Cable Modem Systems            Ethernet Cable Cable TV Technology The New Market System Upgrades Cable Modems Standards Return Path Applications The Combined Corporate and End-User Networking Strategies Security on CATV

Cable Modem Systems and Technology

In the late 1970s, a major battle arose in the communications and computer industries. Convergence of the two industries was happening as a result of the implementation of local area networks (LANs). In the local networking arena, users began to implement solutions to their data connectivity needs within a localized environment. Two major choices were available for their installation of wiring: baseband coaxial cable and broadband coaxial cable.

The Ethernet Cable                                                                                            TOP

The baseband cable was based on Ethernet development, using a 20-MHz, 50-Wfl coax. Designed as a half-duplex operation, Ethernet enabled the end user to transmit digital data on the cable at speeds of up to 10 Mbps. Clearly, the 10 Mbps was maximum throughput, but was attractive in comparison to the technology of twisted pair at the time (telephone wires were capable of less than 1-Mbps bursty data). Moreover, the use of the baseband technology allowed the data to be digitally applied directly onto the cable system. No analog modulation was necessary to apply the data. It was dc input placed directly onto the cable. The signal propagates to both ends of the cable before another device can transmit. This is shown as a quick review in the picture below. To control the cable access, the attached devices used carrier sense multiple access with collision detection (CSMA/CD) as the access control. CSMA/CD allowed for the possibility that two devices may attempt to transmit on the cable at the same time, causing a collision and corruption of the actual data. Consequently, the cable had to be very controlled.

A second alternative at the time was to use the broadband coaxial cable, operating with a bandwidth of approximately 350 MHz on a 75 ohms cable. Broadband systems were well known because they were the same as CATV, which had surfaced in the early 1960s. Therefore, the technology was well deployed and commodity priced. Moreover, the 350-MHz capacity was attractive to the computer industry and the communications industry partisans. The issues began to surface quickly regarding the benefits and losses of using each technique. the picture below shows broadband coaxial cable.

The issue boiled down to analog versus digital and the baseband versus broadband implementations used to achieve this goal. This was a hot issue throughout both industries. The issue included using a broadband cable under the turf of the voice communications departments, whereas the baseband cables were under the primary control of the data processing departments. If one technology was chosen over another, the lines in the sand would be washed away and the convergence of voice and data would force the convergence of the two groups.

The issue was therefore not whether to use a cable, but what type of cable to use so that the LAN would fall under the correct jurisdictional authority within the organization. Unfortunately, control is not the goal of organizations, access and profitability are. As an industry, too much time was wasted over semantics. However, what ultimately rolled out of the bandwidth argument was that the baseband cable systems were better for the LAN. This was the decision of the 1980s, when all traffic on the LAN was geared to data only at speeds of 10 Mbps and less.

CATV has been around since the early 1960s. It is proven technology. In the early days of Ethernet, Digital Equipment Corporation (DEC) rolled out many systems using baseband (Ethernet) cable. However, some organizations needed more than just data on a large localized network. They worked with two major providers at the time to develop the interfaces for the broadband cable systems to attach an Ethernet to the CATV cable.

DEC developed several working arrangements with various suppliers to provide a frequency-agile modem (FAM) to work on the cable TV systems. The CATV companies did not necessarily own the broadband cable. Instead, this cable was locally owned in a high-rise office or a campus complex by the end user. The cable system provided high bandwidth, but it was very complex for the data and LAN departments to understand. The reason is obvious: the broadband coax operated using frequency-division multiplexing (analog techniques), which was beyond the scope of the LAN administrators and the data processing departments. The voice people knew of analog transmission, but they had a hard time with digital transmission in those days. A silent department was in the crux of all the arguments—the video departments within many organizations stayed out of the fight.

As DEC began to roll out various choices, the average user had to justify the connection of the analog technologies (used as a carrier) with the digital data demands of the LAN. Many organizations consolidated voice, data, LAN, and video on a single cable infrastructure on a campus. The industry came up with a specification for 10Broad36 to satisfy the LAN needs over a coax cable. 10Broad36 stands for 10 Mbps on a broadband cable 3,600 meters long. 

The data industry was distraught because this encouraged the use of an analog carrier system to move digital data. Over the years, however, this has been revisited several times. Wang Computer Company developed a proprietary cable system for connecting Wang systems using two broad-band coax cables. Technologically, the system was sound. However, the price and the proprietary nature of the Wang system forced its demise.

Later in the evolution of this service, the term broadband LAN became popularized. Ethernet grew to 100 Mbps and then onto the gigabit range. Justifying this high-speed communication met with resistance until the use of the various fiber and coaxial systems emerged. By taking a quantum leap in the industry, the data and voice departments saw the benefit and need of converging the two services to the desktop and offering voice and video over the LAN. The 10-Mbps Ethernet and coaxial cables could not handle this offering. Moreover, access to the Internet continued with demands to add speed and capacity (voice and video on the Internet). The industry began to seek a new method of bypassing the telephone companies' local loops. A technology already at the door, of course, was CATV. So a new idea emerged: use CATV to support high-speed Internet access and bypass the local loop from telephone companies. Hence, cable modem technology changed the way we will do business in the future.

