How Digital Data Transmission Technology Is Determining the Direction of the Future.
The digital network, however, has been tailored for data. It uses the same subscriber access lines (copper wire), but permits rates up to 56 kb/s, almost four times the speed of the newer breed of very high speed, 14.4-kb/s analog modems. Actually, TI data at 1.544 Mb/s uses these same wirelines. However, the data is regenerated at 6,000-foot intervals.
The bulk of dense, high-speed data transmission takes place over dedicated (leased-line) facilities. Dial-up facilities are used when the length of each transmission is relatively short and a number of dispersed remote locations cover a large geographical territory. Timeshare services represent the classic application of the public switched network for data transmission.
Future digital services are targeting dial-up customer with high-speed, low-error-rate networks. The forthcoming Circuit Switch Digital Capability will move full-duplex 56-kb/s data over the two-wire subscriber access pair, providing a switched network counterpart of the current private-line digital offering.
Much of the world is moving toward new techniques for voice transmission as well as data. The new Integrated Services digital Network will actually digitize the voice signal at the telephone prior to transmission into a fully integrated digital network.
Whereas digital data once required analog technology and media, analog voice transmission of the future will use digital facilities. Before addressing that future, let's look at the past and present: The Digital-Analog differences
Digital service refers to a generic type of transmission between subscriber locations, whereby the carrier accepts formatted data and regenerates, multiplexes and demultiplexes that data prior to distributing it to its final subscriber destinations. The major difference between digital and analog transmission lies in the digital system's ability to reconstruct these data signals, reducing the effects of amplitude and phase distortion. Analog transmission is transparent, exhibiting the additive effects of the interceding distortions. Digital signals are, on the other hand, completely regenerated (and retimed) and at the telco office that terminates the subscriber's access line and throughout the digital transmission system.
Although digital PCM (pulse-code modulation) systems have played an important role in long-haul transmissin since the late 1950s, local distribution for both voice and data remained analog until the early 1970s. During 1974, AT&T initiated its Dataphone Digital Service (DDS), establishing the first nationwide, end-to-end, private-line digital data service. DDS has been used primarily for synchronous data transmission, with service at 2.4, 4.8, 9.6 and 56 kb/s. Prior to the newer, high-capacity offerings, it was also available at 1.544 Mb/s.
DDS continues to gain momentum even though only the four lower-speed rates (2.4 through 56 kb/s) remain. Ninety-six hub cities have been authorized for DDS. It has also been a premium service, sold primarily on its throughput, very low error rate and generally high degree of uptime.
What makes digital transmission an inherently higher-quality service than analog? To understand the advantages of digital transmission, a brief discussion of analog transmission is required.
The voice systems, which predominate, generally center around PCM transmission. Twenty-four voice channels, each of which has a 300 to 3,400-Hz bandwidth, are digitized and subsequently multiplexed at the telco facility to form one 1.544-Mb/s T1 stream. (The chart on the next page shows the development from the channel bandwidths through the T1 data rate.)
The 3,100-Hz bandwidth establishes the data transmission limitation, characterized by Nyquist's criteria. Actually, the upper and lower ends of the channel exhibit so much amplitude and delay distortion that only 2,400 Hz of usable bandwidth remains. The digital baseband data must be modulated to center the transmission spectrum in the band of usefulness. (Current standards choose the band center to be between 1,700 and 1,800 Hz.) Thus, with a double-sideband transmission scheme, the maximum signaling rate is about 2,400 samples per second. For a sinble-level, serial binary system, the maximum rate would be 2,400 b/s. The More-Sophisticated Modems
More-sophisticated data modems actually group multiple bits per sample to increase the effective transmission rate. In this way, a 9,600-b/s modem sends 2,400 samples per second, each of which represents four bits of serial data. However, this method of sending multi-level data is not without penalty. As the number of possibilities for each sample increase, the probabiity that a line impairment will create a data error also increases. For this reason, poor channel response (fidelity) and channel impairments, such as impulse noise, can render a high-speed modem unusable.
