COPYRIGHT 1986, -1995 H & K COMMUNICATIONS, INC., ALL RIGHTS RESERVED
KNET THEORY 1
1.2 Design goals & standards, philosophy and methodology
1.2.1 Design goals1.2.2 National Standards.
1.2.3.1 Manufacturability
1.2.3.2 High reliability design
1.3 Basic external specifications & capabilities
1.4 Comparative analysis of the KNET twisted pair bus architecture
1.4.1 The wiring problem1.4.2 Wireless alternatives
1.4.3 Cable topologies
1.4.4 Multiplexing techniques
1.5 General KNET system theory of operation
1.5.1 Overview1.5.2 KNET devices
1.5.2.1 KNET Phone
1.5.2.2 CO Trunk Interface Card
1.5.2.3 VCIF (KNET Controller InterFace)
1.5.2.4 KNET Controller
1.5.3 KNET Bus
1.5.3.1 Power distribution
1.5.3.2 Control link
1.5.3.3 Voice channel multiplexing and switching
1.2 Design goals & standards, philosophy and methodology
1.2.1 Design goals
The KNET Telephone System was created to bring the performance, convenience and low costs of the modern consumer electronics revolution to the small business and home office user. Current analog loop technology was invented in 1876, touch tone and the 1A2 multibutton key set, in the 1950s. Although modern PBXs and Key systems are more sophisticated, they have debatable performance improvements in light of comparable or increased costs. The stubborn persistence of the analog single line set and the 1A2 Key set in the market is proof of this contention. H & K Communications believed by appropriately applying the full spectrum of modern electronics technology, a business- home office telephone system could be designed with an improvement in costs (direct and life cycle) and performance (improved productivity and convenience).1.2.2 National Standards.
The system conforms to all applicable U.S. standards and design guides. Specifically: EIA Standard RS-464, RS-464-1, RS-470, & RS-478; FCC part 68 and part 15; NEC Article 725; ANSI C62.41/IEEE Standard 587- 1980; ANSI/IEEE Standard 518-1982, 820-1984, 269-1983, 743-983, 753-1983; ANSI C63.4-1981,1.2.3.1 Manufacturability
System components are designed to allow low cost, high volume production. All KNET components use multisource commodity parts. There is a high commonalty of parts among circuit modules to lower the cost associated with handling and stocking different components. The basic KNET System has a minimum of different assemblies to manufacture and inventory. No circuit adjustments or selected components are required. The designs are based on worse case component tolerance runout in order to realize high manufacturing yields.1.2.3.2 High reliability design
All KNET components are reliable and hard to break in the full operating, manufacturing, and service environment. This means power supplies are short proof, or if they do fail, protection circuits will limit KNET bus loading. Failed devices won't disable working devices. Circuit node impedances are held within upper and lower bounds to minimize humidity leakage without compromising power consumption. All circuitry is designed to be ESD (ElectroStatic Discharge) proof (Up to 25KV). All software systems are monitored by watchdog circuitry which will automatically re-boot a crashed system. Misaligning plugs or making connections while powered up won't cause damage. Modular plug connections can be doubled to improve reliability. No batteries are used in devices for memory retention. Softfail architecture means that a controller failure will not immediately terminate a call in progress. The system or phone will recover automaticly from a power fail or disconnection without user intervention.1.3 Basic external specifications & capabilities
The KNET System is a telephone PBX or a Key System, with the following external characteristics: All voice, control, and power distribution occurs through a single common twisted pair bus, the KNET. The voice path capacity of the KNET bus is currently 28 simultaneous, single voice channels, allocated in adjacent pairs to make 14 full-duplex voice paths. These pairs have a 200 Hz to 5khz audio bandwith which is 50% more than standard telephone systems. The bandwith may be further increased with a reduction in number of channels by changing components in KNET devices and with suitable software changes. The maximum range of voice paths is over 3,000 feet on 26 to 22 guage twisted pair. Range is a function of channel frequency and twisted pair gauge. Power is supplied at the CO INTERFACE BOX, or at additional points on the KNET by power boosters. The power source is a 50- 60 volt .5 -1.0 ampere source buffered by a circuit designed to prevent loading of the voice and communication channels. In addition, it has circuitry designed to prevent damage to a standard phone inadvertently plugged into the KNET. Each KNET phone pulls about 2.5 watts maximum (less during idle) and each 3-CO trunk card pulls about 4 watts. Loading calculations indicate about 16 phones evenly distributed along 1000 feet of 22 guage can be driven from a single power source at one end. All KNET devices have low voltage indicators to aid in determining the need and location of additional power boosters. The KNET communication link has a range in excess of 3,000 feet. The link has a basic rate of 9600 bits/s and an effective data rate of 150 to 600 characters (8 bit) per second. It was designed to handle simultaneous, multiple contending devices, quickly. Specifically, it can handle up to 70 independent control messages per second. The link can send a message to or from any of up to 256 devices within 20 ms. The link was designed not to be a bulk data carrier, but to have sufficient capacity to cost effectively handle control and display (32 characters or less) packets for up to 32 Key telephones. The KNET Phone has 49 software assignable