CA1268225A

CA1268225A - Data modem with adaptive synchronized speed change

Info

Publication number: CA1268225A
Application number: CA000536026A
Authority: CA
Inventors: Donald Charles O'connor; Juan Enrique Farias; Raul Fermin Fernandez
Original assignee: Racal Data Communications Inc
Current assignee: Racal Data Communications Inc
Priority date: 1986-06-26
Filing date: 1987-04-30
Publication date: 1990-04-24
Anticipated expiration: 2007-04-24
Also published as: US4780883A

Abstract

ABSTRACT

A method for changing the data transmission rates in a synchronous data communication system including a first modem and a second modem coupled together through a transmission channel, the transmission channel including a first path for transmissions from the first modem to the second modem and a second path for transmission from the second modem to the first modem, includes the steps of determining, at the first modem, that the second path of the transmission channel is adequately high in quality to support an increase in transmission rate.
Next it is determined, at the second modem, that the first path of the transmission channel is adequately high in quality to support an increase in transmission rate. A training sequence is then transmitted at an increased transmission rate from the second modem to the first modem.
Next, a training sequence is transmitted at the increased transmission rate from the first modem to the second modem.

Description

~ 1~68ZZ5 DATA MOD~M WITH ADAP'IIV~ S~NCHRONIZ~D 8PEED CHANGR

Background 1. Field of The Invention This invention relates generally to the ~ield og modems suitable for data communications at varying data tran~mission rates. More partlcularly, this inventlon relates to a data modem which adaptively adJusts its transmission rate in accordance with line quality to optimize data throughput over changing transmission lines.

2. Back~round of The Invention It is known in the art that data communication over telephone transmission lines may be accomplished at data rates which are related to the quality of the transmission line being utilized. That is, higher quality conditioned transmission lines are generally capable Oe transmission rates greater than those of, eor example, simple voice grade transmission 26~;~2S
lines. Line quality varies significantly even among transmission line~
which supposedly have similar conditioning. In addition, the quality of a particular leased circuit may vary from day to day. It is there~ore possible to achieve higher rates of transmission over some lines than others due to variables such as line length, weather, time of day and age of the lines. As such, it is advantageous to provide a mechanism for measuring and monitoring line quality and having data modems adapt to the quality of llnes so that data throughput is maximized for a given set of lines and line conditions.
Modem manufacturers have recogntzed the sbove problem~ and have proposed various circuit~ which attempt to maximize data throughput. For example, U.S. Patent No. 4,1387,511 to 8aran discloses a high speed modem suitable for dialup telephone lines in which the telephone pass-band is subdivided into 64 sub bands each with ~ts own carrier. Each carrier i9 {ndividually amplitude and phase modulated in order to transmit at a relatively low data rate on each carrier. The net result Is a relatively high data rate. Barans modem determines during a training period which portions of the telephone line spectrum may be most effectively utilized to carry data. Those carriers which are most impaired, are dropped in order to reduce the effective data rate while maximizing data throughput for a given set of channel impairments.
Unfortunately, the Baran modem is not currently compatible with existing network control and diagnostic systems such as the Racal-r.qilgo'9 CMS
series of network control and diagnostic equipment. Also, Baran s modem is only able to measure line conditions during its initial training period.
Any line improvements occurring after that training period w~ll not be recognized or accounted for, thus possibly reducing effective utili~ation of Bandwidth.
Other modem manufacturers have provided modems which are

3 o capable of speed changes to adapt to varying line conditions also.
However, such designs hsve generally failed to recogni~e and deal with the fact that the quality of a four wire line can vary such that line quality is high in one direction of transmission (one pair of wires) and low in the other direction. Furthermore, this condition can change so that the J.266~Z~S
situation is reversed. These systems are not designed to function optimally with the above mentioned advanced diagnostic and network control systems. In addition, some such systems may be actually detrimental to data throughput by continually interrupting user data in an attempt to convert to a higher data speed on a marginal or asymmetrical set of transmission lines.
Another problem with such modems is a less than optimal speed change protocol which can result in two mod~ms temporarily winding up at different speeds which causes disruption of user dats until a reinitialization occurs. This results primarily when one modem wh~ch is assigned absolute control over speed increases at any given time blindly assume3 that its upspeed command correctly reaches its counterpart. An upspeed command which is not properly received due to, for example, phase hits on the line can cause such situations. It is also possible that power ~ailures, brown outs and the like can cause neither modem to retain control over increases in speed so that a pair of modems is locked at a speed which is lower than the optimum transmission rate.
The Codex0 model 2660 modem uses an interruptive inband signaling as part of its speed change protocol. No secondary channel signaling is used. While this modem has a mechanism for switching its master/slave relationship similar to the present invention, the protocol for speed changea is less than optimal in th~t it allows one modem to demand upseeed without consulting the other. This can result in two modems operating at different speeds if the command to change speeds is damaged in transmission. It also has 18ss immunity to confusion over master/slave identity or transmission speed in the event of brownout or damage to control signals in transmission.
While the above problems are not exhaustive of those which are encountered in modems having adaptive speed features, they serve to illustrate a substantial need for improvement in this area. The present invention alleviates many of these problems as well as providing a protocol with minimal user data interruption as well as minimizing secondary channel interruption while providing a high degree of reliability in speed change signaling.

_4_ 1Z68Z25 Summary of The Invention It is an object of the present invention to provide an improved data nnodem having an adaptive synchronized speed change feature.
It is an advantage of the present invention to provide an impro~ed data modem having a coordinated adaptive method of adjusting data transmission speed to maximize data throughput over a given set OI
transmission line conditions.
It is another advantage of the present invention to provide a system for changing modem speeds which accounts for tran9ient phenomenon such as amplitude and phase hits.
It is another advantage of the present invention to provide a data modem with adaptive speed adjustment which is compatible and harmonious with secondary channel network management and diagnostic systems.
It is another advantage of the present invention to provide an improved data modem having separate decision mechanisms governing speed increases and speed decreased.
These and other objects and advantages of the invention will become apparent to those skilled in the art upon consideration of the following description of the in~ention.
The invention provides in a data communication system including a first modem and a second modem coupled together through a transmission channel, said transmission channel including a first path for transmission from said first modem to said second modem and a second path for transmission from said second modem to said first modem, a method for coordinating a change in the transmission rates of said first and second modems, the method comprising the ordered steps of:
attempting to train an~ retrain said modems;
determining~ at said first modem, that said second path of said transmission channel is adequately high in quality to support an increase in transmission rate;

