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CABLE WORLD STAFF
DigiPoints, the Society of Cable Telecommunications Engineers monthly Web-based digital data tutorial, has introduced a number of terms and principles to cable telecommunications professionals over the years. Here's a a comprehensive review.
* One component of a digital telephony system is the host digital terminal (HDT), the equipment that interfaces a local digital switch to a hybrid fiber/coax system. It provides the conversions from RF over fiber to the digital inputs required by the switch.
* The interface between the cable modem and the user computer - or other CPE - is an Ethernet - or IEEE 802.3 - connection. With an Ethernet interface, each cable modem appears as a node on an Ethernet LAN. Existing Ethernet network interface cards (NICs) can be used as interfaces to a customer's PC.
* The installation procedure for a cable modem will vary with the particular manufacturer and the standard that has been implemented in the cable modem. Theoretically, when universal serial bus (USB) cable modems are built, they will self-configure, will not require a NIC, and will be ready to operate upon plugging in the USB connection to the PC.
* Digital set-top boxes generally can be grouped into three categories: Digital TV terminal, HDTV interface units and entertainment interfaces. The digital TV terminal processes digital or analog signals that modulate an RF carrier using multibit modulation. Input to the terminal is RF at the carrier channel frequency. Outputs can include RF at channel 3 or 4, baseband video and audio, and digital audio.
* For the large base of customer digital equipment to function, some very sophisticated equipment is needed at the headend to process a digital signal and distribute it to the network serving the subscriber base. Two of these pieces of equipment on the receiving end are the integrated receiver/decoder (IRD) and the integrated receiver transcoder (IRT). The IRD is one type of receiver for modulated signals arriving on a satellite downlink. The IRT receives satellite signals in a digital system.
* One source of locally generated digital signals is the video server. A video server is the multimedia equivalent of a data file server. Ad insertion and video-on-demand are two video server applications. Each is gaining acceptance as an important revenue source for cable operators. Video-on-demand had to wait for compression technology and reliability improvements, as well as storage efficiency.
* Digital signals need to be protected from unauthorized use. Several parts of a digital system are involved in implementing conditional access, of which encryption is an integral part. An encryption algorithm is a mathematical equation containing the message being encrypted or decrypted. The encrypted message is called cipher text. The original, unencrypted message is called plaintext.
* Encryption and decryption aside, digitally coded information is complex, and detection of faulty performance levels needs to be not only rapid but also predictive. That means creating and implementing systems that report troubles automatically. The system for doing this is called network management, which describes an array of hardware, software, protocols and processes.
* Finally, the quality of a digital system must be tested. A spectrum analyzer serves to avoid adverse effects of one signal on another. Constellation diagrams assess the quality of signal modulation.
The entire DigiPoints publication is posted at www.scte.org.
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