Networking.docx

Network Operating Systems PCs use a disk operating system that controls the file system and how the applications communicate with the hard disk. Networks use a network operating system (NOS) to control the communication with resources and the flow of data across the network. The NOS runs on the server. Some of the more popular NOSs are Linux, Microsoft’s Windows Server series (Server 2019, Server 2016, and so on), and macOS Server. Several other companies offer network operating systems as well. Network Resource Access We have discussed two major components of a typical network—servers and workstations—and we’ve also talked briefly about network resources. Let’s dive in a bit deeper on how those resources are accessed on a network. There are generally two resource access models: peer-to-peer and client-server. It is important to choose the appropriate model. How do you decide which type of resource model is needed? You must first think about the following questions: What is the size of the organization? How much security does the company require? What software or hardware does the resource require? How much administration does it need? How much will it cost? Will this resource meet the needs of the organization today and in the future? Will additional training be needed? 322 Networks cannot just be put together at the drop of a hat. A lot of planning is required before implementation of a network to ensure that whatever design is chosen will be effective and efficient, and not just for today but for the future as well. The forethought of the designer will lead to the best network with the least amount of administrative overhead. In each network, it is important that a plan be developed to answer the previous questions. The answers will help the designer choose the type of resource model to use. Peer-to-Peer Networks In a peer-to-peer network, the computers act as both service providers and service requestors. An example of a peer-to-peer resource model is shown in Figure 6.5. Figure 6.5 The peer-to-peer resource model The peer-to-peer model is great for small, simple, inexpensive networks. This model can be set up almost instantly, with little extra hardware required. Many versions of Windows (Windows 10, Windows 8, and others) as well as Linux and macOS are popular operating system environments that support the peer-to-peer resource model. Peer-to-peer networks are also referred to as workgroups. Generally speaking, there is no centralized administration or control in the peer-to-peer resource model. Every station has unique control over the resources that the computer owns, and each station must be administered separately. However, this very lack of centralized control can make administering the network difficult; for the same reason, the network isn’t very secure. Each user needs to manage separate passwords for each computer on which they wish to access resources, as well as set up and manage the shared resources on their own computer. Moreover, because each computer is acting as both a workstation and server, it may not be easy to locate resources. The person who is in charge of a file may have moved it without anyone’s knowledge. Also, the users who work under this arrangement need more training because they are not only users but also administrators. Will this type of network meet the needs of the organization today and in the future? Peer-to-peer resource models are generally considered the right choice for small companies that don’t expect future growth. Small companies that expect growth, on the other hand, should not choose this type of model. A rule of thumb is that if you have no more than 10 computers and centralized security is not a key priority, a workgroup may be a good choice for you. 323 Client-Server Resource Model The client-server model (also known as server-based model) is better than the peer-to-peer model for large networks (say, more than 10 computers) that need a more secure environment and centralized control. Server-based networks use one or more dedicated, centralized servers. All administrative functions and resource sharing are performed from this point. This makes it easier to share resources, perform backups, and support an almost unlimited number of users. This model also offers better security than the peer-to-peer model. However, the server needs more hardware than a typical workstation/server computer in a peer-to-peer resource model. In addition, it requires specialized software (the NOS) to manage the server’s role in the environment. With the addition of a server and the NOS, server-based networks can easily cost more than peer-to-peer resource models. However, for large networks, it’s the only choice. An example of a client-server resource model is shown in Figure 6.6. Figure 6.6 The client-server resource model Server-based networks are often known as domains. The key characteristic of a server-based network is that security is centrally administered. When you log into the network, the login request is passed to the server responsible for security, sometimes known as a domain controller. (Microsoft uses the term domain controller, whereas other vendors of server products do not.) This is different from the peer-to-peer model, where each individual workstation validates users. In a peer-to-peer model, if the user jsmith wants to be able to log into different workstations, she needs to have a user account set up on each machine. This can quickly become an administrative nightmare! In a domain, all user accounts are stored on the server. User jsmith needs only one account and can log into any of the workstations in the domain. Client-server resource models are the desired models for companies that are continually growing, need to support a large environment, or need