WO2002039762A2

WO2002039762A2 - Method of and apparatus for detecting tty type calls in cellular systems

Info

Publication number: WO2002039762A2
Application number: PCT/US2001/044142
Authority: WO
Inventors: Ayman Mostafa; Lundqvist Patrik
Original assignee: Ericsson Inc.
Priority date: 2000-11-07
Filing date: 2001-11-05
Publication date: 2002-05-16
Also published as: WO2002039762A3; AU2002228646A1

Abstract

A cellular system detects, at system level, whether a calling or a called party is a TTY type user, and in response to a positive identification allocates resources to service the TTY mode call. Several techniques are provided for detecting occurrence of TTY mode. According to one, either the calling or called user originates a function code identifying to the cellular system that the call, or part of the call, is in TTY mode. According to another, identifications of subscribers as TTY type user's are entered by service operators, at the time of initiation or change of service, into an HLR and/or a VLR of an MSC, to enable the cellular system to identify TTY type users. It also is contemplated that the cellular system detect specific cues indicative of a user being a TTY type user, and from such cues built and update TTY type user databases that will enable the cellular system to subsequently identify occurrence of a call by a TTY type user.

Description

METHOD OF AND APPARATUS FOR DETECTING TTY TYPE CALLS IN CELLULAR SYSTEMS

BACKGROUND OF THE INVENTION

There is a need to provide enhanced text capabilities for the deaf, hearing impaired and/or speech impaired communities. This is consistent with a social goal of integrating handicapped individuals into traditional society which is, in part, illustrated by two recent statutes: the Americans with Disabilities Act (ADA) and the Telecommunications Act of 1996. Among other things, these statutes require manufacturers and telecommunications service providers to enhance existing approaches used by members of the hearing and/or speech impaired communities to access and utilize telecommunications networks and systems. For instance, the ADA generally requires that handicapped persons have equal access to public facilities. Among other things, 47 U.S.C. §225(b)(1) states that the Federal Communications Commission "shall ensure that interstate and intrastate telecommunications relay services are available, to the extent possible and in the most efficient manner, to hearing impaired and speech-impaired individuals in the United States." Consequently, over the past several years, it has become imperative for telecommunications service providers to develop systems, processes, and apparatus that enable such individuals to use telecommunications networks and systems.

Hearing and/or speech impaired individuals often communicate with others via a telecommunication device for the deaf (TDD). The TDD is primarily a teletypewriter (TTY) electronic device that permits such individuals to communicate over the telephone without the aid of an interpreter. Hearing and/or speech impaired individuals sometimes use

TTY-to-speech relay services to communicate with hearing/speaking persons. These services, such as voice carry-over (VCO) and hearing carry-over (HCO), are generally provided by a group of people who serve as relay operators. Essentially, the relay operators read TTY text from a speaking impaired person and vocalize the message to a hearing person, and also hear a message from a speaking person and type the TTY text to a hearing impaired person.

In addition to using the public switched telephone network (PSTN), the Federal Communications Commission (FCC) requires that hearing and/or speech impaired individuals be able to use TTY devices connected to cellular phones to place emergency 911 calls. To be able to handle such calls, the cellular system must be able to recognize that communication is to occur in a TTY mode via a TTY device, so that the system can reliably accommodate such communication.

Two major solutions have been offered to support TTY users of cellular systems. One is referred to as a vocoder-based solution. A vocoder is hardware/software that uses a codebook to implement a compression/decompression algorithm particular to voice, i.e., a vocoder is a speech encoder/decoder. The vocoder solution, in an attempt to accommodate TTY signals in the cellular system, requires modification of the vocoder software in the user's mobile terminal or phone and in the mobile switching center (MSC). The other solution, referred to as the modem-based solution, does not require any change to either the vocoder software or air interface standards for cellular systems. The mobile solution has recently been adopted by the Global System for Mobile Communications (GSM) Standards Body

TTY/TDD devices transmit characters using 45.45 baud or 50 baud analog Baudot code. At 45.45 baud, each bit has a nominal duration of 22 ms. A character consists of 7.5 bits, comprising 1 start bit, 5 data bits and 1.5 stop bits. There are two character sets, namely letters and figures, and there are two special characters used to switch between the character sets. As transmitted, TTY tones are carrierless frequency shift keying (FSK) signals. A mark or 1 bit is transmitted with 1400 Hz tone and a space or 0 bit is transmitted with 1800 Hz tone.

A problem exists in that current digital cellular technology cannot reliably transmit analog TTY tones without degradation, and therefore cannot reliably detect occurrence of TTY communications. When sending TTY tones in a digital cellular system, the analog TTY tones (and analog voice signals when present) are encoded or compressed at the transmit side by a vocoder, in order to increase air interface transmission efficiency. However, the vocoder and its codebook are optimized for encoding speech signals, not for encoding frequency tones, and in particular not for encoding TTY tones. In consequence, the transmitting vocoder does not accurately encode the TTY tones, and the ensuing decoded TTY tones do not accurately represent the original TTY tones. Because of this transmission impairment and other impairments resulting from phenomena such as Rayleigh fading, the decoded TTY signal at the receive side may degrade to an unacceptable character error rate (CER), which makes detection of TTY mode and transmission of TTY tones through the digital cellular system unreliable.

The problem is not present with analog cellular technology, which does not use vocoders.

SUMMARY OF THE INVENTION

The present invention addresses the situation where a deaf, hearing impaired and/or speech impaired individual uses a TTY terminal to communicate over a cellular network, and must not have his/her communication be distorted and subject to transmission errors. The invention solves the problem by providing a method and apparatus whereby the cellular system recognizes, at system level, that the individual is a

TTY type user. Having made that recognition, the system can then allocate the proper resources to service the TTY user's transmission, so that it will be sent accurately and without distortion. In this connection, the invention provides an apparatus for identifying and servicing TTY type mobile terminal users in a cellular system. The apparatus comprises, in combination with cellular mobile terminals for transmitting messages, a mobile switching center (MSC) for receiving and servicing messages transmitted by the mobile terminals. A detector associated with the MSC is responsive to the MSC receiving a message from a mobile terminal for determining whether a user of the mobile terminal is a TTY type user. If a determination is made that the user is of a TTY type, then a control responsive to that determination operates the MSC in a manner to allocate the proper resources to service the message from the mobile terminal.