,, Z6~3Z2S
transmitting a request to increase speed from said first modem to said second modem;
determining, at said second modem, that said first path of said transmission channel is adequately high in quality to S support an increase in transmission rate wherein the step of determining includes mapping a duration of time wherein said signal quality is deemed to be poor into a count, accumulating said count to produce an accumulated count and comparing said accumulated count to a predetermined maximum 1~ count for a predetermined time line so that upspeed decisions can occur with a moderate amount of transient impairment; and wherein said mapping step includes assigning integer values to time periods of poor signal quality such that larger integers are assigned to longer periods of poor l; signal quality and smaller integers are assigned to shorter periods of poor signal quality, and ~he count is totaled for said time limit;
acknowledging said request to increase speed from said first modem by transmitting an acknowledgement signal from said second modem to said first modem;
squelching transmisslon at said first modem;
transmitting a training sequence at said increased speed from said second modem to said first modem; and transmitting a training sequence at said increased speed 2~ from said first modem to said second modem.
The invention also provides in a data modem, a circuit arrangement for effectins transmission speed changes between a pair of modems comprising in combination:
training means for a.tempting to train and retrain said modem;
first means for establishing the quality of transmission line;
second means for counting retrain attempts as a measure of line quality, 2~S

fallback decision means, responsive to said second means, for producing an output representing a demand for reduction in transmission speed whenever a predetermined number of retrain attempts occur within a selected period of time; fallforward decision means, responsive to said first means, for producing an output represent;ng a request for increase in transmission speed whenever the average signal quality is greater than a predetermine threshold;
establishing means for establishing said data rnodem as having an identit~
designated as either master or slave, including means for determining if said modem is the most recent of said pair of modems to demand a speed reduction, and wherein said modem is designated master in the event it is saidmostrecentrequester;
transmitting means for transmitting requests for increases and demands for reductions in transmission speed over said transmission line, and inhibiting means for inhibiting said transmitting means from tr~nsmitting requests for increases in speed unless said modem's identity is that of master.
In one embodiment of the present invention a method for chan~ing the data transmission rates in a synchronous data communication system including a first modem and a second modem coupled together through a transmission channel, the transmission channel including a first path for transmission from the first modem to the second modem and a second path for transmission from the second modem to the first modem, includes the steps of determining, at the first modem that the second path of the transmission channel is adequately high in quality to support an increase in transmission rate. Next it is determined, at the second modem, that the first path of the transmission channel is adequately high in quality to support an increase in transmission rate. A training sequence is then transmitted at an increased transmission rate from the second modem to the first modem. Next, a training sequence is transmitted at `" ~2~B~25 the increased transmission rate from the first modem to the second modem. Preferably, the determining steps are carried out by comparing the average minimum cost function of a Viterbi decoder with a predetermined thresold and weighting transient phenomenon so that the line quality is measured predominately on the basis of relatively long term averages.
The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself however, both as to organization and method of operation, together with further objects and advantages thereof, may be best understood by reference to the following description taken in conjection with the accompanying drawing.

Brief Description of The_Drawings FIG. 1 is a functional block diagram of a pair of modems incorporating the present invention.
FIG. 2, including FIG. 2A through FIG. 2F is a flow chart describing the operation of the present invention.
FIG. 3 is a hardware block diagram of a modem of the present invention.

Detailed Description of The Invention .
In the preferred embodiment of the present invention a convolutional encoded modem utilizing Viterbi decoding forms the heart of the present data modem. Viterbi decoders are now well known and commonly used in the modem art to achieve maximum likelihood sequence estimation decoding of convolutionally coded data. Such decoders are described, for example, in Forney, "The Viterbi Algorithm", Proceedings of the IEEE, Vol. 61, No. 3, March 1973; Hayes, ~The Viterbi Algorithm Applied to Digital Data Transmission" Journal of Communications Society, Vol. 13, pages 15-20, March 1975, U.S. Patent No. 4,578,800 to Yasuda, et al.; and U.S. Patent No. 4,562,426 to Forney, Jr.
Preferably, the modem of the present invention operates over ~Z6~Z~25 four wire conditioned telephone lines such as 3002, + D-l, or CCITT
M1020 lines. The present invention is especially useful for high speed data modems operated under such conditions at 12,000 BPS and above, but of course this is not to be limiting. In the preferred embodiment, the present modem is capable of operating at either 12,000, 14,400 or 16,800 BPS depending upon the quality of the transmission line. Also, the present modem is preferably designed for compatibility with known network control and diagnostic systems such as those disclosed in U.S. Patent No.
5,385,384 to Rosbury et al. Such diagnostics and control i~unctions, for purposes Oe the present application, are referred to as operating on a TX
channel (a TX channel is a secondary channel preferably utilizing low speed FSK modulation at 75 to 150 bits per s~cond). For purposes o~ the present discussion, the terms "TX" and "TX Equipment" rei~er to systems including a network controller communicating via secondary channel.
User data is modulated over the communication channel utilizing convolutional coding with a ~6 point constellation at 16.8 KBPS, a 128 point constellation at 1~.~ KBPS and a 64 point constellation at 12 KBPS.
Inband network control signals may also be utilized by preceding control signals such as those used to initiate speed changes with a training sequence or other suitable mechanism for encoding control signals into the user data stream. In the preferred embodiment, Racal-Milgo~ T1 inband signaling is utilized. B~iefly, T1 signaling is a form of signaling implemented as an unidirectional message transmission having the following format: lmodem training sequence] [brief squelch~ ~127 symbols of carrier] [sync signal] [short (T1) signaling message at 2400 baud, one bit per symbol] [brief squelch] [modem training sequence] [user data]. While this form of inband signaling is preferred, it will be clear to those skilled in the art that many other forms of signaling may be used to serve an equivalent purpose. Thus, references to Tl signaling herein should be viewed only as illustrative.
The present invention contemplates both upspeed and downspeed conversions (referred to hereinafter as fall forward or failforward and fall back or fallback respectively). Further the present , .

