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Computer Networks Ethan Provensen, Alexander Peter MIS Computer Security Management, Strayer University

Abstract—The purpose of this paper is to show how computer networking is concerned with communication between computer systems. Such networks involves consist of at least two computers, which can be separated by a few centimeters (e.g. via Bluetooth) or thousands of kilometers (e.g. via the Internet). Over the past two decades computers and computer networking have changed drastically. This and many other subjects will be discussed further in this analysis of Chapter 9 - Computer Networks.

Fig. I-2 MAN

Index Terms—Network Topology, Media Access Control (MAC), Network Hardware, Open Systems Interconnection (OSI) Model, Transmission Control Protocol (TCP), and Internet Protocol (IP).

I. INTRODUCTION

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OMPUTER networks can be used for numerous services, both for companies and for individuals. For companies, networks of personal computers using shared servers often provide flexibility and a good price/performance ratio. For individuals, networks offer access to a variety of information and entertainment resources. Networks can be divided up in several different kinds; local area network (LAN) [Fig. I-1], metropolitan area network (MAN) [Fig. I-2], wide area network (WAN) [Fig. I-3], storage area network (SAN) [Fig. I-4], and personal area network (PAN) [Fig. I-5], each with their own characteristics, technologies, speeds and niches. LANs cover a building, MANs cover a city, and WANs can cover a country or continent. LANs and MANs are usually unswitched (i.e. do not have routers); WANs are usually switched [1]. Fig. I-1 LAN

Fig. I-3 WAN

Fig. I-4 SAN

2 Fig. I-5 PAN

Fig. II-1

II. NETWORK TOPOLOGY A Network topology is arrangement or mapping of different network elements. Network topologies can come in different styles of setups; they consist of the physical, signal, and logical topologies between the nodes. A node is a point is a network where part of it branches off or intersects such as a device like a computer. The node has to be able to receive, transmit, and send data and other signals along with being able to process the data itself and do something with the data. A node will either be an end point or a redistribution point for the data on the network and every node has to have a MAC Address or a Data-Link Control Address. When someone examines a Signal and Logical topology there is a slight difference that someone has to look for; thus making it hard to distinguish between the two. The difference between the two is that logical refers to the apparent path of the network and that signal refers to the actual path that the data is transferred through. Physical topology refers to the mapping of the physical connection between networks, it is the layout of the cable or wires that connect the nodes. Some networks such as a local area network (LAN) can consist of both a physical and logical interconnection between the endpoints or nodes. An important thing to help distinguish the two is that physical topology is a real interconnection and logical is a virtual interconnection between the nodes. How the Data flows and is mapped between nodes determines the classification of the physical topology. Some of the classification types include Bus [Fig. II-1], Ring [Fig. II-2, and Star [Fig. II-3] [2].

Fig. II-2

Fig. II-3

3 III. MEDIA ACCESS CONTROL A Media Access Control (MAC) protocol decides when competing nodes may access the shared medium (i.e. the radio channel) and tries to ensure that no two nodes are interfering with each other’s transmissions. In the unfortunate event of a collision, a MAC protocol may deal with it through some contention resolution algorithm, for example, by resending the message later at a randomly selected time. Alternatively, the MAC protocol may simply discard the message and leave the retransmission up to the higher layers in the protocol stack. MAC protocols for wireless networks have been studied since the 1970s, but the successful introduction of wireless LANs in the late 1990s has accelerated the pace of developments. The technical design portion of the standard defines MAC, and the Physical layer. The MAC portion of the standard describes the use of scheduling to make sure that each client is allocated a time slot with the base station. The MAC also describes the use of QoS (Quality of Service) and authentication. The physical layer portion of the standard describes a frequency spectrum from 2 GHz to 66 GHz. The 2 GHz to 11 GHz is intended for non-line of sight usage, and the 10 GHz – 66 GHz range is intended for line of site usage. The physical layer portion of the standard also describes the methods used to communicate with the base stations. TimeDivision Multiplexing (TDM) is used by the base station to communicate with the clients, and Time-Division Multiple Access (TDMA) is used to communicate back to the base stations [3].

may need to purchase for the systems are network interface cards (NICs) [Fig. IV-1], hubs [Fig. IV-2], bridges [Fig. IV3], routers [Fig. IV-4], and switches [Fig. IV-5]. Each machine on the network can be installed with a NIC that connects to the computer’s bus, and to either 10base-T (twisted pair) or coaxial cable. The Ethernet transmits data by means of packets and automatically adjusts to the addition of computers to a net. NICs all have a unique address built in at the factory to facilitate this. After the NICs are installed in each system, installing the driver will be needed for the card by using the Windows Hardware Wizard installation procedures and diskette supplied by the NIC [5]. Fig. IV-1

Fig. IV-2

IV. NETWORK HARDWARE In an indecisive world, network hardware devices uses decision support systems (DSS) to efficiently and effectively route data, in a local area network, with the least amount of errors and inconsistencies. Decision support systems are the brains behind network hardware, and would be near impossible for them to work without the intelligent core of each distinct DSS. There are a variety of network hardware devices: switches, hubs, and routers; and they all use algorithms/procedures to transfer data towards the correct destination. Although there are many more DSS related methods used to route traffic, this paper describes how these three devices use them and each of there functions. Other types of decision support systems that network hardware devices may use are firewall technology, network address translation, and filter tables. The type of network topology in which each of the nodes of the network is connected to two other nodes in the network and with the first and last nodes being connected to each other, forming a ring. All data that is transmitted between nodes in the network travels from one node to the next node in a circular manner and the data generally flows in a single direction only. In a ring network, every device has exactly two neighbors for communication purpose. All messages travel through a ring in the same direction, either “clockwise” or “counterclockwise”. A failure in any cable or device breaks the loop and can take down the entire network [4]. Network hardware upgrades are essential for a successful LAN to work fast and smoothly. The only upgrades that

