Communication Networks/Cable

Cable
Coaxial cable is an electrical cable consisting of a round conducting wire, surrounded by an insulation spacer, surrounded by a foil, surrounded by a cylindrical conducting sheath, usually surrounded by a final insulating layer. It is used as a high-frequency transmission line to carry a high-frequency or broadband signal. Sometimes DC power (called bias) is added to the signal to supply the equipment at the other end, as in direct broadcast satellite receivers. Because the electromagnetic field carrying the signal exists (ideally) only in the space between the inner and outer conductors, it cannot interfere with or suffer interference from external electromagnetic fields.A coaxial cable's self-shielding property is vital to successful use in broadband carrier systems, undersea cable systems, radio and TV antenna feeders, and community antenna television (CATV) applications.



CATV:- In the early years the cable television was known as Community Antenna Television or CATV (now often known as "community access television") is more commonly known as "cable TV." In addition to bringing television programs to those millions of people throughout the world who are connected to a community antenna, cable TV is an increasingly popular way to interact with the World Wide Web and other new forms of multimedia information and entertainment services

HFC:- Hybrid fiber-coaxial systems were provisioned using only coaxial cable. Modern systems use fiber transport from the headend to an optical node located in the neighborhood to reduce system noise. Coaxial cable runs from the node to the subscriber. The fiber plant is generally a star configuration with all optical node fibers terminating at a headend. The coaxial cable part of the system is generally a trunk-and-branch configuration.

Internet Over Cable
Over the period of time the cable system has grew and the cables between the cities are replaced with the high bandwidth fiber. A system with fiber for long distance and coaxial cable to the houses is called Hybrid Fiber Coax (H.F.C) System. The Electro-optical converters are known as fiber nodes. The fiber node can feed multiple coaxial cables due to the high bandwidth of fiber.

A single cable can be shared by many houses while in the telephone system; every house has its own local loop. While programs are broadcast it does not really make any difference whether there are 10 viewers or 10000, but if the same cable is used to provide the internet access, it makes a lot of difference. One user can utilize the other bandwidth. The more the users are the more bandwidth they needed. Cable industry has tackled this problem by splitting long cables and connecting each one directly to the fiber node. The bandwidth from the head end to each fiber node is infinite, so as long as there are not too many users the situation is under control. But as the traffic will increase the more splitting and fiber nodes will be required.





Spectrum Allocation
It is not practically possible to strictly use the cable network only for the purpose of providing internet connection. So there has to be a way of providing internet and cable TV through same cable. Normally cable channel occupy the 54-550 MHz region in which there is FM radio from 88-108 MHz. Each channel is 6 MHz wide. Some of the modern cable operates above 550 MHz, often to 750 MHz which is used as downstream data. The upstream channels are introduced in the 5-42 MHz band and frequencies at the high end for downstream. For downstream channel each 6 MHz channel is taken and modulated it with QAM-64(6 bit), so we get around 36 Mbit/s gross and net 27 Mbit/s payload without overhead. Thus total effective downstream bandwidth is 200 / 6 * 27 = 891 Mbit/s. For upstream QAM-64 does not work because of too much noise and terrestrial microwaves, so QPSK scheme is used which yields 2 bits per baud so we get around 12 Mbit/s gross and 9 Mbit/s net without no overhead. Thus total effective upstream bandwidth is 37 / 6 * 9 = 54 Mbit/s.



Cable Modem
A cable modem is a device that enables you to hook up your PC to a local cable TV line and receive data at about 1.5 Mbit/s. This data rate far exceeds that of the prevalent 28.8 and 56 kbit/s telephone modems and the up to 128 kbit/s of Integrated Services Digital Network (ISDN) and is about the data rate available to subscribers of Digital Subscriber Line (DSL) telephone service. A cable modem can be added to or integrated with a set-top box that provides your TV set with channels for Internet access. In most cases, cable modems are furnished as part of the cable access service and are not purchased directly and installed by the subscriber. A cable modem has two connections: one to the cable wall outlet and the other to a PC or to a set-top box for a TV set. Although a cable modem does modulation between analog and digital signals, it is a much more complex device than a telephone modem. It can be an external device or it can be integrated within a computer or set-top box. Typically, the cable modem attaches to a standard 10BASE-T Ethernet card in the computer. All of the cable modems attached to a cable TV company coaxial cable line communicate with a Cable Modem Termination System (CMTS) at the local cable TV company office. All cable modems can receive from and send signals only to the CMTS, but not to other cable modems on the line. Some services have the upstream signals returned by telephone rather than cable, in which case the cable modem is known as a Telco-return cable modem.



The actual bandwidth for Internet service over a cable TV line is up to 27 Mbit/s on the download path to the subscriber with about 2.5 Mbit/s of bandwidth for interactive responses in the other direction. However, since the local provider may not be connected to the Internet on a line faster than a T-carrier system at 1.5 Mbit/s, a more likely data rate will be close to 1.5 Mbit/s

ADSL versus CABLE
Though the cable and the ADSL are much more like same and no conclusion can be drawn which one is better than other, it really depends upon the circumstances. But some of their differences are as follows:-

1. Cable uses coaxial cable while ADSL uses normal twisted pair, however the much of cable’s bandwidth is wasted on television programs.

2. Cable subscribers share the line connecting them to neighborhood servers; ADSL subscribers share the line connecting them from the regional telephone office to the main telephone office.

3. ADSL users are hardly affected by the number of existing users, since each has a dedicated connection. While if more customers will subscribe for Cable connection the performance will drop. 4. Everyone who has telephone connection may not be able to get ADSL as he is not close enough to companies end office. While if one has Cable and the company is providing Internet access then he can get it.

5. ADSL offers more security then the cable. Any cable user can easily read the packets going down the cable if its cable provider is not encrypting the traffic in both directions.

6. Since the telephone system is more reliable then cable so, ADSL is more reliable then the cable. In the case of cable if one amplifier fails all downstream users are cut off instantly.

Questions
Ques1. The modem constellation diagram given below, QPSK, QAM-16, QAM -64, has data points at the following coordinates: (1,1), (1,-1), (-1,1), (-1,-1). How many bps can a modem with these parameters achieve at 1200 bit/s?

Ans1. There are four legal values per baud, so the bit rate is twice the baud rate. At 1200 baud, the data rate is 2400 bit/s.

Ques2. How many frequencies does a full-duplex QAM-64 modem use?

Ans2. Two frequencies are used, one for upstream and one for downstream. The modulation scheme itself just uses amplitude and phase. The frequency is not modulated.

Ques3. A Cable company decides to provide Internet access over cable in a neighborhood consisting of 5000 houses. The company uses a coaxial cable and spectrum allocation allowing 100 Mbit/s downstream bandwidth per cable. To attract customers, the company decides to guarantee at least 2 Mbit/s downstream bandwidth to each house at any time. Describe what the cable company needs to do to provide this guarantee.

Ans3. A 2-Mbit/s downstream bandwidth guarantee to each house implies at most 50 houses per coaxial cable. Thus, the cable company will need to split up the existing cable into 100 coaxial cables and connect each of them directly to a fiber node.

Ques4. How fast can a cable user receive the data if the network is otherwise idle?

Ans4. Even if the downstream channel works at 27 Mbit/s, the user interface is nearly always 10-Mbit/s Ethernet. There is no way to get bits to the computer any faster than 10-Mbit/s under these circumstances. If the connection between the PC and cable modem is fast Ethernet, then the full 27 Mbit/s may be available. Usually, cable operators specify 10 Mbit/s Ethernet because they do not want one user sucking up the entire bandwidth.