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Certpedia: Terms That Start With O

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  • OFDM (Orthogonal Frequency-Division Multiplexing) - OFDM is a digital multi-carrier modulation scheme, which uses a large number of closely-spaced orthogonal sub-carriers. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation) at a low symbol rate, maintaining data rates similar to conventional single-carrier modulation schemes in the same bandwidth. In practice, OFDM signals are generated using the Fast Fourier transform algorithm. The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions for example, attenuation of high frequencies at a long copper wire, narrowband interference and frequency-selective fading due to multipath without complex equalization filters. Channel equalization is simplified because OFDM may be viewed as using many slowly-modulated narrowband signals rather than one rapidly-modulated wideband signal. Low symbol rate makes the use of a guard interval between symbols affordable, making it possible to handle time-spreading and eliminate inter-symbol interference (ISI). OFDM is used in many applications including DSL and 802.11 a/g wireless networking.

  • Open Relay - Also referred to as an open relay server, an SMTP e-mail server that allows a third party to relay e-mail messages, ie, sending and/or receiving e-mail that is not for or from a local user. Spammers essentially hijack open relays and use them to send out massive quantities of unsolicited email (UCE).

  • OSI Model (Open Systems Interconnection Model) - Developed by the International Standards Organization (ISO), the OSI reference model is a 7 layer networking framework that divides the networking process into logical layers. The 7 layers are: Physical, Data Link (which has sub-layers called Media Access Control and Logical Link Control), Network, Transport, Session, Presentation, and Application. OSI Model

  • OSPF (Open Shortest Path First) - This is a link-state routing protocol that converges faster than a distance vector protocol such as RIP. What is convergence? This is the time required for all routers to complete building of the routing tables. RIP uses ticks and hop counts as measurement, while OSPF also uses metrics that takes bandwidth and network congestion into making routing decisions. RIP transmits updates every 30 seconds, while OSPF transmits updates only when there is a topology change. OSPF builds a complete topology of the whole network, while RIP uses second handed information from the neighboring routers. To summarize, RIP is easier to configure, and is suitable for smaller networks. In contrast, OSPF requires high processing power, and is suitable if scalability is the main concern.
    OSPF E2 Routes vs. E1 Routes
    OSPF Autonomous Border System Routers Explained And Illustrated
    Route Summarization And The OSPF Null Interface

  • Overclocking - This term refers to configuring a computer component run at a higher clock speed than the manufacturer's specifications. The main reason this is done is to increase hardware performance. Overclocking does not come without risks as it can result in system instablity or even cause hardware failure in some situations.

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