buttons and a 32 character alpha-numeric LCD display. A single full-duplex voice path is switchable to any KNET voice channel. In addition, it has a speaker for on hook dialing, monitoring, and ringer. The audio circuitry provides for 16 volume levels, 2 tone generators, 1 white noise generator, numerous cadences, signaling beep, mike, and speaker switching. The KNET 3-CO Cards provide audio interfaces up to 3 Central Office trunks. It provides for trunk and call progress signaling; specifically, ring detect, loop connect, wink detect, touch-tone and dial pulse dialing, and call progress tone energy and cadence detect. It also provides for automatic line loss compensation and touch-tone decoding for automatic extension dialing or remote feature activation. It has conferencing circuitry which allows external and internal conferencing, although internal conferencing may require extra trunk interfaces. It has a full duplex speakerphone circuit per line. And finally, the 3CO Interface has automatic line balancing circuitry to increase line hybrid return loss so as to allow volume control without excessive sidetone, amplified conferencing, and improved speakerphone operation. The VCIF (KNET Controller Interface) provides an isolated communications interface between the KNET Controller (typically a Personal Computer) and the KNET. There is only one VCIF per system controller (there may be back-up controllers). The VCIF performs KNET communication protocol translation to the PC bus of the KNET Controller. The KNET Controller is an unmodified IBM PC compatible computer. The control code is written in "C" and operates under MS-DOS. The Personal Computer can also serve as a powerful interactive terminal for attendant functions, and additional application software and capabilities such as voice mail and SMDR. The KNET Controller can execute other software in the foreground while controlling the KNET in the background.1.4 Comparative analysis of the KNET twisted pair bus architecture
1.4.1 The wiring problem
Currently most PBX, Key systems, or central office systems distribute voice, control, and power with 1-4 pair wiring starred out of a central switch. Old mechanical (1A2) key systems use a 25 pair cable (thick wire) as a bus (also often starred) out of a KSU (Key Service Unit). Modern electronic key systems typically use 2-4 pair (skinny wire) starred out of the KSU. These type systems are sometimes called hybrid key systems because they have PBX structures and related features. Very simple electronic key systems with 3 or less outside lines may use a 4 pair bus (3 CO lines, 1 intercom) wiring scheme without a KSU. In all of the above systems voice is distributed as a single full duplex path per wire pair whether or not the voice signal is digitized. In systems with extra lines, the lines are used for signaling, and power. This wiring produces considerable long term costs in skilled labor to install or to make "moves and changes". With long term system cost reduction as the primary design goal, various digital, frequency, and hybrid multiplexing schemes were investigated. Also, wireless techniques, such as infrared, ultrasound, leaky coaxial cable, induction loop, radio, and power line carrier were considered.1.4.2 Wireless alternatives
Even though a fully wireless system would be ideal, there is still a need for power. Without a line cord, a battery system would need to be recharged or replaced periodically. Assuming a phone has a 10 mW average draw (a very optimistic and costly design goal) and a 20 watt-hour battery (4 "D" cells, probably the maximum practical), the battery would need to be recharged every 3-6 months. Any longer interval with a nickel-cadmium or nickel-hydride battery wouldn't be possible because self- discharge will cause it to lose half its capacity in 3-6 months. Throwaway alkaline batteries might triple the service interval. Another problem with batteries are there poor reliability which can become a significant service problem in any but the smallest system. H & K has had experience with low power drain communication systems and determined that although theoretically possible such a battery system would be too expensive and compromised in performance for use in most business phone stations in a system. Any practical wireless system would still have a power cord either to the unit or a charger. Of course for mobility needs there is reasonable justification for some phones in a business system being cordless. Ignoring power considerations, the wireless techniques of infrared and ultrasound suffer from line of sight and practical range limitations of 20 to 100 feet. Infrared is range limited due to the poor signal to noise ratios of the receiver which is caused by high background ambient noise multiplied by the ultrawide detector bandwith and its very low omnidirectional signal capture area. Ultrasound, on the otherhand, suffers from bandwidth limitations in air (over these distances) of 50 to 100 kHz due to attenuation with increasing frequency. It also suffers from severe multipath and directional propagation characteristics. With enough local relay stations tied into a central switch (like cellular radio), it is possible to get around the limitations of infrared and ultrasound, but to do so is not economically viable for business phone applications. Radio is the most viable wireless technique and includes induction loop and leaky coaxial cable which are near-field antenna techniques. Suitable radio frequency spectrum for this application is presently not available except for some bands on which spread spectrum techniques may be used. There are near term possibilities (5 to 10 years) for spectrum for PCN (Personal Communications Networks) which