;82;ZSi invention contemplates utilization in systems with and without network controllers. In those systems without a network controller, remote modem control ~RMC) may be utilized with secondary channel signaling as well as inband slgnaling to ef~ect speed changes. In those systems hav~ng a network controller, the sy~tem utilize~ a combination of T1 inband signaling and secondary channel signaling to effect speed changes.
Remote Modem Control is a feature common in the art which allow~ a single modem, such as the Omnimode'iD series modem manu~actured by Racal-Milgo~9 to be used to control ~eatures characteri~t~cs and options of remote modems from its control panel. In this RMC environment, the modem's control panel acts as the network controller in a limited capacity. In these environments, the seconclary channel diagnostics and control functions are assumed to have high priority. Thus9 thi~ protocol is designed to disrupt secondary channel data as well as user data as little as 1 5 possible In systems having neither RMC nor a network controller (that is, there is no secondary channel) the speed change protocol i~
implemented with inband T1 signaling in a manner which i9 least disruptive of user dataO However, a high level of acknowledgment Is 2 0 retained to assure that each modem remains aware Oe the other's condition at all times, thus largely eliminating disruptive reinitialization.
The present invention ~inds greater utility in ~our wire leased line environments in which It is possible that one pair o~ wires may be capable of handling a higher speed o~ transmission than the other pair.
There is of course no way to know which pair of wires will be likely to support the higher or lower transmission rates at any given time. It is therefore advantageous to provide a protocol which will enable the modem transmitting on the line with the greatest likelihood of having the lowest capacity to dominate in order to establish an orderly mechanism for such control. To achieve this, one modem is designated "master" and the other is designated "slave". The modem which is designated as the master modem will always initiate a fallforward speed change.
At power up, communication begins at a speed which is establfshed either from the control panel or from the modem's nonvolatile ~~ -8- ~6i3ZZS

memory (that is, the most recent speed used by the modem). Speed increases will occur if possible until the highest tolerable rate is achieved. If any modem in the system Is unable to reliably tran~mit over the channel, it can initiate a ~allback to the next lower speed. Iî the modem is operating in the T1 environment the modem which lflst requested a fallback is designated master and is the only modem which may initiate an increase in speed. In this manner, the modem at the weakest link in the communication channel is generally assured of controlllng the speed of the entire network. In the RMC or TX
environment, the central site modem is always designated master.
Turning now to FIGURE 1, a ~unational block dlagram o~ a data communlcation system including the pre~ent Invent~on i~ shown. The system o~ FIGURE 1 represents a point-to-point data communiaatlon system Includ(ng a modem 10 coupled through a communlcation channel 12 to a second modem 14. Those skilled in the art will recognize that modems 10 and 14 include many other elements not shown but which are standard and well known in the modem art. Modem 10 includes a Viterbi decoder 16 which is used in the modem's receiver to decode the inband transmitted data. In additionf Viterbi decoder 16 produces a binary signal computed from a minimum cost 7unction at its output 18. The signal at output 18 is referred to as "Cost SQ" (Signal Quality) herein. (The term "cost" as used herein iq o~ten re~erred to in the art as "metric" or "distance". The cost as used herein means the magnitude of the euclidean distance between a received constellation point and the ideal constellation point). The cost ~unction at output 18 is obtalned by summing the euclidean distance between the received signal point and the closest ideal point over a trellis path history of the previous N bauds. The average minimum cost is then obtained by integrating this minimum cost over a predetermined number of bauds. The output of the Viterbi decoder is a number that is processed by fallback and fallforward decision logics.
The minimum cost information is provided to fallback decision logic 20 which includes a counter for counting the number of retrain attempts necessitated by data errors in the received data. The output 18 of Viterbi decoder 16 is also provided to a fall forward decision logic block 22 which ~2682ZS
processes the cost SQ signal at 18 to determine if a speed increase is possible. The output of fallback decision logic 20 is a fallback request signal which is transmitted to an auto speed state sequencer 24 the operation of which (along with Pall back deci~ion logic ao and fall forward decision logic 22) is des~ribed by the ~low chart o~ FIGlJRE 2. The output o~ fall forward decislon logic 22 can be viewed as a request tor an increase in speed which is also provided to auto speed state sequencer 24.
The output of 22 is provided in the form of a logic level. Thi~
logic level is obtained by comparing the average minimum c03t ~rom the 0 Viterbi algorithm to ~ predetermined thre~hold. When the ~ignal quality i9 high (therefore the average minlmum cost i3 1Ow) thls comparison results in a logical 1 at the output of 22. When line quallty degrades below the predetermined threshold, the logic level at the output of ~2 drops to a logic 0. Those skilled in the art will recognize that various transiant phenomenon such as phase or amplitudc hits can result in temporary or transient line degradation. Since it is desirable to e~tabllsh transmission speed based upon longer term average line quality, it i8 de~irable to provide a mechanism for accounting for such transient phenomenon In the speed change decision making process. This mechanism operates as follows:
In the preferred embodiment, cost Sq is integrated in the Iallforward deci~ion logic 22 for approximately 8 minutes, 15 mlnutes, 1 hour or 2 hours depending on an integration time strap which ls user selectable. The timer times are selected to be much shorter than a typical time period in which transmission line quality changes (frequently hours, days or even weeks). In this manner, transient phenomenon are given a small weight in determining overall transmission line quality. If the integration timer expired, then a fallforward flag is set and a request to fall forward sent from logic 22 to state sequencer 24. While cost SQ is being integrated the modem receiver can be affected by line impairments (phase hits, amplitude hits9 etc). If the effect of the impairments on the cost SQ is not significant, then the integration will continue with no interruption. If the effect of the impairments is significant then the integration timers will be re-started. Transient impairments will cause - I U~
682~

the cost SQ to become "bad" for a cert~in time intervRl. This time interval is mapped into a count (see Table 1). The count is then added to the count that resulted ~rom previous impairments. Finally the composite count is compared against a threshold (see Table 2), which value depends on the integration time selected by the strsp. I~ the maxlmum count ~s exceeded at any given time then the integration timer is re-started. It will be understood that the values in Tables 1 and 2 were determlned experimcntally to achieve the desired results. Many variat~ons o~ this mapping arrangement may be possible without departing from the present teachings.

