DAAs go for the silicon

The transformer-based telephony data-access arrangement faces strong challenges from optical- and capacitor-based designs that reduce costs and enhance performance.

By Bill Schweber, Executive Editor -- EDN, 2/17/2000

If you think that developing new designs for the conventional analog plain-old-telephone-service (POTS) line is like designing for dinosaurs, think again. The Consumer Electronics Association (www.ce.org) estimates that manufacturers last year sold about 16.8 million desktop and laptop PCs, and nearly every one of them had a V.90/56-kbps modem as a standard feature. In addition, vendors shipped millions of relatively invisible embedded modems within devices such as set-top boxes and home-based controllers plus about 5 million retrofit modems for PCs.

Challenges to the POTS line and associated modem technology certainly exist. For example, two-way cable modems operate independently of the phone line, and digital-subscriber-line-based modem technology uses the POTS line but supports higher data rates. Still, many users like to have a POTS/56-kbps modem link available because of the universal availability, cost, and fundamental reliability of the configuration. Even installations with a satellite-based link for downloading often need a telephone line for the reverse path.

The vital links between a POTS line and a modem, the data-access arrangement (DAA), is a mandatory interface that protects end users' equipment from local-loop disturbances and vice versa. DAAs perform some signal-path and signaling functions for both the loop and the modem. And, although DAA technology is well-established, it is feeling the same silicon rumbles that many other aspects of electronics are sensing. Transformer-based DAAs are facing strong challenges from optically and capacitively coupled, IC-based DAAs, which offer virtues in size, cost, and flexibility. In addition, the silicon DAA can potentially integrate other functions, such as modem codecs and even the lower-baud-rate modems themselves. This integration is desirable for appliance applications with low throughput and low duty cycles.

What's the problem here?

The analog POTS line on the local loop that comes to end-user equipment, whether a phone or a modem, comprises just two wires (Figure 1). These wires are referred to as the tip-and-ring connections for the historical association with the tip and ring sleeve of the phone-connector plug. This two-wire path seems simple, and in some ways it is. However, it's grown more complicated over the years as the phone system has added features and functions to the telephone service.

The conundrum of the tip-and-ring connection is that the same pair of wires serves so many functions, representing signaling for on-hook, off-hook, and ringing; voice and data signals; special features, such as caller ID; and even sequences related to billing for the calls. The inband system does not separate control functions from the voice and data signals themselves. This multifunction confusion of the tip and the ring leads extends even to terminology: People use the word "ring" for both the physical connector lead and the unrelated ringing voltage signal that alerts a phone user to an incoming call.

With the perspective of history, we might choose to redo the entire 100-year-old, two-wire, local-loop system and its many layers of built-up operating complexity, but that scenario is not going to happen. Certainly, using dc-biasing, 20-Hz voltages for ringing, polarity reversal, and on/off timings is about as far from today's techniques for signaling and handshaking as you can get. But the installed base is the reality designers must accept, especially because the system works—and works well (see sidebar "It's a small world, after all...maybe").

Telephony-equipment designers therefore must strive to maintain compatibility between newer functions, such as modems, and the two-wire structure. Although this burden seems difficult for OEM designers, this backward-compatibility constraint has made the life of average phone users relatively painless: Nearly any phone works with an installed line in a true plug-and-play mode. Contrast this with end-user headaches that PC, operating-system, and application-software revisions and updates create.

What's a DAA to do?

A DAA's role is both protection and sleight of hand. First, it protects the phone equipment—in this case, the modem—from line problems, miswiring, and transients without interfering with normal operation. Meanwhile, a DAA must make the central office of the telephone system think that an end user's equipment that is connected at the far end of the local loop is a POTS unit, even if that equipment is an advanced modem. The nonloop side of the DAA also has challenges: It must translate the two-wire POTS interface into a more conventional interface with unambiguous status and control lines.