can be though of as mini-cellular phone/modem systems with ranges of about 1000 feet to a local node. PCN systems may be under the control of a public communications companies which will tariff the use at a rate of about 1/5 to 1/10 the cellular telephone rate. The FCC may decide that there is not enough spectrum space to allow usage of this spectrum without the disincentive of charging per minute especially when effective and lower cost wired techniques are available. Gaining spectrum from the FCC is a longterm bureaucratic process requiring demonstration of technical viability and compelling public priority over other radio needs. Priority usually means there is no other viable way to perform the service without radio; such as in mobile dispatch, aircraft and marine communications, widearea public broadcast, etc. The presence of viable wired telephone systems would disincline the FCC to approve channels for the same purpose. For example, cellular telephone, only recently, got spectrum space a decade after demonstrating technical viability and strong public demand; and then only because, it was more spectrum efficient, used less desirable short range UHF frequencies, and was a premium service capable of serving only 1-2 % of the telephone population. On top of that, it took another 5 years of "legal turf wars" to become commercially available. Radio systems suffer, relative to wired systems, from the major problem of highly variable propagation characteristics (each frequency band has its own peculiar problems). The radio signal may not be able to penetrate through the walls, ceilings, and floors of a building to a nearby extension but is picked up by a similar system 10 times as far away in an adjacent building. Part of the propagation problem is the wide 100 dB+ dynamic range of two-way radio signals. These signal ranges coupled to spectrum crowding make interfering spurious responses expensive to overcome. Spread spectrum techniques can make a significant improvement in these propagation problems but at significant premium in cost and power consumption. Although for cordless and movable (in building) data applications, spread spectrum systems are available but at significant costs. There is some interest in using radio to provide customer access to a telephone carrier by replacing the wire link between building and pole (or pedestal) with a multichannel digital radio link. Such a system may be cheaper and more reliable by eliminating the need for the installation labor by the telephone company, by allowing multiplexing all the way down the local loop, and by improving the resolution of who's at fault when the phone doesn't work. In addition such a system makes it economically possible for another telephone carrier to compete against the local telephone company because it needs only to signup a small fraction (a few per cent) of the total number of customers within the pole coverage range (~1000 to 2000 ft radius) in order to cost justify the pole or pedestal transciever whose capacity would probably increment in about 20 channel steps (a single multiplex digital transciever). In order for the customer to gain radio access to such a service, he may need to mount an antenna with a unblocked radio path; such as, in the attic, through a window, or outside the building. Such radio access to the telephone network systems will not obsolete the need for PBXs or Key systems. In fact, they may spur a need for a multichannel PBX or Key system on the customer's side. Since radio replacement of the pole to customer connection may use digital multiplexed radio, it may be possible to give the customer several channel capacity for very little incremental equipment cost. The radio access telephone carrier would have a very strong incentive to ensure that customer line access equipment have a maximum cost effective capacity (probably at least 2 channels). For a radio access telephone provider there is no cost of individual fixed wire loop per telephone. Since virtually all of the costs will be associated with usage, the radio telephone provider might only charge for usage and eliminate or reduce the fixed monthly fee. This means that the radio telephone provider would like to encourage the use of "virtual" lines in the home, allowing as many users as possible access to the network through a single radio link. Because customers might have access to 3 or more "virtual" lines simultaneously without paying the monthly fee for these lines, they would have an incentive to install a key system phone system to give them access and control of these "virtual" lines. Induction loops (used on low frequencies) and leaky coaxial cables (used on high frequencies) are sort of half-way solutions between radio and wire systems with characteristics of both. They can usually be designed to get around FCC problems. They have better one-way propagation characteristics, but are not secure from nearby radio eavesdropping, or immune from interfering with a similar system in an adjacent office. The dynamic range is similar to regular radio if signals go directly between phones. If all signals go through a relay hooked to the coax or loops, the dynamic range is less but the expense is higher. Also, the coax or loop is not a labor improvement over a simple wired system because the loops or coax needs to be correctly strung, and fed signals at the right points by a skilled technician. Most commercial applications of these techniques are used to augment radio services (such as bringing radio into a mine, tunnel, or basement building or providing local paging) rather than replacing wire based signals. Power line carrier systems which use the AC power line as a transmission medium are alright for low speed local data transfer. However, for voice signals the medium is too noisy due to load switching and poor line balance, and it has insufficient reliable bandwith due to transient and RFI (Radio Frequency Interference) filtering in both power distribution strips and AC loads. Also, the signal range is usually constrained to the outlets fed from a single tap on the power line distribution transformer. The user may not be able to communicate between the two power plugs on either side of a room without adding high voltage bypass capacitors across power lines at a breaker box.1.4.3 Cable topologies
