Computer Networks 4th Ed Andrew S. Tanenbaum [Electronic resources] نسخه متنی

2.7 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. However, an alternative available for fixed networking is now becoming a major player: cable television networks. Many people already get their telephone and Internet service over the cable, and the cable operators are actively working to increase their market share. In the following sections we will look at cable television as a networking system in more detail and contrast it with the telephone systems we have just studied. For more information about cable, see (Laubach et al., 2001; Louis, 2002; Ovadia, 2001; and Smith, 2002).

2.7.1 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 head end, to strengthen it, and a coaxial cable to deliver it to people's houses, as illustrated in Fig. 2-46.

Figure 2-46. An early cable television system.

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) and distributed only on cable. Other cable-only channels followed with 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 their 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.

2.7.2 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 was happening 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 electro-optical converters that interface between the optical and electrical parts of the system are called fiber nodes. Because the bandwidth of fiber is so much more than that of coax, a fiber node can feed multiple coaxial cables. Part of a modern HFC system is shown in Fig. 2-47(a).

Figure 2-47. (a) Cable television. (b) The fixed telephone system.

In recent years, many cable operators have decided to get into the Internet access business, and often the telephony business as well. However, technical differences between the cable plant and telephone plant have an effect on what has to be done to achieve these goals. For one thing, all the one-way amplifiers in the system have to be replaced by two-way amplifiers.

However, there is another difference between the HFC system of Fig. 2-47(a) and the telephone system of Fig. 2-47(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 does not play a role. 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, 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, the more competition 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.

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 5002000 houses, but as more and more people subscribe to Internet over cable, the load may become too much, requiring more splitting and more fiber nodes.

2.7.3 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 coexist on the same cable.

Cable television channels in North America normally occupy the 54550 MHz region (except for FM radio from 88 to 108 MHz). These channels are 6 MHz wide, including guard bands. In Europe the low end is usually 65 MHz and the channels are 68 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 to 750 MHz or more. The solution chosen was to introduce upstream channels in the 542 MHz band (slightly higher in Europe) and use the frequencies at the high end for the downstream. The cable spectrum is illustrated in Fig. 2-48.

Figure 2-48. Frequency allocation in a typical cable TV system used for Internet access.

Note that since the television signals are all downstream, it is possible to use upstream amplifiers that work only in the 542 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 of the traffic is likely to be downstream, 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.

Long coaxial cables are not any better for transmitting digital signals than are long local loops, so analog modulation is needed here, too. The usual scheme is to take each 6 MHz 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 QAM-256, the net payload is about 39 Mbps. The European values are 1/3 larger.

For upstream, even QAM-64 does not work well. There is too much noise from terrestrial microwaves, CB radios, and other sources, so a more conservative schemeQPSKis used. This method (shown in Fig. 2-25) yields 2 bits per baud instead of the 6 or 8 bits QAM provides on the downstream channels. Consequently, the asymmetry between upstream bandwidth and downstream bandwidth is much more than suggested by Fig. 2-48.

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.''

2.7.4 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 V.9x telephone modems) 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) is just starting to replace proprietary modems. The European version 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 10-Mbps Ethernet (or occasionally USB) at present. In the future, the entire modem might be a small card plugged into the computer, just as with V.9x internal modems.

The other end is more complicated. A large part of the standard deals with radio engineering, a subject that is far beyond the scope of this book. The only part worth mentioning here is that cable modems, like ADSL modems, are always on. They make a connection when turned on and maintain that connection as long as they are powered up because cable operators do not charge for connect time.

To better understand how they work, let us see what happens when a cable modem is plugged in and powered up. The modem 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 on-line. 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 modem then 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 accommodate the way the upstream channels operate and to get the timing right. They are divided in time in minislots. Each upstream packet must fit in one or more consecutive minislots. 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 also assigns each modem to a minislot to use for requesting upstream bandwidth. As a rule, multiple modems will be assigned the same minislot, which leads to contention. 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.

On the other hand, if there is contention for the request minislot, there will be no acknowledgement and 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 one thing, there is only one sender (the headend) so there is no contention and no need for minislots, which is actually just time division statistical multiplexing. For another, the traffic downstream is usually much larger than upstream, so a fixed packet size of 204 bytes is used. Part of that is a Reed-Solomon 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. 2-49.

Figure 2-49. Typical details of the upstream and downstream channels in North America.

Getting back to modem initialization, once the modem has completed ranging and gotten its upstream channel, downstream channel, and minislot assignments, it is free to start sending packets. The first packet it sends is one to the ISP requesting an IP address, which is dynamically assigned using a protocol called DHCP, which we will study in Chap. 5. It also requests and gets an accurate time of day from the headend.

The next step involves security. Since cable is a shared medium, anybody who wants to go to the trouble to do so can read all the traffic going past him. To prevent everyone from snooping on their neighbors (literally), all traffic is encrypted in both directions. Part of the initialization procedure involves establishing encryption keys. At first one might think that having two strangers, the headend and the modem, establish a secret key in broad daylight with thousands of people watching would be impossible. Turns out it is not, but we have to wait until Chap. 8 to explain how (the short answer: use the Diffie-Hellman algorithm).

Finally, the modem has to log in and provide its unique identifier over the secure channel. At this point the initialization is complete. The user can now log in to the ISP and get to work.

There is much more to be said about cable modems. Some relevant references are (Adams and Dulchinos, 2001; Donaldson and Jones, 2001; and Dutta-Roy, 2001).

2.7.5 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 do not make any claims 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 their 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. Normally, it is rigidly divided in fixed chunks among the active users by FDM and TDM because voice traffic is fairly smooth. But for data traffic, this rigid division is very inefficient because data users are frequently idle, in which case their reserved bandwidth is wasted. Nevertheless, in this respect, cable access is more like the mobile phone system than it is like the fixed system.

Availability is an issue on which ADSL and cable differ. Everyone has a telephone, but not all users are close enough to their end office 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. This 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.