Benefits of Using T1 and Fractional T1 Carriers and Multiplexers Gilbert Held

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1 Benefits of Using T1 and Fractional T1 Carriers and Multiplexers Gilbert Held Payoff A T1 transmission facility can provide an organization with significant economic savings, a high-quality circuit, and a transmission capacity that is approximately 75 to 150 times greater than that of conventional voicegrade lines. This article reviews the formats associated with T1 circuits in North America and in Europe, presents current carrier offerings, and examines the use of T1 and fractional T1 multiplexers and circuits. Introduction T1 transmission technology dates back to the early 1960s when thet-carrier was used exclusively by telephone companies and was based on the Time-Division Multiplexing of digitized voice. PCM One of the earliest methods employed to digitize voice was pulse-code modulation (PCM). Although pulse-code modulation conversion from an analog to a digital signal can be done in one step by a single integrated circuit chip, economics often dictates the use of two chips in a two-step process. The first step, called pulse-amplitude modulation sampling (see Exhibit 1), samples the analog signal at predefined time increments. The second step, called quantization, converts the height of each sample into a series of Binary digit and assigns a binary code; resulting in the generation of a binary stream of data bits that represents the voice signal. Under the PCM technique, a voice conversation is sampled 8,000 times per second. Pulse-Amplitude Modulation US T1 Facilities In the US, AT&T developed a 24-channel PCM system known as DS1, which multiplexes 24 voice conversations onto one 1.544M-b/s line. In the DS1 system, a seven-level code quantizes the analog signal of each voice channel, permitting 27, or 128, quantizing steps. An analog signal sample is composed of eight bits, seven of which represent a coded quantum step and an eighth bit that is used for signaling. For each frame of 24 analog signal samples, a framing bit is added. The composition of the DS1 frame is written as follows: (7 + 1)* = 193 bits. This equation is illustrated in Exhibit 2. Thus, a DS1 system's data rate of 1.544M b/s is achieved by sampling 193 bits 8,000 times per second. The DSI Frame The name of the service represented by the original DS1 system has changed several times since the early 1960s. In addition, the composition of the framing structure has changed. Five out of every six frames now use a full eight bits for quantizing, and the sixth frame

2 uses seven bits for quantizing and one bit for signaling. The resulting T1 frame structure, illustrated in Exhibit 3, uses 256 quantizing steps to represent the voice signal in five out of every six frames, with the sixth frame using 128 steps to represent the signal. Despite the change in frame composition (i.e., 8* ) its bit structure still equals 193 bits per frame and the data rate remains at 1.544M b/s. This data rate is called a T1carrier in the US and represents a communications facility capable of operating at 1.544M b/s. Common T1 services include AT&T's Accunet T1.5, Accunet Reserved, and Digital Access Crossconnected System/Customer Controlled Reconfiguration (DACS/CCR). The T1 Frame Structure Accunet T1.5 is AT&T's regular, dedicated private-line T1service. It is a Full-DupleX digital transmission facility operating at 1.544M b/s. Accunet Reserved is a switched T1 service that transmits digital signals at a data rate of 1.544M or 3.0M b/s, with the higher data rate achieved by using two 1.544M-b/s channels in parallel. DACS/CCR is a relatively recent offering that permits users to cross-connect 64Kb/s digital subchannels. With DACS/CCR, organizations can control the bandwidth allocation of their T1 facilities by issuing commands to AT&T Central Office, which cause thecentral offices equipment to function as a T1 matrix switch. Economics of Use The growth in the use of T1 circuits in North America and their equivalent E1 circuits in Europe, which is described later in this article, is primarily because of economics. Until T1 became available for use in North America, organizations requiring a large digital bandwidth for transmission had no alternative to using one or more 56K-b/s Dataphone Digital Service (DDS) lines. DDS 56K-b/s service is still provided by AT&T (and equivalent services are offered by other carriers); the economies associated with the use of a T1 carrier will be seen by comparing its current tariff to AT&T's 56K-b/s tariffs. Exhibit 4 lists the monthly recurring charges for an interoffice T1 circuit as of August The costs in that table reflect the charges billed by AT&T and do not include the Local Exchange Carrier cost to route the T1 circuit from AT&T's point-of-presence(pop) to the customer's facility. For simplicity of illustration, various AT&T discounts based on commitments of one to five years of use are not shown, since similar discounts are also available for 56K-b/sDDS service commitments. The European G703/732 Frame Structure Time Slot Type of Information Synchronizing 1-15 PCM-Encoded Speech 16 Signaling PCM-Encoded Speech Exhibit 5 lists AT&T's monthly rate plan for a 56K-b/s DDS circuit, Similar to Exhibit 4, Exhibit 5 costs represent AT&T charges and do not include the cost of the local exchange line from the carrier's point-of-presence to your facility.