A DAA must perform varied and complex functions, including line termination, isolation, hybrid functions, and ring detection. A DAA must also provide a loop switch so that the DAA looks on- or off-hook to the loop; signal the number to dial using dual-tone-multiple-frequency or pulse dialing (a holdover from the early days of automated switchboards); and detect the state of the line and the incoming ringing signal. Other functions include support of full-duplex operation and the two- to four-wire hybrid function; accommodating line impedance for various locales and countries; and performing echo cancellation, or transhybrid balance, to overcome the effect of the sidetone that the phone normally feeds into your ear so that you can hear yourself when speaking into the mouthpiece. DAAs must operate with off-hook loop currents of 15 to 80 mA and on-hook voltages of –24 to –72V with respect to ground while drawing less than 1 mA when in on-hook, or inactive, mode. Of these functions, isolation is the one that makes life difficult, because a DAA must provide isolation yet pass ac- and dc-based signal information (see sidebar "Isolation choices are three").

These problems are just the start. The DAA must also handle layers of additional features on the local loop and telephony interface. In addition to increasingly popular caller ID, functions that you might not normally think of as complex become so in a two-wire local loop. The DAA implements some basic features, and it must also provide status flags and control bits in ways that seem counterintuitive. For example, what happens if a modem is in use while someone is picking up an extension on the same line? As a design engineer, you might assume that an uninterrupted data transmission always has priority, so the other extension should be blocked, but that assumption is wrong. For example, if the DAA and modem are part of a TV set-top box that dials into a central office to report billing information on which cable shows you watched, that data packet has lower priority than a home owner's dialing the police to report a burglary. Based on the line-sense flag of the DAA to the modem, the modem should terminate its transmission and allow the voice call to proceed.

Also consider the vagaries of ac-line-termination impedance. The DAA must match the various local-loop impedances to provide appropriate termination, and the nominal impedance values vary worldwide. In the United States, the loop is nominally 600W, whereas in Britain it is a 300W resistance in series with a parallel combination of 1 kW and 220 nF. To complicate your design, even if you tailor the DAA for each region's nominal line impedance, the local loop is not a carefully controlled transmission line like a 50 or 75W run of coax: A loop's actual impedance can vary high or low by a factor of two. For these reasons, your ability to electrically program the newer DAAs to synthesize different impedances is attractive from a design standpoint.

DAAs attract vendors

Whether you consider an older transformer-based DAA, a newer optically isolated DAA, or the newest capacitively coupled DAA, you need to keep some factors in mind. Unlike some electronic components, DAAs from various vendors differ significantly in function, form, and capabilities (Table 1). For example, some DAAs integrate just core functions and require few external active and passive components, whereas others have more built-in functions. Still others are freestanding, generic POTS devices, and others are part of a chip set that is tightly coupled to a modem/PC configuration.

The DAA2000 chip set from Infineon combines the DL207 line-side IC, the DM207 modem-side IC (both 24-pin TSSOPs), and a pair of IL388 linear optocouplers to yield a generic analog DAA that you can use with modem chip sets and data pumps (Figure 2). Operating at V.90/56-kbps rates, it includes ring detection; hook-switch control; caller ID pass-through; and wake-on ring, whereby the chip set draws only idling current except when it detects ringing.

Along with the components, Infineon offers reference schematics, modem-design guides, and homologation service, for which the company tests your design for conformance to the appropriate public-switched telephone-network standards and gets regulatory agencies to certify it. The company offers a universal schematic that you can use for worldwide operation by setting circuit options with software-based commands or jumpers.

Also taking the optical route is CP Clare with its hybrid CPC5604 Litelink DAA with built-in optocouplers (Figure 3). This PCMCIA-compatible package device supports V.90 operation, is DTR 21-compatible, and requires no other external isolation-related components. Features include ring-detection circuitry and caller-ID signal detection, both the result of snoop circuitry that monitors the line in an on-hook state while drawing less than 2 µA. Many of its features are software-programmable, including line-impedance matching, current/voltage profiles, and current limiting. The device comes in a 32-pin SOIC package.

