Overview of Cable Modem Technology and Services
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Introduction
Cable TV Primer
Cable Modem Access Networks
Cable Internet Service Delivery
Shared Network Platform Performance
Cable Modem Service Availability
Telephone-Return Path Solutions
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Introduction
Residential Internet and online usage has managed to grow tremendously despite maddeningly-slow speeds available through existing dial-up telephone modem connections, typically limited to 33.6 Kbps or less. Touted as an interactive extravaganza, surfing the World Wide Web more typically offers narrowband users a click-and-wait experience. The growing frustration of existing online users is driving demand for higher-speed connections.
Local telephone companies currently offer residential ISDN services that provide connection speeds up to 128 Kbps and they are looking to digital subscriber line technologies (DSL) which can provide downstream speeds reaching 6 Mbps or more. Other alternatives include fast downstream data connections from direct broadcast satellite (DBS), wireless cable providers, and of course, high-speed cable modems.
More than 105 million homes in North America are passed by broadband coaxial cable plant and more than 75 million homes are cable TV subscribers. With near-ubiquitous coverage, coaxial cable connections provide a potentially powerful platform for providing residences and small business with high-speed data access. However, one-way cable television systems must be upgraded into modern two-way networks to support advanced communications services, a technically-complex and capital-intensive proposition.
Cable TV Primer
Cable systems were originally designed to deliver broadcast television signals efficiently to subscribers' homes. To ensure that consumers could obtain cable service with the same TV sets they use to receive over-the-air broadcast TV signals, cable operators recreate a portion of the over-the-air radio frequency (RF) spectrum within a sealed coaxial cable line.
Traditional coaxial cable systems typically operate with 330 MHz or 450 MHz of capacity, whereas modern hybrid fiber/coax (HFC) systems are expanded to 750 MHz or more .
Logically, downstream video programming signals begin around 50 MHz, the equivalent of channel 2 for over-the-air television signals. The 5 MHz - 42 MHz portion of the spectrum is usually reserved for upstream communications from subscribers' homes.
Each standard television channel occupies 6 MHz of RF spectrum. Thus a traditional cable system with 400 MHz of downstream bandwidth can carry the equivalent of 60 analog TV channels and a modern HFC system with 700 MHz of downstream bandwidth has the capacity for some 110 channels.
Cable Modem Access Networks
To deliver data services over a cable network, one television channel (in the 50 - 750 MHz range) is typically allocated for downstream traffic to homes and another channel (in the 5 - 42 MHz band) is used to carry upstream signals.
A headend cable modem termination system (CMTS) communicates through these channels with cable modems located in subscriber homes to create a virtual local area network (LAN) connection. Most cable modems are external devices that connect to a personal computer (PC) through a standard 10Base-T Ethernet card and twisted-pair wiring, though external Universal Serial Bus (USB) modems and internal PCI modem cards are under development.
The cable modem access network operates at Layer 1 (physical) and Layer 2 (media access control/logical link control) of the Open System Interconnect (OSI) Reference Model. Thus, Layer 3 (network) protocols, such as IP traffic, can be seamlessly delivered over the cable modem platform to end users.
A single downstream 6 MHz television channel may support up to 27 Mbps of downstream data throughput from the cable headend using 64 QAM (quadrature amplitude modulation) transmission technology. Speeds can be boosted to 36 Mbps using 256 QAM. Upstream channels may deliver 500 Kbps to 10 Mbps from homes using 16QAM or QPSK (quadrature phase shift key) modulation techniques, depending on the amount of spectrum allocated for service. This upstream and downstream bandwidth is shared by the active data subscribers connected to a given cable network segment, typically 500 to 2,000 homes on a modern HFC network.
See the Cable Data Network Diagrams section for a visual overview of this architecture.
An individual cable modem subscriber may experience access speeds from 500 Kbps to 1.5 Mbps or more -- depending on the network architechture and traffic load -- blazing performance compared to dial-up alteratives. However, when surfing the Web, performance can be affected by Internet backbone congestion.
In addition to speed, cable modems offer another key benefit: constant connectivity. Becuase cable modems use connectionless technology, much like in an office LAN, a subscriber's PC is always online with the network. That means there's no need to dial-in to begin a session, so users do not have to worry aboutr receiving busy signals. Additionally, going online does not tie up their telephone line.
A range of vendors are now offering two-way cable modem products, including 3Com, Cisco Systems, Com21, General Instrument, Motorola, Nortel Networks, Samsung Telecommunications America, Sony, Terayon Communication Systems, Thomson Consumer Electronics, and Zenith. See the Cable Modem Vendors page for more information.
