Signal Scrambling

Signal Scrambling
Video signal scrambling is a deliberate act of changing an electrical signal (often distortion of video, digital voice, or data) to prevent interpretation of the signals by unauthorized users that are able to receive the signal. Because the scrambling process is performed according to a known procedure or algorithm, the received signal can be descrambled to recover the original digital stream through the use of a known code or filtering technique.

In 1971, the first system to using scrambling on a cable system was demonstrated. The first scrambling suppressed the synchronization signal so the video of the television picture was distorted. To decode the scrambled video, the synchronization signal was recreated in the setup box by decoding the correct synchronization signal from another portion of the transmitted signal. Another form of signal scrambling that was used was the insertion a signal that was slightly offset from the channel’s frequency to interfere with the picture.

These early video signal scrambling systems were relatively simple in design. As a result, accessory devices soon became available that allowed consumers to decode the scrambled signals without paying subscription fees. To prevent unauthorized viewing, more sophisticated signal scrambling technologies have been developed.

For digital television signals, video signals can be easily encrypted with a key code. To successfully decode the video signal, the set-top box must contain the decryption key code. For two-way cable systems, this code can be dynamically changed and unauthorized viewing has been greatly reduced.

Cable Modems

A cable modem is a device that MOdulates/DEModulates data signals on a coaxial cable and divides the high data rate signals into digital signals designated for a specific user. Cable modems are often asymetrical modems as the data transfer rate in the downstream direction is typically much higher than the data transfer in the upstream direction. The typical gross (system) downstream data rates range between 30-40 Mbps and gross upstream data rates typically range up to 2 Mbps.

Usually 500 to 2000 users share the gross data transfer rate on a cable system. Cable modems also have the requirement to divide the high-speed digital signals into low-speed connections for each user. In 2001, the average data rates for a cable modem users was approximately 720 kbps.

Cable modems contain a tuner, a demodulator, a modulator, media access control (MAC) section, and a control section. The tuner converts a selected RF channel (high frequency) to the modem baseband (low frequency) signal. The tuner makes adjustments to a frequency (usually between 42 and 850 MHz) for downstream traffic and may convert the upstream traffic to a different RF channel (usually between 5 and 42 MHz).

Early cable modems used a hybrid system that used the cable system for downlink channels and a telephone line for upstream traffic. This was desirable as many of the amplifiers in the cable television system only provided for one-way amplification.

The cable modem receiver contains a demodulator that converts the low frequency received signal into its original baseband digital form and performs error detection and correction. The cable modem may contain a decoder to convert compressed video into a form that can be displayed on the computer monitor. The modulator converts the digital information from the computer into a format suitable for transfer back to the Internet. For hybrid systems, this may be a telephone line audio modem and for two-way cable systems, the modulator converts the data into radio-frequency signals for transmission on the cable system. A control section coordinates the upstream and downstream access operations (called media access control - MAC) of the cable modem. The control section also coordinates the overall operation of the cable modem including how it interfaces to communication devices. For example, the data may be converted to Ethernet format for communication with a personal computer.

Figure 1 shows a block diagram of a typical cable modem system. This diagram shows that the Internet is connected to the head-end of the cable system by a gateway. The gateway adapts the data to and from the Internet into a form that can be transmitted through the cable modem system. The cable modems at the head-end convert the digital signals into RF signals that can be transmitted through the cable network. A single 6 MHz RF television channel is converted to a high-speed data channel (30-40 Mbps) that is transmitted to all the users in the cable modem network. To access data on this channel, each cable modem is assigned a portion of the data channel from the CMTS at the head-end. This diagram shows that multiple RF channels may be used to provide more data transfer capabilities to each customer. When the cable modem at the customer’s location wants to send data, it randomly accesses the system through a return RF channel.

Figure 1: Cable Modem System

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