1.3 Legacy Network Signaling: CAMA, SS7 & Selective Routing

1.3 Legacy Network Signaling: CAMA, SS7 & Selective Routing

Quick Answer: Legacy 9-1-1 call delivery relies on three primary telecommunications signaling protocols: Centralized Automatic Message Accounting (CAMA) utilizing in-band Multi-Frequency (MF) tones over analog/DS0 circuits, Signaling System No. 7 (SS7) utilizing out-of-band Common Channel Signaling (CCS) with ISUP messaging, and ISDN Primary Rate Interface (ISDN-PRI) terminating 23B+D channels into PSAP call-handling equipment. When a call reaches the E9-1-1 Selective Router tandem, the switch buffers the incoming ANI, queries its high-speed Selective Routing Database (SRDB) to identify the Emergency Service Number (ESN), and selects the primary PSAP trunk group. If ANI is missing or unresolvable (data failure), the router invokes Default Routing; if primary PSAP trunks are fully saturated or cut (facility failure), it invokes Alternate Routing.

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1. Architecture of Dedicated Emergency Trunking

In public safety telecommunications, 9-1-1 calls are strictly prohibited from traversing shared, public inter-office trunks (IOTs). If emergency calls competed with commercial traffic on general telephone company trunk groups, a sudden surge in consumer calling (such as Mother's Day, severe weather, or commercial promotions) would generate "All Circuits Busy" conditions, blocking life-safety emergency calls.

To prevent contention, telephone companies engineer dedicated 9-1-1 trunk groups between Class 5 Central Offices, Selective Routers, and Public Safety Answering Points (PSAPs):

• Physical Transport: Dedicated emergency trunks operate over channelized DS0 circuits (64 kbps digital time-slots within a 1.544 Mbps T1 carrier) or legacy 4-wire balanced analog audio pairs.
• Engineering Rule: Every trunk group is sized according to strict traffic engineering formulas to achieve a P.01 Grade of Service, guaranteeing that during the Average Busy Hour (ABH), no more than 1% of emergency calls encounter a trunk busy signal.

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2. Centralized Automatic Message Accounting (CAMA) Protocol Deep Dive

Originally engineered by the Bell System in the 1940s and 1950s to automate toll call billing for long-distance telephone service, CAMA was adapted in the 1970s as the de facto signaling standard for connecting Class 5 Central Offices to E9-1-1 Selective Routers, and Selective Routers to PSAP customer premises equipment (CPE).

Physical Transport & Supervisory Signaling

• Transmission Circuit: Deployed over dedicated 4-wire analog circuits (separate transmit and receive pairs) or dedicated 64 kbps DS0 channels on digital T1 carriers.
• Supervisory Signaling (E&M / Reverse Battery): CAMA uses DC loop signaling to indicate circuit states:
  1. Seizure (Off-Hook): The originating Central Office seizes the idle trunk by applying an off-hook loop closure.
  2. Wink Start Acknowledgment: When ready to receive digits, the Selective Router reverses circuit DC battery polarity for 140 to 290 milliseconds. This pulse is known as a wink start.
  3. Digit Outpulsing: Upon detecting the wink, the Central Office pulses audio tones across the line.
  4. Answer & Cut-Through: When the PSAP answers, a steady reverse-battery condition cuts through the two-way audio path and commences call timing.
  5. Disconnect: Dropping loop current restores the circuit to an idle state.

