds are the same; and IDSL (the I stands for ISDN), which overlays a DSL connection on ISDN. DSL can theoretically deliver data at speeds up to 6 Mbits per second. Typical connections, however provide speeds from 512 Kbps to 1.5Mbps downstream and 128 Kbps upstream. The user is constantly connected over DSL and a DSL line carries both voice and data, enabling that user to remain connected while speaking on the phone.
DSL users may connect via a DSL modem, or directly to their phone switch(router). If the provider requires a modem to connect, the user will need to dial-up to the DSL server in order to use the service.

2.1.3 How does it work?

Data comes in from the Internet to the DSL ISP, such as Verizon DSL. The ISP is connected to a high-speed network thorugh which the data is carried. The data for a particular DSL customer is sent to their nearest phone switch. From the phone switch/router, it travels through the customer's phone line.

Current DSL technology enhances existing voice telephone technology to provide high speed net access. The most popular implementation of DSL currently is ADSL (Asynchronous DSL).
An ADSL circuit works by connecting an ADSL modem on each end of a twisted-pair telephone line, creating three information channels -- a high speed downstream channel (to your home), a medium speed upstream channel (from your home), and a POTS (Plain Old Telephone Service) voice channel. (Note that analog transmission uses only a narrow band of the available frequencies of the copper telephone wire). Thus the modem multiplexes the telephone line bandwidth. The up and downstream data channels are each 100 kHz - 1 MHz while the POTS is from 0-4 kHz.

ADSL modems multiplex the available bandwidth of a telephone line in one of two ways -- Frequency Division Multiplexing (FDM) or Echo Cancellation. FDM is more common because it is less complicated. FDM assigns one band for upstream data and another band for downstream data. The downstream path is then divided by time division multiplexing (TDM) into one or more high speed channels and one or more low speed channels. The upstream path is also multiplexed into corresponding low speed channels. In Echo cancellation, the up and downstream signals are sent on the wire at the SAME frequencies, i.e. they overlap and it separates the two by means of local echo cancellation, a technique well know in V.32 and V.34 modems. With either technique, ADSL splits off a 4 kHz region for POTS at the DC end of the band.

2.2 Cable Modem

2.1.2 What is it?

Cable technology provides a high-bandwidth, always-on connection to the Internet over the same line as cable TV service. Cable access requires installation of a cable modem and an ethernet card in the user's computer. A cable modem is a device that enables you to hook up your PC to a local cable TV line and receive data. The connection can provide speeds ranging from 500 kbps to 2Mbps, even though the theroretical limit is 27 Mbps. Part of the reason is that the local provider may be connected to the Internet on a T-1 line, whose connection speed is no greater than 1.5Mbps and the cable line is shared with other local customers in the vicinity.

2.1.3 How does it work?

The cable company acts as its own ISP. It establishes a connection with the Internet through a router, which receives data over a 1.5-mbps T1 line or 45-mbps T3 line leased from the local telephone company. The router links to the head-end controller (the part of the cable system that transmits TV broadcasts and satellite signals) via fast Ethernet, at rates of up to 100 mbps. The IP packets are converted into signals that can be transmitted, like television broadcasts, over coaxial cable. A cable splitter in each subscriber's home sends one cable to the TV set and another cable to the computer's cable modem. The cable modem then demodulates these downstream signals back to IP packets which are sent via 10baseT through an RJ-45 cable connected to the computer's ethernet card.

Head-End Controller
A cable system's head end receives TV signals via satellite and local broadcast and converts them to signals that can be sent over coaxial cable to subscribers.
To deliver digital data, the head-end controller is needed to modulate IP packets--encode them as a digital signal--and broadcasts the signal down the cable on an unused channel.
The controller interprets the data it receives from individual customers and keeps track of the services offered to each of them. Some head-end controllers let the cable company create different service packages depending on customers' bandwidth needs.

Cable modem
The cable modem demodulates the incoming signal and translates it back into IP packets the computer can understand. The cable modem also sends data upstream to the Internet through the cable system, on a 5- to 40-MHz band. Since all subscribers share this relatively narrow band, the cable modem must be able to transmit data on any 2-MHz channels allocated on the upstream band. The head-end controller instructs the cable modem when to send data upstream and which channel to use. Until the industry adopts standards, the head-end controller and the cable modem in each system most likely must be made by the same manufacturer.

3. Comparison

3.1 Security

Both cable modems and DSL are vulnerable to hacker attacks because they are always-on connections to the Internet that assign a static ip address to the user's machine. They both can be made less vulnerable if the provider can assign dynamic ip addresses, the user frequently turns off their machine, installs personal firewall software and anti-virus programs.

