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In networking, to daisy chain devices it to connect them to each other in a series (cascaded), one after the other. See also Subtending Port.



A collection of data that is organized so that its contents can easily be accessed, managed, and updated.


Data Confidentiality  

The IPSec sender can encrypt packets before transmitting them across a network.


Data Integrity

The IPSec receiver can validate packets sent by the IPSec sender to ensure that the data has not been altered during transmission.


Data Origin Authentication

The IPSec receiver can verify the source of IPSec packets. This service depends on the data integrity service.


Data Link Layer 

Layer 2 of the OSI reference model. This layer passes data between the network layer and the physical layer. The data link layer is responsible for transmitting and receiving frames. It usually includes both the media access control (MAC) protocol and logical link control (LLC) layers.


Daylight-savings Time

This is a period during the late spring, summer and early fall when many countries set their clocks ahead of normal local time by one hour to give more daylight time in the evenings.


Daytime (RFC 867)

A network protocol used by devices for debugging and time measurement. A computer can use this protocol to set its internal clock but only if it knows in which order the year, month, and day are returned by the server. Not all servers use the same format.



To prevent network congestion, Dynamic Bandwidth Allocation (DBA) allows a network device to quickly re-assign unused bandwidth based on traffic requirements.



DCD (Downlink Channel Description) is a WiMAX medium access control layer (MAC) message which describes the physical layer (PHY) characteristics of a downlink (DL) channel.



DCE (Data Communication Equipment) is a device, such as a modem, that converts data between different interfaces (digital and analog for example) and exchanges data with the DTE.



DDI (also called DID, Direct Inward Dial) is a feature that maps a public phone number to an extension number. DDI enables a caller to call an extension number without going through an operator. When people give out their contact number and say it's a “direct line”, often what they mean is that it's a DDI number.


DDNS (Dynamic Domain Name System)

Allows the hosting of a website, FTP server, or email server with a fixed domain name (e.g., and a dynamic IP address.



A DDoS attack is one in which multiple compromised systems attack a single target, thereby causing denial of service for users of the targeted system. See also DoS.



Decryption is the process of taking encrypted data and decoding it so that it becomes readable. The act of restoring an encrypted file to its original status. See also Encryption, Cipher, Plaintext, Ciphertext.



Digitally Enhanced Cordless Telecommunications (DECT) is an ETSI (European Telecommunications Standards Institute) standard for digital cordless phones which specifies how they transmit, receive and process data and voice communications. According to the Generic Access Profile (GAP) standard, any DECT GAP handset should be able to interact with any DECT GAP base station, regardless of manufacturer.


Default Gateway

A device that forwards Internet traffic from your local area network.


Default Router Cache

The default router cache on an IPv6 device contains IPv6 addresses of on-link routers that will help forward packets. An on-link router is directly connected to the device or connected through a switch.


Degraded Array

A degraded hard drive array is one that is damaged by a hard drive failure but can still be repaired by replacing the failed drive. The array is still operational but at risk since all of the array’s data may be lost if a second hard drive fails. Data access may also be slower from a degraded array. It’s recommended that you replace a failed drive and repair the array as soon as you can.



A Dual End Loop Test (DELT) is also called a Loop Diagnostic Mode test or LDM test. Use DELT to perform a loop test where a CPE has been deployed. It provides details about the physical condition of a DSL subscriber’s line and more information than SELT, including line data rate, SNR margin and noise. It uses the CO equipment and the CPE to test the loop in both directions.


Denial of Service

Act of preventing customers, users, clients or other computers from accessing data on a computer. This is usually accomplished by interrupting or overwhelming the computer with bad or excessive information requests.



Data Encryption Standard is a widely-used method of data encryption that uses a private (secret) key. DES applies a 56-bit key to each 64-bit block of data.


Destination Cache

In IPv6, the destination cache is a mapping of the next-hop address to an IPv6 destination address. The next-hop can be the destination itself. The destination cache only shows these entries about destinations to which traffic has been sent recently.