Cost SQ "Bad" Duration Associated Count I.ess than 853 msecs 0 853 msecs to 1.28 secs 1.28 secs to 1.7 secs 2 1.7 secs to 2.13 seos 3 2.13 secs to 2.56 secs 4 2.56 secs to 2.98 secs 5 2.98 secs to 3.41 secs 6 3.41 secs to 3.84 secs 7 3.8d~ secs to 4.27 secs 8

4.27 secs to 4.69 secs 9 4.69 secs to 5.00 secs 10 greater than 5.00 secs Counter is Restarted 6~Z~5 . . .
TABL~ 2 Cost SQ Integration Time Maximum Count 8 minutes 20 15 minutes 30 1 hour 12û
2 hours 240 Speed change requests E13 well as~other information may al80 be manually requested via a control panel 26. Control panel 26 is also coupled to fall ~orward decision logic and ~all back decision logic as well as an EIA RS 232 speed select logic block 2B ~o that control p~nel 2~ may provide configuration inPormation to each of these blocks. Such configuration inPormation is utilized to in~orm the various parts of the modem exactly what so~t strappable con~iguration the mod~m i9 currently in. Control panel 26 is also utilized to override automatic speed change information as desired by the user via its direct connection to the auto speed state sequencer 24. EIA RS-232 speed select logic block 28 serves to process the data rate signal from a DTE per circuit CH o~ a standard RS-232 interface. In the present invention this is accomplished by selecting the data rate which is so~t straæped into the modem from the control panel in response to DTE speed change commands. Speed ~hanges may also be initiated manually by a network controller 30.
Speed change commands from the auto speed state sequencer 24 are encoded for transmission over the communication channel 12 by a command encoder 32 prior to transmission either by a T1 inband transmitter 34 or a T~ FSK transmitter 36 depending upon whether inband signaling or secondary channel signaling is to be utilized for implementing the speed change. Such speed change commands received from modem 14 are received either by a Tl inband receiver 38 or a TX FSK receiver 40 prior to processing by a command decoder 42. When such commands are -12- :~2~8ZZS

decoded by decoder 42 they are forwarded to auto speed state sequencer 24 for further processing.
Modem 14 may be similarly configured to include a ~iterbi decoder 116 having an output 118 coupled to a fallback deci~ion logic block 120 and a fall forward decision logic block 122. ~locks 120 and 122 are coupled to a similar auto speed state sequencer 124. A control panel 126 i5 similarly coupled to blocks 120, 1229 124 and EIA speed select logic block 12a. State sequencer 124 is coupled to a command encoder 132 whlch provide~ speed change commands to inband transmitter 134 and FSK transmitter 136. An inband receiver 138 and a FSK secondary channel receiver 140 ~eed incoming commands to command decoder 142 ~n a manner s3milar to that o~ modem 10.
ln order to rellably e~fect a speed increase, a handshak{ng protocol hav~ng two level~ Oe acknowledgment is establiahed. Al90, In a polnt-to-point communications environment, a master/slave relationship is created between modem 10 and modem 14. In generalt the modem which most recently required a speed decrease is established to be the master modem in the T1 environment. in the TX/RMC environment, the central site modem is always master. Only the master modem may initiate speed increases. In this manner the modem which most recently had the poorest quality line always has control over increases in speed.
According to the present invention, two possible protocols for speed change may be utilized depending upon whether secondary channel is available for use tRMC or TX environment) or not (T1 environment).
To illustrate the speed change protocol in the T1 environment, assume that modem 10 has most recently requested a speed decrease (fallback) so that modem 10 is the master modem and modem 14 is the slave modem. If the line quality as measured by Viterbi decoder 16 of modem 10 is high enough so that modem 10 determines that it can support an increase in transmission speed, it transmits a request to increase speed to modem 14. In this situation where no network controller is present, T1 inband signaling is utilized and the request is transmitted via inband transmitter 34. lf modem 14 is unable to support an increase in transmission rate, it acknowledges the request and responds that it is lZ6~Z25 unable to support a speed increase. At this point, the master/slave modem identities are switched so that modem 14 now controls speed increases. In this manner, speed increases remain under control o~ the modem which ig receiving the poor~st quality signal. In the event that S modem 1~ does not receive the request for speed increase initlally transmitted by modem 10 due to some line impairment, modem 10 waits for 10 seconds to again attempt to increase speed. If a aecond attempt fails, modem 10 will restart its cost SQ integration timer.
When the master receives a trainlng sequence at the new speed it will return a training sequence at the new speed. The master will abort the handshake if it does not receive an acknowledge from the slave within 10 seconds or if after it squelches carrier it does not receiYe a train3ng sequence ~rom the slave within 5 seconds. The slave will abort the handshake if it does not detect th~ squelch within 10 seconds of its transmission of an acknowledgment. I~ the handshake is aborted due to the loss of an acknowledge, the master will retry the handshake. I~ the second attempt fails then subsequent attempts will be inhibited for the duration of the Retry timer.
In this Tl environment, if the modem 14 is able to support a speed increase it tran~mits a positive acknowledgment signal back to modem 10. IJpon receipt oi this positive acknowledgment, modem 10 drops its carrier (squelches its transmitter) as an acknowledgment to modem 14's acknowledgment. Modem 14 then transmits a training sequence at the next higher data rate, followed by a training sequence from modem 10 to modem 14 and normal data communication at the higher rate resumes. This protocol provides minimal user data disruption in the event upspeed cannot be accommodated. When upspeed can be accomplished it provides a protocol which assures that the speed change is well coordinated between both modems so that no loss of master/slave identity occurs and transmission rates are always synchronized.
In the TXtRMC environment, the protocol is as ~ollows. ~Uodem 10 is assumed to be the central site modem and therefore is strapped as the master modem. When signal quality as measured at the central site warrants an increase in data transmission rate, modem 10 transmits a 126F~;2S
request to fsllforward to modem 14 via its FSK secondary channel transmitter 36. This request is destined Por modem 14'g secondary channel recelver 40. Upon requesting a fall forward in speed by modem lO, fall forward decision logic 22 initiates a timer which run~ ~or a predetermined period of time (30 seconds in the pre~erred embodiment) while an acknowledgment from modem 14 is awaited.
If modem 14 i~ unable to support an increaae in transmission rate, it will not respond to modem 10's request. I~ modem 10 does not receive a positive acknowledgment within 30 ~econds it retransmits the request ~one t~me~. If modem 10 again does not receive or reply w3thin 30 seconds, it assumes that modem 10 cannot operate at the requested data rate. Modem lO will then restart its cost SQ integration timer.
Subsequent retries can only be made a~ter the duration o~ this timer (8, 15, 60 or 120 mlnutes). Therefore, rather than repeatedly Interrupt the 9econdary ahannel~9 limited available transmisslon band with appsrently fruitless attempts to Increase In speed, modem 10 will wait for a much longer period of time (8, 15, 60 or 120 minutes, strap selectable in the pre~erred embodiment) to see if line conditions change to allow modem 14 to increase in speed. The exact times of the above timers may be varied to optimize operation in various environments. This form of negative acknowledgment (N0 17.EPLY) has the distinct advantage that user data is never interrupted by an unsuccessful attempt. A second advantage of this methodology is that it results in minimal interruption of network control functions.
The fall forward integration timers in the master and slave modems are not exactly synchronized. For a successful auto speed handshake, the slave modem is the first to send a training sequence at the new speed. Hence the master modem starts to integrate the computational fall forward flag before the slave. The offset in the two timers could be as much as a minute. Therefore the integration timer in the slave is set to be 2 minutes less than the master (i.e. 58 minutes rather than 60 minutes). The integration timer is reset by either a successful or unsuccessful fall forward attempt or the computational flag being false.