3 AT&T T1 Interoffice Monthly Channel Charges Monthly Recurring Charges Circuit Miles Fixed Charge Per-Mile Charge $2,500 $ , , Exhibit 4 and Exhibit 5 show that at distances beyond 100 miles, the cost of a 1.544M-b/s T1 circuit is only a few times greater than the monthly cost of a 56K-b/s Dataphone Digital Service circuit. Yet, the T1 circuit provides an organization with over 24 times the bandwidth available from the use of a 56K-b/s DDS circuit. Organizations can easily cost justify the use of a T1 circuit to replace a few 56K-b/sDDS circuits. In addition to the significant potential economies of scale afforded by the replacement of a group of 56K-b/s DDS circuits with one T1 line, the additional bandwidth of the T1 line enables organizations to add such applications as videoconferencing and digitized voice to their network without incurring an additional line charge. Thus, the growth in the use of highspeed T1 and E1 circuits has gathered momentum as organizations have begun to take advantage of high-speed digital transmission facilities to integrate their previously separate voice and data networks. European T1 Facilities The European T1 facility, technically referred to as a G703/732 channel and commonly referenced in Europe as an E1 facility is a 32-channel system that encodes and multiplexes voice signals. Under the 32-channel system, 30 channels digitize voice signals from incoming telephone lines and the remaining two channels provide signaling and synchronization information. Each channel operates at 64K b/s, using eight bits per sample at a rate of 8,000 samples per second; thus, 64K b/s times 32 channels results in a composite data rate of 2.048M b/s. Exhibit 6 lists the composition of the 32 channels in the G703/732 frame structure. There are several key differences between the North American T1 frame structure and the Eurpopean G703/732 structure. One major difference is that North American T1 systems derive their 544M-b/s data rate from 24 channels, whereas the European G703/732 system uses 32 channels operating at 64K b/s to produce a 2.048M-b/s data rate. Another difference is that whereas the G703/732 system provides a separate 64K-b/s channel for the synchronization function, the T1 system uses the 193rd bit in each frame for this function. Finally, the T1 system uses the eighth bit in every sixth frame for signaling; the G703/732 system again uses a separate 64K-b/s channel for this function. Exhibit 7 compares the signaling characteristics of the T1 and G703/732 systems. Although some T1 multiplexers can operate correctly in both Europe and North America, many T1 multiplexers are designed for operation only in North America and will not perform properly if used in Europe. AT&T 56K-b/s DDS Monthly Charge

4 Mileage Band Fixed Charge Per-Mile Charge $ $ , T1 and G703/732 Signaling Characteristics Characteristics T1 G703/ Composite Data Rate (M b/s) Number of Channels Channel Data Rate (K b/s) Synchronization Frame Bit Channel 0 Signaling Eighth Bit in Sixth Frame Channel 16 T1 Multiplexers When T1 became available for subscriber use in 1980, its use was at first limited to voice networks because of the lack of equipment to efficiently combine voice and data on one T1 facility and because telephone company equipment was designed exclusively for the combination of 24 voice channels onto one T1 circuit. Because one T1carrier costs less than 20 analog circuits, and a voicegrade line provides only about 6% of the capacity of a T1 facility, many communications equipment manufacturers developed T1 multiplexers that are designed to multiplex both voice and data economically onto a composite T1 channel. In addition to multiplexing data, most T1 multiplexer manufacturers offer users a variety of optional voice digitization modules that can digitize voice signals at data rates ranging from the standard 64K-b/sPCM rate as low as 9.6K b/s. Such modules can increase (by a factor of two to four or more) the number of voice channels that can be transmitted on a T1 line under PCM digitization technique. T1multiplexers enable organizations to effectively integrate voice, data, and video information onto a common T1 facility. Before the ways T1 multiplexers can be employed in a network can be examined, a discussion of the various voice digitization modules commonly available is necessary to understand the benefits and limitations of these modules. Voice Digitization Modules In a conventional PCM digitization process, 24 voice channels can be carried on a North American T1 facility. To increase the number of voice signals that can be carried on this facility, a variety of voice digitization techniques has been developed to include adaptive differential pulse-code modulation (ADPCM) and continuous-variable-slope delta modulation (CVSDM) as well as several less widely employed schemes, such as Time- Assigned Speed Interpolation and differential PCM (DPCM).

