0-9  A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z dark fiber Fiber-optic cables that have been laid but have no illuminating signals in them. The opposite of lit fiber. data analyzers A systematic investigation of the data and their flow in a real or planned system. data encryption standard (DES) A secret key cryptography method developed by the U.S. National Security Agency that converts plaintext to ciphertext then back to plaintext uses a 56-bit key. The DES consists of 16 rounds of operations that mix the data and key together so that every bit of the ciphertext depends on every bit of the data and every bit of the key. Triple DES is an enhancement to DES that provides more security than standard DES. data networking Communications process whereby data bytes are transported and exchanged across network elements. data over cable service interface specification (DOCSIS) A set of standards for transferring data via cable TV. DOCSIS is managed by Multimedia Cable Network System, an organization formed by four major cable operators. DOCSIS is expected to increase the market for cable modems. data transmission The sending of data from one place to another by means of signals over a channel. datacom The transfer of information between functional units by means of data transmission according to a protocol. data-link layer (Layer 2) The second level of the open systems interconnection (OSI) model. Often referred to as Layer 2, the data-link layer is responsible for connecting two nodes across a physical link. Layer 2 is the only layer in the OSI model that makes a connection between the physical connection of Layer 1 and upper application layers that utilize virtual connections. days sales outstanding (DSO) The number of days required to collect accounts receivable based on sales over a given period and the current accounts receivable balance. All things being equal, the lower a company's DSOs, the quicker it collects on credit it extends customers and clients. DSOs are calculated by dividing the accounts receivable balance by the average sales per day. debt to capital A useful measure in determining the proportion of borrowed funds in relation to the total funds of a company. A higher debt to capital ratio lends to a more leveraged company. This figure is calculated by dividing debt by the sum of debt and equity. This is usually presented as a percentage using the data from the most recent reporting period. Over leverage can be detrimental to the shareholder, who is paid last in case of bankruptcy. decibel (dB) A logarithmic comparison of power levels, defined as 10 times the base 10 logarithm of the ratio of the two power levels. One-tenth of a Bel. deferred revenue Refers to cash that has been received by a firm for products and/or services that have not yet been rendered by the company, and thus cannot be recognized as revenue according to generally accepted accounting principals (GAAP). Deferred revenue is a liability to the company until the product has been delivered or the services have been performed. A significant growth in deferred revenues should be investigated, as it may indicate operational problems. demultiplexer Networking equipment that pulls a digital or analog signal from a single transmission channel or circuit into multiple signals. dense wavelength division multiplexing (DWDM) A technology that puts data from different sources together on an optical fiber, with each signal carried on its own separate light wavelength. Using DWDM, more than 80 separate wavelengths or channels of data can be multiplexed into a light stream transmitted on a single optical fiber. Since each channel is de multiplexed at the end of the transmission back into the original source, different data formats transmitted at different data rates can be transmitted together. Specifically, Internet (IP) data, SONET data, and ATM data can travel at the same time within an optical fiber. DWDM offers very high transport capacity and added flexibility in routing and protection. depreciation Referred to as the expense matched with the use of an asset over its life, diminishing the asset's book value. Depreciation is said to be a non-cash expense because it is a tax and accounting allocation of the cost of the asset over a period time, theoretically matching the asset's liquidation value. Since depreciation (and amortization) are accounting rules rather than actual cash outlays, it shows up as an expense on the income statement but is added back in the statement of cash flows. detector An active optical component that receives an optical signal and converts it into an electrical signal. dielectric A substance in which an electric field may be maintained with zero or near-zero power dissipation, i.e., the electrical conductivity is zero or near zero. In other words, a dielectric material is a substance that is an efficient supporter of electrostatic fields but a poor conductor of electricity. diffractive optics Optical elements (such as diffraction gratings) that use diffraction, rather than refraction or reflection, to control wave fronts. digital The use of binary code to represent information. Analog signals, like voice or music, can be encoded digitally by sampling the signal many times a second and assigning a number to each sample. Information that is recorded and transmitted digitally is reproduced more precisely and it lowers costs of electronic circuitry. digital CLEC (D-CLEC) Competitive local exchange carrier (CLEC) that focuses specifically on data services and usually utilizes and in most cases uses DSL technology as its means of supplying broadband connectivity. digital cross connect An electronic cross-connect that has access to the lower-rate channels in higher-rate multiplexed signals and can electronically rearrange (cross connect) those channels. Dispersion The name given to any effect wherein different components of the transmitted signal travel at different velocities in the fiber, arriving at different times at the receiver. digital return path Refers to the route used for signals, going from the user to the control system ore headend, carrying the user's requests in a fully interactive system. digital signal-0 (DS0) North American digital hierarchy signaling standard for 64 Kbps (one channel). digital signal-1 (DS1) Consists of 24 channels (DS0 signals) multiplexed into a 193-bit frame. Transmission rate is 1.536 Mbps with 0.008 Mbps reserved for overhead. The term is synonymous with T1. [see also t-carrier system]. digital signal-3 (DS3) North American digital hierarchy signaling standard for 44.736 Mbps, or 28 simultaneous DS1 signals. Synonymous with T3. [see also t-carrier system]. digital signal processing (DSP) Refers to various techniques for analyzing and manipulating signals as digital data. digital subscriber line (DSL) Considered an attractive alternative to integrated services