1 5~ 68ZZS

In the case of a positive acknowledgment, modem 14 will reply with positive ACK using T1 inband signaling. This presents no problem interrupting user data since the modems must be retrained at the new data rate anyway.
SIn the preferred embodiment, the modem may be strap selectable to request fall forward when the line quality is such that the expected block error rate at the new data rate is either D/1000 or 511000 a~ desired by the user. Retrains are institllted at block error rates of either 30/1000 or 500/1000 (u~er strappabl~). Thi~ ha~ been ~ound to 10provide adequate hysteresi~ to prevent unnecessary speed changes under marginal line conditions.
If the upspeed which is ef~ected i9 not the maximum speed o~
the modem, the aforement~oned processes m~y be repeated 1~ the l~ne quallty is ~apable of supporting ~urther speed increases. By utilizing the 15aforementioned protocol9 both user data and secondary channel communication interruption is minimized by use of no acknowledgment equating to a negative acknowled~ment in the TX/RMC environment and minimizing user data interruptions in both the TX/RMC and T1 environments.
20If after a period of time, either modem 10 or 14 determines that it can no longer support a higher data rate, either modem may initiate a speed ~all back. Although 8 fall forward is a speed change which must be mutually agreed upon by both modems 10 and 14, a speed fall back ls a non-negotiable speed change which may be instituted on 25demand by either modem upon determining that it cannot support the data rate presently being used. This inability to support the higher data rate, is determined in the preferred embodiment by counting the number of retrain requests occurring over a period of time. In the preferred embodiment, the fall back criteria may be strap selectable to either four 30(4) retrain attempts within one (1) minute or six (6) retrain attempts within fifteen (15) minutes, but these specifics are not to be limiting.
These criteria have been established experimentally and may not be optimal under all conditions. They have, however, been found suitable for a wide range of general applications. Since either modem may demand a - 16- ~Z~;~Z25 fallback and since retraining is required for fallback, T1 signaling i9 preferably always used for fallback. There i5 no need to disrupt the secondary channel at all when implementing a fallback. Slnce a Pallback Is not negotiable, user data mu~t be interrupted so it is pre~erable to u~e inband signaling.
Slnce the Pall ~orward process is dependent upon which modem is designated master, it is lmportant that the condition o~ both modems being designated as slave be voided. This is accompllshed in the pre~erred embodiment by having each modem at power up always start at a known transmission rate and gradually increase speed ti~ possible) through the nforementioned process until the most rapid data rate tolerable by the transmission line~ is establishedO Further, at power up and subsequent to a brown out or a power out or ai~ter change in strapping, a rein~ti~lizatlon of modem master/slave relationsh~p i3 established. Whenever a modem i8 powered up, it sends an initialization command sequence to the other modem telling that modem to assume a ma~ter or ~lave identity opposite that of the modem transmitting the initialization command ~equence. In this manner, it is always assured that one modem will assume the identity of the master and one will assume the identity of slave. ht the conclusion of a successful speed change handshake in the TX environment, the central modem sends a speed change alert message to the network controller to inform the network controller o~ the new data rate.
The speed change operation is controlled prim~rily by fall back decision logic blocks 20 and 120, eall forwsrd decision logic blocks 22 Rnd 122 in conjunction with the auto speed state sequencers 24 and 124. The operation of these functional blocks as well as their relationship to other functional blocks is described in the flow chart shown in FIGURE 2. For ease of illustration and description FIGURE 2 has been subdivided into FIGURES 2A through 2F.
The process begins at start block 200 after which a series of three checks 202, 204 and 206 are made to deterrnine if any tests are active, if auto speed has been disabled or if the external clock has been enabled. A positive response to any of these three queries leads the process to block 208 of FIGllRE 2B where the signal quality (SQ~

-17- 12~i~22S

integration timer i9 initialized. Once the signal qunllty integration t3mer i9 initlalized all other timers, flags and command registers are initlalized at step 210 snd the process ends at 2l2. It will be understood that ~ince the process Is repetitive, control always passes ~rom 212 to 200.
Stste sequencer 24 (or 124) may assume any o~ five poss~ble states 1 through state 5 of FIGURE 2A. When negative response3 are received in response each of tests 202, 204 and 206 the route taken by the proeess at the output of deci~ion block 206 i9 determined by the ~tate o~
the state sequencer at that point.
l~ the state sequencer i5 in 3tate 1, wh~ch repre~ents an idle state where no commands are being sent or received, then process control goes to deci~ion block 212 o~ FIGURE 2B If a command is received at 212 control o~ the process goes to decision block 214 o~ FIGURE 2D. This is command c~used by input ~rom the ~ontrol p~nel or the RS a3a interface. Otherwise, control passes to decision block 213 o~ FIGURE
2E~. If there has been a ~trap change or power up, a ~tatu3 command (reintialize~ master/slave relationship) i~ set up at 215. An acknowledgment timer (implemented In 24) which allows ~or a predetermined time period to wait for an acknowledgment to the speed change command is started at block 220. The state o~ the machin2 is changed to state 2 at 222 and a T1 command i~ sent at 224. The proeess then goes to step 212. I~ a strap or power has not ch~nged then central passe~ to decision block 216 of FIGURE 2B. If a modem speed command is to be sent at block 216 the command is set up at 218. If a modem speed command is not to be sent at 216, then state sequeneer 24 inspects the EIA speed selector logic 28 to determine if it enabled at step 226. If the EIA speed select is not enabled at 226 then control passes to decision block 228. If a fall back command is not to be sent at 228 then the modem checks to see i~ it is the master or the slave at 230. If it is the slave at 230 then control passes to decision block 232 of FIGURE 2C.
Otherwise control passes to block 234. If a fall forward command is not to be sent at 234, control sirnilarly passes to 232 of FIGURÆ 2C. If the fall forward command is to be sent, then the speed is checked at 236 to determine if the speed is greater than 9.6 KBPS and less than 16.8 KBPS.

