CA2004476A1 - Adaptive rate control for echo cancelling modem - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An echo cancellation modem for full-duplex communication over a channel with a remote device in accordance with one of several possible modulation schemes suitable for different channel qualities, respectively. The modem includes an echo canceller for reducing echo components appearing in the channel signal while leaving a residual echo component in the echo reduced channel signal, a monitor for determining the quality of the channel based on an analysis of the residual echo component, and a controller for selecting an acceptable modulation scheme, based on the analysis of the monitor. The modem selects the acceptable modulation scheme prior to data communication and it also requests changes to the appropriate level of modulation during data communication when the monitor indicates that the channel quality has changed significantly. The modem requests the remote device to operate at the selected rate and begins operating at the selected rate only if the remote device concurs. In another aspect, the modem includes a fall forward flag which is used to prevent contention between the modem and the remote device which would occur if the modem repeatedly requested an increase to a higher level modulation scheme which the remote device cannot support. In another aspect, the modem includes re-initialization logic which selects the appropraite thresholds used to monitor the quality of the channel depending upon whether the modulation option is coded or uncoded.

Description

~da~tive ~ate Control ~or Echo Cancellina Modem Backaround o~ the Invention This invention relates to modems.
A modem is a device for transmitting and receiving digital in~ormation over a bandwidth-limited channel, such as a telephone line. Typically, two modems are required to communicate over the channel in a given direction, one modem located at one end of the channel to transmit a signal and another modem at the other end of the channel to receive the signal. The transmitting modem employs an appropriate modulation technique to convert the digital signal into a ~orm which may be transmitted over the channel and the receiving modem demodulates the received signal to recover the digital information.
Data transmission can occur in both directions over a channel and it can be in either hal~-duplex mode or full-duplex mode. In half-duplex mode, communication occurs only in one o~ the directions at any given time.
In ~ull-duplex mode, communication occurs in both directions at once. In both modes, the rate at which data can be transmitted depends critically on the quality of the channel, that is, on the amount and character of noise which the channel adds to the transmitted ~ignal.
Typical channel noise includes idle noise, which is present even when no signal is being transmitted, and signal induced distortion. Noise impairs the quality o~
the channel and reduces the ~ignal-to-noise ratio of the transmitted signals. Thus, a higher noise level on the channel, or equivalently, lower channel quality, generally means that a lower communication rate mu~t be used in order to achieve a reliable, error-~ree transfer o~ data.

1 ~:, 0 4 . i~ O R A~vI ~ F I S ~ R I C ~ A P, D S O N P O ':=~
21)~:)4476 In ~ull-duplex ~ode, ther~ 1~ an addltlonal imp~lr~ent to channel quality be~d~ channel nolse. ~hs addlt~onal impalrm~nt is referr~d to a~ ~cho. some o~
th~ tr~n~mlt~ed ~gnal inevitably roed~ directly back to the rscei~or side oS the ~nding modom whore it app~ars a~ an und~lred n~ax echo slgnal that lnter~er~ wi~h the r~c-lved sisnAl s~nt by a r~moto ~od-~. Sn addit~on, becau~e impedance mi~matches in thQ network are unavoldable, ~ome of the sendlng modem'~ transmitted ~gnal is re~lected bac~ ~rom tho dlstant modem and ~rom any other point at whlch there ls an impedancQ m~omatch to th~ 6ending modam where ~t ap~ears ~s a far echo sign~l which ~urth~r corrupts the ~lgnal tr~nsmitted from the di~tant modRm. The combinatlon or the noar and ~ar lS echo are reforred to a3 the co~blned ~cho.
Methods have been developed ~or ~epa~atoly dealing w~h th~ two type~ of channel ~palr~ent, nam~ly, the channel di~tortion and the combinod ~oho. For oxA~ple, ~od-m~ ar- av~llablo ~hlch havo th- abllity to ~ ct the co~mun~o~tion rat~ ~or hal~-dupl-x oo~munication ba--d upon ~he quallty o~ tho chann~l. 80mo o~ ~he~- mod-m~
doterm~ne tho quality of the channel by mea~urlng the amount of noi~e which is pre~ent ln the receivsd 61gnal.
Then, ~as~d upon a negot$ation with tho other mode~ wlth whlch they are co~munlcatlng, the~e modem~ ~elect ~ro~
among sev~r~l avallable com~unicat~on rate~ tho part~cular rate whlch will ylel~ the do~lred signal-to-noi~ ratlo, i.e., the deolred per~ormance given the noise present on the chsnnel.
Modems are available which include an echo cancel~r to improve full-duplex communicatton. In princlple, the echo canc~l~r operate~ by generatlng r-pllc~ of the near and ~r echo ~lgnal~ whlch Are then O-~htracted ~ro~ the received ~lgnal to yl~ld a rel~tlvely ect,s-~r~e aign~i.

Metho~ of dealing with channel impairments generally con~orm to specific communlcatlon protocols to assure that they can communicate with other modems which are made by other manufacturers and which may not incorporate the same methods of dealing with channel impairments.
One important organization which develops such protocols is the CCITT group within the International Telecommunication Union (ITU). The CCITT has promulgated or reaommended various protocols for half-duplex and full-duplex communication. For example, a current recommendation for the family of 2-wire, duplex modems is the V.32 specification. The V.32 speciflcation defines the signaling sequences and timing of the sequences which two modems must use in order to establish and continue communications with each other. Since such standards are widely adopted by modem manufacturer~, it i~ important that any feature~ or capabilitie~ which are developed for n~wer modems con~orm to such ~pecifications.

Summarv of the Invention In general, in one aspect, the invention features an echo cancellation modem for receiving data signals from a remote device over a channel in accordance with one of several possible modulation schemes suitable for different channel qualities, respectively, while simultaneously sending data signals to the remote device over the same channel; the modem includes an echo canceller for reducing echo components appearing in the channel ~ignal while leaving a re~idual echo component in the echo reduced channel signal; a monitor for determining the quality of the channel based on an analysis of the residual echo component; and a controller for selecting an acceptable modulation scheme ba~ed on the analysis done by the monitor.
Preferred embodiments include the following Z . 0 4 . i3 ~ i3 A ~`~I ~ F I S ~ R T C H A R. D S O N P Cl , 2(~0~7~

reatures. Tho ~odo~ lnclud~ circuitry for ~u~llzlng and demodulatln~ the echo r~duced ~ignal~ and the monitor ~nalyz~ tho residual echo component ba~od on th~
~qu~llzod and dc~odulated echo roduced Jignal. Thoro 19 al~o control circuitry ~or caus~ng the monitor to analyze th~ ro~l~ual echo componont while tho mode~ 18 tranam$tting a s$gnal and the re~ote d~vico i8 not sending a 6ignal. ~he quallty o~ the ch~nnel 18 dotormined ~ on a line~r combinatlon o~ thQ s-p~r~t~
analysos o~ the channal di6tortion And tho residual Qcho componont. Tho co~troll~r include~ mQan~ rOr reque~t~ng the r~mote d~v$ce to shl~t to a h~ghor level modulatlon sch~me and m~ans for ~hl~ting to the highor leve~
modulation scheme i~ the romote device concur~. Th~
1~ controller also lncludo~ means ~or r-questlng ths romoto dovlce to ~hl~t tO A lower l~v-l modulation ~che~e and ~ean~ for the mo~em to unilaterally ehl~t to tho lowor l-vel modulation ~cheme for tr~n~ml--lon or d~a ~lgn~l~
to th- r-mote d~vlce. ~ho modulatlon och-m~- ~nclu~o dlf~o~ent modulation r~too, and ood-d ~nd uncoded ~odulatlon ~ode~. ~he controller ~urthor include~ ~oans ~or causing the mod~ to tran~mlt data s$gn~1~ u~ing tho sa~e ~odulation Rcheme as the remot- dovice U90~ ror it8 transm$~slon~.
In gener~l, in another a8pect~ the invontlon f~atur~s a ~ethod ~or u~o ln a modem o~ the klnd which recel~es data ~ignals ~rom a romote dovlc- ov~r a ch~nnQl in accordance with one of a rang~ o~ avAll~b1e ~odulation ~chQ~es sultable, ro~p~ot~voly, ~or a ranqe o~ dlff~rent channel ~ualltieo, a high~r l~vol modulatlon oohem- bolng 6ultabl- ~or a hlgher channol q~allty, the ~ thod include~ det~rminlng whethor the curr~nt channel quallty wo~ld support a higher level ~o~ulation ~che~ th~n th~
curr~nt modulation s~heme; i~ oo, s~ndlng a requ-ot tho 3~ re~ote devlce ~o use a teoired ~lqher levol modul~t~on 1 Z, Cl 4 . ~ C~ ji AI~ 7 ~ R I C ~ A F~ D S O N F~ o i~i sohemQ; and con~lt~oning the modam not to la~er r-quo~t a hlgher lovel modul~t~on ~cheme i~ a high~r l~vel mod~lation sch~e c~nnot be supporto~, ~ro~r~d e~bod~mont~ l~clude tho ~ollowing ~oatures The condition~ng ~ ~asod upon tho modulatlon schomo chosen ~y tho other dev~o ln ro~pon~e to th~
requo~t sent fro~ tho modem ~ tho r-mote devlco concurs ln ~hs U5~ o~ th~ d~lred ~odulation ~oheme and i~ ~here r~mains a hlgher l~vol 30dulatlon whe~e usable by tho modem, then tho mode~ is condltionod to latQr reque~t a highor levol modulation ~ohom ~f the chann~l gual~ty lat~r improves; lf th~ rcmote devloo doe~ not concur in th~ u~e of the de~ired modulatlon schomo or i~
th4re i8 not hlghor l~vel modul~tlon ~chemo u~able by the 1~ modom, then the modom lg conditionod to xe~rain from alter r~quest~ng a hlgh~r l-vol modul~tion ochomo 1~ th~
channel quallty later improv~s In goneral, ln another ac~-ct, the inventlon fo~ture~ ~onltor~ng ths quallty o~ the chann-l ba~ed on recoived dAt~ signals, determlning when a chang4 ~o B
hl~hor level ~odulation schem~ would b- perm$colble ~a~od on when th~ ~onitor~d channel gu~llty exceed~ a thro~hold, ~nd ~ettlng the thro~hold to ono value with re~pect to a c~nge to a coded modulatlon ~od~, Qnd to a different value w~th re~psct to a ch-nge to ~n uncod~d mod~lation mode Pref~rred embo~iment~ o4 tho ~nvontion include ths ~ollowin~ foaturo~ The ~od~m i8 inlt~all2ea ~lng ono threshold corre~pondlng to one o~ tho ~odulatlon mo~e~, and reinitlallzed ucing the oth-r thre~hold ln res~on~s to a r-que~t fro~ tho re~ote de~lco to ~wltch to tho othor ~od~
In general, ln ~nothsr ~oct, the lnv4ntion f~uros ~onltoring the qUAlity o~ the channol, wlth an scho pros~nt, and ~elect~ng a sign~l point oon~tellat~on 1 Z . Cl 4, i3 ~ Cl æ A~ ~ F I S H ~ R ~ C ~ A }:~ D S O N P Cl 8 2~ L7~