In-Band Multi-Frequency (MF) Signaling Mechanics

CAMA transmits signaling data directly over the audio voice pair (in-band). It does not use consumer DTMF touch-tones. Instead, it uses Multi-Frequency (MF) signaling, combining two discrete audio frequencies selected from a pool of six tones (700, 900, 1100, 1300, 1500, and 1700 Hz):

$\text{Transmission} = [\text{KP}] + [\text{Information Digit}] + [\text{ANI Digits}] + [\text{ST}]$

1. Key Pulse (KP): Pulsed at 1100 Hz + 1700 Hz for approximately 100 ms to prepare receiving registers.
2. Information Digit (Class of Service): Identifies line class (0 = standard line, 1 = multi-party line, 2 = ANI failure, 9 = test call).
3. ANI Digits: Traditional CAMA transmits 7 digits (NXX-XXXX). Modified/Extended CAMA transmits 10 digits (KP + 0 + NPA + NXX + XXXX + ST) or uses a two-burst sequence (NPA followed by NXX-XXXX) to support area code overlays.
4. Start Pulse (ST): Pulsed at 1500 Hz + 1700 Hz to indicate completion of the digit stream.

Architectural Limitations of CAMA

• Unidirectional Operation: CAMA trunks are strictly one-way (inbound to the PSAP). Call takers cannot originate outbound calls over CAMA facilities.
• Call Setup Latency: Wink start, tone pulsing, and audio cut-through add noticeable setup delay (commonly a few seconds) before voice connection occurs.
• In-Band Tone Bleed: Tones travel across the speech path; premature audio cut-through can result in audible MF beeps.
• Data Capacity Constraints: CAMA cannot transmit alphanumeric text, caller names, or coordinates over the voice channel; location delivery requires an out-of-band query to an ALI database.

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3. Signaling System No. 7 (SS7) & Out-of-Band Common Channel Signaling

As telephone networks modernized, Signaling System No. 7 (SS7) superseded CAMA between digital Central Offices and modern Selective Routers.

Common Channel Signaling (CCS) Architecture

SS7 completely decouples voice audio from signaling control:

• Bearer Channels (DS0): Digital 64 kbps voice paths carry purely user speech.
• Common Channel Signaling Links (56/64 kbps): High-speed packet-switched data networks operating in parallel carry call setup, supervisory control, and routing messages for thousands of circuits.

SS7 Network Nodes & ISUP Call Flow

SS7 utilizes three primary nodes: Service Switching Points (SSP) (originating/tandem switches), Signal Transfer Points (STP) (packet switches in redundant mated pairs), and Service Control Points (SCP) (centralized databases).

Emergency calls execute setup via the ISDN User Part (ISUP) protocol:

1. Initial Address Message (IAM): The originating SSP reserves an idle voice trunk (identified by a Circuit Identification Code - CIC) and transmits an IAM packet containing the full 10-digit Calling Party Number (CPN/ANI), Called Party Number (9-1-1), and Originating Line Information (OLI).
2. Address Complete Message (ACM): The Selective Router confirms circuit reservation and returns an ACM packet indicating that the destination is ringing.
3. Answer Message (ANM): When the PSAP answers, an ANM packet triggers instantaneous voice cut-through.
4. Release (REL) & Release Complete (RLC): Structured packet handshake that terminates the call and returns the trunk to idle.

SS7 Advantages over CAMA

• Faster Setup: SS7 exchanges ISUP messages out-of-band, so call setup is typically much faster than in-band MF pulsing and the voice path carries no signaling tones.
• Native 10-Digit ANI: Eliminates number truncation and modified CAMA complexity.
• Continuous Circuit Telemetry: SS7 continuously monitors link integrity and voice trunk continuity, automatically routing around facility failures.

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4. ISDN Primary Rate Interface (ISDN-PRI) Demarcation

While SS7 handles inter-office trunking, ISDN Primary Rate Interface (ISDN-PRI) is the premier digital standard terminating high-density trunk groups from the Selective Router directly into PSAP call-handling equipment (CPE).

The 23B+D Channel Architecture

In North America, an ISDN-PRI operates over a channelized 1.544 Mbps T1 carrier structured into 24 discrete 64 kbps time-slots:

• 23 B-Channels (Bearer Channels): Channels 1 through 23 carry clear-channel digital voice audio (G.711 PCM sampled at 8 kHz with 8-bit resolution).
• 1 D-Channel (Delta Channel): Channel 24 is dedicated to out-of-band call signaling using the ITU-T Q.931 protocol.