digital network (ISDN), DSL is a modem technology that enables high-speed delivery of data, audio and video in digital form over the existing telephone (copper networks) infrastructure. Provides multimedia services at speeds of more than 100 times that of a 56-Kbps modem. The basic network architecture calls for two modems, one placed at the customer premise and one at the local telephone switching office (or central office), that transmit digital signals using a different portion of the frequency spectrum than analog voice for signal transmission over the copper plant. The modem at the central office digital subscriber line access multiplexer (DSLAM) can handle multiple DSL lines, and thus digital signals, which it multiplexes for communications out of the global IP footprint. DSLAMs can transmit signals at various speeds. [see also xDSL] digital subscriber line access multiplexer (DSLAM) A multiplexer co-located at a central office that concentrates data for transmission via DSL. dilution The change in earnings per share or book value per share as a result of converting all warrants and options and adding them to the shares outstanding. This is an alternate representation of a company's earnings per share, which is often used by analysts since it represents the lowest potential earning per share (see Diluted EPS). Dilutive activities such as acquiring a company in a pooling transaction with a higher multiple can also result in reducing the overall EPS. direct goods Raw materials incorporated in an organization's finished products. One organization's finished goods become another's direct goods in a multi-level supply chain. Examples include chemicals, steel, circuit boards, and carburetors. discount P/E analysis A valuation method used in evaluating or comparing investments. The discount rate refers to either a cost of equity (for an equity valuation) or a cost of capital (for a firm valuation). discounted cash flow valuation (DCF) A valuation method used in evaluating or comparing investments or purchases. To the extent that reasonable estimates of the cash flows and applicable multiples can be made, the DCF is considered one of the most sophisticated valuation methods. In calculating a DCF, first, the present value of the free cash flow is calculated over the period in which future projections can be reasonably estimated. The terminal value is then calculated to represent the continuing value after the explicit forecast period. There are two methods of DCF Equity and Firm Valuation. Under the equity valuation method, the value of equity is obtained by discounting expected cash flows to equity, i.e., the residual cash flows after meeting all expenses, tax obligations and interest and principal payments, at the cost of equity. Under the firm valuation method, the value of the firm is obtained by discounting expected cash flows to the firm, i.e., the residual cash flows after meeting all operating expenses and taxes, but prior to tax-adjusted debt payments, at the weighted average cost of capital (WACC). The discount rate refers to either a cost of equity (if performing an equity valuation) or a cost of capital (if performing a firm valuation). The discount rate used should reflect both the riskiness and the type of cash flow being discounted. Under the equity method, the cost of equity reflects the rate of return required by equity investors of the firm based on the return that the investors would have been able to alternately obtain for an investment with the same risk level. WACC is used in the firm valuation method, which reflects the opportunity cost to the different providers of financing, weighted by their market value proportions to the company's total capital. Terminal Value is then commonly estimated, using a multiple of earnings, to reflect the continuing value after the explicit forecast period. In an equity valuation model, the exit multiple may be the P/E ratio. In firm valuation models, the exit multiple is often of EBIT or EBITDA. Analysts who use these multiples argue that it saves them from the dangers of having to assume a stable growth rate and that it ties in much more closely with their objective of selling the firm or equity to someone else at the end of the estimation period. An alternate DCF valuation method is to use the perpetuity growth method. This methodology is useful when the future projected cash flows cannot be predicted. This method determines valuation by taking today's cash flows and assuming that these cash flows will grow at a certain rate in perpetuity (i.e., "forever"). The calculation is equal to (today's cash flows / (discount rate - growth rate)). The proper growth rate should not be overestimated, as even though the immediate, short-term growth spurt may be very high, this growth rate is likely not sustainable over the long-term. discounted forward P/E   disk operating system (DOS) Software that sits at the center of software applications and their interaction with hardware. It organizes how a computer reacts with disks, input/oiiiutput devices, and processors. dispersion The spreading out of light pulses as they travel in an optical fiber. dispersion compensation Passive optical components that compensate for the effects of dispersion on an optical signal. distortion An unwanted change to a signal caused by outside interference or imperfections of the transmission systems. distributed-feedback lasers (DFB) An interjection laser diode, which has a Bragg reflection grating in the active region in order to suppress multiple longitudinal modes and enhance a single longitudinal mode. dividend discount model The dividend discount model is a specialized case of equity valuation, and the value of a stock is the present value of expected future dividends. This valuation method is inappropriate for companies that do not pay dividends, which may be the case for many new technology companies that are reinvesting their funds back into the company rather than paying it out to the shareholders. domain naming system (DNS) The mechanism used on the public Internet and private intranets to translate domain names into numeric IP addresses. A DNS server computer maintains a database for translating host names (e.g., www.epoch.com) into IP addresses (e.g., 206.143.298.475), and vice versa. dynamic random access memory (DRAM) A readable and rewriteable memory commonly found in computers. DRAM stores data in a "cell" composed of a capacitor and a transistor. The capacitor in a DRAM cell can only hold a charge for a few milliseconds, so it needs to be continually refreshed to retain the data. Its counterpart, static RAM (SRAM), does not require refreshing and delivers quicker performance, but it is more expensive. dynamic spectral equalizer (DSE) A device that equalizes the optical power in each channel of a DWDM network through attenuation (i.e., reducing the signal strength).