Fig. IV-3

Fig. IV-4

4 Fig. IV-5

V. OSI MODEL Networks software consists of protocols, or rules by which process can communicate. Protocols can be either connectionless or connection-oriented. Most networks support protocol hierarchies, with each layer providing services to the layers above it and insulating them from the details of the Protocols used in lower layers. Protocol stacks are typically based either on the OSI model or the TCP/IP model. Both of these have network, transport, and application layers, but they differ on the other layers. The Open Systems Interconnection (OSI) model is a reference tool for understanding data communications between any two networked systems. It divides the communications processes into seven layers. Each layer both performs specific functions to support the layers above it and offers services to the layers below it. The three lowest layers focus on passing traffic through the network to an end system. The top four layers come into play in the end system to complete the process [6]. The OSI is the accepted model that describes the how the communications for a computer network should be designed. OSI was developed as part of the Open Systems Interconnect Initiative. Basically, OSI divides the functions of a protocol in to seven layers. This makes the reasoning easy to follow and the system much more reliable. The implementation of several OSI layers is often referred to as a Stack or a TCP/IP Stack. The layers are often referenced in descending order because layer 7 is the first layer that an end user sees. A detail of each layer in descending order follows: ¾ Layer Seven of the OSI Model - The Application Layer of the OSI model is responsible for providing end-user services, such as file transfers, electronic messaging, e-mail, virtual terminal access, and network management. This is the layer with which the user interacts. ¾ Layer Six of the OSI Model - The Presentation Layer of the OSI model is responsible for defining the language that enables two network hosts to communicate. Encryption and compression should be Presentation Layer functions. ¾ Layer Five of the OSI Model - The Session Layer of the OSI model is responsible for establishing process-to-process communications between networked hosts.

¾ Layer Four of the OSI Model - The Transport Layer of the OSI model is responsible for delivering messages between networked hosts. The Transport Layer should be responsible for fragmentation and reassembly. ¾ Layer Three of the OSI Model - The Network Layer of the OSI model is responsible for establishing paths for data transfer through the network. Routers operate at the Network Layer. ¾ Layer Two of the OSI Model - The Data Link Layer of the OSI model is the interface between network software and hardware. This layer is also responsible for communications between adjacent network nodes such as, hubs, switches, and routers. ¾ Layer One of the OSI Model - The Physical Layer of the OSI model is at which communication between devices actually takes place. The physical layer includes hardware devices that encode and decode bit streams and the transmission lines that transport them [7]. VI. TCP/IP At first glance, TCP/IP (Transmission Control Protocol/Internet Protocol) may seem baffling. Many other protocols, such as NetBEUI and IPX/SPX, require no configuration. TCP/IP is different. Due to the seemingly endless number of options that can be configured within TCP/IP, many people become intimidated at first. In reality, however, TCP/IP is not very difficult, but gaining some understanding of the configuration is important. The most basic element of TCP/IP is the IP address. The IP address is a number that is unique to each computer. If a computer’s IP address is known, then that computer can communicate with that computer from anywhere in the world. Since TCP/IP is the protocol that the Internet uses and since Internet servers are located all over the world, TCP/IP must be routable. Thus, when trying to access an IP address, the computer must be able to tell whether or not that IP address is located on a specific local network. If the desired address is located on a specific local network, there will not be a problem reaching it. If it is not on a specific local network, TCP/IP must know which network the IP address is located on in order to reach the address [8]. The network number represents the network that contains a given IP address. Looking through the various tabs of the TCP/IP properties sheet, there is no field that allows specification of the network number. Instead, the network number is part of the IP address. An IP address is composed of a network number and a computer number. A computer can distinguish those two numbers because of something called the subnet mask. The subnet mask is located in a field directly below the IP address on the TCP/IP properties sheet. A simple subnet mask would be something like 255.255.0.0 [9].

5 VII. CONCLUSION Computer Networks are the primary means of intercomputer communications. Simply, specialized computer networks can be broken into five parts: Network Topology, Media Access Control (MAC), Network Hardware, Open Systems Interconnection (OSI) Model, Transmission Control Protocol (TCP), and Internet Protocol (IP). Each of these parts plays their specific role in creating a successful computer network enterprise. ACKNOWLEDGMENT Ethan Provensen would like to thank the author of his textbook “Systems Architecture”, Stephen Burd, for his written information that was referred to immensely throughout this presentation. He would also like to thank Prof. Peter for his immense patience and cooperation. It has been greatly appreciated. REFERENCES [1] S. D. Burd, Systems Architecture Fifth Edition. Course Technology, 2006 [2] S. D. Burd, Systems Architecture Fifth Edition. Course Technology, 2006 [3] S. D. Burd, Systems Architecture Fifth Edition. Course Technology, 2006 [4] S. D. Burd, Systems Architecture Fifth Edition. Course Technology, 2006 [5] S. D. Burd, Systems Architecture Fifth Edition. Course Technology, 2006 [6] S. D. Burd, Systems Architecture Fifth Edition. Course Technology, 2006 [7] S. D. Burd, Systems Architecture Fifth Edition. Course Technology, 2006 [8] D. Teare, K. Hutton, & M. Schofield, Cisco Press, Designing Cisco Enterprise Campus Architecture Models. http://www.ciscopress.com. (2009) http://www.ciscopress.com/articles.asp?p=1398754 [9] S. D. Burd, Systems Architecture Fifth Edition. Course Technology, 2006