5 Adaptive Differential PCM The Adaptive Differential Pulse Code Modulation technique uses a transcoder to reduce the eight-bit Plug-Compatible Manufacturer samples into four-bit words while retaining the 8,000-sampling-per-second PCM rate. This technique results in a voice digitization rate of 32K b/s, which is one-half of the PCM voice digitization data rate. Under the ADPCM technique, the use of four-bit words permits only 15 quantizing levels; however, instead of representing the height of the analog signal, each word contains the information required to reconstruct the signal. This information is obtained by circuitry in the transcoder that adaptively predicts the value of the next signal based on the signal level of the previous sample. This technique is known as adaptive prediction, and its accuracy is based on the fact that the human voice does not change significantly from one sampling interval to the next. Unfortunately, currently, there are no standard ADPCM algorithms and most T1 multiplexer vendors offering such modules use proprietary transcoder algorithms. Because of this lack of ADPCM algorithm standardization, T1 multiplexers from one vendor will probably be incapable of directly passing digitized voice signals into a T1 multiplexer manufactured by another vendor; therefore, ADPCM data channels will need to first be converted back to voice analog signals and then redigitized before being multiplexed again. However, if venders adopt a pending American National Standards Institute standard called ANSI T1Y1, this incompatibility problem can be eliminated. Continuous-Variable-Slope Delta Modulation The Continuous-Variable-Slope Delta Modulation digitization technique compares the analog input voltage with a reference voltage. When the input voltage is greater than the reference voltage a binary 1 is encoded, and a binary 0 is encoded when the input voltage is less than the reference level. This permits a one-bit data word to represent each sample. The incoming bit stream at the receiver represents changes to the reference voltage and is used to reconstruct the original analog signal. Each 1 bit causes the receiver to add height to the reconstructed analog signal, while each 0 bit causes the receiver to decrease the analog signal in set increments. If the reconstructed signal were plotted, the incremental increases and decreases in the height of the signal would result in a series of changing slopes, hence the name of this technique: continuous-variable-slope deltamodulation. Because only changes in the slope or steepness of the analog signal are transmitted, a sampling rate higher than the PCM sampling rate is required to recognize rapidly changing signals. CVSDM usually samples the analog input at either 16,000 or 32,000 times per second and, using a one-bit word for each sample, transmits data at a rate of either 16K b/s or 32K b/s. Other Continuos-Variable-Slope Delta Modulation data rates are obtainable by varying the sampling rate. Some T1 multiplexer vendors offer a CVSDM option that permits sampling rates from 9,600 to 64,000 samples per second, resulting in a CVSDM data rate ranging from 9.6K b/s to 64K b/s, respectively. Voice signals are usually easily recognized at 16K b/s and higher rates; however, lower sampling rates such as 9.6K b/s reduce the quality of the reconstructed voice signal. T1 Multiplexer Employment Modern T1 multiplexers are microprocessor-based time-division multiplexers designed to combine data, voice, and video from various sources onto a single communications circuit

