Alternative Technologies and Delivery Systems for Broadband /SON Access Rapid deployment of B-ISDN depends upon collaboration among potential transport, delivery, and service providers - especially for on-demand video.

Jack Terry

JACK TERRY is a Senior

Consultant and Technical

Director at Bell-Northern

Research (BNR) in Ottawa,

Canada.

58

ack in the early 1970s, during the conception of the Integrated Ser­vices Digital Network (ISDN), there was a vision that eventually all infor­mation would be transported and rout­ed in digital form. Multilevel

modulation methods and removal of redundant infor­mation were also predicted. Telecommunications engineers thought that all this would happen well before 1980; in reality, the move to digital has taken longer. Over the past few years, telephony switching and data transmission have indeed become digital. Now, sooner than the industry expected, tele­vision is also going digital.

Another major telecommunication trend is the reversal of telephony and television in their use of wired and wireless transmission media. Television bandwidth needs reuse of spectrum, made available now using coax cable and fiber trans­mission. Personal communication demands wireless for portability.

Broadband ISD N (B-ISD N) has always been con­sidered a telephone network provider domain. How­ever, recently the North American cable television industry has become aware of the tremendous band­width potential of its embedded (now almost ubiquitous) coax cable distribution systems. Moreover, the now digital direction of high-defi­nition television (HDTV) has alerted the cable TV industry to the potential digital capabilities of its coax, not just for television channel capacity but also for data, voice, and telemetry services.

Analog fiber optic transmission systems are already being deployed rapidly and universally by cable TV operators in their local and trunk distri­bution networks to reduce service outages, decrease maintenance costs, and increase video per­formance. In effect, this will provide an already-paid­for B-ISDN capability.

Fiber feeder cables being deployed by cable TV operators typically contain a large proportion of spare fiber strands. These may be used for increased analog video channel capacity or upstream paths for two-way services. However, these fiber strands are very likely to be used to expand, at very low cost using lasers carrying only digital

0163-6804/92/$03.00 1992© IEEE

signals, to provide increased B-ISDN and more TV channel capacity

8-/SDN Driving Forces

Switched on-demand video is expected to soon become a major consumer of bandwidth.

Two-way video offers telecommuting, televisiting, tele-education, and telecommuting services, which together could represent a further substantial bandwidth demand.

High-speed data services, for example, to pro­vide the fiber distributed data interface (FDDI) between locations, or even operate a computer-aided design (CAD) terminal remotely from its local area network (LAN), are likely new applications for B-ISDN. Graphic images such as X-rays or CAT scans in the medical field and high-resolu­tion color imaging in the printing and publishing business need extremely high bit rates.

However, digital video is likely to become the dominant B-ISDNbandwidth driver, particularly for on-demand entertainment services, as well as for interactive visual services in residential and business applications.

Television is Rapidly Going Digital Television is increasingly going digital in cable TV and satellite delivery of video entertainment. Digital modulation is needed in broadcasting to utilize gaps of utilized spectrum, which are normally prohibited because of cross-interference. Pro­posed quadrature amplitude modulation (QAM) schemes do not create cochannel or adjacent channel crosstalk patterning in existing analog TV channels.

The perceived driving force for more channel capacity is HDTV simulcasting. Television has also started to become digital in the studios for storage, editing, and transmission to service providers. Digital storage in today's analog VCRs creates anoth­er opportunity.

Digital television depends critically on both video compression and digital modulation technologies. Thus, it is appropriate to review video quality, decoder cost versus bit rate, and transmission bandwidth.

IEEE Communications Magazine • August 1992

ARRIS 10191

Figure la shows a scale of video quality, rang­ing from slow TY phone to perfect or lossless transmission. The quality curve shows the pro­jected bit rates needed to achieve these subjective qualities implemented in 1993 technology.

The dotted cost curve in Fig. lb shows esti­mated system-packaged receiver plus decoder sil­icon costs (in 1993 technology) for different bit rates; however, this is only the user equipment cost.

The solid optimized system cost curve shows that to deliver digital video and audio to the television set, including transmission and decoding, in cable TV or telephone company (telco) fiber-to-the­curb (FTTC) networks, a bit rate of around 8 Mb/s gives the lowest cost and offers at least S-VHS quality (no "ghosts," "echoes," "snow," or interference patterns).

