Cable Modem

01/11/1999

Pentti Peisa
Electrical and Communications Engineering Dept.
Helsinki University of Technology
ppeisa@cc.hut.fi

Ganesh Sivaraman 93796P
Computer Science and Engineering Dept.
Helsinki University of Technology
ganeshs@cc.hut.fi

Abstract

With the idea of Modulation Demodulation it became possible to send and receive data between computers over analog telephone lines by using telephone modems. But the drawback of telephone modem is the bandwidth. With more and more hardware capabilities available for multimedia there is a constant need of faster and cheaper means of transfering both audio and video over network. This essay gives an overview of one such medium. This essay explains working, a comparative study with other mediums, and future trends of Cable Modems.

Contents

1 Introduction

2 Working of Cable Modem

2.1Basic Layout

2.2 Data Trasmission

2.3 System Architecture

3 Comparative Study

References

1 Introduction

A "Cable Modem" is a device that allows high speed data access (such as to the Internet) via a cable TV (CATV) network. A cable modem will typically have two connections, one to the cable wall outlet and the other to a computer (PC) [5].
Cable modems not only provide fast data transfer but will also deliver other performance enhancements. First (and most notable) is the ability to use the modem and watch television at the same time. This is possible because unlike telephone modems (where the line is busy during the total time of data transfer),cable modems use only a fraction of the available bandwidth of cable systems. So this makes cable modem to provide multiple services like surfing the Internet,TV programmes and usage of telephone all at the same time. The bandwidth advantage also gives cable modems instant access to the Internet. and thus there is no need of dial-up and login process as in case of telephone modems. So the service is available all the time. [6]

Contents

2 Working of Cable Modem

Cable modem is a device that does two conversions. One at the PC end ie converting the digital signal coming from PC to analog one for transmitting over the upstream of the cable and the second one is to convert the analog downstream signal of cable to digital one for PC. Cable modem connects to the same coaxial cable that carries the television programmes. For transmission of data the cable modem accesses a specific frequency or channel that the cable operator keeps aside for data transmission. This way the same cable can be used for both purposes[8].

2.1 Basic Layout

Cable modem has five components:

1.the headend,
2.the trunk cable,
3.the distribution (or feeder) cable in the neighborhood,
4.the drop cable to the home and in-house wiring, and
5.the terminal equipment (consumer electronics).

  
Fig 1 Cable modem layout [5]

After it converts the incoming signal to digital signal, the digital signal is transmitted from the cable modem to the PC over a 10BaseT Ethernet line. The 10BaseT line is the same used in a local area network (LAN). While the cable modem receives data at up to 30 Mbps, communication over the 10BaseT line between the cable modem and the PC is at 10 Mbps. The function of the cable modem is to connect connect broadband (i.e., the cable television network) to Ethernet.

Typically a cable modem sends and receives data in two slight different ways.In the DOWNSTREAM direction, the digital data is modulated and then placed on a typical 6MHz television carrier, between 42 MHz and 750 MHz. For this various modulations techniques can be used. The two most popular ones are QPSK (upto 10Mbps) and QAM64 (upto 36Mbps). This signal is placed on either side without disturbing the cable television video signals.
The UPSTREAM channel (also called return channel or reverse path) is more tricky. In most two-way acitvated cable networks, the UPSTREAM is transmitted between 5 and 40 MHz. But this spectrum is more moisy due to interferences from a number of sources such as HAM radio, CD radios, and also impulse from house appliances. For these reasons manufacturers prefer QPSK technique over others as it is more robust scheme for a noisy environment though it is slower than QAM technique.

2.2 Data Transmission

From various sources (broadcast transmissions, satellite-delivered programming, and local television stuido productions) the signals are received and processed at the headend. TV siganls being EM impluses or waves need some medium to propagate. For broadcast transmissions air acts as the medium and for the cable system the TV signals are carried by a special type of cable. These cables carry signals through metal wires such as coaxial cable. Each TV signal travels at a different frequency inside the cable, and so coaxial cable acts as a self-contained spectrum. Thus this helps those households that cannot receive broadcast transmission ie over the air.

2.3 System Architecture

The present system arhcitecture is of the tree and branch type. In this type the cable system uses one source point to send signals out to thousands of subscribers via a system of trunk, feeder, and drop cables. Trunk is a large cable which carries the siganls through the city. Feeder cables are of smaller diameter which connect to the trunk cable and brach off into the local neighbourhood. Drop cables are the ones which connect the subscribers TV with the feeder cable.Sometimes it is possible that the TV or VCR tuner is not able to tune itself to all the channels. This is due to incompatibilty between the tuner and cable. For this reason a converter is placed between the cable and the tuner of the TV or VCR. Hence the name tree and branch type of architecture.It is the most efficient and economical method to transmit a package from the headend to all the subscriber.

Contents

3 Comparative Study

This is a comparison of time taken to transfer 10MB file on different mediums. [11]

References