-18- ~Z~ Z25 If so, the fall forward command is set at 238. Control then passes to decision block 240 where it is determined whether or not the TX FSK
transmitted is enabled. That is, it is determined whether or not the system is utilizing secondary channel capabilitiesO If so, the acknowledgment timer (implemented in 24) is set at 242, state 2 is set at 246 and the TX command is sent (using secondary channel3 at 248 and the process goes to step 212.
If TX is not enabled at 240, then the modems designation is changed to master at 250. The acknowledgment timer is set at 252 and state 2 is set at 254. The T1 command to change speeds is sent at 256 and control passes to block 212. If at step 228, a fallback command i9 to be sent, the speed is inspected 258 to determine if the speed is greater than or equal to 14.4 KBPS. If so, the fallback command is set up at 260.
Turning now to FIGURE 2C (which represents the decision process for setting either an upspeed or downspeed flag), at step 232 a check is made to deterrnine if the retrain timer is currently running. The retrain timer is a fi~ced duration timer implemented in 24 which establishes a timer interval in which a predetermined number of retrains may occur without requiring a fall back in ~odem speed. If the timer is running at 232, a check is made to determine if the number of retrains requested has e2cceeded a predetermined (strap selectable) threshold at 266. If so, a fall back reques. command is initiated at 268 and control is passed to block 270. If the retrain timer is not running then control passes to 233. If a retrain attempt has occurred then the retrain timer is started in step 235 and control passes to block 270. Control is passed to block 270 in the event of a negative response at blocks 233 or 266.
DCD (data carrier detect) is inspected at block 270. If DCD if off (indicating that the remote modem is squelched), the cost signal qu~lity is set to bad at 272 and control passes to block 274. Otherwise, control passes directly from 270 to 274.
At 274 the cost signal quality is checked to see if it is good. If not, control passes block 276. If so, control passes to 278 where a check is made to determine if the cost signal quality was bad previously. If so~
the duration mapped into the count (at 22) as described previously at 682;~5 230. At 278 if the cost signal quality was not bfld block 280 i9 bypassed and control goes directly to block 282.
At step 276 the duration on retrain timer o~ step Z32 i9 checked to determine if it is greater than or equal to the predetermlned threshold. If so the duration and the count Is satur~ted at 284 and control is passed to decision block 282. If the duration is not greater than or equal to the threshold at 276 then the duration is mapped into the count at 286 prior to passing control to block 282. At block 282 if the count is greater than or equal to the threshold the integrators are initialized at 288 and control is passed to ~12. If the count i~ less than the threshold at 282 and the signal quality integration/retry timer (implemented in 22) has expired at 290 a fall forward l~ requested at 292. If the count is less than the threshold at 232 and the signal quality timer has not expired at 290 control i5 passed directly to 212 bypRssin~
2g2.
Turning now to FIGURE 2D which describes how command~ are processed if an invalid command is received at block 214 control passes to block 208 of FIGURE 2B. Otherwise control passes to 300. If a status command is received at 300 the master/slave status of the modem is updated at 3Q2 after which control passes to block 208 of FIGURE 2B. If a status command is not received at 300 control passes to 304 which checks to see i~ a modem speed command has been received. I~ so a positive acknowledgment command is set up at 306 a carrier detect off timer is started at 303. State is set to five at 310 and the T1 command i5 sent at 312 after which control is passed to block 212. The carrier detect off timer of step 308 (implemented in 24) is used to time a duration for waiting for detection of transmitter squelch at the remote modem. In the preeerred embodiment this timer is set at 6 seconds but this is not to be limiting.
If a modem speed command is not received at 304 control passes to 314 where it is determined if a fall back command is received.
If so the status of the modem is set to slave at 316 after which control is passed to block 306. rf a fall back command is not received at 314 control passes to 318 where it is determined whether or not a fall forward i8~Z5 command has been received. If not, control passes to block 20~ Oe FIGURE 2B. If a ~all forward command is received at 318, a fall forward flag is inspected at 320 to determine whether or not it is active. If so, control passes to 306. The fall forward flag is used by state sequencer 24 to determine that it is appropriate to request command encoder 32 to transmit a speed increase request or to accept a speed increase request from the remote modem.
If the fall forward flag is not active at 320 then step 322 determines whether or not the TX secondary channel is enabled. 1~ it is, control passes to block 208 ot FIGURE 2B. lf it is not enabled at 322 the modem is set to master 324, a negative acknowledgment command is set up at 326 and the T1 command is transmitted at 328. Control is then passed to block 208 of FIGURE 2B.
If state sequencer 24 (or 124) is in either state 2 or 3 after step 206 of FIGURE 2A the state sequencer's actions are governed primarily by the flow chart of FIGURE 2E. If the state sequencer is in state 2, which represents an acknowledgment processing state (that is, a speed change command has been sent and an acknowledgment is e2~pected) then control is passed to step 340 where it is determined whether or not an acknowledgment has been received. If not, control is passed to 342 to determine if the acknowledgment timer has expired. (If the acknowledgment timer has e~cpired, the acknowledgment timer is started at 344, the state sequencer is set to state 3 and 346 and the T1 or TX
command is retransmitted at 348.) State 3 represents the acknowledge processing for a second attempt. Control then passes to block 212. If the acknowledgment at 342 is not positive, control passes direetly to block 212 by passing steps 344, 346 and 348.
If an acknowledgment is received at 340 the validity of the acknowledgment is inspected at 350. If the acknowledgment is invalid at 350 control passes to block 344 (second attempt is initiated) otherwise, control passes to block 35~. ~t block 352, the acknowledgment is inspected to determine if it is positive. I it is positive, the local modem speed is changed at 354, the transmitter is squelched at 356. The squelch timer is- started at 358 and the state sequencer is set to state 4 at 360.

~6~Z~:5 .,, lf a positive acknowledgment is not received at 352, control passes to block 362 where it is determined if a fall forward commsnd is being sent. If so, control passes to 364 to determine if TX is enabled. If TX is enabled then control passes to block 208 of FIGURE 2B. Otherwise the modem is set to slave at block 366. If the decision at block 362 is no, control passes to block 208 of FIGURE 2B.
If the state sequencer is in state 3, which indicates that an invalid acknowledgment was received in state 2 and an acknowledgment is now being awaited in response to a retransmitted fall forward request, then control is passed to block 370 where it is determlned whether or not an acknowledgment has been received. If not, and the acknowledgment timer has not expired"n block 372, then control i~ passed to 212. I~ the acknowledgment timer has expired at 372 taken control is passed back to block 208 of FIGURE 2B.
If an acknowledgment is received at 370 then its validity i9 inspected at 374. If it is invalid, then control is passed to block 208 of FIGURE 2B otherwise, control is passed to 376. If acknowledgment is not positive at 376 then control is passed to block 362.
If the state sequencer i9 in either state 4 or state 5, its actions are governed by the flow charts of FIGURE 2F. If the state sequencer is in state ~ it is indicative of a valid acknowledgment having been received by the master (initiator) modem in state 2 or 3 and the transmitter is squelched awaiting receipt of a training sequence from the remote modem. Once the machine is in state 4, control is passed to block 380 2 5 where it is determined whether or not a training sequence has been received. If so, control is passed to 382 where it is determined whether or not TX is enabled. If TX is not enabled, control passes to block 208 of FIGURE 213. If however, TX is enabled indicating that secondary channel diagnostics are being utilized, and a network controller is present in the system, then a speed Mayday is generated at block 384 prior to passing control to 208. The speed Mayday generated by 384, informs the network controller of the new data rate (see the Rosbury et al., patent).
If a training sequence is not received at 380, then the squelch timer is inspected at 386 to determine whether or not it has expired. If -22- ~26~ZZS

