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Enterprise Architecture What are Computer Networks Michele Hermosura, Alexander Peter, Ricky Elum B.S. Business Management, University of Maryland

Abstract—This paper summarizes Computer Networks and discusses how computer networks design relates to Zachman’s Framework. Computer networks include topologies, routing, media access control, networking devices, and protocol suites used in a network. Index Terms—LAN, WAN, OSI, TCP/IP

I. INTRODUCTION

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networks are defined by S. Burd as hardware and software that enables multiple users and computer systems to share information, software, and other resources[1]. Computer networks include many elements not mentioned in the definition including the layout of the network both physical and logical, the addressing or routing protocols and the standard needed to communicate across the network. A network node is a workstation, server, printer, or other computing device on the network [2]. OMPUTER

II.

NETWORK TOPOLOGY

In order to connect computers and allow them to communicate a physical and logical connections are required. The physical and logical topologies for a network can be different, but the physical and logical connection is required. The network topology is the organization of the network devices, physical routing of network cabling, and the flow of messages from one computer to another [3]. Connections to nodes or hardware devices can be shared or point to point connections. Shared connections allow communication to other hardware devices on the network through a single communication path, such as a cable. Point to point connections require connections to and from every node to communicate. Point to point only makes sense when there are only two or three nodes. Figure 1 is an example of point to point network.

Manuscript received September 6 2009

Fig. 1. Point to point network topology. Shows the complexity with seven workstations and the need for large amounts of cabling. The two major disadvantages of using the point to point topology on larger networks are the massive cables and cable trays to support all of the point to point cabling and difficulty to troubleshoot and repair cables and nodes during outages. Figure 2 shows nodes connected through a shared cable. This is considered the physical topology or the physical placement of cables and devices [4].

Fig. 2. Shared Network Topology The logical topology as defined by Stewart, Tittel, and Chapple “is the grouping of networked systems into trusted collectives [5]. The physical and logical topologies don’t always look the same. The basic topologies are ring, star, bus and mesh [6] [7]. The topologies are differentiated by the routing of the cables, the length of the cables, the performance of the cables and connected devices, and how often communication between devices or nodes fail [8]. The ring topology is physically connected in a ring as show

2 in Fig 3. Each system is connected to a point on the ring [9]. Communication on the ring goes in one direction, which means only one device at a time can transmit on the ring. If a computer wants to send a message to another computer on the ring, the sender must have a token to transmit. A computer wanting to send message on the ring would listen on the network and then grab the transmit token when its available. Once the computer has the token it attaches its message and sends it, each computer on the network looks at the token to determine if the message is address to them, if it is not it passes the token on to the next device in the ring until it reaches the intended recipient. This is one of the disadvantages of the ring network topology. The other is every device and cable on the ring must be working for the ring to allow the computers on the ring to transmit and receive messages.

referred to as the backbone or trunk cable [11]. There are two type of bus configurations the linear and tree. The mesh topology is any physical or logical configuration that does not fit into the ring, star, or bus definitions. An example is shown in Fig. 5.

Fig. 5. Mesh Network Topology does not fit characteristics of the ring, star, or bus.

III. NETWORK ARCHITECTURES

Fig. 3. Ring Network Topology The star topology has a center hub or switch to connect all of the hardware devices. If one segment or computer fails the star remains operational. If the center hub fails then the star fails. This single point of failure is one of the disadvantages of the star topology [10]. The star is shown in figure 4. The advantages to the star topology are the small amount of cable required to configure the topology and the star is easy to troubleshoot.