In the case of cable modems also, is that the cable line is shared with others in your area, which makes it easy for a neighbor to snoop ip packets destined for your computer. However, a lot of companies are now encrypting the data stream as it enters the network.

3.2 Cost

At this point in time, cable Internet access is cheaper than DSL on average, although rates are falling for each constantly, due to competition.

Typical cable modem service costs $39.95-49.95 per month depending upon whether the subscriber already has cable service, with installation costs ranging from $69-$99 depending upon whether a network interface card has to be installed as well. Many companies frequently have marketing deals when they offer free installations in a bid to gain more customers

Typical DSL service costs between $40 and $80 for home users depending upon the provider, and $80 to $500 for business users. The installation costs range between $100 and $200 which include a network card and DSL modem.

3.3 Ease of Setup

Cable modems are generally regarded as being easier to setup than DSL. There have been well-documented horror stories in the media about the problems involved in getting DSL up and running for new subscribers. This is probably due to the lack of experience of the phone companies in this area. Two years ago, cable took months to install. The problems with installation of DSL have to do with the interplay between the phone company, the DSL provider, and (often) the ISP used for service.
As DSL use becomes more widespread and popular, these problems should be alleviated. The

3.4 Performance

DSL performance is affected by the distance of a subscriber from a central switching station; the further away, the slower the connection. In most instances, ADSL and SDSL connections must be located within 18,000 feet of the telephone company's central office. Most starter DSL accounts offer at least 384 kbps download/128-kbps upload access. The higher the bandwidth, the higher the cost.

Cable performance is affected by the number of people connected on a particular line. Since it is a shared line, the more local subscribers there are, the slower the connection. Most cable providers set a limit on how many customers are connected to a particular line so they can guarantee a minimum download/upload speed.

Cable DSL

Security Potentially less secure than DSL Better security

Cost Generally cheaper; price break if use gets cable service as well More expensive; for business users, higher speeds get pricey.

Setup Setup can be difficult Easier to setup

Performance Line shared with others in neighborhood; speeds vary accordingly Depends on location from central switching office

	Cable	DSL
Security	Potentially less secure than DSL	Better security
Cost	Generally cheaper; price break if use gets cable service as well	More expensive; for business users, higher speeds get pricey.
Setup	Setup can be difficult	Easier to setup
Performance	Line shared with others in neighborhood; speeds vary accordingly	Depends on location from central switching office

State of Market

Technology	Dec. 1999	June 2000	% Change
ADSL	185,950	325,901	+75
Cable ( Coaxial)	879,671	1,434,237	+63
Cable (Fiber)	307,315	301,551	n.m

According to a Dept. of Commerce survey(endnote), 4.5 % of U.S. households (10.7% of online households) had broadband access. Kinetic Strategies states that by Sept. 2000, there were about 3.8 million cable-modem subscribers in North America and projects that this number will increase to 20 million by 2004. According to TeleChoice, there were about 1.7 million DSL lines in service by Sept. 2000, and it projects that this number will grow to about 9.6 million by 2004.

Conclusions

It is hard to make predictions about which broadband technology will come to dominate the marketplace. In addition to cable and DSL, other technologies are now becoming available to the mass-market consumer, such as satellite, fixed wireless, albeit at a lower rate. These technologies will see growth in areas where cable and DSL are not readily available , or prohibitively expensive to deploy (rural areas in the developing world come to mind).

In the U.S., however, it is safe to say that the two main ways for consumers to gain access to the broadband info-super highway will be cable and DSL. Cable will likely stay dominant in the residential marketplace for a while, where cable infrastructure is firmly in place, as long as the cable companies continue their aggressive rollout of services. The AOL-Time Warner merger, ATT &T's purchase of TCI and MediaOne (http://news.cnet.com/news/0-1004-200-4878620.html) seem to assure a bright future for cable modem access. For example, the AOL Time-Warner merger makes it possible to provide cable modem service to the 25 million or so AOL subscribers through Time Warner's RoadRunner cable modem service. AT&T is also poised to offer broadband cable access as part of an all-in-one communications package for the residential consumers consisting of telephone and Internet access.

DSL companies are not sitting on their heels either. In the business market , DSL will likely dominate. This is because business operators will probably not look to cable companies to provide service as the infrastructure is not already in place. Phone companies who offer DSL will also aggressively market it as part of an all-in-one package of telephone, Internet and wireless service.

Hosted by www.Geocities.ws