uCertify Articles » CCENT http://www.ucertify.com/article Just another WordPress weblog Wed, 04 Nov 2009 00:00:00 +0000 http://wordpress.org/?v=2.8.4 en hourly 1 What is Video over IP? http://www.ucertify.com/article/what-is-video-over-ip-3.html http://www.ucertify.com/article/what-is-video-over-ip-3.html#comments Mon, 02 Nov 2009 00:00:00 +0000 uCertify 1401641 Download free practice test for Cisco CCENT exam. Website : http://www.mkyong.com Become Cisco CCNA certified. -->

Video over IP is a TCP/IP application that passes video traffic over the data network inside IP packets by using application protocols. The requirement of bandwidth varies from 300/400Kbps to 10Mbps. As high-definition video over IP is also in demand now, the bandwidth requirement will increase for video over IP applications. It requires medium to high bandwidth and low delay, jitter, and loss requirements.

Get certified in first attempt download 640-802 - CCNA: ICND1 and ICND 2 simulation. Download practice question and study guide for 640-822 for exam. Share/Bookmark]]> http://www.ucertify.com/article/what-is-video-over-ip-3.html/feed 0 What is OSPF? http://www.ucertify.com/article/what-is-ospf.html http://www.ucertify.com/article/what-is-ospf.html#comments Sun, 01 Nov 2009 00:00:00 +0000 uCertify 2101094

Open Shortest Path First (OSPF) is a routing protocol that is used in large networks. Internet Engineering Task Force (IETF) designates OSPF as one of the Interior Gateway Protocols. A host uses OSPF to obtain a change in the routing table and to immediately multicast updated information to all the other hosts in the network.

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The OSI Model

The International Organization for Standardization (ISO) developed the Open Systems Interconnection (OSI) reference model in 1977. The OSI model is a conceptual framework that can be used to get a better understanding of the complex interactions taking place among the various devices in a network. The purpose of the OSI model is to demonstrate how the network entities should work together for communication.

The OSI model organizes communication services into seven groups called layers. The layers are as follows:

  • Layer 1, The Physical Layer: The physical layer is responsible for packaging and transmitting data on the physical media. This layer conveys the bit stream through a network at the electrical and mechanical level.

  • Layer 2, The Data-Link Layer: The data-link layer is responsible for error free transfer of data frames. This layer provides synchronization for the physical layer.

  • Layer 3, The Network Layer: The network layer controls the operation of the subnet. It determines the physical path that data takes on the basis of network conditions, priority of service, and other factors. The network layer is responsible for routing and forwarding data packets.

  • Layer 4, The Transport Layer: The transport layer ensures that messages are delivered in the order in which they are sent and that there is no loss or duplication. It ensures complete data transfer.

  • Layer 5, The Session Layer: The session layer establishes a communication session between processes running on different communication entities in a network and can support a message-mode data transfer. It deals with session and connection coordination.

  • Layer 6, The Presentation Layer: The presentation layer serves as the data translator for a network. It is usually a part of an operating system and converts incoming and outgoing data from one presentation format to another. This layer is also known as syntax layer.

  • Layer 7, The Application Layer: The application layer serves as a window for users and application processes to access network services. It handles issues such as network transparency, resource allocation, etc. This layer is not an application in itself, although some applications may perform application layer functions.

Each communicating entity is equipped with these seven layers. When a message is sent between two communicating entities, data flows down through each layer in the sending entity and flows up through each layer in the receiving entity.

As the message travels down the sending entity layer stack, each layer it passes through (physical layer is an exception) adds a header. These headers contain pieces of control information that are read and processed by the corresponding layer on the receiving entity layer stack. As the message travels up the stack of the receiving entity, each layer strips the header added by its peer layer.

For example, two networked applications, based on Windows and Macintosh operating systems, are communicating with each other. At layer 7, the Windows application requests data from the Macintosh application. The request is sent to the Windows application’s layer 6. This layer receives the request as a data packet. It then adds some information, known as header, and passes the packet down to layer 5. The process continues until the request reaches the physical layer.

Note: Physical layer does not add a header.

The physical layer sends this request packet to the Macintosh application through the network transmission media. When the Macintosh application receives this packet, the header that was added at the data-link layer of Windows application is stripped at the data link layer of the Macintosh application. The Macintosh data-link layer performs the tasks requested in the header and passes the request packet to the next higher layer, i.e., the network layer. The process is repeated until the Macintosh application’s application layer receives the packet and interprets the request.

The DoD Model

The Department of Defense developed a four-layer reference model in 1970s for the DARPA Internetwork Project. DoD is a reference model that determines the way in which messages should be transmitted between any two-communication entities in a network. In DoD model, the two end points in a network are divided into layers. As data moves through the layers, header information is added (for transmission) or removed (for reception). The DoD model consists of the following four layers:

  • Layer 1, Network Interface Layer: The network interface layer is the lowest layer in the DoD model. This layer is responsible for putting frames on the wire and pulling frames off the wire. It is equivalent to the data-link and physical layers of the OSI model.

