D-E

DAL

Distribution Abstraction Layer. A component used to hide the distribution of functions across multiple boards.

data plane

In the classical planar networking architecture model, this is the core function of a system. Functions on this plane provide data forwarding, switching, and packet processing based on the information built by the control plane.

demultiplexer

The software that uses the information in the packet header to direct the packet upward through one or more protocol stacks.

detailed design

The stage of a software development project in which the high-level modules are broken down into submodules, internal and global data structures are specified, and the processing logic outlined, where applicable. This is the phase before the coding phase.

device driver

The software used to permit the applications to utilize a device without knowing all the details of the hardware.

differential timer block

A timer block which stores only the difference in timer ticks from its previous block in the timer list. If there are two timer blocks required with one to time out in 10 seconds and the other in 15 seconds, the first block will have a timer count of 10, and the second will have a timer count of 5.

distance vector protocol

A type of routing protocol which advertises reachability information to its immediate neighboring routers, based on which they construct their routing tables. Examples are RIP and IPX RIP. Contrast with a link state protocol like OSPF.

DMA

Direct Memory Access. A technique used to transfer data directly between the network controller and memory without processor intervention.

DRAM

A generic term used to describe various types of Dynamic Random Access Memory, including Synchronous DRAM (SDRAM), Double Data DRAM (DDR DRAM), Rambus DRAM (RDRAM) and Quad Data Rate DRAM (QDR DRAM).

DSL

Digital Subscriber Loop. Used for high- speed communication in the 'last mile,' which is the link from the service provider to the subscriber (enterprise or home). Variants include ADSL (Asynchronous DSL), SDSL (Synchronous DSL), and their enhancements.

DVMRP

Distance Vector Multicast Routing Protocol. A protocol specified by the IETF for multicast routing in the Internet.

dynamic allocation

An allocation scheme in which the assignment is done dynamically.

edge routers

Routers used on the 'edge' of the service provider network. These routers interface via WAN links to the various customer premise routers. Due to the large number of customer connections, these boxes are usually high end/chassis based, with a large number of interfaces.

EEPROM

Electrically Erasable and Programmable Read-Only Memory.

encapsulation

A technique in which successive layers add a header to the outgoing packet.

encryption

The process of transforming data such that only the intended receiver of the data will be able to read it.

end node

A node which originates and terminates traffic in a communications network.

Ethernet

A Local Area Network (LAN) technology using CSMA/CD. Ethernet can operate at 10, 100, 1000, or 10000 Mbps.

Ethernet address

A 6-byte value uniquely identifying an Ethernet interface. Ethernet frames use the destination and source Ethernet addresses at the beginning of the frame.

event

A form of notification which is to be acted upon. There are several categories of events: timer events, message-queuing events, internal or local events, frame reception events, and so on.