Cable Modems: Cable TV Meets the Internet
John G. Shaw
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).
The cable modem itself is comprised of the following major components; the Tuner, the Demodulator, the Burst Modulator, the Media Access Control (MAC) Mechanism, the Interface, and the Central Processing Unit (CPU). External cable modems have an on-board CPU to handle instruction processing. Internal cable modems are being developed that will use the PC’s CPU much like the way internal dial-up modems do. The cable modem’s tuner connects directly into the CATV outlet. For two-way data transfer, a tuner must have a two-way diplexer to break out the upstream and downstream traffic (Ostergard, 1999).
The Cable Modem’s Demodulator receives the downstream IF signal from the tuner and, as the name implies, demodulates it. The Demodulator is composed of an A/D converter, a QAM64/256 demodulator, MPEG frame synchronization, and Reed Solomon error correction. Downstream data is framed according to the MPEG-TS (transport stream) specification. The frame format for this specification is a 188/204 byte block, with a single fixed sync byte in front of each block. The Reed-Solomon error correction algorithm reduces the block size from 204 bytes to 188 bytes, which leaves 187 bytes for MPEG header and payload (Ostergard, 1998).
The upstream data traffic is modulated by the cable modem’s Burst Modulator. The Burst Modulator feeds the cable modem’s Tuner, performs Reed Solomon encoding of each downstream burst, performs QPSK or QAM16 modulation on the designated upstream frequency, and D/A conversion. The Burst Modulator’s output signal is fed through a variable output amplifier, so the signal level can be adjusted to compensate for cable loss(Ostergard, 1998).
Both the upstream and downstream traffic travels through the cable modem’s Media Access Control mechanism. The MAC mechanism’s functions are fairly complex. The MAC mechanism’s main purpose is to implement MAC protocols under the direction of the CMTS. MAC protocols are used to time-share the cable media among the various cable modems in a cable data network. The MAC processes can be implemented in hardware, or a combination of software and hardware. Both the CMTS and the MAC mechanism implement MAC protocols to perform ranging procedures to compensate for cable media delays and line losses. The CMTS also interfaces with the MAC mechanism in each cable modem to assign upstream frequencies and upstream time slots. The CMTS controls data traffic on the cable network through the use of a special control channel. When the cable modem is turned on, it scans all its assigned channels to locate the control channel, which can be identified by its unique header signal. The CMTS control channel tells each subscriber’s cable modem when it can transmit, on which frequency band, and for how long. The data that passes through the MAC mechanism goes into the computer interface of the cable modem, which is 10Base-T Ethernet for the majority of current cable modems (Ostergard, 1998).
A cable data system is comprised of many different technologies and standards. The first generation of cable modems used various proprietary protocols that made it impossible for the CATV network operators to use multiple vendors cable modems on the same system. 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 (Van Matre, 1999).
The cable operators were anxious to get into the high-speed data business as soon as possible, and became impatient waiting IEEE 802.14. So, the cable operators combined their purchasing power to jump-start the standards process. In January 1996, cable operators 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 1996 (Van Matre, 1999).
MCNS released its Data Over Cable System Interface Specification (DOCSIS) for cable modem products to vendors in March 1997. Afterwards, IEEE released its standard, but by that time, the cable operators had already wed themselves to the MCNS DOCSIS standard. To date, more than 20 vendors have announced plans to build products based on the MCNS DOCSIS standard. The cable companies MediaOne (formerly Continental Cablevision), Rogers Cablesystems, and CableLabs, also signed on to the 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. Even though DOCSIS is the dominant US cable data network standard, it has yet to be formally certified by any independent standards body. The DOCSIS requirements are now managed by CableLabs. A CableLabs certification program administers vendor equipment compliance to the DOCSIS requirements and interoperability tests. Standardized DOCSIS cable modems started shipping in limited quantities in the third and fourth 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 (Van Matre, 1999).
The differing cable modem specifications advocated by IEEE 802.14 and MCNS reflect the priorities of each organization. IEEE 802.14 is a vendor-driven group, and 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 were in an extreme hurry to get into the high-speed data market. To achieve its objectives, MCNS sought to minimize technical complexity and develop a technology solution that was adequate for its members’ needs (Van Matre, 1999).
