1.5 Signaling, Transport Protocols & Transmission Systems
Key Takeaways
- Digital voice uses pulse code modulation: 8,000 samples per second × 8 bits per sample = a 64 kbps DS0, and 24 DS0s form a 1.544 Mbps T1.
- In-band MF signaling carries ANI on CAMA trunks, DTMF is touch-tone keypad signaling, and SS7 and ISDN carry signaling out of band for faster setup and richer call information.
- ISDN BRI provides 2 bearer channels plus a 16 kbps D channel, while North American PRI provides 23 bearer channels plus a 64 kbps D channel on a T1.
- Data transport evolved from Frame Relay and ATM to IP and MPLS; carrier-managed MPLS with QoS commonly carries ESInet traffic, and SIP sets up NG9-1-1 sessions.
- Trunk concentration lets many lines share fewer trunks, so 9-1-1 trunk groups and selective routers must be engineered for grade of service and redundancy.
1.5 Signaling, Transport Protocols & Transmission Systems
Quick Answer: Voice and data move through 9-1-1 networks as analog signals (continuous waveforms) or digital signals (discrete bits). Digital telephony samples voice 8,000 times per second at 8 bits per sample, producing the 64 kbps DS0 channel that builds into a T1 (1.544 Mbps) and larger circuits. Signaling tells the network how to set up a call: in-band methods such as MF (used on CAMA trunks) and DTMF (touch-tone), and out-of-band methods such as SS7 and ISDN. Data networks moved from Frame Relay and ATM to IP and MPLS, and NG9-1-1 uses SIP over IP to set up multimedia emergency calls. Trunk concentration lets many lines share fewer trunks, which is why trunk groups must be engineered for a grade of service.
1. Analog vs. Digital Transmission
| Characteristic | Analog | Digital |
|---|---|---|
| Signal form | Continuous waveform that mirrors the voice | Discrete values (ones and zeros) |
| Noise | Noise and distortion build up with distance and amplification | Regenerators restore clean bits, so quality holds over distance |
| Capacity | One conversation per pair or channel unless frequency-division multiplexed | Many conversations share a circuit through time-division multiplexing |
| Examples | POTS local loop, analog two-way radio, older CAMA trunks | T1/E1, SONET, ISDN, IP networks, P25 digital radio |
From Voice to Bits: Pulse Code Modulation (PCM)
- Sampling: The voice signal is sampled 8,000 times per second, a little more than twice the highest frequency in the roughly 4 kHz telephone voice band (the Nyquist principle).
- Quantizing and encoding: Each sample becomes an 8-bit value. North America uses mu-law companding; most other countries use A-law.
- Result: 8,000 samples × 8 bits = 64 kbps, one DS0 channel. This is the same rate as the G.711 codec used in VoIP and NG9-1-1.
The Digital Hierarchy
| Level | Channels | Rate | Notes |
|---|---|---|---|
| DS0 | 1 | 64 kbps | One voice channel |
| T1 (DS1) | 24 DS0s | 1.544 Mbps | North American standard; framing adds 8 kbps to the 1.536 Mbps payload |
| E1 | 30 voice + 2 overhead channels | 2.048 Mbps | International standard |
| DS3 (T3) | 28 T1s (672 DS0s) | 44.736 Mbps | Carrier transport |
| SONET OC-3 | 3 × STS-1 | 155.52 Mbps | Fiber rings; OC-1 is 51.84 Mbps |
SONET rings can switch traffic around a fiber cut in about 50 milliseconds, which is why carriers use them for critical 9-1-1 transport.
2. Signaling Methods
In-Band Signaling
- Multi-Frequency (MF): Pairs of tones sent over the voice path between switches. CAMA 9-1-1 trunks use MF to pass ANI to the selective router and PSAP (1.3).
- Dual-Tone Multi-Frequency (DTMF): The touch-tone keypad. Each key sends two tones at once, one from a low group (697, 770, 852, or 941 Hz) and one from a high group (1209, 1336, 1477, or 1633 Hz). PSAPs meet DTMF in automated menus, voicemail and conferencing systems, and some alarm and paging interfaces.