Another vendor using hybrid fabrication to put the optoisolator into the package is Mitel with the MH88437-P DAA (Figure 4). This 28-pin device supports 56-kbps data rates and lets you program the terminating-line impedance, network balance, and dc-termination characteristics via external components. It includes a two- to four-wire hybrid function and an integral loop switch. Mitel uses a resistor/capacitor combination to limit DAA susceptibility to common-mode signals. A high-value resistor, matched with a low-cost, low-voltage capacitor, is trimmed for optimum performance and common-mode rejection.

Other vendors have chosen the capacitative route for isolation. Analog Devices offers the AD1803 modem codec plus AD1804 DAA chip set, which acts as a front end to a fax/data modem. You can program virtually all the parameters of this chip set via software and meet worldwide standards with one design with programmable speakerphone, voice, and modem functions. The capacitive barrier uses two capacitors to couple both data and clock information between the ICs.

Further pushing the integration level, Analog Devices uses the same AD1804 DAA but with AD1807 DSP to complete a V.90 modem that spans from the RJ-11 phone jack to the host-computer PCI interface for what the industry calls a controllerless modem. A complete design using this pair occupies less than 0.4 in.² (260 mm²) of pc-board area.

Silicon Laboratories, too, uses IC technology linked by a pair of capacitors for its 56-kbps DAA chip sets (Figure 5). Their Si3034 chip set comprises the 16-pin Si3014 and Si3021 ICs and meets nearly all international standards while delivering programmable values for ac and dc termination, ring-detection threshold, and ringer impedance, along with two- to four-wire hybrid, ring detector, caller ID, and loop-current-monitor functions. The 3.3V ICs interface to a DSP and controller to provide the complete modem function with 3000V isolation.

The company has also developed a DAA version that complies with the AC'97 Revision 2.1 specification. The Si3038 chip set includes the Si3014 and Si3014 chips, targets PC-based soft modems, and offers the same 84-dB dynamic range on its transmit and receive paths that the Si3034 chip set specifies (Figure 6). You can daisy-chain as many as eight DAAs for multiline operation and software-select between a variety of filter cutoff frequencies and even between digital FIR- and IIR-filter implementations.

By restricting operation to 2400 bps and lower, which satisfies many appliance-class appliances, Silicon Laboratories integrated the microcontroller, DSP, analog front end, DAA, and voice codec into a pair of ICs. The result, the Si2400 Isomodem, uses less power, requires less board space, and costs less than using a separate DAA and modem components.

Another vendor, Krypton Isolation Inc, provides the K²960GW DAA chip set, which uses a pair of active ICs and six capacitors for the isolation barrier. The 16-pin K²961C contains the transmitting and receiving buffers, ring-detection and power-down circuits, off-hook and caller-ID control-input circuits, and other functions. The 28-pin K²963C has the ine terminations, hybrid converter, transhybrid loss-gain control, and complementary functions of the K²951C device. As with the other chip sets, you can get V.90/56-kbps data rates with this combination; active power consumption is less than 20 mW.

Using the Krypton Isolation technology and also providing AC'97 modem support is the STMicroelectronics ST-MC97 chip set, which comprises the ST7597MC analog front end and the ST952 DAA line interface, along with capacitive isolation. The DAA includes a programmable line interface, a hook-switch driver, a ring indicator, a caller-ID interface, dual-tone multiple-frequency generation and detection, and wake-on-ring functions.

Conexant (formerly Rockwell Semiconductor) has also chosen the capacitive method for the SmartMC chip set for V.90 operation. This AC'97-compatible pairing includes enhanced features, such as caller ID and monitoring of local-extension status without going off-hook. The device set comprises a host-side device in a 48-pin TQFP and a line-side device in a 32-pin TQFP.

You can compare the optical and capacitive DAAs with a more established transformer-based device by examining Xecom Inc's generic DAA for V.90 operation. The device comes in a 68-pin PLCC package that measures less than 1 in.² (25 mm²) and 0.170 in. (4.3 mm) high, including the transformer; the device itself uses only 18 pins on the package. The XEV90 includes a line-current-holding circuit, hook-switch control, a ring indicator, pulse- and tone-dialing operation, and 6-dB insertion loss and achieves THD of better than –85 dB. You use this device with a 3 or 5V supply.

