Cable Modems and ADSL
Kim Maxwell
Independent Editions
March 1998
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.
Figure 1. Cable Modem Network Architecture
Copper Access Nodes. Figures 2 and 3 show alternative architectures for accessing Internet and other IP-based networks using existing copper telephone lines. Each involves connecting ADSL modems at both ends of a subscriber's telephone line, the same one used for POTS. ADSL operates at frequencies above POTS, and a POTS splitter leaves POTS service unaffected, even if ADSL loses power or fails for some inexplicable reason. The premises ADSL modem, equipped with a Packet Mode interface, connects directly to the Ethernet port of a personal computer or a local Ethernet hub.
In Architecture A, the Starter Kit Internet Platform (SKIP), each central office would be equipped with an Internet Router and an Ethernet switch or comparable packet based concentrator. The switch serves to concentrate access lines into expensive router ports. Exploiting typical usage statistics, a single DS1 (1.544 Mbps) pipe from the router to the Internet could support five or more simultaneous users at the same speed, and as many as 50 subscribers, assuming no more than 10% of them wanted access at any one time. A DS3 (45 Mbps) pipe could support 1500 subscribers, the likely maximum required of any one central office. (It is likely, howeve, that early designs will be conservative, with 750 users assumed for each DS3 and something in the order of 2000 users for each OC3.) Note that signals do not pass through any other telco switching equipment, nor does the system need any new signaling protocol for connection -- it uses the existing TCP/IP stack. (The arrangement is in effect a private line service, just like cable modems.) The benefit of the SKIP network is that it can be designed with existing equipment and deployed in the short term, measured in months.
Figure 2. Starter Kit Internet Platform
In Architecture B, the ATM Internet Model (AIM), a network provider locates an ATM switch in a LATA (or similar topology) and connects it to end office access nodes via DS1, DS3, or OC3 transmission links. Each access node concentrates individual ADSL lines onto a single transmission link. Usage statistics once again favor significant levels of concentration (and concomitant dispersion of core network costs over a large number of users). A single switch with 155 Mbps capacity could service 1000 simultaneous users operating at 1.5 Mbps peak, and more than 10,000 subscribers. The benefit of the AIM network is that it is the first step towards a Full Service Network and can be easily scaled upward to support video applications as they materialize. The downside of AIM is short term deployment. It should be noted that SKIP and AIM are not mutually exclusive. Access concentrators can be developed that replace Ethernet switches in the SKIP configuration and then migrate towards access nodes for an AIM network.
Figure 3. ATM Network Architecture
NOTEMany Internet users today experience busy lines attempting dial-up access. With any of the technologies presented here, users will not be blocked from initial access, and the consequence of traffic congestion will be slower response time or longer file transfer delays, not service interruption.
Basic Network Distribution
HFC. Basic CATV lines pass 180 million residences in the world now (90 million in the U.S., with 60 million subscribers), but the preponderance of these lines are old, one-way, and coaxial only, often connecting thousands of subscribers from one headend over massive tree and branch networks. Estimates vary, but upgraded CATV into HFC format has probably only been accomplished in the U.S. for about 6 million subscribers. All major U.S. CATV companies have upgrade programs underway, however. All recent CATV installations in the UK, now passing a million homes, have been two way. Other European countries with large CATV infrastructures, such as the Benelux countries and Germany, have older systems, and all Asian networks of any size are older coaxial only. Several telephone companies, notably Pacific Telesis in the U.S. and Telstra in Australia, have begun massive deployment programs for HFC.
Upgrading from one-way coax to HFC will cost about $200 per home passed (just changing amplifiers for two way service to get started will cost in the order of $25 per home passed). Note that CATV does not serve small businesses.