Cable Internet Service Delivery
To get into the high-speed Internet services business, cable operators must do more than simply install cable modem gear. Rather, they must build a sophisticated end-to-end IP networking infrastructure in each community they serve that is robust enough to support tens of thousands of data subscribers. That includes items like Internet backbone connectivity, routers, servers, network management tools, as well as security and billing systems. In essence, cable operators are faced with the task of building some of the world's largest "intranets," a serious engineering and operations challenge.
Cable operators are focused on providing high-speed intranet access instead of straight Internet access for a simple reason: a network connection is only as fast as its slowest link. Clearly, the benefit of a 1-Mbps cable link is lost if a subscriber tries to access content stored on a Web server that is connected to the Internet though a 56-Kbps line. The solution to this dilemma is to push content closer to the subscriber, ideally right down to cable headend. This is done by "caching" or storing copies of popular Internet content on local servers, so when a cable modem subscriber goes to access a Web page, he or she will be routed to the server in the headend at top-speed, rather than being required to voyage out onto the congested Internet.
A number of companies are offering comprehensive networking and systems integration services to cable operators entering the high-speed Internet business.
@Home Network., which is jointly owned by cable operators AT&T Broadband & Internet Services (formerly TCI)., Cox Communications Inc., Comcast Corp., Rogers Cablesystems Ltd., Shaw Communications Inc. and others, as well as venture capital firm Kleiner Perkins Caulfield & Byers, has built a high-speed data backbone and caching infrastructure to distribute broadband Internet services through affiliate cable systems.
The Road Runner Group, a joint venture between Time Warner Cable, MediaOne, Microsoft and Compaq, has developed another broadband Internet service. Other companies, such as High Speed Access Corp. and SoftNet Systems are offering turnkey Internet packages specifically designed for small cable system operators.
Shared Network Platform Performance
Most cable modem systems rely on a shared access platform, much like an office LAN. Becuase cable modem subscribers share available bandwidth during their sessions, there are concerns that cable modem users will see poor performance as the number of subscribers increases on the network. Common sense dictates that 200 cable data subscribers sharing a 27-Mbps connection would each get only about 135 Kbps of throughput -- virtually the same speed as a 128-Kbps ISDN connection -- right? Not necessarily.
Unlike circuit-switched telephone networks where a caller is allocated a dedicated connection, cable modem users do not occupy a fixed amount of bandwidth during their online session. Instead, they share the network with other active users and use the network's resources only when they actually send or receive data in quick bursts. So instead of 200 cable online users each being allocated 135 Kbps, they are able to grab all the bandwidth available during the millisecond they need to download their data packets -- up to many megabits per second.
If congestion does begin to occur due to high usage, cable operators have the flexibility to allocate more bandwidth for data services. A cable operator can simply allocate an additional 6 MHz video channel for high-speed data, doubling the downstream bandwidth available to users. Another option for adding bandiwdth is to subdivide the physical cable network by running fiber-optic lines deeper into neighborhoods. This reduces the number of homes served by each network segment, and thus, increases the amount of bandwidth available to end users.
Cable Modem Service Availability
After years of technical trials, large cable operators finally began widespread deployments of cable modem services in late 1996. Cable Datacom News publisher Kinetic Strategies Inc. estimated that cable operators were commercially offering high-speed Internet services to 25 million homes in North America as of April 15, 1999 and had attracted more than 700,000 paying subscribers. See Cable Modem Market Stats and Projections for more information.
Cable operators are typically charging between $40 and $60 per month for the service, which includes cable modem rental and unlimited Internet access.
The most important factor in the deployment of two-way cable data services is the availability of high-quality two-way HFC plant, which to date, has been limited. A few large MSOs have been particularly aggressive in investing the required $200 - $250 per home passed to make two-way HFC upgrades, including Time Warner Cable, MediaOne, Comcast Corp., Cox Communications and Rogers Cablesystems. Not surprisingly, these MSOs have also been most successful in deploying two-way cable data services.
For the latest list of cable modem deployment locations, see Commercial Cable Modem Launches in North America and Select International Cable Modem Trials and Commercial Launches.
The lack of rapid, ubiquitous cable system upgrades is the major limitation in the widespread deployment of two-way cable modems. For at least the next three years, local cable systems and their subscribers will be divided into a world of two-way cable modem "haves" and "have nots."
Telephone-Return Path Solutions
A number of vendors are working to serve this one-way majority of cable systems by offering cable modems that use RF cable spectrum for fast downstream transmission and a telephone modem to handle upstream communications over the public telephone network. These vendors include 3Com, General Instrument, Hybrid Networks Inc., and New Media Communication Ltd.