The simplest and least costly telephone interconnect topology is a single twisted pair bus. However, due to multiplexing difficulty, virtually all modern telephone systems use star topologies with twisted pair wiring. Such systems place the switching circuitry at the hub of the star rather than in each phone as in KNET. Mechanical key systems, the only significant exception, place most switching in each key set using the common KSU for auxiliary functions and using an expensive 25 pair cable as a common interconnect bus. If compared against the KNET system's distributed switching, a maximally loaded central switch with standard phones or a plain key system may offer a slightly lower switching cost per phone. If not fully loaded, the fixed cost of the central switch or KSU may make the KNET less expensive. Most compelling though, any disadvantage KNET has in capital cost against star topologies is overwhelmed by the life cycle costs. Industry figures indicate the cost to install the average business phone is $100-$150 per phone and it is moved once a year at an average cost of $75+. The KNET bus structure should at least cut these figures in half by significantly lowering labor and material costs through reductions in cable lengths, wiring complexity, and number of connectors. It should also be possible for a large percentage of users to install or make "moves and changes" themselves with considerably less disruption or delay in service. Future downward cost spirals of electronic equipment and upward spiraling labor costs will make this analysis even more viable through the next decade. More complex wiring schemes were considered. Some of these provided somewhat lower equipment costs but suffered increases in typical life cycle costs. Despite the apparent advantages of the single twisted pair bus preliminary analysis indicated it could be potentially less reliable than a star structure. In a star structure an unreliable cable or connector will cause only a single phone or trunk to fail, but in a bus structured system such a problem may cause the whole system to malfunction. Device malfunctions can produce bus failures. (Ring topologies do even worse since node failures are usually fatal to system operation.) Consequently, the KNET bus structure was designed to be as reliable as a star structure. If a KNET device fails there are a number of protective mechanisms designed into it to prevent disabling the KNET. Cabling problems are the most likely source of failure for the KNET. Since connector opens are the overwhelming source of cable problems, these typically limit failure to individual phones. (Most of these are due to modular cable crimp failure or conductor breakage due to flexing cycles.) The only time shorts are likely to occur are during installation or changes in wall wiring. Shorts will not harm the KNET which has automatic resettable fusing. Consequently, it is not expected that the bus topology of KNET will result in lower reliability than star wired phone systems.1.4.4 Multiplexing techniques
Digital techniques were thoroughly investigated and found not to be optimal for local voice distribution. Digital voice coding techniques although expensive may be appropriate in large systems. In such systems they provide immunity from incremental degradations that analog signals experience when going through numerous long distance links and switches. Small systems typically have less need for noise immunity because noise pickup tends be a function of system size and complexity. Large systems have larger wiring "antennas" and more sources for cross talk. 1) Digital coding in current telephone systems Digital voice encoding tends to be expensive due to the added cost of analog to digital conversion, anti-aliasing filtering and time slot assignment circuitry. This fixed cost per phone may be offset by savings on switching and intersystem trunking costs if the system is large enough. Digital switching and trunk multiplexing techniques have a lower rate of cost increase with size although they have a higher minimum system cost than analog techniques. Because telephone system circuitry per phone increases with system size, at some break even point (~100 phones) the savings on digital coding offset the conversion and minimum system costs. (This analysis doesn't apply to long distance trunking where bandwith and repeater spacing considerations may give analog systems cost and capacity advantages.) In fact all low to moderate cost phone systems of less than 60 phones use analog switching techniques. These systems have no worse and often better (greater bandwith and lower distortion) audio compared to digital systems. On such systems the main source of audio degradations are the analog loop trunk lines from the central office. Only systems large enough to bring digital trunk carriers (more than 24 trunks) directly into the switch can avoid this problem. There is an industry trend on high end digital systems to do the analog to digital conversion at the phone and send the digital data via proprietary high speed modems to the central switch. This is also the goal of ISDN (Integrated Services Data Network) which will be provided by the local telephone companies. This saves nothing on costs, but adds the expense of the modems. It does provide digital communication connect capability at the phone but usually with a several fold increase in cost over a data only connect system. Perhaps the only viable advantage these systems have over standard systems is that the integrated data path allows implementing sophisticated feature phone or voice/data functions. Some of these systems potentially can save some money by avoiding separate cabling for the data system. However, analysis of typical users suggest this saving is small due to the following factors: 1) Phones outnumber terminals by more than 2 to 1 in most environments. 