FEATURES
On The Road To PCS
by Peter Rysavy and Craig J. Mathias
By most estimates, more than 90 percent of the traffic on the U.S. cellularphone network is voice, but data transmission on cellular systems is growingrapidly. With the right modems and a little cooperation from your carrier,it's easy to send faxes, move e-mail and even be a mobile remote LAN node.
Moreover, the data-only Cellular Digital Packet Data (CDPD) service is beingrolled out in many parts of the country, offering low-cost access over TCP/IP.Still, it's probably no secret to most big-city cellular phone users thatcellular networks are beginning to run out of capacity as demand increasesand price
s decline. So, mo
st cellular operators are looking into digitalcellular as a way to boost both capacity, and, in the bargain, offer evenmore comprehensive and capable data services. Want voice, data, fax, pagingand mobility all in one system with one handset? Digital cellular couldbe it.
Of course, the big news in wireless is Personal Communications Services(PCS)--a new spectrum located in frequency bands recently auctioned by theFCC. Having paid big bucks (more than $7 billion) for the right to use thesenew frequencies, PCS carriers are working feverishly to define and deploytheir all-digital integrated voice/data systems. Although volume PCS deploymentis a few years away, network managers responsible for purchasing wirelessdata services need to prepare. But, since digital-cellular data servicespoint the way for PCS data capabilities, the transition sh
ouldn't be tooonerous.
Before the sky gets too blue, it is important to recognize the hurdles thatmust be overcome before digital cellular
data services become
the norm.These challenges include competition from other wireless data networks (includingpacket radio systems and the emerging satellite-based services) and a seeminglyunwieldy number of different digital cellular standards.
Starting From Analog
The current cellular phone system, known asAdvanced Mobile Phone Service (AMPS) was, believe it or not, designed inthe 1960s--long before modems and faxes were commonplace. Nonetheless, witha modem supporting a cellular-compatible over-the-air protocol, users canget reliable circuit-switched data connections with uncompressed speedsof 9.6 Kbps, and, with good radio conditions, 14.4 Kbps. One drawback toachieving high performance, however, is that both sides of the connectionrequire cellular-enhanced modems. If one side uses a conventional landlinemodem, connection reliability and throughput can suffer.
Cellular service providers recently began to address this problem by deploying"modem banks" at Mobile Switch
ing Centers. This lets c
ellularmodem protocols terminate at the switch and conventional landline modemprotocols operate over the rest of the connection. Digital cellular systemswill also take advantage of such modem pools.
We should also mention that CDPD, which is similar in concept to the packet-radionetworks run by ARDIS (Lincolnshire, Ill.) and RAM Mobile Data (Woodbridge,N.J.), is really a retrofit to the existing analog cellular system, movingdigital packets over otherwise unused cellular channels. Packet servicesare best used when bandwidth demands are infrequent or bursty. Constantdemand or interactivity is best served by a circuit connection using modems.
Digital Cellular Enters
Two technologies are contending for the U.S.digital cellular market: Time Division Multiple Access (TDMA), promisinga threefold increase in capacity over analog, and C
ode Division MultipleAccess (CDMA), with an even greater increase in capacity--perhaps as muchas 10 times more than analog. TDMA de
ployment is already underway
in a numberof markets, while CDMA should see installation this year. Individual cellularoperators will decide which digital system they will deploy.
Two TDMA digital cellular standards have recently been released by the TelecommunicationsIndustry Association (TIA): IS-135 (TDMA Services, Async Data and Fax) andIS-130 (TDMA Radio Interface, Radio Link Protocol 1). They govern how datacalls are handled within TDMA. CDMA data capabilities are defined in theIS-99 standard (Data Services Option Standard for Wideband Spread SpectrumDigital Cellular Systems).
A mobile computer will connect to the digital cellular telephone using asimple serial connection, rather than an external modem, and will use thefamiliar "AT" commands implemented in most conventional landlinemodems. Users will be able to move data and faxes at 9.6 Kbps with bothTDMA and CDMA, and possibly up to 14.4 Kbps with CDMA. Digital cellularsystems will also support a "short message service,
" similar infunction to pa
ging, but providing reliable delivery. In fact, compared withdata over analog cellular, data sent via digital cellular should offer significantlymore reliability and greater immunity to RF problems such as noise and hand-offdelays.
Both TDMA and CDMA digital cellular are designed to coexist with AMPS, sothat a single cell (the area covered by one base station) can support bothanalog and digital service. Most digital cellular phones marketed in theU.S. also have an analog capability and are often referred to as "dualmode." But phones supporting both TDMA and CDMA are unlikely to appear.As carriers deploy PCS, we'll probably see dual-mode PCS/cellular phones--suchas a CDMA phone that operates both at 1.9 GHz and the conventional cellularfrequencies at 800 MHz.
Unfortunately, truly global digital standards are unlikely. In Europe, the
Global System for Mobile Communications (GSM) digital cellular standardis already widely deployed, and it is planned for
more than 70 countriesworldwide.