About 15 Mb/s is needed to achieve videodisk quality. However, using 64-QAM modulation, videodisk quality needs only 3 MHz of band­width. Thus, two S-VHS quality channels can be transmitted in an existing 6-MHz National Tele­vision Standards Committee (NTSC) radio frequency (RF) channel. It is also possible to carry five "movie" (24 frames/s) programs within such a channel bandwidth.

Video Quality Evolution Although Fig. la refers to NTSC or PAL formats, the quality levels at and above S-VHS can only be obtained using bandwidths beyond that of TV RF channels. Digitized and moderately compressed (to about 14 Mb/s) NTSC- or PAL-format video offers potentially higher (perhaps double) horizontal resolution and artifact-free video to commonly avail­able monitor-style TV sets having direct video inputs.

Currently available program material and studio camera, production, and storage equipment already provides at least 750 lines of horizontal resolu­tion. Optionally, further improvement in picture quality can be attained at the TV set by using higher-frame-rate noninterlaced display techniques. Digital signal processing can be used to increase apparent vertical resolution.

Thus, digital TV offers overall quality and res­olution close to HDTV without the need to change studio equipment or transmission bandwidth. All existing program material is fully compatible. In the home, quality depends on the receiver used. Regular RF TVs will deliver ghost-free, vir­tually noiseless, color-precise pictures. Monitor-style TVs offer even higher horizontal resolution.

Future 33-in. TVs with added digital signal processing (DSP) and higher-frame-rate nonin­terlaced displays could offer the same fineness of resolution as a 48-in. HDTV set, but at perhaps one tenth the cost. Techniques are known today for doubling vertical resolution using existing cameras and other studio equipment to provide a signal that is compatible with present TV sets. In Europe, there is a move toward changing the shape of TV tubes to display wider-aspect-ratio pictures using existing channel bandwidths and resolution. Combining this wide screen strategy with some of the above digital higher resolution possi­bilities to provide extended definition TV (EDTV) could potentially defer the need, and thus expense, of HDTV for some time while providing a smoother path for its eventual introduction. However, such top­ics go beyond the intended scope of this article.

IEEE Communications Magazine• August 1992

~­ Subjective video quality (NTSC/ PAL format) ~~

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.,. VJtS ($P)-i.

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• Figure 1. Digital video quality cost and bandwidth.

Regardless of the choice of video quality, the key issue today is how to deliver tomorrow's digi­tal TV and other B-ISDN services into the home using existing ubiquitous transmission facilities such as twisted pairs and coax cables with efficient dig­ital modulation.

Digital Modulation QAM is an efficient, spectrally compatible, and pop­ular modulation scheme for digital delivery on media having limited bandwidth, such as coax or twist­ed-pair cables and wireless.

QAM using a full matrix of modulation points and practical filters provides a spectral efficiency of about 0.84*Log2(n) b/s/Hz, where n is the number of possible modulation states for a single symbol. For example, a 4-b/symbol signal having 16 possible states ( 16-QAM) has a spectral efficiency of approximately 3.36 b/s/Hz.

Amore optimal arrangement uses a subset of pos­sible matrix points, usually in a more circular rather than rectangular grouping, as shown in Fig. 2. In this example, 32 points out of a possible 64 are used. Such a "32-QAM" scheme provides around 4.2 b/s/Hz. Thus, three very-high-quality video signals could be transmitted within a con­ventional RF TV channel. Higher-order circular QAM schemes can be used to achieve even greater spectral efficiencies, but these require lower dis-

119 Mb/$ . ~· 31 .. t.-tHE

t

59

2

Digi,tal

television

transmission

offers many

of the

attributes

of HDTV

but at much

lower cost

and with

existing TV

set and

studio

system com­

patibility.

60

tortion and noise in the transmission channel-pos­sible in the future as fiber penetration extends closer to the user and transmission distances in coax and twisted pair decrease.

Spectral compatibility is improved if the digi­tal channel spectrum is made to look like flat band-limited noise. This can be achieved by pseudo­randomizing the digital stream prior to QAM modulation. Four-level vestigial sideband modu­lation has also been proposed. However, this scheme has a somewhat lower spectral efficiency than 16-QAM and departs significantly from the desired flat noise spectrum.

Digital Formats The primary requirements for formatting digital sig­nals are simplicity, low cost, and open-endedness to provide flexibility for a wide range of present and future (as yet undefined) services.