Under the MCNS DOCSIS specifications, to enable transparent transfer of Internet Protocol messages across a cable system, three of the protocol layers of the International Organization for Standardization’s (ISO) 7 Layer Open System Interconnect (OSI) Reference Model are used. These three layers are the Network Layer, Data Link Layer and the Physical Layer. The functions of each layer are described below (Salent,1999).
Network Layer The Network Layer uses the Internet Protocol (IP), which enables IP traffic to be seamlessly delivered over the cable modem platform to end-users.
Data Link Layer The Data Link Layer is comprised of three sublayers: a Logical Link Control (LLC) Sublayer, which conforms to Ethernet standards, a Link-Security Sublayer that supports the basic needs of privacy, authorization, and authentication, and a Media Access Control Sublayer, suitable for cable system operation, that supports variable-length protocol data units (PDU).
Physical Layer The Physical Layer which defines the upstream and downstream modulation format. There is minimal coupling between physical and higher layers which accommodates the incorporation of future physical layer technologies.
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 and 16QAM, virtually the same as MCNS (Van Matre, 1999).
The MAC sublayer provides the general requirements for many cable modem subscribers to share a single upstream data channel for transmission to the network. These requirements include collision detection and retransmission, timing and synchronization, bandwidth allocation to cable modems at the control of CMTS, error detection, error handling, and error recovery, as well as procedures for registering new cable modems.
For the MAC sublayer, 802.14 specified Asynchronous Transfer Mode (ATM) as its default solution from the head-end 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 (Van Matre, 1999).
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.
Initially, cable operators were solely 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. However, QoS guarantees were added under DOCSIS version 1.1 (Van Matre, 1999)..
The Link-Security Sublayer insures the privacy of cable modem user data by encrypting link-layer data between cable modems and CMTS. Security is a major concern with cable modems because the total bandwidth is shared by all cable modems in a local loop. This means all downstream data is received by all the cable modems in a loop. Each cable modem uses the Ethernet frame format to filter out the data it needs from the downstream of data. The CMTS encrypts the payload data of link-layer frames transmitted on the cable network. The Security Association (SA) assigns a set of security parameters including keying data to a cable modem. All of the upstream transmissions from a cable modem travel across a single upstream data channel and are received by the CMTS. In the downstream data channel, the CMTS must select appropriate the appropriate SA parameters based on the destination address of the target cable modem. Baseline privacy employs the data encryption standard (DES) block cipher for encryption of user data. The encryption can be integrated directly within the MAC hardware and software interface.
Cable modem technology offers tremendous advantages. A cable modem user can get the performance of a T-1 line at a fraction of the cost. Current cable modem service connection speeds are much greater than that of a dial-up ISP at roughly the same price. Dial-up ISPs offer 56 Kbps connections for around $20 per month. Emerald Coast Cable TV, the MSO for the Fort Walton Beach, Florida area currently charges $30 a month for unlimited Internet access with a cable modem. A cable modem offers speeds between 500 Kbps to 1.5 Mbps. Even the low end of this range is an order of magnitude faster than a 56 Kbps connection. Cable modems currently retail for approximately $300, but the prices are forecast to drop rapidly. Emerald Coast Cable TV charges $15 a month to rent a cable modem.
In addition to their blazing speed, another advantage of cable modems is constant connectivity. Cable modems are online as soon as the computer is turned on. The cable modem user does not have to dial-in to begin an online session. There are no busy signals and tied up telephone lines like there are with dial-up modems. Another advantage of cable modem technology is that it has tremendous upgrade capacity. Twisted pair telephone lines have already used up a sizeable portion of their inherent bandwidth capacity (Halfhill, 1996). On the other hand, MSOs have already created a tremendous amount of shared bandwidth with their upgrades to HFC networks. Furthermore, as the number of cable modem users grows, and too many users try to share the available bandwidth, the cable operators have the capability to add more. Many MSOs have six optical fibers in their cable bundles and are only currently using two of them. The MSOs could “light up” these unused fibers and greatly increase the amount of bandwidth to be shared. Another option is to allocate additional 6 MHz channels for high-speed data. Still, 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 (Medin, 1999).
Another advantage of cable modems is that they are not the traffic bottlenecks on a cable data network. The cable modems have tremendous throughput capacity. It is the other components of the cable network and the Internet itself that slow traffic down. Some of the items that slow down a cable data network are the 1.5 Mbps T-1 cable Internet connection, the Ethernet PC interface card, current PC technology, and plain old Internet congestion. But all these items are being upgraded to allow ultra-fast data traffic. The Internet is going through growing pains and there is still a lot of growing to do (Medin, 1999).
