Data over Cellular

Enhanced Throughput Cellular makes you forget that your data is riding on an analog connection

John Bryan

The cellular-phone network was built with voice communication in mind. Thus, the transmission glitches and pauses caused by handoffs are merely an annoyance, because the human brain is great at inferring what it doesn't actually hear.

The explosion in portable computing, however, has created an explosion of interest in wireless data communication--particularly in cellular data communication, because cellular is the most popular and accessible form of wireless communication now available. The problem is that the mere annoyances that plague voice transmission over cellular are anathema to data communication. Dropped or missing bits or bytes cause retries or, in the worst case, a lost connection.

Cellu lar transmission is a circuit-switched technology. In it, cellular transceivers switch between frequencies to avoid interference as new calls come on the air or simply to acquire a better signal-to-noise ratio on a transmission channel. Because the transmission is wireless, it is susceptible to all types of atmospheric conditions, emissions, and electronic interference. The act of switching itself also produces a momentary cessation of the transmission signal, which interrupts the data stream.

Last year, AT&T Paradyne (Largo, FL), a data and communications technology firm, introduced ETC (Enhanced Throughput Cellular) as a means of solving not only data-continuity problems, but speed, error-correction, reliability and call setup, and handshaking issues as well. Other protocols (e.g., MNP level 10 and V.42 error correction) deal with these issues, but none as comprehensively as ETC. With more than a dozen cellular providers supporting ETC, it is quickly becoming a checkoff item for cellular data communi cation.

Modem Standards

Modem standards are numerous, and they come with confusing names and numbering schemes. The ITU-TSS, which recently replaced the CCITT, sets the V.xx standards. The ITU-TSS is located in France, so its designations, like those of the CCITT, are French. The ``bis'' designation, for example, means ``second,'' as in the second iteration of a standard. Thus, V.22 is the original 1200-bps standard, while V.22bis is 2400 bps. V.32 covers 4800 and 9600 bps. V.32bis adds 7200 bps, 12 Kbps, 14.4 Kbps, and some fast rate-negotiation parameters to the speed switching. In particular, if either modem specifies a transmission speed, the connection must maintain that speed or disconnect.

V.42 is an error-correction protocol based on LAP-M (Link Access Procedure-Modem), which in turn is basically HDLC (high-level data-link control), the original bit-oriented synchronous link-layer protocol used in data connections. MNP level 10 is another error-correction scheme. It's a de facto st andard, widely used but not formalized by a committee.

Cellular communication is organized on OSI's seven-layer network model. ETC's big advantage over protocols like MNP level 10 and V.42 is that it operates on both layer 1 (i.e., the physical layer) and layer 2 (i.e., the link layer). The physical layer is largely concerned with how the data is modulated for transmission. Today's modems use simultaneous amplitude and FM. The link layer is where handshaking and error control are handled.

To realize all the benefits of ETC, a modem that supports both V.32bis and V.42 is required. V.32bis is used because it handles the handshake setup between modems, and it is easier and faster to take advantage of the handshake than to duplicate the effort. As the handshake takes place, the sending modem determines if ETC is supported by the modem on the receiving end. If it is supported by both modems, they work together to determine the highest data transfer rate the circuit is likely to support. Only after th is has been established do you get a connect message.

V.42 is put to similar use. Standard implementations of it require the sending modem to repeat all frames transmitted after a flawed frame is received, regardless of whether subsequent frames are error-free. (This is also the case with MNP level 10.) On average, a modem will have received two additional frames by the time a retransmission request reaches the originating modem. Thus, all three frames would need to be retransmitted. However, ETC uses V.42's selective reject feature and requires the retransmission of only frames with errors.

ETC Enhancements

Besides working with existing protocols, ETC brings many improvements to the table. One of the primary benefits is Transmit-Level Control. A typical (i.e., non-ETC) modem's amplitude modulation may often exceed the cellular channel's transmission capabilities. This results in a condition called clipping, a harmonic distortion that damages the signal.

