Telephony

ISDN user-network call control: SETUP, CALL PROCEEDING, ALERTING, CONNECT, DISCONNECT, RELEASE. Information elements carry bearer capabil…

Primary Rate ISDN: T1 23B+D or E1 30B+D with a 64 kb/s D-channel for Q.931/LAPD. Timeslots map B-channels; framing and line coding come f…

ITU umbrella for packet multimedia: H.225.0 call signaling/RAS, H.245 media control, and RTP/RTCP media. Gatekeepers provide admission an…

Text-based signaling for creating and managing sessions. Methods (INVITE, ACK, BYE, REGISTER…) and response codes resemble HTTP; SDP bodi…

Textual session description: media lines, codecs (rtpmap/fmtp), ICE candidates, and direction attributes. Carried in SIP, WebRTC, and MGC…

End-to-end media transport for audio/video: sequence numbers, timestamps, SSRC, and payload type. Usually over UDP; profile and format sp…

SS7 Message Signal Units carry user-part payloads over MTP. Level-2 framing wraps length, sequence/ACK bits, and SIO/SIF; Level-3 routing…

SS7 call-control user part for trunk setup: IAM, ACM, ANM, REL, RLC and related messages. Circuit identification codes bind signaling to …

Reliable, ordered byte-stream transport with ports, sequence/ack numbers, windowing, and congestion control. Three-way handshake and four…

SS7 link-layer framing with flags, sequence numbers (BSN/FSN), FIB/BIB retransmission bits, and FISU/LSSU/MSU SU types. Alignment procedu…

SS7 network layer providing routing labels (DPC/OPC/SLS), linkset/route management, and discrimination to user parts. It carries ISUP, SC…

SS7 connectionless and connection-oriented transport with global title translation (GTT) for number-based routing. It underpins TCAP and …

Transaction and dialogue layer over SCCP for intelligent-network and mobile services. Begin/Continue/End/Abort components carry MAP, CAP,…

GSM/UMTS Mobile Application Part over TCAP for location update, authentication, SMS, and subscriber data management. Operation codes and …

CAMEL Application Part for prepaid and intelligent services in mobile networks. InitialDP and subsequent event reports drive SCP interact…

Intelligent Network Application Part for fixed-network service control over TCAP. CS-1/CS-2 operations resemble CAP but target wireline S…

Stream Control Transmission Protocol — a multi-stream, multi-homed transport used heavily by SIGTRAN and some WebRTC data channels. Chunk…

SIGTRAN adaptation that replaces MTP2 over SCTP while preserving MTP3 peer semantics between IP-connected SS7 nodes. Link status and sequ…

SIGTRAN adaptation that backhauls MTP2 user primitives over SCTP between a signaling gateway (SGP) and application server (ASP). It separ…

SIGTRAN MTP3 user adaptation over SCTP that carries ISUP/SCCP payloads with routing keys for IP-SS7 interconnect. ASPs and SGPs exchange …

SIGTRAN SCCP User Adaptation delivering SCCP-user data over SCTP without a full MTP3 stack on the application server. Common for TCAP app…

ISDN Basic Rate Interface providing two 64 kb/s B-channels plus a 16 kb/s D-channel for Q.931 signaling (2B+D). S/T and U interfaces diff…

Channel Associated Signaling and MFC-R2 multi-frequency compelled trunk signaling used on legacy TDM interconnects. Line signaling may be…

Time-division multiplexed digital trunks such as T1 (24×DS0) and E1 (32 timeslots) framed per G.704 and related recommendations. Framing,…

64 kb/s PCM audio codecs for telephony: μ-law (PCMU, North America/Japan) and A-law (PCMA, elsewhere) compress linear samples with logari…

Companion control protocol to RTP providing sender/receiver reports, SDES identity, BYE, and extension feedback. Report intervals scale w…

Secure RTP/RTCP with encryption and authentication, keyed from SDES, DTLS-SRTP, or MIKEY. A rollover counter (ROC) and key derivation pro…

RTP payload format for named telephone events such as DTMF digits, replacing unreliable in-band tones. Event, end bit, volume, and durati…

Real-time fax over IP using UDPTL or RTP to carry T.30 facsimile phases across packet networks. Gateways translate between PSTN fax modem…

Media Gateway Control Protocol in which Call Agents issue CRCX/MDCX/DLCX and signal requests to subordinate media gateways. SDP in the te…

H.248/Megaco media-gateway control using terminations, contexts, and descriptors for large carrier gateways. It succeeds MGCP conceptuall…

Cisco Skinny Client Control Protocol (SCCP) for phone registration and call control toward CUCM. Station messages drive lamp, ringer, and…

AAA and policy protocol that succeeds RADIUS in LTE/IMS and many 5G control functions. AVPs, application IDs (Gx, Gy, Cx, S6a, and others…

DNS-based mapping of E.164 telephone numbers to URIs via NAPTR records (often sip: or mailto:). Used in number portability, VoIP peering,…

Session Traversal Utilities for NAT — binding requests discover reflexive addresses and keep NAT mappings alive for ICE. Attributes carry…

Traversal Using Relays around NAT — a STUN extension that allocates relayed transport addresses when direct peer paths fail. Permissions …

Interactive Connectivity Establishment gathers local, reflexive, and relay candidates, then runs STUN checks to pick working media paths.…

Application-defined signaling — often SIP, or JSON over WebSocket — that exchanges SDP offers/answers and ICE candidates for WebRTC peers…

TCP protocol between External Short Messaging Entities (ESMEs) and SMSCs for submitting and delivering SMS. PDUs include bind, submit_sm,…

SIP MESSAGE method for pager-mode instant messaging and some RCS-related flows. Bodies carry text or CPIM; it is not a substitute for MSR…

Digital Mobile Radio — an ETSI TDMA LMR standard with two slots in a 12.5 kHz channel for voice, data, and trunking control. Tier I/II/II…

APCO Project 25 public-safety digital radio with FDMA Phase 1 and TDMA Phase 2 common air interfaces. IMBE/AMBE vocoders, trunking contro…

H.323 call signaling and RAS: Q.931-derived Setup/Alerting/Connect on the Call Signaling Channel plus gatekeeper discovery/registration. …

H.323 multimedia control protocol for capability exchange, master/slave determination, and logical channel open/close. Fast Start and H.2…

Local Number Portability lets a subscriber keep an E.164 number when changing carriers. Originating switches query an NPDB (often via TCA…

Calling Name Delivery presents a display name for the calling party at the terminating set. Classic wireline dips query a CNAM/LIDB datab…

Line Information Database holds per-line attributes used by calling-card, collect, third-number billing, and CNAM services. Queries trave…

North American Advanced Intelligent Network defines detection points, triggers, and SCP transactions (often over TCAP) for number transla…

Toll-free / 8XX service translates a dialed toll-free number to a routing number or carrier via an SCP (SMS/800, RespOrg databases) over …

ANSI-41 (IS-41) is the North American mobile application protocol family over TCAP for AMPS/TDMA/CDMA mobility: registration, authenticat…

User Datagram Protocol — connectionless transport with ports and an optional checksum. It is the basis for DNS, RTP, QUIC, DHCP, and RoCE…

Datagram Transport Layer Security adapts TLS to unreliable transports with retransmission, explicit sequence numbers, and record epochs. …