Japan is also deploying an all-digital system called PersonalHandyPhone System (PHS), and both GSM and PHS support enhanced data servicefunctions. Both are incompatible with the TDMA and CDMA systems plannedby North American cellular operators.
At the physical (radio signal) layer, voice, fax and data services trafficare all just "data," when carried across a digital cellular network.As traffic moves up the network protocol stack, however, the differencesbetween them matter, and the way they must be handled varies considerably.The digital cellular data standards specify link-level protocols that transportdata reliably across the inherently unreliable airlink. For a call directedtoward the mobile station, one phone number will be used for voice calls,and another number for fax and data calls. For outgoing calls, the mobiledevice will issue a command to the digital cellular telephone to specifythe type of service desired.
Integration with existing w
ireline systems represents a slightly
differentchallenge. Although the base station can efficiently receive a data transmissionfrom a mobile unit, it must still communicate that data to a landline modemon the other side of the telephone connection. Consequently, a modem poolis required at the mobile switching center so that the data can be forwardedusing conventional modem protocols such as V.32 and V.42, as well as Group3 fax. Ultimately, this internetworking function will also support ISDNand many wide area data networks.
Dark Clouds
The primary motivation for digital cellular deploymentis not enhanced data services. Rather, it is to solve a capacity problemsome carriers have in some markets. Therefore, we don't expect digital cellularto become ubiquitous. Moreover, the market will demand the continued availabilityof analog cellular for quite some time, making analog cellular the
lowestcommon denominator for the foreseeable future. This not only protects theinstalled base, but also will
provide the only way for a roaming user
withone digital system to operate in a region served by a carrier using theother.
Plus, the TDMA and CDMA data standards have only recently been finalized.Infrastructure equipment to support these data standards should soon beavailable from companies such as AT&T, Motorola (Schaumburg, Ill.),Qualcomm (San Diego, Calif.), Hughes Network Systems (Germantown, Md.) andEricsson (Paramus, N.J.). The big question is how soon carriers will makedata services available. CDMA carriers appear to be more aggressive, andplan on providing data services soon after they launch service. Since itis unlikely that digital cellular systems will be universally deployed overthe next five years, demand for data services on digital cellular will likelytake some time to develop.
Still, one way to look at digital-cellular data services is as a bridgeto PCS. In fact, some cellular carriers may find it more lucrative to investin PCS than to rebuild their AMPS infrastructure
s using digital technology.The name of the
cellular game, after all, is building the subscriber base.Many carriers see PCS as a way to access new markets, whereas digital cellularwill only let them increase their capacity in existing markets. While thiscould limit carrier interest in digital cellular, it is, on the other hand,highly likely that data services on PCS will closely resemble those beingdefined for digital cellular. Three of the seven air interfaces definedfor PCS are variants of existing digital cellular standards, namely TDMA,CDMA and GSM. Consequently, these systems will likely inherit the data capabilitieswe've discussed, initially supporting circuit-switched asynchronous datacommunications.
Researchers and standards bodies associated with TDMA, CDMA and GSM arealso looking at packet data capabilities. In fact, the link layer of thedata service for CDMA has been designed so that CDPD p
rotocols can be usedat higher layers. Synchronous or isochronous data capabilities for digitalcellul
ar networks may also eventually become availa
ble for applicationsusing video and multimedia. Here, GSM already has an advantage, with itsISDN signaling and easier connection to ISDN networks.
Higher data rates on digital cellular are also a possibility. TDMA, CDMAand GSM support the concept of channel aggregation for higher data rates.TDMA will allow three channels to be combined for user data rates of 28.8Kbps, and CDMA could ultimately provide user rates as high as 64 Kbps.
In the end, services developed to work with digital cellular should be easilyported to PCS when PCS systems are broadly deployed. While the all-but-certaingradual phase-out of analog cellular service will likely take more than10 years, digital cellular systems and their associated data services arethe inevitable replacement. In Europe, where GSM already is broadly available,data usage is growing rapidly. Indeed, the demand for data services willbe a key driver of future wide-area wireless growth.
Digital cellu
lar is an important parallel development with PCS
. The technologicalsuperiority of digital, including increased capacity, lower cost, enhancedfeatures and reliable data, is a clear motivator for both carriers and users.However, upgrading existing networks to digital will take time and willnot always be the highest priority for cellular carriers. While the datacapabilities of digital cellular networks have attractive technical features,it is unlikely these data capabilities will be broadly available for perhapsthree years or more. Still, the eventual deployment of high-capacity wirelessdata services is assured, and as workforce mobility continues to challengethe creativity of network managers everywhere, cellular and PCS carriersare finding that there's much more to their networks than voice.
Peter Rysavy is the principal of Rysavy and Associates, Seattle, Wash.Rysavy and Associates consults with companies d
eveloping personal and mobilecommunications solutions. He can be reached at rysavy@halcyo
n.com. CraigJ. Mathias is a principal with Farpoint
Group in Ashland, Mass. FarpointGroup provides advisory and systems-integration services in emerging communicationstechnologies. He can be reached at farpoint@world.std.com.
February 13, 1996
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