CABLE TELEVISION




CABLE TELEVISION

We have now studied both the fixed and wireless telephone systems in a fair amount of detail. Both will clearly play a major role in future networks. But there is another major player that has emerged over the past decade for Internet access: cable television networks. Many people nowadays get their telephone and Internet service over cable. In the following sections we will look at cable television as a network in more detail and contrast it with the telephone systems we have just studied. Some relevant references for more information are Donaldson and Jones (2001), Dutta-Roy (2001), and Fellows and Jones (2001).

Community Antenna Television

Cable television was conceived in the late 1940s as a way to provide better reception to people living in rural or mountainous areas. The system initially consisted of a big antenna on top of a hill to pluck the television signal out of the air, an amplifier, called the headend, to strengthen it, and a coaxial cable to deliver it to people’s houses, as illustrated in Fig. 2-50.

CABLE TELEVISION

In the early years, cable television was called Community Antenna Television. It was very much a mom-and-pop operation; anyone handy with electronics could set up a service for his town, and the users would chip in to pay the costs. As the number of subscribers grew, additional cables were spliced onto the original cable and amplifiers were added as needed. Transmission was one way, from the headend to the users. By 1970, thousands of independent systems existed.

In 1974, Time Inc. started a new channel, Home Box Office, with new content (movies) distributed only on cable. Other cable-only channels followed, focusing on news, sports, cooking, and many other topics. This development gave rise to two changes in the industry. First, large corporations began buying up existing cable systems and laying new cable to acquire new subscribers. Second, there was now a need to connect multiple systems, often in distant cities, in order to distribute the new cable channels. The cable companies began to lay cable between the cities to connect them all into a single system. This pattern was analogous to what happened in the telephone industry 80 years earlier with the connection of previously isolated end offices to make long-distance calling possible.

Internet over Cable

Over the course of the years the cable system grew and the cables between the various cities were replaced by high-bandwidth fiber, similar to what happened in the telephone system. A system with fiber for the long-haul runs and coaxial cable to the houses is called an HFC (Hybrid Fiber Coax) system. The electrooptical converters that interface between the optical and electrical parts of the system are called fiber nodes. Because the bandwidth of fiber is so much greater than that of coax, a fiber node can feed multiple coaxial cables. Part of a modern HFC system is shown in Fig. 2-51(a).

Over the past decade, many cable operators decided to get into the Internet access business, and often the telephony business as well. Technical differences between the cable plant and telephone plant had an effect on what had to be done to achieve these goals. For one thing, all the one-way amplifiers in the system had to be replaced by two-way amplifiers to support upstream as well as downstream transmissions. While this was happening, early Internet over cable systems used the cable television network for downstream transmissions and a dialup connection via the telephone network for upstream transmissions. It was a clever workaround, but not much of a network compared to what it could be.

However, there is another difference between the HFC system of Fig. 2-51(a) and the telephone system of Fig. 2-51(b) that is much harder to remove. Down in the neighborhoods, a single cable is shared by many houses, whereas in the telephone system, every house has its own private local loop. When used for television broadcasting, this sharing is a natural fit. All the programs are broadcast on the cable and it does not matter whether there are 10 viewers or 10,000 viewers. When the same cable is used for Internet access, however, it matters a lot if there are 10 users or 10,000. If one user decides to download a very large file, that bandwidth is potentially being taken away from other users. The more users there

are, the more competition there is for bandwidth. The telephone system does not have this particular property: downloading a large file over an ADSL line does not reduce your neighbor’s bandwidth. On the other hand, the bandwidth of coax is much higher than that of twisted pairs, so you can get lucky if your neighbors do not use the Internet much.

CABLE TELEVISION

are, the more competition there is for bandwidth. The telephone system does not have this particular property: downloading a large file over an ADSL line does not reduce your neighbor’s bandwidth. On the other hand, the bandwidth of coax is much higher than that of twisted pairs, so you can get lucky if your neighbors do not use the Internet much.

The way the cable industry has tackled this problem is to split up long cables and connect each one directly to a fiber node. The bandwidth from the headend to each fiber node is effectively infinite, so as long as there are not too many subscribers on each cable segment, the amount of traffic is manageable. Typical cables nowadays have 500–2000 houses, but as more and more people subscribe to Internet over cable, the load may become too great, requiring more splitting and more fiber nodes.

Spectrum Allocation

Throwing off all the TV channels and using the cable infrastructure strictly for Internet access would probably generate a fair number of irate customers, so cable companies are hesitant to do this. Furthermore, most cities heavily regulate what is on the cable, so the cable operators would not be allowed to do this even if they really wanted to. As a consequence, they needed to find a way to have television and Internet peacefully coexist on the same cable.