6 that operates at 1.544M b/s in North American and 2.048M b/s in Europe. Exhibit 8 below lists the typical input channel data rates accepted by most T1 multiplexers. The data communications managers should note that although digitized voice is treated as synchronous input, its digitized data rate can vary considerably based on the type of optional voice digitization modules offered by the T1 multiplexer vendor. Typical T1 Multiplexer Channel Rates Type Data Rates Asynchronous 110; 300; 600; 1,200; 1,800; 2,400; 3,600; 4,800; 7,200; 9,600; 19,200 Synchronous 2,400; 4,800; 7,200; 9,600; 14,400; 16,000; 19,200; 32,000; 38,400; 40,800; 48,000; 50,000; 56,000; 64,000; 112,000; 115,200; 128,000; 230,400; 256,000; 460,800; 700,000; 756,000 Voice 9,600; 16,000; 32,000; 48,000; 64,000 As interface to an organization's Private Branch exchange in most T1 applications provides one or more tie lines through the use of two T1 multiplexers and a T1 carrier facility. Exhibit 9 illustrates a typical T1 multiplexer application in which voice, video, and data are combined in one T1 carrier facility. In this example, the PCM digitization channel modules used in the T1 multiplexer resulted in 10 voice channels on the private branch exchange (PBX) interface using 640K b/s of the available 544M-b/s T1 operating rate. A Typical T1Multiplexer Application This example also required the organization's conference room to be connected to a distant location for video conferencing. The 700K-b/s input to the T1 multiplexer in Exhibit 9 represents the required data rate for a digitized video conferencing signal. Furthermore, the organization has two data centers and a private branch exchange (PBX) at each end to permit computer-to-computer transmission to occur at 128K b/s. Finally, the organization's 12 data terminals, each operating at 4.8K b/s at one site, required access to the computer located at the other end of the T1 link. If the T1 multiplexer channels operate at 64K b/s, the video conferencing will require 11 channels, the 12 data terminals will occupy one 64K-b/s channel, and the 10 voice conversations will require ten 64K-b/s channels, leaving two 64K-b/s channels for future requirements. Since the cost of 10 tie lines and a few leased lines to support the data terminal traffic usually equals the cost of a T1 carrier facility, the employment of T1multiplexers eliminates the bandwidth cost for video conferencing andwideband computer-to-computer transmission. T1 Multiplexer Features Exhibit 10 lists some of the more important features of a T1 multiplexer. The data communications manager must realize that the data rates supported for asynchronous, synchronous, and voice transmission can vary considerably from multiplexer to multiplexer (see Exhibit 8 and must be examined accordingly to ensure that the proposed equipment can support the user's requirements. What channel interfaces a T1 multiplexer

7 can support must also be determined. Although most vendors support the Electronic Industry Association (EIA) RS-232C interface as well as the North American T1 interface, some vendors may not support other interfaces. For example, the more modern RS-422/3 interface, which is equivalent to the International Telegraph and Telephone Consultative Committee V.10 and V.11 interface and is not commonly included in communications equipment manufactured in the US, may not be supported by some T1 multiplexer vendors. Similarly, the V.35 wide-band interface, the Mil-Std-188 current-loop interface, and the European T1 interface (G703/732) may not be supported. Multiplexer Features to Consider Multiplexer Channel Rates Asynchronous Synchronous Voice Channel Interfaces EIA RS-232C (CCITT V.24) RS-422, RS-423 (CCITT V.10, V.11) V.35 G703/732 Mil-Std-188 T1 Another issue to consider is the total number of channels that a T1 multiplexer can support as well as the manufacturer's constraints regarding the mixture of such channels. Whereas some T1 multiplexers cannot support more than 48 independent channels, other T1 multiplexers can support either 512 or 1,024 channels. Some T1 multiplexers are designed so that asynchronous data channels actually use a 64K-b/s synchronous channel time slot on the aggregate T1 link, regardless of the asynchronous data rate. Other T1 multiplexers may more efficiently service asynchronous data traffic, thus permitting a larger number of channels to be supported by the multiplexer. Data communications managers should be wary of uninodal T1multiplexers that can be used only on pont-to-point lines; these cannot provide the user with routing flexibility. T1 multiplexers that offer multinode capability permit users to network the multiplexers to several locations. Exhibit 11 illustrates an example in which three T1 multinodal multiplexers are used to interconnect three distinct locations via three T1 lines. When networked as shown in Exhibit 11, some users at Location C might be routed to Location A and other users might be routed to Location B. Networking of Multinodal T1Multiplexers Fractional T1 Circuits As its name implies, a Fractional T1 circuit provides an end-user's organization with some fraction of the bandwidth of a T1 circuit. Current fractional T1 offerings start at 64K b/s, equivalent to a single DS-0 channel. Other fractional T1 offerings include 128K b/s (two

