Competing For That Last Mile

A comparison of high-speed data technologies: Cable vs. xDSL

Executive Summary:

Tired of the World Wide Wait? The advent of the Internet has taken the residential communication technology to its knees. Even with the fastest of the modems available today, Internet access is by no means "a breeze" even for the least demanding of the web surfers. Two emerging high-speed data access technologies on the horizon are capable of satisfying the information hunger: Cable TV (CATV) and Digital Subscriber Line (DSL) technology. The Cable TV strategy uses the cable television infrastructure t o send data to and from the computer while the Digital Subscriber Line technology uses the regular telephone lines to achieve the same. Supporters of either technology are widespread, with telephone companies on one side for the DSL technology and the c able companies on the other for the cable technology. This paper makes an attempt at comparing the two competing technologies with the hope that such an exercise will bring some understanding on the issues at stake.


Cable Television has been a household keyword in America for some time now. Yet with the advent of the Information Super Highway and users looking for speedier access, the cable television system will perhaps attain new heights. The main advantage of th e cable system lies in the fact that it has the capacity of transmitting data at very high rates, on the order of 10 to 36 Mbps which is approximately 400 to 1200 times faster than a regular 28.8 Kbps modem. Although most computers today will not be able to handle such as network speed, but CATV opens up new markets for on-the-spot multimedia, video conferencing, video-on-demand and other data hungry applications that were not previously possible over the network. Digital Subscriber Line (DSL) technology on the other hand, is the weapon that telephone companies hold to counter the cable technology. DSL is a new modem technology that enables digital transmission over regular telephone wires that will exceed current analog signal technology (modems) over 20 to 200 times and potentially much higher. The main advantage of the DSL technology is that a it can use the existing telephone infrastructure to reach millions of customers. Yet the question remains. What will be the prevailing local loop technology for the 21st century? Will the cable or the telephone companies prevail? That is the question that many want an answer. Here is more information,

Basic Modem Technology

Cable modems will serve as the interface between a client computer and the cable network connecting to the cable outlet on one side and to the PC through an Ethernet 10BaseT. Cable modems modulates and demodulates signals like a regular PC modem but it al so functions as part radio, part bridge, part router and part coder among other functionality. Cable modems send and receive signals in slightly different fashions. One the downstream (ie: to the PC) direction, digital data is modulated and placed on a typical 6 MHz television carrier. On the upstream direction (also known as the "reverse path" i n a two-way cable system), the signal is transmitted between 5 and 40 MHz, which is quite noisy. Interference from HAM, CB, Impulse Noisy from home appliances in addition to loose connectors and poor cabling degrades the quality of the upstream signal ag gregated as it travels upstream. To overcome the poor signal, QPSK modulation (up to ~10 Mbps) is utilized which has the disadvantage of being "slower" compared to the QAM (up to ~36 Mbps) modulation. Fortunately, the asymmetry of the data transfer spee d matches the typical "browsing" on the World Wide Web: more information is retrieved than submitted. ADSL modems connects both end of a twisted pair and creates three separate channels of communication: a high speed downstream channel, a medium speed duplex channel and a regular POTS (Plain Old Telephone Service) channel. The high-speed channel has a bandwidth from 1.5 Mbps to 6.1 Mbps while the duplex channel rages from 16 to 640 Kbps. (Note: the numbers vary depending on the quality of the wire, distance, manufacturers among others) As it is implied in the name upstream and downstream speed is asym metrical.

Basic Network Technology

A high-speed data access system based on the Cable TV network operates on Hybrid Fiber/ Coax (HFC) systems and it utilizes a "Tree and Branch" network topology as opposed to the Star topology for xDSL. The HFC system consists of the head-end functioning as the Central Office (CO), the trunk cables which send data through a large metropolitan area where the Optical Network Unit (ONU) is located, the feeder cables which takes these signals to smaller communities, and finally the drop lines which takes the se signals to the customerís premises and finishes with a connection to a terminal equipment (Usually consumer electronics). A single modem sits at each customerís premise and a single modem sits at the ONU location of each coaxial line. For a typical I nternet access configuration IP routers would be installed at each head-end. Digital Subscriber Line (DSL) technology uses the Copper Access Node model as the network architecture having the properties of a star topology. The Central Office holds the IP Router and an Ethernet switch. In turn, the Ethernet switch is connected to an array of ADSL modems. These modems are then connected through the POTS (Plain Old Telephone Service) wire to a matching ADSL modem at each customer premise. Various other implementations of DSL exist as show in the following table.

NameMeaningData RateModeApplications
V.221, V.32, V.34Voice Band Modems1200 bps to 28,800 bpsDuplexData communications
IDSLDigital Subscriber Line160 kbpsDuplexISDN service, Voice and data comm
HDSLHigh data rate Digital Subscriber Line1.544 Mbps 2.048 MbpsDuplex DuplexT1/E1 service Feeder plant, WAN, LAN access, server access
SDSLSingle line Digital Subscriber Line1.544 Mbps 2.048 MbpsDuplex DuplexSame as HDSL plus premises access for symmetric services
ADSLAsymmetric Digital Subscriber Line1.5 to 9 Mbps 16 to 640 kbpsDown UpInternet access, video on demand, simplex video, remote LAN access, interactive multimedia
VDSLVery high data rate Digital Subscriber Line13 to 52 Mbps 1.5 to 2.3 MbpsDown UpSame as ADSL plus HDTV

Key Issues and Analysis:

The main battles between the two competing high-speed digital transmission technology lies are to be fought in the area of the network infrastructure, deployment, scalability, standards, cost and user susceptibility.