~er mo~ulation based on the monitored ch~nnel quality.
In pr~err~d embodim~nt~, one ignal po~nt con~t~llation co~prise~ more than 2~ point~, wh-r~ N ls th~ numb~r or b~t~ per ignal~ng int-rval to be ~ent.
In g~neral, ln anoth~r aspoct, the in~Q~tiOn ~eatur~ a method ~or ~tting an lnitial r-copt$on rate ~or an echo aan~ellatlon mod~m Or the k$nd whlch r-ceives data 61gnala ~rom a remote de~ic~ at one of se~eral pos~ible rat4~ ~nd which i8 ~b~c~ to an lnit~al train~ng procedur~ prior to inltial r~ceptlon o~ data ~nals; thQ method includes monitorlng the ~uality of the channel durlng the inltlal train~ng procedure, and ~mmediat~ly setting the inltlal rat~ to a valuo corre~ponding to the rat~ cu~tainablo on th~ channel ba~ on its monitor~d quality.
The invent~on allow~ a mo~em to 5Qlect a~ w~ll as adju~t the rat~ at whlch the mod-m c~mmunicA~e~ ln rull-du~lex o~eration over th~ channol with tho remoto dovice ba~od upon a ~a~ur- o~ the ov~rall channel quality wh~ch ~nclud-a a m~acure re6i~uAl echo ~a well as ~he ¢hannel dl~tortion~. In addltlon, ~lnce the ch~nnel qu~llty determlnation in the preferred embod~ment ls mAde uslng ths receiv~d ~ignal after it ha6 been proce~s~d by egualizer-demodu~3tor, th~ mode~ more accurately e~tlm~te~ th~ lmp~ct o~ tho comblned channel lmpslr~ent~
on th~ ~lgnal point~ wh~ch aro u~ed to decide what th~
~ctual tr~nsm~tted ~ign~l point~ woro. Mor~over, the abllity o~ the ~odsm to sep~r~tely measure the lmpairm~nts du~ to ~e~ ual echo ~nd chann-~ distortlon giv~ th~ modem th~ capab~llty o~ se~ectln~ the optlmum communio~tion rat6 du~ng th- start-up ~equence de~ined by th~ CC~ V.32 ~p~cl~lcatlon.
In ~ddlt1on, th~ inventlon provldeo a meChA~iBm ~or avoiding con~ent~on between the modem and the x~mote device. In other words, nelther t~o mod-m nor the remoto 1 ~:, C) 4 . i3 ~ O i3 A~ C F I S ~ C ~ A F~ D S O ~J F 1 Cl 2~0~76 d4vice w~ll rep~atedly r~quest a $all forward to ~ hlgher mo~ulation schem~ when the oth-r modem ~ither doe~ not concur with th~ req~e~t when ~ir~t sont or ~ lncapablo of opera~lng at a h~gher ~odulatlon ~ch-me. 8y prevonting repo~ted unpro~uctlve r-traln~, data communicat~on i~ not neodlo~ly int-rrupted ~h~ ln~entlon o~2er~ the addltlonal adv~nt~ge o~
mak~ng it po~ibls to util~zo a w~der range e~ modulatlon ~chomoa ror data communicat~on. That ~s, oven though the 1~ thre~holds ~or determin~ng when a ~all rorw~rd to hlghar modulatlon depend upon whother t~e mo~ulation i~ unaoded or coded, the lnvont~on provldeB a mechanl6m ~or assuring that ~all ~orward dec~lon6 are m~d- uslng the approprlate thros~olds.
lS other advantages and ~eatures will become apparent ~rom th~ ~ollowing d~crlpt~on of the preforrQ~
e~bodl~nt, and ~rom tho clalms We flrst brl-~ly descrlb- the drawlng-.
Flg. ~ i~ a funct~onal block di~gram of ~n echo c~noelllng ~odem which ombodi8~ th- lnventlon;
Flg. 2 i~ a bloc~ d~gram Of ~ ~ory regi~ter~ ln the modsm of Flg. l;
Flgs. 3a-d deplot "leaky" integrator~ lncl~ded ~n tho fall-~orward and fall-bac~ ~on~or shown in Fig. l;
Flg. ~ is a flow ch~rt or the oporatlon O~ the rats ad~u8tm~nt ~ogic ~hown in Flg. 1~
Flg. S is a eign~l tlming di~gram Or tho CCITr V.32 8psc~1cation; and Flg. 6 i~ a high l~vel block dlagram o~ a mod-m whlch ombodia~ the inventlon.