8 DS-0 channels), 256K b/s (four DS-0 channels),384k b/s (six DS-0 channels), and 768 b/s (12 DS-0 channels). Economics of Use Although a T1 circuit is very economical in comparison to a few 56K-b/s DDS circuits, for many organizations the 1.544M-b/s bandwidth is more than they require. Recognizing this, many communications carriers have structured fractional T1 tariffs to encourage organizations to obtain the transmission bandwidth they require at a more economical cost than obtaining a T1 and, in many instances, even a 56K-b/s DDS circuit. Exhibit 12 below lists AT&T's Accunet Spectrum of Digital Services (ASDS) interoffice channel charges for different types of fractional T1 lines. This tariffs was also in effect in August 1993 and, similar to the tariffs presented in Exhibit 4 and Exhibit 5, does not include access connection charges to AT&T'sPoint Of Presence. AT&T Accunet Spectrum of Digital Services Tariff Monthly Charges Per Channel Channel Rate Fixed Charge Per-Mile Charge (K b/s) 56/64 $270 $ , , , , , , , , In comparing the cost of a 56K-b/s circuit previously listed in Exhibit 5 to the fractional T1 costs listed in Exhibit 12, it is easy to predict the eventual demise of DDS as fractional T1 service becomes available in additional locations throughout North America. For example, the monthly cost of a 56/64K-b/s fractional T1 channel is significantly less than the cost of a 56K-b/s DDS circuit. Thus, asfractional T1 service becomes available throughout North America there may be a corresponding decrease in the use of 56K-b/s DDS circuits. Since the cost of several higher bandwidth fractional T1 circuits is also less than the cost of 56Kb/s DDS, it is quite possible that organizations will replace existing 56K-b/s DDS circuits with higher speed Fractional T1 circuits and use the additional bandwidth to add voice and videoconferencing applications while still saving circuit costs. Methods of Access One of the more interesting and perhaps confusing aspects of the use of a fractional T1 circuit is the method of accessing the long distance communications carrier's fractional T1 transmission facility. If the use of a 64K b/s fractional T1 transmission facility is required in many areas within the US, a digital 64K b/s local loop can be obtained from the local

9 serving telephone company. It will be routed to the point-of-presence where the local telephone company interconnects to the long distance communications carrier. In this situation the only communications equipment the customer requires is a combined Data Service Unit/Channel Service Unit which would be located at each end of the fractional T1 local loop connecting two offices of the organization. Exhibit 13 illustrates the routing structure associated with the use of a 64K b/s fractional T1 circuit. 64K b/s Fractional T1Routing Structure In Exhibit 13,the Office Channel Unit located at the serving telephone company office is the name used for a data service unit/channel service unit (DSU/CSU) located at that office. The data service unit/channel service unit (DSU/CSU) is normally employed on AT&T's DDS (DDS) and equivalent carrier digital services at data rates ranging from 2.4K b/s to 56K b/s. The data service unit/channel service unit (DSU/CSU) terminates the digital line, provides loopback control by responding to predefined digital codes, and amplifies and reshapes digital signals. The data service unit/channel service unit (DSU/CSU) offered by many communications vendors can be modified to operate at 64K b/s and provide a lowcost mechanism to access a 64K b/s fractional T1 circuit. At the point-of-presence the 64K b/s data stream is multiplexed onto a full T1 circuit by the long distance communications carrier and routed through the carrier's network. At the distant end of the circuit the 64K b/s data stream is demultiplexed from the T1 circuit and presented to the local telephone company at the distantpoint of presence. From that location the 64K b/s data stream is transmitted by the local telephone company's office channel unit (OCU) to the customer's data service unit/channel service unit (DSU/CSU). If the customer intends to use a fractional T1 circuit at a data rate above 64K b/s, access to the long distance carrier'sfractional T1 facility is both more complex and more expensive than access at 64K b/s. Currently, all telephone companies providing access to long distance communications carrier Fractional T1 facilities require the end-user to obtain a full T1 local loop to the point of presence. This means that a mechanism is necessary to define what fraction of the local loop T1 circuit contains DS-0 data that will be multiplexed by the long distance carrier. The required mechanism can be a T1 multiplexer or a fractional T1 Channel Service Unit. A T1 digital circuit is similar to a DDS circuit in that the line must be terminated, the digital signal must be reshaped, and loopback codes must be recognized. A combined data service unit/channel service unit (DSU/CSU) is used to terminate a connection to a DDS circuit. The Digital Service Unit portion of the data service unit/channel service unit (DSU/CSU) converts unipolar signals produced by Data Terminal Equipment to the bipolar signal used on DDS. This function is built into T1 multiplexers, which eliminates a requirement for a DSU when transmitting data on a T1 circuit. Thus, T1 circuits require the line to be terminated with a T1CSU. For the sake of simplicity, this article has avoided illustrating the T1 CSU. In actuality, each of the T1 multiplexers in Exhibits 9 and 11 would connect to a T1 CSU, which in turn would terminate the T1 transmission line. In addition to performing the previously mentioned functions, the T1 CSU formats the T1 line by taking 192 bits from the T1 multiplexer and adding a framing bit whose value is based upon the framing format supported by the T1 line. Returning to the methods available to access a fractional T1 circuit at a data rate above 64K b/s, it is possible to use a T1 multiplexer or a fractional T1 CSU. With a T1 multiplexer, the network configuration required to access a long distance carrier's fractional

