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Cable Modems: Cable Tv Meets The Internet Essay, Research Paper

Cable Modems: Cable TV Meets the Internet

John G. Shaw

IS 3348

October 2, 1999


The Telecommunications Act of 1996 opened the way for cable TV (CATV) companies to become full-fledged telecommunications companies, offering two-way voice and data communications services, in addition to television programming. After passage of the Act, the cable companies were eager to expand into the new fields of business that had been opened to them, especially the rapidly growing Internet Service Provider (ISP) business. The biggest hurdle facing the cable companies is that cable television systems were designed for one-way traffic, and must be upgraded into modern two-way networks in order to support advanced communications services. This is an expensive and technically complex undertaking. In addition, interfaces allowing subscriber’s PCs to access the Internet via the CATV cable had to be developed. These interface devices are called cable modems. Cable modems are designed to take advantage of the broadband capability provided by the cable TV infrastructure, enabling peak connection speeds many times faster than conventional dial-up connections.

Cable Modems, Cable TV Meets the Internet

Cable modems have only recently been introduced for private commercial use. Cable modems and the cable data networks they are a integral part of hold the promise of providing a great deal of communications bandwidth for the private user. Greater bandwidth equals greater speed in the realm of the Internet. The Internet has only been around for private use for a relatively short period of time, nonetheless, it has grown quite rapidly. It appears that the Internet will continue to grow at a rapid pace. People will begin to use the Internet for more and more applications. Networking will eventually be a part of the most minute parts of our daily lives. New Internet applications will undoubtedly require greater data speeds, and cable data networks are a tremendous step forward in providing that speed. Cable modem technology is still in its infancy, but it has already revolutionized Internet “surfing”. Cable modems are providing connection speeds that people only dreamed about a short time ago. However, on a greater scale, as more and more people start using cable modem service, the cable companies will have to continue upgrading their networks to keep up with increased demand. Eventually, fiber-optic cable will reach into individual homes. This breakthrough development will increase bandwidth by orders of magnitude, and it is cable modem that has already started this process.


“Cable Modems, Cable TV Meets the Internet” is an informative overview of cable modems and cable data systems. Extensive research was done to investigate how cable modems work, and how cable modems fit into a cable data system. The cable industry was only allowed to enter the ISP business less than three years ago. Because cable modems are relatively new devices, and cable data network technology has advanced rapidly, the latest up-to-date sources of information had to be used to provide accurate information. Recent magazine articles and Internet sites had the most current information. The information in hardcover books was obsolete and dated. After researching the subject, the results of the research were presented in the paper. The references used as sources of information for the paper are cited.


Cable modems are proven technology. Cable data networks provide tremendous speed as well as upgrade potential.


The material presented here shows that cable modem technology is robust and has tremendous potential to continue growing. Cable modems are just another step to the total networking of everyday life. This development is still a long way off. But, it is bound to happen. It will happen sooner, rather than later

Residential Internet usage has grown rapidly despite the frustratingly slow speeds available through conventional dial-up telephone modem connections. These voiceband connections are limited to 56 Kbps or less. Surfing the ‘Net with a dial-up modem is usually a click-and-wait experience. There is a tremendous demand for faster Internet connections. The Telecommunications Act of 1996 opened the way for cable TV (CATV) companies to become full-fledged telecommunications companies, offering two-way voice and data communications services, in addition to television programming (Clark, 1999). Cable companies that offer these extended services are known as Multiple Service Operators (MSO). The aspiring Multiple Service Operators realize there is a sizable market of Web surfers who feel a “need for speed”, and they want to be the ones to meet that need. Cable modems are devices that allow high-speed access to the Internet by way of a cable television network. Cable modems work much the same way as traditional dial-up telephone modems, but cable modems are much more powerful. Instead of using telephone lines as the connection medium to the Internet, cable modems use the cable that carries cable TV programming as its connection medium. Cable modems are designed to take advantage of the broadband capability provided by the cable TV infrastructure, enabling peak connection speeds many times faster than dial-up connections. More bandwidth equals more speed. A cable modem subscriber may experience access speeds from 500 Kbps to 1.5 Mbps or more, depending on the cable network architecture and traffic load (Halfhill, 1996). With their blazing speed, cable modems are able to rapidly download large audio and video files, providing true multimedia capability. In addition to speed, cable modems offer another key benefit: constant connectivity. Cable modems are online as soon as the computer is turned on. This is possible because cable modems use connectionless technology, much like an office LAN (Ostergard, 1998). There is no need to dial in to begin a session, so there are no busy signals and no need to tie up their telephone line. Also, with prices ranging from $40 and $60 per month, which includes cable modem rental and unlimited Internet access, cable modem Internet service is extremely cost effective when compared to other high-speed data systems.