Several techniques are contemplated for the apparatus of the invention to enable a determination to be made, at the MSC level, as to whether a mobile terminal user is a TTY user, so that proper resources can then be allocated to service the user's call. According to one technique, the detector for determining whether a mobile user is a TTY user is responsive to the MSC receiving from the mobile terminal a function code indicative of a TTY type user, which function code may be transmitted via a fast access control channel (FACCH) message or a slow access control channel (SACCH) message. According to another technique, the detector is responsive to the MSC receiving from the mobile terminal a service code indicative of a TTY type user. The MSC has a database that includes a home location register (HLR) and a visitor location register (VLR), in which is stored mobile terminal user profile data, and in a further embodiment the stored user profile data includes identifications of those users who are TTY type users. The MSC can then draw upon such identifications to determine whether any particular user is a TTY type user, and thereby service such user's call appropriately. A control system is provided for entering into the database such user profile data, advantageously in a first instance by a cellular operator at the time a TTY user subscribes for or changes service. The control system is also contemplated for entering and updating the database user profile data automatically in response to sensing occurrences, at the MSC, of messages transmitted from mobile terminals that are indicative of such mobile terminal users being TTY type users. Should a determination be made at the MSC that the mobile terminal user is a TTY type user who has been assigned a digital channel, it is further contemplated that the MSC can force a handoff from the digital channel to an analog channel, which is advantageous since an analog channel does not introduce distortion to a TTY signal.

According to a method of the invention, a process is provided for identifying and servicing TTY type mobile terminal users in a cellular system. Included are the steps of transmitting messages from cellular mobile terminals, and receiving and servicing the messages with a mobile switching center (MSC). Also included are the steps of determining, in response to receiving at the MSC a message transmitted by a mobile terminal, whether a user of the mobile terminal is a TTY type user, and in response to a determination that the user is a TTY type user, operating the MSC to allocate resources to service the message in TTY mode.

The method contemplates several techniques for identifying TTY type calls.

According to one, the determining step is responsive to the MSC receiving from the mobile terminal a function code indicating that the mobile terminal user is a TTY type user. The function code may be transmitted via a fast access control channel (FACCH) message or a slow access control channel (SACCH) message. It also is contemplated that the MSC receive from a mobile terminal a service code indicative of a TTY type user.

Alternatively, there can be the step of storing identifications of TTY type mobile terminal users, so that the determining step then comprises determining whether there is a stored indication of the mobile terminal user as a TTY type user. In this case, the storing step can comprises storing mobile terminal user profile data, including data identifying TTY type users, that advantageously is entered in a first instance by a cellular service operator. Additionally, the stored user profile data can be developed and updated automatically by storing identifications of individual TTY type users in response to the determining step making the determination that such mobile terminal users are TTY type users. If a determination is made that a user is a TTY type user, and if the TTY type user's mobile terminal has been assigned a digital channel, the MSC can force a handoff from the digital channel to an analog channel. Other objects, advantages and features of the invention will become apparent upon a consideration of the following description thereof, when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figs. 1 a-c are block diagrams showing implementation in a digital cellular system of a modem tone solution for transmitting TTY tones;

Fig. 2 is a block diagram of a cellular system of a type with which each of a function code identification technique, an HLR/VLR user identification technique, and a service code identification technique of the invention may be employed for system level detection of TTY mode;

Fig. 3 is a block diagram of an alternate embodiment of the Fig. 2 cellular system;

Fig. 4 is a block diagram of a mobile switching center (MSC) of a cellular system, showing implementation of an intelligent cues technique of the invention for system level identification of TTY users, and

Fig. 5 is a flow chart showing operation of an analog channel handoff technique contemplated by the invention.

DETAILED DESCRIPTION

The Federal Communications Commission (FCC) requires wireless service providers to support emergency calls, e.g. 911 calls, for users with hearing and/or speech disabilities using devices such as TTY/TDD devices coupled to cellular handsets.

TTY/TDD devices transmit characters using 45.45 baud or 50 baud analog Baudot code.

Analog cellular channels readily transmit analog Baudot code without degradation.

However, digital cellular channels are not able to transmit analog Baudot code without introducing degradation, and two major solutions have been offered for decreasing degradation of TTY tones transmitted in digital cellular systems. One is referred to as the vocoder solution and the other is called the modem solution. Each solution requires for its operation that the occurrence of TTY tones or TTY mode be detected, but neither satisfactorily solves the problem of reliably detecting TTY mode.

Vocoders used in digital cellular systems, in combination with bit errors caused by radio channels, is the cause of the degradation introduced to TTY tones. Vocoders are used for speech encoding/decoding and consist of hardware/software that implements a compression/decompression algorithm particular to voice signals. When a vocoder of a transmit side terminal receives an analog voice or TTY signal to be sent over an air interface, it encodes the signal to provide a compressed digital signal for efficient transmission over the air interface. The problem of degradation of TTY signals occurs because vocoders and their codebooks of adaptive code gains are optimized for voice signals, and therefore are not well adapted to encode TTY signals. Thus, the decoded

TTY signal that occurs at a receive side terminal is not necessarily an accurate representation of the original TTY signal, and is not necessarily recognized by the receive side terminal as a TTY signal. In the vocoder solution, also known as a no gain solution, the vocoder algorithm has a new block known as tonal detector, which detects the presence/absence of TTY tones and takes appropriate action. The vocoder effects compression/decompression (encoding/decoding) using a codebook of code transformations and an adaptive code gain, and if the tonal detector detects TTY/TDD tones, the adaptive code gain is forced to zero at the transmit side. This makes adaptive codebook bits (ACB) not relevant to an encoder of the transmit side vocoder, and the TTY information then occupies the bits reserved for the ACB. On the receive side, a decoder of a vocoder monitors the adaptive code gain bits looking for frames with zero gain. The decoder maintains a history of 11 frames, and when a TTY character is detected in response to a selected number of frames having adaptive code gain bits with zero gain, the receive side decoder regenerates the tones corresponding to that character.

Because the time span of a TTY TDD character is at least 8 speech frames or 165 ms (1 Baudot character is 7.5 bits and 1 bit is 22 ms), the character is repetition coded and sent 7 times by the transmit side encoder to the receive side decoder, allowing the receive side to better be able to correctly regenerate the character despite any frame errors due to channel conditions. Should there be an unmodified vocoder in the receive side, which has no knowledge of TTY encoding changes made by a vocoder in the transmit side, the receive side vocoder will decode and reconstruct the TTY Baudot signal with fixed codebook and linear prediction filter as though the encoder in the transmit side were not modified. This gives cellular system interoperability in the case of unmodified terminals or vocoder function in a mobile switching center (MSC) communicating with a TTY-modified vocoder in the remote end. All changes are confined to the vocoder software.

Disadvantages of the vocoder solution are that vocoder standards have to be changed, as well as vocoder software in the caller's terminal and in the MSC. Also, while the solution is specific to vocoder technology, details of algorithms have to be applied to every coding technology. This puts limits on available vocoder bits and on creativity in the R&D of voice coding. Further, the solution has to be implemented in all models of hardware existing in the market.