Label multiplexing, similar to that used in Asynchronous Transfer Mode (ATM) networks, provides rate adaptation, or bandwidth on demand. Within a common structure, bit rates per service can range from a few bits per second for telemetry and signaling to perhaps 10 Mb/s for a high-quali­ty digital video channel, or even 52 Mb/s or more for business data carried in Synchronous Optical NETwork (SONET) STS-1 or even OC3 format. This means that in low-cost access systems, SO NET formats should be carried within ATM struc­tures rather than the more conventional method of carrying the bursty ATM signals within SONET structures.

The issue of carrying several "channels" of digi ta! video within a single QAM stream versus pro­viding a separate QAM signal per video service is still a subject of debate for digitally overlaid cable TV networks.

The statistical advantage of ATM-multiplex­ing a number of digital video channels together is significant. However, although this technique offers greater overall channel capacity, it still results in the need to provide a separate digital TV "tuner" per channel being received-an issue that recedes as each TV receiver, VCR, and personal comput­er receives its own digital signal directly, instead of using shared TV set-top converter functions.

Fiber feeder

-->.-.f1 .... ,_____

Bridger amplifier

.i •.""~J

•Figure 2. QAM matrix points.

Delivery Networks

Fiber-fed cable TV coax and telco-provided fiber­fed short twisted pairs both offer potential B­ISDN delivery. Each has at least 95 percent ofN orth American TV homes connected or passed, and each has the potential to provide spectrum reuse. In the wireless domain, both broadcast TV and satellite transmitter stations can also provide B-ISD N services, but on a more limited spectral basis. These wireless systems offer almost instant uni­versal coverage, but presently require telephone loop paths for upstream slow-speed data, control, sig­naling, and authentication.

Cable TV 8-/SDN Delivery Fiber Deployment Both large multiple system operators (MSOs) and several small single-network cable TV opera­tors are rapidly installing analog fiber optic trunks and feeders to within a maximum of one or two miles from the home to dramatically reduce service outages and maintenance cost and increase picture quality. Payback for such systems in

Coax distribution

Spare fiber strands

1.11111111~. 450MHz

•Figure 3. Fiber feeders in cable-TV distribution.

IEEE Communications Magazine • August 1992

3

terms of reduced maintenance can occur within a very few years [1 ]. When subscriber retention and advertising revenue is taken into account, the payback period may be even shorter.

Figure 3 shows fiber systems used to reduce the number of tandemed bridger amplifiers to five or less. This is a major reduction from the typical string of up to 30 or 40 amplifiers that often exists prior to the introduction of fiber trunks and feeders. Normally, a maximum of two line-extender amplifiers can also exist between a bridger amplifier and the home.

Fiber cable TV installations usually include the provision of a large ratio of spare "dark" fiber strands. The rationale for this is that to provide for future on-demand services, smaller numbers of customers should be served from each fiber. Upstream paths will also be needed for control, two­way video and audio, data, and personal commu­nication network (PCN) services. Additional fiber strands may also be used to create ring structures, and thus reduce service outages even further. The capital cost of providing these spare fiber strands is minimal, often much less than $10/household.

Digital Capabilities Today's coax cable TV feeder and distribution systems can carry at least 16-QAM or higher­order modulated digital signals above, below, or with­in conventional TV channels. A large number of narrow digital bands can each be used to carry a single digital stream. Alternatively, a smaller number of digital bands (for example, as shown in Fig. 4) can each be made to carry a number of digitally multiplexed signals. In either case, the width of each digital band can be assigned to suit any individual service.

Existing coax bridger and line extender ampli­fiers can easily carry QAM signals and provide very low bit error rate (BER) performance, provided that the sum of the peaks of existing amplitude-modu-

•Figure 4. Co-existence of QAM digital signals in cable-TV distribution.

lated (AM) VSB analog TV channels do not themselves create amplifier overload. To achieve a low BER, the required QAM signal level is typically 10 dB below that of the analog TV channel carriers.

