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acknowledgment---Notification sent from one network device to another to acknowledge that some event (for example, receipt of a message) occurred. Sometimes abbreviated ACK.
all-routes explorer packet---Explorer packet that traverses an entire SRB network, following all possible paths to a specific destination. Sometimes called all-rings explorer packet.
AppleTalk---Series of communications protocols designed by Apple Computer consisting of two phases. Phase 1, the earlier version, supports a single physical network that can have only one network number and be in one zone. Phase 2, supports multiple logical networks on a single physical network and allows networks to be in more than one zone.
APPN---(Advanced Peer-to-Peer Networking) Enhancement to the original IBM SNA architecture. APPN handles session establishment between peer nodes, dynamic transparent route calculation, and traffic prioritization for APPC traffic.
backoff---The (usually random) retransmission delay enforced by contentious MAC protocols after a network node with data to transmit determines that the physical medium is already in use.
BECN---(backward explicit congestion notification) Bit set by a Frame Relay network in frames traveling in the opposite direction of frames encountering a congested path. DTE receiving frames with the BECN bit set can request that higher-level protocols take flow control action as appropriate.
BNN---(boundary network node) In SNA terminology, a subarea node that provides boundary function support for adjacent peripheral nodes. This support includes sequencing, pacing, and address translation.
CIR---(committed information rate) Rate at which a Frame Relay network agrees to transfer information under normal conditions, averaged over a minimum increment of time. CIR, measured in bits per second, is one of the key negotiated tariff metrics.
cluster controller---In SNA, a programmable device that controls the input/output operations of attached devices. Typically, an IBM 3174 or 3274 device.
CoS---(class of service) An indication of how an upper-layer protocol requires a lower-layer protocol to treat its messages. In SNA subarea routing, CoS definitions are used by subarea nodes to determine the optimal route to establish a given session. A CoS definition comprises a virtual route number and a transmission priority field. Also called ToS.
data-link control layer---Layer 2 in the SNA architectural model. Responsible for the transmission of data over a particular physical link. Corresponds roughly to the data-link layer of the OSI model.
DLCI---(data-link connection identifier) Value that specifies a PVC or SVC in a Frame Relay network. In the basic Frame Relay specification, DLCIs are locally significant (connected devices might use different values to specify the same connection). In the LMI extended specification, DLCIs are globally significant (DLCIs specify individual end devices).
DLSw---(data-link switching) An interoperability standard, described in RFC 1434, that provides a method for forwarding SNA and NetBIOS traffic over TCP/IP networks using data-link layer switching and encapsulation. DLSw uses SSP instead of SRB, eliminating the major limitations of SRB, including hop-count limits, broadcast and unnecessary traffic, timeouts, lack of flow control, and lack of prioritization schemes.
DSAP---(destination service access point) SAP of the network node designated in the Destination field of a packet.
DTE---(data terminal equipment) Device at the user end of a user-network interface that serves as a data source, destination, or both. DTE connects to a data network through a DCE device (for example, a modem) and typically uses clocking signals generated by the DCE. DTE includes such devices as computers, protocol translators, and multiplexers.
E1---Wide-area digital transmission scheme used predominantly in Europe that carries data at a rate of 2.048 Mbps. E1 lines can be leased for private use from common carriers.
explorer frame---Frame sent out by a networked device in a SRB environment to determine the optimal route to another networked device.
FECN---(forward explicit congestion notification) Bit set by a Frame Relay network to inform DTE receiving the frame that congestion was experienced in the path from source to destination. DTE receiving frames with the FECN bit set can request that higher-level protocols take flow-control action as appropriate.
FEP---(front-end processor) A device or board that provides network interface capabilities for a networked device. In SNA, an FEP is typically an IBM 3745 device.
FRAD---(Frame Relay access device) Any network device that provides a connection between a LAN and a Frame Relay WAN.
fragmentation---Process of breaking a packet into smaller units when transmitting over a network medium that cannot support the original size of the packet.
Frame Relay---Industry-standard, switched data-link layer protocol that handles multiple virtual circuits using HDLC encapsulation between connected devices. Frame Relay is more efficient than X.25, the protocol for which it is generally considered a replacement.
FST---(Fast Sequenced Transport) Connectionless, sequenced transport protocol that runs on top of the IP protocol. SRB traffic is encapsulated inside of IP datagrams and is passed over an FST connection between two network devices (such as routers). Speeds up data delivery, reduces overhead, and improves the response time of SRB traffic.
HDLC---(High-Level Data Link Control) Bit-oriented synchronous data link layer protocol developed by ISO. Derived from SDLC, HDLC specifies a data encapsulation method on synchronous serial links using frame characters and checksums.
HPR---(High-Performance Routing) Second-generation routing algorithm for APPN. HPR provides a connectionless layer with nondisruptive routing of sessions around link failures, and a connection-oriented layer with end-to-end flow control, error control, and sequencing.
LACK---(local acknowledgment) Method whereby an intermediate network node, such as a router, responds to acknowledgments for a remote end host. Use of local acknowledgments reduces network overhead and, therefore, the risk of time-outs. Also known as local termination.
latency---Delay between the time a device requests access to a network and the time it is granted permission to transmit. Delay between the time a device receives a frame and the time that frame is forwarded out the destination port.
LLC---(Logical Link Control) Higher of the two data-link layer sublayers defined by the IEEE. The LLC sublayer handles error control, flow control, framing, and MAC-sublayer addressing. The most prevalent LLC protocol is IEEE 802.2, which includes both connectionless and connection-oriented variants.
LOCADDR---Local address.
local explorer packet---Packet generated by an end system in an SRB network to find a host connected to the local ring. If the local explorer packet fails to find a local host, the end system produces either a spanning explorer packet or an all-routes explorer packet.