Fig. 4. Star Network Topology The bus topology has a single cable connecting all devices on the network, similar to Fig. 2. The cable is sometimes

The addressing in routing of message on a network tells us how information moves from a computer in one location to a computer in another location. This addressing and routing is described in terms of network size. The smallest configuration is called a local area network or LAN. The LAN is a group of connect computers and devices within a very small geographic area. Examples include in an office or building. LANs use what called routing tables to make decisions about routing. [12] There are different types of LAN technologies; examples include Ethernet and Fiber Distributed Data Interface (FDDI). Ethernet uses collision detection and avoidance by having nodes take turns communicating on the network. FDDI uses high-speed token passing and has two rings to allow traffic to flow in opposite directions. Failed segments are automatically removed; this overcomes the problem of bringing the entire network down when a device or cable fails. [13] Packets are delivered using protocols. Packets in a local area network are delivered using the media access control address (MAC address). Each hardware device has a unique hardware number or MAC address. WANs normally use Internet Protocol (IP) addressing to deliver packets. Routing messages on a LAN depends on the logical topology [14]. On the logical bus topology the devices on the network see the network as a linear bus or one cable. On the logical ring the network is seen as a one way connection around a circle. In the logical star configuration a switch would send traffic from one device to another by looking up the recipients address. The message is then placed on the intended recipient’s transmission line [15]. In wide area networks each node knows the IP address and locations of devices connected to them. It also knows some of the neighboring devices IP addresses. A node attached to a LAN and a WAN would know the LAN IP’s and the IP’s of

3 the devices it’s connected to on the WAN. Wide Area Networks or WANs cover larger geographic areas. An example of a WAN is several LAN’s located throughout the United States connected together. WANs include dedicated and temporary lines. Examples include leased lines, standard modems, ISDN, and DSL connections [16]. Another type of WAN connection is using virtual connections through packet switching. The physical hardware used to connect the WAN to the LAN is a switch, router, or border connection device. The border connection device also known as the channel service unit/data service unit (CSU/DSU) [17]. The CSU/DSU translates the information from the WAN language to LAN language so that the LAN router knows what to do with messages received. The actual WAN connection technologies are X-25, Frame Relay, and ATM [18]. X.25 and Frame Relay use packet switching which is dedicated virtual circuit(s). ATM uses cellswitching. Cell switching is fixed length data messages sent over WAN lines. There are several common facts about WANs [19]: - The largest most used WAN is the Internet. - Protocols are required to communicate across a WAN. - Connection between geographically separate segments can be point to point or packet switching. - Central WAN nodes must know the physical route to connect directory servers. If the server is moved all directories have to update their addresses. - Directory and answers must be stored, which requires large storage areas. - WANS deal with massive amounts of queries and responses. - Information must be copied to all servers. The hardware devices used in WANs are routers, switches and modems. The routers direct the traffic and connect the different networks, WAN switches create the virtual connections between routers, and if needed the modem does conversions. WANs cover large geographic areas and they help businesses share information. LAN’s and WAN’s were described above, but there are other types of network architectures for example metropolitan area networks (MAN) which is normally within a city or town, peer-to-peer two or more equal computers communicating, client-server used in Windows 2000, and intranet which are used to share information on a LAN, most intranets resemble web pages but only nodes on the inside of the LAN have access to information. The last is the Internet which is really a type of WAN [20]. IV. LAN MEDIA ACCESS CONTROL The purpose of media access control is to prevent collisions, decrease the number of re-transmissions, and reduce data loss. Collisions create interference or static on transmission lines or media [21]. Collisions can be either

detected or avoided through the use of media access controls. The type of media access control used in a network determines how nodes access the transmission media or line. Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is one type and token passing is another type of MAC protocol. CSMA/CD detects collisions on the transmission line and token passing attempts to avoid collisions. [22] CSMA/CD is used in bus topologies [23]. CSMA/CD works like this a node on the bus will listen for traffic if it hears nothing it will transmit its traffic, then listen again for noise, if the node hears noise it stops transmitting and starts listening for traffic on the network. It waits a random amount of time before transmitting again. CSMA/CD simplicity is its major advantage. The disadvantages of CSMA/CD are its wasteful use of the network data transfer capacity [24]. Collisions waste resources. Another disadvantage is when there is a lot of traffic there is an increase in the number of collisions, which means the network nodes are not delivering messages but spending time waiting to send and resending. Networks that use token passing, like the ring topology, do not listen for traffic on the network, they wait for a token to be available. In token passing only the node with the transmit token can send traffic onto the network. All other nodes have to receive only and wait for the token to be passed again for transmission [25]. Other MAC protocols include CSMA/CA both AppleTalk and IEEE 802.11 wireless use collision avoidance. [26]. Polling is another type of MAC. Polling uses a master slave mode and its used in synchronous data link control (SDLC) [27] V. HARDWARE In addition to the actual computers and servers on a network, connection devices are needed to actually make it a network. The hardware is what is needed to create the topology, determine the addressing, routing and MAC. There are many types of network hardware including Network Interface Cards (NICs), hubs, bridges, routers, and switches. The standard types are summarized in Table I. Other types include gateways, proxies, brouters, repeaters, amplifiers and security devices like firewalls. Network devices are updated as the technology gets faster and more capable. These changes bring better and more advanced features. Many network functions are combined into one device.