Destination Filter

A destination filter sifts traffic going through the switch based on the destination MAC addresses.


Device Access Key

The Device Access Key (DAK) is a password that identifies a powerline network. All powerline devices with the same DAK can join the same powerline network.


Device Driver

A system file that lets other programs interact with a piece of hardware. You should never try to locate and install or uninstall device drivers yourself since they are modifications to an operating system at the core level.


Device Filters

Device Filters decide whether or not to allow passage of a data packet and/or to make a call. Device filters act on raw data from/to LAN and WAN and serve as a limited firewall to your device.


Device Filter Rules

Device filter rules are filter rules that treat a packet as a byte stream as opposed to an IP or IPX packet. You specify the portion of the packet to check with the Offset (from 0) and the Length fields, both in bytes.


Device HA

Device High Availability (HA) allows a device to be available more of the time by having a backup device take over if the primary device goes down.



Dynamic Frequency Selection (DFS) allows an IEEE 802.11a access point (AP) to use radio channels in the frequency range normally reserved for radar systems. Radar uses radio signals to determine the location of objects for military, meteorological or air traffic control purposes. As long as the DFS-enabled AP detects no radar activity on the selected channel, it uses the channel to communicate. If it detects radar activity on the selected channel, it automatically instructs its wireless clients to move to another channel, then resumes communications on the new channel. DFS (Dynamic Frequency Selection) allows an AP to detect other devices in the same channel. If there is another device using the same channel, the AP changes to a different channel, so that it can avoid interference with radar systems or other wireless networks. See also IEEE 802.11h.



A DHCP (Dynamic Host Configuration Protocol) server can assign your device an temporary IP address, subnet mask, DNS and other routing information when it’s turned on.



A Dynamic Host Configuration Protocol (DHCP) pool is the range of IP addresses that a DHCP server assigns to clients that request IP addresses. For example, computers can request an IP address when starting up.


DHCP Relay

Dynamic Host Configuration Protocol Relay is a function that allows DHCP data to be forwarded between the computer that requests the IP address and the DHCP server.


DHCP Relay Agent Information (Option 82)

This feature has a device add information to client TCP/IP configuration requests that it relays to a DHCP server. The information details where on the device the request was received (such as the slot ID, port number and VLAN ID) and helps the DHCP server authenticate the source of the DHCP requests. Option 82 allows you to specify a string of additional information for the device to add.


DHCP Server

This is a device that uses DHCP (see DHCP) to assign addresses to nodes on a LAN.


DHCP Snooping

With DHCP snooping, a system obtains a client's MAC-IP address information (in the reply messages from a DHCP server). This feature prevents subscribers from assigning their own static IP addresses that may conflict with DHCP-assigned IP addresses. If DHCP snooping is part of IP Source Guard, the system also drops DHCP packets from untrusted ports and learns dynamic bindings from trusted ports.


DHCP Unique IDentifier (DUID)

"The DHCP Unique IDentifier (DUID) is generated from the MAC address, time, vendor-assigned ID and/or the vendor's private enterprise number registered with the IANA. Each DHCP client and server has a unique DUID, which is used for identification when the


Dial Backup

Dial backup is an auxiliary WAN connection that you can use if your primary WAN link goes down.


Dial Up

This is the process of setting up a connection through a switched network. It also describes a type of Internet service where you have to connect (like a call) to your ISP for each session.


Dialing Plan

A VoIP line card uses dialing plans to identify specific types of phone numbers dialed by a user, and to process the number before transmission by deleting or adding digits according to the relevant rule. The dial plan can also forward the call to a spec.



DIAMETER is a type of AAA (Authentication, Authorization and Accounting) server which supersedes RADIUS. DIAMETER uses the TCP or SCTP protocol and supports IPSec or TLS security.



Differentiated Services is a class of service (CoS) model that marks packets so that they receive specific per-hop treatment at DiffServ-compliant network devices along the route based on the application types and traffic flow.



The use of a binary code to represent information, such as 0/1, or on/off.