The modem solution, also called a modem tone solution, has been adopted by the Global System for Mobile Communications (GSM) Standards Body. The modem solution permits transmission within a digital speech channel by means of using improved modem signals, requires no change of mobile cellular standards and enables existing cellular equipment to be used. The modem solution provides for TTY information to be transmitted as a new analog signal, by transforming the analog TTY signal to a new analog FSK signal that can reliably pass through a vocoder and a time division multiple access (TDMA) air interface without significant degradation. At a receive side, the new FSK signal is then converted back to the original TTY tones. To allow for error correction and for a more robust protocol, the bit rate of the TTY signal is raised at the transmit side. Fig. 1 a shows one manner in which the modem solution may be implemented in a digital cellular system. A TTY/TDD tonal detector 20 and a new modulator 22 are embodied in a smart cable or adapter 24 in the transmitter side. The adapter connects between a TTY/TDD device 26 and a radio modem 28, only a speech encoder 30 of a transmit side cellular mobile terminal and a speech decoder 32 of a receive side of an air interface being shown. The new modulator 22 comprises a channel encoder 34, an interleaver + sync pattern generator 36 and a modem^" tone modulator 38, as seen in Fig. 1b. At the receiver side, a TTY unit 40 includes a new demodulator 42 and a TTY/TDD tone generator 43. The new demodulator 42 comprises a modem tone demodulator 41, a de-interleaver + sync pattern receiver 44 and a channel decoder 45, as seen in Fig. 1c. The speech decoder 32 and TTY unit may be embodied in either a mobile switching center (MSC) or in a receive side cellular mobile terminal. At the transmitter and receiver sides, the adapter 24 and the TTY unit 40 include respective bypass means 46 and 48 that allow analog signals that are not detected to be TTY/TDD tonal signals, i.e., voice or data signals, to bypass the TTY/TDD tonal detector 20 and new modulator 22, and to bypass the new demodulator 42 and the TTY/TDD generator-43.

Upon occurrence of TTY tones at the transmitter side, the TTY/TDD tonal detector

20 detects and converts the TTY tones to binary information bits. As seen in Fig. 1 b, the information bits are input to the channel encoder 34 of the new modulator 22, which generates at its output a multiple of the input bits, such that one input bit results in a plurality of output bits of the same 0 or 1 state as the input bit. This introduces redundancy to the signal and provides repetition encoding. The repetition-encoded bits from the channel encoder are input to the interleaver + sync pattern generator 36, which interleaves or scrambles the bits and provides a sync pattern for synchronizing the receiver side with the transmitter side. The output from the interleaver + sync pattern generator 36 therefore includes an expanded over time and scrambled representation of the TTY information bits, which enables the information bits to be more accurately recovered and read should signal interference occur. The interleaver + sync pattern generator output is applied to the modem tone modulator 38, which generates a new analog signal comprising frequency shift keying (FSK) tones that consist of simultaneously occurring pairs of tones representing the 0s and 1 s of the TTY digital output bits from the interleaver + sync pattern. The FSK tones comprise four frequencies, 400 Hz, 600 Hz, 800 Hz and 1000 Hz, which as combined in pairs have been found to degrade much less, when passing through a speech encoder of a vocoder, than do conventional TTY tones that occur singly at 1400 Hz and 1800 Hz. The FSK tones from the new modulator 22 are applied to the speech encoder 30 of the radio modem 28 and output from the speech decoder 32 to the modem tone demodulator 41 of the new demodulator 42 of the TTY unit 40 of the MSC. The output from the modem tone demodulator is applied through the de-interleaver + sync pattern receiver 44 and the channel decoder 45 to the TTY/TDD generator 43. The TTY/TDD generator restores the TTY tones.

Thus, in the modem solution the TTY tonal signal from the TTY/TDD device 26 is converted to a new analog signal, i.e., an FSK signal that experiences less degradation in passing through a voice encoder than would the original TTY tones under similar radio channel conditions. Also, the FSK signal is encoded with 1/7 repetition code for error correction and detection.

Advantages of the modem tone solution are that no change is required to the software (SW) of current cellular mobile terminals. Existing TTY terminals can be used with a smart cable adapter, new mobile terminals can integrate the smart cable adapter functionality, and no changes to cellular system standards are required.

Each of the vocoder and modem solutions requires reliable detection of TTY tones to determine the occurrence of TTY mode. If the TTY/TDD tonal detector 20 fails to detect occurrence of TTY mode, unmodified TTY tones will pass through the transmit side vocoder and be degraded. On the other hand, if the TTY/TDD tonal detector falsely detects voice signals as TTY tones, the voice signals will either be encoded with the adaptive code gain forced to zero in the case of the vocoder solution, or will be passed through the new modulator in the case of the modem solution, and suffer voice quality degradation. Presently, both the vocoder and modem tone solutions rely on signal processing before the speech vocoder to detect TTY mode, by correlating tones and their duration with TTY tones, i.e., 1400 and 1800 Hz ±5% for 20 or 22 msec ±0.4 msec. The reliability of this TTY mode detection technique leaves room for improvement.

In overcoming the disadvantages of prior TTY mode detection, the invention provides several techniques for positively identifying, at the network or system level, whether a calling or a called party is a TTY type user. The invention contemplates that a determination be made whether a user's mobile terminal is capable of supporting TTY calls, i.e., if it is either a mobile terminal with a TTY mode implemented in it or a mobile terminal without a TTY mode implemented in it, but attached to a TTY smart cable. Advantageously, detection of TTY mode occurs at cellular system level, which eliminates the need for TTY tonal detection to occur at a user's mobile terminal.

USE OF FUNCTION CODE

According to a function code TTY user identification technique contemplated by the invention, the particular mode of operation of the user's mobile terminal, i.e., voice only or TTY mode, is transmitted by the user's terminal via the air interface to the MSC. It being understood that TTY mode is also capable of supporting voice, this is accomplished by using a function code similar to those used in call conferencing or call forwarding, but unique to TTY mode. A function code, such as * 123, is originated by the user's terminal and sent via a fast access control channel (FACCH) message or a slow access control channel (SACCH) message to an MSC. The function code identifies to the MSC that the call, or part of the call, is in and needs to be handled in the TTY mode. In response to receipt of the TTY mode function code, the MSC allocates the proper resources to service the TTY mode call, which includes routing the call through a TTY unit or TTY function until another and different function code is received by the MSC. The TTY mode may be displayed on a screen of the user's mobile terminal upon switching from regular voice mode (default) to TTY mode. An advantage of the function code user identification technique is that it can support hearing carry-over (HCO) and voice carry-over (VCO) mode requirements of the TTY forum.

As is apparent, operations identified herein as being performed in an MSC can be performed in other fixed network components. For consistency herein reference will be made to the MSC relative to implementation of the invention in the fixed network side.