Whichever approach or vendor you choose to investigate further, keep a few design tips in mind. A part's data sheet is critical, because it shows the components and the mandatory layout you must use to make your design meet regulatory approval. If you modify the layout, you may have a design that works perfectly but fails safety or regulatory requirements. These data sheets and associated application notes are usually long and educational, explaining both the requirements of the public switched-telephone network (PSTN) in various countries as well as the operating specifics of a vendor's device. A less complete data sheet means you have to work harder to convince yourself that the vendor knows the thicket they are helping you through. You can further educate yourself by studying the DAA's central-office counterpart, the subscriber-line interface circuit (SLIC) (see sidebar "When a DAA meets a SLIC").

Be sure to compare your needs to the level of design and approval support the vendor offers. Designing DAAs and related circuits for the various PSTNs differs significantly from designing a complex analog interface for a thermocouple or resistance-temperature detector because of regulatory and approval issues. The vendor's support and understanding of the national and homologation issues makes a major difference in your frustration level and time to market. You may want to visit vendors' application-support labs and qualification areas to better understand what they offer in addition to the components themselves.

Also, study data sheets to see how many and what type of external components each DAA implementation requires. Some external components in the circuit-protection areas are unavoidable given today's technology. Meanwhile, the number of conventional external passive and active components differs among DAAs, and you need to look at the final bill of materials to judge complexity, cost, and board space. A good data sheet or application note provides a full bill of materials. Always watch for the adjective "complete" in front of the noun "DAA," because vendors' concepts of "complete" differ. Some DAAs require numerous inexpensive components, whereas others need fewer but more expensive ones. Data sheets also point to useful references, and many, such as the Telecommunications Industry Association (www.tiaonline.org), are associated with telephony and modems.

It's a small world, after all...maybe The telephony-standards world is fragmented—unlike that of communications, in which standards, such as Global System for Mobile (GSM) communications for cellular phones, apply to designs worldwide. With telephony, each country has its own standards because of both the historical roots of phone service and the desire to protect the local phone market from outside competition. Each country's local technical governing authority, the Postal, Telegraph, and Telephone (PTT) office, promulgates its own standards for the public switched-telephone network (PSTN). Soon, you are grappling with the Federal Communication Commission's Part 68 in the United States, Jate in Japan, CTR 21 in Europe, and countless other specifications. The differences in local technical standards range from minor to severe and affect many of the signaling conditions on local-phone loops. Essential billing signals, too, differ widely as you cross borders, as do optional functions, such as caller ID. In some cases, the standards differ slightly in factors such as loop current; in other cases, sharply conflicting or irreconcilable differences exist among countries' standards. But the good news about this fragmentation is that homologation is replacing it. Countries are harmonizing many standards across their boundaries, so you may be able to meet one broad interconnection standard and meet compliance in many countries. For example, in July 1998, the European Commission adopted CTR 21, a standard for nonvoice equipment connected to the PSTN. If you meet this standard, you meet approval in 20 countries, thus avoiding 20 approval processes. Even if the standards are not compliant worldwide, they are moving closer together with fewer outright conflicts. This situation makes it more practical for you to use one data-access-arrangement circuit design to accommodate many, if not all, of these standards by substituting some component values in a design or by electronically programming some performance attributes in a circuit after installation.