Digital Certificate

A digital certificate is an electronic ID card that proves the senders identity. A digital certificate is issued by a certificate authority.


Distinctive Ring

A VoIP line card may have a list of ring patterns based on country codes that the SIP proxy server or H.248 Media Gateway Controller (MGC) can have the VoIP line card send to the connected telephones.



A DLCI specifies the channel and destination that frame relay traffic will use.



The Digital Living Network Alliance (DLNA) is a group of personal computer and electronics companies that works to make products compatible in a home network.



DLS (Direct Link Setup) allows two wireless devices to communicate with each other directly in the same wireless network. The packets will not go through the AP, which can increase throughput. This feature is applicable only when both wireless clients are in Infrastructure mode and the AP or wireless router has DLS enabled.



Discrete Multi-Tone (DMT) modulation allows a VDSL device to adapt to the bit rate based on the line condition.



A DMZ (Demilitarized Zone) is a network that makes public servers visible to the outside world and physically separates them from the LAN, thus making the LAN more secure. Removes the router's firewall protection from one PC, allowing it to be "seen" from the Internet.



Destination NAT changes the destination address of packets so they can be routed to the proper destination.


DNS (Domain Name Server)

The IP address of your ISP's server, which translates the names of websites into IP addresses.


DNS Cache

DNS cache is the temporary storage area where a router stores responses from DNS servers.


DNS Server Address Assignment

Use DNS (Domain Name System) to map a domain name to its corresponding IP address and vice versa. The DNS server is extremely important because without it, you must know the IP address of a device.



The Data Over Cable Service Interface Specification (DOCSIS) also called Cable Modem (CM) is an international standard that defines how cable modems transmit and receive data over a cable network.


Dolby Digital

Dolby Digital is one of several audio compression technologies (codecs) produced by Dolby Laboratories. Dolby Digital (also called AC-3) is the most common version. It contains up to six discrete channels of sound. Five channels for normal-range speakers (right front, center, left front, right rear and left rear) and one LFE (Low Frequency Effects) channel for the subwoofer. This is often abbreviated as 5.1. The Dolby Digital format also supports mono and stereo usage.



A domain is a group of computers that are part of a network and share a common directory database. See also Workgroup.


Domain Name

The unique name that identifies an Internet site. Domain Names always have two or more parts that are separated by dots. The part on the left is the most specific and the part on the right is the most general. A specific name for a network of computers.


Domain Name System

The Domain Name System (DNS) maps domain names to their corresponding IP addresses and vice versa. The DNS server is important because without it, you must know the IP address of a networking device before you can access it. Domain Name System (DNS) links names to IP addresses. When you connect to websites on the Internet you can type either the site’s IP address or its DNS name.


Domain Zone

A domain zone is a fully qualified domain name without the host. For example, is the domain zone for the fully qualified domain name.



The goal of DoS attacks is not to steal information, but to disable a device or network on the Internet. See also DDoS.


DoS Prevention

DoS prevention is to protect against DoS (Denial of Service) attacks such as SYN flooding and Ping of Death. A device can use filtering actions to determine when to start dropping packets that may potentially be associated with a DoS attack. DoS prevention lets a switch use the configured thresholds or the filtering actions to determine when to drop packets that may potentially be associated with a DoS attack.


Dot-decimal Format

See dotted-decimal notation.


Dotted-decimal Notation

This is the writing out of a decimal number (base-10) using periods (dots or decimals) to separate it into parts. This is commonly used for IP addresses. Also referred to as dot-decimal format.


Double-Tag PVC (DTPVC)

DTPVCs (Double-Tag Permanent Virtual Circuits) add double VLAN tags to untagged frames received from an xDSL subscriber on the specified PVC (also called channel). These double VLAN tags consist of an inner c-tag (customer tag) and an outer s-tag (service).


Down Array

A down array is a hard drive array that is damaged and cannot be fixed. The array cannot be used and the data is lost. Replace the failed drive or drives and recreate the array, volumes, and sharing configuration.