The function code technique is not limited to initiation from the transmit side in the call originating case. It can also be initiated from the receive side in the call terminating case. Here, a called cellular TTY user, after the call is set up and the called user receives a call alert, can force the call to the TTY mode by keying the function code to the system via a FACCH message.

Use of the function code technique for informing the MSC of the need to handle all or part of a call in the TTY mode, for example to support the HCO/VCO mode requirement of the TTY forum, is an effective, simple and versatile means for selecting and precisely controlling modes of operation. A particular advantage is that implementation of the technique in existing cellular systems does not require air interface standards changes.

Fig. 2 illustrates a digital cellular system that is adapted to utilize the function code technique of the present invention to enable system level TTY mode detection. The modem solution is used in the cellular system and a smart cable or adapter 50 interconnects a mobile terminal 52 and a TTY terminal 54. The smart cable performs the same functions as does the adapter 24 in Fig. 1 , and converts a TTY signal from the TTY terminal to an analog FSK signal, the frequencies of which experience less degradation in passing through a vocoder than would the original TTY tones. The mobile terminal 52 uses an antenna 58 to engage in radio-frequency communication with a base station (BS)

56 having an antenna 60.

The BS 56, in turn, is connected by radio frequency or hard-wired communications links to a mobile switching center (MSC) 62. The MSC 62 processes requests for service from the mobile terminal 52 and routes calls to and from the mobile terminal 52 and other destinations. The BS 56 connects to the MSC 62 through an exchange terminal circuit (ETC) 64. The ETC 64 is an interface that provides compatibility between a T1 line from the BS 56 and a group switch (GS) 66 of the MSC

62. A time division multiple access (TDMA) signal from the BS 56 is applied through the

ETC 64 to the GS 66, which sorts out the frames of the TDMA signal for sending to their proper destinations. A TTY unit 68 and a transcoder adapter board (TRAB) 70 of the

MSC 62 are coupled to the GS 66. The TTY unit 68 implements a TTY function, which includes converting TTY tones to FSK signals that can reliably pass through a vocoder and a TDMA air interface, and converting FSK signals to TTY tones. The TRAB 70 is, generally, a speech vocoder that converts complex voice to normal voice and that is capable of handling multiple channels simultaneously. The TRAB 70 also detects occurrence of function codes representative of TTY mode and voice mode and controls the TTY unit accordingly to implement or to cease implementing the TTY function. The

MSC 62 utilizes a home location register (HLR) 72 and a visitor location register (VLR)

74, each of which may be part of the MSC 62 or implemented in stand-alone fashion.

The HLR 72 stores, among other data, permanent data for subscribers that is independent of the current location of the subscribers. The VLR 74 stores current data for subscribers, such as the current or most recently known location of the mobile terminal for a subscriber, the on/off status of the mobile terminal, security parameters, etc. The MSC 62 connects the mobile terminal 52 to a number of other networks via an

ETC 76, which networks can include a public switched telephone network (PSTN) 78 as shown, other cellular stations, a local area network (LAN) and/or a wide area network (WAN). Included in the PSTN is TTY equipment 80 with which the TTY terminal 54 can communicate, such as TTY equipment of a 911 emergency service provider.

In performance of the function code embodiment of the invention, the TRAB 70 recognizes occurrence of a function code and controls the TTY unit 68 accordingly. If a TTY caller initiates a call in TTY mode, then at call setup a function code is sent to and detected by the TRAB 70, identifying to the TRAB 70 that the call is to proceed in TTY mode. The TTY mode function code can be sent automatically at call setup or it can be sent in response to the caller keying the mobile terminal 52. In response to receiving the TTY mode function code, the TRAB 70 controls the TTY unit to perform a TTY function, i.e., to convert the FSK signals received from the caller to TTY tones for transmission to the TTY equipment-80 in the PSTN 78.

A primary purpose of system level TTY mode detection is to ensure that TTY users are able to reliably send and receive TTY calls in a cellular environment, for example to access emergency services, such as 911 services. Since such services are normally PSTN subscribers, the function code technique as described has been concerned with reliable use of TTY mode in communications between a cellular subscriber and a PSTN subscriber. Because a vocoder is employed on the cellular side, but not on the PSTN side, in response to a TTY message from the cellular side to the PSTN side, the TTY function performed by the TTY unit 68 of the MSC 62 will comprise converting the FSK signals from the cellular subscriber to TTY tones for transmission from the MSC 62 to the PSTN subscriber. However, in response to a TTY message from the PSTN side to the cellular side, the TTY function performed by the TTY unit of the MSC 62 will comprise converting the TTY tones from the PSTN to FSK tones for transmission from the MSC 62 to the cellular side.

Generation of a function code representative of TTY mode and voice mode can be initiated by either the caller or the called user, which is advantageous in the case of

HCO or VCO calls. In either case, operation of the function code technique proceeds in the manner as above described for the situation where a TTY cellular subscriber initiates a call.

It can happen that a TTY type call is established between two cellular subscribers, in which event a vocoder is present in each of the send and receive sides. In this case, either no TTY function or two TTY functions must be performed in the MSC 62. If two TTY functions are performed in the MSC62, then after decoding incoming FSK signals from a transmit side to TTY tones, the TTY tones are encoded back to FSK signals for transmission to a receive side. The function code technique is simple to implement and provides a versatile means for selecting a mode of operation and for precisely informing an MSC of a need to handle all or part of a call in TTY mode. The technique readily accommodates the HCO/VCO mode requirement of the TTY forum, and implementation of the technique in existing cellular systems does not require air interface standards changes.

Fig. 3 shows an alternate configuration of a cellular system where TTY mode detection occurs at fixed network level. Fig. 3 is similar to Fig. 2, and like reference numerals are used to denote like components. The primary difference is that an MSC 82 includes a TRAB/TTY 84, which essentially is a TRAB of a type as in Fig. 2, except that its software (SW) has been modified to also perform the TTY function. While a TRAB/TTY upgrade would be required for existing cellular systems, advantages of the Fig. 3 embodiment are that no extra GS connections are needed for a separate TTY unit, no configuration of the GS is required for TTY calls, and the system is transparent to the MSC, since all mobile calls are routed via the TRAB/TTY 84.

TTY USER IDENTIFICATION

According to TTY user identification techniques contemplated by the invention, a cellular system includes databases for identifying TTY users and potential TTY users.

One approach to building such databases is to include TTY user and potential TTY user identifications in an HLR and/or a VLR of an MSC. Another is to build TTY user registers based upon artificial intelligence cues derived from calls initiated and received. The two approaches can be combined to yield a robust TTY detection capability.