The solution is to build on frequency division multiplexing. Cable television channels in North America occupy the 54–550 MHz region (except for FM radio, from 88 to 108 MHz). These channels are 6-MHz wide, including guard bands, and can carry one traditional analog television channel or several digital television channels. In Europe the low end is usually 65 MHz and the channels are 6–8 MHz wide for the higher resolution required by PAL and SECAM, but otherwise the allocation scheme is similar. The low part of the band is not used. Modern cables can also operate well above 550 MHz, often at up to 750 MHz or more. The solution chosen was to introduce upstream channels in the 5–42 MHz band (slightly higher in Europe) and use the frequencies at the high end for the downstream signals. The cable spectrum is illustrated in Fig. 2-52.

CABLE TELEVISION

 

Note that since the television signals are all downstream, it is possible to use upstream amplifiers that work only in the 5–42 MHz region and downstream amplifiers that work only at 54 MHz and up, as shown in the figure. Thus, we get an asymmetry in the upstream and downstream bandwidths because more spectrum is available above television than below it. On the other hand, most users want more downstream traffic, so cable operators are not unhappy with this fact of life. As we saw earlier, telephone companies usually offer an asymmetric DSL service, even though they have no technical reason for doing so. In addition to upgrading the amplifiers, the operator has to upgrade the headend, too, from a dumb amplifier to an intelligent digital computer system with a high-bandwidth fiber interface to an ISP. Often the name gets upgraded as well, from ‘‘headend’’ to CMTS (Cable Modem Termination System). In the following text, we will refrain from doing a name upgrade and stick with the traditional ‘‘headend.’’

Cable Modems

Internet access requires a cable modem, a device that has two interfaces on it: one to the computer and one to the cable network. In the early years of cable Internet, each operator had a proprietary cable modem, which was installed by a cable company technician. However, it soon became apparent that an open standard would create a competitive cable modem market and drive down prices, thus encouraging use of the service. Furthermore, having the customers buy cable modems in stores and install them themselves (as they do with wireless access points) would eliminate the dreaded truck rolls.

Consequently, the larger cable operators teamed up with a company called CableLabs to produce a cable modem standard and to test products for compliance. This standard, called DOCSIS (Data Over Cable Service Interface Specification), has mostly replaced proprietary modems. DOCSIS version 1.0 came out in 1997, and was soon followed by DOCSIS 2.0 in 2001. It increased upstream rates to better support symmetric services such as IP telephony. The most recent version of the standard is DOCSIS 3.0, which came out in 2006. It uses more bandwidth to increase rates in both directions. The European version of these standards is called EuroDOCSIS. Not all cable operators like the idea of a standard, however, since many of them were making good money leasing their modems to their captive customers. An open standard with dozens of manufacturers selling cable modems in stores ends this lucrative practice.

The modem-to-computer interface is straightforward. It is normally Ethernet, or occasionally USB. The other end is more complicated as it uses all of FDM, TDM, and CDMA to share the bandwidth of the cable between subscribers. When a cable modem is plugged in and powered up, it scans the downstream channels looking for a special packet periodically put out by the headend to provide system parameters to modems that have just come online. Upon finding this packet, the new modem announces its presence on one of the upstream channels. The headend responds by assigning the modem to its upstream and downstream channels. These assignments can be changed later if the headend deems it necessary to balance the load.

The use of 6-MHz or 8-MHz channels is the FDM part. Each cable modem sends data on one upstream and one downstream channel, or multiple channels under DOCSIS 3.0. The usual scheme is to take each 6 (or 8) MHz downstream channel and modulate it with QAM-64 or, if the cable quality is exceptionally good, QAM-256. With a 6-MHz channel and QAM-64, we get about 36 Mbps. When the overhead is subtracted, the net payload is about 27 Mbps. With QAM256, the net payload is about 39 Mbps. The European values are 1/3 larger.

For upstream, there is more RF noise because the system was not originally designed for data, and noise from multiple subscribers is funneled to the headend, so a more conservative scheme is used. This ranges from QPSK to QAM-128, where some of the symbols are used for error protection with Trellis Coded Modulation. With fewer bits per symbol on the upstream, the asymmetry between upstream and downstream rates is much more than suggested by Fig. 2-52

TDM is then used to share bandwidth on the upstream across multiple subscribers. Otherwise their transmissions would collide at the headend. Time is divided into minislots and different subscribers send in different minislots. To make this work, the modem determines its distance from the headend by sending it a special packet and seeing how long it takes to get the response. This process is called ranging. It is important for the modem to know its distance to get the timing right. Each upstream packet must fit in one or more consecutive minislots at the headend when it is received. The headend announces the start of a new round of minislots periodically, but the starting gun is not heard at all modems simultaneously due to the propagation time down the cable. By knowing how far it is from the headend, each modem can compute how long ago the first minislot really started. Minislot length is network dependent. A typical payload is 8 bytes.