LU---(logical unit) A type of addressable unit in an SNA network. The LU is the port through which the end user accesses both the SSCP provided services, and communicates with other LUs at other nodes.
MAC---(Media Access Control) Lower of the two sublayers of the data-link layer defined by the IEEE. The MAC sublayer handles access to shared media, such as whether token passing or contention will be used.
MTU---(maximum transmission unit) Maximum packet size, in bytes, that a particular interface can handle.
NAU---(network addressable unit) An SNA term to describe the three entities which are addressable in an SNA network: SSCP, PU, and LU.
NCP---(Network Control Program) In SNA, a program that routes and controls the flow of data between a communications controller (in which it resides) and other network resources.
NetBIOS---(Network Basic Input/Output System) API used by applications on an IBM LAN to request services from lower-level network processes. These services might include session establishment and termination, and information transfer.
OUI---(Organizational Unique Identifier) 3 octets assigned by the IEEE in a block of 48-bit LAN addresses.
PAD---(packet assembler/disassembler) Device used to connect simple devices (like character-mode terminals) that do not support the full functionality of a particular protocol to a network. PADs buffer data and assemble and disassemble packets sent to such end devices.
priority queuing---Routing feature in which frames in an interface output queue are prioritized based on various characteristics such as packet size and interface type.
propagation delay---Time required for data to travel over a network, from its source to its ultimate destination.
PU---(physical unit) A type of addressable unit in an SNA network. Each node in the network has a PU, which provides services to control the physical configuration and the communication system resources associated with the node, and also to collect maintenance and operational statistics.
PVC---(permanent virtual circuit) Virtual circuit that is permanently established. PVCs save bandwidth associated with circuit establishment and tear down in situations where certain virtual circuits must exist all the time.
QLLC---(Qualified Logical Link Control) Data-link layer protocol defined by IBM that allows SNA data to be transported across X.25 networks.
QoS---Quality of Service.
queuing delay---Amount of time that data must wait before it can be transmitted onto a statistically multiplexed physical circuit.
RIF---(Routing Information Field) Field in the IEEE 802.5 header that is used by a source-route bridge to determine through which Token Ring network segments a packet must transit. A RIF is made up of ring and bridge numbers as well as other information.
RII---(Routing Information Identifier) Bit used by SRT bridges to distinguish between frames that should be transparently bridged and frames that should be passed to the SRB module for handling.
RSRB---(remote source-route bridging) A method of encapsulating SRB traffic over WAN links.
RSVP---(Resource Reservation Protocol) Protocol that supports the reservation of resources across an IP network. Applications running on IP end systems can use RSVP to indicate to other nodes the nature (bandwidth, jitter, maximum burst, and so forth) of the packet streams they want to receive. RSVP depends on IPv6. Also known as Resource Reservation Setup Protocol.
SAP---(service access point) A logical address that allows a system to route data between a remote device and the appropriate communications support.
SDLC---(Synchronous Data Link Control) SNA data-link layer communications protocol. SDLC is a bit-oriented, full-duplex serial protocol that has spawned numerous similar protocols, including HDLC and LAPB.
SNA---(Systems Network Architecture) An architecture designed by IBM to provide a unified systems design for their communication network products.
SNAP---(Subnetwork Access Protocol) Internet protocol that operates between a network entity in the subnetwork and a network entity in the end system. SNAP 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.
SRB---(source-route bridging) Method of bridging originated by IBM and popular in Token Ring networks. In an SRB network, the entire route to a destination is predetermined, in real time, prior to the sending of data to the destination.
SSAP---(source service access point) SAP of the network node designated in the Source field of a packet.
SSP---(Switch-to-Switch Protocol) Protocol specified in the DLSw standard that routers use to establish DLSw connections, locate resources, forward data, and handle flow control and error recovery.
star topology---LAN topology in which end points on a network are connected to a common central switch by point-to-point links. A ring topology that is organized as a star implements a unidirectional closed-loop star, instead of point-to-point links.
T1---Digital WAN carrier facility. T1 transmits DS-1-formatted data at 1.544 Mbps through the telephone-switching network, using AMI or B8ZS coding.
TDM---(time-division multiplexing) Technique in which information from multiple channels can be allocated bandwidth on a single wire based on preassigned time slots. Bandwidth is allocated to each channel regardless of whether the station has data to transmit.
ToS---Type of service.
traffic shaping---Use of queues to limit surges that can congest a network. Data is buffered and then sent into the network in regulated amounts to ensure that the traffic will fit within the promised traffic envelope for the particular connection. Traffic shaping is used in ATM, Frame Relay, and other types of networks. Also known as metering, shaping, and smoothing.
unnumbered frames---HDLC frames used for various control and management purposes, including link startup and shutdown, and mode specification.
virtual circuit---Logical circuit created to ensure reliable communication between two network devices. A virtual circuit is defined by a VPI/VCI pair, and can be either permanent (PVC) or switched (SVC). Virtual circuits are used in Frame Relay and X.25.
VTAM---(Virtual Telecommunications Access Method) Set of programs that control communication between LUs. VTAM controls data transmission between channel-attached devices and performs routing functions.
WAN---(wide-area network) Data communications network that serves users across a broad geographic area and often uses transmission devices provided by common carriers. Frame Relay, SMDS, and X.25 are examples of WANs.
WFQ---(weighted fair queuing) Congestion management algorithm that identifies conversations (in the form of traffic streams), separates packets that belong to each conversation, and ensures that capacity is shared fairly between these individual conversations. WFQ is an automatic way of stabilizing network behavior during congestion and results in increased performance and reduced retransmission.
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