Glossary
Telecom & VoIP Glossary
Clear, standards-based definitions of 39 terms used in VoIP, SIP trunking, codecs, wholesale voice, telecom billing, mobile networks and GSM gateways. Each entry explains what the term means, how it works and how it relates to running mobile lines as managed SIP gateways.
GSM gateways
VoIP & SIP
- DTMFDual-Tone Multi-FrequencyDTMF (dual-tone multi-frequency) is the keypad tone signalling used for IVRs and PINs. In-band, RFC 4733 and SIP INFO methods compared.
- JitterPacket delay variationJitter is the variation in packet arrival time on a VoIP call. How it is measured, what jitter buffers do, acceptable levels and how to reduce it.
- NAT TraversalSIP NATNAT traversal lets SIP and RTP cross routers and firewalls. Why NAT breaks VoIP, and how STUN, TURN, ICE, rport and SBCs fix one-way audio.
- RTPReal-time Transport ProtocolRTP (Real-time Transport Protocol, RFC 3550) carries the audio of VoIP calls. Learn RTP headers, RTCP, payload types, ports and common RTP problems.
- SBCSession Border ControllerA session border controller (SBC) secures and controls SIP traffic at the network edge: topology hiding, NAT, transcoding, DoS protection and interop.
- SIPSession Initiation ProtocolSIP (Session Initiation Protocol, RFC 3261) sets up, manages and ends VoIP calls. Learn SIP methods, response codes, ports and how SIP trunks use it.
- SRTPSecure Real-time Transport ProtocolSRTP (RFC 3711) encrypts and authenticates VoIP audio. How SRTP works, SDES vs DTLS-SRTP key exchange, and why it needs TLS for signalling.
- TLSTransport Layer SecurityTLS encrypts SIP signalling on port 5061. Learn how SIP over TLS works, certificates, TLS 1.2 vs 1.3, and why it should be paired with SRTP.
- VoIPVoice over Internet ProtocolVoIP (Voice over Internet Protocol) carries phone calls as data packets over IP networks. How it works, the protocols used and what affects call quality.
- WebRTCWeb Real-Time CommunicationWebRTC lets browsers and apps make encrypted voice and video calls without plugins. How it works, ICE, DTLS-SRTP, Opus, and how it connects to SIP.
Codecs
- CodecVoice codecA voice codec encodes speech into digital data for VoIP calls. Compare G.711, G.729, G.722 and Opus by bitrate, audio bandwidth and quality.
- G.711PCMA / PCMUG.711 is the 64 kbit/s ITU-T voice codec used by the PSTN and most SIP trunks. A-law vs µ-law, bandwidth per call and when to use it.
- G.722HD voiceG.722 is the ITU-T wideband HD voice codec: 50–7000 Hz audio at 64 kbit/s. How it works, the 8000 Hz SDP quirk, and HD voice over gateways.
- G.729G.729aG.729 is an 8 kbit/s ITU-T voice codec used to save bandwidth on VoIP and wholesale links. Annex A and B, bandwidth, quality and DTMF caveats.
- OpusOpus codecOpus (RFC 6716) is an open, royalty-free audio codec from 6 to 510 kbit/s, mandatory in WebRTC. How it works, FEC, and Opus vs G.711 and G.722.
Numbers & trunks
- DIDDirect Inward DialingA DID (direct inward dialing) number routes external calls straight to an extension, IVR or queue over a SIP trunk. How DIDs work and how to route them.
- DODDirect Outward DialingDOD (direct outward dialing) lets PBX users call external numbers directly, showing an authorised caller ID. How DOD works on SIP trunks.
- ExtensionPBX extensionA PBX extension is an internal number for a phone, user or feature. How SIP extensions register, how they are dialled and how they map to DIDs.
- IVRInteractive Voice ResponseIVR (interactive voice response) answers calls with menus that callers navigate by keypad or voice. How IVRs work, DTMF input and design tips.
- PBXPrivate Branch ExchangeA PBX (private branch exchange) is a business phone system that connects internal extensions and routes calls to trunks. IP PBX, cloud PBX and features.
- SIP TrunkSIP trunkingA SIP trunk connects a PBX to a carrier or gateway over IP for inbound and outbound calls. Registration vs IP auth, channels, DIDs and failover.
Switching
- Class 4 SoftswitchTandem switchA Class 4 softswitch routes high volumes of calls between carriers (transit/tandem). Features, LCR routing, CDRs and how it differs from Class 5.
- Class 5 SoftswitchLocal exchange switchA Class 5 softswitch serves end users with calling features: extensions, voicemail, IVR, hosted PBX and forwarding. How it differs from Class 4.
- SoftswitchSoftware switchA softswitch is software that routes and controls voice calls on IP networks, replacing hardware exchanges. Class 4 vs Class 5, functions and examples.
Wholesale voice
- ACDAverage Call DurationACD (average call duration) is billed minutes divided by answered calls. How to calculate ACD, what it tells you about route quality and how to use it.
- ASRAnswer-Seizure RatioASR (answer-seizure ratio) is the percentage of call attempts that are answered. How to calculate ASR, typical values and what a drop means.
- Call OriginationVoice originationCall origination delivers calls from the phone network to your VoIP service, usually via DIDs. How origination works and how it differs from termination.
- Call TerminationVoice terminationCall termination is delivering a call to the network of the person being called. How lawful termination, termination rates and interconnects work.
- PDDPost-Dial DelayPDD (post-dial delay) is the time from dialling until the caller hears ringing. How PDD is measured in SIP, typical values and common causes.
- Wholesale VoIPWholesale voiceWholesale VoIP is the carrier-to-carrier market for voice minutes. How rate decks, routes, quality metrics and licensed interconnects work.
Billing
- CDRCall Detail RecordA CDR (call detail record) logs every call: numbers, times, duration, route and cause code. What CDRs contain and how they drive billing and analytics.
- Telecom BillingVoIP billingTelecom billing turns call records into charges: rating, prepaid and postpaid accounts, invoicing and reconciliation. How VoIP billing systems work.
SIM infrastructure
- Remote SIM GatewayRemote SIMA remote SIM gateway uses SIM cards held elsewhere, in a SIM bank, over IP. How it works, legitimate uses, and why operators restrict it.
- SIM BankSIM serverA SIM bank holds many SIM cards in one place and connects them remotely to devices. Legitimate uses, how it works, and the regulatory context.
- SIM BoxSIM box fraudA SIM box is a gateway that routes international calls through local SIMs to avoid termination fees. Why SIM-box bypass is illegal and how it is detected.
Mobile networks
- IMSIP Multimedia SubsystemIMS (IP Multimedia Subsystem) is the 3GPP SIP-based core that delivers VoLTE, VoWiFi and VoNR calls. Key functions: P-CSCF, S-CSCF, HSS and TAS.
- VoLTEVoice over LTEVoLTE (Voice over LTE) carries mobile calls as IP packets over 4G via the IMS core. How it works, HD codecs, and why it matters after 2G/3G shutdowns.
- VoWiFiWi-Fi CallingVoWiFi (Wi-Fi Calling) carries operator calls over Wi-Fi via an ePDG into the IMS core. How it works, requirements, handover and gateway use.