HLR/VLR TTY USER IDENTIFICATION

As seen in Figs. 2 and 3, an MSC utilizes an HLR 72 and a VLR 74, each of which may be part of the MSC or implemented in stand-alone fashion. The HLR 72 stores, among other data, permanent data for each subscriber that is independent of the current location of the subscriber. The VLR 74 stores current data for each subscriber, such as the current or most recently known location of the mobile terminal for the subscriber, the on/of status of the mobile terminal, security parameters, etc.

The HLR/VLR TTY user identification technique contemplates that the data for subscribers, which is stored in the HLR and/or VLR of an MSC, include an identification of those subscribers who are TTY type users. This identification can be part of the information included in the subscriber's profile that is entered by a service operator into the HLR and/or VLR at the time of initiation of a cellular service subscription or a change of service. By adding a TTY classification to the subscriber's profile, the HLR or VLR then contains data about whether the calling or called user is capable of supporting a TTY call. This information is available to the serving MSC, which uses the information to determine whether or not a call needs to be routed through a TTY function. Adding a new class of subscriber/user in the HLR or VLR is a simple and systematic means for standardizing recognition of TTY mode users and supporters. The TTY classification information can be used for originating as well as terminating call cases, and once the caller or the called subscriber is identified as a TTY user, the call is automatically routed by the MSC through the TTY function. This technique, when combined with an artificial intelligent cue approach described below, results in a robust TTY call detection scheme. ARTIFICIAL INTELLIGENT CUE TTY USER IDENTIFICATION

In an artificial intelligent cue TTY user identification embodiment of the invention, a TTY user database is automatically updated by the cellular system whenever the system detects occurrence of a TTY call by a cellular subscriber. The cellular system utilizes elected cues to identify TTY type calls, and in response to positive identifications of TTY calls, automatically builds and updates a database for TTY subscribers.

Fig. 4 illustrates an MSC 86 that is adapted to perform the artificial intelligent cue TTY user identification technique of the invention. The MSC includes a GS 88 that communicates with a BS and a PSTN. A TRAB 90 and a TTY unit 92 are coupled to the GS, as is a control system in the form of an artificial intelligence (Al) register 94, which is a software block that includes a telephone number analysis (NA) register 96 and a memory 98. An HLR 100 and a VLR 102 are coupled to the Al register, as are the TTY unit 92 and the communication paths to and from the BS and the PSTN. Upon occurrence of selected cues that identify TTY type users, the Al register builds databases in the HLR and/or VLR of such users and stores the users' telephone numbers in the NA register 96. Then, in response to occurrence of future calls from such users, which can be identified by means of the telephone numbers stored in the NA register, the Al register controls the TTY unit to perform a TTY function. A cues input 104 to the Al register enables an operator to enter into the Al register the type of cues to be utilized. Some such cues the system can utilize are:

1. When a TTY adapter or smart cable is inserted between a user's TTY terminal and mobile terminal, and the mobile terminal senses the connection, a special code is sent to the TTY unit 92 of the MSC 86 at call setup for a call originating case.

Alternatively, the mobile terminal can be configured to automatically send the special code at call setup. For example, the new FSK can be sent to the TTY unit in the MSC.

Upon detection of the TTY tones, the TTY unit informs the Al register 94 of the occurrence of TTY mode, so that the TTY user's telephone number and user identification can be stored in the NA register 96 and in the HLR 100 and/or the VLR 102. Alternatively, a TTY user identification prefix or a code can be sent via a FACCH or a SACCH message to the MSC 86, which would cause the user's telephone number and TTY user identification to be stored in the NA register and in the HLR and/or the VLR. For a call terminating case, the called user can send, or the user's terminal can automatically send, the special code or prefix after the call is set up, which would result in the Al register storing similar identification information about the called party as is stored for a calling party. In essence, once the tones, prefix, special code or other indication is received by the cellular system, one or both users are entered into system memory as potential TTY users. All fixture originating or terminating calls by either user are then automatically routed to the TTY function performed by the TTY unit of the MSC.

The cellular system keeps a running record of TTY users' profiles, updating the record when a call is originated or terminated by a TTY user. If the recorded cellular profile of the calling or called party indicates that the user is a TTY user, the system adds the other party to its record as a potential TTY user.

2. As part of a record keeping procedure, when the called party is an emergency agent, e.g., 911 , the invention contemplates that the cellular system keep a record of a calling TTY user's telephone number, since it is likely that the emergency agent will return a call back to the TTY user within a reasonable time. When the emergency agent originates a call back to the TTY user, the record stored by the Al register 94 causes the cellular system to route the call through the TTY unit or function. The cellular system identifies this situation by keeping a record of the initial emergency call originated by the TTY cellular user and the TTY user's telephone number, and by examining and matching the B telephone number (the number called by the emergency agent) in any follow up call by the emergency agent. Another approach is for the cellular system to remember the identification of the TTY user who initiated the 911 call and the time the call was made, and for a selected time thereafter to route all calls made to the TTY user through a TTY unit.

The artificial intelligent cue user identification technique, when combined with the HLR/VLR user identification technique, yields a robust TTY call detection scheme.

USE OF SERVICE CODE IN CALL SETUP

According to a service code identification technique contemplated by the invention, at call setup a new service code indicates to the fixed side of the cellular system that the service type initiated by the cellular user is TTY mode. In addition to voice and data service codes as are used today in origination and termination messages, a TTY code is added to the service code information element of the air interface. This requires a change to air interface standards. The service code technique can be used with the cellular system of Fig. 2. Upon recognition of a TTY mode service code, the call is routed through the TTY unit 68 for performance of the TTY function.

The service code identification technique can also be used with the cellular system as shown in Fig. 1, since the service code transmitted by the calling user is received by the called user; and would therefore provide to the called user a positive indication that the call is in TTY mode.

FORCE A HANDOFF SIGNALING TO AN ANALOG CHANNEL

This technique is based upon the fact that analog cellular channels do not degrade TTY tones because analog cellular channels do not utilize vocoders. Here, upon detecting a TTY call, the cellular system forces a handoff to an analog channel, if analog resources are available at the BS. To ensure that resources are available, cellular service providers can allocate analog transceivers to support TTY calls. Another approach is to allow cellular users terminals to be set to only accept analog voice channel assignments and to assign all 911 calls to analog voice channels. Fig. 5 shows operation of the analog channel handoff technique of the invention, which contemplates that calls that otherwise would be assigned a digital channel be assigned an analog channel. Performance of the technique requires, of course, that analog channels be available. Beginning with a TTY user's mobile terminal in an idle condition at a block 110, upon the user initiating a call, after call setup in block 112 a determination is made at a block 114 of the MSC whether there is a service code indication of TTY mode. If there is, then the BS is controlled to assign an analog channel to the call at a block 116, and voice and/or TTY transmission proceeds over the analog channel at a block 118. If there is no service code indication of TTY mode, then the cellular system assigns the call to either an analog or digital channel. Should the call be assigned to an analog channel as determined at a decision block 120, then voice and/or TTY transmission proceed over the analog channel at the block 118. On the other hand, if the call is assigned a digital channel, then a determination is made at a decision block 122 whether the call is TTY mode. If it is not, it proceeds over the digital channel as a data or voice call at a block 124. However, if it is determined that the call is TTY mode, then the system forces a handoff of the call to an analog channel at a block 126.

