Internetworking Terms

ITU-T standard for a physical layer interface between DTE and DCE. V.24 is essentially the same as the EIA/TIA-232 standard. See also EIA/TIA-232.

ITU-T specification describing procedures for call setup and tear down over the DTE-DCE interface in a PSDN.

ITU-T standard serial line protocol for bidirectional data transmissions at speeds of 4.8 or 9.6 kbps. See also V.32bis.

ITU-T standard that extends V.32 to speeds up to 14.4 kbps. See also V.32.

ITU-T standard that specifies a serial line protocol. V.34 offers improvements to the V.32 standard, including higher transmission rates (28.8 kbps) and enhanced data compression. Compare with V.32.

ITU-T standard describing a synchronous, physical layer protocol used for communications between a network access device and a packet network. V.35 is most commonly used in the United States and in Europe, and is recommended for speeds up to 48 kbps.

ITU-T standard protocol for error correction using LAPM. See also LAPM.

volts alternating current.

voice activity detection. When enabled on a voice port or a dial peer, silence is not transmitted over the network, only audible speech. When VAD is enabled, the sound quality is slightly degraded but the connection monopolizes much less bandwidth.

Digital certificate for which the binding of the data items can be trusted; one that can be validated successfully.

value-added network. Computer network or subnetwork (which is usually a commercial enterprise) that transmits, receives, and stores EDI transactions on behalf of its customers.

See VBR.

variable bit rate. QoS class defined by the ATM Forum for ATM networks. VBR is subdivided into a real time (RT) class and non-real time (NRT) class. VBR (RT) is used for connections in which there is a fixed timing relationship between samples. VBR (NRT) is used for connections in which there is no fixed timing relationship between samples but that still need a guaranteed QoS. Compare with ABR, CBR, and UBR.

Virtual Communications Address. The standard and extended programming APIs for the Cisco VCO/4K product use a byte message scheme to facilitate communications between a controlling host application and the VCO/4K. Both source and destination VCA bytes are used to label and track communications between VCO/4K systems and host applications.

virtual channel connection. Logical circuit, made up of VCLs, that carries data between two end points in an ATM network. Sometimes called a virtual circuit connection. See also VCD, VCL, and VPI.

virtual circuit descriptor.

virtual channel identifier. 16-bit field in the header of an ATM cell. The VCI, together with the VPI, is used to identify the next destination of a cell as it passes through a series of ATM switches on its way to its destination. ATM switches use the VPI/VCI fields to identify the next network VCL that a cell needs to transit on its way to its final destination. The function of the VCI is similar to that of the DLCI in Frame Relay. Compare with DLCI. See also VCL and VPI.

virtual channel link. Connection between two ATM devices. A VCC is made up of one or more VCLs. See also VCC.

virtual circuit number. 12-bit field in an X.25 PLP header that identifies an X.25 virtual circuit. Allows DCE to determine how to route a packet through the X.25 network. See also LCI and LCN.

Virtual Central Office. VCO represents the Cisco VCO/4K product, an open, host-controlled, telephony switch capable of providing a wide range of enhanced services in the telecommunications market.

volts direct current.

very-high-data-rate digital subscriber line. One of four DSL technologies. VDSL delivers 13 to 52 Mbps downstream and 1.5 to 2.3 Mbps upstream over a single twisted copper pair. The operating range of VDSL is limited to 1,000 to 4,500 feet (304.8 to 1,372 meters). Compare with ADSL, HDSL, and SDSL.

Data segment of an SNA message. A vector consists of a length field, a key that describes the vector type, and vector-specific data.

very easy rodent oriented netwide index to computer archives. Gopher utility that effectively searches Gopher servers based on a user's list of keywords.

See VIP in the "Cisco Systems Terms and Acronyms" section.

variance factor. One of three link attributes exchanged using PTSPs to determine the available resources of an ATM network. VF is a relative measure of CRM normalized by the variance of the aggregate cell rate on the link.

Voice interface card. Connects the system to either the PSTN or to a PBX. Compare with WIC. See also PBX and PSTN.