Cable modem technology does have some disadvantages. Cable data networks are still in their infancy and are going to experience some growing pains as the rest of the Internet is upgraded to handle more traffic. Most of the disadvantages of cable data systems the result of legacy issues within the CATV systems. However, the cable companies were aware of these disadvantages before they got into the business, and they know how to overcome the problems. The solutions to the most of the problems of cable data networks are known. But, it will take a great deal of money to implement these solutions. The MSOs have invested already invested billions in creating cable data networks, and they are willing to invest more to remain competitive with the telephone industry.
The biggest legacy issue induced problem facing cable modem technology is that CATV systems were originally designed to carry TV programming from the cable operator to the subscriber’s home. The CATV systems were never intended to be used for two-way or point-to-point communications. The MSOs have the capability to upgrade their systems to deliver two-way data communications by switching to HFC plant, but there are no easy solutions to the point-to-point communication dilemma. The tree and branch configuration of a CATV system is not conducive to point-to-point communication. There was no need for sophisticated switching systems, like those used in a telephone system, when the CATV systems were first developed. Unfortunately for the MSOs, a switching network is essential for point-to-point communication.
Another disadvantage of cable data systems is slow upstream communication. But, fortunately for the MSOs, the high-speed telephone data technologies also have this problem. The root of the problem is that CATV systems were primarily designed for downstream delivery of TV programming. So, the majority of the coaxial cable bandwidth of a CATV system was dedicated to downstream traffic. There was very little bandwidth set aside for upstream traffic. To further complicate matters, the cable modem manufacturers are making their products use much narrower bandwidth, 2 MHz, for upstream communication.
The issue of shared bandwidth is also a disadvantage for the cable data systems. If the MSOs do not upgrade capacity as more people sign up, cable modem access speeds may become slower. Upgrading capacity is a fairly straightforward exercise for the MSOs. It is just a question of if the MSO’s are willing to invest the money to make the upgrades. Another shared bandwidth issue is security. All the downstream traffic in a cable data network goes to all the cable modems in the network and all the upstream bandwidth is shared by all the cable modems in a local loop. IP and network protocols are used to make sure traffic is secure and routed properly. Many of the non-standardized first generation cable modems did not have encryption capabilities, so users were able to access other user’s traffic. Second generation modems and CMTS equipment are built according to the DOCSIS specification, which contains security and data encryption requirements. Still another disadvantage of using a cable modem is that the user does not have a choice of ISPs. The only ISP available to a cable modem user is the local cable company. This is because cable TV lines do not have ‘common-carrier’ status as do phone lines. However, there are some efforts underway to change this.
The recommended application for cable modems is for private, not commercial use. Because of the shared bandwidth issue, most MSOs will not allow a subscriber to host a server with their cable modem. A cable modem subscriber who tried to host a server would use a great deal of the shared bandwidth, which would be detrimental to the other subscribers on the loop. But, for a private user, cable modem systems definitely offer the most bandwidth for the money. There are approximately 75 million cable TV subscribers in North America (Medin, 1999). As of August 1999, cable operators were offering two-way high-speed Internet service to about half of these customers (Medin, 1999). More than one million of these subscribers have signed up for cable modem service.
The future prospects of the cable modem are excellent. The current cable modem technology is stable and improvements are being developed at a rapid pace. Soon there will be internal cable modems on the market that will not need the Ethernet interface. There is tremendous upgrade capacity available to the MSOs and it appears that the MSOs are willing to make the capital investments required to make the upgrades. The MSOs have already moved the fiber-optic cable close to the end-users. It remains to be seen if the MSOs will move the fiber-optic cable all the way to the end-user. The coax portion of the HFC network that goes the “final mile” to the home is limiting factor to broad bandwidth. People are forecasting that the house of tomorrow will be totally networked. A great deal of bandwidth, more than there is available now, it going to be needed to accomplish this. Right now, there is speculation that as more and more people start using broadband access, there will come a point where there are so many high-speed users that the Internet’s backbone will be “broken”. But, this will not happen if backbone capacity is upgraded. If the Internet backbone is not upgraded, upgrading the cable data networks will be like building a new super fast off-ramp to the highway. Cable modem users will be able to get on the Internet highway much faster. But, they will find a traffic jam on the highway when they get there. Fortunately, the costs associated with upgrading the Internet backbone will be small in comparison to the cost of reworking other parts of the Internet to improve performance. Also, there is the issue of cable modems changing Internet usage patterns. As networks perform better, people are going to demand more and more out of them. To use the overused highway analogy, current Internet traffic is composed generally of small cars (e-mail) and some trucks (graphics) and only rarely a convoy of trucks (large audio/video files or programs). Traffic flows smoothly most of the time. However, if the composition of Internet traffic shifts heavily towards audio and video files, it would mean more and more truck convoys will be on the highway, taking up whole lanes and reducing the space available for other traffic. The future looks bright for cable modems, but the powers that be must insure the Internet can handle the increased traffic that will result from the growth of cable data networks.