Forcing a handoff from a digital to an analog channel is a simple and effective expedient for handling calls in TTY mode, since analog channels handle TTY calls as well as voice calls and no distortion of the TTY signals occurs.

While embodiments of the invention have been described in detail, one skilled in the art thereof may devise various modifications and other embodiments without departing from the spirit and the scope of the invention, as defined by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. Apparatus for identifying and servicing TTY type mobile terminal users in a cellular system, comprising: cellular mobile terminals for transmitting messages; a mobile switching center (MSC) for receiving and servicing messages transmitted by said mobile terminals; a detector associated with said MSC and responsive to said MSC receiving a message from a mobile terminal for determining whether a user of said mobile terminal is a TTY type user, and a control responsive to a determination that the user of said mobile terminal is a TTY type user for operating said MSC to allocate resources to service said message from said mobile terminal in TTY mode.

2. Apparatus as in claim 1 , wherein said detector associated with said MSC is responsive to said MSC receiving from said mobile terminal a function code indicative of a TTY type user and transmitted via a fast access control channel (FACCH) message for determining that the user of said mobile terminal is a TTY type user.

3. Apparatus as in claim 1 , wherein said detector associated with said MSC is responsive to said MSC receiving from said mobile terminal a function code indicative of a TTY type user and transmitted via a slow access control channel (SACCH) message for determining that the user of said mobile terminal is a TTY type user.

4. Apparatus as in claim 1 , wherein said detector associated with said MSC is responsive to said MSC receiving from said mobile terminal a service code indicative of a

TTY type user for determining that the user of said mobile terminal is a TTY type user.

5. Apparatus as in claim 1, wherein said MSC includes a home location register (HLR) and a visitor location register (VLR) in one or both of which is stored identifications of TTY type mobile terminal users, and wherein said detector associated with said MSC for determining whether the user of said mobile terminal is a TTY type user determines if the user is identified in one or both of said HLR and VLR as a TTY type user.

6. Apparatus as in claim 5, including means for entering mobile terminal user profile data into one or both of said HLR and VLR, including data identifying TTY type users.

7. Apparatus as in claim 5, including a control system for entering into one or both of said HLR and VLR identifications of TTY type users in response to sensing the occurrence, at said MSC, of messages transmitted from mobile terminals that are indicative of such mobile terminal users being TTY type users.

8. Apparatus as in claim 1 , wherein said control responsive to a determination that the user of said mobile terminal is a TTY type user operates said MSC, if said mobile terminal has been assigned a digital channel, to force a handoff from said digital channel to an analog channel.

9. Apparatus for identifying and servicing TTY type mobile terminal users in a cellular system, comprising: adapters for operating cellular mobile terminals of TTY type users to transmit messages that include a signal indicative of operation of said mobile terminals in TTY mode; a mobile switching center (MSC) for receiving and servicing messages transmitted by said mobile terminals, and a control associated with said MSC and responsive to said MSC receiving from a mobile terminal said signal indicative of TTY mode for causing said MSC to allocate resources to service messages from said mobile terminal in TTY mode.

10. Apparatus as in claim 9, wherein said adapter operates said mobile terminals to send a said signal indicative of TTY mode at call setup.

11. Apparatus as in claim 9, wherein said signal indicative of TTY mode comprises a function code.

12. Apparatus as in claim 9, wherein said signal indicative of TTY mode comprises a function code transmitted to said MSC via a fast access control channel (FACCH) message.

13. Apparatus as in claim 9, wherein said signal indicative of TTY mode comprises a function code transmitted to said MSC via a slow access control channel (SACCH) message.

14. Apparatus as in claim 9, wherein said adapter also operates said mobile terminals to transmit messages that include a signal indicative of operation of said mobile terminals in voice mode, and wherein said control associated with said MSC is responsive to receiving from a mobile terminal said signal indicative of voice mode for causing said MSC to cease allocating resources for servicing messages from said mobile terminal in TTY mode.

15. ^■ Apparatus as in claim 9, wherein said signal indicative of TTY mode comprises a service code.

16. Apparatus as in claim 9, wherein said mobile terminals, when operating in TTY mode, transmit encoded Baudot code, and said resources allocated by said MSC to service TTY mode messages comprise means for decoding said encoded Baudot code.

17. Apparatus as in claim 9, wherein said MSC establishes communication between said mobile terminals and a public switched telephone network (PSTN).

18. Apparatus for identifying and servicing TTY type mobile terminal users in a cellular system, comprising: adapters for operating cellular mobile terminals of TTY type users to transmit a message that includes a function code indicative of operation of said mobile terminals in

TTY mode; a mobile switching center (MSC) for receiving and servicing messages transmitted by said mobile terminals, and a control associated with said MSC and responsive to said MSC receiving from a mobile terminal said function code indicative of TTY mode for causing said

MSC to service the messages from said mobile terminal in TTY mode.

19. Apparatus as in claim 18, wherein said function code is transmitted to said MSC via a fast access control channel (FACCH) message.

20. Apparatus as in claim 18, wherein said function code is transmitted to said MSC via a slow access control channel (SACCH) message.

21. Apparatus as in claim 18, wherein said adapters also operate said mobile terminals to transmit messages that include a function code indicative of operation of said mobile terminals in voice mode, and said control associated with said MSC is responsive to said MSC receiving from a mobile terminal said function code indicative of voice mode for causing said MSC to service the messages from said mobile terminal in voice mode, whereby said cellular system supports hearing carry-over (HCO) and voice carry-over (VCO).

22. Apparatus as in claim 18, wherein said mobile terminals, when operating in TTY mode, transmit encoded Baudot code, and said control associated with said MSC and responsive to said MSC receiving said function code indicative of TTY mode causes said MSC to decode said encoded Baudot code.

23. Apparatus as in claim 18, wherein said MSC establishes communications between the mobile terminal users and public switched telephone network (PSTN) users.

24. Apparatus as in claim 18, wherein the mobile terminal TTY type users are connected by said MSC for communication with PSTN users, and wherein said control associated with said MSC is also responsive to said MSC receiving from the PSTN users a function code indicative of operation by the PSTN users in TTY mode for causing said MSC to service messages from both the mobile terminal users and said PSTN users in TTY mode.

