6.2 Accessibility: TTY/TDD, Text-to-911 & Real-Time Text (RTT)

6.2 Accessibility: TTY/TDD, Text-to-911 & Real-Time Text (RTT)

Quick Answer: Universal accessibility in public safety telecommunications is federally mandated under Title II of the Americans with Disabilities Act (ADA, 28 CFR § 35.162). The statute requires PSAPs to deliver direct, equal access to emergency services for individuals who are deaf, hard of hearing, or have speech disabilities without forcing reliance on third-party relay services. DOJ guidance calls for TTY capability at every call-taking position, periodic testing, and TTY checks on silent calls. Modern emergency networks expand accessibility through Text-to-911—governed by the J-STD-110 standard, Text Control Centers (TCC), and FCC bounce-back rules—and Real-Time Text (RTT, RFC 4103), which transmits text character-by-character in full-duplex conversational rhythm without a "send" button.

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1. Federal Statutory Mandate: ADA Title II & DOJ Enforcement

Emergency telephone communications systems are fundamental public safety lifelines. When Congress enacted the Americans with Disabilities Act (ADA) of 1990, it placed explicit, non-negotiable mandates on state and local governmental entities under Title II (28 CFR § 35.162).

Direct, Equal Access Requirements

The ADA Title II regulation specifies that state and local government agencies providing emergency telephone services (9-1-1) must provide direct, equal access to persons who use telecommunications devices for the deaf (TTYs) or computer-based text systems:

• "Direct" Access Defined: Disabled callers must be able to dial 9-1-1 directly and communicate immediately with the PSAP call taker. An agency cannot require individuals who are deaf, hard of hearing, or speech impaired to route their emergency calls through an intermediate relay service (such as 7-1-1 Telecommunications Relay Service or private commercial operators) before reaching public safety.
• "Equal" Access Defined: The service provided to callers using assistive technology must be functionally equivalent in every dimension to that provided to voice callers. This encompasses:
  • Equivalent telephone answering speed and queue priority
  • Equal response features (transfer, hold, conference, and call-back capabilities)
  • Equal operational quality, testing rigor, and staff training

Department of Justice (DOJ) 9-1-1 Settlement Agreements

The U.S. Department of Justice (DOJ) Civil Rights Division actively investigates PSAP compliance with Title II. Through technical assistance guidance and settlement agreements with individual 9-1-1 centers, the DOJ has described the practices it expects:

1. Console Integration: Every individual call-taking position must be equipped with functioning TTY/text capability. Requiring a telecommunicator to walk to another room, fetch a standalone portable TTY device, or transfer a call to a "dedicated ADA console" undermines direct, equal access.
2. Mandatory Staff Training: Telecommunicators must receive comprehensive training on TTY, text systems, and relay services upon hiring, followed by periodic refresher training.
3. Unannounced Testing Requirements: Agencies must conduct regular, unannounced live TTY test calls across all operational shifts and consoles (on a recurring schedule). Tests are conducted without advance warning to verify that staff can recognize TTY tones, query silent lines, and handle TTY calls promptly.

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2. Legacy TTY/TDD Architecture & Baudot Protocols

For decades, direct access relied on Teletypewriters (TTY), also referred to as Telecommunications Devices for the Deaf (TDD). While digital IP text is supplanting analog TTY, legacy Baudot signaling remains embedded in public safety CPE and appears extensively on the ENP exam.

[Standard TTY / Baudot Communication]

Caller:    Q WHAT TIME FIRE TRUCK ARRIVE GA
           (Transmits 45.45 baud FSK tones across copper loop)
           
PSAP:      TRUCK ON SCENE 2 MIN GA
           (Telecommunicator types response; ends with GA)
           
Caller:    THANK U SKSK
           (Both parties transmit SK before hanging up)

Technical Characteristics of Baudot Code

• Baudot Signaling: TTY devices communicate using Baudot code, an archaic 5-bit character encoding system operating at a data rate of 45.45 baud (approximately 6 characters per second).
• Modulation Scheme: Utilizes Frequency Shift Keying (FSK) audio tones across standard analog voice circuits:
  • Mark (Binary 1): 1400 Hz
  • Space (Binary 0): 1800 Hz
• Half-Duplex Operation: Baudot communications are strictly half-duplex. Only one party can transmit at a time. If both parties type simultaneously, the audio tones collide and cancel out, rendering the text on both screens completely scrambled and unreadable.
• ASCII Alternative: Some modern TTY terminals support standard ASCII code operating at 110 or 300 baud in full duplex, but 45.45 baud Baudot remains the baseline universal protocol for legacy PSTN interoperability.

