REGIONAL NETWORKS FOR BROADBAND CABLE TELEVISION OPERATIONS by Donald Raskin and Curtiss Smith ABSTRACT There is a clear trend toward regional aggregation of local cable television operations. Simultaneously, the broadband communications operations that once were cable TV are now implementing systems to distribute digital multiplexes of entertainment video, highspeed connections with the Internet and local servers via cable modems, commercial enterprise LAN/WAN s, PCS-over-cable and telemetry, in addition to the traditional analog TV service. Both of these trends have a strong influence on many of the architecture and equipment decisions being made by cable operators. One of the outcomes of these trends has been a shift toward using standards-based digital transport ( and SDH) for the backbone interconnections within these regional networks. Using service examples, this paper discusses the reasons for this new direction. REGIONAL NETWORKS A generic regional network is shown in Figure 1, which also introduces some of the terminology used in the broadband cable industry. Generally, all of the signal sources are collected at the main head-end. After processing, the signals are distributed over a backbone to primary hubs. The primary hubs feed a number of secondary hubs, which distribute to fiber nodes. All of the distribution between the main head-end and the nodes uses singlemode fiber. The nodes perform the optical-to-electrical conversion and begin the distribution over coaxial cable. Each of the head-ends and hubs provides distribution to its immediate vicinity, as well. In all but the smallest networks, the backbone is a bi-directional ring using baseband digital transport. The focus of this paper is the choice between the different techniques for that transport.
Primary Hub 1 Redundant Fiber Primary Hub 2 Main Head-End 0.5-1 million HP 20,000 HP Secondary Hub 100,000 HP Primary Hub 4 Fiber Secondary Hub Secondary Hub Primary Hub 3 Node Node Fiber Node Coax Node 500-2000 HP Figure 1. Diagram of regional network (HP = homes passed). There are many reasons why cable networks are becoming regionalized. The primary driver has been the increasing complexity of the head-end, as the service offerings have grown in both number and variety. The specific impacts of this increasing complexity are Equipment costs have risen due to higher channel counts and to wholly new items, such as routers and servers. In many cases, the signals produced by the electronics deployed in one head-end can be reused by all the others. Space requirements have increased both inside the head-end, due to the additional equipment, and outside, due to the required satellite dishes. Consolidation of head-ends makes construction of one large building and dish farm feasible.
The achievement of high network availability has become a necessity with the higher service intensity. As a result, more personnel with specific skills and training are needed to operate and maintain the electronics. This can be provided more readily in a single location. Billing systems have become more elaborate as the service menu has broadened. Accordingly the billing equipment has become more complex and costly, thus favoring centralization. As evidence of the growing complexity of the service offering, Table 1 shows a list of the signals received at the main head-end and how they are ultimately transmitted over coax to the home. Table 1. Broadband cable service offerings. Signal Received at Main Head-End Analog satellite (clear) Analog satellite Access control system Analog satellite Analog satellite Digital satellite multiplex Digital satellite multiplex Digital satellite multiplex Analog off-air Analog off-air Local origination, central ad insertion Baseband digital (MPEG-II, I/P pkts) Telephony Music Choice/DMX Sega Server/VOD Signal Distributed onto Coaxial Cable System Analog, non-scrambled Analog, scrambled Analog out-of-band Analog, scrambled with SAP or BTSC Same w/ local SAP Analog, non-scrambled Analog, scrambled Digital multiplex Analog, non-scrambled Analog, scrambled Analog, non-scrambled Modulated carrier digital Modulated carrier digital Modulated carrier digital Modulated carrier digital Modulated carrier digital SERVICE SCENARIOS There are four alternative techniques that can be used for the backbone rings. The first -- using 1550 nm analog transmission for broadcast services -- is very cost-efficient in cases where it can be applied, but it has limited extent and is not very "friendly" to narrowcasting or to two-way services. In many cases it is viewed as an inexpensive interim solution to be used for a few years while more extensive network plans are being finalized. This paper concentrates on the remaining three approaches:
1. digital transport 2. Standards-based transport 1 without video compression 3. Standards-based transport with video compression In order to understand the trade-offs between these techniques, we will discuss of deployments of each in five service applications: a) A digital multiplex of entertainment TV, distributed to subscribers as individual, non-scrambled analog channels b) A digital multiplex of entertainment TV, distributed to subscribers as individual, scrambled analog channels c) A digital multiplex of entertainment TV, distributed to subscribers as a digital multiplex d) Cable modem access to the Internet e) Digital video on demand, downloaded to local servers and distributed as a digital multiplex. The examples will be used to illustrate the equipment and infrastructure requirements for proprietary and standards-based digital backbones. 1. NON-SCRAMBLED ANALOG We discuss first the case of analog entertainment video that is delivered by satellite to the main head-end as an encrypted digital multiplex (QPSK modulated L-band microwave). 2 The multiplex is demodulated to baseband and decrypted in a satellite receiver unit. Typically two different processes are needed in the main head-end for (a) local distribution and (b) transport to hubs. For local distribution, each program in the signal needs to be AM- VSB modulated and upconverted to its assigned channel. In most cases, the modulation is done initially in the satellite receiver unit up to a standardized intermediate frequency (), which is 44.75 MHz in NTSC systems. A separate upconverter takes inputs and converts them to the appropriate radio frequency () channel. Before we discuss the options for transporting signals to remote hubs, we need to establish one of the key performance requirements the signal to noise (SNR) requirement for the digital transport system. Since the digital transport precedes a hybrid fiber/coax (HFC) distribution system, its SNR requirement is determined by the end-of-line carrier to noise (C/N) requirement. Figure 2 shows how the end-of-line C/N varies with digital SNR 1 For brevity, we will use to signify standards-based. Readers should understand this as referring both to and to SDH. Similarly, references to elements in the non-synchronous hierarchy, such as DS-3, should be interpreted as including E-n. 2 The discussion would be similar for direct VHF/UHF off-air signals and for analog satellite delivery, only the specific component names would differ.
for five cases of HFC plant C/N. For reference, an HFC plant consisting of a fiberoptic transmitter-receiver link, node amplifier, two distribution amplifiers and two line extenders has a C/N of approximately 50 db. This means that for an end-of-line requirement of 49 db, the digital SNR needs to be better than 57.5 db. It also means that an infinite SNR will not raise the end-of-line in this case by as much as 1 db. End-of-line C/N 52.0 50.0 48.0 46.0 44.0 42.0 50 55 60 65 70 75 Digital SNR HFC C/N 51 50 49 48 47 Figure 2. End-of-line carrier-to-noise performance for HFC plant after digital transport. As shown in Figure 3, there are three basic schemes for transporting the signals to remote hubs. Until recently, the most commonly used method has been proprietary digital transport of the signal. Generally, this involves a ten-bit analog-to-digital (A/D) conversion, which results in a digital stream of approximately 150 Mbps. Sixteen of these streams can be multiplexed into a 2.5 Gbps stream for transport over a single fiber. At the remote hub, this stream is demultiplexed, converted (D/A) back to analog and upconverted to. Ten-bit encoding (and Nyquist sampling) ensures a digital SNR of better than 61 db. A more recent variant of this scheme is to use transport of the digitized signal. In this method, the output of the A/D is an STS-3 (155 Mbps), which means that sixteen programs can be carried on an OC-48. This has the advantages of standards-based transport, which we will discuss below, but it requires that the equipment be very lowcost. An alternate method that is more efficient in its use of equipment and of fibers, is to feed the baseband analog video and audio signals from the satellite receiver into a digital encoder that compresses them. In this way, it is possible to put two programs onto a single DS-3 while maintaining a 60 db SNR. With this compression, as many as 96 programs can be transmitted on a single fiber. 3 After demultiplexing at the remote hub, the DS-3 signal is decoded and then modulated onto carriers. As indicated in Figure 3, six times as many fibers are needed for either of the uncompressed transport 3 Keep in mind that 22 Mbps video compression is a much smaller degree of compression than is being discussed for digital transport to the home.
methods. On the other hand, the modulators required to put the baseband signals on channel at the remote hubs in this method are somewhat more expensive than the upconverters used with uncompressed transport. Integrated L-BAND Satellite Receiver/ Demodulator BASE- BAND Local Distribution Encoder A/D A/D 1 FIBER 6 FIBERS 6 FIBERS Decoder D/A D/A Modulator Figure 3. Non-scrambled analog distribution and transport. 2. SCRAMBLED ANALOG Scrambling of the video and audio requires that a scrambling processor be inserted into the signal stream and that a low-rate data stream be carried for controlling converters (Figure 4). Uncompressed transport can be used for the signals out of the scrambling processors, because the A/D and D/A conversions aren t dependent on knowing anything about the input signals. Scrambled signals present a difficulty for compression, however, because one of the keys to compression is making use of foreknowledge of the signal structure, such as the details of the sync pulse and blanking intervals. As a result, either
the scrambled signal is transported uncompressed or the scrambling must be done at the remote sites. L-BAND Integrated Satellite Receiver/ Demodulator A/D A/D 6 FIBERS 6 FIBERS D/A D/A BASE- BAND Encoder Scrambler Local Distribution 1 FIBER Decoder Scrambler Figure 4. Scrambled analog distribution and transport.