Coax amplifiers with increased channel capac­ity for service upgrades are specified, in terms of signal output with low intermodulation, for a maximum load of AM VSB television channels. Under this "normal" condition there is a frequent probability of peak overload, but this is rarely detectable by television viewers. Unfortunately, the impact of such overloads on digital services carried on the same coax distribution is much more serious. However, if a reasonably large pro­portion of the service upgrade amplifier band­width is used for the lower-level digital services, the peak signal level in these amplifiers is such that overload rarely, if ever, occurs. Thus, only a small degree of forward error conection is needed to protect digital TV and other B-ISDN services. In the case of analog fiber feeder systems, the same peak overload design criteria exist. Often, such fiber systems are engineered to each carry fewer channels than the coax systems they feed; thus, two or more fibers are needed to provide a single analog feeder capacity .

... <---Headend ---->•f4--Feeder~i"'j<..------,Distribution

~mmu1. ~ 450 MHz

services

nQAM: nQAM modulator

E/O: Electro-optic interface O/E: Optoelectric interface

Bridger amplifier

• Figure 5. Digital service delivery in a cable-TV network.

IEEE Communications Magazine• August 1992

450MHz

61

4

A short

twisted pair

loop has the

potential to

deliver at

least three

high quality

digital video

channels.

Many such

loops can be

served by a

single, fiber

ring-fed,

curbside

module.

62

Input

Tul'\er/ Q"61 ~·

Remote control

Video output

Audio outputs

• Figure 6. A digital set-top converter.

The introduction of fiber to within perhaps a max­imum of seven amplifiers from the home allows the possible use of higher-order QAM signals, perhaps as high as 128-QAM. Thus, digital capac­ities in excess of 3 Gb/s, equivalent to over 300 very-high-quality digital TV channels, can be pro­vided with adequate reserve capacity for data or other B-ISDN services.

Figure 5 contains video coders for digitizing premium/switched programs. The n-QAM modu­lators offer flexibility in terms of QAM levels, thus providing for future capacity upgrades. Upstream functions (not shown) include reverse amplifiers and filters (contained in the bridger ampli­fier housings), plus one or more digital fiber feeders to carry these signals to the headends.

Digital services can be introduced across a network incrementally and without cost or distur­bance to existing non-digital subscribers. Only those users requiring the digital services need digital set-top converters.

Service Expansion While some digital service capacity can be provid­ed within the analog fiber feeder shown in the top portion of Fig. 5, the spare fiber strands can be used to provide more on-demand service capacity by extending these directly to some or all of the bridger amplifiers. In effect, this allows the provision of an individual spectrum of digital services to each group of 100 to 200 homes, in addition to the delivery of a common set of AM VSB television chan­nels. The combined spectrum fed to each receiv­ing equipment is also shown in Fig. 5.

Since the fiber strands providing the spectrum re-use service extension carry only QAM digital signals, the cost of the required laser transmitters is perhaps only 15 percent that of those used for ana­log services. Using such low-cost lasers, an incre­mental capital cost of providing digital service expansion is potentially less than $10/household. In cases where the number of analog channels is already high, the provisioning of digital services may be realized using entirely separate digital fiber feed­er strands. Ultimately, in North America, about 500,000 digital fiber feeders would be needed to fully utilize available coax capacity.

The substitution of digital spectrum at each bridger amplifier requires a simple passive filter.

Home (or Business) Equipment

Figure 6 shows the functions needed to receive and present a digitally carried TV signal to a monitor-style television set. Most of the functions can be achieved using today's state-of-the-art cus­tom silicon technology. A reasonable cost objec­tive should not exceed, by much, that of an equivalent analog TV set-top converter. The addition of a digital decryption function is essen­tial for pay-per-view (PPV) applications.

The QAM receiver and decryption chips can also be used in data interface cards contained in PCs or workstations. The QAM modulator function need­ed for upstream data is relatively simple to achieve.

Upstream Flow Control The issue of upstream digital signal contention can be resolved by broadcasting enabling signals or tokens, unique to each user equipment, from the bridger amplifier outputs. Upstream func­tions contained in each bridger amplifier housing should include a small amount of input buffering for flow control of the ATM-like signals.

Short Twisted Pair 8-/SDN Delivery

Telco-provided twisted pair drops and home telephone wiring have considerable digital capa­

bilities using QAM if their lengths are kept short. Figure 7 shows how this capacity varies with length and order of QAM.

B-ISDN services can be more easily provided on a separate twisted pair for each home. Howev­er, where a second pair does not exist, it is possi­ble to combine existing telephone and B-ISDN within a single pair.