Unfortunately for the cable companies, it is not just a simple matter of attaching cable modems to their subscriber’s PCs and letting them surf away at light speed. To get into the high-speed Internet Service Provider (ISP) business, a CATV company must build an expensive and complex IP networking infrastructure. This network has to be able to support thousands of subscribers. Building cable data network involves addressing such items as Internet backbone connectivity, routers, servers, network management tools, as well as security and billing systems (Salent, 1999). Furthermore, CATV data systems are comprised of many different technologies, so standards governing cable modems had to be developed which would allow products from different vendors to be interoperable. But, the biggest hurdle facing the cable companies is that cable television systems were designed for one-way traffic, and must be upgraded into modern two-way networks in order to support advanced communications services (Medin, 1999). This is an expensive and technically complex undertaking.

CATV systems were originally designed to deliver broadcast television signals to subscribers’ homes. In the cable industry, this is known as downstream traffic. The Head-end is the central distribution point for a CATV system. Video signals are received at the Head-end from satellites or other sources, frequency modulated to the appropriate channels, and then transmitted downstream through the cable medium into the subscriber’s homes. The subscriber’s television tuner, or set-top cable converter box, demodulates the signal back to a video image. To insure 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 cable line. The older coax-only cable systems typically operate with 330 MHz or 450 MHz of capacity (Ostergard, 1998). While the newer, more expensive hybrid fiber-optic/coax (HFC) systems can operate at 750 MHz or more (Ostergard, 1998). HFC networks combine both fiber-optic and coaxial cable lines. About half of the cable subscribers in North America are connected to HFC cable systems. HFC networks cost much less than a pure fiber-optic network, but provide many of fiber’s reliability and bandwidth benefits. The fiber-optic portion of the HFC network is a star configuration where optical fiber feeder lines run from the cable head-end to groups of 500 to 2,000 subscribers (Van Matre, 1999). These groups of subscribers are called cable nodes or cable loops. A trunk-and-branch configuration of coaxial cable runs from the optical-fiber feeders to reach each subscriber.

Because CATV systems were originally designed primarily to send signals downstream, only a small amount of the available bandwidth was allocated for upstream transmissions. There is very little need for upstream communication in CATV system that is used solely for television signal transmission. The allocated upstream bandwidth is a narrow 5 to 42 MHz band residing at the lower end of the cable TV RF spectrum (Barnes, 1997). Downstream cable TV program signals begin at 50 MHz, which is the equivalent of channel 2 for over-the-air television signals. Each standard television channel occupies 6 MHz of RF spectrum. So a traditional coaxial 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 110 channels (Salent, 1999).

To deliver two-way data transmission over a cable network, one unused 6 MHz television channel, in the 50 – 750 MHz range is typically allocated for downstream data traffic. Another unused 6 MHz channel, in the 5 – 42 MHz range, is used to carry upstream data. Whenever someone clicks on a hyperlink, sends e-mail, or uploads files, they are sending data upstream. Unfortunately, the upstream band is subject to all sorts of interference that can garble data. This shortcoming makes it close to impossible to use a coax-only cable system for two-way high-speed data traffic. Coaxial cable picks up noise from motors, CB radios, microwave ovens, and other appliances. Ham radio and VCRs can interfere tremendously with upstream data. Only CATV systems that have been upgraded to HFC plant are capable of high-speed two-way data transfer. The use of optical fiber reduces noise and increases the upstream bandwidth, facilitating upstream data transmission. Optical fiber can also transmit signals over much longer distances before requiring amplification. To send the data over the HFC network, laser transmitters convert signals sent from the head-end into optical signals. At each cable node, a laser receiver reconverts the signals so they can again be transmitted over tree-and branch configured coaxial cable plant, which goes into each individual house.

The most important factor in the deployment of two-way cable data services is the availability of high-quality two-way HFC plant. But upgrading to HFC is very expensive. It costs a cable company $200 – $250 per home to upgrade to HFC plant (Clark, 1999). Some cable companies that have not upgraded to HFC are offering cable modems that use the RF coaxial cable spectrum for fast downstream transmission and a traditional dial-up modem to handle upstream communications over the public telephone network. However, telephone-return modems do not provide some key benefits available with two-way cable modems, such as ultra-fast upstream speeds, constant connectivity, and not tying up a subscriber’s telephone line.