1 Z. 0 4. 13 53 1 Z: 4 4 F~ I ~ F ~ S~ ~ R T C~SARD SON P O Z
Z(~)4~7~i ~L~ and O~ration Re~errlng to Fig.l, in a two-wlre, ~ull-duplox ~¢ho cancelllng mo~em 3 having an adapt~ve rate ey~t~, ~n lnput llne 2 rec~lves a data bit trea~ ~rom a DT~
g ~d~gltal t~rminal equlpment), not ~hown. A ~rambler-~od~r-mapp~r 4 proce~s~B the dat~ blt ~tre~m by ~a) rando~lzlng the blt~ 50 that no blt pattern i~ ~or-ll~ely to occur than any other pattern, ~b) en¢oding groups o~ blts lnto cymbolo according to the partioul~r code belng uDed, and ~c) mapplng the symbol~ onto a ~gnal constollatlon be~ng u~ed. ~he output of the ~crambl~r-encoder-mapper is a complex ~lgnal ~ln Flg. 1 compl~x slgn~ls ar~ depicted by thla~-r llne~) whlch goe~
to a tran~mittar 6 wh~re it i~ flltered and u~-d to mo~u~te a carrler to produco a real analog tran~mit 61gn~1 7 wh$ch may be tr~nomltted ov~r a e~gn~l ch~nnel 8, ~u¢h ~ a tolephon~ line, to a remote mo~em (not ~hown~. 3e~0re b~ing transmitted ov~r th- ch~nn~l 8, the tran-mlt ~i~nAl p~Qs through a hybrl~ clrcult 10, ~ybrid circ~lt 10 lg ~ho ~od-m'~ lnt-r~ace to the c~nnol 8 And ~erve~ to separate tran~mltt-d ~gnal~, whloh are sent out over th~ channel, and received ~iqnals, wh~ch have b~n aent to modem 3 ~ro~ a romote ~od~m over the same channsl. IdeAlly, the hybrld 10 dlrects all of the tran~mit signal on~o chann~l 8 and d~vQr~s only the arrlvlng recolved 8~ gnal to a r-ceiver i6 wlth~n the modem. In re~llty, ~omo of the transmit ~lgnal ~ed6 through th~ hybrid 10 onto th~ path o~ ~e r-c~ved ~lgnal where 1t Bpp~ar~ a- a no~r ~oho whlch com~lnes w~th th~ roc~lved ~ign~l ~rom tho remoto modom to produc~ a roceived signal (REC) 13. ~he REC 13 may al~o lnclu~e a far echo.
~o ollm~n~te th~ n~Ar and ~ar echo~ ~ro~ REC 13, th~ modem aloo incLude~ an adaptlvo echo csncellor 12 an~
a comblner 14~ Iyplcally~ the ~d~ptlve el~mont~ o~ echo _Z . 0 4, ~ 4 4 P ~ ~ F ~: S H & ~ T C ~ A R D S O N P 0 3 Z(~ 76 canceller 12 ar~ train~d, u~ing a le~t-m-An-~quar~
~LM8) algorith~, durlng ~n ln~t~al trainlng sequenco, ~rlor to data co~muniaation, so that the eaho ¢ancolle~
12 ~-n4rate~ a~ lm~t4d echo slgnal lS whlch closely ~pyroxl~a~e~ the o~mbinod e¢ho sign~l preo~nt ln REC 13.
Th- co~bin~r 1~ e~oentially subtract- the otlmated ocho ~lgnal 15 ~ro~ REC 13 to produce an echo-cAnc~llod ~ignal ~EC) 17. ~he aom~lnor 14 al~o g~n~rate8 an err~r slqnal (r-pre~ontlnq tho ~rror b-tweon the aotual ¢omblnod oho and the o~t~ated oc~o slgnal 15) which th- eaho canceller 12 u~ to cont~nually leAxn about the ch~racter~at~ CB 0~ ~he channel ~o that lt can lmprove the accur~cy Or th~ estimate~ echo ~lgnal.
The ~C ~gnal 17 goe~ to the recelver 16 where it lS i8 pro~o~sod to gene~ato an o~tput data stre~m of blts 32 roprosentlng the bit stre~m 6ent to th- modem 3 by the r-mot- modem, Th~ rec~iver ~6 includec an Ad~ptlve quallz-r-d-modulator 18 wh~¢h tr~n-~on~- EC 17 to a oo~l-x r~celved signal ~CREC) 19 r~pr-~ontlng, on A
~lgn-llng lnt~rv~l by ~ignal~n~ lntorval ba~lo, th-~ucc-~iv~ ignal~ compri~ing EC 17. Any corroctlon- to do.pt~ ve ~lo~erlta o~ th~ ~uAllz~r-d~m~dulatc~r la are hanaled by t~ control ~ox 20.
C~EC 19 at the outpu~ of the equall~or-d mo~ulator 18 l~ proce6~d ln one o~ three way~, dependlng upon tho co~unlcatlon r~te and dependlng uyon tho mod- o~ ~he ~gnal, i.e., whe~her ~he ~lgnal wa- produc~d ~y coded or uncoded modulat~on.
I~ th~ ~ignal 18 codod, a Vlterb$ algor~thm decod~r 22 com~utes, a~ter a delay, a ~lnal ~ocl~ion 23 ~or eaoh recelved o~gnal. ~hQ op~ration o~ the ~lterbl doooder 2~ i8 expla~n~d generally ln Fornoy, "Th~ Vlterbl Algorithm", Proceedlngs o~ the l~EE, Vol. 61, No. 3, March 1973, an~ as applied to ~od-m op~ratlon~ in U.S.
Patent 4~562,426 ~nt~tlod Symbol Coding ApparatU~, to Forney. The Viterbl algorithm decoder 22 also sends a tentative decision 25 to the equalizer-demodulator initializa-tion and update control 20, and it send~ a minimum metric (MINMET 27) and a trelli~ discrepancy (TD 29) to a fall-forward and fall-back monltor 24 ("the FF/FB monitor").
Uncoded signala are proce~ed di~erently. I~ the received signal has been eommunieated uslng a 4800 bit/see rate, then 4-pha~e deei~ion logie 26 make~ a hard decision using a 4-point signal constellation to arrive at decision 31 ~or eaeh received ~ignal polnt. And, i~
the signal is eommunieated using a 9600 bit/see rate, then 16-point decision logie 28 make~ a hard decl~ion on a 16-point signal con~tellation to arrive at deeision 33 for eaeh reeeived signal point. In all three ca~es, the dee~ions 23, 31 and 33 (in the rorm o~ bit~) are ~ent to deeoder ~nd de~erambler 30 whieh produees the output data ~troam 32.
The 4-pha~e deeision logie 26 and the 16-point deel~ion logie 28 al~o generate a 4-point deei~ion index 37 and a 16-point deeislon index 39, re~peetively, which are uaed by the FF/FB monitor 24. The~e aspects o~ the 4-phase decision logie 26 and the 16-point de¢i~ion logic 28 are explained more ~ully below.
The reeeiver 16 also includes memory 41 which stores the variables that are used by the varioue components in the receiver during operation. Some o~ the regi~ters within the memory 41 are shown in Fig. 2. The relevance o~ particular reglsters will be explained below in connection with the receiver sub~y~tem whieh utilize~
the register.
Inltial training and all retraining o~ the modem i5 controlled by n training control 34 and a tran~mitter training generator 36. When training or retraining occurs, the trainlng generator 36 ~end~ a predetermined 1 Z . C) 4 . S3 C~ 1 1 c, o A~ 76R ~ C H A R D S O N P 1 2 signal s~quence, a~ speci~led ~y the appllcable communlcation protocol, ~o the tran~mlttor 6 for tran~mi~ion over ths channel 8 to th~ ~e~ote modem. To ro~pont to tr~ln~q ~quences ~0nt by the remot~ mod~m, g th~ modem also include~ a train~ng d~toctor 38 whlch mo~itor~ ~C 17 to detect th~ pre~enco o~ a trsinlng ~e~uonce ln the sign~l from the remoto modem. If the tr~lning d~t~ctor 33 detect3 a tra~ning ~equ~nce, it cau~s an RX traln g~nerator control ~0 to ooordinate the operat~on of the modem during th~ p-riod o~ tr~in~ng and to synchronlze the transmitter 6, the re¢~lver 16, and the echo-canceller 12.
~ur~ng the initlal training p~rlod, whlch occurs 600n aftor the modem ha~ made contact with a remoto lS modem, initial rate loglc ~2 deter~ine~ tho combined l~pairment o~ the ch~nnel, inaludlng the re~idual echo (d-~lned ao ~he echo wh~ch remains a~ter the comblnor 14 ha- canoelled mo~t o~ the echo). In~ut- to th~ inltlal r~te loglc 42 ~or thl~ d~teroinatlon are pro~ld-~ ~y the 4-pha~e decislon logic 26 and the quallzer-d~modulator 18. Th~ ~quallzer-domodulator 18 g-n-rates the complox recoiv4d slgnala on llne 43and th~ 4-phase docl~lon logic 26, which ls programmed to "know" the polnt~ in the ~gnal con~tellation u~ed ~or train~ng, gener~toe tho aotual ~gnal polnts whlch wexe sont. Th- lnitlal rat~
log~c 42 comput~ the ~quar~d distance b~tw~en each rec~lved clgnal point and the corrQ6ponding actual ~iqn~l point. The initlal rat4 logic 42 then av-rage~ tho co~puted e~uared dl~tance over many recelve~ signal point- to arrive a~ an ~t~mate of th- im~airment to the cha~nel 8. Flnally, bas~d upon the o-ti~d chann-l im~alrment, the $nitlal rate logio 42 oompute~ a r~te code 4S, which re~lects the combln-d imp~irm nt o~ the channel.
The init$al xate logic 42 u~es two dlf~erent 1 Z . O 4, ~i 53 1 1 : O i~i A~ F ~ S ~ &~ R I C H A R D S O N P 1 :~i Z(~04476 - ~2 -m~thod~ to dete~mlne the comblned channel impairment, dependlng upon whether the modem iB uBlng hal~-duplex t~a~ning or ~ull-~uplox train~ ng . ~h~ ~ gn~lcance o~
thl- dl~tination wlll ~eaom~ more ~pparent lat~r. I~ the g modem 1~ using hal~-~uplex t~aining, the inltlal rate loqic 42 soparat41y determinec an e~tlmate o~ th4 channel dlctort~on and an est~mate o~ th~ re~l~ual echo. Thsn, tho ln~t~al rate logic 42 combino- thos~ two e~tim~tes to arriv~ at an estimate o~ the comblnoa impalrm~nt o~ the chann~ , on the othor hand, th~ modom i6 uqing full-duplex training, th~ inital rato logic 42 dlréctly determine~ the est~mate o~ the combined impa~rment Or the channal, wh~ch inclu~e~ th~ residual echo, w~th~ut first arriv~ng at eeparate estimat4a ~or tho channel dictortion and the residual echo.
To apprecla~e tho di~Serenc~s in th~ ~odem'~
o~eratlon during ~ull-duplex and hal~-du~lex tr~lning, it i~ helprul to r~Ser to co~Gunication b~tw~en a local ~od~m, wh~ch inltlato~ da~a communlcatlon ov-r a channol, ao ~nd a r-mot~ modem, whlch r~pond~ to tho loa~l mode~.
Wh-n hal~-duplex trainlnq occur~ ln o~ch direction ln preparatlon ~or ~Ull-dUplOX data co~ounicat~on, the tralning generally ha~ at least two perlo~ during whic~
half-duplox train~ng occurs. For ex~mple, ln on~ period, the ro~ote ~odem tran mit~ ~ tr~inlng ~ignal whllo the local modem rom~ins s~lent. In the othor period, th~
local modem transmits a tralnlng sign~l whil- the remotQ
modem remalns silent. The initial ~ato logic 42 u~
eAch of theso pe~iod~ o~ th~ tr~ining ~o determln~ a different c~mpon~nt of the cdmblnod lmpalroent to t~e chann~l.
When the remot~ modem iB tranemittlng the tralnlng signal and th~ local mode~ is silent, the local mod-m i~
not produc~ng an echo signal. ~hu~, it~ recelvor ~lmply racolvo~ the signal transm~tted by t~o r-mote modem, 1 Z . O 4, i3 53 1 1 : O ~i A~ c F I S ~ ~ R I C ~ A F~ D ~i: O N P 1 4 2~04~t76 which includo~ the channel ~pairment8 attr~u~able to idlo noi~Q and slgnal induced di~tortlon. The equal1z~r-dRmodulator ~8 u~ the r~c~ived sign~l to traln itB
~d~ptivo ~lement~ and aleo convert~ the rec41ved 8 lnto cREc 19. A4BUming that th~ hal~-duplox tralnlng i~
w lng a 48CO bit~c~c ~lgnal, CR~C lg OO~pXlB~ ~lgnal pointc that are clustered about the ~our point~ in the slgn~l con~t~llation being w ~d. The 4-ph~se docision logic 26 convort3 tho received ~ignal~ to the actual 51gn~1 points by maklng a hard d~ci~lon on tAe ~our point aon~tellation. Then, tho initial rato logic 42 ~easuras th~ disporsion of the recalv~d signal~ around tho actual signal point6. It does this by calculatlng the squ~rea dlstance o~ the recel~ed 6ignals from the ae~ual sign~l polnts. The init~al rate logic 42 average~ the squared di~tance moa6urements over many slgnal intorvals to prod~ce th~ ~ollowing ~uantity: N_HD, whlch 1B a mea~ro o~ tho di~tortlon wh~n th~ local ~o~-m 18 recolvlng a hal~duplex tralnlng signal 88nt by the remot~- ~ode~.
~hat 1~, N_~D ~s a m~a~ura o~ the channel i~pairments, excl~ding tho echo.
Dur$ng the othox period of hal~-dupl~x tr~inlng, wh-n the 10CA1 modem 1~ transmittlng th- tralnlng signal and th~ r-mote mode~ iQ silen~, the local modem r~ceives an echo signal plu8 tho l~le noise o~ the channel.
A~uming that th~ echo cane~ller 12 1~ ~ir~t tralned, u~lng ths approprlat~ training soquence, tho combinor 14 then goneratos EC 17. Th~ equ~l$z~r-demodulator 18, which w~ tralned durlng the pr~vious porlod o~ h~
duplex tra$nlng and has ~roz~n lt~ adaptlv~ element~, convertc this to the C~C 19. 8~nc~ the remot4 ~odem ls not tr~n~mitt$ng a signal, the roc-lved signal~ sro clu6terod abou~ the or$gln. The relovan~ par~m~ter~ of the 4-pha~ deci~ion log~c 2~ ar- chang~d during thl~
3~ pha~ o~ op~rat~on ~o ~hat the 4-ph~s- logic 26 gon~rate~

1 Z. Cl 4 . i3a 1 1 : fi i3 A~`~$ ~ F I S }I ~ R I C ~1 A R D S O~J P 1 ~, Z ~ '7'~

th~ actu~l signal polnts by ~old$ng the r~c61~ed slgnals lnto the origln. Th~ initial ~at~ logic 42 thon ~ea~ur~
th4 dlcper~ion o~ the recelved signal~ about the orlgin.
Again, thls 15 accompl~h~d by measuring the ~qu~red dl~tance betw~en the rec~lved elgnal~ and the or~gln.
~ho ~nitlal rate logi~ 42 thqn averag~ tho ~qua~ed dl~tance me~urem~nts over many oignal lnt-rval~ to produce tho ~ollowing quantity: SQRERR~,ho, which 1~ a measure o~ the no~se slgnal raceived by th~ local modem lo when lt is ~ran~mittlng a half-duplex tralnlng signal.
~hat 18, SQRE~R~oho iB a measure of the scho slgnal plus the idle noise on thQ channel.
A~ter the inltial rat~ loglc 42 hA~ obtained mea~urem~nt~ of the channQl distortion and of the echo, 1~ it combtnes these two mqasurem~nta in th~ following way to obtaln an estlmate of the co~blned impairment o~ the channel 8.
SqRERR~o~ N_~D
whore ~ 1~ a ~oaling ¢on-tant u~ed ~o el~minat~ the contributlon o~ the idle noise.
~he ~callng ~a~or is nece~sary because idle nol~o is lncl~ded in ~oth measUrements SQRERR~Cbo and N HD. A
simple ~um o~ the two mea~ur~ment~ would yield a re~ult which under~timates ~he quallty of the ch~nnel 8. In ono p-rtlcular e~bod~ment, ~ i8 ~electo~ to be ~, whlch re~l~ct~ an a~umptlon that each compo~-nt o~ nol~- on the ¢hannel, namely, echo, idle noi~e, ~nd slgnAl lnduced dl~ortlon, contAin approximatoly ~qual energy. $hls prov~s to yleld ~ood m`odem por~or~nce.
A~ not~d earller, when th~ loc~l mod~m i~ u~lng full-duplex trainlng, the loc~l modem can determlno dlrectly th~ comb~ned ~mpairment o~ the chann~l without computlng ~p~rato components o~ lt. Dur~ng the ~Ull-duplex t~aining, ~he local mod~ roc~e~ both a tran~mitted signal ~rom a remot~ modom and th~ eaho 1 Z . C~ 4 . i3 5~ O &' A~I -~ F I S H ~ R ~: C ~ A ~ D S O ~ P 1 Fj 2(~ 476 signal cau~cd by it~ t~anc~itted slgnal to thQ remote modom. Con~equQntly~ EC 17 ~e~locts tho co~blned lD~pai~m~nt of t~e channel, including signal induced d~ sto~tlon, idlc noise and echo. CR~C 19 produced by the ~qualizer-d~modulator 18 comprlse~ recelv-d ~ignal8 which aro clustered about the actual signal point~ ., ths ~o~r slgnal poln~s o~ the signal oon~t-llatlon. Ao be~o~e thq lnltlal r~te logic 4Z dete~ml~os th~
d~persion. ~n other wo~ds, it me~sur~ th- squ~r~d d~tanc~ betwe~n th~ rec~lved olgnalc and the actual s~gnal points and avsr~ge~ this m-a~ura~on~ ov~r many ~ignal lnterval~. The resulting quantity 1~
8QRERRCo~b~ , wh~ch is ~ meacure of the noise ~ignal roceiv~d by the local modem during full-duplox op~ration.
~hat i~ SQRER~co~b~n~d io a mea~ure of the combined impairment o~ tho ch~nn~l lncludlnq idl~ noio~ gn~l lnduc-d dioto~tlon, and echo.
~n both the halt-tUpleX and the ~Ul1-d~plOX
m-'chodl~, th- lnlt$al ra'ce loglc 42 conv-rta the ro~ul.tlng m-~-ure oS co~bined lr~palrment to r~te cod- 45. lt doe- thie by uolng the ~ollowing t~ano~o~tlon~:
Rate codQ ~ QUAN~S~RERR~ho I ~N_HD}
R~te code - ~UAN~S~RERRco~b~ }
QUAN ~ a quantlzatlon funct~on whlch se~s ~ho thr~eholds ~o~ ~lecting d~er~nt ~lgn~l con~tell~tion~.
Rat~ cod~ 45 g~ne~at~d by the QUAN ~unctlon 18 pass~d to a ra~e dacislon algorithm 44 whlch uses lt to ~elect thQ signal con6tellatlon ~or data communic~tion over the channel. I~ e~ect, the rate d~ci~ion logic 44 in com~in~tlon wlth ths QUAN ~unot~on ~termi~o~ th~
pArticular signal constellatlon from among thQ signal co~st~llatlo~ avail~bl~ to t~e ~odo~ whlch will yield opti~m perto~mance, ~hat 1~, they sel~ct tho ~ignal constellation whlch wlll yield the hlghest ~ta tr~nQmi~sion spe~d while ~till providing the de~ired I

