S
SAP	Service access point. Field that is part of an address specification.
SAR	Segmentation and reassembly. In ATM, the process of dividing PDUs into 48-byte pieces of payload data at the source for transport and then reassembling them into a stream at the destination.
SAS	Special exchange subscriber (as in SAS loop-start and ground-start). Identical to FXS loop-start and ground-start in functionality, but makes different use of the A and B bits. (SAS-transmitted B bits are complementary to FXS-transmitted B bits. SAS-received A bits are complementary to FXS-received A bits.)
SCCP	Skinny (or Simple) Client Control Protocol. Cisco-proprietary protocol that defines call-connection methods and signaling between IP phones and a router. Allows IP phones to coexist in an H.323 environment. Savings in memory size, processor power, and complexity makes the protocol desirable.
SCN	Switched-circuit network. Network that carries traffic within channelized bearers of predefined sizes.
SCTP	Stream Control Transmission Protocol. Protocol for general IP transport.
SDP	1. Session Definition (or Description) Protocol. Protocol that defines multimedia services. SDP messages can be part of SGCP and MGCP messages. 2. Session Data Protocol. Protocol that describes multimedia sessions for the purposes of session announcement, session invitation, and other forms of multimedia session initiation.
secondary dial tone	Additional transmission of dial tone after one or more specified digits have been dialed on an outgoing call. Typically used for external calls and is intended to reassure the caller that the dialing of an external number can continue after the dialing of an access digit. Remains on until a subsequent digit is received from the phone.
SED	Stream editor.
sequential ephone hunt group	Hunt group in which ephone-dns ring in the left-to-right order in which they are listed when the hunt group is defined. The first number to ring is always the first number in the list (the number that is farthest left in the list). See also peer ephone hunt group.
session expiration	Time at which an element considers the call timed out if no successful INVITE-request transaction occurs first.
session interval	Largest amount of time that can occur between INVITE requests in a call before a call is timed out, as conveyed in the Session-Expires header.
SGCP	Simple Gateway Control Protocol. Protocol that controls VoIP gateways using an external call control element (called a call agent). Used to establish, maintain, and disconnect calls across an IP network.
shared ephone-dn	An ephone-dn that appears on more than one phone.
signaling gateway	Gateway that supports only signaling traffic (no bearer traffic). Transmits PSTN signaling at the edge of an IP/ATM network and backhauls the signaling to a media gateway controller. For example, a gateway that terminates SS7 A-links is a signaling gateway. Sometimes designated SG.
silent ring	Audible ring and call-waiting beeps are suppressed for incoming calls. Visible cues are the same as for a normal ring, including the flashing ((< icon in the phone display and the flashing red light on the handset.
simple forking	Forking scenario in which all of the branches of a fork use the same voice codec compression. See also complex forking.
simple mixing	Situation in which a mixer receives multiple streams of the same codec type and combines them into one stream that is sent to the telephony interface.
single-number voice and fax	See fax detection.
SIP	Session Initiation Protocol. Protocol, developed as an alternative to H.323, that equips platforms to signal the setup of voice and multimedia calls over IP networks.
SIP proxy server	SIP proxy server. Sometimes designated SPS. See proxy server.
SIP session	Session that includes a set of multimedia senders and receivers and the data streams that flow between senders and receivers. A SIP multimedia conference is an example of a session. The called party can be invited several times by different calls to the same session.
SIP URL	Session Initiation Protocol Uniform (or Universal) Resource Locator. Used in SIP messages to indicate the originator, recipient, and destination of the SIP request. Takes the basic form of user@host, where user is a name or telephone number, and host is a domain name or network address.
SLA	Service-level agreement. Contract between a wholesaler and a service provider that specifies the connectivity, performance, and availability levels that the wholesaler guarantees.
SLIP	Serial Line Internet Protocol. Protocol for point-to-point serial connections using a variation of TCP/IP. Largely superseded by PPP.
SM	Session manager.
SMTP	Simple Mail Transfer Protocol. Simple ASCII protocol that describes the exchange of e-mail between two message-transfer agents using TCP/IP.
SNAP	Subnetwork Access Protocol. Protocol that operates between a network entity in the subnetwork and a network entity in the end system. Specifies a standard method of encapsulating IP datagrams and ARP messages on IEEE networks. The SNAP entity in the end system makes use of the services of the subnetwork and performs three key functions: data transfer, connection management, and QoS selection.
SNMP	Simple Network Management Protocol. Protocol, used in TCP/IP networks, for monitoring and controlling network devices and managing configurations, statistics collection, performance, and security.
socket listener	Software provided by a socket client to receives datagrams addressed to the socket.
soft keys	Patterns on an IP-phone LCD display that represent keys with labels indicating the particular actions that a phone user can take by pressing the display where they appear. Display of these keys changes according to the current activity of the phone. See also buttons.
SONET	Synchronous Optical Network. Standard format for transporting a wide range of digital telecommunications services over optical fiber. Characterized by standard line rates, optical interfaces, and signal formats.
source IP group	VoIP-side grouping of signaling characteristics that are associated with incoming H.323 or SIP calls.
span	Full-duplex digital transmission line between two digital facilities.
SPE	Service-processing element.
speech	Encoded data delivered via an RTP stream. Excludes fax, modem, and DTMF relay. Includes DTMF digits that are sent as inband audio when DTMF relay is not enabled.
speed dial	Ability to automatically make a call to a number by using a code or special button rather than dialing the complete number. See also local speed dial and personal speed dial.
SPI	Service-provider interface.
SPRT Protocol	Simple Packet Relay Transport Protocol. Simple packet-based protocol layered on UDP/IP that provides reliable in-sequence delivery of data across the IP network. Used in modem relay for fax.
SPVC	Soft permanent virtual circuit.
SRST	Survivable remote-site telephony.
SSCS	Service-specific convergence sublayer. One of the two sublayers of any AAL. SSCS is service dependent and offers assured data transmission. The SSCS can be null as well, in classical IP-over-ATM or LAN-emulation implementations.
ssinfo	Structure in a raw message that contains all decoded supplementary service information.
stateful proxy server	SIP proxy server that remembers incoming and outgoing requests, provides reliable retransmission of proxied requests, and returns the best final responses.
stateless proxy server	SIP proxy server that forgets all information after a request or response is processed. It merely forwards requests and responses.
static payload	Payload that is assigned a specific RTP format and is generally grouped for specific applications—for example, audio and video conferencing. See also dynamic payload.
static route	Routes that have been manually provisioned into a proxy server. Generally, there is no dynamic information about the existence, availability, or capacity of this route or it would have been discovered using TGREP. Based on policy within the location server, static routes may be injected into TRIP, in which case they are also considered local routes to that location server.
store and forward	Function whereby a message is transmitted to some intermediate relay point and temporarily stored before forwarding to the next relay point.
store-and-forward fax	See T.37.
stream	One of multiple media branches that constitute a fork.
subrate	Less than the standard rate of transmission, which is defined at the voice-grade rate of 64 kbps.
SUS message	ISUP Suspend message.
suspend message	ISDN Suspend message.
SVC	Switched virtual circuit. Virtual circuit that is dynamically established on demand and torn down when transmission is complete. Used in situations where data transmission is sporadic. Called a switched virtual connection in ATM terminology. See also virtual circuit.
switchover	Transfer of control from the active to a standby router when the active router fails, is removed from the network, or is manually taken down. See also failover.