Analog & the local loop
| Term | Definition |
|---|---|
| POTS | Plain Old Telephone Service. A single analog line over a copper pair, powered from the central office. |
| PSTN | Public Switched Telephone Network. The circuit-switched telephone network as a whole. |
| Local loop | The copper pair from the central office to the customer premises. |
| Tip and ring | The two conductors of a telephone pair, named for the parts of the operator's plug they once touched. |
| Central office (CO) | The exchange building housing switching equipment and the battery plant. |
| Wire center | The serving area of a central office. The unit in which copper retirement is planned. |
| Demarc | Demarcation point. The boundary between carrier and customer responsibility. |
| NIU / smart jack | Network Interface Unit. Carrier-owned equipment at the demarc, often supporting remote loopback testing. |
| Loop start | Supervision where the set seizes the line by closing the loop. Susceptible to glare. |
| Ground start | Supervision where the set grounds one conductor and waits for acknowledgement. Preferred for PBX trunks. |
| Glare | Collision when both ends seize a line simultaneously. |
| DTMF | Dual-Tone Multi-Frequency. Touch-tone dialing — one low and one high tone per digit. |
| Centrex | PBX-like features delivered from the carrier's central office rather than on-premises equipment. |
| DACT | Digital Alarm Communicator Transmitter. The fire alarm panel component that dials the central station. |
| ATA | Analog Terminal Adapter. Converts an analog line to SIP. |
| μ-law / A-law | Companding schemes for 8-bit PCM voice. μ-law in North America and Japan, A-law elsewhere. |
TDM & the digital hierarchy
| Term | Definition |
|---|---|
| TDM | Time Division Multiplexing. Sharing a circuit by allocating repeating time slots. |
| DS0 | The 64 kbps base channel — 8,000 samples per second at 8 bits. |
| DS1 / T1 | 1.544 Mbps carrying 24 DS0s plus 8 kbps of framing. DS1 is the signal; T1 is the carrier system. |
| Fractional T1 | A subscription to a subset of a T1's channels, with specific timeslots assigned by the provider. |
| E1 | The 2.048 Mbps equivalent used outside North America — 32 timeslots, 30 bearer. |
| DS3 / T3 | 44.736 Mbps, equivalent to 28 DS1s. |
| Channelized | Individual DS0s are separately addressable. Required for voice. |
| Clear channel | The full payload presented as one undivided pipe. |
| D4 / SF | Superframe framing — 12 frames per group. No in-service error monitoring. |
| ESF | Extended Superframe — 24 frames, adding CRC-6 error checking and a Facility Data Link. |
| AMI | Alternate Mark Inversion line coding. Needs ones-density stuffing, reducing data channels to 56 kbps. |
| B8ZS | Bipolar with 8-Zero Substitution. Preserves clear-channel 64 kbps. |
| Robbed-bit signaling | In-band supervision that steals the least significant bit of designated frames. |
| CSU/DSU | Channel Service Unit / Data Service Unit. Terminates a digital circuit at the premises. |
| Channel bank | Equipment multiplexing 24 analog lines onto a T1 and back. |
| DACS | Digital Access and Cross-connect System. Grooms DS0s between circuits. |
| LOS / LOF / AIS / RAI | T1 alarms: loss of signal, loss of frame (red), alarm indication (blue), remote alarm indication (yellow). |
Signaling
| Term | Definition |
|---|---|
| CAS | Channel Associated Signaling. Supervision carried within the voice channel. |
| CCS | Common Channel Signaling. Signaling on a dedicated channel, separate from voice. |
| ISDN | Integrated Services Digital Network. Digital voice and data with common channel signaling. |
| PRI | Primary Rate Interface. 23B+D on a T1; 30B+D on an E1. |
| BRI | Basic Rate Interface. 2B+D with a 16 kbps D-channel. |
| B-channel / D-channel | Bearer channel carrying media; Delta channel carrying signaling. |
| NFAS | Non-Facility Associated Signaling. One D-channel controlling B-channels across multiple spans. |
| Q.931 | ISDN layer 3 call control — SETUP, ALERTING, CONNECT, DISCONNECT. |
| Q.921 / LAPD | ISDN layer 2 link protocol for the D-channel. |
| QSIG | PBX-to-PBX signaling with feature transparency across nodes. |