During initialization, the headend assigns each modem to a minislot to use for requesting upstream bandwidth. When a computer wants to send a packet, it transfers the packet to the modem, which then requests the necessary number of minislots for it. If the request is accepted, the headend puts an acknowledgement on the downstream channel telling the modem which minislots have been reserved for its packet. The packet is then sent, starting in the minislot allocated to it. Additional packets can be requested using a field in the header.

As a rule, multiple modems will be assigned the same minislot, which leads to contention. Two different possibilities exist for dealing with it. The first is that CDMA is used to share the minislot between subscribers. This solves the contention problem because all subscribers with a CDMA code sequence can send at the same time, albeit at a reduced rate. The second option is that CDMA is not used, in which case there may be no acknowledgement to the request because of a collision. In this case, the modem just waits a random time and tries again. After each successive failure, the randomization time is doubled. (For readers already somewhat familiar with networking, this algorithm is just slotted ALOHA with binary exponential backoff. Ethernet cannot be used on cable because stations cannot sense the medium. We will come back to these issues in Chap. 4.)

The downstream channels are managed differently from the upstream channels. For starters, there is only one sender (the headend), so there is no contention and no need for minislots, which is actually just statistical time division multiplexing. For another, the amount of traffic downstream is usually much larger than upstream, so a fixed packet size of 204 bytes is used. Part of that is a ReedSolomon error-correcting code and some other overhead, leaving a user payload of 184 bytes. These numbers were chosen for compatibility with digital television using MPEG-2, so the TV and downstream data channels are formatted the same way. Logically, the connections are as depicted in Fig. 2-53.

CABLE TELEVISION

ADSL Versus Cable

Which is better, ADSL or cable? That is like asking which operating system is better. Or which language is better. Or which religion. Which answer you get depends on whom you ask. Let us compare ADSL and cable on a few points. Both use fiber in the backbone, but they differ on the edge. Cable uses coax; ADSL uses twisted pair. The theoretical carrying capacity of coax is hundreds of times more than twisted pair. However, the full capacity of the cable is not available for data users because much of the cable’s bandwidth is wasted on useless stuff such as television programs.

In practice, it is hard to generalize about effective capacity. ADSL providers give specific statements about the bandwidth (e.g., 1 Mbps downstream, 256 kbps upstream) and generally achieve about 80% of it consistently. Cable providers may artificially cap the bandwidth to each user to help them make performance predictions, but they cannot really give guarantees because the effective capacity depends on how many people are currently active on the user’s cable segment. Sometimes it may be better than ADSL and sometimes it may be worse. What can be annoying, though, is the unpredictability. Having great service one minute does not guarantee great service the next minute since the biggest bandwidth hog in town may have just turned on his computer.

As an ADSL system acquires more users, their increasing numbers have little effect on existing users, since each user has a dedicated connection. With cable, as more subscribers sign up for Internet service, performance for existing users will drop. The only cure is for the cable operator to split busy cables and connect each one to a fiber node directly. Doing so costs time and money, so there are business pressures to avoid it.

As an aside, we have already studied another system with a shared channel like cable: the mobile telephone system. Here, too, a group of users—we could call them cellmates—share a fixed amount of bandwidth. For voice traffic, which is fairly smooth, the bandwidth is rigidly divided in fixed chunks among the active users using FDM and TDM. But for data traffic, this rigid division is very inefficient because data users are frequently idle, in which case their reserved bandwidth is wasted. As with cable, a more dynamic means is used to allocate the shared bandwidth.

Availability is an issue on which ADSL and cable differ. Everyone has a telephone, but not all users are close enough to their end offices to get ADSL. On the other hand, not everyone has cable, but if you do have cable and the company provides Internet access, you can get it. Distance to the fiber node or headend is not an issue. It is also worth noting that since cable started out as a television distribution medium, few businesses have it.

Being a point-to-point medium, ADSL is inherently more secure than cable. Any cable user can easily read all the packets going down the cable. For this reason, any decent cable provider will encrypt all traffic in both directions. Nevertheless, having your neighbor get your encrypted messages is still less secure than having him not get anything at all.

The telephone system is generally more reliable than cable. For example, it has backup power and continues to work normally even during a power outage. With cable, if the power to any amplifier along the chain fails, all downstream users are cut off instantly.

Finally, most ADSL providers offer a choice of ISPs. Sometimes they are even required to do so by law. Such is not always the case with cable operators.

The conclusion is that ADSL and cable are much more alike than they are different. They offer comparable service and, as competition between them heats up, probably comparable prices.



