F E A T U R E
`
`o o ver e u or:
`
`While we wait for 3G to emerge,
`another 2.SG data-over-cellular
`system has emerged from the
`labs and is currently in field trials
`in several locations. GPRS is an
`extension of the GSM system,
`and uses the same channels, the
`same modulation, and the same
`network backbone as the existing
`GSM network.
`
`By Doug Grant, Esben
`Randers, and Zoran Zvonar
`
`T here is often a long
`
`lag time hecwcen the
`secclcmcnc of :1 new
`communication proto(cid:173)
`col o r standard and
`the availability of chips with which
`co implement chat standard. General
`Packet Radio Service (G PRS) is a
`\\ elcomc exception to that rule.
`Enabling wireless users to gain net(cid:173)
`work access at speeds over 100 kbps.
`Gl'RS takes advanrnge of advances
`in cellular chipsets and the existing
`GS.\ I network infrastructure.
`
`The G \ I digital cellular stan(cid:173)
`dard is \I e ll-entrenched in most of
`the wo rld. Industry ana lyses predict
`that over h:df the cellular phones
`sold in the world in 2000 will be
`GS.\1, \\ ith the remainder divided
`among IS-95 code division multiple
`access (CD.\IA), 18-136 time di\·i(cid:173)
`~ion multiple access (TO;\ IA), pcr(cid:173)
`:.onal digital cellular (PDC) (in
`Japan only), and analog systems
`:,uch as the J\.1-. IP system used in
`the US.
`'etwork operacors in mo t
`ufchc \IOrld use the original G, .1\ 1
`spectrum allocation ac 900 I\ I Hz.
`ddicional spectrum at I ,800 \lll z is
`used for the (,S.\I derivative called
`
`DCS- 1800, and ch is band is used
`in many countries. 'ome coumrics
`"ith cellular allucacion at 450
`.\11 lz may bcgin deploying GS.\I
`in chat band as 11 ell co replace old
`;malog networks. In many area\ of
`the Cnircd Scares, there arc GS\I
`·ysrcms operating in the 1,900-
`\ IHz PCS frequency band.
`Considering the man) options
`available, dual-band G \ I phones
`arc quite commonplace now. ,\
`fc1\ tri-band (900/ J.800/1. 900
`.\ I Hz) phones are al\o available.
`allo11 ing a GS I s ubscriber co use::
`the same phone almost anywhere
`in the world.
`Circuit-switched voice call\ arc
`till thc mo\t commonly used ser(cid:173)
`vices in cellular sy seems. However,
`mobile users are beginnin.e; to turn
`to the cellular necwork for wireless
`daca and I nce rnet access. There arc
`Cll'O bas ic modes of data access over
`a ll'irc lc:.s network: circuit switched
`and packet Sll'itchcd. A ty pical
`example of 1·ircuit-switched access
`i, the u,cr \I ho plugs a modem into
`a connector on a cellular phone
`instead of a \1 ired phone jack, and
`dials chc phone numher of the J. P
`or ncrll'ork accc~s poi nc. The con(cid:173)
`nection i~ a dedicated connection,
`and the u,er is billed, using the
`same method as that used for :t
`1·oicc call. by the minutcs of usage.
`,-\ network operator cannot use th:H
`channel for any other user" hilc
`this user is connected.
`
`38 C:0\1\ll S: ICA I 101\ yq fo'\IS DESIG,
`
`W\\ w.<:sdm~tg.corn
`
`0001
`
`Rosetta-2006
`
`

`
`Packet switching makes use of
`data's bursty nature. Data screams
`are broken up into packecs, each
`packet is then quickly routed co its
`descinacion over a shared medium.
`An early example of chis cedmolo(cid:173)
`gy on a c~llular network is the cel(cid:173)
`lular digital packet data (CDPD)
`protocol. This system, operated
`over the AMPS network in the US,
`provides up co 19.2 kbps of raw
`data speed over the AM PS net(cid:173)
`work. Each user time-shares chan(cid:173)
`nels with other users, allowing the
`operator to sell the same channel co
`ocher users. Billing is done on a
`cents-per-packer basis, indepen(cid:173)
`dent of the time spent on line.