The Cable Modem Termination System (CMTS) is the central device for connecting the cable TV network to the Internet. The CMTS resides at the cable head-end. All the traffic to and from the cable modems in a cable data network travel through the CMTS. The CMTS connects to an IP router that sends and receives the data from the rest of the Internet. The CMTS interprets the data it receives from individual customers and keeps track of the services offered to each of them. The CMTS also modulates the data received from the Internet so that the head-end equipment can send it to a specific subscriber. Some Cable Modem Termination Systems provide the capability to let the MSO create different service packages depending on customers’ bandwidth needs (Clark, 1999). For example, a business service can be programmed by the CMTS to receive, as well as transmit, with high bandwidth, while a residential user may be configured by the CMTS to receive high bandwidth downstream traffic and limited to low bandwidth upstream traffic.

Cable data network architecture is similar to that of an Ethernet Local Area Network (LAN) (Halfhill, 1996). Current cable modem systems use Ethernet frame format for upstream and downstream transmissions. Basically, the cable operators are building some of the world’s largest “intranets”. Cable operators are concentrating on providing high-speed intranet access instead of straight Internet access because a network connection is only as fast as its slowest link. The head-end at most MSOs usually connect to the Internet via a T1 line, which has a data rate of 1.5 Mbps, significantly slower than a cable modem, which can theoretically deliver 30 Mbps (Brownstein, 1997). But, the Internet is only as fast as the slowest server. The benefit of a 1.5 Mbps T1 Internet connection 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.

Thus, the bottlenecks for Internet traffic in a cable network system are usually the gateway to the Internet, as well as the Internet itself. The cable companies’ solution to this problem is to move the Internet content closer to the subscriber. Many popular Web sites are cached on the cable operator’s server. So, when a cable modem subscriber goes to access a popular Web page, he will be routed to the server in the head-end at top-speed. If a site isn’t cached, however, the head-end server has to go looking for it out on the congested Internet, just as a conventional ISP’s server does. Cable modem subscribers should see high speeds (multiple MBit/sec) as long as they stay within the local cable network system. However, data transfer rates can slow down considerably when the user needs to venture out onto the Internet.

Like LANs, cable modem systems rely on a shared access platform (Ostergard, 1999). All the cable modem subscribers in a cable loop share available bandwidth to the head-end. Everyone on the local cable loop shares the same cable, which can carry about 30 Mbps total bandwidth. So as more subscribers hook up cable modems, more users will be sharing the same amount of bandwidth. Because of this, there are concerns that cable modem users will see poor performance as the number of subscribers increase on the network. If congestion does begin to occur due to high usage, the cable operators do have the capability to upgrade bandwidth capacity. A cable operator can easily allocate an additional 6 MHz video channel for high-speed data, doubling the downstream bandwidth available to users. Another option for adding bandwidth is to subdivide the physical cable network by running fiber-optic lines deeper into neighborhoods. This reduces the number of cable modems served by each node segment, and thus, increases the amount of bandwidth available to subscribers.

Based on bandwidth alone, it would seem that 200 cable modem subscribers sharing a 27-Mbps connection would each get approximately 135 Kbps of throughput, which is not much better than a 128-Kbps ISDN connection (Salent, 1999). However, 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 150 Kbps, each user is able to use all the bandwidth available during the short period of time they need to download their data packets.

Another bottleneck in cable data networking is the interconnection currently being used between the cable modem connect and the subscriber’s PC. A splitter is used to split the coax cable in the subscriber’s home into two lines, one for the TV set and another for the cable modem. Cable modems are external devices that connect to the coax cable by way of a standard “F” port connector (Barnes, 1997). Ethernet10Base-T twisted-pair wiring and RJ-45 connectors are used to connect the cable modem to the PC. The twisted pair wiring from the cable modem connects to the RJ-45 jack of a 10Base-T Ethernet card that has been installed in the subscriber’s PC. While cable modems can receive data at speeds up to 30 Mbps, the PC itself is limited by its Ethernet interface. Ethernet theoretically runs at 10 Mbps but is usually much slower, typically a maximum of 4 Mbps (Barnes, 1997). Because most home computers do not have an Ethernet card installed, cable operators must typically install one when connecting a new customer for cable modem service.