1 Z . 0 4, ~ 5~ o i~ A~vI ~ F;~ I C H A R D ~; O N _ P 1 ~ignal-to-nolse ratlo ln vi~w o~ the combln~d lmpai~ent of the chann~l The 4UAN map~ th~ m~a~urement o~ th~
combln4d impalrm~n~ of the channel onto th- rate code lnd~x and the rat~ decision logic 44 u~- th~ lndex to identi~y the slgnal constollatlon In on~ embod~ment, the ~UAN runctlon genorates an even numb-r ~hich io ~rom 0 to ~ ~ho numb-r~ de~ign~to the rollowlng 8 - any ~ignal conct~llation ~or a cignal to noiss ratlo which 1~ less than thQ 16 polnt 9600 uncoded conot~lla~lon, includlng the 32 point 9600 ¢oded constellatlon;

6 - any ~lgnal con~tell~tlon ~or ~ ~lgn~l to noise ratio whlch is les~ than the 32 polnt 9600 coded constellation;

4 - any ~lgnal constellatlon ~or ~ ~lgn~l to nol~a ratlo whlch 1~ 1e-B th~n th- 4 polnt 4800 unaoded con-tella~lon~

2 - any ~lqnal con~tellatlon ~or a slgnal to nol~e ratlo whlch i~ le~ than the 2 polnt 2400 uncode~
conctell~tion ~nd 0 - di~connect A~ter the initlal co~unicAtlon rate has ~e~n s~lected and data communlcation h~ ~ogun, the FF/FB
monltor 24 as~ume~ regpons~b~llty for ~onitoring the ~u~llty of the channel during data co~munlcat~on ~
noted earller, to ~onltor tho quallty o~ tho ch~nnel the FF/FB monltor 24 rolles upon input tro~ the Vlterbi ~lgor~thm 22, the 4-pha~o d-ci~ion loglc 26, or the 16-po~nt deci3ion logic 28, dep~nding, of course, on tho data communicat~on rate boing u~od At tho tlme and upon 1 Z . 0 4, ~ 0 i~i A h~ F I S ~ ~ R I C ~ A R ~ S O ~ P 1 i~ _ 2(~0~L476 whether th~ signal 1~ codsd or uncoded. 1~ the overall chann~l quality ha~ i~proved or d-terlorated ~lgnl~$cantly, the FF/~B monitor 24 as~erts a ~all-~orw~rd ~FF) indi¢Ator 47 or a ~all-back ~FB) indicator 51, ~e~pectlYely. A rat~ Ad~Ustment logic 46 monltor6 the ~F lndicAtor~ 47 and the F3 lndlcators 51 and ~texmines when a r~train at A dif~erent xate i~
de~ir~ hus, the FFI~B monitor 24 snd th~ rate ad~ust~Qnt logic 46 work together to ~daptlvely ad~u~t the ¢o~municatlon rat~ reque~t~d by the modom ln responsa to change~ in th~ quallty o~ the channel. Wh~n the rate ad~ust~ent loglc 4~ ~etermine~ that a retr~ln should occur, it s~nds a set tratn slgnal 49 ~SET TRAIN1 to the trainlng control 34 there~y initiatlng a retrain.
1~ Wh~n the r~ceived signal i~ codod, the FF~FB
monltor 24 relie~ on the min~mu~ metrlc~ (MI~MET 27) and tho trolll~ di~crepancl~6 ~D 29) provided ~y the Vlterbi ~l~orlt~m 22 to ~onlto~ channel gu~llty. In o~-enc-, th~
minl~ metrlc 27 1~ a cumulatlvo hiotory oS how ZO accur~t~ly the t-ntatt~ d~cl~ion~ 25 gon-r~ted by the Vltex~l slgorith3 22 ha~e 4stlmsted th- received ~lgnsls.
Thu~, the mlnl~um m~txic 2t i9 a good lndlcator o~ the o~rall a¢curacy of ths receiver 16. The trellls discroPanCy 23, on tho other hand, ~ a m~a-ure o~ the accurac~ of lnd~vid~al tentatlvo deciJion~. ~hus, tho trelli~ dl~crepanoy 29 $g a good ln~ioAto~ o~ the ocourrenc~ of errors.
In on~ embodiment, an 8-~tat- cod~ which i~
opecifl~d ~n th~ V.32 rocommend~t~on i~ u~d. Using that cod~, the Viterbi al~orlthm 22 computes the m~nlmu~
motric 27 ~or each reca~v~d sl~nal by ~irst determining the metricc ~o~ all o~ th~ ~ignal polnt6 o~ the ~lgnal con-tellatlon. ~e metr~c ~or a ~iqnal polnt ln the con8tellatl0n i8 equal to the minlmu~ ~quared d~stance between th~t point and th~ roce~ved ~lgn~l. Thon, the 1 ~ . 0 4, i~ 0 ~3 A~,I ~ F T S ~ & R I C H A R D S O N P 1 ~3 2(~L47~i algorithm 22 comput~ a path m~tric ~or ~ach o~ the eight stat~ ln the state pro~r~ssion diagram ~"th~ tr-lll~) ~or thQ modulatlon code. The path metric ~or A ~t~e iQ
the ~u~ o~ th~ motrlcs along a path th~ough the trellis s which termlnates at thAt ~tate. Ea¢h o~ th~ ~ight stAte~
ha~ only rour paths leading to that 6tate rro~ wlthin the trell~s, ono o~ which has a low~r path metrlc than the oth~r three. For each atatQ, the algorithm 22 ~-1QCtS
the path with th~ lowo~t path motr~c, which 15 the most l$~ely path to that ~tata. Flnally, the algor~t~ 22 select~ tho mlnlmum mRtric 27 ~ro~ among the survlvlng path~. ~he min~mum metric path identi~i~d durlng a qlv~n symbol lnterval, that ls, the tlmo betwo-n the tran~mlsslon of each successive slgn~l, permit~ a ~nAl decison to bo made re~arding the receiv~d slgnal sev~ral ~y~bol lntervals earll~r, e.g. 16 5ymbol int4rvals ~cr th~ eight state code. ~h~ mlnimum ~otr~c 27 roflect~ the probab~lity of error r~lating to th- path and thu~
ind~cate~ the quality o~ the path.
The Vit~rbl algorlthm 22 furth-r computec the trell~- tlsorspancy 29 by compar~ng the t~ntative decl~lon regard~ng the rocelv-d s~nal ~o the final d~cl~lon ~rr$ved at s~veral 3ymbOl int~rvals later. I~
the d~¢ision~ are th~ same, ind~cating that the tentAtlve Zs dec~son wa~ corr~ct, the trelli~ dlscrep~ncy 29 ~ o;
wher~a~, if th~ doclsion~ are not the ca~e, ln~lcatlng that ths tentativ~ de~clon wa~ lncorreot, the trelll~
dl~crepancy 29 ia 1.
A~ter ~aoh n~w~slgnal ls recsiv~, the algorlthm 22 updato~ th~ ~etr$c~ and ths path metrlc~ ~or each o~
thQ eigh~ states and provldes a n~w m~nlmum ~etric 27 and a new trellls di~cre~ancy 29 to th~ FF/FB mon$tor 24 ~or ~he corre~onding tontatiVQ decl~ion 25 which w~s ~ads ~ev~ral s~mbol inte~vals ~arlie~
3S The FF/FB monl~or 24 ~mploys two dl~erent "lcaky Z . 0 4 . a ~ 0 8 A ~I ~ F I S ~ & R I C ~ A ~ D S O N P ~ O
2~n4~ 6 integrators to oep~r~tely integrate t~e seque~ce~ of trelll~ discrep~ncies and minimum mQtrlcs whlch lt recelv~ ~rom the Vlterbl algorlthm 22. Functlonal repr~entatlons Or ~ha "leaky" integrator~ ~or the mlnl~um ~etric and trell~s dlso~epancy ~equsn¢e~ are illu~tr~ted ln FIGS. 3a And 3b, r-s~ect~vo1y. In general, tho "lesky" lntegrator produce~ an output whioh i~ ~qual to the value Or the relev~nt var~abl~ ~or tho current aymbol interval plU5 ~ tlmeo the output fro~
the previous ~ymbol ~n~erval, where "~ les~ than one.
In the ca~o o~ th~ ~qu~nce o~ ~lnlmum motr~cs 27, thQ
FF/FB monitor 24 compare~ th~ output or the "leaky"
integrator (shown ln Fig. 3a) durlng each 6ymbol interval to a threshold THl. I~ tha output falls below the threshold ~l, the FF/FB monitor 24 a~erto the FF
indicator 47, lndicating that ths quality of the channel 8 ha~ l~proved. Oth~rwi~e, lt doe~ not s~ert the FF
lndloator 47~ In the oa~e o~ tho ~-qu-na- o~ trolli3 dlocr-panaie~, the FF/FB monitor 24 com~re~ the output o~ th~ aky" lntegrator (ghown ln Flg. 3~) durlng each ~y~bol interval to a thre~hold ~2. 1~ the output r~ses abovo th~ threshol~ T~2, th~ FF/FB ~onitor 24 a~orts thQ
FB indicator 51, indicating that the qual~ty of the ohannel 8 ha~ d~teriorated. Otherw$~e, lt do~ not 2S ass~rt th~ FB ~ndlc~tor 51.
The th~o~holds TH~ and TK2 and the tlme con~tant~
~or th~ "loaky" int~gratore, wh~ch ar- ogual to (l a) 1, are solect~d ~o achieve the ~odom perrorm~nce wh$ch is des1r~d. ~h~ modem aXA~ines the FF and FB indlc~tor~ 47 and 51 during a wlndow o~ t~m~ and doeo not request any change~ in th~ com~unication rate untll tho window ha~
explrod. ~he duration o~ thl~ window a~ect~ the magnitudo o~ th~ tl~e constants whlch ar~ approprlato.
tf tho durAtlon o~ the window i~ ~Qlected to ~e a~out t~n mlnutes, then t~me con~tsnts whlch ~all wlthln the r~nge 1 ~:, Cl 4, ~; ~.3 1 1 : O i~i A~VI ~; F I S ~ ~ ~ I C ~ A R D ~: O N P c 1 Z~ 76 o~ lOo to 2Fo milli~cond~ produc~ good ~od~m por~orm~nce.
When the receivad ~i~nal ia uncod4d, th~ FFIFB
~onitor 24 rslle~ on tha d~¢i~ion indexe~ 37 generat4d by th~ 4-pha~ d~cision logic 26 or the doclsion lnd-x~ 3 q nerated by 16-point decl~on logic 28, d-pendlng upon th~ communication rat~. Th~ FFIFB monitor 24 uses a ~irct "lo~y~ integr~or 56 and a socond ~leaky~' lntegrAtor 58 to monitor the decl~ion $ndQxe~. Th~ fl~st ~o lntegrator 56 and the second lntegator 58 ar- illu~trat~d in FlGS. 3C and 3d, re~poct~vely.
For slgnal~ s~nt at 4800 blt~ c, tho 4-pha~e decl~ion logic 26 produc~s the 4-polnt decl61On lndex for each receivQd signal based upon how ~ar the ~ignal ln CREC 19 i~ grom it~ co~responding ~lgnal point ln the ~lgnal constellatlon. In e~sct, th~ 4-pha~ doc~slon logic 26 place~ two concentrlc square~, namely, an inner ~u~ra ant ~n outor ~qu~r~, around ach polnt ln tho ~lgn~l con-t-llAtion. ~ th~ rec-lv~d e~gnAl ~all~
~lthin ~n lnner sguaro, the 4-phase d-cision logic 26 co~d~ a un~t ~tQp 5~ gnal to the ~rat integr~tor 56 in tho FF/~B monltor 24 and a null signal to th~ cecond integr~tor 58 in the FF/F8 monitor 24. ~owe~er, 1~ the r~ceiv~d signal ~alls OUtsidQ o~ the outer square, the 4-pha~e deci~ion loqio 26 send~ a un~t ~tep ~lgnal to the second integrator 56 and a null slgnal to ~he ~rst lnt~grator ~6. O~ courBQ~ i~ th~ slgnal fall~ betwQon the inner and the outex squares, then null ~lgnal~ aro sent to ~Oth the f~rst lntegrator 56 and t~e -eaond lntegrator 58.
T~e 16-point deci~lon logi¢ 28 oporatos in a w~y whioh i~ ~lmllar the way t~ 4-pha~s ~ecl~on lo~lc 26 operates. That ~5~ each polnt ln the 16-poln~ s~gnal oon~tellation i3 surround~d by an ~nnex ~g~are and An outer square and th~ 16-point dec~lon loglc 28 1 Z . Cl 4 . ~ 5~ 0 i3 A ~I ~ F I S 1~ & F~ ' C ~ A R D S O ~ F' ~ 2 2(~0~7~i - Zl -aotermln~s wh~re th~ rec~ived ~lgnal f ~118 with r~s~ect to th~ two a~t~ o~ squar~. Llke the 4-point doclolo~
log~c 26, i~ the rec~lved signal ~allA wlth~n an lnnQr ~qu~re, the 16-po~nt d~clslon logi¢ 2a ~ondo a unit st~p ~ignAl to the ~ir5t intQgrator and a null ~l~nal to the ~econd lntegrator 58- I~ th~ rec~i~o~ ~iqnal fAllD
out~l~o of the outer sqaure~, ~he 16-point d-ci~ion logic 28 send~ a unit s~ep to the seCond integrator 58 and a null signal to the ~irst integrator 56. A rocei~ed 61gnal ~alling ~etwe~n the inner and outQr ~quar~ m~ns that both inte~rators rec~lv~ null ~lgnals.
The ~irst and second int~grator~ 56 snd 58 each produce output~ wh~ch are ts~t~d agaln~t corre~ponding thre~hold~ the output o~ the ~lr~t integrator 56 1~ r~ s-S abov~ a thre~ho~d TH3, then the F~IFB monitor 24 a~6erts the FF indicator 47; othorwi~e it doe~ not. And lr tho output o~ the 5econd lntegrator 58 ~l~e~ above a thr-~hold ~H4, th~n th~ FF/FB monltor 24 ~ rt~ the ~B
lndlcat4r 51~ otherw~e it do~ not.
~ wlth the other integr~tor~ in th- FF/FB monltor 24, the th~e~holds TH3 and ~4 and th- tlm~ con~tant~ for the flrst and second intsgrator~ 56 ~nd 5a ~re ~-lected to achleve the mod~m p~r~orm~nce whlch 1~ desirod.
~he ~unction of the rate ad~u~tm-nt loglc 46 whlch act~ on the FF ~nd FB lndicator~ 47 ~nd Sl wll~ now bo dowribed. Fig. 4 ~hows the ~teps o~ tho algorithm which th~ rate adjustment logic 46 exocutes. A~ter a ~ign~l i8 rQceiv~d, the rate ad~ustment ~ogic 46 bogins executlon ot th~ ~topo o~ tho algor~th~ (~t~p 100). Flr~t, the logic 46 tosts i~ the F3 lnaic~tor 51 1~ rted ~stop 11~). Ir th4 F8 ind~cator Sl i~ orted, ~ rotr~in countor $s incremented (st~p 120) and an error window ls ah~ck~d (~tep 130). The retrain ¢ounter record~ the occ~rr~nces of poor channe~ guallty. The ~rror wlndow 19 a sllding window wh~ch is g-neratet by the logl¢ 46 and ~: . Cl 4 . a ~ 1 1 : o ~ A ~,I ~ F I S ~ 8.. R I C ~ A R D S O ~ P z 3 2~ 7S j whlah monitora th~ ~et~ain countor to dotect wh~n two 2-6~ccnd periods o~ continuou~ FB indication occur during a t-n ~acond lnterval. When such a sQqUOn~ Or poor ~hannol ~ua~lty ha~ be~n d~toct~d, t~- loglc 46 term~n~te~ the error w~ndow. I~ the checX o~ tho error window lndicatas that it ha~ expired, then th~ logic 46 cend6 a set ~all-back retraln glgnal to the tra~n~ng control 34 ~t~ 140), thereby inltlatlng a complete retrain of local ~odem. During tho ~n-ulng rotraln all o~ the adaptive alements in both mode~s are rec~t and a nQW com~unication rate $~ s~lsct~ bAsed upon the lndlcatlon that the current rate could not be ~u~talned.
The rato initial rate logic 42 detormins~ a n~w co~unication rate, WhiCh i8 gener~lly lower than tn4 previou~ commun~cat~ on rate. The new communlcAtlon rate may not bo lower ~f the cauee o~ the rotr~ln wa~ A
temporary drop in tha ~uallty of the channel which no longer exl~t8 by th~ ti~e that the lnltla~ rato loglc 42 mea~urc~ the channel ~ua~lty.
A~ter the ~et ~ ack rotrAln 1~ ~ont, th- rate ~d~u~t~nt logic 46 also roset6 both a ~ forw~rd counter and a ~all-~orward tim~r to zoro (8top 1~0) and then exit~ tho algorithm ~step 160) ~O that ~t i8 roady to re~pond to the next r~c~iv~d signal. A~ wlll bo de~crlb~d in more tetail shortly, the fall-forward counter and th~ ~all ~orward tl~r are used to evalu~te how good the channel quality i~. Th-re~oro, whonever a ~all-back r~train i~ r~quest~d, ~t 1~ approprl~te that tho1r content~ b~ reinitializ~d to zero, ae i~ dono.
I~ it i9 d~ter~ined in et~p 130 that tho error wlndow h~ not explred, the r~te ~d~u-t~-nt log~¢ 46 dQc~ement~ a fall~orward count~r (step 170) and exi~9 the algorithm (~tep 180).
Back ln step 110 1~ the FB indicator 51 wa~ not 3S a~orted, thon tha rate ad~ustment logle 46 deorement~