`
`2Gto3G
`Second-gene ration (digital) cellular
`networks such as GSM, IS-95
`COMA, IS-136 TOMA, and PDC
`would seem co have a natural
`advantage in transmitting data due
`to their digital natu re. However,
`since these ne tworks were original(cid:173)
`ly developed to provide increased
`capacity and features for voice traf(cid:173)
`fic, data services were not a high
`priority. ow, however, as the
`Internet becomes more and more
`ubiquitous as a medium, access
`(preferably at reasonable speeds) to
`that medium has become necessary.
`See Table l for a comparison of
`these air interface specifications.
`The existing GSM network
`provides data access at speeds
`up to 14.4 kbps. This was consid(cid:173)
`ered a reasonable speed when the
`system was developed, but users
`are now accustomed to at lease V.90
`dial-u p speeds (56 kbps maximum),
`and many are now using ADSL or
`cable modem connections co get
`even higher rates. The evolution
`of the wireless network co provide
`data races in the same range is
`being accomplished in cwo stages,
`which are often referred to as
`"2.SG" and "3G."
`3G services will offer data
`races char rival che best wired da(cid:173)
`ta races now available to the con(cid:173)
`sumer. Speeds up ro 2 Mhps
`will be available to mobile users,
`depending on their distance from
`a base, and whether they are physi-
`
`www.csdm ag.com
`
`APRIL ZOOO 39
`
`0002
`
`

`
`cally in motion wh ile connected.
`These data races are sufficient co
`allow fast Web surfing and screaming
`video feeds with modest resolution.
`The reg ulatory bodies have
`been struggling to gain consensus
`on the 3G system for several yea rs,
`wi th an agreement on the air inte r(cid:173)
`face only reached in lace 1999.
`Disputes an_!i disagreement over
`the optimum technology and inte l(cid:173)
`lectual property ownership have
`considerably slowed the process.
`Even now, the compromise agree(cid:173)
`me nt on the radio standards s hows
`several different and incompatible
`air interfaces. le remains for the
`ne twork operators and infrastruc(cid:173)
`ture man ufacturers co make choices
`and begin developing plans for true
`3G rollou r.
`The so-called "2.SG" systems
`represent an intermediate upgrade in
`data rates available co mobile users.
`There are several of chem, and while
`there is no clear consensus ofwhac
`constitutes 2.SG, a few systems cer(cid:173)
`tainly qualify.
`E nhanced Data for GSM
`Evolution (EDGE) is a representative
`2.5G system. It uses the GSM or IS-
`136 network backbone, and allows
`data races up to 384 kbps. This is
`accomplis hed by switching co a diffe r(cid:173)
`ent modulation scheme than either
`IS-136 or GSM currently uses, to get
`a higher number of bics per transmit(cid:173)
`ted symbol. In the case of use on TS-
`136 networks, EDGE requires a
`wider channel than the 30 kHz
`presently used.
`EDGE wi ll be deployed by net(cid:173)
`work operators in some GSM coun(cid:173)
`tries and also in the US by JS-136 and
`GS i operators. As of yet, there is no
`specific date sec for the deployment.
`The IS-95 (also known as
`cdmaOne) syste m includes an evo(cid:173)
`lutionary path co 3G. T he first step,
`known as " I x RTT," is a straightfor(cid:173)
`ward upgrade co cdmaOne using
`
`the same channels and mod ulation
`system, but using modified d igita l
`processing. It allows data rates up
`co 144 kbps, and is being tested in
`several locations as of th is article's
`preparatio n. The 3G extension,
`known as 3xRTT, wi ll allow highe r
`dl'!ti1 races by using three combined
`CDMA rad io channels. When it is
`fu lly impleme nted, the 3xRTT ver(cid:173)
`sion (also k nown as cdma2000) wil l
`offer the 2-Mbps data speeds tout(cid:173)
`ed by 3G designers.