V.32bis-compliant cellular m odems have a flat-frequency response, but the cellular network gives the frequencies from 600 to 3000 Hz, the ordinary vocal range, a 6-decibel boost. ETC deemphasizes the same bandwidth area by 6 dB, which results in more consistent data transmission. The theory and the effect are similar to the function of Dolby noise reduction in stereo tape recording and playback equipment.

ETC also monitors the cellular channel. Auto-rating is a technology that works in conjunction with the V.32bis standard to measure disruptive phenomena, like harmonic distortion, phase jitter, and signal-to-noise ratios. The measured figures are translated into a cumulative absolute number, which is compared to a table of acceptable levels for any given transmission speed.

While V.32bis supports speeds of 4800 bps to 14.4 Kbps, ETC will let the modem drop even lower, to the V.22 standard of 1200 bps. ETC skips the V.22bis 2400-bps standard, because a quirk in the modulation algorithm causes it to yield worse results than V.32bis at 4800 bps. Auto-rating is capable of adjusting the transmission speed, in certain cases, as often as every 5 seconds, although a more realistic number is about every 15 seconds. This reduces the risk of dropped calls while simultaneously taking advantage of the fastest possible transmission rates. The net effect of this protocol is the efficient use of air time, which is the expensive component of cellular communication.

ETC also breaks up data to be sent into smaller frames than most error-correcting protocols. ETC frames are 32 bytes in length, one-quarter the standard V.42 setting. (There is a modem command that enables the manual selection of smaller frame sizes, but who has ever used it?) Using smaller frames has two positive results in error correction. First, frames that the receiving modem determines need replacing are smaller and can be sent in less time than their larger predecessors. Second, a modem can send more frames before receiving an acknowledgment from the receiving device.

This is another timesaving device. The transmit/acknowledge cycle takes time to complete over the length of the cellular circuit, and while it is transpiring, no data can be transmitted. Even though 15 32-byte frames represent about half the data of seven 128-byte frames, the net result of a longer transmission window is more data sent.

Finally, ETC increases the number of retries a modem will make when attempting to send a frame with errors. Currently, standard modems will retry about eight times before breaking the connection. ETC makes 20 attempts, which results in a significant percentage in the percentage of maintained calls.

Deploying ETC

ETC will provide improvement when present in only a single modem in a connection, but it isn't as significant as when it's present in both. If ETC is present in both sending and receiving modems, test results have shown that its overall reliability is nearly 20 percent greater than when it is present at only one end of the connection. One fac tor to note here is that to provide any improvement, MNP level 10 is required on both ends of the connection, while ETC may be present at only one end. This is the theory behind cellular carrier's modem pools (see the text box ``Modem Pools'').

ETC provides improvement in transfer rates for all file sizes, but a real bonus of the protocol is that significantly larger files can be sent in under 1 minute. None of the other protocols now available can beat the 1-minute threshold, even for files as small as 4 KB, which means that the same call will cost twice as much without ETC. Obviously, as the file transfers get larger, the percentage of savings is not as dramatic. However, the average on-air time for the transfer of a 100-KB file is 3 minutes, 57 seconds, for an ETC-ETC connection.

Real-World ETC

ETC is available in AT&T products like KeepInTouch, which is a PCMCIA cellular modem card, and Dataport and Comsphere, which are desktop system cards. For pre-ETC customers of these products, the technology is downloadable at no charge to the end user. AT&T has licensed ETC, and several modem vendors have also done so. Among the hardware vendors are Compaq, Intel, and Xircom. The list of the service providers who support it reads like a who's who of cellular communication in the U.S., and several of the companies are involved in communications operations worldwide.

Intel's Cellular Faxmodem was the first ETC-compliant product available from a third-party vendor. The Cellular Faxmodem for PCMCIA, descriptively if unimaginatively named, was introduced in June. It is compatible with both wireless and wired communications and is capable of 14.4 Kbps, the fastest V.32bis speed. Besides supporting ETC, the Cellular Faxmodem supports MNP level 10, enabling error correction in communication with modems that also support MNP level 10 but do not comply with V.42.

