Network Layer example essay topic

Methods of sharing transmission media CSMA / CD is short for Carrier Sense Multiple Access / Collision Detection, a set of rules determining how network devices respond when two devices attempt to use a data channel simultaneously (called a collision). Standard Ethernet networks use CSMA / CD. This standard enables devices to detect a collision. After detecting a collision, a device waits a random delay time and then attempts to re-transmit the message. If the device detects a collision again, it waits twice as long to try to re-transmit the message. This is known as exponential back off.

CSMA / CA is short for Carrier Sense Multiple Access / Collision Avoidance, a network contention protocol that listens to a network in order to avoid collisions, unlike CSMA / CD that deals with network transmissions once collisions have been detected. CSMA / CA contributes to network traffic because, before any real data is transmitted, it has to broadcast a signal onto the network in order to listen for collision scenarios and to tell other devices not to broadcast. Token Passing Scheme This technology is used for token ring systems. Its incorporation along with complimentary fault-tolerance capabilities yield a LAN with a fair amount of sophistication, manageability and reliability. In this channel access method, a small signal called a token regularly visits each device. The token gives permission for the device to transmit if it needs to.

If a transfer of data is needed, the device receives a set amount of time to broadcast its data. When it is done, the machine then retransmits the token to another machine giving that recipient permission to transmit, and so the system continues. This mechanism ensures opportunity for all devices to gain access to the LAN. Because of its predictable behavior, token scheme LANs offer the advantage of priorities, where a certain group of devices may have enhanced access to the LAN if warranted. As traffic demand increases on a token LAN, the overall throughput of data rises as well until a point is reached where the networks simply cannot accommodate anymore. Because the throughput characteristics of token LANs are so predictable, and because of the characteristics of traffic demand vs. throughput, these systems are ideal for heavy traffic situations.

However, the complexity of such a LAN does come at some cost. Token systems require overhead to carry out their many functions including fault-tolerance. Plus, token ring systems are considerably more expensive than Ethernet systems. Factors weighing in deciding which system to choose should include traffic demand and budgetary restraints. Data Transfer Protocols Data Link Layer The main task of the data link layer is to take a raw transmission facility and transform it into a line that appears free of transmission errors in the network layer. It accomplishes this task by having the sender break the input data up into data frames (typically a few hundred bytes), transmit the frames sequentially, and process the acknowledgment frames sent back by the receiver.

Since the physical layer merely accepts and transmits a stream of bits without any regard to meaning of structure, it is up to the data link layer to create and recognize frame boundaries. This can be accomplished by attaching special bit patterns to the beginning and end of the frame. If there is a chance that these bit patterns might occur in the data, special care must be taken to avoid confusion. The data link layer should provide error control between adjacent nodes. Another issue that arises in the data link layer (and most of the higher layers as well) is how to keep a fast transmitter from drowning a slow receiver in data. Some traffic regulation mechanism must be employed in order to let the transmitter know how much buffer space the receiver has at the moment.

Frequently, flow regulation and error handling are integrated, for convenience. Network Layer The Network layer's basic purpose is to decide which physical path the information should take to move from its source to its destination. Determining the path to take is called routing. The network layer determines how messages are routed within the network. It provides transport entities independence from routing and relay considerations, including the case where sub networks (relay-only) nodes are used. All relay functions are operated within or below this layer.

This layer is responsible for communication between adjacent nodes in the network, and for routing of packets. where there are alternative paths through the network. It essentially takes a message from the host machine, splits it up into packets and organises the transmission of packets across the network to the desired destination. In doing so it is responsible for the sequencing and flow control of the packets. Larger wide-area networks typically offer a number of ways to move a string of characters (put together by the Data Link Layer) from one geographical location to another. The third layer of the OSI model, Network Layer, decides which physical pathway that data should take, based on network conditions, priority of service, and other factors Routing may be static or it may be determined at the start of each conversation. It can even be dynamic, being determined for each individual packet, depending on network load.

The network layer is also responsible for accounting. in terms of keeping a record of the number of bits and / or packets sent between different nodes. This could be used for billing users of the network. Finally, the network layer is responsible for interconnection between different networks. This can be a complex problem as the networks may use different standards for protocols. The network layer is built on the data link layer and carries packets of information across the network. in a long haul network, a WAN, several data links may be used together, and the network layer is responsible for routing the packets of information from point to point through the network.

OSI Seven Layer Model Explain each of the Layers and its applicability to Commercial Networks The seven layers of the OSI Basic Reference Model are (from bottom to top): 1. The Physical Layer describes the physical properties of the various communications media, as well as the electrical properties and interpretation of the exchanged signals. Ex: this layer defines the size of Ethernet coaxial cable, the type of BNC connector used, and the termination method. 2. The Data Link Layer describes the logical organization of data bits transmitted on a particular medium. Ex: this layer defines the framing, addressing and check summing of Ethernet packets.

3. The Network Layer describes how a series of exchanges over various data links can deliver data between any two nodes in a network. Ex: this layer defines the addressing and routing structure of the Internet. 4. The Transport Layer describes the quality and nature of the data delivery. Ex: this layer defines if and how retransmissions will be used to ensure data delivery.

5. The Session Layer describes the organization of data sequences larger than the packets handled by lower layers. Ex: this layer describes how request and reply packets are paired in a remote procedure call. 6.

The Presentation Layer describes the syntax of data being transferred. Ex: this layer describes how floating point numbers can be exchanged between hosts with different math formats. 7. The Application Layer describes how real work actually gets done. Ex: this layer would implement file system operations. Network Configurations Computer networks are basically of two types - client / server networks and peer-to-peer networks.

A client / server network is a network architecture where any one of the computer or processes on the network can act as either a server or a client. Although the term 'client / server computing' can be used to describe can be used to describe a network, it is really more than just that. The term can also be applied to describe software architecture. Some of the popular client / server applications can be found on the Internet and they include email programs, FTP programs and web browsers. Email client such as Microsoft Outlook and Pegasus allow users to download email messages from email servers such as Eudora Internet Mail Server and Microsoft Exchange Server. Internet browsers such as Microsoft's Internet Explorer, Netscape and Opera operate on the same principles by connecting to the dedicated web servers and download web pages for viewing.

Servers can also run operating systems such as Windows 95 and 98 and can be connected to by computers running the same operating system or even Window NT Server (however this is not recommended). Desktop PCs, minicomputers and some of the newer mainframes can act as servers in the client / server network. Peer-to-peer networks are a type of network where each computer connected to the network has equivalent capabilities and responsibilities, hence the name 'peer'. Unlike the client-server network, there is no dedicated server in the peer-to-peer network; each computer on the network is capable of being both a server and a client. This allows the computers to pool their resources, such as disk drives, CD-ROM drives, scanners and printers, and make them accessible to each computer on the network.

Peer-to-peer networks because of their simple architecture are really just practical for a small number of computers. Peer-to-peer networks can be built using either Ethernet (10 Base T cabling and a hub) or Fast Ethernet (a thin coaxial backbone -10 Base 2). Both methods work best with a maximum of 16 computers or less that are not separated by great distances (small to medium local area networks), more so with 10 Base T cabling. Ethernet speeds are up to 10 Mbit per second and 100 Mbit per second for Fast Ethernet.