Issues in Handling Cable Signals Within Digital Optical Interconnect Networks

ISSUES IN HANDLING CABLE SIGNALS WITHIN DIGITAL OPTICAL INTERCONNECT NETWORKS James 0. Farmer Electronic System Products An Antec Corporation Business Edward J. Callahan Antec Corporation INTRODUCTION headends may be connected. Two architectures are shown. Figure 1 (A) shows a single master As cable systems seek to add telephony headend connecting with three hubs, or sub setvices and interconnect headends for opera headends. Fiber cables interconnecting the tional efficiencies, they will migrate toward use headends are routed two ways, so if one path is of digital fiber optics for the interconnections. interrupted, for example by a car hitting a pole, This is true even of systems which expect to the other path can be used. Figure 1 (B) shows continue to deliver analog programming to the a modified concept in which the headends are home for the immediate future. When develop connected in rings. Signals travel around the ing a digital optical interconnection system, one ring in both clockwise and counterclockwise may choose either proprietary systems, or may directions, passing through each headend on the select compatible standard systems available way to the next. In this case two master from a number of manufacturers. Here-in the headends are used. Signals are routed com case is made for the latter approach, in which pletely around the ring in two directions. As in standard systems, originally developed for (A), if one path is interrupted, the path around telephony applications, are used to transport ....---*~HU video and ancillary signals from one headend location to another. Interface issues include not only video, but also audio setvices, addressable data, con trol and new data setvices. The versatility of the network is enhanced by integrating cable television's needs with standard multiplexing A. INTERCONNECTION systems. Multiplexing is explained, and the USING DIVERSE ROUTING hierarchies in common usage are introduced. Finally, a short space is devoted to describing some of the quality issues in hybrid ana log/digital networks. INTERCONNECfiNG HEADENDS B. SONET RING WITH DUAL Headend interconnection is becoming MASTER HEADENDS popular today to allow efficiencies of operation, more controllable advertising insertion and Figure 1. Options in Interconnection improved reliability. Figure 1 shows how 1994 NCTA TECHNICAL PAPERS --325 the ring in the other direction is available to occurs during transmission or switching.i Of carry signals. Further, if something happens at course we can and do pick up distortion in the one headend, there is a second master headend process of digitizing (and compressing) a sig to take over, so that service is interrupted, if at nal. That distortion is a trade-off in how few all, to only one node. bits we use to represent the signal, the quality of the signal and our ingenuity in compressing the video. TO DIGITAL FIBER OPTIC TRANSMISSION SYSTEM In the future the industry is likely to begin switching sig nals more, as we subdivide Figure 2. Simplified Master Headend systems into fiber nodes to which we can route different The preferred method used to transmit signals and as we provide more customized the signals is the SONET ~ynchronous Optical cable features and transmission path back-up. Network. It is a fiber optic based system with These will require more switching and routing digital transmission of all signals. Figure 2 of signals. These operations can be done much shows the basic conditioning of a video signal better if the signals are in digital format, since at the master headend. Signals are converted to we don't have to worry about signal degrada baseband and supplied to encoders, which tion. convert to digital and compress the signal. At this time MPEG compression is not being used Video file servers, big brothers to the due to the cost of the encoders, though this file servers used in local area networks (I.ANs) may change in time. The digital signals are today, will, in future years, become commonly multiplexed (described below) onto a single used for storage of programs. The programs data stream for transmission throughout the will then be transmitted to subscribers, either on network. At each node, the signals are passed a subscription basis or on demand. File servers through with provision to drop and add signals store video digitally, so in many cases it will if necessary. The signals are demultiplexed and make sense to transmit the signals in digital converted to NTSC for modulation and con form, at least part way to the subscriber. The ventional transmission to subscribers. trick will be to balance the cost of transmission against the cost of converting the signal back to WHY DIGITAL SIGNAL TRANSMISSION? NTSC. Within the network, there are excellent Once we have a signal digitized and reasons for transmitting TV signals