2) Although adding data paths to a location is a major cabling problem, another major problem is adding additional voice paths to a location. 3) Because of wiring uncertainty, multiline cable of sufficient capacity is usually run anyway to any phone or terminal termination to allow a reasonable increase in users. 4) Voice/data PBXs can usually only service a subset of the common data terminals. They do not provide universal connectivity. 2) Digital coding on a bus based system Digital transmission on a bus based system is the basis for a number of commercial LAN (Local Area Network) data communication products. Telephone PBXs have been experimentally demonstrated on such products. (With 50 milliseconds of voice buffering a 10 Mbits/s Ethernet can support 60 simultaneous conversations.) Such high speed bus systems suffer from high equipment costs ($1000+ per tap), costly cabling and connections, cable sensitivity to kinks and termination quality, and without repeaters the inability to branch and go beyond a limited range (1500 feet). To send voice, the added cost of analog to digital conversion would also be incurred. For such systems to share data communication, they would need to limit data access to avoid causing voice delay disruptions. A number of digital systems optimized for voice were considered in attempt to get around some of the above problems. However, two fundamental problems appear to make a digital bus have poor cost to performance. The first major problem of a long twisted pair digital bus is its range/bandwith limits. Twisted pair attenuation increases with bandwith, for 22 gauge being 19 dB per mile at 400 kHz and increasing to 31 dB per mile at 2 MHz. Since digitizing the voice signal typically increases its bandwith by tenfold, doing so reduces the bus capacity manyfold over FDM techniques. Of course higher bandwith cabling such as coaxial cable or glass fiber can solve this problem but only with significant (X 10) increase in material and labor costs. Techniques such as multilevel encoding, pulse amplitude modulation (PAM), or pulse position modulation can reduce the bandwith but make the signals significantly more vulnerable to pulse reflections. The second major problem of a long digital bus (longer than a 1/4 wavelength of the highest frequency) is noise and pulse distortion errors due to reflections from impedance mismatches. This problem gets worse as the bus length increases due to the rise in signal dynamic range caused by attenuation. This means digital bus terminations or taps must be made with care and branches are prohibited unless expensive adaptive echo canceling circuitry is used. Such circuitry is increasingly expensive and power hungry with rising frequency. ISDN interfaces are expensive because of the need for 140 kbit/sec echo cancelers. Echo canceler designs for a 20 channel system at 1+ Mbits/s would require nanosecond signal processing. This would result in prohibitive costs per device and raise the power consumption per device beyond practical line powered levels. The difficulty of controlling digital reflections due to impedance discontinuities is the major reliability problem of high speed bus LANs such as Ethernet. In fact, LAN designers have backed away from both of the above major problems by various performance compromises such as lowering the bit rate (Appletalk, HP-IL, Cheapernet), going back to star wiring and doing bus contention on a short bus (Cheapernet, Star Net), going to daisy chained ring topologies (IBM token ring, HP-IL), or encoding data on a radio frequency subcarrier (Sytek, Wangnet). Long (more than 1/4 wave) digital buses of indeterminate lengths also suffer from other problems. One is uncertainty in propagation time which makes digital time slot assignment more complex than on a short bus. To get around this requires storing data, and sending minimum length data blocks so as minimize propagation dead time. Also, each device needs to generate its own synchronization from data on the bus. Probably, the more proven technique than time slot is packet switching. (Most LANs are packet switched.) Theoretically, this can double the throughput over a time slot system because most conversation is half-duplex, and the non-talking side of the conversation is wasted in a dedicated time slot system. However, packet switch systems incur overhead from packet addressing, control, propagation time, and collisions. This can be mitigated by making the data blocks long enough. The major problem (in addition to the two above) with packet switch systems is the high cost of the complex control circuitry at each connection to the bus. Practical digital multiplexed twisted pair bus systems are possible if the length and channel capacities are limited. Such systems typically have much less capacity and range than the KNET system. Phillips has demonstrated such a digital system of medium cost (?) which has a capacity of 8 half-duplex (4 full- duplex) channels over a range of 