Isolation choices are three Historically, the data-access-arrangement (DAA) function used a transformer for galvanic isolation and to meet the conflicting requirements of the interface. The transformer is a good choice, too, because designers understand it and its subtleties; it offers a detailed, comprehensive track record of designability, manufacturability, high isolation, and reliability; and you can tailor it to meet different impedances, interwinding capacitances, and other factors. Transformers do have several drawbacks in DAA applications, however. They are larger in both footprint and thickness than DAA and modem ICs. This size factor becomes even more of a drawback as system designs shrink and Internet-enabled appliances, such as vending machines and home products, need telephone-line access. Another transformer virtue is that you can design it with a set of fixed parameters. This virtue becomes a weakness, however, when you want a DAA to adapt to telephone lines with different impedances and other line parameters that you find even in one country, as well as the regulatory requirements in different countries. Also, unlike silicon-based designs, transformers lack the potential for dramatic volume-driven cost reductions that ICs offer. Even a device as apparently simple as a transformer has variations in DAA applications. The dc-loop current flows through a wet transformer's primary winding, easing overall design. However, this approach increases return loss and distortion, so may limit the DAA to low-speed, full-duplex or high-speed, half-duplex modes. In some applications, such as Internet appliances, this result is not necessarily a limitation. Conversely, dry transformers require zero dc current in the primary winding, which gives better transformer performance but means you need to add a dc-holding circuit. Optical-coupling and capacitive-coupling techniques, each with relative strengths and weaknesses, also allow you to achieve galvanic isolation of analog signals. Optoisolators can pass dc levels through the barrier, which is necessary for phone-line functions, such as ring detection. However, optocoupler performance is inconsistent among units; it shifts with temperature, so you need to include gain and other compensations. Linear optocouplers, which use a feedback path to stabilize performance and provide a better transfer function, are more costly than standard optocouplers and need additional gain. They have wider, flatter bandwidths than nonlinear optocouplers—a virtue for minimizing distortion in the entire passband. In comparison, capacitors are smaller, offer more stable performance, cost less, and provide higher isolation than do optocouplers. However, capacitors block dc, so you need to use additional design tricks to pass dc. Also, you cannot integrate the capacitors onto the IC itself, which you can do with optocoupler-based DAAs. However, adding a couple of external capacitors is not a severe problem for most designs, because you must install several other nonintegratable, high-voltage and transient-protection components outside the IC.

When a DAA meets a SLIC A data-access arrangement (DAA) and its associated modem do not exist alone on the local loop. At the central-office (CO) end of the loop, the subscriber-line-interface circuit (SLIC) provides the necessary signals, timing, and control functions for the plain-old-telephone-system line. The SLIC provides battery, overvoltage protection, ring trip, supervision, hybrid two-to-four-wire conversion, and test (BORSHT). Because the SLIC is provided by the telephone company and is essential to maintain performance of the loop, it must meet more stringent technical requirements than the DAA or whatever else is on the user end of the loop. SLICs and DAAs are complementary functions with considerable overlap, and some DAA vendors also offer SLIC components. The requisite functions of these components, although similar at first look, differ enough that implementing the technologies requires different techniques. For example, SLICs act as power drivers as they send ringing signals down the line and supply loop power (battery feed) to the far end. DAAs, on the other hand, act more like receivers and use the supplied loop power. SLIC designs have different priorities from those of DAAs, because you usually install SLICs in relatively dense CO racks, and service technicians, rather than modem end users, usually maintain them. The SLIC qualification process is more stringent than that for DAAs, because marginal SLIC performance affects all potential users of a loop and raises maintenance costs for the phone company. To better understand what the DAA is doing and why, though, study a detailed SLIC data sheet. Intersil (www.intersil.com), for example, offers the HC55180 through HC55 family of SLICs, which operates at voltages as high as 100V, so it can send a high-potential ringing voltage through local loops as long as 5000 ft (1500m), equivalent to 500W dc resistance. Mitel Semiconductor (www.mitelsemi.com), a DAA supplier, offers the MT91600 SLIC, which targets shorter loops and allows –22 to –72V battery operation for the loop feed. Silicon Laboratories (www.silabs.com) recently introduced the Si3210 IC, which integrates the basic SLIC, codec, and dc/dc-converter controller into a 38-pin TSSOP CMOS device.

Author info

You can reach Executive Editor Bill Schweber at 1-617-558-4484, fax 1-617-558-4470 bill.schweber@cahners.com.

 

 

ACKNOWLEDGMENT

Thanks to Tom Pinto of Infineon Technologies North America, Anjum Tanveer of Analog Devices Inc, Alastair Waite of Mitel Semiconductor, and Brad Fluke of Silicon Laboratories for their insight and comments on DAA designs and developments.