Suprisingly, this seemingly simple procedure presents a major bottleneck in the cable modem installation process. First, the user’s computer must have an ISA or PCI card slot available in their computer for the Ethernet adapter. Also, the card installation often requires configuration work within the operating system settings to prevent conflicts with other hardware devices. Due to the complexity, cable operators are often forced to send a specialized computer technician to handle Ethernet card installations, a process that can take more than 20 minutes per subscriber. Also, the requirement of opening each customer’s PC to install hardware creates a potential liability for the cable operator. Eager to avoid the Ethernet card headache, cable operators have searched for an alternate approach. The solution they have found lies is a device nicknamed a “dongle,” which is a Universal Serial Bus (USB) adapter. USB is a “plug-and-play” technology for connecting peripheral devices to computers, including modems, keyboards, printers and scanners (Van Matre, 1999). USB ports are external interfaces, so there’s no need to open the computer to install a USB device. External Universal Serial Bus (USB) modems and internal PCI modem cards are under development.

Cable modems receive data much faster than they can send it. Cable modem manufacturers have designed their modems to use less than a full 6 MHz carrier channel for upstream traffic. Typically 2 MHz wide bands are used for upstream data traffic. Cable TV networks transfer data using sophisticated digital modulation schemes which greatly increase the amount of data that can be sent. 64-state quadrature amplitude modulation (64 QAM) is digital modulation technique used for sending data downstream over a coaxial-only cable network. A single downstream 6 MHz television channel may support up to 27 Mbps of downstream data throughput from the cable head-end using 64 QAM transmission technology. HFC networks are able to implement 256 QAM, which supports 36 Mbps of downstream data throughput. However, 64QAM and 256 QAM are susceptible to interfering signals, making them unable to support noisy upstream transmissions. Quadrature Phase-Shift Keying (QPSK) is a digital frequency modulation technique used for sending data upstream over coaxial cable networks. QPSK is suitable for sending data upstream over a cable data network because it is fairly resistant to noise. Depending on the amount of cable RF spectrum allocated, upstream channels may deliver 500 Kbps to 10 Mbps, using 16 QAM or QPSK modulation techniques, with 16QAM being the fastest transfer method of the two (Salent, 1999).

Upstream cable modem traffic is always sent in bursts. Each modem transmits upstream bursts in time slots. These time slots can be designated as reserved, contention, or ranging slots. As the name implies, a reserved slot is a time slot that is reserved to a particular cable modem. No other cable modem is allowed to transmit in this reserved time slot. The CMTS allocates the reserved time slots to the various cable modems under its control through a bandwidth allocation algorithm. Reserved slots are normally used for longer data transmissions (Ostergard, 1998).

Contention time slots are open for all cable modems to transmit in. If two cable modems attempt to transfer simultaneously in the same contention slot, their packets collide and the data is lost. The CMTS detects the collision and signals that no data was received, which makes the each cable modems try to retransmit the data after waiting a random length of time.

Ranging is the process of automatically adjusting transmit levels and time offsets of individual cable modems. Ranging is performed to insure that bursts coming from different modems line up in the right time slots and are received at the same power level at the CMTS. A uniform power level for bursts reaching the CMTS facilitates collision detection. If two cable modems transmit at the same time, but one is much weaker than the other one, the CMTS will only detect the strong signal and assume that no collision took place. If the two colliding upstream signals are the same strength, they will both be detected by the CMTS as garbled. The CMTS will then know that a collision took place and will instruct the cable modems to retransmit their packets (Ostergard, 1998).

Ranging slots are also used to compensate for the differences in physical distance between the CMTS and each of the cable modems. The large geographic reach of a cable data network poses special problems as a result of the transmission delay between users close to head-end versus users at a distance from cable head-end. To compensate for cable losses and delay as a result of distance, the CMTS performs ranging, which allows each cable modem to assess its time delay in transmitting to the head-end. Large CATV networks can experience long delays in the millisecond range. The ranging protocol compensates for these delays by moving the “clock” of each cable modem forward or backward to make up for they delay. Ranging is performed periodically by the CMTS for each cable modem under its control. Three consecutive time slots are set aside for ranging. The CMTS commands the cable modem to transmit in the second time slot. The CMTS then measures the transmission time and gives the cable modem a small positive or negative correction value for its local clock. The two time slots on either side of the second time slot are required to insure that other traffic does not interfere with the ranging burst (Ostergard, 1998).

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