300 meters of twisted pair. The KNET's use of FDM (Frequency Domain Multiplexing) solves all of the above problems efficiently, economically, and without loss of relevant performance. Because all 28 KNET voice channels are below 427 kHz a total bus length of 5,000 feet or more is possible. Most important though, any topology is allowed with special terminations unnecessary. Impedance reflections still occur but typically only cause fixed amplitude variations of less than 6 dB in the voice carrier. This is because reflections which have significant phase shift must travel a distance which causes an attenuation of 2 to 6 dB. Worse case reflective interference occurs when a signal at the maximum KNET frequency of 435 kHz reflects from a 1/4 wave open causing phase inversion. At this frequency the signal is attenuated about 2.5 dB traversing the 1/2 wave (1/4 down & a 1/4 back) 22 guage distance of 650 feet. Because of phase inversion the reflected signal subtracts from the forward signal and reduces its amplitude 12 dB. For lower frequency signals such as a 100 kHz the attenuation for the 1/2 wave distance which is 2800 feet (reflects cable phase velocity) increases to 5 dB, which means the resulting interference is less, causing only about 7 dB of attenuation. Because the system has a dynamic range in excess of 20 dB, these levels of signal fluctuation shouldn't be a problem except when operating at the upper KNET frequencies and the maximum range of 5,000 feet where the attenuation is about 15 dB. Should the system diagnostics sense such a condition, it is possible to assign lower frequency channels to avoid the problem. Smaller guage wire will have less reflection problems but will have greater attenuation which may limit the range on the upper frequencies by a few thousand feet. Practically, very few installations should push these limits or encounter these problems. The flexible KNET topology is not a problem for the control link even though it is a bus based system because the bit wavelength exceeds 35,000 feet. Due to the nature of the modified NRZ coding worse case reflections off of line end opens are in phase with the originating transition and potentially can only interfere with a following bit transition after 1.3 bit intervals of delay. Such a reflection would travel over 40,000 feet of twisted pair with a loss in excess of 30 dB. (This is composed of 24 dB of 22 gauge cable loss and at least two reflections or traverses past the VCIF load with at least 3 dB per pass.) This reflection is 10 dB less than the minimum receive signal threshold which is 20 dB below the maximum signal. The KNET, although ideal for local phone distribution in a building, is not recommended as a central office local loop replacement. This is due to insufficient range and crosstalk problems. Although the KNET voice and control channels have a maximum range equal to most local loop distances, the power range is significantly less. In a local loop application the KNET would typically require several remote power boosters. However, the most serious drawback to the local loop application would be the high cross talk between KNET cables in the same multipaired distribution cable. Crosstalk isolation in such cables is adequate over several miles for audio frequencies but becomes a serious problem for the higher frequency KNET voice channels and is further compounded by the twisted pair loss at those frequencies. If need be, these problems might be cost effectively overcome by using heavier guage, lower loss, shielded twisted pair cabling. Of course, none of these things should be problematic in the business-home office environment where the KNET technology provides a more efficient telephone distribution system than the standard local loop technology. It does demonstrate that technology appropriate for one environment is not necessarily optimum for a related environment.1.5 General KNET system theory of operation
1.5.1 Overview
The KNET multiplexes power, control, and many voice channels on a single twisted pair bus by frequency domain multiplexing (FDM). The frequency bands are 0-200 Hz for power, 1-14 kHz for baseband communication control link, and 49-427 kHz for voice. KNET devices receive power and control over the KNET. They have frequency agile receiver-transmitters which are used to perform audio switching and communication between devices over the voice channels. The KNET controller assigns frequencies and sets audio characteristics for the device. In addition, it supports terminal functions.1.5.2 KNET devices
1.5.2.1 KNET Phone
The KNET Phone basically contains 1) a special power buffering circuit in conjunction with 2) a power supply, 3) a baseband control (or communications) link modem, 4) a controlling microprocessor, 5) keyboard and display components, 6) a receiver-transmitter and 7) a frequency determining programable synthesizer, 8) programable audio circuitry, and 9) programable tone generators and attenuators. The microprocessor (8049) is the local intelligence for the phone. The processor performs two principle functions, 1) communication link protocol handling and 2) input/output command processing. The KNET Phone's microprocessor transmits and receives commands to and from the controller via the baseband control link modem. In order to reduce communication and system complexity (and consequential cost), the KNET devices only talk to the controller and never to each other. (They do sense when another device is transmitting but ignore the content.) KNET devices also use a complete primitive command set (much like a microprocessor) which allows building higher level phone/terminal functions by stringing commands together. In response to commands from the KNET