not, control passes to 212. If the squelch timer has expired, however, the transmitter is unsquelched at 388 prior to passing control to block 208.
If the state sequencer is in state 5 which is indicative of the slave (respondent) modem awaiting squelch so that a training sequence can be sent, then control is passed to block 392. At bloak 392 carrier detect is inspected to determine whether or not it is off. If carrier detect is off at 392 then the modem speed is set according to the speed command received at step 394 and a training sequence is transmitted at 396 90 that communications can be established at the modifled speed. Control is then passed to block 2a8 of FIGURE 2B If carrier detect is not off at 392 and the squelch timer (implement~d in 24) has not expired in 398 then control is passed to 212. If the squelch timer has expired in 398, control is passed to block 208 of FIGURE 2B.
In the preferred embodiment the present invention ls implemented utilizing microprocessor technology and the above state sequencer is in the form of a micro computer. However, this is not to be limiting as one skilled in the art will recognize that a dedicated state machine could be designed to provide the functions described above. In the preferred embodiment of the present invention, the variou~ timers described in conjunction with the flow chart operation are implemented in software also. It will be clear to those skilled in the art, however, that the preferred values of these timers may be modified without departing from the spirit of the present invention. It will also be clear that such timers may also be implemented in hardware using known circuitry.
Turning now to FIGU~E 3, a hardware block diagram of the preferred embodiment of the present invention is shown Data Terminal Equipment (DTE) 402 is coupled via a standard connection such as RS-232 to an interface/timing circuit 404 and a modem controller 406.
Interface/timing circuit 404 is coupled to a computational processor 408 which includes a transmit microprocessor 410. Transmit microprocessor 41~ is preferably a custom microprocessor such as that disclosed in U.S.
Patent No. 4,541,045 to l~romer, III., -Transmit microprocessor 410 communicates with received processor A 412 and received processor B 414 via a mailbox RAM 416.

~23- 126~3Z~5 modem controller 406 is coupled to TX/RMC controller 418, option controller 420, and a common random access memory 422. Option controller 420 is eoupled to a control panel 424 for reeeiving user input.
Modem eontroller 406 is also coupled to an analog/digital interface 426 whieh is eonnected to interfaee/timin~ cireuit 404 as well as eomputational processor 408. TX/RMC controller 418 is coupled to an external network controller 30 in the preferred embodiment. TX/RMC
controller 418 provides and receives serial data to and from a low speed FSK modem 430. Filters 432 and 434 are used to filter serlal low speed FSK data to and from modem 430.
Reeeived data passes through an isolation transformer 440 and is amplified by an amplifier 442 prior to being divided between filter 434 for proeessing by the low speed FSK modem and a Pilter 444 whieh îilters the user data and applies it to an automatie gain eontrol eireuit 448.
AGC eireuit 446 drives and analog to dlgital eonverter 448 whieh provides digital data to analog/digital eonverter 448 vla interfaee 426.
Outgoing user data i5 flrst eonverted to unalog signals by digital to analog eonverter 450 prior to Iiltering by filter 452 and eombining at adder 454. The output of adder 454 is amplified by an amplifier 456 and provided to transmit isolation transformer 460.
The interfaee/timing eireuit 404 serves to generate transmit and reeeive eloeks for the DTE as well as providing serisl to parallel eonversion ~or the transmit data and parallel to serial data for the reeeived data. Interfaee/timing eircuit 404 also ineludes logie and other 2 5 cireuitry for per~orming digital loop tests as is known in the art.
Transmit mieroprocessor 410 eonfigures the interfaee/timing cireuit 404 and reads the transmit data and writes the received data through its parallel interface thereto. The analog/digital interface 426 generates the baud clocks and some of the bit clocks used by the interface/timing circuit 404.
The analog/digital interface provides bit and baud clock generation as well as interface between the modem controller and the transmit processor. The modem controller 406 configures the analog/digital interface through a parallel interface. The transmit , ,; . . ~
,.

-2~-IL26~ZZS

processor sends and receives configuration and status information from the modem controller 406 through the analog/digital inter~ace.
The transmit processor 410 of computatlonal processor 408 scrambles the transmit data read from the inter~ace/timing circuit 404 S and encodes the scrambled transmit data. Transmit microprocessor 410 also generates transmit symbols from the encoded transmit dats and generates the transmit samples for the ~digital to analog converter.
Transmit microprocessor 410 also controls the AC:C based on energy in~ormation derived from the samples from the analog to dlgital converter iû and from the received processor. Transmit microproce~sor 410 also controls the received timing generation on the analog/dlgital ~nter~ace based on information coming ~rom the re~elved processors. Transmit microprocessor 410 is also used to wrlte re~e~ved data to the interface/tlm~ng circuit 404 and exchange statu~ and con~iguration information with the modem controller 406. Received processors 412 and 414 get received samples, conf3guratlon and status information ~rom the transmit processor through the mailbox RAM ~60. Clock in~ormatlon i~
recovered from the received channel and the rece~ved samples are phase split and equalized and then sliced into symbols. Viterbi decoding (the functional blocks of FIGURE 1 associated with the pre~ent speed change invention) is implemented in proces~or ~14 to convert the symbols into data in a known manner. Unscrambling is also implemented in processors A and B on the decoded data. Processor~ 412 and 414 also update equalizer taps based on Viterbi decoder cost function and calculates signal quality and fall forward flags based on the Viterbi decoder cost ~unction.
Modem controller 406 sends configuration iniormation to The analog/digital interface 426, interface/timing circuit 404 and the receive and transmit processors. Transmitted and receive related DTE s3gnals based on DTE interface and transmit processor information are processed by the modem controller also. Modem controller 406 also generates master timing for the comn~on RAM access and implements the auto speed state sequencer and fallback and fallforward decision logic (20 and 22). Modem controller 406 also controls the tests that the system can perform .