`While we all wait for 3G to
`emerge, another 2.5G data-over-cel(cid:173)
`lular system has emerged from the
`labs and is currently in fie ld trials in
`several locations. The GPRS is an
`extension of the GSM system, and
`uses the same channe ls, same mod u(cid:173)
`lation, and same network backbone
`as the existing GSM network. This
`makes deployment a re latively sim(cid:173)
`ple matter, at lease compared with
`the installation of a completely new
`infrastructure at every level. With
`GSM operators in 140 countries,
`GPRS offers the promise of a high(cid:173)
`speed data connection for users in
`most of the world.
`
`The mobile terminal
`The re are several classes of GPRS
`mobile terminal. T hey are classified
`by how many of the GSM slots arc
`used in each di rection - more sloes
`mean higher data speed. C urre ntl y,
`GPRS infrastructu re is operational
`in some networks, with comme rcial
`service planned for lace 2000. Sub(cid:173)
`scriber eq ui pment will cake several
`forms, and will be ready when the
`networks are ready.
`Today's GSM phone is, well, a
`phone. It handles circuit-switched
`voice traffic, and through the con(cid:173)
`nector on the bottom of the unic, it
`can also be used for circuit-switched
`data. However, new devices are
`emerging which can benefit from
`non-voice wire less connecciviry, s uch
`
`TABLE 1: Comparison of GSM, IS-136, and EDGE Air Interface Specifications
`
`_Chan_nel _b11~dth
`Raw channel l)it "'tE!
`
`User bll rate
`
`OSM
`
`200 kHz
`
`270.833 kbps
`14.4 kbps
`
`R•• - - - - - - · - - - - -
`
`Modulation
`
`GMSK
`
`11-138
`
`EDGE
`
`48.6 kbps
`
`19.2 kbps canaJog
`CDPD)
`n/4 OQPSK
`
`812.5 kbps
`Up to 384 kbps
`
`8-PSK
`
`as PDA-cypc computi ng devices.
`Even a laptop computer can make
`use of a wire less modem card in a
`PC-card form factor, with no need
`for voice capability ac all.
`sing
`circuit-switched connections, data
`rates of 14.4 kbps are now available.
`lultislot operation in circuit-swi tched
`mode is possible, and allows higher
`data rates. This high-speed circuit(cid:173)
`switched data (HSCSD) mode will
`be offered by some operators, and
`allows data rates up co 57.6 kbps. It
`is likely to be more expensive than
`GPRS service.
`GPRS terminals, or mobile sta(cid:173)
`tions, fall into various phases, classes,
`and mulcislot classes, documented
`in GSM 05.02, An nex B. Annex R
`is s hown in Table 2 in its e ntirety.
`There are two phases of GPRS. A
`Phase 1 GPRS terminal operates in a
`point-to-poi nt mode. Phase 2 allows
`point-co-multipoint operation. Since
`Phase 2 req uires additional changes
`ac the ne twork lcvd , it will probably
`lag behind Phase I in deployment.
`See Figure l for a diagram of the
`mobile station. 1
`
`lable definitions
`For HSCSO, only mul tislot classes 1
`through 18 are recognized. A mobile
`station with a highe r mulcislot class
`number will indicate a s uitable mul(cid:173)
`cisloc class less than 19 for HSCSD
`applications (sec GS I 04.08). The
`following is a list of defi nitions for
`Table 2.
`
`• Rx: Rx describes the maximum
`number of receive time sloes (TS)
`that the m obile stat io n can use
`per T DMA frame. T he mobile
`station must be ab le to support all
`integer values of receive TS from
`0 co Rx (de pe nd ing on the ser(cid:173)
`vices su pported by the mobile sta(cid:173)
`tion). The receive TS need not be
`contiguous. For type l mobile sta(cid:173)
`tio ns, che receive TS shall be allo(cid:173)
`cated within window of size Rx,
`and no transmit TS shall occur
`between receive TS within a
`TOMA frame.