Controller, the microprocessor sets the receiver-transmitter to an assigned channel. (The controller will send commands to another KNET device to set it to mating frequencies so as to form a voice path connection between devices on the KNET.) It sets the audio circuitry to the appropriate state; such as, turning the speaker on, setting volume or ring cadence, etc. It scans and debounces the keyboard, sending the struck key number to the controller. It drives the display by passing on the received ASCII data directly to the smart alpha-numeric display. It also handles various housekeeping and diagnostic status messages. The KNET Phone's power supply converts the more efficiently transmitted 50 volt line voltage to the 5 and 12 volts required by the digital and analog circuitry. Special power buffer circuitry provide noise, load, and protective isolation from the KNET.1.5.2.2 CO Trunk Interface Card
The CO Trunk Interface Card provides audio connection path and line interfaces to 3 CO (Central Office) trunks. It contains much of the circuitry (some duplicated for each trunk) and software of the KNET Phone. It is principly different in that it doesn't have a keyboard and alpha-numeric display but it does have additional audio circuitry for DTMF generation and decoding, line isolation, line supervision, and conferencing circuitry. The CO Trunk Interface housed in the CO BOX, along with the KNET power supply, and common equipment card.1.5.2.3 VCIF (KNET Controller InterFace)
There is only one VCIF (KNET Controller InterFace) per KNET system. (There can be a total of 256 KNET Phones, CO Trunks, or other devices.) The VCIF has three functions. These are 1) communication protocol conversion and 2) buffering KNET messages to the KNET Controller, and 3) and watchdogging the KNET Controller. The VCIF has a microprocessor which in conjunction with a baseband modem and support circuitry acts as the bridge or gateway between the KNET and the KNET Controller. This frees the controller from immediately servicing the VCIF when it receives a message. The VCIF handles much of the low level KNET communication protocol which requires rapid time critical response such as acknowledge response, retransmissions, error checking etc. In addition, the VCIF provides various KNET status functions to the Controller. To improve controller reliability from line power fluctuations or software bugs, a VCIF watchdog program monitors the KNET Controller and forces a reset on a system crash. (The VCIF and all KNET devices have their own hardware watchdog monitor which resets it if its own software crashes.) To aid in system troubleshooting the VCIF provides diagnostic indicators to help determine which box or cable has a problem.1.5.2.4 KNET Controller
The KNET Controller is the central intelligence for the KNET system. Typically it is a commodity IBM compatible personal computer. Unlike the KNET devices whose simple control programs are small assembly language programs optimized for speed and limited memory, the KNET Controller's much larger program is written in "C", a high level language. The use of such a high level language facilitates program development on complex programs required for telephone controllers and allows program portability to other types of computers. By concentrating system intelligence and all adjustable parameters in a single powerful computer and by using phones with a complete primitive command set, software changes are easier, and high virtual phone intelligence is possible at a much lower system cost.1.5.3 KNET Bus
1.5.3.1 Power distribution
Standard phone systems typically send power for the phone set down the same twisted pair that the audio path uses. The power is usually 50 volts current limited to 60 mA. The phone only draws power when it goes off hook. Then, it only uses a fraction of this power because it clamps the line to 2-8 volts. The bulk of the power is wasted in the line or the current limited source. KNET power is 50-60 volts, bus distributed from an electronically protected voltage source which drives efficient switching power supplies in all KNET devices. KNET Phones although consuming more power (KNET Phones consume 2.5 watts) than standard phones, typically load the system power supply less. KNET power is sourced and picked off the KNET through electronic choke buffer circuits which act to prevent the power circuits from loading the control or voice channels. Because of the slow response of these circuits (less than 200 Hz), KNET devices with sudden load changes need to energy buffer with large electrolytic capacitors. These same buffer circuits also act to prevent noise from the KNET device switching supply from getting into the control or voice channels. All KNET devices draw power from the net through full wave rectifier bridges. This makes KNET wiring polarity insensitive and ensures correct control link data polarity.1.5.3.2 Control link