Table I Types of Hardware Device

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The NIC has either Ethernet or token ring connectors. Ethernet with RJ-45 connector is most common [28]. The NIC works at the physical layer of the OSI model. The OSI model is discussed later in this paper. A NIC is required at the physical layer to communicate between the computer and the network. Hubs operate in the physical layer of the OSI model. Hubs can be the center point in a star topology. Hubs are simple devices that save money in cable costs, but they don’t know how to manage traffic on the network. Hubs are cheap and easy to hook up to computer. Bridges connect devices at the physical layer of the OSI model. The basic function of the bridge is to repeat information between two network segments or networks. Bridges look at the MAC address in the header [29]. Some bridges work in the data link layer of the OSI model. Bridges act as dividers for network segments; this segment dividing reduces congestion on the network [30]. Bridges are devices that learn the MAC address which helps to control traffic. Bridges either directly connect two different networks or it translate between two different networks. Routers route and forward packets or traffic among networks [31]. They work at the network layer of the OSI model. Routers are intelligent and they can see more of the data frame than bridges, switches and hubs. Routers look at traffic, analyze it and remember where it goes. Routers work using interfaces and access control lists, which means the router can distinguish between different types of traffic and route it to the correct segment. Switches connect network devices in a way that reduces the number of collisions because each connection is considered a separate LAN connection [32]. Network performance is increased with switches by creating Virtual LANs or VLANS. The VLAN allow only one sender and one receiver [33]. Basic switches work at the data link, network, or transport layers of the OSI model. VI.

NETWORK STANDARDS

There two basic network standards Open Systems Interconnection (OSI) model and the Transport Control

Protocol/Internet Protocol (TCP/IP). The OSI model was developed in the 1970’s by International Standards Organization [34]. The OSI model consists of seven layers application, presentation, session, transport, network, data link and physical. In layers five through seven the data is called a datastream, in layer four segment or datagram, packet in layer three, frame in layer two, and bits in layer one. Each of the layers has a separate purpose and works with different protocols. Communications between nodes is between there layers. Application layer for a node on the network can communicate with the application layer for any of the other nodes on the network. Communication is done in the upper layers and transmission is done in the lower layers. The application layer is the layer closest to the user it provides the applications protocol’s ability to communicate with the other protocols in the other layers of the OSI model. This layer can be looked at as the layer that interfaces with the user application or operating system. The user application are not in this layer, what is in this layer is the protocols that are needs for the applications to work [35]. Hypertext Transfer Protocol (HTTP) is an example of a protocol needed to run Internet Applications. Other protocols are shown in Table II. The presentation layer ensures transmitted data is properly understood by the distant end of the communication. It formats and structures the data. File and data formats such as JPEG image, MPEG video, MIDI sound, documents, and email operate in this layer [36]. The presentation layer does the encryption and compression, which means it interfaces between the network and the applications. The session layer is responsible for the communication session. The communication session can be simplex - one way, half-duplex - two way just not at the time, or full duplex – two way and message can be sent at the same time [37]. This is the communications between two nodes. The session layer is responsible for maintaining this link. The transport layer is responsible for the integrity of the connection and controlling the session described above. The logical connection between two nodes is established by the transport layer. This layer does segmentation, sequencing, error checking, control of data flow, error correction, and multiplexing. TCP and UDP are two examples of protocols used at the transport layer. TCP is considered connectionoriented and UDP is considered connectionless. Connectionoriented means a connection is established between two nodes before data is transmitted. The connectionless UDP does not establish the connection before transmitting data. TCP has the ability to detect errors where UDP does not. The advantage to UDP is that there is less overhead when transmitting data. The network layer adds the routing and addressing information to the data [38]. Information like the source and destination IPs are added to the segment from the transport layer to make the packet [39]. Routers work in the network layer there are many protocols that can work in this layer, but they are sometimes divided into two categories distance vector and link state [40]. Distance vector knows the destination networks and the number of hops to get there. Link state uses

