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 The number of samples taken per unit time, i.e., the rate at which signals are sampled for subsequent use, such as for modulation, coding, and quantization. Deprecated synonym sampling frequency.
 Service access point - In an Open Systems Interconnection (OSI) layer, a point at which a designated service may be obtained.
 A satellite system or a part of a satellite system, consisting of only one satellite and the cooperating Earth stations.
 Service data unit - In layered systems, a set of data that is sent by a user of the services of a given layer, and is transmitted to a peer service user semantically unchanged.
 Signaling in which the whole or a part of one or more channels in a multichannel system is used to provide for supervisory and control signals for the message traffic channels. (188) Note: The same channels, such as frequency bands or time slots, that are used for signaling are not used for message traffic. Contrast with common-channel signaling.
 A network device that provides service to the network users by managing shared resources. Note 1: The term is often used in the context of a client-server architecture for a local area network (LAN). Examples are a printer server and a file server.
 In the Open Systems Interconnection--Reference Model (OSI--RM), a capability of a given layer, and the layers below it, that is provided to the entities of the next higher layer and for a given layer, is provided at the interface between the given layer and the next higher layer.
 A time-dependent variation of a characteristic of a physical phenomenon, used to convey information.
 Signaling in which two conductors are used for a single channel, and a center-tapped coil, or its equivalent, is used to split the signaling current equally between the two conductors. SX signaling may be one-way, for intra-central-office use, or the simplex legs may be connected to form full duplex signaling circuits that function like composite (CX) signaling circuits with E & M lead control.
 SIP, the session intitation protocol, is the IETF protocol for VOIP and other text and multimedia sessions, like instant messaging, video, online games and other services.

The Session Initiation Protocol (SIP) is an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences. It is a text-based protocol, similar to HTTP and SMTP.

SIP invitations used to create sessions carry session descriptions that allow participants to agree on a set of compatible media types. SIP makes use of elements called proxy servers to help route requests to the user's current location, authenticate and authorize users for services, implement provider call-routing policies, and provide features to users. SIP also provides a registration function that allows users to upload their current locations for use by proxy servers. SIP runs on top of several different transport protocols.
 A variable-duration window that allows a sender to transmit a specified number of data units before an acknowledgement is received or before a specified event occurs. An example of a sliding window in packet transmission is one in which, after the sender fails to receive an acknowledgement for the first transmitted packet, the sender slides the window, i.e., resets the window, and sends a second packet. This process is repeated for the specified number of times before the sender interrupts transmission. Synonym (loosely) acknowledgement delay period.
 In a distributed-queue dual-bus (DQDB) network, a protocol data unit (PDU) that consists of 53 octets used to transfer segments of user information has the capacity to contain a segment of 52 octets and a 1-octet access control field, and may be either a pre-arbitrated (PA) slot or a queued arbitrated (QA) slot.
 In networks using carrier sense multiple access with collision detection (CSMA/CD), the length of time that a transmitting station waits before attempting to retransmit following a collision. Note: Slot time varies from station to station.
 The Transmission Control Protocol/Internet Protocol (TCP/IP) standard protocol that facilitates transfer of electronic-mail messages, specifies how two systems are to interact, and specifies the format of messages used to control the transfer of electronic mail.\r\n
 Simple Network Management Protocol (SNMP): The Transmission Control Protocol/Internet Protocol (TCP/IP) standard protocol that is used to manage and control IP gateways and the networks to which they are attached, uses IP directly, bypassing the masking effects of TCP error correction, has direct access to IP datagrams on a network that may be operating abnormally, thus requiring management, defines a set of variables that the gateway must store, and specifies that all control operations on the gateway are a side-effect of fetching or storing those data variables, i.e., operations that are analogous to writing commands and reading status.
 Signal-to-noise ratio - The ratio of the amplitude of the desired signal to the amplitude of noise signals at a given point in time. SNR is expressed as 20 times the logarithm of the amplitude ratio, or 10 times the logarithm of the power ratio. SNR is usually expressed in dB and in terms of peak values for impulse noise and root-mean-square values for random noise. In defining or specifying the SNR, both the signal and noise should be characterized, e.g., peak-signal-to-peak-noise ratio, in order to avoid ambiguity.
 start-of-text character A transmission control character that precedes a text and may be used to terminate the message heading.
 In an Internet Protocol (IP) address, an extension that allows users in a network to use a single IP network address for multiple physical subnetworks. The IP address contains three parts: the network, the subnet, and host addresses. Inside the subnetwork, gateways and hosts divide the local portion of the IP address into a subnet address and a host address. Outside of the subnetwork, routing continues as usual by dividing the destination address into a network portion and a local portion.
 A collection of equipment and physical transmission media that forms an autonomous whole and that can be used to interconnect systems for purposes of communication.
 A comparison of checksums on the same data on different occasions or on different representations of the data in order to verify data integrity. Synonym sum check.
 In communications systems, a mechanical, electro-mechanical, or electronic device for making, breaking, or changing the connections in or among circuits.
 A communications network, such as the public switched telephone network, in which any user may be connected to any other user through the use of message, circuit, or packet switching and control devices
 synchronous idle character A transmission control character used in synchronous transmission systems to provide a signal from which synchronism or synchronous correction may be achieved between data terminal equipment, particularly when no other character is being transmitted.
 synchronization pulse - A pulse used to achieve or maintain synchronism. Note: The term synchronization pulse is usually applied to analog signals, whereas the term synchronization bit is usually applied to digital data streams. Synonym sync pulse.
 A multiplexing scheme in which timing is obtained from a clock that controls both the multiplexer and the channel source.