`• Tx: T x describes the maxim um
`number of cran mit TS chat the
`mobile station can use per TOMA
`frame. The mo~ile srnrion muse be
`able to support all integer values of
`transmit TS from O co Tx (depend(cid:173)
`ing on the services su pported by
`
`40 COMMUNICATION SYSTEMS DESIGN
`
`www.csdmag.com
`
`{
`
`0003
`
`

`
`II
`
`DSP
`memory
`
`DMA
`
`ADSP
`218x
`DSP
`
`1/0
`
`.c: ~
`Ill 0
`111 E
`Ii: GI E
`
`Auxiliary
`AID, D/A
`
`FIGURE 1: Block diagram of a typical GSM mobile station
`
`TABLE 2: GPRS Multislot Classes.1
`
`Multislot class
`
`Maximum number of &lot &
`sum
`Tx
`
`1
`2
`1
`
`3
`3
`4
`
`lype
`
`Minimum number of slots
`Tlb
`T,.
`T,.
`2
`4
`2
`3
`1
`3
`1
`1
`
`3
`3
`3
`
`{
`
`C)
`
`C)
`
`c)
`
`2
`
`- ~--~)__
`2
`c)
`2
`
`c)
`
`-
`
`1
`1
`1
`
`1
`1
`1
`
`the mobile station). The transmit
`TS need noc be contiguous. For
`type 1 mobile stations, the transmit
`TS shall be allocated within win(cid:173)
`dow of size Tx, and no receive TS
`shall occur between tra nsmit TS
`within a TOMA frame.
`• Sum: Sum is the cotal number of
`upli nk and downlink TS that can
`actually be used by the mobile sta(cid:173)
`tion per TOMA frame. The mobile
`station must be able to support all
`combinations of integer values of
`Rx and T x TS where 1 <= Rx + Tx
`<= Sum (depending on the services
`supported by the mobile station).
`Sum is not applicable to all classes.
`• 1~n: T,. relates to the time needed
`for the mobile station co perform
`adjacent cell signal level measure(cid:173)
`ment and co get ready to transmit.
`For type 1 mobile stations, it is che
`minimum number of TS chat will
`be allowed between che end of the
`previous transmit or receive TS
`and the next transmit TS when a
`measurement is to be performed
`between. In practice, the minimum
`time allowed may be reduced by
`amount of timing advance. For
`type 2 mobile stations, it is not
`applicable. For circuit switched
`mulcislot configurations as defined
`in subclause 6.4.2.1, T,2 is not
`applicable.
`• T,1r· T,b relates co che cime needed
`for che mobile station to gee ready
`co transmit. T his minimum re(cid:173)
`quirement will only be used when
`adjacent cell power measurements
`are not requi red hy t he service
`selected. For type 1 mobile sta(cid:173)
`tions, it is the mi nimum number
`of TS that will be allowed be(cid:173)
`twee n the e nd of the last previous
`receive T S and the immediately
`following transmit TS, or between
`the previous transmit TS and the
`next transmit TS when the fre(cid:173)
`quency is changed in between.
`In practice, the minimum time
`allowed may be reduced by the
`amount of the ti ming advance.
`For Type 2 mobile station, it is
`the mi nimum number of cime sloes
`that will be allowed between the
`end of the last tra nsmit burst in
`a TDMA frame and the first trans(cid:173)
`mit burst in the next TOMA
`frame.
`• T,.0 : T,. relates ro the time neerled
`for the mobile station co perform
`
`42 COMMUNICATION SYSTEMS DES IGN
`
`www.csdmag.com
`
`0004
`
`

`
`adjacent-cell signal-level measure(cid:173)
`ment and co get ready to receive.
`For tyj)e 1 mobile stations, it is the
`minimum number of TS that will
`be allowed between the previous
`transmit/receive TS and the next
`receive TS when measurement is
`co be performed between. For type
`2 mobile stations, it is the mini(cid:173)
`mum number of timeslots that will
`be allowed between the end of the
`lase receive burst in a TDMA frame
`and the first receive burst in the
`next TDMA frame.