Since demand for consumer cable data services is being driven primarily by the need for faster downstream speeds, telephone-return path cable modems could prove to be a viable means for cable operators without two-way plant to rapidly enter the residential high-speed data market. However, telco-return modems do not provide some key benefits available with two-way products, such as ultra-fast upstream speeds, constant connectivity and not tying up a subscriber's telephone line.
Nonetheless, some cable operators see the opportunity to go into business as full-service ISPs, offering one-way cable modems, traditional dial-up Internet access at 28.8 Kbps, and two-way cable modems over portions of plant that have been upgraded to two-way HFC. The goal with this strategy is to quickly grab share of a local market for residential Internet access, providing the opportunity to migrate subscribers to different dial-up, one-way or two-way modem service tiers over time.
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Cable Modem Standards and Specifications
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Cable operators have long believed success in the high-speed data business would require that cable modems be interoperable, low-cost and sold at retail like telephone modems and data network interface cards. This way, MSOs could avoid the capital burden associated with purchasing cable modems and leasing them back to subscribers, and consumers would be able to choose products from a variety of manufacturers.
The Institute of Electronic and Electrical Engineering's (IEEE) 802.14 Cable TV Media Access Control (MAC) and Physical (PHY) Protocol Working Group was formed in May 1994 by a number of vendors to develop international standards for data communications over cable. The original goal was to submit a cable modem MAC and PHY standard to the IEEE in December 1995, but the delivery date slipped to late 1997.
Tired of waiting for the IEEE 802.14, cable operators combined their purchasing power to jump-start the standards process. In January 1996, cable MSOs Comcast, Cox, TCI, and Time Warner -- operating under a limited partnership dubbed Multimedia Cable Network System Partners Ltd. (MCNS) -- issued a request for proposals (RFP) to retain a project management company to research and publish a set of interface specifications for high-speed cable data services by the end of the year. MSOs MediaOne (formerly Continental Cablevision) and Rogers Cablesystems, and CableLabs, also signed on to the MCNS Data Over Cable Service Interface Specification (DOCSIS) RFP. Together, this coalition represents the majority of the North American cable industry, serving 85% of U.S. cable subscribers and 70% of Canadian subscribers.
MCNS released its Data Over Cable System Interface Specification (DOCSIS) for cable modem products to vendors in March 1997. To date, more than 20 vendors have announced plans to build products based on the MCNS DOCSIS standard.
A Tale of Two Standards: MCNS vs. IEEE
The differing cable modem specifications advocated by 802.14 and MCNS reflect the priorities of each organization. A vendor-driven group, 802.14 has focused on a creating a future-proof standard based on industrial-strength technology. The MSO members of MCNS, on the other hand, are far more concerned with minimizing product costs and time to market. To achieve its objectives, MCNS sought to minimize technical complexity and develop a technology solution that was adequate for its members' needs.
At the physical layer, which defines modulation formats for digital signals, the IEEE and MCNS specifications are similar. The 802.14 specification supports the International Telecommunications Union's (ITU) J.83 Annex A, B and C standards for 64/256 QAM modulation, providing a maximum 36 Mbps of downstream throughput per 6 MHz television channel. The Annex A implementation of 64/256 QAM is the European DVB/DAVIC standard, Annex B is the North American standard supported by MCNS, while Annex C is the Japanese specification. The proposed 802.14 upstream modulation standard is based on QPSK (quadrature phase shift keying) and 16QAM, virtually the same as MCNS.
For the media access control (MAC), which sets the rules for network access by users, 802.14 has specified Asynchronous Transfer Mode (ATM) as its default solution from the headend to the cable modem. MCNS went a different route, using a scheme based on variable-length packets that favors the delivery of Internet Protocol (IP) traffic. Although the MCNS MAC is based on packets and the IEEE specifies fixed ATM cells, both cable modem solutions specify a 10Base-T Ethernet connection from the cable modem to the PC.
IEEE 802.14 committee members say they chose ATM because it best provides the quality of service (QoS) guarantees required for integrated delivery of video, voice, and data traffic to cable modem units. The group saw ATM as a long-term solution that would provide the flexibility to deliver more than just Internet access.
MCNS members didn't buy the argument. Cable operators are clearly focused on delivering high-speed Internet services to consumers and believed ATM would add unnecessary complexity and cost to cable modem systems. By supporting a variable-length packet implementation, MCNS members plan to capitalize on the favorable pricing associated with Ethernet and IP networking technology.
Standardized DOCSIS cable modems started shippining in limited quantities in the third and fopurth quarters of 1998 with wider availability expected in the first quarter of 1999.
No major vendors are currently building modems based on the initial IEEE standard
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