5 maps to determine the shortest path. Examples of each include RIP and OSPF [41]. The data link layer puts the packet in a format it can be transmitted. The format depends on the network hardware or standard being used. Network standards are discussed later in this paper. The MAC address is added in this layer. The MAC address is a 6 byte binary address written in hexadecimal. MAC addresses are unique to each piece of equipment. Address Resolution Protocol (ARP) which takes an IP address and resolves the MAC address and Reverse Address Resolution Protocol (RARP) which converts the MAC to the IP address are two protocols used in the data link layer. Switches and bridges work at this layer using the MAC addresses. The physical layer takes the frame from the data link layer and turns it into bits for transmission [42]. The physical components including the physical link, NIC, hubs, repeater, and amplifiers work in this layer. The protocols and standards at this layer are EIA/TIA-232, EAI/TIA-449, X.21, HighSpeed Serial Interface, Synchronous Optical Network, V.24 and V.25 [43]. The basic functions at this layer are: [44] -

throughput control synchronization management of line noise medium access determination on the type of signal to use

The TCP/IP model in appearance is less complex than the OSI model. TCP/IP model was developed by the Department of Defense in the 1960’s. TCP/IP has four layers application, host to host, internet, and network access. The protocol stack was developed by Department of Defense and sometimes is referred to as the DOD net or DARPA net [47]. Table III shows how TCP/IP looks next to the OSI model. Table III OSI Model Layers Next to TCP/IP Model Layers

As defined by S. Harris and J. Stewart, E. Tittel and M. Chapple in the CISSP Exam Guide and CISSP Study Guide respectively Table III shows data, protocols and basic functions of the OSI Model [45] [46]. The OSI model is framework for networking, similar to Zachman’s framework in that is assists businesses in defining requirements and answers the what, how, where, who, when, and why for creating a LAN or WAN and determining the requirements to actually communicate across the network.

Table II Summary of OSI Model

The TCP/IP is based on communications using TCP and IP. The internet uses both these protocols for the majority of internet communications. Internet communications include HTTP and HTTPS to access the Web, FTP and TFTP which allow large file transfer and SMTP for E-mail. IP is connectionless protocol, meaning there is no verification of the whether the destination exists. IP’s can be assigned to each node statically, which is permanent or dynamically where each time the machine accesses the network it receives and IP. IP’s consist of 32 bit addresses IPv4 or 128 bit addresses called IPv6. IPv6 was developed because IPv4 addresses were running out [48]. An example of the IPv4 and IPv6 are listed below: - 128.25.9.1 (IPv4) - FF05:0:0:0:0:0:0:101 (IPv6)

6 IP tables contain information about IP addresses of nodes it communicates directly with, and the physical address of those nodes [49]. TCP is connection-oriented protocol it checks for errors or loss during transmission. TCP can provide the following features: -

management functions verifying receipt data integrity checks message flow control

TCP uses a handshake to ensure connection is established and data is delivered. Fig. 8. Shows how the virtual connection in TCP is set up using the handshake. To make it simple the first node says hello are you there, the second nodes says Hello back and says I am here, then the first node says Good, let’s talk.