At least three high-quality digital TV channels (or as many as seven movie-quality channels) per loop can be achieved using 32-QAM and a maxi­mum loop length of 800 ft. At this loop length in typical suburban home densities, at least 24 such loops can be served by a single fiber-ring­fed optoelectronic curbside module, as shown in Fig. 8.

The fiber rings can be used to provide large volumes of highly reliable service using scrambled

.l!! ~ 100 ~ Mb/s

10

Mb/s 100 200 300 500 1000 ft ft ft ft ft

# 24 AWG loop length

• Figure 7. Broadband perfonnance of short twist­ed pair loops.

IEEE Communications Magazine• August 1992

5

binary transmission. Normally all signals flow in one direction, with idle ("dark") timeslots provided for upstream traffic or control. In the event of fiber damage, two-way transmission can be main­tained in each part of the separated ring using time-compression multiplexing techniques. Alter­natively, the overall system design could use wavelength division to provide bidirectional traf­fic in the ring.

Individual twisted-pair-service delivery elimi­nates the need for QAM receiver frequency agili­ty or decryption functions in the home equipment, since spectrum and access control is provided in the curbside interface module. Only a single QAM receiver is needed per twisted-pair drop for all TV and data channels. Thus, the cost of home equipment is potentially less than that of cable TV service delivery. A "picture in picture" fea­ture could even be included in the video decoder at a small incremental cost.

Home wiring requires only a low-cost active split­ter at the home entry point plus plug-in resistive ele­ments at the outlets.

Using this proposed system, delivery of digital television or other B-ISD N services over short twist­ed-pair loops can compete very well with cable TV's digitally overlaid coax in terms of capacity, home equipment cost, and quality of service. The fiber ring with individual twisted pairs for each home is superior in terms of continuity of service. The only drawback is that the capital and maintenance cost of the fiber and curbside optoelectronic modules must be borne entirely by the video dial tone and other B-ISDN services. Thus, there is a "log-jam" situation: the need for new service revenue to pay for service provisioning.

TV Broadcast Digital Delivery

A s stated earlier, channels considered taboo for added AM VSB services, due to cochan­

nel and adjacent channel interference problems, can be utilized when digital QAM modulation is used. This available channel capacity with digital compression could result in at least a fourfold increase in program capacity within most urban and sub­urban communities. An order of magnitude less transmitter power is needed, compared with TV channel peak power, and, using 16-QAM and 9-Mb/s video coding compression, two TV programs can fit within a 6-MHz channel.

Asymmetrical data services could be provided using telephone lines for lower-speed upstream data and/or command signals.

Satellite Digital Delivery Satellite TV transmission has already made the switch to digital for PPV services. The receiving antenna and its installation (or automatic positioning system) and higher-frequency receiver fr on tends contribute to additional subscriber costs. However, satellite­delivered video/B-ISDN is very rapidly and uni­versally deployable. Spectrum reuse, although possible using spot beam approaches, is today relatively difficult and potentially costly. Forupstream signaling, the telephone network remains the best choice, although in the longer term, polled upstream transmission to a satellite would offer bet­ter service due to the avoidance of conflict with telephone calls.

IEEE Communications Magazine • August 1992

Central office

• Figure 8. A fiber ring B-ISDN delivery system.

Services

Services and functionality for the end user ultimately create most of the business revenue. Services to ven­dors, such as advertisers, provide additional rev­enues. However, at the end of every "value chain" it is the end user or end chooser who must be satisfied.

A variety ofnew information and interaction ser­vices, such as electronic shopping, banking, reser­vations, education, and library searches, have been tried, with mixed commercial success. How­ever, the North American videotape rental business currently has annual revenues of around $12 billion. If the inconvenience of time and travel to collect, and later return, the videotape could be eliminat­ed, it is predicted that the size of this business could be doubled. (A proven nonelectronic business solution used to resolve the VCR tape collec­tion/return problem is the "Pizza + Video" deliv­ery service!)

If sufficient spectral capacity and low enough cost of digital TV receiver/converter functions could be achieved, on-demand entertainment in the form of impulse PPV (IPPV) could become the most significant "dial-tone video" service, which in turn could pay for B-ISDN "delivery" network(s).