Unlike standard phone systems which communicate to and from the phone with voice band tones (touch tone, dial tone, busy, ring back, re-order, etc.), the KNET System uses a dedicated digital channel (control link) to KNET Phones, CO Trunk Interfaces, VCIF, and all other devices connected to the KNET. This control link has a much higher signaling rate and is continuously usable, not just during connection setup. This control link passes control information between the KNET controller via the VCIF and the KNET devices (phones and trunk interfaces). All control messages are strictly to and from the controller, the system intelligence. KNET devices are like "dumb" terminals and take all commands from the controller. The only independent actions KNET devices take are for maintaining control link communications integrity and allowing for graceful call disposition following link or controller failure. The communications control link is a 9.6 (actually 9.579) kbits/s CSMA (Carrier Sense Multiple Access) bus topology LAN (Local Area Network). The communications control link uses an alternate phased impulse coding technique which results a significantly simpler modem circuit than other modulation/coding techniques without compromising system performance. More robust coding techniques were found to be unwarranted in the KNET environment which suffers only limited transmission impairments of phase shift, group delay, frequency response, attenuation, noise, and distortion. The low speed of 9.6 kbits/s makes for termination and bridged tap insensitivity. Any reflections caused by such transmission line impedance changes die out well under a bit time and thus do not cause bit errors. Analysis suggests than any line impairment that would significantly disrupt control communication would also disrupt a standard telephone signal using the same line. The KNET communications protocol is simple, efficient, and robust. It uses a standard bit rate and asynchronous byte format (one start bit and two stop) to allow the use of standard communications components and testers. Even though synchronous coding can provide improved noise margins, its extra complexity was not warranted because of the high average signal to noise ratio of the channel. Also, because of the short length of KNET data blocks, the normal coding efficiency improvement provided by synchronous protocols would not have been available. Data blocks need to be longer than 5 bytes to gain improvement from synchronous protocols by eliminating start and stop bits in exchange for adding one or more synch bytes. All correctly received (no parity, framing, time-out, or check sum errors) transmissions are acknowledged. Failure to receive acknowledgements causes retransmission. Packets have low overhead and recover rapidly from collisions and burst noise so that system response is fast with many users accessing the channel.1.5.3.3 Voice channel multiplexing and switching
The voice channels are multiplexed using radio type Frequency Domain Multiplexing (FDM). Audio is encoded on the 28 voice simplex channels using companded amplitude modulation. Amplitude modulation (AM) is used over frequency modulation (FM) because it is more efficient in the twisted pair radio environment. Wideband FM only achieves quieting improvement over AM at the expense of additional bandwith. The twisted pair does not have sufficient bandwith without paying a range and termination handicap to justify wideband FM. Narrow band FM is used in the VHF and UHF mobile environments where the frequency of multipath fading is high. At these fading frequencies AM's automatic gain control (AGC) can't keep up while FM limiters are more effective at suppressing the amplitude modulation of rapid multipath fading. Because the KNET twisted pair is not a rapidly changing multipath environment, expensive (requiring linear phase delay filters), narrow band FM is unnecessary. And, by using audio companding techniques, the desirable signal to noise ratios of wideband FM is possible even at maximum range. The voice channels are 14 kHz apart. Adjacent channels (one for each direction) are paired to form a full duplex channel. Channel bandwidth is 7 kHz and passes both sidebands of the audio signal from 200 to 5000 Hz. A doubling of channels is possible by using single sideband or suppressed sideband techniques but with a significant penalty in cost (due to more expensive filtering). The voice channels were kept below 500 kHz to avoid potential interference from AM Broadcast stations. Above 500 kHz it becomes harder to suppress pickup due to two factors: 1) The twisted pair balance deteriorates significantly. This determines the coupling between common (longitudinal) mode (the dominant antenna pickup mode) and differential signals (KNET is differential). 2) Common mode pickup of these potentially interfering signals also increases significantly because wiring runs approach 1/2 wave dimensions. There are radio sources (usually navigation signals) below 500 kHz which might interfere on a channel. This is a low probability event requiring long (more than a 1000 feet) KNET runs and close proximity to the station (less than 2-3 miles) to be noticeably audible (although not necessarily disruptive). If such an interferer is disruptive, the associated channel can be blocked from the system with only a small loss of capacity, or it can be given a "last to be used" priority. The main limitation to range is the loss associated with increasing frequency (about 35 dB at 435 kHz down 10,000 ft. of 22 gauge). Beyond about 40 dB of dynamic range spurious signals become difficult and expensive to overcome. Typically most spurious responses are 70 dB below the level of the transmitter loaded into a long line. This means that with worst case loading on the top-most channel, interfering spurs if present will be 35 dB below the signal. Because of the channel frequency generation scheme virtually all spurs (including those from the switching supply) fall between channels where filtering and companding make a 35 dB down spur minimally audible. The KNET safely complies with FCC part 15 radiation limits. All signals are below 500 kHz which gets around the tightest specified band from 500 kHz to 30 MHz. All conducted common mode signals are at least 30 dB below maximum FCC specifications. All digital circuitry is moderate speed CMOS, which along with short bus runs accessed at low duty cycle ensures low RFI.Here's how many times this document has been accessed
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