The MAC and LLC standard are a part of the data link layer. The most common standards used today in networks are 802.11 wireless and 802.3 Ethernet. Both are defined in the physical layer of the OSI model. IEEE 802.3, 802.4, 802.5, and 802.6 spans from the MAC section of the data link layer into the physical layer of the OSI model. The IEEE number such as 802.11 can have letters added to the end to indicate additional requirements in that standard. [51]. Ethernet which is defined by IEEE 802.3 can be divided into four categories: -

10 MBPS Ethernet 100 Mbps Ethernet Gigabit Ethernet 10 Gigabit Ethernet

These categories are further subdivided and defined by cable or laser type and maximum cable length. These are important facts to take into consideration when creating a network. Ethernet is a very popular LAN standard, the only real competition is has today is wireless. IEEE 802.11 is popular today and all computing devices have the ability to be wireless. This standard is further defined by speed and frequency some examples included in Fig. 9. [52].

Fig. 8. TCP Handshake The TCP/IP protocol stack is similar to the OSI model, it uses the same protocols across its layers. It is a matter of preference which one is used when developing a network or applications for your networks. It’s important to know the different layers whether from the OSI or TCP/IP model so that the correct hardware and software is purchased. VII.

NETWORK STANDARDS

Institute of Electrical and Electronic Engineers (IEEE) created the IEEE 802 standards, which describe the standards for networking hardware and software. Those standards include: [50] -

802.1 – Media access control (MAC) 802.2 – Logical link control (LLC) 802.3 – CMA/CD and Ethernet 802.4 – Token Bus 802.5 – Token Ring 802.6 – Metropolitan area networks (MANs) 802.7 – Broadband LANs 802.8 – Fiber optic LANs 802.9 – Integrated data and voice networks 802.10 – LAN/MAN security 802.11 – Wireless LANs 802.12 – Demand priority 802.14 – Cable TV broadband networking 802.15 – Wireless personal area networks 802.16 – Broadband wireless access

Standards are influenced by business, technology and government. Many organizations contribute to the defining or standards including ANSI, ISO, and the U.S. FCC [53].

VIII. RELATIONSHIP TO ZACHMAN’S FRAMEWORK It takes some knowledge to determine whether a business needs a LAN, WAN, or maybe just a connection to the Internet. Once an organization decides to build a LAN decisions need to made first on how much to spend, where will the new equipment be located, who will take care of it, and many more questions must be answered just to get started. These questions should be answered at all levels of an organization. Using Zachman’s framework the scope, business model, logical, physical, detailed diagrams and the functioning network in the organization. The planners would define from a high level the need for a network to communicate within the organization or to other organizations. The owner would define the business model for the chosen network. The designer would draw the logical models of the network. The builder would create the physical model of the network within the organization. A contractor or systems designer would meet the requirements defined by the planner, owner, designer and builder and install the network and provide the as-built drawings. What’s left should be the functioning enterprise. [54]

7 IX.

CONCLUSION

Computer networks are fairly complex, most business have implemented LANs without really understanding the technology behind the network. Successfully designing and implementing networks requires some knowledge of networking and the protocols used to implement the applications that a business needs to be successful. Business should weigh the cost of implementing a LAN or upgrading a LAN against their return on the investment. It takes some knowledge to determine whether a business needs a LAN, WAN, or maybe just a connection to the Internet. Once an organization decides to build a LAN decisions need to made first on how much to spend, but where will the new equipment be located, who will take care of it, and many more questions must be answered just to get started. ACKNOWLEDGMENT M.A. Hermosura thanks the authors of the books titled “Systems Architecture”, “CISSP Certified Information Systems Security Professional Study Guide”, “Principles of Computer Security”, and “All-In-One CISSP Exam Guide” for the information found in them and referenced throughout this paper. Also thanks to Professor Peter for this unique experience using the IEEE format. REFERENCES [1] [2] [3] [4] [5]

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8 She is employed as a civilian with the United States Navy in Norfolk, VA where she holds the position as Information Assurance Manager. Prior to this employment she was on active duty with the United States Navy where she achieved the rank of master chief petty officer. She served twenty three years with the navy. Ms. Hermosura is certified Comptia Security + and Comptia Network +.