Standard TTY Operational Abbreviations

Because 45.45 baud communication is slow and half-duplex, the deaf community and public safety telecommunicators utilize standardized shorthand codes to manage conversation turn-taking:

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3. Mandatory TTY Interrogation of Silent Calls

A critical failure mode in emergency communications is assuming that a silent 9-1-1 call is an accidental dial or empty line. Under federal ADA regulations and NENA standard operating procedures, every silent 9-1-1 call must be challenged with a TTY interrogation probe.

[Incoming 9-1-1 Call Presents Total Silence]
       │
       ▼
[Telecommunicator Voice Query: "9-1-1, What is your emergency?"]
       │ (No audible response received)
       ▼
[MANDATORY TTY CHALLENGE]
 Telecommunicator activates TTY and sends standard probe:
 "[AGENCY NAME] 911 GA" (in Baudot tones)
       │
       ├───────────────────────────────┬───────────────────────────────┐
       ▼                               ▼                               ▼
[Baudot Response Appears]       [No Response to TTY]            [Audio Sounds Detected]
Proceed with TTY interrogation  Follow standard abandoned       Mute room mic; listen
and dispatch assistance         call callback protocol          for open line distress

The Operational Rationale

Callers who are deaf, hard of hearing, or experiencing severe speech loss cannot respond to verbal queries. Furthermore, older acoustic-coupler or electronic TTY devices do not automatically generate tones upon answering; they remain completely silent until the caller or the call taker strikes a key on the keyboard. If a telecommunicator answers a silent call, asks "9-1-1, where is your emergency?" and disconnects after hearing silence, they may have abandoned a citizen in mortal danger.

The Interrogation Protocol

1. Telecommunicator receives an open line with silence or faint clicking.
2. Telecommunicator announces the voice challenge: "9-1-1, what is your emergency?"
3. Hearing no response within 2 to 3 seconds, the telecommunicator immediately engages the console TTY function.
4. The telecommunicator sends a standardized greeting query in Baudot: "[Agency Name] 9-1-1 GA".
5. If the caller is using a TTY, the incoming tones trip the terminal decoder and the caller types their response.
6. Only after both voice and TTY queries elicit no response may the call taker proceed with abandoned call callback workflows.

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4. Text-to-911 Deployment, Architecture & Gateway Routing

While TTY fulfilled the statutory requirements of the 1990 ADA, consumer telecommunications transitioned overwhelmingly to digital messaging. To bridge this gap, the FCC, public safety associations (NENA/APCO), and major nationwide wireless carriers established modern Text-to-911 frameworks standardized under ATIS/TIA J-STD-110.

[Consumer Cellular Phone: SMS to "9-1-1"]
                   │
                   ▼
   [Wireless Carrier SMSC Gateway]
                   │
                   ▼
   [Commercial Text Control Center (TCC)]
   (Validates location & routes via SR or ESRP)
                   │
       ┌───────────┴───────────┬───────────────────────────┐
       ▼                       ▼                           ▼
[Web Portal / Browser]   [TTY Translation]       [Direct IP / NG911 i3]
(Interim standalone)     (CAMA 45.45 baud)       (MSRP over SIP on ESInet)
(No CAD integration)     (High latency/truncation) (Full console integration)

The Regulatory Framework & The FCC Bounce-Back Rule

In December 2012, the four nationwide wireless carriers made a voluntary commitment with NENA and APCO to support text-to-911, and the FCC then adopted binding rules, now codified in 47 CFR § 9.10(q):