3. DIGITAL VIDEO TO THE HOME With the advent of MPEG-2 compression, cable operators are now delivering digital video multiplexes to the home. The multiplex of programs is delivered to the head-end by satellite exactly as in the analog case just discussed, but it is carried on the HFC plant as 64- QAM or 256-QAM digital signals modulated onto carriers. At the head-end, a transcoder is used in place of the analog satellite receiver. The transcoder can demodulate the QPSK satellite signal, decrypt the baseband digital stream and modulate it to QAM. It is possible to do an uncompressed A/D conversion of the QAM/ signal, 4 so the two methods for transport already discussed (proprietary and ) can be applied here, as well (Figure 5). Alternatively, the baseband digital stream can be taken from the trans- Integrated L-BAND Satellite Receiver/ Transcoder BASEBAND MULTIPLEX DS-3 A/D A/ D Local Distribution 1 FIBER 3 FIBERS 3 FIBERS DS-3 D/A D/A Modulator Figure 5. Distribution and transport of digital video programs. 4 Digitization of a signal that is already digital may seem unaesthetic, but it is functional.
coder either before or after decryption and fed into a DS-3 port in the system. Since the maximum data rate in the digital signal from the satellite is 38.8 Mbps, 5 it fits easily within a DS-3 tributary. At the remote hubs, the baseband signal can be manipulated to change the mix of programs or to insert new ones within the multiplex. This is not practical with transport. Note that since the multiplex takes a full DS-3, the ratio of fiber compression is 3:1 in this case. 4. CABLE MODEMS In a cable modem system, routers and servers are located in the main head-end and in the primary hubs. Communication between these units is universally done with standard WAN interfaces, which makes transport on the backbone a fairly straightforward choice. In fact, in a network design providing 1 Mbps bi-directional access for cable modems, we have concluded that OC-48 connectivity is required between primary and secondary hubs, as well. For systems that carry video on a proprietary network, the use of for data transport is problematical because it means that two separate networks are being operated in parallel. This means that network management programs and maintenance skills must be duplicated. 5. VIDEO SERVERS For video-on demand (VOD) service, a set of servers is located at the main head-end and requests are sent upstream from subscribers. From the transport point-of-view, it is similar to a cable modem service, since it is very much an interactive, narrowcast service. Thus transport between the main head-end and the individual modulators appears to be the method of choice. DISCUSSION In comparing transport options, there are many important trade-offs to be considered, such as: Cost Investment cost Vs operating costs Are there sufficient fiber counts Opportunity cost of fiber 5 This rate is consistent with 4-6 MPEG-2 video programs (or more with statistical multiplexing) or with two HDTV signals at 19.2 Mbps each.
Space Is there sufficient room in the remote hubs for the equipment Flexibility Is local ad insertion required Is local program insertion required Is the program line-up the same at all hubs What additional services need to be planned for Management Systems Can more than one management system be supported Will commercial customers need access to the management system The relative importance of these issues to any particular operation will, of course, depend on the specific goals of that system. Thus it is not productive to go through detailed analyses in the abstract. It is sufficient to say that as with nearly all other equipment decisions in the communications industries there is not likely to be a one size fits all solution. CONCLUSION The incorporation of equipment into cable operations has begun as a method for transporting data (and voice) traffic. It seems inevitable that this trend will grow and that the selection of for transporting video services will become increasingly common. This will be encouraged by (a) the increasing importance of data services to the overall cable operation, (b) the need to increase bandwidth readily as service penetration grows, (c) the opportunity for interconnection to public networks for video delivery and (d) the desire for sourcing from multiple vendors. ACKNOWLEDGEMENTS The authors would like to recognize the contributions of Rich Brown, who was instrumental in establishing the digital transport program within our company. They also express their appreciation for the support of Fujitsu Network Systems and ABL Canada, Inc.