Initially, on-demand PPV could be provided on a 15-min. maximum wait basis. However, IPPV involving the individual switching of ser­vices could open up further possibilities, e.g., any program being viewed could be "paused" to accommodate interruptions, thus allowing the user to continue viewing without losing continu­ity. Previous days' programs could also be made avail­able on a broadcast or near-on-demand basis. Interactive services for individuals or groups could include car/appliance/home repair, cook­ing, and education.

Two-way dial-tone video, providing interac­tion between two or more geographically separat­ed users, creates other business opportunities. One example is tele-education, in which students or groups of students could interact with the instruc­tor or educator. Teleconsulting, medical advice, more "how-to" services-the list of potential opportu­nities is limited only by business imagination. However, it is unlikely that any individual or combination of these "special" or two-way ser­vices alone could produce sufficient revenue to pay for B-ISDN delivery. It is the high-volume custom entertainment market that will create the economic base for B-ISDN as we move towards the end of this century.

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6

Collabora-

tion between

potential

B-ISDN

transport,

delivery, and

se11J1ce

providers

is the key

to rapid,

effective, and

economic

deployment

of B-ISDN

64

Network Partitioning and Ownership

Evolution of video networks and other B-ISDN services is likely to create new network and busi­ness partitioning. Studio material, information video databases, advertising, etc. are likely to reside in ded­icated video warehouses [2] interconnected to each other, program sources, and delivery net­work headends. National fiber networks comprising high-capacity transmission links and switching nodes are the likely vehicles for providing digital video transport.

Summary

Video transmission, processing, and storage are now rapidly going digital. Broadband dig­

ital delivery is expected to move towards a com­mon QAM approach in cable TV coax and fiber, telco twisted pairs, and wireless. Digital television transmission offers many of the performance and feature attributes of HDTV but at much lower cost, sooner, and with existing TV set and studio sys­tem compatibility.

Deployment of fiber optics into cable TV net­works is paid for very rapidly through existing ser­vice maintenance savings. Customer satisfaction is also increased due to reduced service outages and video quality improvement. The same fiber optics deployment also offers a low-cost near-term digi­tal infrastructure for B-ISDN! Digital "spectrum reuse," using the already installed "spare" fiber strands, costs far less than analog because laser linearity performance needed to carry QAM digi­tal signals is much easier to achieve.

Short twisted-pair loops can potentially deliver at least three switched high-quality digital video chan­nels, or at least six movie-quality channels. Such loops can be served by a single fiber-ring-fed curbside mod­ule.Twisted-pair B-ISDN delivery is superior to that of cable TV in terms of continuity of service,

competes well in capacity, and requires simpler home receiving equipment. However, the B-ISDN delivery system cost must be carried entirely by the new services, a significant business barrier.

Broadcast TV stations and satellites can also be used effectively and rapidly to deploy B-ISDN services. Broadcast station video spectral reuse offers a fourfold capacity increase in most communities using digital techniques.

Future digital television sets will require single QAM and video compression standards, regard­less of who delivers the services. Digital set-top converter technologies will migrate into television sets, V CRs, PCs, etc. Eventually, as is now happening in today's analog cable TV era, digital set-top converters will themselves become redundant.

The potential B-ISD N delivery network providers have a set of complementary business, technical, and embedded-base capabilities. Collaboration between potential B-ISDN transport, delivery, and service providers is the key to rapid, effec­tive, and economic deployment of B-ISDN, par­ticularly for on-demand video.

References [I] K. Casey, "The Economic, Performance and Strategy Factors of Fiber

Optics in Cable Television," Fiber Optics Plus 1992, San Diego, Calif., Jan. 1992.

[2] A. T. Futro, "Digital Compressed Video On-Demand, An Alternative Sys­tem View," Cablelabs-Specs International, vol. 2, no. 1, Jan. 1992.

Biography JACK TERRY [SM] is a Senior Consultant and Technical Director at Bell-North­ern Research (BNR) in Ottawa, Ontario, Canada. He is currently responsible for the development of advanced system architectures and technologies required for broadband switching, transmission, and access systems. His previous product development responsibilities at BNR have included the management and technical direction of stored pro­gram digital switching and access systems and their component tech­nologies. Prior to joining BNR in 1974, while at Marconi Communication Systems in England, Jack contributed in the fields of broadcast video trans­mitter and studio equipment design, digital switching, PCM transmission, and stored program controlled systems. He is a member of SCTE and Sigma Xi.

IEEE Communications Magazine• August 1992

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