VLAN ID. The identification of the VLAN, which is used by the standard 802.1Q. Being on 12 bits, it allows the identification of 4096 VLANs.

Virtual Integrated Network Service. NOS developed and marketed by Banyan Systems.

See VIP in the "Cisco Systems Terms and Acronyms" section.

Instance of a unique virtual interface that is created dynamically and exists temporarily. Virtual access interfaces can be created and configured differently
by different applications, such as virtual profiles and virtual private dialup networks.Virtual access interfaces are cloned from virtual template interfaces.

See network address.

See 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. In ATM, a virtual circuit is called a virtual channel. Sometimes abbreviated VC. See also PVC, SVC, VCD, virtual route, and VPI.

In ATM, a connection between end users that has a defined route and endpoints.
See also PVC and SVC.

See VIP in the "Cisco Systems Terms and Acronyms" section.

See Virtual Networking Services in the "Cisco Systems Terms and Acronyms" section.

Logical grouping of virtual circuits that connect two sites. See also virtual circuit.

Entity in an SRB network that logically connects two or more physical rings together either locally or remotely. The concept of virtual rings can be expanded across router boundaries.

In SNA, a logical connection between subarea nodes that is physically realized as a particular explicit route. SNA terminology for virtual circuit. See also virtual circuit.

Logical grouping of devices that share a common Layer 3 subnet.

A logical interface configured with generic configuration information for a specific purpose or configuration common to specific users, plus router-dependent infor-
mation. The template takes the form of a list of Cisco IOS interface commands that are applied to virtual access interfaces, as needed.

A portion of a physical interface that has the following characteristics: address space containing only one VPI and all VCIs underneath, bandwidth that is rate limited by hardware (VI), and ownership by a controller that uses it to interface to another peer controller.

Process of implementing a network based on virtual network segments. Devices are connected to virtual segments independent of their physical location and their physical connection to the network.

Hidden, self-replicating section of computer software, usually malicious logic, that propagates by infecting—that is, inserting a copy of itself into and becoming part of—another program. A virus cannot run by itself; it requires that its host program be run to make the virus active.

virtual LAN. Group of devices on one or more LANs that are configured (using management software) so that they can communicate as if they were attached to the same wire, when in fact they are located on a number of different LAN segments. Because VLANs are based on logical instead of physical connections, they are extremely flexible.

virtual LAN internetwork. Internetwork composed of VLANs. See also VLAN.

visitor location register. A database that contains temporary information about subscribers who roam into an area controlled by another MSC. The VLR communicates with the HLR of the subscriber to request data about that subscriber.

variable-length subnet mask. Capability to specify a different subnet mask for the same network number on different subnets. VLSM can help optimize available address space.

Virtual Media Access Control.

See Virtual Networking Services in the "Cisco Systems Terms and Acronyms" section.

Voice over ATM. Voice over ATM enables a router to carry voice traffic (for example, telephone calls and faxes) over an ATM network. When sending voice traffic over ATM, the voice traffic is encapsulated using a special AAL5 encapsulation for multiplexed voice.

Dial peer connected via an ATM network. VoATM peers point to specific VoATM devices.

video on demand. System using video compression to supply video programs to viewers when requested via ISDN or cable.

Voice over Frame Relay. VoFR enables a router to carry voice traffic (for example, telephone calls and faxes) over a Frame Relay network. When sending voice traffic over Frame Relay, the voice traffic is segmented and encapsulated for transit across the Frame Relay network using FRF.12 encapsulation.

Dial peer connected via a Frame Relay network. VoFR peers point to specific VoFR devices.

Voice over HDLC. Voice over HDLC enables a router to carry live voice traffic (for example, telephone calls and faxes) back-to-back to a second router over a serial line.

Dial peer connected via an HDLC network. VoHDLC peers point to specific VoHDLC devices.

See VIC.

Voice over Frame Relay enables a router to carry voice traffic (for example, telephone calls and faxes) over a Frame Relay network. When sending voice traffic over Frame Relay, the voice traffic is segmented and encapsulated for transit across the Frame Relay network using FRF.12 encapsulation.

