uCertify Articles » Network+ 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 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:

Share/Bookmark]]> http://www.ucertify.com/article/what-are-osi-and-dod-models-of-network-communications-2.html/feed 0 What is 802.11g? http://www.ucertify.com/article/what-is-80211g.html http://www.ucertify.com/article/what-is-80211g.html#comments Wed, 28 Oct 2009 00:00:00 +0000 uCertify 1401425

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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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

Share/Bookmark]]> http://www.ucertify.com/article/what-is-eirp-2.html/feed 0 What is IEEE 802.11b? http://www.ucertify.com/article/what-is-ieee-80211b-2.html http://www.ucertify.com/article/what-is-ieee-80211b-2.html#comments Fri, 28 Aug 2009 00:00:00 +0000 uCertify 1401452

The IEEE 802.11b standard applies to wireless LAN networks that use radio waves as the transmission medium. These networks each have a separate media access control mechanism, known as Carrier Sense Multiple Access with Collision Avoidance (CSMS/CA), that works like the CSMA/CD mechanism used by the Ethernet. The difference is that in this protocol, when a computer sends its signals, the receiving computer performs a cyclical redundancy check and sends an acknowledgement message to the sending computer if no error is detected.

The IEEE 802.11b networks run at the transmission speed of 11 Mbps with a fallback to 5.5, 2, and 1 Mbps in the 2.4 GHz bandwidth. The radio transmission medium used by these networks is called Direct Sequence Spread Spectrum (DSSS).

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A routing protocol specifies how routers communicate with each other for distributing routing information between routers on a network. It helps each router determine the best path to deliver packets by knowing about all other routers connected to the network.

There are two categories of routing protocols, namely Interior Gateway Protocol (IGP) and Exterior Gateway Protocol (EGP).

An Interior Gateway Protocol (IGP) refers to a routing protocol that exchanges routing information between gateways within a single autonomous system. The routing information is used by the Internet Protocol (IP) or other network protocols to specify how to route transmissions. Following are the Interior Gateway Protocols (IGPs):

  • IS-IS
  • OSPF
  • IGRP
  • EIGRP
  • RIP (RIPv1 and RIPv2)

Intermediate System-to-Intermediate System (IS-IS) is a link-state routing protocol standardized by the ISO. It supports variable length subnet masks (VLSM), uses multicast to discover neighboring routers using hello packets, and supports authentication of routing updates.

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.

Interior Gateway Routing Protocol (IGRP) is a Cisco proprietary distance vector Interior Gateway Protocol (IGP). It used by Cisco routers to exchange routing data within an autonomous system (AS). This is a classful routing protocol and does not support variable length subnet masks (VLSM). IGRP supports multiple metrics for each route, including bandwidth, delay, load, MTU, and reliability.

EIGRP is a Cisco proprietary protocol. It is an enhanced version on IGRP. It has faster convergence due to use of triggered update and saving neighbor’s routing table locally. It supports VLSM and routing summarization. As EIGRP is a distance vector protocol, it automatically summarizes routes across Class A, B and C networks. It also supports multicast and incremental updates and provides routing for three routed protocols i.e. IP, IPX, and AppleTalk.

RIPv1 was designed to fulfill dynamic routing needs of a network. It uses MAC-level broadcasts, which are not desirable in the current scenario because all nodes must process all broadcasts. It does not support VLSM and route summarization.

RIPv2 is a classless protocol. It improves upon RIPv1 with the ability to use VLSM and route summarization. It supports route authentication and multicasting of route updates. It is used in small to medium size networks. RIPv2 uses triggered updates to speed up convergence. It supports variable length subnet masks (VLSM).

Variable Length Subnet Masks (VLSMs) allow administrators to divide a given IP address space into more than one subnet. It helps in efficient use of IP addressing. A classless routing protocol is required to deploy VLSM. Classful protocols such as RIP v1 do not support VLSM.

An Autonomous system (AS) is a group of networks under a single administration and with single routing policies. Each AS is assigned a unique number in order to differentiate it from other autonomous systems. The assigned unique number can be from 1 to 65,535. The Internet Assigned Numbers Authority (IANA) is responsible for assigning these numbers. There are public and private AS numbers like private and public IP addresses.

Administrative distance defines the reliability of a routing protocol. The router uses administrative distance in order to choose the best path when there are two or more different routes on a router to the same destination using different routing protocols. Each routing protocol’s administrative distance value decides which routing protocol is more reliable. The lower the administrative distance value, the more reliable the routing protocol.

Protocol Administrative distance value
IGRP 100
OSPF 110
IS-IS 115
RIP 120

Note: The administrative distance value for static route is 0.