Copper. There are, by ITU estimates, 640 million copper telephone lines in the world today, of which 70% connect residences, the balance connecting businesses and pay phones. Despite emerging technologies for non-copper based telephony, the world-wide copper loop plant is still projected to exceed 900 million lines by 2001. In the U.S approximately 80% of these line can accommodate ADSL at approximately 1.5 Mbps, and 50% can support rates of 6 Mbps or more. Many other countries have more favorable cable length statistics. However, some countries or regions have very old loop plants, and the percentage of copper lines that are actually usable for megabit access may be well below 80%. Variable rate ADSL modems, with minimum speeds below at megabit, may enable connection to all users.
Issues
Cable Modems and ADSL have comparable capabilities and both can be built into broadband IP-based infrastructures starting as early as 1997 (assuming successful trials and kink-straightening in 1996). However, other issues remain. It is likely that all of them will pale before the commercial benefits of ubiquitous access enjoyed by telephone companies and ease of network deployment enjoyed by CATV companies, but they must be considered, by operators and users alike, as the information superhighway begins, finally, to take some shape.
Security
All signals go to all cable modem users on a single coaxial line, creating serious prospects of intended or inadvertent wiretapping. ADSL, on the other hand, is inherently secure. Intended wire tapping requires invading the line itself (often underground) and knowing the modem settings established during initialization -- not impossible, but very difficult. Encryption and authentication will be important parts of both systems, but vital for cable modems. (Several cable modem vendors have put encryption into their modems.)
Reliability
Cutting a CATV line in the street or losing above ground cable in a storm will bring down all users on that line. A single streaming transmitter on a CATV line will bring down all users on that line (this problem just needs network management attention, but it must be attended to). Amplifiers in CATV networks have been problems in the past. An ADSL modem failure only affects one subscriber, and telephone lines are legendary for reliability, rain or shine.
Stability
The first user of a cable modem on a given line will have excellent service. Each additional user creates noise, loads the channel, reduces reliability, and generally degrades the quality of service for everyone on the line. Quality of service will also degrade as Internet users on a line shift from text and low graphics to high graphics and multimedia, an inevitable trend if the Internet is in any way successful. ADSL itself suffers no degradation based on traffic or number of users in the access network. However, ADSL must work into an access concentrator of some sort, which will encounter congestion during peak hours. Indeed, if the concentrator output is not greater than the speed of a single cable modem, it will have identical degradation. However, it is probably easier to add concentrator capacity than split coax nodes, the comparable remedy for HFC lines.
Home Wiring.
Personal computers are seldom located in a home adjacent to the television, or television coaxial cabling. Personal computers, especially ones desiring Internet access, typically sit near a telephone line. Cable modems will usually require some new wiring in the home. ADSL for PC access may at some circumstances be installed without new wiring. The exact distribution of these circumstances will not be known until many units have been deployed.
Standards and technology status
Several efforts are underway to standardize cable modems, particularly ones by IEEE 802.14 and Cable Labs (now joined by ADL in Cambridge). However, it is quite likely that quite a few will be deployed before a standard is agreed, and new transmission ideas still surface. It should be noted that CATV, as a business, has not history of standards development or enforcement.
ADSL, meanwhile, is a standardized, scalable technology that will live in its present form for decades. The telephone business is standards driven, and standards organizations such as ITU and T1 have long and stable histories.
And the Winner is ...
Both, but ADSL will dominate. Both technologies will come into commercial service at about the same time -- mid 1997. They deliver comparable capabilities. The inherently lower network costs of cable modems compared to ADSL access systems will be offset by higher infrastructure costs incurred by upgrading exising plant, a cost telephone companies do not have to bear. In any event, network costs for ADSL systems will be sufficiently low that telephone companies will be able to match CATV pricing strategies, if necessary. However, telephone companies are already connected to the entire customer base; CATV passes a small fraction today, and won't pass more than 40% by 2000. Even with a tie in territories covered by both enterprises, telephone companies will achieve 70 - 80% market share over-all, in the U.S. If dial up modems can serve as an example, once central office infrastructure has been fully deployed (no more than three years), ADSL and cable modems can grow from low millions to tens of millions very quickly.
Copyright 1998