Frequently Asked Questions

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Ans: The traditional telephone system, even if it someday gets multigigabit end-toend fiber, will still not be able to satisfy a growing group of users: people on the go. People now expect to make phone calls and to use their phones to check email and surf the Web from airplanes, cars, swimming pools, and while jogging in the park. Consequently, there is a tremendous amount of interest in wireless telephony. view more..
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Ans: When two computers owned by the same company or organization and located close to each other need to communicate, it is often easiest just to run a cable between them. LANs work this way. However, when the distances are large or there are many computers or the cables have to pass through a public road or other public right of way, the costs of running private cables are usually prohibitive. view more..
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Ans: Now that we have studied the properties of wired and wireless channels, we turn our attention to the problem of sending digital information. Wires and wireless channels carry analog signals such as continuously varying voltage, light intensity, or sound intensity. To send digital information, we must devise analog signals to represent bits. view more..
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Ans: We have now studied both the fixed and wireless telephone systems in a fair amount of detail. Both will clearly play a major role in future networks. But there is another major player that has emerged over the past decade for Internet access: cable television networks. Many people nowadays get their telephone and Internet service over cable. view more..
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Ans: In this chapter we will study the design principles for the second layer in our model, the data link layer. This study deals with algorithms for achieving reliable, efficient communication of whole units of information called frames (rather than individual bits, as in the physical layer) between two adjacent machines. By adjacent, we mean that the two machines are connected by a communication channel that acts conceptually like a wire (e.g., a coaxial cable, telephone line, or wireless channel). view more..
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Ans: We saw in Chap. 2 that communication channels have a range of characteristics. Some channels, like optical fiber in telecommunications networks, have tiny error rates so that transmission errors are a rare occurrence. But other channels, especially wireless links and aging local loops, have error rates that are orders of magnitude larger. view more..
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Ans: To introduce the subject of protocols, we will begin by looking at three protocols of increasing complexity. For interested readers, a simulator for these and subsequent protocols is available via the Web (see the preface). Before we look at the protocols, it is useful to make explicit some of the assumptions underlying the model of communication. view more..
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Ans: To introduce the subject of protocols, we will begin by looking at three protocols of increasing complexity. For interested readers, a simulator for these and subsequent protocols is available via the Web (see the preface). view more..
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Ans: In the previous protocols, data frames were transmitted in one direction only. In most practical situations, there is a need to transmit data in both directions. One way of achieving full-duplex data transmission is to run two instances of one of the previous protocols, each using a separate link for simplex data traffic (in different directions). view more..
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Ans: In the previous protocols, data frames were transmitted in one direction only. In most practical situations, there is a need to transmit data in both directions. One way of achieving full-duplex data transmission is to run two instances of one of the previous protocols, each using a separate link for simplex data traffic (in different directions). view more..
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Ans: Within a single building, LANs are widely used for interconnection, but most wide-area network infrastructure is built up from point-to-point lines. In Chap. 4, we will look at LANs. Here we will examine the data link protocols found on point-to-point lines in the Internet in two common situations. The first situation is when packets are sent over SONET optical fiber links in wide-area networks. view more..
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Ans: Network links can be divided into two categories: those using point-to-point connections and those using broadcast channels. We studied point-to-point links in Chap. 2; this chapter deals with broadcast links and their protocols. view more..
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Ans: Network links can be divided into two categories: those using point-to-point connections and those using broadcast channels. We studied point-to-point links in Chap. 2; this chapter deals with broadcast links and their protocols. view more..
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Ans: Network links can be divided into two categories: those using point-to-point connections and those using broadcast channels. We studied point-to-point links in Chap. 2; this chapter deals with broadcast links and their protocols. view more..
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Ans: Network links can be divided into two categories: those using point-to-point connections and those using broadcast channels. We studied point-to-point links in Chap. 2; this chapter deals with broadcast links and their protocols. view more..
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Ans: We have now finished our discussion of channel allocation protocols in the abstract, so it is time to see how these principles apply to real systems. Many of the designs for personal, local, and metropolitan area networks have been standardized under the name of IEEE 802. A few have survived but many have not, as we saw in Fig. 1-38. Some people who believe in reincarnation think that Charles Darwin came back as a member of the IEEE Standards Association to weed out the unfit. view more..
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Ans: In any broadcast network, the key issue is how to determine who gets to use the channel when there is competition for it. To make this point, consider a conference call in which six people, on six different telephones, are all connected so that each one can hear and talk to all the others. It is very likely that when one of them stops speaking, two or more will start talking at once, leading to chaos. view more..
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Ans: At the same time that switches were becoming popular, the speed of 10-Mbps Ethernet was coming under pressure. At first, 10 Mbps seemed like heaven, just as cable modems seemed like heaven to the users of telephone modems. But the novelty wore off quickly. view more..




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