Infrastructure and Deployment

Copper wires are the lifelines of the xDSL technology. There are currently 700 million existing copper lines in the world with 100 million of them in the US. This fact alone will be a point up for the telephone companies which will gain an excellent time -to-market competitive advantage over the cable companies. Top this with the fact that generally, telephone companies are in healthier financial standing than most cable companies, the battleground seems to be lost for the cable advocates. Furthermore, X DSL technology can be deployed on a subscriber-by-subscriber basis thus lowering the risk and the investments involved. More than 40 companies worldwide have conducted successful trials of xDSL technologies. Many regional Bell operating companies will h ave xDSL services deployed by mid-1997 time frame and many ISPs (Internet Service Providers) are capitalizing on this broadband technology and are preparing for the big time. One draw back that the xDSL technology face is its extreme distance sensitivity . A customer will need to be within 18,000 cable feet of an ISP offering xDSL connections to the Internet. On the other hand, infrastructure and deployment issues are more problematic for the cable TV based network. Although 96.6 million homes in America are cable enabled currently, a major obstacle in cable modems deployment for high-speed access is the need to convert the CATV plant to handle two-way traffic. This upgrade process is expensive and slow, and yet there are hundreds of cable modems test, trials and projects being conducted worldwide. Approximately seventy of these trials have been upgraded to commercial deployment. In September 1996 LANCity and Zenith modems shipped a total of about 70,000 modems over 650 locations worldwide. Motorola announced in February 1997 that they have shipped 50,000 modems to 80 different locations. Other cable com panies are deploying hybrid cable system, such as RTTís "Virtual Two-way Cable" system. Instead of sending the data upstream, the data is sent downstream, collected and sent back to the metro area. Deploying such a model is more cost effective (10 times cheaper) than setting up two way cables but the data speed would have to be compromised to some degree. The cable companies are definitely idling. Many of them are teaming up with high-tech hardware manufacturers and network managers to push broadband cable modems. Motorola and Sun Microsystems have announced recently of the alliance to create an end-to-end package, complete with digital modems, network servers, network management, installation and servicing to see to both cable TV and telephone compa nies.


As both HFC CATV technology and the DSL technology matures and both with imminent successful commercial deployments, it seems that DSL might be win in the short term but "content inflation" might kill the DLS approach in the end. The infrastructure advan tage for DSL seems favor its success in the short run, but as applications programs becomes more demanding the DSL technology might run out of steam. According to a study by Communications Information and Referral (CIR), "Broadband modem-to-service provi der interconnection can be improved by replacing Ethernet with Fast Ethernet, or ATM and the latest versions of DSL promise up to 53 Mbps over short distances. But CIR's study also notes that in the end, fiber has a far greater information capacity than a ny copper-based systemÖWith HFC, all you have to do is to have fewer homes per hub or devote more cable channels to data. Or you can make use of new capacity on dark fiber. Taking this approach, you can pump any amount of data you choose into homes or off ices." In short, "fiber technology is inherently superior to any copper technology."


MCNS consists of ComCast, Cox Communications, TCI, Time Warner, Continental Cablevision, Rogers Cablesystems, CableLabs and Arthur D. Little, has been established to create standards for data communications over Hybrid Fiber / Coax systems. MCNS has worke d with the IEEE 802.14 to establish a MAC/PHY layer standard. MCNS has already completed the cable modem RF specification and recognized as a standard setting body of the CATV industry with a commanding market share of 80%. Seventeen cable and networkin g vendors have indicated that they will build cable modem products conforming to the MCNS specifications. These companies include 3Com, Bay Networks, Cisco Systems, Com21, GI, Hewlett-Packard, Intel, Motorola, NEC America, Panasonic, Sharp Electronics, Toshiba, US Robotics and Zenith. In addition to new MCNS-compliant chipsets to be produced by Broadcom, ComStream, Lucent and VLSI Technology among others. The American National Standards Institute (ANSI) and The European Technical Standards Institute (ETSI) have recently approved ADSL standards at rates up to 6.1 Mbps. These standards will ensure interoperability between DMT-based equipment regardless of l ocation. Standards for xDSL in general are still immature and will depend on various factors such as field trials, customer preferences and market momentum.


Cable companies have higher infrastructure and deployment costs at the start. Thatís one advantage to the telephone companies which could balance that with the higher equipment cost. But as more equipment manufacturers are entering both the cable and A DSL modem arena and more chip-based modems manufactured the cost of installation, equipment and monthly charges would become more insignificant.

Monthly Connection Fee$30-100$50$30 - $50 (BRI)
$400+ (PRI)
Modem$200 and up $500 and up
Installation Fee$150$100 - $150$100-$150 (BRI)
$1400 (PRI)

User Susceptibility

Which technology would an average computer user select will depend of a combination of cost, performance, ease of installation, and service availability. Service availability seems to be the most dominant factor at this point. The other three variables seems to be comparable and if not will be so shortly. Once again, the infrastructure advantage of the DSL technology will seem to gain another edge.


Who is the winner then? Although the HFC cable modem system seems to be a more reasonable choice for the future as far as bandwidth is concern, the lack of existing usable infrastructure might prevent it from becoming ubiquitous in the short to medium ti me frame. This, of course, assumes that the xDSL technology does not fumble in anyway. In terms of speed, installation, cost, and reliability both technologies are viable options for the future. It is hard to say which technology will dominate the mark et eventually. Perhaps, they might co-exist to complement one another. Nonetheless, it is a Win-Win situation for the millions of Internet users who are tired of congestion and speed restrictions on the Information Highway. Cheers! There is hope after all.