The New Market                                                                                                TOP

The cable television companies are in the midst of a transition from their traditional core business of entertainment video programming to a position as a full-service provider of video, voice, and data telecommunications services. Among the elements that have made this transition possible are technologies such as the cable data modem. These companies have historically carried a number of data services. These have ranged from news and weather feeds, presented in alphanumeric form on single channels or as scrolling captions, to one-way transmission of data over classic cable systems.

Information providers are targeting upgraded cable network architecture as the delivery mechanism of choice for advanced high-speed data services. These changes stem from the commercial and residential data communications markets. The PC and LAN explosions in the early 1980s were rapidly followed by leaps in computer networking technology. More people now work from home and depend on connectivity from commercial online services (such as AOL, CompuServe, and Prodigy) to the global Internet.

Increased awareness has led to an increasing demand for data service and for higher speeds and enhanced levels of service. Cable is in a unique position to meet these demands. There appears to be no serious barriers to cable deployment of high-speed data transmission.

The cable platform is steadily evolving into a hybrid digital and analog transmission system. Cable television systems were originally designed to optimize the one-way, analog transmission of television programming to the home. The underlying coaxial cable, however, has enough bandwidth to support two-way transport of signals. The picture below  shows the hybrid network.

Growth in demand for Internet access and other two-way services has dovetailed with the trend within the industry to enhance existing cable systems with fiber-optic technology. Many cable companies are in the midst of upgrading the Hybrid Fiber/Coax (HFC) plant to improve the existing cable services and support data and other new services. Companies are taking different approaches to online service access. For some applications, customers may be accessing information stored locally at or near the cable headend or regional hub such as the @Home services being offered in many cities. This may be temporary until wide-area cable interconnections and expanded Internet backbone networks are in place to allow information access from any remote site.

Cable Modems                                                                                                            TOP

Digital data signals are carried over radio frequency (RF) carrier signals on a cable system. Digital data utilizes cable modems, devices that convert digital information into a modulated RF signal and convert RF signals back to digital information. The conversion is performed by a modem at the subscriber's premises and again by headend equipment handling multiple subscribers. See the picture below  for a block diagram of the cable modem.

A single CATV channel can support multiple data streams or multiple users using shared LAN protocols such as Ethernet, commonly in use in business office LANs today. This is where Ethernet networks can be applied to the broadband coaxial networks. Different modulation techniques are being tried to maximize the data speed that can be transmitted through a 6-MHz channel. Comparing the data traffic rates for different types of modems shows why the cable modem is so popular under today's environment. The table below shows a comparison of a file download of 500KB using different techniques.                                                                                                                    TOP

Careful traffic engineering is being performed on cable systems so that data speeds are maximized as customers are added. Just as office LANs are routinely subdivided to provide faster service for each individual user, cable data networks can also be custom tailored within each fiber node to meet customer demand. Multiple 6-MHz channels can be allocated to expand capacity as well.

Some manufacturers have designed modems providing asymmetrical capabilities, using less bandwidth for outgoing signals from the subscriber. CATV companies in some locations may not have completed system upgrades. Therefore, manufacturers have built migration strategies into such modems to allow for eventual transmission of broadband return signals when the systems are ready to provide such service and customers demand it. The table below  provides a representative sample of the way data speeds are provided on cable modems.

Standards                                                                                                        TOP

Modems are available today from a variety of vendors, each with their own unique technical approach. These modems are making it possible for cable companies to enter the data communications market now. In the long term, modem costs must drop and greater interoperability is desirable. Customers who buy modems that work in their current cable system need assurance that the modem will work if they move to a different geographic location served by a different cable company. Further, agreement on a standard set of specifications allows the market to enjoy economies of scale and drives down the price of each individual modem. Ultimately, these modems will be available as standard peripheral devices offered as an option to customers buying new personal computers at retail stores. The cable companies and manufacturers came together formally in December 1995 to begin working toward an open standard.

Leading U.S. and Canadian cable companies were involved in this development toward an open cable modem standard. Specifications were to be developed in three phases and then be presented to standards-setting bodies for approval as standards. Individual vendors were free to offer their own implementations with a variety of additional, competitive features, and future improvements. A data interoperability specification will comprise a number of interfaces. The resultant specification is called the Data Over Cable Service Interface Specification (DOCSIS).

Some interfaces reside within the cable network. Several of these system-level interfaces will also be specified to ensure interoperability.