1 ~ . 0 4, ;~ n o~ F I S H ~ R I C I~I A R D S O N _ P 2 ~
2~044~6 the ratrain counter ~3tep 190). Slnc~ the log~c 46 relles on the ret~aln oountor a~ an lndlcatlon oS how bad tho channol quallty i3, it is appropr~to that the r-traln oountor roSlect instances when aaaept~ble signal quallty 18 detected.
~ xt, tho rate ad~u~tment logic 46 determina~
wh~ther the FF indicator 47 i~ a-~rtod ~-top 200).
Lt 1~ 60t, the log~c 4C lncromont~ th- fall-forward countor (st~p 210) and chsck~ to 800 1~ tho FF indlc~tor 47 ha8 been as~ertad for At least 90~ oS a ten ~nut~
window ~stop 220). In this ~tep, the content~ o~ two r4gl~t~rs, namely, a FF_ESTIMATE re~ister 48 and ~
FF THRES~O~D reglster 50, shown in F~g. 2, are compared.
T~e FF ESTIMATE r~g$ster 48 18 a counter wh$ch records -how many received slgnals during the ten m$nute lnterva re~ulted in as~orting the FF ~ndlcator 47 and th~
FF_THRES~OL~ register 50 conta~ns a number whlch corr~pond~ to the 90% thr-~hold tq-t. The 90% thre~hold t~-t 1~ the criterion usod to ~ot-r~lne wh-ther th-ao quAlity 0~ th~ ¢hannel ha~ improvod ~uSSlclontly to sup~ort an incre~so ln the communicatlon rate.
ln ~tep 200 i~ tho FF ln~lcator 47 i8 not a~serted, tho rato ad~ustment loglc 46 d-croments tho fall-~orward count~r and branches to ~tep 220 where th~
93% thr~hol~ tes~ is conductsd ~step 205).
lr lt pa~6e~ the so~ thre~hold te-t, th-n th-logio 46 sets a DES~RED RA~E vari~ble, which i~ stored in a DEsIaED RA~E r~gi~tor 70, equal to a commun~cat~on rat~
whlch i6 one level hlgher than the current commun$c~t~on r~te (step 220). The curr~nt communleAtlon rate 1 stored as a CUR~E~$ RA~E var~a~le in ~ CURRENT RAT~
regist~r 72 ~hown in Flg. 2. A~ter that, the loglc 46 looks at th~ ~all ~crward t$~r to determine 1~ ~ho ten ~inute wlndow hac expired tstep 240). ~ thQ ton ~inu~e 3s window has exp~red, th~n th~ logic 46 deter~ines the 1 Z. 0 4, ~ 53 1 1: 0 ~ A~ ~ F I S~ & R I C:HAR~I SON P ~ 5 .
2~0~7~i condit~on o~ a ~all-rcrward (FF) ~lag whlch i8 stor-d ln a ~ orward ~F~) ~lag r~g~st~r 68 ~hown in Flg. 2 (~tep 250). I~ the ten minute has not expired, the logic 46 exit~ ~ro~ th~ algorit~ ~step Z45~, Ir the 90% threshold te~t 1~ not paa~ed in ~tep 2~0, th~ rat~ Ad~ust~ent loglo 4~ Bet- th- DE8IRE~ RATE
v~r~abl~ equal to th~ CURRENT RATE var~able (~tep 225) and then ex~t~ (step 235).
The FF flag provide~ a mechanism ~or re~olving rato sett~ng cont~nt~on b~tw~n tho local ~ode~ and ~h~
re~ote mod~m, as will be d~cribed b~low. lf the ~F rlag is tru~, the log~c 46 set5 a ~all-rorward retraln ~tep 260) and wa~ts to see ~ th~ ~all-~orward reque~t is acceptQd by the remoto mod~. Oth~rw~e, the logic 46 1~ ~xlts ~rom th~ algorithm ~step 255).
~he ~all-forward retrain cau~e~ the training ¢ont~ol 34 to ~lgnal th~ ~emot~ ~od~m that an increa8e ln rat- ~ deairet. Dur~ng the ~all-~orw~rd retrain, th~
~d~ptlve el-monte ln both modem~ kopt A~ they w~r~
betor- the r-tra~n an~ the ~odem~ ~orely u-- th- r-train to requ-~t a hig~er communlc~tlon r~te, which 18 ~poci~led by the DES ~ED RA~E varlAble s~ored in the DES~RED RA~E r~gi6ter 70.
Flnally, lf the ~all-~orward reque~t 1~ accQpted 2S ~t-p 270), ths rate ad~u-tmont lo~ic 46 ro~ots th- ~all-~orw~rd ¢ountor and the ~all-~orward tlmer for tho noxt ten minut~ period t~tCp 2~0). Then, the rate ad~ustm~nt loglc 46 oxlt~ the algorithm so thst lt is ~repared to re-pond to the n~xt signal point (~tep 290~. Ir th~
~all-forward rsquost is not accepted, th~ loglc 46 exit~
from tho algorthim (step 275).
~n summary, A mod~m whl~h implements tho rato ad~ustm~n~ ~lgorithm de~cribed abov~, op~rAte~ accordtng to the ~ollowing rul~s. I~ w~thln ny ten ~cona p~r~od t~e mode~ detects two per~oda of ~oor ignal quall~y, 1 Z . ~ O L O i3 A~I ~ F 1: S ~ R I C H A X 3 S O N P ~: ~
Z0~ 76 each la~ting ~t laast two sQconds, thon the modem will fo~ce a ~ back retrain to occur. Durlng the ra ba~k retrain both modemc completely retraln thelr ad~pt~ve ole~0nt~ and s~lect a new, gen0rally lower, communicatlon ra~e whlch accommodat4s the deterlorstlon in channel guallty which Sorced tho retraln. on the other hand, i~ the modem detects a su~taino~ period o~
good signal quality, it wlll lndicate by a Sall-~orward r-train that an increaSQ in the co~munlcAt~on rate ~s de~ired. A rotrain in w~ich the rate 1~ $ncr-ased occur~
only if both modems can sustain the increa~e and durlng that retraln tho adap~i~e ole~ents of neithQr modom are changed. The modem det~rm~ne~ when a sustalned per~od of good signal qu~ y has occurred by ~onitorlng th~ FF
lndicator 47 during wlndows having a ten minute duratlon, I ~ thc FF ~ ndicator 47 ha~ ~en a~s~rt-d for at least 9o~
of the te~ ~tnute inter~al ~and i~ the FF flag 1~ true), the ~od-m wlll roquest a rate increaeo, A~ noted a~ove, the FF ~lag p~-v-nt~ wa~to~ul cont~ntto~ between tho loaal mo~em And th~ r-mote m~dem.
W~t-ful contentlon ocau~s wh~n on- modom contlnues to a~k ~or An increase to a rate hlgher than i9 6upported by the other modem. It al~o occurs when one modem contlnue~
to experisnce improved channel guallty while the oth~
2S ~odem dooc not. In oither cass, a r-que~t for a hlgher rate would not resu~t in a hl~her rAte boing ~ cted.
Thus, rspeatsd retralns used to communicato the reque~t ~or th- h~gher rate would ~nnece~-~rily disr~pt data communicatlon.
~o ~void the unneceC~ary dl~rupt~on, sach modem has ~ re~istsr whic~ stores a eorreoponding FF flag. In t~e local modem, here deslgnat~d the CA~ mo~m, the FP
flag 1~ e~t by flag ~tti~g logio 80 hown in F~g. 2.
The flag ~ettlng loglc 80 eets th~ orwar~ g ln aoaordanco with the following rult. If the hlg~eet Z ! 0 4 . i3 ~ 2 ~1 A ~ c F I S H ~ ~ I C: H A Ft D S O N P 0 2(~0~76 co~mon rate availab~e ln the CA~L modem and the AN8WER
~odem ~8 groater than the DESIRED RATE ~n~ the DESIRED
~ATE ~ Qqual to tho CURREN~ ~A~E, then the FF ~lag in th~ CA~ ~od-m i~ true; other~l~o it ~ tal~e. ~n the r~otQ modo~, whlch ~hall bo re~err-d to A~ the ANSWER
modo~, t~e FF ~laq 15 set in accordance with a difforent rul~. ~t th~ CUR~ENT RA~E ig l e8e than the DE~I~ED RATE
lndicated by t~e ~ALL ~o~em, then the FF ~ in th~
AN8WER modqm le true; othe~wi~o it 1~ ~alee.
Th- way in which th- FF flags avold contentlon can be more ~a~ily appr~clated by looking a~ an exampl~. At the conclus~on of the ~tart-up or lnlt~al training sequence whlch pr~cedes data com~unicatlon, the AN8WER
modem arrlve~ at a communicatlon rat~, ref-rred to ac the CU~REN~ RATE, which 15 ba~et upon lt~ m~urem-nt o~
chAnnel quallty, tho cA~ modem'~ me~ure~t of ch~nn~l qu~llty, and the communio~tlon rate~ av~lla~le to th-AN8wEa mode~. ~ tho CURREN~ ~ATE 1~ lq-- thAn th-m~xlmum ~v~ le rat- ~n tn~ CALL mod-m ~nd lt 1~ o qy~l to th- DESIRE~ RATE roqu~-t-d by the ~ALT' modem, th~n th- FF rlAg in the CAL~ modem wlll ~e ~ot to tru~, Ao do-cribed in ¢onnectlon with step 240 in Flg. 4, thls me~no that the CAL~ mod-~ will b~ p-r~ltted to r~gue-t a fall-forwar~ ~etrain undor the ~ppropra~t~ ~lgn~l 2~ condltion~. That ~, lS the CAL~ mo~em x~erlonce~ 8 p~riod of l~prov~d ~$gnAl quallty whl¢h m~-t~ th~ tee~
for a f~ forward, $t wlll be peroitted to $naroe~- the DESIRED RATE to a hlgher l~vel and then reque~t a ~
forward retr~in durlng~Whlch tho n-w D~8I~ED RATE w~ll be commUniCAtod to th~ AN8WER modom.
~otic~, howe~or, that once th- new DE5IRED RATE 1-~ot and communlcatad to tho A~8WER mod-m and tho AN6WER
nodem dooc not increa~ 1tB commun~catlon rato to th~ n~w 3ES~ED ~EVEL, the t~ot ~or th~ F~ Slag ~ B no lo~ger atl~ied. Now, the DESI~ED RA~E ho longer ~quals tho 1 Z 0 4 8 9 1 1 _Z ~1 A:;~ ~ F I S H &. 1:~ I C 1~1 A ~ D ~: O N P O ~:
Z(~