25. Apparatus for identifying and servicing TTY type mobile terminal users in a cellular system, comprising: adapters for operating mobile terminals of TTY type users to transmit messages that include a service code indicative of operation of said mobile terminals in TTY mode; a mobile switching center (MSC) for receiving and servicing messages transmitted by said mobile terminals, and a control associated with said MSC and responsive to said MSC receiving from a mobile terminal said service code indicative of TTY mode for causing said MSC to service the messages from said mobile terminal in TTY mode.

26. Apparatus as in claim 25, wherein messages transmitted by said mobile terminals are encoded frequency shift keying (FSK) tones and said control associated, in response to receipt by said MSC of said service code, causes said MSC to decode the encoded FSK tones.

27. Apparatus for identifying and servicing TTY type mobile terminal users in a cellular system, comprising: mobile terminals for transmitting messages; a mobile switching center (MSC) for receiving and servicing messages transmitted by said mobile terminals, said MSC comprising a database for storing user profile data, including identifications of TTY type users of mobile terminals; a detector associated with said MSC and responsive to said MSC receiving a message from a mobile terminal for determining whether a user of said mobile terminal is identified in said database means as a TTY type cellular user, and a control responsive to a determination that the user of said mobile terminal is identified as a TTY type user for operating said MSC to service messages from said mobile terminal in TTY mode.

28. Apparatus as in claim 27, including a control system for entering into said database user profile data, including data identifying TTY type mobile terminal users.

29. Apparatus as in claim 28, wherein said control system enters such data by a cellular service operator at the initiation of subscriptions or changes of service by TTY type mobile terminal users.

30. Apparatus as in claim 27, including a sensor for sensing the occurrence, at said MSC, of messages from mobile terminals that are indicative of TTY mode transmissions, and a control system responsive to said sensor for entering into said database identifications and updated identifications of the users of such mobile terminals as TTY type users.

31. Apparatus as in claim 30, wherein said mobile terminals of TTY type users transmit a code indicative of TTY mode in operation of said mobile terminals, and said sensors sense occurrence of said code.

32. Apparatus as in claim 31 , wherein said code indicative of TTY mode comprises tones with good TTY correlation properties.

33. Apparatus as in claim 31 , wherein said code indicative of TTY mode is transmitted to the MSC via a fast access control channel (FACCH) message.

34. Apparatus as in claim 31 , wherein said code indicative of TTY mode is transmitted to the MSC via a slow access control channel (SACCH) message.

35. Apparatus as in claim 30, wherein said control system for entering into said database also enters into said database identifications, as potential TTY type users, of mobile terminal users and of public switched telephone network (PSTN) users with whom the identified TTY type users communicate.

36. Apparatus as in claim 27, wherein said MSC further includes a control system responsive to an identified TTY type user calling an emergency agent, e.g., 911 , for temporarily storing the telephone number of the TTY type user and the time the call to the emergency agent was made, and responsive to sensing a return call by the emergency agent to such TTY type user for servicing the message from the emergency agent in TTY mode.

37. Apparatus as in claim 30, wherein said MSC control system, responsive to an identified TTY type user calling an emergency agent, e.g., 911, temporarily stores the telephone number of the TTY user and the time the call to the emergency agent was made, and for servicing in TTY mode all calls made to such TTY type user for a selected time thereafter.

38. Apparatus for identifying and servicing TTY type mobile terminal users in a cellular system, comprising: cellular mobile terminals for transmitting messages; a mobile switching center (MSC) for receiving and servicing messages transmitted by said mobile terminals; a first detector associated with said MSC, and responsive to said MSC receiving a message from a mobile terminal, for determining whether a user of said mobile terminal is a TTY type user; a second detector, responsive to a determination that the user of said mobile terminal is a TTY type user, for determining whether said mobile terminal has been assigned a digital channel or an analog channel, and a control, responsive to a determination that said mobile terminal of the TTY type user has been assigned a digital channel, for forcing a handoff to an analog channel.

39. A method of identifying and servicing TTY type mobile terminal users in a cellular system, comprising the steps of transmitting messages from cellular mobile terminals; receiving and servicing the messages transmitted by the mobile terminals with a mobile switching center (MSC); determining, in response to receiving at the MSC a message transmitted by a mobile terminal, whether a user of the mobile terminal is a TTY type user, and operating the MSC, in response to a determination that the user of the mobile terminal is a TTY type user, to allocate MSC resources to service the message in TTY mode.

40. A method as in claim 39, wherein said determining step is responsive to the MSC receiving from the mobile terminal a function code indicative of a TTY type user for determining that the user of the mobile terminal is a TTY type user.

41. A method as in claim 39, wherein said determining step is responsive to the MSC receiving from the mobile terminal a function code transmitted via a fast access control channel (FACCH) message and indicative of a TTY type user for determining that the user of the mobile terminal is a TTY type user.

42. A method as in claim 39, wherein said determining step is responsive to the MSC receiving from the mobile terminal a function code transmitted via a slow access control channel (SACCH) message and indicative of a TTY type user for determining that the user of the mobile terminal is a TTY type user.

43. A method as in claim 39, wherein said determining step is responsive to the MSC receiving from the mobile terminal a service code indicative of a TTY type user for determining that the user of the mobile terminal is a TTY type user.

44. A method as in claim 39, including the step of storing identifications of TTY type mobile terminal users, said determining step, in response to receiving at the MSC a message transmitted by a mobile terminal, determining whether there is a stored indication of the mobile terminal user as a TTY type user.

45. A method as in claim 44, wherein said storing step comprises storing mobile terminal user profile data, including data identifying TTY type users, that is entered by a cellular service operator.

46. A method as in claim 44, wherein said storing step comprises storing identifications of individual ones of the TTY type users of mobile terminals in response to said determining step determining that such mobile terminal users are TTY type users.

47. A method as in claim 39, wherein said operating step, in response to a determination that the user of the mobile terminal is a TTY type user, causes the MSC, if the mobile terminal has been assigned a digital channel, to force a handoff from the digital channel to an analog channel.

48. A method of identifying and servicing TTY type mobile terminal users in a cellular system, comprising the steps of: operating cellular mobile terminals of TTY type users to transmit messages that include a signal indicative of operation of the mobile terminals in TTY mode; receiving and servicing the messages transmitted by the mobile terminals with a mobile switching center (MSC); determining, in response to receiving at the MSC a message transmitted by a mobile terminal, whether the message includes a signal indicative of operation of the mobile terminal in TTY mode, and causing the MSC, in response to a determination that the mobile terminal is operating in TTY mode, to allocate MSC resources to service the message in TTY mode.

49. A method as in claim 48, wherein said operating step operates the mobile terminals to transmit a signal indicative of TTY mode at call setup.