• PSAP Readiness Registry: Text-to-911 is implemented on a PSAP-by-PSAP basis. A 9-1-1 authority must formally notify wireless carriers through the FCC Text-to-911 Registry or another valid request. Covered text providers must begin delivering texts to that PSAP by June 30, 2015 or within six months of a valid request, whichever is later.
• The FCC Bounce-Back Rule (47 CFR § 9.10(q), in effect since September 30, 2013): If a consumer attempts to send an SMS text message to "9-1-1" in a geographic jurisdiction where the serving PSAP has not yet deployed text-to-911, the text provider must automatically return a bounce-back text message to the sender advising them to make a voice call:

  "Please make a voice call to 911. There is no text service to 911 in this area at this time."

The Text Control Center (TCC) & J-STD-110

Because cellular Short Message Service (SMS) is an asynchronous, store-and-forward mobile carrier protocol that cannot natively interface with public safety telephone networks, the industry deployed specialized aggregation hubs called Text Control Centers (TCC) (operated by vendors such as Comtech Telecommunications and Intrado) under the J-STD-110 standard:

1. The carrier's Short Message Service Center (SMSC) routes the "911" message to the TCC.
2. The TCC queries the carrier's Location Information Server (LIS) or GMLC/MPC to determine the mobile device's coarse location.
3. The TCC interrogates routing tables (or queries an NG911 ECRF) to identify the authoritative PSAP.
4. The TCC converts the SMS message and establishes a session with the PSAP.

Text Delivery Architectures

PSAPs utilize one of three architectural models to receive Text-to-911:

1. Web Browser / Portal (Interim Solution): The TCC routes incoming text messages over the public internet to a secure web portal displayed on an administrative computer monitor in the PSAP. Severe limitations: It creates an operational island, is completely unintegrated with CAD or CPE, cannot automatically seize lines, and delays telecommunicator notification.
2. TTY Translation (Archaic Ingress): The TCC converts incoming SMS text into 45.45 baud Baudot audio tones and delivers the call across dedicated analog CAMA trunks to the PSAP's legacy TTY terminals. Severe limitations: Slow transmission, character limits, frequent tone dropouts, and trunk tie-ups.
3. Direct IP / MSRP over SIP (NG911 i3 Standard): The modern, permanent standard defined in NENA i3 specifications. The TCC initiates a Session Initiation Protocol (SIP) session across the private Emergency Services IP Network (ESInet). Once the SIP handshake completes, text packets are streamed bi-directionally using the Message Session Relay Protocol (MSRP - RFC 4975) directly into the call-taker's unified multimedia CAD/CPE console.

Public Education: "Call if You Can, Text if You Can't"

Public safety leaders maintain strict public messaging: "Call if you can, text if you can't." Text messaging is inferior to voice for several critical reasons:

• Location Latency: SMS text messages often deliver only coarse cell-tower centroid data rather than rapid, dynamic device-based hybrid location (DBHL).
• Loss of Acoustic Context: Voice calls allow call takers to evaluate vocal inflection, background screams, gunfire, vehicle noises, and subtle environmental clues.
• Store-and-Forward Delay: SMS messages can be delayed in cellular carrier queues during network congestion, whereas voice packets travel in real-time.

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5. Real-Time Text (RTT) & IETF RFC 4103

While SMS Text-to-911 represents a major milestone, it remains a store-and-forward technology: the user types a full message, hits "send," waits for carrier transmission, and the recipient receives a block of text. In high-stress emergencies, this introduces deadly delays.

The next generation of accessibility is Real-Time Text (RTT), standardized under IETF RFC 4103 (RTP Payload for Text Conversation) and RFC 5194.