  • Layer 2, Internet Layer: The Internet layer provides three services – a connectionless delivery service , a mechanism to break data into individual packets or frames on the transmitting side and put them back together on the receiving side, and the routing functions necessary to interoperate with other networks. It is equivalent to the network layer of the OSI model.

  • Layer 3, Transport Layer: The transport layer provides communication sessions between connected computers. This layer is responsible for error detection and correction. It is also known as host-to-host or transmission layer and is equivalent to the transport layer of the OSI model.

  • Layer 4, Application Layer: The application layer serves as a window for users and application processes to access network services. Several standard TCP/IP utilities and services exist at the application layer. It is equivalent to the application, presentation, and session layers of the OSI model.

Comparison between the layers of the OSI and DoD models

The following figure compares the layers of the OSI model with the layers of the DoD model:

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The 802.11g standard, defined by IEEE, is an extension to the 802.11b standard of wireless network. It operates in 2.4-GHz band and brings data rates up to 54Mbps using Orthogonal Frequency-Division Multiplexing (OFDM) technology. Since the 802.11g standard is backward compatible with 802.11b, an 802.11b device can interface directly with an 802.11g access point.

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Wide Area Network (WAN) represents a network spanned in a large geographic area. WANs connect two or more LANs separated by a large distance. To connect LANs in a WAN network, carrier networks such as telephone companies provide the services. The basic types of connections or circuits WAN services use are circuit-switched, cell-switched, packet-switched, and dedicated connections.

Download free practice test for 640-802 CCNA: ICND1 and ICND 2 exam. Click here to get free 640-822 CCNA: ICND1 exam practice questions. Share/Bookmark]]> http://www.ucertify.com/article/what-is-wan.html/feed 0 What are private IP addresses? http://www.ucertify.com/article/what-are-private-ip-addresses-2.html http://www.ucertify.com/article/what-are-private-ip-addresses-2.html#comments Tue, 15 Sep 2009 00:00:00 +0000 uCertify 2500235

Each IP node requires an IP address globally unique to the IP internetwork. When a number of Intranet hosts use the Internet, each IP node on the private network requires a globally unique IP address. For the hosts within an organization that do not have a direct access to the Internet, IP addresses that do not duplicate already-assigned public addresses are required. These addresses are part of the reserved private address space defined by the ISPs and are never assigned as public addresses. Such addresses are known as private addresses.

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Effective Isotopic Radiated Power (EIRP) is the power of the signal transmitted from the antenna of an access point (AP). EIRP is calculated as follows:

EIRP = The radio’s power output + Increase in power caused by the antenna – Power lost in the cabling

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Beacon is the process through which a wireless client learns about each access point (AP) and their SSIDs. When a client moves, this process helps it to re-associate with a new AP when the old AP’s signal gets weaker. The AP sends a periodic Beacon frame that lists the AP’s SSID and other configuration information.

Share/Bookmark]]> http://www.ucertify.com/article/what-is-beacon-2.html/feed 0 What is a cell-switched connection? http://www.ucertify.com/article/what-is-a-cell-switched-connection-3.html http://www.ucertify.com/article/what-is-a-cell-switched-connection-3.html#comments Sat, 29 Aug 2009 00:00:00 +0000 uCertify 1401617 Download free practice test for Cisco CCENT exam. Author : Yong Mook Kim Pass CCNA in first attampt. Website : http://www.mkyong.com Download free practice test for Cisco CCNA: ICND1 and ICND 2 exam. -->

A cell-switched connection is a packet-switched connection that uses digital circuits. Cell-switched connections use fixed-length (53 bytes) packets to transmit information. These packets are called cells. ATM and SMDS are examples of cell-switched connections.

Download free practice test for 640-816 CCNA: ICND2 exam. Click here to download 640-822 test study guide and practice question. Share/Bookmark]]> http://www.ucertify.com/article/what-is-a-cell-switched-connection-3.html/feed 0 What is CDP? http://www.ucertify.com/article/what-is-cdp-2.html http://www.ucertify.com/article/what-is-cdp-2.html#comments Sat, 29 Aug 2009 00:00:00 +0000 uCertify 1401622 Pass CCENT in first attampt. Website : http://www.mkyong.com Download free practice test for Cisco CCNA exam. -->

Cisco Discovery Protocol (CDP) is a Cisco proprietary protocol. It works at the data link layer. CDP is enabled by default on all Cisco devices. Devices directly connected to one another receive CDP messages. A Cisco device does not forward a message to any device that is behind it. CDP messages are generated as multicast messages by a Cisco device on each of its active interfaces after every 60 seconds.

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