Jim --
I'll leave the cable vs. DSL debate to you and SteveG. As you can see from his response, if I say the sky's blue, he'll say it's grey. My emotional bank refuses withdrawals on his behalf.
For a good DSL article, check out:
nwfusion.com
Do a search under "mcclimans" for his most recent "Keeping Current on DSL" article, dated 12/15/97. I think you have to register, but it's worth it.
Several helpful URLs on the DSL vs. Cable debate:
adsl.com
adls.com
<<<Cable Modems and ADSL
Kim Maxwell
Independent Editions
Two candidate modem technologies have emerged over the past year for switched data com-munications services at megabit data rates. Cable Modems operate over two-way hybrid fiber/coax and provide user rates as high as 10 Mbps. ADSL Modems (for Asymmetric Digital Subscriber Lines) operate over existing copper telephone lines and provide rates as high as 9 Mbps. Both technologies address vast markets for Internet access, remote LAN access for work at home and telecommuting, distance learning, and special network access for the hundreds of millions of personal computers in place today and to be sold over the next ten years.
EXECUTIVE SUMMARY
ADSL modems have been proven now in the field. Despite the current negative press about cable modems, which suffer more from premature announcements than killer flaws, they too will be made to work over the modem-hostile envirorment of CATV. Neither represents a perfect solution for ubiquitous megabit access. But users will quite likely be more than satisfied with either, as megabit access offers so much more power than today's modems, or even ISDN, that users will be initially stunned at the performance, then wonder how they got along without it.
Cable modems may offer more raw speed than ADSL, but that advantage is compromised by inevitable reductions in available cable modem speed. Cable modems share a line with tens of other users; as more users join a line, the capacity available to any one inevitably drops. The top speeds of both technologies will not be usable for years anyway. Internet server speeds, network delays, and personal computer limitations will hold usable rates at or below 2 Mbps for some time. ADSL offers higher security and reliability profiles. Both technologies are at about the same state of maturity and integration. Cable modems may offer a less expensive network solution because of its shared architecture, but that differential is more than offset by infrastructure costs required to upgrade existing networks to HFC.
The largest advantage of ADSL, and it is a significant one, is the number of telephone lines already installed that can support ADSL compared to the number of HFC lines available, or prospectively available with network upgrades. Today the global ratio is in the order of 400 million to 6 million, or about 60 to 1. Aggressive upgrades will not improve the ratio to better than 10 to 1 in the next five or six years. Even in the United States the ratio today is in the order of 20 to 1, and will not likely get better for CATV suppliers than 3 to 1 over the next five or six years.
Two hundred million personal computers will have been sold by the end of 1996. At present run rates, another 240 million will be added by 2001 as PCs start to approach the global population of televisions. Small offices and residences will absorb at least 25% of them, or 100 million. Forester has projected 6 million cable modems will be installed by the year 2000. With SKIP architectures and suitable pricing, telephone company connections could be triple that number, yielding an altogether reasonable figure of 25 million personal computer users operating at megabit rates as the century turns.
BASIC MODEM TECHNOLOGIES
Cable Modems. While cable modems come in many forms, the most typical create a downstream data stream out of one of the 6 MHz TV channels that occupy spectrum above 50 MHz (and more likely 550 MHz) and an upstream channel carved out of the currently unused band between 5 and 50 MHz. Using 64 QAM, a downstream channel can realize about 30 Mbps (the quoted speed of 10 Mbps refers to pc rates associated with Ethernet connections). Upstream rates vary considerably from vendor to vendor, but speeds in low megabits should be available on good HFC systems. The downstream channel is continuous, but divided into cells or packets, with addresses in each packet determining who actually receives a particular packet. The upstream channel has a media access control that slots user packets or cells into a single channel. To avoid collisions, the system gates each upstream packet onto the network with control signals embedded in the downstream information stream. (Some cable modem configurations divide the upstream into frequency channels and allocate a channel to each user. Others combine the two multiplexing methods. A few modem companies are proposing techniques like spread spectrum or code division multiplexing to provide more robustness in the presence of ingress noise, the dominant difficulty on HFC networks.) Cable modem rates do not depend upon coaxial cable distance, as amplifiers in the cable network boost signal power sufficiently to give every user enough. Variation in cable modem capacity will depend rather on ingress noise in the line itself and the number of simultaneous users seeking access to a shared line.
ADSL. Asymmetric Digital Subscriber Lines locate modems on either end of existing copper telephone lines. As the name suggests, they realize downstream speeds up to 9 Mbps, but upstream speeds up to 640 kbps As ADSL operates point-to-point, it does not need media access control, and each user gets the full rate available continuously. However, ADSL modem speeds do depend upon line distance, and the longer lines found today may support speeds no greater than 1.5 Mbps. The average line, however, will support speeds up to 6 Mbps. Variable rate ADSL modems will adapt to line length, offering high speed service to almost all telephone subscribers.
NOTE
The Internet and IP systems generally function in an Available Bit Rate (ABR) mode, and are therefore graceful about accommodating various and varying speeds. Furthermore, most Internet servers today operate at 56 kbps, and power servers seldom operate at speeds above T1, limiting useful data rates to 1.5 Mbps for some time. Remote LAN access may use higher speeds, but the performance difference between 1.5 and 6 Mbps (the best realizable Ethernet speed) is small compared to the performance difference between 28.8 kbps and 1.5 Mbps. This inherence scalability benefits both cable modems and ADSL -- 10 Mbps cable modems will not exhibit effective speed deterioration until a large number of users attempt simultaneous transmission, and ADSL will give excellent service even at speeds below 1.5 Mbps.
BASIC NETWORK ARCHITECTURES
Hybrid Fiber/Coax (HFC) As outlined in Figure 1, cable modems operate over hybrid fiber/coax (HFC) networks, comprised of fiber feeder from a so-called head-end and branch coaxial cables installed from the Optical Network Unit (ONU) to customer premises, with as many as 100 users on any one cable line. Even these shorter reach coax lines need a few amplifiers, which must be two way to permit upstream signals to pass. A Cable Modem sits in each subscriber premises, and a single modem fits at the ONU location of each coaxial line (this is identical, conceptually, to multidrop polling systems used in SNA networks). In a typical Internet access configuration an IP router would be situated at each head end. Note that the shared cable arrangement acts as a data concentrator, a sort of distributed access node, obviating any further concentration to make efficient utilization of expensive router ports. In some cases a service provider may also locate cache memory and proxy servers at the router point to smooth traffic and to compensate for congestion within the Internet and slow access rates to other information resources.
Most CATV systems today are not HFC. Rather they are tree and branch compositions of coaxial cable, sometimes serving as many as 10,000 customers from a single headend, with one-way amplifiers that preclude any upstream data flow. Since 1993 many CATV lines have been installed with two-way amplifiers, creating an upstream path from 5 to 45 MHz. However, the sheer size of these networks and the noise and channel problems with so many subscribers attached to a common line make high speed upstream channels unattainable after a few subscribers have joined the line.
Upgrading a coax system from unidirectional to bi-directional may be accomplished by physically replacing amplifiers, at a cost around $25 per home passed. Upgrading from coax to HFC requires more work -- laying fiber, installing ONUs, rerouting any coax not convenient to the ONU, and replacing the few remaining coax amplifiers (they cannot be eliminated altogether). Costs vary, but $200 per home passed seems to be a decent operating assumption. >>> |