[SMS Text-to-911 (Store-and-Forward)]
Caller Types: [H-E-L-P - H-E - H-A-S - A - G-U-N] ──► [Hits SEND] ──► (Carrier Delay) ──► PSAP Displays Full Block
Total Time Elapsed: 15 - 30 seconds before telecommunicator reads word "GUN"

[Real-Time Text (RTT - IETF RFC 4103)]
Caller Types:  H ... E ... L ... P ... [Space] ... H ... E ...
PSAP Sees:     H ... E ... L ... P ... [Space] ... H ... E ...
(Characters stream instantly across IP network as keys are struck - NO SEND BUTTON)
Telecommunicator initiates dispatch within 3 seconds while caller continues typing

Architectural Mechanics of RTT

• Character-by-Character Streaming: RTT transmits text bi-directionally across the IP network character-by-character (or in micro-bursts of a few milliseconds) as keystrokes occur. There is no "send" button.
• Conversational Rhythm & Full-Duplex: Both the caller and telecommunicator can type simultaneously. Characters appear on both screens in dedicated conversational windows, perfectly capturing the human tempo, cadence, and urgency of speech.
• Pre-Arrival Comprehension: If an intruder breaks into a home and the caller types "He is kicking down the bedroom door," the telecommunicator reads each word in real time. If the caller is attacked and drops the phone halfway through the sentence, the PSAP already has the critical information on screen. Under SMS, that unsent text would be lost forever.
• Emotional & Cognitive Telemetry: Call takers can observe backspaces, corrections, hesitation pauses, and frantic typing speed, offering psychological insight into the caller's immediate threat level.
• Simultaneous Voice and Text: In an RTT SIP session, voice (RTP) and text (RFC 4103) travel concurrently over the ESInet. A caller who can hear but cannot speak can listen to the telecommunicator's soothing voice through the headset while typing their responses back in RTT.

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6. Telecommunications Relay Services (TRS, VRS & STS)

Federal telecommunications policy provides specialized relay services to facilitate telephone communication for individuals with disabilities under Title IV of the ADA:

Telecommunications Relay Services (TRS - 711)

• Operational Concept: A telephone service that allows persons with hearing or speech disabilities to place and receive calls via a Communications Assistant (CA). Dialing 7-1-1 connects the user to a relay center where the CA reads TTY text aloud to a hearing party and types spoken words back to the TTY user.
• PSAP Interaction: When a TRS CA calls 9-1-1 on behalf of a citizen, the CA identifies themselves with an operator ID number and announces: "This is a relay call." Telecommunicators must speak directly to the caller, not the CA (e.g., say "What is your address?" rather than "Ask him for his address").

Video Relay Services (VRS)

• Operational Concept: A high-speed broadband video service used by individuals whose primary language is American Sign Language (ASL). The caller connects via video camera to a certified ASL interpreter CA. The interpreter translates sign language into spoken voice to the 9-1-1 call taker, and translates the dispatcher's voice back into ASL over video.
• The Location Trap: ENP candidates must be aware that the caller ID displayed at the PSAP on a VRS call may reflect the corporate headquarters or regional center of the video relay provider (which could be in another state), rather than the physical address of the citizen signing from home. Telecommunicators must verbally verify the caller's physical location immediately.

Speech-to-Speech (STS)

• Operational Concept: Designed for individuals with severe speech disabilities (e.g., cerebral palsy, ALS, Parkinson's). A specially trained CA listens to the caller's speech and repeats the caller's exact words clearly to the telecommunicator without rephrasing or interpreting.

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7. Operational Traps & ENP Exam Watch

• Every Silent Call Gets TTY: Never disconnect from a silent call without transmitting a TTY query. Silent lines may be a deaf individual awaiting a Baudot handshake.
• Baudot Collision: Baudot is half-duplex. If the call taker types while the caller is typing, both transmissions are garbled. Always wait for "GA" before responding.
• FCC Bounce-Back Mandate: The bounce-back rule is a wireless carrier obligation under 47 CFR § 9.10(q). If a PSAP cannot receive SMS, the carrier's network must send an immediate automated bounce-back text advising a voice call.
• RTT vs. SMS: RTT has no send button and streams character-by-character (RFC 4103). SMS is store-and-forward.
• Direct Equal Access: Requiring a deaf caller to dial 7-1-1 to reach 9-1-1 denies the direct access that ADA Title II requires (28 CFR § 35.162).