| SS7 | Signaling System No. 7. The out-of-band signaling network between carriers. |
| ISUP | ISDN User Part. SS7 inter-switch call setup — IAM, ACM, ANM, REL. |
| SCCP / TCAP | SS7 layers enabling addressing and database transactions. |
| SSP / STP / SCP | SS7 elements: switching point, transfer point, control point (database). |
| Point code | An element's address on the SS7 network. |
| SIGTRAN | SS7 transported over IP using SCTP. |
| LNP / LRN | Local Number Portability; Location Routing Number identifying the switch now serving a ported number. |
| CNAM | Calling Name. Looked up by the terminating carrier — never carried with the call. |
| DNIS / ANI | Dialed Number Identification Service (the number called); Automatic Number Identification (the number calling). |
IP voice
| Term | Definition |
|---|---|
| SIP | Session Initiation Protocol (RFC 3261). Establishes, modifies and ends sessions. Carries no media itself. |
| SDP | Session Description Protocol. Negotiates codecs and media addresses inside SIP messages. |
| RTP / SRTP | Real-time Transport Protocol carrying the media; SRTP is the encrypted form. |
| SBC | Session Border Controller. A back-to-back user agent mediating signaling and media at the network border. |
| B2BUA | Back-to-Back User Agent. Terminates one session fully and originates a new one. |
| CAC | Call Admission Control. Enforces concurrent session limits per trunk group. |
| Topology hiding | Preventing internal network detail from appearing in outbound SIP headers. |
| SIP normalization | Rewriting headers to reconcile differing SIP dialects between systems. |
| Transcoding | Converting between codecs. Resource-intensive and quality-degrading. |
| Media anchoring | Routing RTP through the SBC rather than directly between endpoints. |
| SIP trunk | A logical, contracted connection to a provider carrying a number of concurrent sessions. |
| SIP ALG | Application Layer Gateway in a firewall that rewrites SIP. A frequent cause of subtle call failures. |
| REFER | The SIP method requesting a transfer. Carrier support varies. |
| OPTIONS ping | SIP OPTIONS used as a trunk heartbeat. |
| G.711 / G.729 | Codecs: G.711 is toll quality at DS0-comparable bitrate; G.729 is compressed with a quality cost. |
| T.38 | Fax over IP. Must be verified end to end — never assumed. |
| Jitter buffer | Buffer smoothing packet arrival variation, at the cost of added latency. |
| BYOC | Bring Your Own Carrier. Using your own carrier with a cloud platform rather than the vendor's telephony. |
Voice quality
| Term | Definition |
|---|---|
| MOS | Mean Opinion Score. Voice quality on a 1–5 scale. |
| MOS-CQE / LQO / LQS | Estimated (computed from network parameters), objective (measured from audio), and subjective (human-rated) MOS. |
| E-model | ITU-T G.107. Computes an R-factor from network and codec parameters and maps it to estimated MOS. |
| R-factor | The E-model transmission rating, roughly 0–100. Above 80 is generally acceptable. |
| GoB / PoW | Good or Better; Poor or Worse. Listener-proportion measures derived from R. |
| POLQA | ITU-T P.863. Full-reference perceptual quality measurement, wideband and full-band capable. |
| PESQ | ITU-T P.862. POLQA's narrowband predecessor. |
| PLC | Packet Loss Concealment. Masks small losses; ineffective against bursty loss. |
| G.114 | ITU-T guidance on one-way delay — around 150 ms as the comfortable limit. |
Contact center
| Term | Definition |
|---|---|
| CCaaS | Contact Center as a Service. Cloud-delivered contact center platform. |
| ACD | Automatic Call Distributor. Queues and routes contacts to agents. |
| IVR / IVA | Interactive Voice Response; Intelligent Virtual Agent — the conversational successor. |
| WFM / WFO | Workforce Management (forecasting and scheduling); Workforce Optimization (the wider suite including quality). |
| AHT | Average Handle Time — talk plus hold plus after-call work. |
| ASA | Average Speed of Answer. |
| Service level | Percentage of contacts answered within a target time, e.g. 80/20. |
| Abandonment rate | Share of contacts that leave the queue before being answered. |