The future holds a lot of competition for the cable modem industry. There are other high-speed data technologies on the horizon which are going to be in fierce competition with cable data networks. Right now, cable modems offer the simplest, fastest and cheapest broadband access. But this could easily change in the future. The phone companies are looking to give the cable companies some more competition in the high-speed Internet access market. Asynchronous Digital Subscriber Lines (ADSL) are just starting to come on line. ADSL speed should be comparable to cable modems. However, speed will depend on the user’s distance to the next telephone company switch. A potential ADSL customer can no be any further from 15,000 feet from the switch if 26-gauge twisted-pair wire is used. However, this distance increases to 18,000 feet if 24-gauge wire is used (Clark, 1999). With ADSL, the closer one is to the telco switch, the higher the connection speed. Like a traditional dial-up modem, ADSL will provide the user with a dedicated, always on, line to his ISP. Because there is a dedicated connection, there will not be any shared bandwidth concerns like there are for cable modems. ADSL uses filters to split the existing copper phone line into three separate frequency channels. A 0- to 4-KHz band carries the traditional analog telephone signal and a higher frequency band is used for upstream data transfers at rates up to 640 Kbps. The rest of the band is used to transmit data downstream, either from the Internet or as video-on-demand, at speeds of up to 8 Mbps. ADSL is likely to be more expensive than cable data access and it is too early to determine how fast ADSL will be in the real world (Clark, 1999)
Another source of competition for the MSOs is a satellite-based, high-speed Internet service called DirecPC, which is available through Hughes Network Systems (Clark, 1999). For $499, U.S. customers can purchase a kit that includes a satellite receiving dish, a PC interface card, and software. This price does not installation. With this service, upstream transmissions are sent via dial-up connections to the ISP. The upstream information requests go to the DirecPC satellite network operations center, which broadcasts the requested data down to the users from a Galaxy IV Satellite at rates up to 400 Kbps. This service starts at $9.95 per month, but goes up to $129.95 per month for the premium package. With the basic $9.95 rate, the user will also be charged 60 cents for each megabyte of information downloaded from the satellite (Clark, 1999).
Another future source of competition for cable modems is wireless cable. Wireless cable sounds like an oxymoron, but wireless cable TV is already an established business. Wireless cable works by using a satellite system to broadcast multichannel TV programming to homes equipped with dish antennas that operate in the 2- to 2.6-GHz range (Clark, 1999). The system can also be used to transmit data as well. The downstream data from the satellite is received by the dish, which passes the information through coaxial cable to a special modem that converts the signal into IP data the computer can computer understand. This process works for downstream data, but upstream data transmission involves using a standard analog telephone line. The industry is working to establish a wireless upstream path, but a viable solution is at least two years away (Clark 1999).
Cable modems are the wave of the future. Of all the current broadband technologies, cable modems offer the most benefits. Cable modems are simple to operate, do not require a great deal of extraneous hardware, offer the fastest connection speeds, and are the least expensive of all the alternatives. There are some disadvantages to cable modems and cable data systems, but there are solutions to these disadvantages. Just as computers have already done, networking will become a large part of everyday life. Better networking means having more speed, and having more speed takes having more bandwidth, and cable data networks provide that bandwidth. As more people begin to use cable modems, the MSOs will have to upgrade the capacity of the system. This means they will have increase the fiber-optic portions of the HFC networks. As the fiber-optic portions of the MSO’s HFC networks grow larger, fiber-optic cable will get closer and closer to the home. Cable modems are the harbinger of things to come because they will drive the need for fiber-optic to the home. Fiber-optic to the home is only a matter of time, and when this happens the Internet will become even bigger than anyone has imagined.
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