Return Path                                                                                                    TOP

The portion of bandwidth reserved for return signals (from the customer to the cable network) is usually in the 5- to 40-MHz portion of the spectrum. This portion of the spectrum can be subject to ingress and other types of interference, so cable systems offering two-way data services have been designed to operate in this environment.

Industry engineers have assembled a set of alternative strategies for return-path operation. Dynamic frequency agility (shifting data from one channel to another when needed) may be designed into modems so that data signals may avoid unwanted interference as it arises. Other approaches utilize a gate that keeps the return path from an individual subscriber closed except for those times when the subscriber actually sends a return signal. Demarcation filters, different return laser types, and reduced node size are among the other approaches, each involving tradeoffs between capital cost and maintenance effort and cost.

Return-path transmission issues have already been the subjects of two years of lab and field-testing and product development. The full two-way capability of the coaxial cable already passing most U.S. homes is now being utilized in many areas and will be available in most cable systems soon. Full activation of the return path in any given location will depend on individual cable company circumstances, ranging from market analysis to capital availability.

The spectrum used for the forward and reverse paths is shown in the picture below as an indication of the frequencies available and the overall management of the system. This also shows that additional 6-MHz channels can be set aside to handle the data traffic on the cable modems and the cables themselves.

Applications                                                                                                        TOP

Cable modems open the door for customers to enjoy a range of high-speed data services, all at speeds hundreds of times faster than telephone modem calls. Subscribers can be fully connected, 24 hours a day, to services without interfering with cable television service or phone service. Among these services are

• Information services Access to shopping, weather maps, household bill paying, and so forth

• Internet access   Electronic mail, discussion groups, and the World Wide Web

• Business applications Interconnecting LANs or supporting collaborative work

• Cable commuting   Enabling the already popular notion of working from home

• Education  Enabling students to continue to access educational resources from home

The promises of advanced telecommunications networks, once more hype than fact, are now within reach. Cable modems and other technology are being deployed to make it happen. Regardless of the technology selected, the main goal is to get the high-speed data communications on the cable adjacent to the TV and entertainment. This gives the CATV companies the leverage to act in an arbitrage situation, competing with the local telephone companies who have dragged their feet in moving high-speed services to the consumer's door.

The use of a single PC on a cable system is okay for the telecommuter (or cable commuter now), but what about the small office or home office where more than a single PC is connected? The picture below  shows an example of various ways the CATV connection can be accomplished. This figure uses an example of local home networking with two PCs connected to a single cable modem. Most of the providers have instructions on how to accomplish this and require a home user (or small user) to download additional software to accommodate the dual connection on a single modem. The second alternative is to have a router connected to the cable modem, such as a branch-office router. The network is attached to the router, and the router is responsible for handling the dispersing of the traffic onto the cable system. This can be a very effective use of the link. Next in the figure is a connection to a hub, such as a 10- or 100BASE-T connection into a LAN hub. CATV providers state this is not supported and will likely not work. However, it has been done and works well for a small office or home office connection. Using the connection directly into the hub from the cable modem makes the modem available to more users instead of just a single PC. The hub will act as a bridging function onto the modem and concentrate the traffic through the individual devices. These configurations all work, but the providers do not support problems if they arise. You are on your own if it does not work.

When CATV systems are used, they are shared high-speed Ethernet backbone access to the Internet or other connections. One must be aware that on a shared cable, the PC is a peer to all others on the same cable, although they are in physically different locations. With 500 connections, many people will acquire the service from the CATV suppliers. The CATV company installs according to the appropriate technology, not according to security parameters. This is okay because they are merely providing the bandwidth to gain access. It is the end user's responsibility to turn off all the leaks in the local system (the PC). By default, when you run the Microsoft Windows environment and the appropriate networking software, the shares on the PC are turned on. The end user must therefore go in and turn them off. This means that if the shared services are not turned off, a user down the street or across the town can double-click on the network neighborhood icon and see all the other PCs connected to the cable. Not only can he or she see the devices, but the user can also double-click on the PC and see the resources available on that PC. From there, when a remote device has double-clicked on your PC, the user can open your drives and see your files. Unless some provisions have been taken to block this access, the intruder (used as a method of entry only) can read, write, edit, or delete your files. Worse yet, while the intruder is on your system, you do not even know he or she is there.

Many users who have cable modem service from the cable companies are not aware of the risks. Worse, the installation personnel on these systems do not totally understand or forget to point out the risks. Therefore, the user leaves the PC on 100 percent of the time, day and night, leaving access to the computer totally available. The cable modem is available 100 percent of the time, making the computer a target for hackers and the mischievous, without the permission or the knowledge of the penetrated computer owner.

Be aware of the risks and find out how to shut these open doors before leaving your computer on the network. Do not assume you are secure just because you shut your system off when not using it. When you do log on, you are exposed, and the perpetrator can get on your system while you are on it too.

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