`
`• T rl,: T,b relates ro the time needed
`for the mobile station to gee ready
`to receive. T his minimum require(cid:173)
`ment will only be used when adja(cid:173)
`cent cell power measurements are
`not required by the service select(cid:173)
`ed. For type l mobile stations, it is
`the minimum number of timeslots
`that will be allowed between the
`previous transmit TS and the next
`receive TS, or between the previ(cid:173)
`ous receive TS and the next re(cid:173)
`ceive T S when the freq uency is
`changed in between. For type 2
`
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`44 COM MU, ICATI01 SYSTEMS DESIGN
`
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`{
`
`mobile stations, it is the minimum
`number of timeslots that will be
`allowed between the end of the last
`receive burst in a TOM/\ frame
`and the fi rst receive burst in the
`next TDMA frame.
`
`GPRS terminal classes
`There are three classes of GPRS ter(cid:173)
`minals. A Class C terminal can oper(cid:173)
`ate in either circuit-switched or pack(cid:173)
`et-switched mode, but the mode is
`manually selected. A Class B termi(cid:173)
`nal can operate in circuit-switched
`mode and be in GPRS idle mode at
`the same time. When a GPRS termi(cid:173)
`nal is in idle mode, it can make its
`presence known ro the network. If
`the network wishes to send data ro
`the terminal (or the terminal wishes
`co send data co the network), it must
`then switch co the standby state
`and then co the ready state, du ring
`which it can t ra nsfer data. A Class
`B termi nal must be able to rewrn
`co circuit-switched mode quickly.
`Class A terminals are capable of fu ll,
`simultaneous operation in circuit(cid:173)
`switched and packet modes. Most
`manufacturers are tending towards
`development of Class B and C
`equipment at chis time.
`The mul t islot classes are more
`complicated to explain -
`the re arc
`29 of them. First, recall chat GSM
`operation divides ti me into eight
`TS. This slot strucwre enables
`TDMA operation.
`ormal voice
`calls use three slots: one for the
`mobile-co-base transmission (for(cid:173)
`ward link), one for the base-to(cid:173)
`mobile transmission (reverse link),
`and one for the monitor slot (when
`the mobile device can monicor
`neighboring cells). Thus, several
`users can use the same radio chan(cid:173)
`nel, each transmitti ng during a dif(cid:173)
`ferent time sloe than the others.
`T he mobile station never transmits
`and receives at the same time. In
`contrast to analog systems, the IS-
`95 CDMA system and several pro(cid:173)
`posed 3G systems are full-duµlc.:x
`systems that use frequency division
`du plex (FDD) technology.
`The 29 GPRS multisloc
`classes arc differen tiated on the
`nu mber of transmit sloes and
`receive sloes used . .Classes 13
`through 18 are denoted as type 2,
`and are special, since they are
`based on the assumption that the
`
`www.csdmag.com
`
`' I
`
`0005
`
`

`
`mob ile station can tra nsmit and
`receive simultaneously. Since near ly
`all c urre nt GSM mobi le imp le men(cid:173)
`tations make extensive u e of the
`partitioning, inccgracion, and power
`a, ings afforde d by TD IA opera(cid:173)
`tion, it w ill probably be some time
`before mobile stations in these
`c lasses wi ll appear. ~ I ulcislot classes
`l chrough ' 12 and 19 th ro ugh 29 arc
`all type l classes, mea ning that
`simultaneous trnnsmit a nd receive
`is nor req 11 ired.
`J\l u ltislo t classes 19 through
`29 permit up co e ight t ra nsmi t and
`e ig ht receive slo ts, which involves
`add itional work at the system level.
`However. multisloc classes I through
`12 are readi ly accom modated with
`min or c ha nges co the infrascruccure.
`These classes wi ll be the first co be
`deployed commercially.
`There are some ad d itional
`demands on the mobile station in
`m ulcisloc ope ratio n. Fi rst, each sloe
`requires its own equalization. In
`GSJ\l voice operation, each burst (i n
`one slot) carries a seq uence of bi ts
`in mid-b urst that is used fo r clrn nncl
`equaliza tion. T his cq ual i1.acion is
`ach ieved using a D P and requires
`a la rge amount of rime an d J\ II PS
`co accom plish. In G PRS, each sloe
`needs equalization. and this places
`an add itional bu rden on the DSP,
`meaning more ~JlPS a nd more
`power co ns umption. Second, the
`channel-tu ni ng synthesizer needs to
`be able to settle faster, since fewer
`unused lots arc ava ilable to allow
`the PL L to se ttle. This requ irement
`dictate chat the S) nchesizc r u e the
`fraccional-N architecLUre, a nd m ust
`have the loop filter opcimized for
`fasc seccl i ng.