Out-of-Band Signaling
- Signaling System 7 (SS7): A separate packet network, built from signal transfer points, service switching points, and service control points, that sets up and tears down calls, carries calling number information, and supports database lookups such as number portability. Because signaling stays off the voice path, calls set up faster and more information travels with them.
- Integrated Services Digital Network (ISDN): Digital access that separates bearer (B) channels from a signaling (D) channel:
- Basic Rate Interface (BRI): 2 B channels (64 kbps each) + 1 D channel (16 kbps) = 144 kbps.
- Primary Rate Interface (PRI): In North America, 23 B channels + 1 D channel (64 kbps) on a T1. PRI trunks are common for PBX connections and some PSAP call-handling equipment.
Intelligent Networks
The Advanced Intelligent Network (AIN) moved call-processing logic out of individual switches into centralized databases (service control points) reached over SS7. That design enabled services such as toll-free number translation and local number portability lookups.
3. Packet Data Networks: Frame Relay, ATM, IP, and MPLS
| Technology | How It Works | Strengths | Status in 9-1-1 |
|---|---|---|---|
| Frame Relay | Variable-length frames over permanent virtual circuits (PVCs) identified by DLCIs, with a committed information rate (CIR) | Cheaper than dedicated lines for bursty data | Legacy; once used for ALI and CAD data links, now largely retired |
| Asynchronous Transfer Mode (ATM) | Fixed 53-byte cells (5-byte header + 48-byte payload) over virtual circuits with defined service classes | Predictable delay for voice and video | Legacy carrier backbones |
| Internet Protocol (IP) | Connectionless packets routed hop by hop, best effort unless QoS is added | Universal and flexible; carries voice, video, text, and data | Foundation of ESInets and NG9-1-1 |
| MPLS | Routers forward packets by short labels assigned at the network edge (often called "Layer 2.5") | Traffic engineering, QoS classes, private VPNs, fast reroute | Common carrier-managed ESInet transport |
Session Initiation Protocol (SIP)
SIP is the signaling protocol that sets up, changes, and ends IP multimedia sessions. In NG9-1-1, SIP carries the call to the ESRP and PSAP with location attached, while the media travels separately over RTP (3.3). SIP trunks also connect modern PBXs and call-handling systems to carriers.
4. Trunk Concentration and Selective Routing
Trunk concentration means many subscriber lines share a smaller number of trunks, because not everyone calls at once. It saves money but creates the possibility of blocking:
- A concentration ratio (for example, ten lines per trunk) works only if traffic studies show the busy-hour load fits the trunk group.
- 9-1-1 trunk groups are engineered to a P.01 grade of service using Erlang B, and planners watch for events such as storms, major incidents, and TDoS attacks that create surges far above the normal busy hour.
- The selective router is itself a concentration point: many end offices converge on one tandem, so its redundancy, alternate routing tables, and default routing are critical (1.1, 1.3, and 1.6).
5. Operational Traps & ENP Exam Watch
- 64 kbps Comes From 8,000 × 8: Know how the DS0 rate is derived and that 24 DS0s make a 1.544 Mbps T1.
- MF vs. DTMF: MF passes digits between switches on trunks such as CAMA; DTMF is the subscriber's touch-tone keypad.
- SS7 Is Out-of-Band: SS7 signaling travels on a separate network, unlike MF tones on the voice path.
- BRI vs. PRI: BRI is 2B+D with a 16 kbps D channel; North American PRI is 23B+D with a 64 kbps D channel on a T1.
- ATM Cells Are Fixed Size: ATM cells are always 53 bytes, while Frame Relay frames vary in length.
- MPLS Is Not the Public Internet: MPLS networks are carrier-managed with QoS, which is why ESInets often use them.
A standard DS0 digital voice channel carries how much data, and how is that rate derived?
Which transport technology forwards packets using short labels assigned at the network edge and is commonly used for carrier-managed ESInet connections?
How does a North American ISDN Primary Rate Interface (PRI) differ from a Basic Rate Interface (BRI)?