Exterior Gateway Protocol (EGP) is a protocol that exchanges routing information between different autonomous systems. It is commonly used between hosts on the Internet to exchange routing table information. Border Gateway Protocol (BGP) is the only active EGP.

BGP stands for Border Gateway Protocol. It is an interautonomous system routing protocol and is a form of Exterior Gateway Protocol (EGP). This protocol is defined in RFC-1267 and RFC-1268. It is used for exchanging network reachability information with other BGP systems. This information includes a complete list of intermediate autonomous systems that the network traffic has to cover in order to reach a particular network. This information is used for figuring out loop-free interdomain routing between autonomous systems. BGP-4 is the latest version of BGP.

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To perform a physical installation of a router, take the following steps:

  1. Connect LAN cables to the LAN ports.

  2. If using an external CSU/DSU, connect the router’s serial interface to the CSU/DSU and the CSU/DSU to the line from the WAN service provider. And if using an internal CSU/DSU, connect the router’s serial interface to the line from the WAN service provider.

  3. Through a rollover cable, connect the router’s console port to a computer to configure the router.

  4. Connect the power cable from the power outlet to the power socket of the router.

  5. Switch on the router.
Share/Bookmark]]> http://www.ucertify.com/article/how-to-perform-a-physical-installation-of-a-router.html/feed 0 What is Voice over IP? http://www.ucertify.com/article/what-is-voice-over-ip.html http://www.ucertify.com/article/what-is-voice-over-ip.html#comments Fri, 10 Jul 2009 00:00:00 +0000 uCertify 1401640

Voice over IP (VoIP) is a TCP/IP application that passes voice traffic over the data network inside IP packets by using application protocols. A single VoIP call takes less than 30kbps of bandwidth over a WAN. VoIP requires low delay and very low jitter to produce good quality VoIP traffic. It does not require resending lost packets, as it would be useless by the time it was recovered.

Share/Bookmark]]> http://www.ucertify.com/article/what-is-voice-over-ip.html/feed 0 IEEE standards for Wireless LAN (WLAN)? http://www.ucertify.com/article/ieee-standards-for-wireless-lan-wlan.html http://www.ucertify.com/article/ieee-standards-for-wireless-lan-wlan.html#comments Fri, 10 Jul 2009 00:00:00 +0000 uCertify 1401704

IEEE

Institute of Electrical and Electronic Engineers (IEEE) is a society of technical professionals. It promotes the development and application of electro-technology and allied sciences. IEEE develops communications and network standards, among other activities. The organization publishes a number of journals, have many local chapters, and societies in specialized areas.

WLAN

Wireless Local Area Network (WLAN) is a network that enables devices to connect to the network wirelessly. WLAN uses radiated energy, commonly called high-frequency radio waves, to communicate amongst nodes. IEEE defines standards for WLAN through 802.11 family.

IEEE standards for WLAN

Various standards defined by IEEE are described below:

802.11

The IEEE 802.11 standards define wireless local area network (WLAN) computer communication in the 5GHz and 2.4GHz public spectrum bands. These specifications define an over-the-air interface between a wireless client and a base station or access point. The 802.11 specifications also define standards among wireless clients. These specifications address both the Physical (PHY) and Media Access Control (MAC) layers and are tailored to resolve compatibility issues between manufacturers of wireless LAN equipment.

802.11a

The IEEE 802.11a standard for WLAN uses the U-NII spectrum at 5GHz. It uses Orthogonal Frequency Division Multiplexing (OFDM) encoding class. The maximum speed supported by 802.11a standard is 54Mbps.

802.11b

The 802.11b standard, defined by IEEE, is an extension of the 802.11 standard of wireless network. It is also referred to as 802.11 High Rate or Wi-Fi. It operates in 2.4GHz band and brings data rates up to 11Mbps.

802.11g

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.

802.11i

The 802.11i standard of IEEE specifies security mechanism for Wireless LAN (WLAN). The standards include authentication and encryption.

Share/Bookmark]]> http://www.ucertify.com/article/ieee-standards-for-wireless-lan-wlan.html/feed 0 What is 100BaseTX? http://www.ucertify.com/article/what-is-100basetx.html http://www.ucertify.com/article/what-is-100basetx.html#comments Thu, 09 Jul 2009 00:00:00 +0000 uCertify 1401451

The 100BaseTX networks are known as fast Ethernet. These are the most widely used networks now-a-days. 100BaseTX Ethernet uses two of the four wire-pairs in the category 5 UTP cable. The number 100 used in the name indicates that these networks run at the speed of 100 Mbps using category 5 UTP cable. The topology used in these networks is star. The maximum cable segment length of 100BaseTX is 100 meters. The following table lists the 100BaseTX specifications:

Maximum segment length 100 meters
Speed 100 Mbps
Topology Star
Cable UTP (Cat. 5)
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