Downlink Port

This port connects to the uplink port of another device when the devices are cascaded. Also known as a subtending port.



To receive a file transmitted over a network.


Download Service

Some devices offer a download service, which can download files from the Internet directly to the device. You do not have to download to your computer and then copy to the device. This can free up your computer’s system resources.


Downstream Bandwidth Utilization

In load balancing, downstream (incoming) bandwidth utilization is the measured downstream throughput as a ratio of the available downstream bandwidth.


Downstream Broadcast Blocking

Some DSLAMs can block downstream broadcast packets from being sent to specific VLANs on specified ports. This helps reduce downstream bandwidth requirements on a subscriber line.


Downstream Policing

Downstream policing per queue applies a bandwidth limit to downstream traffic. You can define different maximum downstream bandwidth for packets in different queue. See also Downstream Policing.



A VDSL signal may interfere with other services (such as ISDN, ADSL or ADSL2 provided by other devices) on the same bundle of lines due to downstream far-end crosstalk. DPBO (Downstream Power Back Off) can reduce performance degradation by changing the PSD level on the VDSL switch(es).



The Differential Phase-Shift Keying (DPSK) digital modulation technique is used in IEEE 802.11b wireless networks. See also Phase-Shift Keying and IEEE 802.11b.



The Differentially-encoded Quadrature Phase-Shift Keying digital modulation technique is used in IEEE 802.11b wireless networks to operate at 2 Mbps. See also DPSK and QPSK.



In OSPF, a Designated Router (DR) keeps track of link state updates in their area and make sure that Link State Advertisements (LSAs) are sent to the rest of the network. See OSPF, BDR, Area, LSA.



Dynamic RAM stores information in capacitors that must be refreshed periodically.



The DiffServ Code Point value determines the forwarding treatment (or PHB) that each packet gets across the DiffServ network.


DSCP-to-IEEE 802.1p Priority Mapping

DiffServ is a class of service (CoS) model that marks packets with DiffServ Code Points (DSCPs) so that they receive specific per-hop treatment at DiffServ-compliant network devices along the route. DSCP-to-IEEE 802.1p mappings allow a device to prioritize all incoming traffic based on the DSCP value according to the mapping table.



Digital Subscriber Line technologies enhance the data capacity of the existing twisted pair wire that runs between the local telephone company switching offices and most homes and offices. There are actually several types of DSL service, ranging in speeds from 16 Kbps to 52 Mbps. The services are either symmetrical (traffic flows at the same speed in both directions) or asymmetrical (the downstream capacity is higher than the upstream capacity). DSL connections are point-to-point dedicated circuits, meaning that they are always connected. There is no dial-up. There is also no switching, which means that the line is a direct connection into the carrier’s frame relay, ATM (Asynchronous Transfer Mode) or Internet-connect system.



A Digital Subscriber Line Access Multiplexor (DSLAM) is a network device, usually at a telephone company central office, that receives signals from multiple customer Digital Subscriber Line connections and puts the signals on a high-speed backbone line using multiplexing techniques. Depending on the product, DSLAM multiplexers connect DSL lines with some combination of asynchronous transfer mode ATM, frame relay or IP networks.



A Digital Signal Processor (DSP) carries out the mathematical operations used in converting a signal into digital output. This is a circuit that is specially designed for digital signals in processor-intensive applications, such as wireless communications links and image processing. DSP circuits are often used in consumer products, such as mobile phones, faxes and digital TVs.



Direct-Sequence Spread Spectrum (DSSS) is the modulation technique used in IEEE 802.11b wireless networks. Frequency transmissions with a redundant bit pattern resulting in a lower probability of information being lost in transit.



The DTE (Data Terminal Equipment) is a computer or terminal that is connected to a DCE.



A DTIM (Delivery Traffic Indication Message) is used to tell wireless clients in power-saving mode that a packet is to be sent to them. A message included in data packets that can increase wireless efficiency.


DTIM Period

A DTIM period (in beacon intervals) indicates how many broadcast and multicast packets can be transmitted to wireless clients between two DTIMs.