digitally. compressed, the next effort is route the signal Digital transmission can be less costly when to a given subscriber, along with other signals. applied intelligently. Because fiber transmit Simultaneously other signals will be routed to ters for baseband digital signals don't have to other subscribers. The process of combining all control distortion (be linear), they are poten of the signals for transmission is the subject of tially lower in cost. Receivers are also poten this paper. We call this combining process tially less costly. Of course realization of the "multiplexing." cost savings will depend on volume, which cable television may drive in the future. Another reason for digital transmission is that, within limits, no video degradation 1994 NCTA TECHNICAL PAPERS --326 INTRODUCTION TO DIGITAL more than a different way of putting signals MULTIPLEXING together on a single cable. Rather than putting each signal on its own unique RF carrier fre· As we move into a world in which video quency and multiplexing by combining the is carried in digital form, we encounter the term signals on a cable, we assign each signal to its multiplexing. This is another way of saying own unique time slot, switching from one that we are combining signals. In the cable signal to another at the appropriate time. AMPLITUDE FDM SIGNAL TO DISTRIBUTION SYSTEM 000 ~~~----~---+FREQUENCY CH 2 CH 78 A. FREQUENCY DIVISION MULTIPLEXING (COMBINING) AMPLITUDE DIGITAL SIGNAL 1 --+ TOM SIGNAL TO DIGITAL SIGNAL 2--+ TRANSMISSION SYSTEM 0 (MAY OR MAY NOT BE 0 ETC. MODULATED ON RF DIGITAL SIGNAL 78 __:_. CARRIER) ~--...J...._--L----L----1~--L-+TIME (OR WHATEVER) MULTIPLEXER ("MUX") SYNC (SEE TEXT) 1 1 DIGITAL SIGNAL 1 --+0 DIGITALSIGNAL2-+0 ~ DIGITAL SIGNAL 78 --+0 (OR WHATEVER) B. TIME DIVISION MULTIPLEXING Figure 3. Concept of Multiplexing television world, we are accustomed to combin Figure 3 compares time and frequency ing RF signals in a combiner, which puts many division multiplexing. In (A) we are doing signals together on a single cable. In traditional frequency division multiplexing at a headend, cable television, these signals are put together by sending each signal (on its own frequency) using a technique called frequency division to a combiner. The spectrum diagram shows multiplexing (FDM). In FDM, we put several what the resulting signal looks like. We have signals together on one cable by assigning each the familiar diagram of each channel in its place its unique band of frequencies. The same idea on the cable. In order to recover one of the is practiced in the digital world, except instead signals, we tune it with circuits that can differ of employing FDM we normally use time divi entiate one frequency band from another. sion multiplexing (TDM). TDM is nothing 1994 NCTA TECHNICAL PAPERS --327 In contrast, (B) shows time division arbitrary numbers: we shall encounter them multiplexing, as practiced in the types of sys again. tems discussed here. Each digital signal (a series of ones and zeros coming one after the SPECTRUM other) is applied to a multiplexer (commonly called a "mux"), which serves an analogous The spectrum of a digital signal extends function to the combiner in a conventional to at least one half the bit rate of the data headend. The amplitude vs. time diagram stream: if we transmitted a one followed by a shows the result. A short segment (one or zero, we would have one cycle of a square more bits) of Signal 1 is transmitted, followed wave. In some cases, we want to filter the by a short segment of Signal 2 and so on. signal so we have nothing above the minimum Notice that, as in all digital systems, everything frequency required to get ·data through. In has the same amplitude: amplitude data is other cases, we would handle the signal as a carried as a series of bits which represent the square wave, with a spectrum much wider than numerical amplitude. In order to recover the required. individual signals, we apply the data stream to a demultiplexer ("demux"), which takes the signal Figure 4 shows a sample sequence of a back apart, into its individual data streams. The baseband digital signal and the resulting spec demux does this by switching Signal 1 to its trum. In (A) we see a sequence of bits that "bin," then switching Signal 2 to its bin and so starts out with the sequence 1010, followed by on. In order to let the demux know when a a sequence 11001100, then a random sequence. particular signal is being sent, we must add The 1010 sequence represents the highest synchronization (analogous to vertical and frequency the system will be called on to horizontal sync in TV) to the data stream. The transmit. As shown, two bits, a one followed sync is shown in the diagram. Depending on by a zero (or vice-versa), make up one cycle of the system, sync may be part of each data a periodic waveform.

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