uCertify Articles » N10-003 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 are OSI and DoD models of network communications? http://www.ucertify.com/article/what-are-osi-and-dod-models-of-network-communications-2.html http://www.ucertify.com/article/what-are-osi-and-dod-models-of-network-communications-2.html#comments Sat, 31 Oct 2009 00:00:00 +0000 uCertify 1401102

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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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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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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A media tester, also called a media certifier, is an electronically programmed device that is used to perform a variety of tests on an installed cable. It tests for shorts, open circuits, transposed wire pairs, attenuation, and various types of crosstalk. A media tester compares the results with standards programmed into the unit, and displays the result in the form of pass or fail readouts for each test.

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Internet Control Message Protocol (ICMP) is an integral part of IP. It is used to report an error in datagram processing. The Internet Protocol (IP) is used for host-to-host datagram service in a network. The network is configured with connecting devices called gateways. When an error occurs in datagram processing, gateways or destination hosts report the error to the source hosts through the ICMP protocol. The ICMP messages are sent in various situations, such as when a datagram cannot reach its destination, when the gateway cannot direct the host to send traffic on a shorter route, when the gateway does not have the buffering capacity, etc.

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The network topology is the pattern in which the network medium is connected to the computers and other networking components. The following are the major topologies associated with local area networking:

  • Bus topology: Bus topology is a type of physical network design where all computers in the network are connected through a single coaxial cable known as a bus. This topology uses minimum cabling and is therefore the simplest and least expensive topology for small networks. In this topology, 50-ohm terminators terminate both ends of the network. A Bus topology network is difficult to troubleshoot, as a break or problem at any point along the cable can cause the entire network to go down.

  • Ring topology: Ring topology is a type of physical network design where all computers in the network are connected in a closed loop. Each computer or device in a Ring topology network acts as a repeater. It transmits data by passing a token around the network in order to prevent the collision of data between two computers that want to send messages at the same time. If a token is free, the computer waiting to send data takes it, attaches the data and destination address to the token, and sends it. When the token reaches its destination computer, the data is copied. Then, the token gets back to the originator. The originator finds that the message has been copied and received and removes the message from the token. Now, the token is free and can be used by the other computers in the network to send data. In this topology, if one computer fails, the entire network goes down.

  • Star topology:Star topology is a type of physical network design where each computer in the network is connected to a central device, called a hub, through an unshielded twisted-pair (UTP) wire. Signals from the sending computer go to the hub and are then transmitted to all the computers in the network. Since each workstation has a separate connection to the hub, it is easy to troubleshoot. Currently, it is the most popular topology used for networks.

  • Mesh topology: Mesh topology is a type of physical network design where all devices in a network are connected to each other with many redundant connections. It provides multiple paths for the data traveling on the network to reach its destination. Mesh topology also provides redundancy in the network. It employs the full mesh and partial mesh methods to connect devices. In a full mesh topology network, each computer is connected to all the other computers. In a partial mesh topology network, some of the computers are connected to all the computers, whereas some are connected to only those computers with which they frequently exchange data.

  • Wireless topology: Wireless topology is a type of physical network design where no cable is used as a network medium. The wireless topology eliminates the need for cables. A wireless LAN consists of a transceiver unit, called a wireless access point, that is connected to servers or directly to the network and other devices using a standard cabled network protocol, such as Ethernet or Token Ring. In the wireless network, client computers communicate with the network-attached transceivers by using their own transceivers. The most common wireless media are infrared light and radio waves.
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10Base5 is an IEEE standard for baseband Ethernet coaxial cable. It transfers data at 10Mbps, which can travel a maximum distance of 500 meters. 10Base5 is also known as thick Ethernet, ThickWire, and ThickNet. The characteristics of 10Base5 are summarized in the table given below:

Parameters Value
Cable diameter 0.4 in. (1.01 cm)
Maximum segment length 1,640 ft. (500 m)
Maximum network length (with 4 repeaters) 8,200 ft. (2,500 m)
Maximum connections (taps) per segment 100
Minimum connection (tap) spacing 8.2 ft. (2.5 m)
Share/Bookmark]]> http://www.ucertify.com/article/what-is-10base5.html/feed 0 What is IEEE 802.1X authentication? http://www.ucertify.com/article/what-is-ieee-8021x-authentication.html http://www.ucertify.com/article/what-is-ieee-8021x-authentication.html#comments Sun, 05 Jul 2009 00:00:00 +0000 uCertify 1401342

The IEEE 802.1X standard defines a method of authenticating and authorizing users to connect to an IEEE 802 LAN. It blocks users from accessing the network on the failure of authentication. IEEE 802.1X supports the Extensible Authentication Protocol-Transport Level Security (EAP-TLS) and Protected EAP-Microsoft Challenge Handshake Authentication Protocol version 2 (PEAP-MS-CHAP v2) protocols. In the IEEE802.1X authentication system, an access point receives a connection request from a wireless client and forwards the request to the RADIUS server. The RADIUS server then uses the Active Directory database to determine whether the client should be granted access to the network.

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Extensible Authentication Protocol-Transport Level Security (EAP-TLS) is an authentication protocol, which provides mutual authentication, integrity-protected negotiation of cryptographic service providers, and a secret key exchange between two systems that use public key cryptography. EAP-TLS works on a network that is configured for public key infrastructure (PKI) and uses certificates for authentication. These certificates can be stored on computers or on smart cards.

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