Q.931 Signaling & Information Elements

Call supervision on ISDN-PRI uses structured Q.931 packets: SETUP (carries 10-digit ANI, Bearer Capability, and Channel ID Information Elements), CALL PROCEEDING, ALERTING, CONNECT, and DISCONNECT/RELEASE.

Non-Facility Associated Signaling (NFAS)

In large communications centers requiring multiple T1 circuits, NFAS enables a single D-channel on a primary T1 to control B-channels across up to 20 separate T1 lines (up to 478 B-channels). A backup D-channel on a secondary T1 ensures seamless failover if the primary circuit experiences a physical cut.

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5. The E9-1-1 Selective Router Tandem Switch & Routing Matrix

The Selective Router (SR) is a carrier-grade tandem switch (such as a Lucent 5ESS or Nortel DMS-100) that aggregates emergency traffic from Central Offices and routes it to regional PSAPs.

Selective Routing Database (SRDB) Mechanics

The router operates using memory translation tables:

1. ANI Ingestion: The SR buffers the incoming 10-digit or 7-digit ANI from the CAMA or SS7 trunk.
2. SRDB Indexing: The SR performs an in-memory lookup in the Selective Routing Database (SRDB), associating the ANI with a unique Emergency Service Number (ESN).
3. Trunk Translation: The SR references its trunk translation table to map the ESN to the outgoing dedicated trunk group connected to the Primary PSAP.
4. Outpulsing: The SR seizes an idle channel in that trunk group and delivers the call and ANI to the PSAP.

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6. Selective Router Routing Hierarchy & Failover Logic

To ensure that life-safety calls are never lost, the Selective Router executes a deterministic failover hierarchy:

1. Primary Selective Routing: Normal operation. The incoming ANI matches an SRDB entry, resolves to an ESN, and terminates at the primary PSAP.
2. Default Routing (Data Failure Mode): Triggers when the router cannot identify the caller's telephone number or location. This occurs when ANI is missing, unlisted, corrupted by line noise, or arriving from trunks lacking ANI capability. Because selective routing cannot be performed, the router executes a hardcoded trunk table: every call arriving over that specific incoming Central Office trunk group is steered to a pre-assigned Default PSAP responsible for that wire center's general territory.
3. Alternate Routing (Facility / Capacity Failure Mode): Triggers when dedicated trunks to the primary PSAP are saturated (All Trunks Busy - ATB) or physically severed. The router automatically diverts calls to a designated backup PSAP, secondary facility, or 10-digit administrative rollover number.
4. Night Service: Scheduled time-of-day diversion, rerouting calls to a consolidated 24-hour dispatch center.

Trunk Glare Protection

In telephony, glare occurs when both ends of a two-way trunk attempt simultaneous seizure, causing call collision. To eliminate glare, legacy 9-1-1 networks mandate that CAMA and basic PSAP emergency trunks operate strictly as one-way incoming circuits to the PSAP.

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7. End-to-End Signaling Comparison Matrix

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8. Operational Traps & ENP Exam Watch

• MF Tones vs. DTMF Tones: Do not confuse consumer DTMF (touch-tones) with Multi-Frequency (MF) tones used on CAMA trunks. CAMA uses 2-of-6 inter-office frequencies (700, 900, 1100, 1300, 1500, 1700 Hz) framed by KP and ST.
• The Wink Start Handshake: A CAMA trunk requires the receiving switch/PSAP to return a momentary reverse-battery pulse lasting 140 to 290 milliseconds. Without a wink, the originating switch times out and logs an ANI failure.
• Default Routing vs. Alternate Routing Root Causes: Default Routing = Data Failure (missing or garbled ANI). Alternate Routing = Facility / Capacity Failure (trunks saturated or severed).