The Datagram Transport Layer Security (DTLS) protocol provides dataflow protection for datagram-based applications. It is based on the TLS protocol.


Dual Band

Dual band technology can operate in both the 2.4 GHz and 5 GHz radio bands which helps minimize interference. Operation may be selectable (one band at a time) or simultaneous, doubling available wireless bandwidth and network capacity.


Dual Firmware Block Structure

Devices with a “dual firmware block structure” have one “main block” and another “backup block”. You can save the current firmware into the backup block before you upload new firmware. If the firmware in the main block gets corrupted, the device tries to boot from the backup block automatically so the service is not interrupted.


Dual-mode Phone

A dual-mode phone is a telephone that uses two communications methods for calls, messaging or other services. For example, a dual-mode mobile phone may use the GSM cellular network for standard PSTN calls and SMS text messaging, as well as a local wireless LAN connection for VoIP calls, E-mail and Internet access.


Dual Personality Interfaces

This is a combination of two ports. One is a mini GBIC slot and the other is an Ethernet port. You can connect both ports for redundancy but only one will be active. Typically this is a feature available on Ethernet switches or routers.



The DHCP Unique IDentifier (DUID) of a DHCP client, the client's hardware type for example, is added in client DHCP requests that a networking device relays to a DHCP server.


Duplexing (Disk)

Disk duplexing, like disk mirroring, copies data to a duplicate hard disk. Duplexing also duplicates the adapter controllers that control the hard drives. With mirroring the separate disks rely upon a common controller, so access to both copies of data is threatened if the controller fails. With duplexing, the duplicate controller(s) enables continued data access as long as one of the controllers continues to function. See also RAID.


Digital Versatile Disc. An optic disc with the same physical size as a CD but with significantly greater storage capacity.



DVMRP is a protocol used for routing multicast data within an autonomous system (AS).



DVMRP grafts attach a branch back onto the multicast delivery tree.



DVMRP probes are used to discover DVMRP neighbors on a network.



DVMRP prunes trim the multicast delivery tree(s).


DVMRP Report

DVMRP reports contain DVMRP source routing information.


Dynamic Bandwidth Allocation (DBA)

In a PON network, dynamic bandwidth allocation (DBA) allows the OLT to quickly re-assign unused bandwidth based on traffic requirements and link priority.


Dynamic Binding

In IP source guard, a dynamic binding is learned by snooping DHCP packets. See also Binding.


Dynamic Channel Selection (DCS)

Dynamic Channel Selection (DCS) is a feature that allows an AP to automatically select the radio channel upon which it broadcasts by scanning the area around it and determining what channels are currently being used by other devices.


Dynamic DNS

With Dynamic DNS support, you can have a static hostname alias for a dynamic IP address, allowing the host to be more easily accessible from various locations on the Internet. You must register for this service with a Dynamic DNS service provider to use this service. It is a temporary IP address assigned by a DHCP server.


Dynamic Jitter Buffer

A dynamic jitter buffer helps smooth out the variations in delay (jitter) for voice traffic. This helps ensure good voice quality for your conversations.


Dynamic Link Aggregation

The IEEE802.3ad standard describes Link Aggregate Control Protocol (LACP), which is a protocol that dynamically creates and manages trunk groups. When you enable LACP link aggregation on a port, the port can automatically negotiate with the ports at the remote end of a link to establish trunk groups. LACP also allows port redundancy, that is, if an operational port fails, then one of the "standby" ports becomes operational without user intervention.


Dynamic VPN Rule

A dynamic IPSec VPN rule does not specify the remote IPSec router’s IP address or domain name. This allows a remote IPSec router with a dynamic IP address to initiate a VPN tunnel to the local IPSec router (the one upon which you configured the dynamic IPSec VPN rule). Only the remote IPSec router can initiate a dynamic VPN tunnel.


Dynamic Turbo


Dynamic turbo is a feature of Super G wireless networking technology that enables channel bonding (using two radio channels instead of one) for improved transmission speed.