CU~R~N~ RATE. The~fo~e, the CA~ modem mu~t al~o ~ot ~t~ FF ~lag to ~al~e. Thl~ mean~ that evon t~ough th~
CA~ modom may contlnu~ to expe~ionce improv-~ ~ignal qual~ty, it may no~ continue to requ--t tho hlgher rat~
3 th~ough re2ea~ed ~all-~orward ~etra~n~. ~ndoed, a- long A~ th~ FF ~lag in the CA~ modom ~emain~ ~a~ all-~orwa~d retraln re~uoots will ~e prohlbl~ed.
In th~ ANSWER ~odem tho saon~rio le slLghtly differont. At th~ conclu8ion o~ the ~tart-up ~quence and under the assumption~ statod abov~, th~ CURRENT RA~E
15 equal to tho lnitlal D~S~RED RA~E roquestsd by the CALL modem. Therefor~, the ~F ~lag ln th~ AN5WSR modo~
i5 set to ~al6e. As ~ong as lts ~F f ~ag romain~ ~Alse, the ANSWER modem may not r~quest a Sall-~orwar~ retr~l~
lS v-n though it may oxper~-nce a poriod o~ $mproved ~lgnal quality. However, after the CALI~ mode~ ~-nds a retrain r~qu~ting a n~w ~ES~ED RA~E which 18 h~gher than th~
CURRE~ RATE and ir the AN#WER ~ot-m tlll ro~pond~ wlth ~ low~r com~unic~ion rate, th~ AN~WE~ mo~e~ mu~t ~et ~t~
FF ~l~g to tru~. T~en~ when the ANSWER modom exporienc~s lmprov~d olgn~l qyal~ty, it may inltl~t- a rsll-~orward r~train wh~ch increase~ tbe communicatlon rate to a h~gher levol.
A~ter th~ co~munication r~to ha- b-en incr~a~ed ln respon~ to lmp~o~ed signal guallty, then each modem r-compute- the corrospondlng FF ~lag b~ea upon the aboYe-dO~Crlb~d rulo5 Then, th pzoco-~ de~crl~ed abovo m~y occur again. Bo~h mo~em~ can set the ~F ~lag to ~al~ the communicatlon rate i~ th- hig~e~t rate ~ustalnable by eit~er modsm. Only one modem cAn Bot lts FF rlag to b~ true at any giv~n tlm . In thl~ way, th-FF flag~ o~ both modems prevont elthor mo~em fro~
regue~ting a fall-~orward in communicatlon ~ate a~ter the other modem ha~ provided ~vldence that it c~nnot oupport ~uch a.n increa~ in ~ate.