| Occupancy | Proportion of logged-in time agents spend handling contacts. |
| Erlang | A unit of traffic — one Erlang is one resource occupied continuously for the measurement period. |
| Erlang B | The formula relating traffic, trunk count, and blocking probability. Used for trunk sizing. |
| Grade of service | Target blocking probability, e.g. P.01 for 1%. |
| Busy hour | The consistently busiest hour. The correct basis for sizing — averages are not. |
| Screen pop | Automatic display of customer context when a contact is answered. |
| RespOrg | Responsible Organization. The entity managing a toll-free number's routing records. |
AI & automation
| Term | Definition |
|---|---|
| NLU | Natural Language Understanding. Extracting intent and entities from what someone said. |
| Intent | The goal behind an utterance — what the customer is trying to achieve. |
| Entity / slot | A specific value extracted from an utterance, such as an account number or date. |
| ASR / WER | Automatic Speech Recognition; Word Error Rate, its accuracy measure. |
| Containment rate | Share of interactions handled end to end by automation without escalation. Should mean resolved, not merely stopped. |
| Deflection rate | Share of inquiries that never entered a human queue. Does not imply the customer was helped. |
| Resolution rate | Share where the customer's problem was actually solved. Frequently far below deflection. |
| FCR | First Contact Resolution. Solved in one interaction with no repeat contact. |
| Escalation / handoff | Transfer from automation to a human. Correctness includes whether context transferred. |
| Barge-in | The caller speaking over a prompt. A common turn-taking failure point. |
| Agentic AI | An AI system that takes actions in other systems, not only produces text. |
| Groundedness | Whether a response is supported by an actual retrieved source rather than generated freely. |
| Hallucination | A confident, fluent, false statement. In a contact center, frequently an invented policy. |
| Prompt injection | Adversarial input attempting to override an AI system's instructions. |
| Evaluation set | A labeled collection of real interactions used to measure quality reproducibly across releases. |
| Holdout set | Evaluation data withheld from the build team, so results measure generalization rather than fit. |
Regulatory & compliance
| Term | Definition |
|---|---|
| Section 214 | The FCC discontinuance authorization required to fully retire a service. |
| Copper retirement | Carrier decommissioning of copper facilities in favor of fiber or wireless. |
| Grandfathering | Closing a service to new customers while existing customers move to a retirement track. |
| MLTS | Multi-Line Telephone System. The category to which the 911 obligations attach. |
| Kari's Law | Requires direct 911 dialing without a prefix, plus on-site notification. |
| RAY BAUM'S Act §506 | Requires a dispatchable location with 911 calls. |
| Dispatchable location | Validated street address plus detail such as building, floor or room, and a callback number. |
| PSAP | Public Safety Answering Point. The 911 call center. |
| STIR/SHAKEN | Caller ID authentication framework — cryptographic signing and verification of calling party information. |
| Attestation A / B / C | Full, partial, and gateway attestation. Determines how downstream carriers treat your calls. |
| RMD | Robocall Mitigation Database. Requires filing and annual recertification. |
| TRACED Act | The legislation underpinning US caller ID authentication requirements. |
| PCI DSS v4.0.1 | Payment card security standard, mandatory since 31 March 2025. |
| PAN / CVV | Primary Account Number; Card Verification Value. CVV must never be stored post-authorization. |
| DTMF masking | Real-time suppression of keypad tones during card entry so digits never reach agent, recording or transcript. |
| CDE | Cardholder Data Environment. Everything in PCI scope. |
| QSA | Qualified Security Assessor. The party who validates PCI compliance. |
| AHJ | Authority Having Jurisdiction. The body that inspects and signs off life-safety systems. |
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