Digital transmission systems utilize local distribution systems designed specifically for data. While the media used remains twisted-pair wire, no analog-to-digital conversion is required. Therefore, the bandwidth for digital transmission is not constrained by the 3,100-Hz voiceband limitation. There is no requirement for modulation/demodulation coupled with the compression techniques used in voice-band modem technology. As an example, DDS makes seven-eighths of the full 64-kb/s local-distribution data rata (56 kb/s) available for subscriber-to-central office communications.
Bi-polar data encoding effectively eliminates the DC spectral component. With a 50 percent bi-polar signaling, the transmitted energy is contained within zero and the data rate, in cycles per second. Thus, a 9,600-b/s DDS transmission exhibits no energy at 0 Hz o 9,600 Hz and peaks at 4,800 Hz. Digital data distribution techniques overcome the bandwidth limitations that hinder the use of analog voice-band data transmission.
Prior to 1981, AT&T, via the various operating companies, provided virtually all of the communications equipment for digital service. The terminating equipment took the form of the Western Electric 500A-type Data Service Unit (DSU). In the fall of 1981, the standard means of attachment to the various digital networks became the channel service unit (CSU). These devices have been referred to, generically, as network channel terminating equipment.
From October 1981 through December 1982, the customer could choose between a telco-provided 500B-type data service unit or purchase his or her own DSU to interface with the telco's CSU. During 1983, due to the outcome of Computer Inquiry II, the Bell operating companies (BOCs), as service providers of AT&T, were not permitted to provide equipment that could be considered customer premises equipment (CPE). Since the demarcation for digital services had become the CSU, the 500B-type DSU truly entered the competitive arena.
During June 1983, the FCC declared all digital network channel terminating equipment to be CPE. This meant that the responsibility of providing the CSU or "CSU-line" device now resided with the subscriber, not the telco. On November 18, 1983, the first set of "direct-connection" tariffs went into effect. Both DDS and the higher-capacity 1.544-Mb/s digital service were amended by these revised tariffs that unbundled the CSU from the service. The Digital Service Terminations
Digital service, as currently provided, terminates in one of two ways:
Channel Service Unit--A telco-provided CSU performs certain line-conditioning functions, such as access-line equalization and signal reshaping. Also, the CSU incorporates a line-loopback test capability to respond to a command transmitted from the serving telco office, which causes the CSU to return the received line signal back to the telco. This loopback is used in troubleshooting procedures to isolate system problems.
Integrated Channel Service Unit/Data Service Unit--Newer instatllations place the responsibility of providing the CSU function upon the service subscriber (the end user). The user may either provide a CSU into which a data service unit must interface or a combination CSU/DSU device. The equipment that the subscriber supplies must be listed as compliant for operation with the appropriate digital service. In the future, equipment to terminate digital services will be certified under the FCC Part 68 Registration program. The terminating equipment that the user selects will have two levels of interoperable test capability with the serving telco central office.
The first level concerns the line-loop-back test response, described previously. This is the same test currently performed by all telco-installed CSUs. An optional, second-level test capability involves both stand-alond DSUs and CSU/DSU combination devices. This test actually results in a full digital loopback, up to the terminal equipment's serial, digital interface. The telco-serving test center can transmit a command code structure, utilizing an alteration in the bi-polar signaling mehtod, to instruct the remote DSU or CSU/DSU device to enter a digital loopback mode. This alteration in signaling format is called "bi-polar violations." The test is described by telcos as "remote terminal" loopback, since all terminations of digital service are considered "remotes" from a digital system's view.
How is digital service ordered? The scenario for installing such service proceeds as follows:
A customer requiring service such as DDS first contacts the service provider. This may be a BOC, an independent telco or AT&T Communications. Since DDS is very frequently used for "inter-LATA" data, AT&T might be the provider. The user would order service, in this case, from AT&T and would receive an approximate service installation date. The user must then purchase or lease either a CSU and DSU or a combined device from a communications equipment manufacturer, one of the telcos or an equipment distributor.