50. A method as in claim 48, wherein the signal, indicative of TTY mode comprises a function code.

51. A method as in claim 48, wherein the signal, indicative of TTY mode comprises a function code transmitted via a fast access control channel (FACCH) message.

52. A method as in claim 48, wherein the signal indicative of TTY mode comprises a function code transmitted via a slow access control channel (SACCH) message.

53. A method as in claim 48, wherein said operating step also operates the mobile terminals to transmit messages that include a signal indicative of operation of the mobile terminals in voice mode, said determining step determines whether a message transmitted by a mobile terminal includes a signal indicative of operation of the mobile terminal in voice mode, and said causing step is responsive to a determination that the mobile terminal is operating in voice mode to cause the MSC to cease allocating resources to service messages from the mobile terminal in TTY mode.

54. A method as in claim 48, wherein the signal indicative of TTY mode comprises a service code.

55. A method as in claim 48, wherein the mobile terminals, when operating in TTY mode, transmit encoded Baudot code, and wherein said causing step causes the MSC to allocate MSC resources that decode the encoded Baudot code.

56. A method of identifying and servicing TTY type mobile terminal users in a cellular system, comprising the steps of operating cellular mobile terminals of TTY type users to transmit a message that includes a function code indicative of operation of the mobile terminals in TTY mode; receiving and servicing the messages transmitted by the mobile terminals with a mobile switching center (MSC); determining, in response to receiving at the MSC a message transmitted by a mobile terminal, whether the message includes a function code indicative of operation of the mobile terminal in TTY mode, and causing the MSC, in response to a determination that the mobile terminal is operating in TTY mode, to allocate MSC resources to service the message in TTY mode.

57. A method as in claim 56, wherein the message transmitted by the mobile terminals includes a function code transmitted via a fast access control channel (FACCH) message.

58. A method as in claim 56, wherein the message transmitted by the mobile terminals includes a function code transmitted via a slow access control channel (SACCH) message.

59. A method as in claim 56, wherein said operating step also operates the mobile terminals to transmit messages that include a function code indicative of operation of the mobile terminals in voice mode, said determining step determines whether a message transmitted by a mobile terminal includes a function code indicative of operation of the mobile terminal in voice mode, and said causing step is responsive to a determination that the mobile terminal is operating in voice mode to cause the MSC to cease allocating resources to service messages from the mobile terminal in TTY mode.

60. A method as in claim 56, wherein the mobile terminals, when operating in TTY mode, transmit encoded Baudot code, and wherein said causing step causes the MSC to allocate MSC resources that decode the encoded Baudot code.

61. A method as in claim 56, wherein the mobile terminal TTY type users are connected by the MSC for communication with public switched telephone network

(PSTN) users, said determining step is responsive to the MSC receiving from the PSTN users a function code indicative of operation by a PSTN user in TTY mode, and said causing step is responsive to a determination that a PSTN user is operating in TTY mode to allocate resources to service messages from both the mobile terminal user and the

PSTN user in TTY mode.

62. A method of identifying and servicing TTY type mobile terminal users in a cellular system, comprising the steps of operating cellular mobile terminals of TTY type users to transmit a message that includes a service code indicative of operation of the mobile terminals in TTY mode; receiving and servicing the messages transmitted by the mobile terminals with a mobile switching center (MSC); determining, in response to receiving at the MSC a message transmitted by a mobile terminal, whether the message includes a service code indicative of operation of the mobile terminal in TTY mode, and causing the MSC, in response to a determination that the mobile terminal is operating in TTY mode, to allocate MSC resources to service the message in TTY mode.

63. A method as in claim 62, wherein the messages transmitted by the mobile terminals are encoded frequency shift keying (FSK) signals, and said causing step, in response to a determination that the mobile terminal is operating in TTY mode, causes the MSC to allocate resources to decode the encoded FSK signals.

64. A method of identifying and servicing TTY type mobile terminal users in a cellular system, comprising the steps of operating mobile terminals to transmit messages; receiving and servicing the messages transmitted by the mobile terminals with a mobile switching center (MSC); storing in a database mobile terminal user profile data, including data identifying TTY type users; determining, from the stored data and in response to receiving at the MSC a message transmitted by a mobile terminal, whether a user of the mobile terminal is a TTY type user, and causing the MSC, in response to a determination that the user of the mobile terminal is a TTY type user, to allocate MSC resources to service the message in TTY mode.

65. A method as in claim 64, including the step of entering user profile data into the database by a cellular service operator at the initiation of subscriptions or changes of service by TTY type mobile terminal users.

66. A method as in claim 64, including the steps of sensing the occurrence, at the MSC, of a message from a mobile terminal that is indicative of a TTY mode transmission from the mobile terminal, and in response to occurrence of such a message entering into the database identifications and updated identifications of the TTY type user of the mobile terminal.

67. A method as in claim 64, wherein said mobile terminal operating step operate the mobile terminals of TTY users to transmit messages including a code indicative of TTY mode, and said sensing step senses the occurrence of TTY mode by sensing the code.

68. A method as in claim 67, wherein the code indicative of TTY mode comprises a code transmitted to the MSC via a fast access control channel (FACCH) message.

69. A method as in claim 67, wherein the code indicative of TTY mode comprises a code transmitted to the MSC via a slow access control channel (SACCH) message.

70. A method as in claim 67, wherein the code indicative of TTY mode comprises tones with good TTY correlation properties.

71. A method as in claim 66, wherein said entering step enters into the database identifications, as potential TTY type users, of mobile terminal users and of public switched telephone network (PSTN) users with whom identified TTY type users communicate.

72. A method as in claim 64, including the steps, responsive to an identified TTY user calling an emergency agent, e.g., 911 , of temporarily storing the telephone number of the TTY type user and the time the call to the emergency agent was made, sensing a return call by the emergency agent to such TTY type user, and in response to sensing a return call causing the MSC to service the return call in TTY mode.

73. A method as in claim 64, including the steps, in response to an identified

TTY type user calling an emergency agent, e.g., 911 , of temporarily storing the telephone number of the TTY user and the time the call to the emergency agent was made, and causing the MSC to service in TTY mode all calls made to such TTY type user for a selected time thereafter.

74. A method of identifying and servicing TTY type mobile terminal users in a cellular system, comprising the steps of operating cellular mobile terminals to transmit messages; receiving and servicing messages transmitted by the mobile terminals with a mobile switching center (MSC); determining, in response to the MSC receiving a message from a mobile terminal, whether a user of such mobile terminal is a TTY type user; ascertaining, in response to a determination that the user of the mobile terminal is a TTY type user, whether the mobile terminal has been assigned a digital channel or an analog channel, and causing, in response to a determination that the TTY user's mobile terminal has been assigned a digital channel, a handoff from the digital channel to an analog channel.