1 Z :~1 4, a ~ 1 1 ~ ~ A~5 ~ F ~ S1~ ~ R ~ Cl-IA~D SON P o :~
Z~)4~76 rho oom~un~cation protoco~ bo~ween mode~
do~ln-~ ~y the ~C~TT V.32 ~pecl~catlon. A tlmlng dlagra~ illu~tratlng a start-up o~ initl~l tr~nln~
~egue~ce as do~ln~d ln the V.32 ~ecl21catlon 20r the ~AL~ ~odem and th~ A~SWER modem 18 deplctea ln Flg. S.
In Aocord~ne- with the inv~ntion, the CAL~ modem and the AN~ER mode~ u~e the start-up or initial training ~-q~en~e to d~termlne and ~-lect the lnitlal communic~tlon rat~ ba~ed upon tho overall quallty o~ the chann~l a~ ~e~sured separately by ~ot~ ~odem~. T~e tart-up or $nltial tra~ning comprlso~ two qlo~e~y linke~
~-quonce~, nam~ly, a signal s~quonce rOr the CALL mode~, whlch lo illu~tra~ed ln the top Or Flg. 5, and a ~lgnal ~oquence ~¢r th~ ANSWER modem, wh~ch le illustrated in 1~ the ~ottom o2 Flg. 5, The operation o~ th- CAL~ modem ~nd the A~8WER mode~ during thi~ period are a~ ~ollow~.
For purposes o~ thQ 20110wlng di~cu~slon, lt le ~e~um-d thst the CAL~ modem and tho ANSWER ~odo~ ~r~ of th~ type hown in Flg. 1. Ther~r-, the ~ecl~lc ub~y-t-~ wi~hi~ each modem are ldent~fled by r d -rrl~g to th- oo~r~spondl~g ~ub~y~em~ ln F 1 g, 1. Ho~evor, to cle-rly dl~tlngui~h b~twe~n the CAL~ mo~-~ and the ANSWER
mod-m when ro~erri~g to tho eub~y~t-~o, a eufrlx of "a"
or "b" wlll bo appende~ to the iden~lcatlon nu~-r~
d-p-~ding upon wh~ther tho r~fer~nce i~ to th~ CALL modem or th~ A~SWER modem, ~e~pe¢t~ly.
In~t~ally, th~ CALL ~odem eet~liehe~ contact with t~e AN8WER mode~ and aw~lts a reepon~e lndlcating conta¢t has b~en made. T~ AN~WEa modem ~e~no~ledges the contact by connecting to the ch~nnel 8 and e~ndlng an AN8 tone 60 whil~ ~t the ~e tl~e conditlonin~ ~t- ~er~bl~r 4b and desorA~bler 30b. ~he CAL~ modem r--pond~ to th~ AN~ tone 60 by ~l~o conditloning itc ~cramb~er 4~ ~nd d-~cra~blor 30~.
3S . A~ter the conneotlon has been o~tabll-hed, both ~9~S~ 1 1 : Z 9 A~ 7Ic F T S H ~ Ft I C H A R D S O N P o 4 ZO~ 7~

modomo ~-nd pr~det~rmined seguence~ o~ ~ignal stateq to o~ch oth~r to furth~r condit~on their r-ceivera. Tho CAL~ ~odem transmits a ~equ~nce o~ signal ~t~tes a-~ign~t~d in F~q. 5 aa AA ~ollowed by ¢C. ~n th-meantime, the h~8WE~ modem een~o a ~ r-nt oequ-nc~ o~
~ign~l ~t~t~s d~81gn~ted ao AC, ~A an~ AC. ~oth signal 8e~UQnC-C contain ~ignal ph~e reverealo whlch ar~ used by the Cor~eopanding rec~lv~ng ~odem to determine the roun~ trl~ delAys, ~ and ~, for the chann~l 8. At ~he conclus~on of those ~equencos or s~gnal state~, both mo~em~ cea~ transmission.
In tho next pha~e whlch ie designated a~ eegment 1 in Fig. 5, a p~riod o~ halS-duplex tralning beglno.
Durlng this phae~, th~ GALL modem 18 silent wnlle th~
1~ ANSWER moaem transm~t~ preseloc~ed ~lgnal ~-quence~. The AN8WER mod~m send8 a training sequence 64 which is ~n uncod-d 4800 bit/soc scram~led e$gnal whlch the CA$L
modom U~09 to tra~n lt~ ~ocoivor 16~ ~nd whlc~ the hNSW~R
~od-m u~-~ to t~ain lt~ ~cho c~nc~ r lZb.
A~t-~ th~ CALL mode~ ha~ tr~ln-d lts rooeiver 16a ~o~ ~ ~ult~bl- a~ount o~ t$me, it u--~ tb- romalnder o~
th~ traln~ng ~oquence 64 in segment 1 to m-a~ure the qual~ty of the chann~l durlng halr-~uplox oper~tlon. ~n other word~, the CA~L modem mQasure- the ldle nol~ and a5 the ~lgnal lnduced di~to~tlon with no cho pre~ent. By the nd oS ~egment 1, the ~nitl~l r~to ~oglc 42a ln the CALL modem will have comput-a and storo~ the quant~ty ~D de~cr~bed sarlier. ~e duratlon of the training sequence 6~ may b~ selectsd so th~t thero is ~uf~oient t~me ~or tho CA~ ~od~m to ~o~h tr~n ito roceivor ~nd computo tho channel distort~on. Moreo~er, by oxt-nding the duration o~ the tralning ~equence 64, the CA~ mode~
can lnore~se the num~er o~ recol~ed olgnnl point~ u~ed to co~pute N_XD, th~reby ~mproving ~ccur~cy.
At the conclu~lon of the tralnin$ ~squonce ~4, ~he 1 Z', Cl 4., ~ ~1 _ 1 1 : ~ 53 A~I ~ F T S ~ 8~< R I C ~ A R D S O N P 0 6 Z()~)~476 AN~WER mod~ tran~mlts a rat~ 4ign~ which indlcatoo th- data rates, ¢odlng, and any special opora~lonal mod~c arallnble ln the ANSWER mod~m an~ lt~ oclAted DTE.
When th~ C~ modem r~celve~ the ~1~ lt ~reeze~ it~
adAptiVe elemRnts and sends a signAl sequence de~lgnat~d a~ an 8 sl~nal for a pe~$od o~ time e~ual to the round tr~p delay, N~, o~ the channel 8. Upon detocting the s ~ignal, the ANSWER modem ceaSQ5 transmi~ion o~ Rl.
Th~ modom~ now enter s~ont 2, whlch ~ Another period o~ hal~-duplex ~rainlng. Durlng Segment 2, tho CA~ modem transmlts anoth~r tr~inlng soquence ~6 to the ANSWER mod~m while the ANSWER modem remaln~ 611ent. The CAL~ modem uses ths trainlng ~equonce 66 to traln it-~cho canaeller ~2a whlle the ANSWER ~odem use~ tho lS training seq~ence 66 to train its rsceiver 16~. A~te~
the o¢ho cancellor 12a i~ trained, the lnltlal rate logic 42a ln th- ~ALL mod~m u~e~ tho remAlnd-r o~ the tr~lnlny ~qu-noo 66 to dete~mln~ the nols- ln th- rocolv-d ~gnal, whlch inolude~ roeldual echo a- w-ll a~ ldl~
nol6e fro~ th~ channel. By th~ end o~ Segment 2, tho ~nltlal rate logic will have computod an~ ~tor~
SQ~ERR~ho . Then, using the two nols- mea~ur~-nt~, each obtain-d from a corresponding period of half-duplex trainlng, the lnltlal rate logic 42a detormine~ a rat~
2S coda ~or the channel by co~putlnq the followlng ~orm~la:
QUAN~SQR~RR.Cho ~ ~N_HD}. The rato code 1- p~ssed to the rato decl~on logic 46a which selects th~ co3munica~on rat~ that the.CA~L modQm can su~taln over th~ channel.
The CA~ modem seto th~ DE8IRED RAT~ v~rlable to equal the solectod rato and ~ends tho do-~red rate to the ~N~WER modem ~8 a rate s~gnal ~2.
When th~ ANSWER mod4m recelveo the rate slgnal ~, the st~rt-up sequenco ent~r~ segment 3, which ls a p~r~od o~ full-duplex communlcatlon. ~hroughout 8-gment 3, the CAL~ modem cont~nues to son~ the rate ~gn~l ~2~ wh~l~

1 Z. C14. ~ 53 1 1: ~ ~3 A M ~ F I S~ Q R I C:~ARDSO~I P O Pi 20n~476 th~ ANSWER modem tran~mlte a training ~ignal 6a. The ANSWER mo~em use~ thi~ ~oriod to determlne th- comb~nod i~par$ment to the channel and to compute th~
com~un~o~lon rAte whlch it can su~taln ov-r th- chann-l, S ~hAt 1~, the initial rate log$c 42b ~eter~ln~
8QRERRCo~b~ and then co~put~ a ~ate code ualng the ~ollow~ng ~o~mula QUAN~SQRERR~o,b~d~ T~e rat~ code i~
pa~sea to th~ rat~ decl~on logic 46b whlch aol~ct~ th~
communlcatlon rate whlch th- AN8WER mo~o~ can 6ustaln ~he ANSwER modem thon 3elocts the lower or the communication rate it haa computed and R2 and send6 t~is selocted rate to th~ CA~L modem as a r~te ~ignal R3, which repreaents the rate at which the two ~odoms wlll bogin data co~munioate w~th eaoh other lS One advantage o~ the inventlon is partlcularly h~hlighted by ~ho abovo desc~lption o~ moaom oporat~on ~u~lng the s~art-up sequence~ Becaus- the CAL~ modom ~
~bl~ to aom~ute t~- co~bln~ lm~alrm-n~ to th- ahann-l by co~b~nin~ tho ocpar~t-ly d~termlned r--idual ooho and cha~n~l d~tort~on, the CAI~ mods~ ~nd th~ AN~W~R mod-~
are abl~ ~o arriv- ~t t~e.optl~um co~munic~tlon r~to ror the ahannel durlng th~ ~t~rt-up procedure de~lned by the V.32 p~cl~lcatlon. Thu~, the ~od~ms do not exp~ri~nce d-graded p-rrormance elther ln th- for~ of a low d~te ~5 tran-fer rate, i~ the initial dat~ rat- 1~ lower th~n the optlmum r~to for thQ ch~nn~l, or ln the form o~ a high blt errror rate, if the inltial rate ~e hlgher than the op~mu~ rate.
~he mode~ alco include~ two regl-ter~ whlch ~tor~
mod- in~ormation, namely a CUR~ENT_MOD~ reglst~r 52 and a P~S~_~ODE r~g~at~r 54, whlch ar- ~hown ln Fig. 2. ~od~, which re~e~s to whether th~ modulatlon u~od ln tr~n5mittlng thQ signal 18 code~ or uncod~, i8 ~p~c~ a during tho t~ainlng ~equence ~n whioh a particul~r 3S co~m~nlc~tlon rate i8 r~quested or ~ at~d. Th~ mo~em 1 Z, 0 4, a ~ AM ~ F I SH a~ R ~ CHARD SON P o Z0~)44'76 u-o~ tho PAS~_MODE r~gister S4 and the CURRENT_MODE
r~gl~t~r 52 to elect th~ ~roper cr~te~ia ~or detor~lnlng wh-n to authorlze or to reques~ A ~all ~orw~rd ln the oommunlcat~on rat~ when a ch~nge ln mode ie involv~d.
Thls 1~ part~cularly importan~ b~cau~o uncoded tran~mls~lon r~qui~o~ ~etter channel quAlity at a glven co~munication rA~e than do~s cod~d tr~nsml~lon.
Con~equently, the crlterla u~od ~or determlning when a 2all Sorward to an uncoded tran~mis~ion 1~ de~irable are not appropriate Sor d~term~ning when a fsll forward to a coded trans~is~lon ~ 5 desl~abl~.
~o make tho proper s~l~ction, the mod~m op-rates ~8 follow~. The modem u~e8 the PAST MODE roglster 5~ to store the mode sp~ci~led ln tho prevlous roceived trai ning signal and lt uses the CU~ T MODE regi~ter 52 to tor4 the mode ~pecif led ln the current trainlng s-quonce. A~ter a ~rainlng ~equenco ha~ occurred, the PA8~_MO~E r~gl-tor 54 idcntl~les th- previous ~od- And th~ CURRE~T_MODE r~g~-ter 52 ld-ntl~ th~ mode ~p-c~led ln the mo~t r~cent trainlng ~-quence A
qompar~tor 82 in the modem co~par-~ th- contentc o~ ~oth regl-t-r~ 5~ ~nd 54 lf the cont-nt~ o~ tho two r~ t~r- ~re dlf~erent, lnd~catlng th~t a ch~nge ln mode i~ being roque~t~d, r4-initializlng log~c 84 wlthin the modem r~oetc the FF_E5~IMATE regl~t-r 48 to zero and modlfi-e the parameter~ in the 4-ph~e decl~lon log~c 26 which d~ne the ~lze Or th~ inner square th~t i~ d to dot~¢t when a ~all ~orward from th- 4800 bit/~ec rat~ 1 d--~rable On the other hand, 1~ the contont~ of the CURREN~_MOD~ reglster 52 and the PA~_MODE r~giot~r 54 Are tho ~ame, indlcatinq th~t no ~od- chAnge 1B be~ng r~que~ted, then th~ ~odem leav~ tho cont-nt~ of th~
FF_E8~TMATE rsgls~er 48 undi~tur~-d and doe~ ~ot motify ~ny par~moters of the 4-phaso dect~lon logic 26 The modem al~o ~6e~ tho PAST ~OD~ regleter 54 and 1 Z, 0 4. i3 9 1 1 ~ A~I ~ F I S ~ 8... R I C ~ A RD S ON F' o 13 Z()~)~476 the CUR~E~_MOD~ ~g~t~r 52 during tho inltlal tralning to d~tormine the appropriat4 paramet~r~ ue~d to aelect th~ ~nitlal communication rate In~y are u~ed ln th~
am- way a~ doscrlbed above ~he modem is implemonted by a ~ultlple pro~cor achltocture, as shown in Fig 6 ~h~t i8 ~ lt ha~ a general ho~t procQs~or 86, whlch per~ormc over~ll control and tata movem~nt Sunctions; a signal procQ~ing l~ment 88, which performs ths functlons o~ the transmltter 6, th~ echo-cancellQr 12 and the comb$ner 14; an~ anothe~
elgnal proces8inq element 92, whlch performs the ~unction~ o~ the rece~ver 16 A receiver o~ gonerally this type is deacribed in U S patent appllcatlon seria~
number 586,681 entitled Processor Inter~a¢~ C~rcuitry, to Quro~hi ~t al ~iled ~a~ch 6, 198~, incorporated herein by ref ~renae .
Othor embodim4nts are wlthin the ~ollowing claim8.
rOr xampl~, th~ local and remotc dovlco~ oould opera~e a~ymm-trloally, i ~, u~lng dl~eren~ co~munlcation rat~
ao in dirt-rcnt directlon~ In ~uch ~n em~odlment, th~
local d~vlC~ ooul~ determlne ite pr-rerr-~ rat~ o~
~ecoptlon And ln~truct tho remot- d-vlc~ to lmploment that rate, rogardless o~ tho romot- device'c pre~orred rat- of receptlon and r~qardl~s OS whother the new ~ate 1~ highor or lower than the rate at Whlch the local d-~$ce 1~ transmitting to the rsmote devico ~ik~wi~, th~ r-moto dQv$ce could detormins 1te proterred rato o~
receptlon and in~t~uct the local devlc~ to imploment that rate a~ the local devlce' 8 tran~ml~on r~te reg~rdle~
3~ of whother the local dQV$C~ concura that the now rat~ ia it~ preferred rate o~ roce~t~on What i8 claimed i9:

Claims (28)

1. An echo cancellation modem for receiving data signals from a remote device over a channel in accordance with one of several possible modulation schemes suitable for different channel qualities, respectively, while simultaneously sending data signals to the remote device over the same channel, said modem comprising an echo canceller for reducing echo components appearing in the channel signal while leaving a residual echo component in the echo reduced channel signal, a monitor for determining the quality of the channel based on an analysis of the residual echo component, and a controller for selecting an acceptable said modulation scheme based on the analysis of said monitor.

2. The modem or claim 1 further comprising circuitry for equalizing and demodulating the echo reduced signal and wherein said monitor analyzes the residual echo component based on the equalized and demodulated echo reduced signal.

3. The modem of claim 1 further comprising control circuitry for causing said monitor to analyze said residual echo component while said modem is transmitting a signal and said remote device is not sending a signal.

4. The modem of claim 1 wherein said monitor further determines the quality of the channel based on a separate analysis or the channel distortion imparted when the modem is receiving a signal and is not sending a signal.

5. The modem of claim 4 wherein said monitor determines the quality of the channel based on a combination of the separate analyses of the channel distortion and the residual echo component.

6. The modem of claim 5 wherein said monitor determines a linear combination of said channel distortion and said echo component.

7 . The modem of claim 1 wherein said controller includes means for requestion the remote device to shift to a higher level modulation scheme and means for shifting to the higher level modulation scheme if the remote device concurs.

8. The modem of claim 1 wherein said controller includes means for requesting the remote device to shift to a lower level modulation scheme and means for the modem to unilaterally shift to the lower level modulation scheme for transmission of data signals to the remote device.

9. The modem of claim 1 wherein said modulation schemes include different modulation rates.

10. The modem of claim 1 wherein said modulation schemes include coded and uncoded modulation modes.

11. The modem of Claim 1 wherein said controller further comprises means for causing said modem to transmit data signals using the same modulation scheme as the remote device uses for its transmissions.

12. A method for use in a modem of the kind which receives data signals from a remote device over a channel in accordance with one of a range or available modulation schemes suitable, respectively, for a range of different channel qualities, a higher level modulation scheme being suitable for a higher channel quality, said method comprising determining whether the current channel quality would support a higher level modulation scheme than the current modulation scheme, if so, sending a request the remote device to use a desired higher level modulation scheme, and conditioning the modem not to later request a higher level modulation scheme if a higher level modulation scheme cannot be supported.

13. The method of claim 12 wherein said conditioning is based upon the modulation scheme chosen by the other device in repose to the request sent from the modem.

14. The method of claim 12 wherein said conditioning step comprises if the remote device concurs in the use of the desired modulation scheme and if there remains a higher level modulation scheme usable by the modem, then conditioning the modem to later request a higher level modulation scheme if the channel quality later improves, and if the remote device does not concur in the use of the desired modulation scheme of if there is no higher level modulation scheme usable by the mode, then conditioning the modem to refrain form later requesting a higher level modulation scheme if the channel quality later improves.

15. A method for use in a modem of the kind which receives data signals from a remote device over a channel in accordance with one of a range of available modulation schemes which are suitable, respectively, for different levels of channel quality, a higher level modulation scheme being suitable for a higher channel quality, said modulation schemes including coded and uncoded modulation modes, comprising monitoring the quality of the channel based on received data signals, determining when a change to a higher level modulation scheme would be permissible based on when the monitored channel quality exceeds a threshold, and setting the threshold to one value with respect to a change to a coded modulation mode, and to a different value with respect to a change to an uncoded modulation mode.

16. The method of claim 15 further comprising initializing said modem using one threshold corresponding to one of the modulation modes, and reinitializing said mode using the other threshold in response to a request from the remote device to switch to the other mode.

17. A method for use in an echo cancellation modem of the kind which communicates in full-duplex with a remote device over a channel in accordance with one of a ranged of available modulation schemes suitable, respectively, for different levels of channel quality, a higher level modulation scheme being suitable for a higher channel quality, wherein different modulation schemes which operate at the same communication rate may use different signal point constellations and wherein the quality of the channel is determined in part by the presence of an echo, the method comprising:
monitoring the quality of the channel, and selecting a signal point constellation based on the monitored channel quality.

18. The method of claim 17 wherein one said signal point constellation comprises more than 2N points, where N
is the number of bits per signaling interval to be sent.

19. A method of setting an initial reception rate for an echo cancellation modem of the kind which receives data signals from a remote device at one of several possible rates and which is subject to an initial training procedure prior to initial reception of the channel during the monitoring the quality of the channel during the initial training procedure, and immediately setting the initial rate to a value corresponding to the rate sustainable on the channel based on its monitored quality.

20. The method of claim 19 further comprising reducing echo components appearing in the channel signal while leaving a residual echo component in the echo reduced channel signal, and wherein the quality of the channel is determined based on an analysis of the residual echo component.

21. The method of claim 19 further comprising analyzing said residual echo component while said modem is transmitting a signal and said remote device is not sending a signal.

22. The method of claim 19 wherein the quality of the channel is further determined based on a separate analysis of the channel distortion imparted when the mode is receiving a signal and is not sending a signal.

23. The method of claim 19 wherein the quality of the channel is determined based on a combination of the separate analyses of the channel distortion and the residual echo component.

24. A modem which receives data signals from a remote device over a channel in accordance with one of a range of available modulation schemes suitable, respectively, for a range of different channel qualities, a higher level modulation scheme being suitable for a higher channel quality, said modem comprising:
quality monitoring logic for monitoring the quality of the channel, a fall forward flag register for storing a fall forward flag, conditioning logic for conditioning the fall forward flag to indicate whether an opportunity exists for the modulation scheme to fall forward to a higher level modulation scheme than the current modulation scheme, rate adjustment logic responsive to the quality monitoring logic for determining that the channel will support a fall forward to A higher level modulation scheme, rate setting logic responsive to the rate adjustment logic for sending a fall forward request to the remote device wherein the rate setting logic sends the fall forward request only if the fall forward flag is conditioned to indicate that an opportunity to fall forward exists.

25. The modem as defined in claim 24 wherein the conditioning logic conditions the fall forward flag based upon the modulation chosen by the remote device in response to an earlier request to fall forward sent by the modem.

26. The modem as defined in claim 24 wherein a current modulation scheme is selected in response to an earlier request by the modem to use a desired modulation scheme and wherein the conditioning means conditions the flag in accordance with the following rule:
If the highest modulation scheme available to the modems is higher than the current modulation scheme, And if the desired modulation scheme is equal to the current modulation scheme, Then the conditioning logic conditions the flag to indicate that an opportunity to fall forward to a higher modulation scheme exists, Otherwise the conditioning logic conditions the flag to indicate that a fall forward to A higher modulation scheme does not exist.

27. A modem which receives data signals from a remote device over a channel in accordance with one of a range of available modulation schemes witch are suitable, respectively, for different levels of channel quality, a higher level modulation scheme being suitable for a higher channel quality, said modulation schemes including coded and uncoded modulation modes, the modem comprising:
quality monitoring logic for monitoring the quality of the channel based on received data signals, fall forward logic for determining when a change to a higher level modulation scheme would be permissible based on when the monitored channel quality exceeds a threshold, and initialization logic for setting the threshold to one value when the higher modulation scheme is a coded modulation mode, and to a different valuewhen the higher modulation scheme is an uncoded modulation mode.

28. The modem as defined in claim 27 wherein the initialization logic sets the threshold to one value corresponding to one of the modulation modes, the modem further comprising:
renitialization logic for reinitializing the fall forward logic to the other mode in response to receiving a request from the remote device to fall forward to a modulation scheme of the other mode.