Once the service is installed, the CSU or CSU-like device simply attaches to the four wires (transmit pair and receive pair) that terminate the service. If a CSU has been purchased, then a DSU must also be provided by the user to convert the bi-polar signals into serial, digital data. If an integrated CSU/DSU has been purchased, the user simply connects it to the four wires that terminate the service and to his data terminal equipment.
From its inception, digital transmission services, such as DDS, have been oriented toward very high data throughput at very low error rates. DDS is the first service ever tariffed with guarantees for uptime and error-free seconds.
In order to maintain service offerings like DDS, elaborate testing systems have been established by the telcos to pinpoint problsms anywhere in the network. One centralized test facility, known as Automated Bit Access Test Systems (ABATS), can test the network up to and including the subscriber's data service unit. ABATS, owned by AT&T Communications, is operated out of Chicago. It actually controls remote Bit Access Test Systems at the various serving test centers. By accessing the bit stream, ABATS can actually cause loopbacks at the remote DSU.
While the ability to track a digital network problem helps to maintain these services, complete control of the tests resides with the telco technicians at the serving test center. When a subscriber suspects a problem, he must first call a repair number. At that point, the telco will begin the diagnostic procedure that tracks data flow all the way through the digital network.
Some users are not completely comfortable with this concept, since centralized telco diagnostics remove control from them. A communications manager may want to be certain that the problem involves the network, rather than one of the other elements of the system, before calling for telco service. Also, if the network appears to be functioning properly, telco testing can only tell the manager that some other system element may be at fault. While telco testing may have eliminated a major element, the system is still not operational, and manual procedures are required to find the fault and re-establish operation.
For these reasons, some communications managers have remained with their current analog data systems. Obviously, the solution to this problem lies in the addition of user-controlled diagnostic capabilities for digital services. These diagnostic systems are now available from communications equipment manufacturers. But the nature of DDS dictates that there is no true auxiliary or "side" channel capability, as there may be with multiplexing) bandsplitting and protocol approaches are possible, the solution lies in a newly proposed offering, appropriately named DDS with Secondary Channel. The Primary, Secondary Channels
By running overspeed, both a full-rate primary channel and an auxiliary channel can be provided by TDM. The proposed data and ine-signaling rates are shown in the accompanying chart. By utilizing the secondary channel for diagnostics, the elements of network control so popular with analog data communication systems can be incorporated into the newer digital systems.
Two new digital services have been proposed and are actually being tested. The first is called Local Area Data transport (LADT), while the second is Circuit Switched Digital Capability (CSDC).
LADT provides a means to share facility resources by utilizing packet-switching technology. Customers have the choice of dial-up or dedicated access, depending on the amount of data to be transmitted. Dial-up access is provided via 212-type 1,200-b/s full-duplex data sets. Once the user dials into the access node, his or her data is combined with other subscribers' data to form a dense aggregate. Dedicated access is provided via a full-duplex private-line 4,800-b/s data set, with simultaneous voice and data capabilities.
Using LADT, customers with "bursty" terminal data can share telco facilities and achieve both reduced operating costs and a means of sending data to various locations, without the contention problems of the public switched network.
CSDC, on the other hand, provides a high-speed switched alternative to 56-kb/s private-line DDS. With full-duplex data transmission at the 56-kb/s rate, subscribers will be able to run high-speed facsimile, CAD/CAM, CPU to CPU, and backup for DDS on a dial-up basis. Using a transmission technology called time-cimpression multiplexing, full-duplex 56-kb/s transmission is accomplished over the two-wire subscriber access lines.
As the technology advances, there can be no question that digital implementation will eventually replace older, analog systems. This will have an impact on both voice and data transmission. But the movement toward higher data rates will create increasing interest in new data transmission applications.
A revolution in communications technology is taking place. It is called Integrated Services Digital Network (ISDN) in most countries and Integrated Digital Services in others. But the thrust toward high-speed, digital communications services is identical. The result will be a unified capability to handle voice, high-speed facsimile, CPU-to-CPU communications, and full-motion video applications, utilizing a well-integrated telecommunications network.
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|Date:||May 1, 1984|
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