See VoIP.

Voice over IP. The capability to carry normal telephony-style voice over an IP-based internet with POTS-like functionality, reliability, and voice quality. VoIP enables a router to carry voice traffic (for example, telephone calls and faxes) over an IP network. In VoIP, the DSP segments the voice signal into frames, which then are coupled in groups of two and stored in voice packets. These voice packets are transported using IP in compliance with ITU-T specification H.323.

Dial peer connected via a packet network; in the case of Voice over IP, this is an IP network. VoIP peers point to specific VoIP devices.

virtual path. One of two types of ATM circuits identified by a VPI. A virtual path is a bundle of virtual channels, all of which are switched transparently across an ATM network based on a common VPI. See also VPI.

virtual path connection. Grouping of VCCs that share one or more contiguous VPL. See also VCC and VPL.

virtual private dial-up network. Also known as virtual private dial network. A VPDN is a network that extends remote access to a private network using a shared infrastructure. VPDNs use Layer 2 tunnel technologies (L2F, L2TP, and PPTP) to extend the Layer 2 and higher parts of the network connection from a remote user across an ISP network to a private network. VPDNs are a cost effective method of establishing a long distance, point-to-point connection between remote dial users and a private network. See also VPN.

virtual path identifier. 8-bit field in the header of an ATM cell. The VPI, together with the VCI, identifies the next destination of a cell as it passes through a series of ATM switches on its way to its destination. ATM switches use the VPI/VCI fields to identify the next VCL that a cell needs to transit on its way to its final destination. The function of the VPI is similar to that of the DLCI in Frame Relay. Compare with DLCI. See also VCD and VCL.

See VCI and VPI.

virtual path link. Within a virtual path, a group of unidirectional VCLs with the same end points. Grouping VCLs into VPLs reduces the number of connections to be managed, thereby decreasing network control overhead and cost. A VPC is made up of one or more VPLs.

Virtual Private Network. Enables IP traffic to travel securely over a public TCP/IP network by encrypting all traffic from one network to another. A VPN uses "tunneling" to encrypt all information at the IP level.

A VPN routing/forwarding instance. A VRF consists of an IP routing table, a derived forwarding table, a set of interfaces that use the forwarding table, and a set of rules and routing protocols that determine what goes into the forwarding table. In general, a VRF includes the routing information that defines a customer VPN site that is attached to a PE router.

Virtual Reality Modeling Language. Specification for displaying three-dimensional objects on the World Wide Web. Think of it as the 3-D equivalent of HTML.

virtual source/virtual destination.

vendor-specific attribute. An attribute that has been implemented by a particular vendor. It uses the attribute Vendor-Specific to encapsulate the resulting AV pair: essentially, Vendor-Specific = protocol:attribute = value.

See VSC in the "Cisco Systems Terms and Acronyms" section.

Virtual Switch Interface.

A VSI master process implementing the master side of the VSI protocol in a VSI controller. Sometimes the whole VSI controller might be referred to as a VSI Master but this is not strictly correct.

1. A device that controls a VSI switch, for example, a VSI label switch controller.

2. A process implementing the master side of the VSI protocol.

Voice Services Provisioning Tool. Provides end-to-end configuration for IP, trunk groups, trunks, routes, and dial plans for VSC3000 and VISM. Also known as Dart.

virtual telecommunications access method. Set of programs that control communication between LUs. VTAM controls data transmission between channel-attached devices and performs routing functions. See also LU.

Virtual Tributary level n. SONET format for mapping a lower-rate signal into a SONET payload. For example, VT-1.5 is used to transport a DS-1 signal. See also DS-1 and SONET.

Virtual Terminal Protocol. ISO application for establishing a virtual terminal connection across a network.

virtual type terminal. Commonly used as virtual terminal lines.

virtual wavelength path. A VWP is a group of one or more channels between source and destination nodes. The term virtual indicates that the signal path can actually travel on different physical wavelengths throughout the network. All channels of the VWP transit the same path through the network.