`C lass 12 is rhe most demanding
`of the multislor classe . It allows
`the mob ile station co use up to four
`receive and four transmi t sloes, as
`long as the um of active transmit
`and receive sloes in a frame is Ii, c .
`If the mobile is operated wirh four
`active transmit slots, the battery life
`wil l be significancly shortened due to
`the much higher power consumption
`of transmit sloes compared co receive
`s lots. This also means that th e power
`amplifier muse be ca pable of opera(cid:173)
`tion i: t a 50% du ty cycle (four ~lots
`out of e ight), in contra c co the 12.5%
`d uty cycle (o ne slot out of eight) of a
`GS~ I voice rerminal.
`
`w,, w.csdm.,~.com
`
`In an effort to red uce battery drain
`and to cake ad,·ancage of the usually(cid:173)
`asymmetric nature of a data connec(cid:173)
`tion. many manufacturers arc mo,·ing
`coward classes of operation rhat rcsul t
`in a lower transmit dury cycle. Class IO
`seems like the preferred class, with a
`good trade-off of battery life and data
`speeds. ~lost chipsecs arc capable of
`either class 10 or cb ss 12 operation -
`it is the RF power amplifier and the
`power source (batter) or ocher) chat
`ne::e::ds additional investigation.
`
`Putting It all together
`le will be incerescing co sec how
`GPRS deployment impacts the evo(cid:173)
`lution of wireless Internet con nec(cid:173)
`tions. Geccing speeds greacer chan
`100 kbps will be a huge improve(cid:173)
`ment o,·cr the currently-available
`speed~. We will all be able co read
`our e-mail much foster through this
`con nection -
`even faster chan the
`V.90 modems we all use now when
`craveli ng. T he big question is
`wheth,;r rhis is fast enough for the
`services beyo nd e-mail chat will be
`offe red by network operato rs in the
`ncar-cc rm. Fortunaccly, with the
`technology becoming available in the
`form of readv-co-go chipsecs and soft(cid:173)
`ware, we won 't have co wait coo long
`to fi nd out the answer.
`
`D1J11g Gn1111 r~:ori:s fo, the RF r111d r.::ire/ess
`systems b11si11ess 1111il 01 f\ 110/og Devices. He
`holds 11 BSEI•: degrer from thr U11i-r.•r1,il)•
`of I .oral!, ""d tilleudn/ gradu"'" school nl
`.Vor1h,11s1em U11if.:ersity. I le ru11 be ro11tuc1rd
`tit do11g.p.n111t®F111ulog.rom.
`
`E.<bm Rrmdrrs ~·orh ffJr th, RF t111d u:·irr:le.rJ
`sy.ffems b11sim:.c.r 1111i1 r11 Jl11ulog Devires. H,
`m11 be reuched r11 esbe11.m11du@r111alog.ro111.
`
`7.om,, Zvo,1111· is the systn11s tk'i:elopmeut
`111r111uger i11 the ro1111111111irutio11J divisio11 of
`A1lf1log Devices. He rerl'ived r1111t1sters i11 el,•c-
`11irul e11J!,i11eeri11gfro111 U11it:n,il)• of Belgmde.
`Yugoslm:ir,. u11d PhD from .\'ortheustrm
`U11ive1"Si~1·, Bos/011. Hr am be co111ucted m
`zom11.::wo11ar@r111u/og.co111.
`
`References
`
`1, From Annex B of the ETSI EN 300
`908 Ver. 6.4.1 (C) ETSI, reprinted with
`permission. Ttle original standard may
`be obtained from ETSI PublicaUon
`Office. publlcatlons@etsl.fr. Tel: 33 (0)4
`92 94 42 4 .
`
`Source Code
`Protocol
`Solutions
`
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`
`~
`
`INVERNESS SYSTEMS, Inc.
`Networkl11g Tecb11ologies for a Paster World
`
`APRIL lOOO 45
`
`0006