The retail price for the Cellular Faxmodem is $349; an RJ-11 phone cable is a $99 option. The modem is compatible with AT&T's cellular phones and Nokia Consumer Electronics' Technophone. It is not, however, directly compatible with the line of cellular phones from Motorola Codex, the best-selling cellular products in the U.S. Some third-party vendors make products that let you attach an ETC-compliant modem to just about any type of phone, but a direct connection to a Motorola phone through its MC2 bus requires a Motorola modem or a KeepInTouch card.

The Motorola Difference

Motorola's products do not feature ETC. Instead, Motorola has developed its own proprietary protocol called ECC (Enhanced Control Cellular), which is designed to provide the same advantages as ETC. ECC works, like ETC, at both the physical and link layers of the cellular network model. Unfortunately, no further information on ECC was available at press time, including whether it is compatible with other standards, what its specific benefits are, and the availability of licenses from Motorola.

Xircom is an industry leader in the development of LAN and WA N (wide-area network) technology for the mobile market and a technological partner of Motorola. However, Xircom is going with ETC. Jim Soriano, senior engineering director at Xircom, says, ``Our products feature a Motorola 68302 processor and plenty of RAM and are capable of supporting multiple simultaneous protocols; space and compute power are not a problem. Xircom simply feels that ETC is the best protocol for the type of marginal conditions that cellular sometimes delivers.''

The LAN/WAN group at Xircom is devoted to providing remote connections to LANs and other types of networks. It developed and is marketing two ETC products, the CreditCard Ethernet + and the Pocket Ethernet +. As its name implies, the CreditCard Ethernet + is a PCMCIA product, while the Pocket Ethernet + plugs into a system's parallel port. Both are unique in that they provide an Ethernet interface and a V.32terbo-compatible modem. (Terbo is a word play on the V.32ter standard, which wasn't passed. V32terbo is capable of 19.2-K bps modulation.)

Xircom chose ETC because AT&T is actively marketing and supporting the product; the secrets of MNP level 10 and ECC, on the other hand, remain veiled. Xircom has left its options open, though. If ECC becomes an important standard, you will be able to download the technology onto Xircom products. ETC will be available as a software upgrade in the fourth quarter at no charge for those who own the CreditCard Ethernet + and the Pocket Ethernet +.

The Cellular Future

The fact that cellular today is a virtual circuit-switched technology adds to the necessity of a product like ETC. Signals sent in an open-broadcast format will always be more susceptible to various types of interference than data carried over a wire.

ETC is just one way to safeguard the transmission of data over a cellular connection. Part of its popularity comes from the fact that ETC isn't required on both ends of a connection. You can buy an ETC modem and still benefit even if your cellular carrier doesn 't use it on the other end of the connection. The same can't be said of ETC's biggest rival, CDPD (Cellular Digital Packet Data).

CDPD is a connectionless digital transmission technology that uses some of the unused bands in the cellular frequency spectrum to transmit digital information. Because it breaks information into packets, it can take advantage of standard digital compression and error-correcting techniques. It also doesn't need the long setup time an analog cellular connection does.

Many of the same companies that support ETC also support CDPD. These technologies are not mutually exclusive; one cellular base station can support them both. Because it is required by both parties in a transmission, CDPD technology will roll out far slower than will ETC. Both will undoubtedly exist side by side until after the turn of the century, when the analog cellular network makes the transformation to all-digital technology.

Because of its open-standards nature, and the backing of companies li ke AT&T, IBM, Intel, and others, ETC looks like a ready-made success. There is no question that it represents a significant boon to the many thousands of cellular users around the country, and that it makes cellular data communication a viable tool for the mobile business professional.

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Illustration: How Wireless Cellular Works For the greater part of its journey, a data packet sent by way of cellular uses the public switched-telephone network. The only components that require wireless capabilities are the mobile system and the cellular switching office, where the wireless-wired modem pairs reside.

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John Bryan is a freelance technology writer and consultant based in San Jose, California. You can send him E-mail on the Internet or BIX at editors@bix.com .