10 T1 facility is similar to that shown in Exhibit 14. In this example data is transmitted through a T1multiplexer in increments of 64K b/s to correspond to the fractional T1 data rate obtained from the long distance carrier. The T1multiplexer at the customer's site produces a 1.536M b/s data stream, although the actual data carried is significantly less than that data rate. The CSU takes the 1.536M b/s data stream and adds 8K b/s of framing data to produce the T1 line rate of 1.544M b/s. The local telephone company passes the T1 data stream to the long distance carrier, which uses a Digital Access and Cross-connect System to separate the DS-0s carrying data from the DS-0s not carrying data. The digital access and cross connect system (DACS) then feeds N 64K b/s DS-0s carrying data to the long distance carrier's T1 multiplexer for transmission through that carrier's network. Using a T1 Multiplexer to Access a Fractional T1 Circuit Since the cost of a T1 multiplexer can easily exceed $10,000, the use of this device to access a low-speedfractional T1 circuit can be prohibitive. Recognizing this fact, several manufacturers of CSU have added fractional T1 capability to their products. In actuality, a fractional T1 CSU is a limited function T1 multiplexer combined with a CSU in one housing. Typically, the fractional T1 CSU accepts two to four data streams operating at multiples of 64K b/s and multiplexes the data for transmission on a T1circuit. The typical cost of a fractional T1 CSU is under$4,000, which represents a significant savings in comparison to obtaining a T1 multiplexer and a stand-alone CSU to access afractional T1 circuit. Recommended Course of Action T1 and fractional T1 facilities are a cost-effective way to combine separate voice and data networks. Because the key to effective use of T1 and fractional T1 facilities is the T1 multiplexer and fractional T1 Channel Service Unit. Data communications managers must evaluate such equipment carefully. Key parameters to focus on are the multiplexer's compatibility with T1 carrier facilities, the types of voice digitization modules available for use with the T1 multiplexer, the range of asynchronous and synchronous data rates supported by the multiplexer and the number of ports supported on the fractional T1 CSU. If T1 multiplexers are to be used in North America, the data communications manager must investigate their internal multiplexing format carefully. To be compatible with most AT&T services, the multiplexer should employ a byteinterleaving multiplexing process instead of a bit-interleaving process, which may not be compatible with Accunet offerings. However, because AT&T's Bell Laboratories tests all non-at&t communications equipment sold for use with Accunet facilities, vendors can simply be asked for a copy of a report concerning the compatibility of their equipment to the T1 facility they intend to use. Since T1 and fractional T1 carriers are end-to-end digital facilities, managers who upgrade to such facilities will, in addition to saving money, obtain a better quality circuit than that offered by analog facilities. Author Biographies Gilbert Held Gilbert Held is chief of data communications for the US Office of Personnel Management. He is an internationally recognized lecturer and has written 11 books and more than 50 technical articles. Held received the Karp award for best conference paper at Interface 84 and at Interface 87. He has a BSEE degree from Pennsylvania Military College, an

11 MSEE degree in computer science from New York University, and MBA and MSTM degrees from American University in Washington DC.

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