, . ... . .

``" :126;~ZZS
Option controller 420 implements control panel control as well as keeping and managing system configuration information. Option controller 420 also controls the common RAM access for any options and manages commanication between the modem and the options.
The TX/RMC controller ~18 operate~ in one of two mode~. In the TX mode, it serves to communicate to the network controller through a serial interface on a central site or through secondary channel on a remote site. It also serves to translate network diagnostic and management commands into modem commands for putt~ng a modem into various test mode~ and requesting StEltUS information or generating alarms. In the RMC mode the TX/RMC controller 418 serves to communicate to remote modems through the secondary channel. It also serves to translate RhlC commands into modem commands vice versa for conducting tests, requesting status inforrnatJon or generating alarms. In the preferred embodiment, received processors 412 and 414 are preferably general purpose microcomputers based upon the Texas Instruments x TMS
3020 series microprocessors but this is not to be limiting as other high speed microprocessors could be utilized to implement the present invention.
2 0 THUS it is apparent that in accordance with the present invention an apparatus that fully satisfies the objectives, alms and advantages is set forth above. While the invention has been described in conjunction with a specific embodiment, it is evident that many alternatives, modifications and variations will become apparent to those skilled in the art in light o~ the foregoing description. Accordingly, it is intended that the present invention embrace all such alternatives, modifications and variations as fall within the spirit and broad scope of the appended claims.

Claims

WHAT IS CLAIMED:

1. In a data communication system including a first modem and a second modem coupled together through a transmission channel, said transmission channel including a first path for transmission from said first modem to said second modem and a second path for transmission from said second modem to said first modem, a method for coordinating a change in the transmission rates of said first and second modems, the method comprising the ordered steps of:
attempting to train and retrain said modems;
determining, at said first modem, that said second path of said transmission channel is adequately high in quality to support an increase in transmission rate;
transmitting a request to increase speed from said first modem to said second modem;
determining, at said second modem, that said first path of said transmission channel is adequately high in quality to support an increase in transmission rate wherein the step of determining includes mapping a duration of time wherein said signal quality is deemed to be poor into a count, accumulating said count to produce an accumulated count and comparing said accumulated count to a predetermined maximum count for a predetermined time line so that upspeed decisions can occur with a moderate amount of transient impairment; and wherein said mapping step includes assigning integer values to time periods of poor signal quality such that larger integers are assigned to longer periods of poor signal quality and smaller integers are assigned to shorter periods of poor signal quality, and the count is totaled for said time limit;
acknowledging said request to increase speed from said first modem by transmitting an acknowledgement signal from said second modem to said first modem;
squelching transmission at said first modem;
transmitting a training sequence at said increased speed from said second modem to said first modem; and transmitting a training sequence at said increased speed from said first modem to said second modem.

2. The method of claim 1, further comprising the steps of:
establishing, at either one of said first and second modems, that either path of said transmission channel is inadequately high in quality to support the current transmission rate;
transmitting a training sequence at a decreased transmission rate from said one modem to the other of said first or second modem; and transmitting a training sequence at said decreased transmission rate from said other modem to said one modem.

3. The method of claim 2, wherein said establishing step includes the steps of:
counting a first number of retraining attempts occurring within a second predetermined time period;
comparing said first number with a first predetermined maximum count; and ensuring that said first predetermined count is exceeded by said first number.

4. The method of claim 2, wherein said one modem is designated master and said other modem is designated slave, and wherein future increases in transmission rate are required to be initiated by said master.

5. The method of any one of claims 1 to 3, further including the steps of:
starting a first timer of first predetermined duration upon transmitting said request; and retransmitting said request if an acknowledgement has not been received prior to expiration of said first timer.

6. The method of any one of claims 1 to 3, further including the steps of:

starting a first timer of first predetermined duration upon transmitting said request; and retransmitting said request if an acknowledgement has not been received prior to expiration of said first timer and further including the steps of:
restarting said timer upon retransmitting said request;
starting a second timer of second predetermined time period if said first timer expires without receipt of said acknowledgement, said second time period being much longer in duration than said first time period; and inhibiting further retransmitting of said request until after second timer has expired.

7. The method of any one of claims 1 to 3, wherein said request to increase speed is transmitted via inband signaling.

8. The method of any one of claims 1 to 3, wherein said determining steps are carried out by evaluating the average minimum cost output of Viterbi decoder.

9. The method of any one of claims 1 to 3, wherein said mapping is approximately given by:

and said time limit and maximum count are approximately selected from:

10. In a data modem, a circuit arrangement for effecting transmission speed changes between a pair of modems comprising in combination:
training means for attempting to train and retrain said modem;
first means for establishing the quality of transmission line;
second means for counting retrain attempts as a measure of line quality;
fallback decision means, responsive to said second means, for producing an output representing a demand for reduction in transmission speed whenever a predetermined number of retrain attempts occur within a selected period of time;
fallforward decision means, responsive to said first means, for producing an output representing a request for increase in transmission speed whenever the average signal quality is greater than a predetermined threshold;
establishing means for establishing said data modem as having an identity designated as either master or slave, including means for determining if said modem is the most recent of said pair of modems to demand a speed reduction, and wherein said modem is designated master in the event it is said most recent requester;
transmitting means for transmitting requests for increases and demands for reductions in transmission speed over said transmission line; and inhibiting means for inhibiting said transmitting means from transmitting requests for increases in speed unless said modem's identity is that of master.

11. The data modem of claim 10, wherein said transmitting means includes a low speed secondary channel FSK tranmitter.

12. The data modem of claim 10, wherein said transmitting means includes an inband transmitter.

13. The data modem of claim 11, wherein said transmitting means includes an inband transmitter.

14. The data modem of any one of claims 10 to 12, wherein said establishing means includes means for determining it said modem is the most recent of a pair of modems to transmit a negative acknowledgement to a speed change request, and wherein said modem is designed master in the event it is most recent.

15. The data modem of any one of claims 10 to 12, further comprising:
means, coupled to said establishing means, for detecting application of a power source, and wherein said establishing means establishes the identity of said modem as well as a remote modem, upon application of a power source.

16. The data modem of any one of claims 10 to 12, further including a network controller and means for alerting said network controller of speed changes.

17. The data modem of any one of claims 10 to 12, further including:
means for acknowledging requests for increases in transmission speed;
means for squelching a transmitter in response to said acknowledgement; and means for transmitting training sequences at a variety of transmission rates.