centralized security. Server-based324networks offer the flexibility to add more resources and clients almost indefinitely into the future. Hardware costs may be higher, but with the centralized administration, managing resources becomes less time consuming. Also, only a few administrators need to be trained, and users are responsible for only their own work environment. If you are looking for an inexpensive, simple network with little setup required, and there is no need for the company to grow in the future, then the peer-to-peer network is the way to go. If you are looking for a network to support many users (more than 10 computers), strong security, and centralized administration, consider the server-based network your only choice. Whatever you decide, always take the time to plan your network before installing it. A network is not something you can just throw together. You don’t want to find out a few months down the road that the type of network you chose does not meet the needs of the company—this could be a time-consuming and costly mistake. Network Topologies A topology is a way of laying out the network. When you plan and install a network, you need to choose the right topology for your situation. Each type differs from the others by its cost, ease of installation, fault tolerance (how the topology handles problems such as cable breaks), and ease of reconfiguration (such as adding a new workstation to the existing network). There are five primary topologies: Bus Star Ring Mesh Hybrid Each topology has advantages and disadvantages. Table 6.1 summarizes the advantages and disadvantages of each topology, and then we will go into more detail about each one. Table 6.1 Topologies—advantages and disadvantages Topology Advantages Disadvantages Bus Cheap. Easy to install Difficult to reconfigure. A break in the bus disables the entire network. Star Cheap. Very easy to install and reconfigure. More resilient to a single cable failure More expensive than bus Ring Efficient. Easy to install Reconfiguration is difficult. Very expensive Mesh Best fault tolerance Reconfiguration is extremely difficult, extremely expensive, and very complex. Hybrid Gives a combination of the best features of each topology used Complex (less so than mesh, however) 325 Bus Topology A bus topology is the simplest. It consists of a single cable that runs to every workstation, as shown in Figure 6.7. This topology uses the least amount of cabling. Each computer shares the same data and address path. With a bus topology, messages pass through the trunk, and each workstation checks to see if a message is addressed to it. If the address of the message matches the workstation’s address, the network adapter retrieves it. If not, the message is ignored. Figure 6.7 The bus topology Cable systems that use the bus topology are easy to install. You run a cable from the first computer to the last computer. All of the remaining computers attach to the cable somewhere in between. Because of the simplicity of installation, and because of the low cost of the cable, bus topology cabling systems are the cheapest to install. Although the bus topology uses the least amount of cabling, it is difficult to add a workstation. If you want to add another workstation, you have to reroute the cable completely and possibly run two additional lengths of it. Also, if any one of the cables breaks, the entire network is disrupted. Therefore, such a system is expensive to maintain and can be difficult to troubleshoot. You will rarely run across physical bus networks in use today. 326 Star Topology A star topology branches each network device off a central device called a hub or a switch, making it easy to add a new workstation. If a workstation goes down, it does not affect the entire network; if the central device goes down, the entire network goes with it. Because of this, the hub (or switch) is called a single point of failure. Figure 6.8 shows a simple star network. Figure 6.8 The star topology Star topologies are very easy to install. A cable is run from each workstation to the switch. The switch is placed in a central location in the office (for example, a utility closet). Star topologies are more expensive to install than bus networks because several more cables need to be installed, plus the switches. But the ease of reconfiguration and fault tolerance (one cable failing does not bring down the entire network) far outweigh the drawbacks. This is the most commonly installed network topology in use today. Although the switch is the central portion of a star topology, some older networks use a device known as a hub instead of a switch. Switches are more advanced than hubs, and they provide better performance than hubs for only a small price increase. Colloquially, though, many administrators use the terms hub and switch interchangeably. Ring Topology In a ring topology, each computer connects to two other computers, joining them in a circle and creating a unidirectional path where messages move from workstation to workstation. Each entity participating in the ring reads a message and then regenerates it and327hands it to its neighbor on a different network cable. See Figure 6.9 for an example of a ring topology. Figure 6.9 The ring topology The ring makes it difficult to add new computers. Unlike a star topology network, a ring topology network will go down if one entity is removed from the ring. Physical ring topology systems rarely exist anymore, mainly because the hardware involved was fairly expensive and the fault tolerance was very low. You might have heard of an older network architecture called Token Ring. Contrary to its name, it does not use a physical ring. It actually uses a physical star topology, but the traffic flows in a logical ring from one computer to the next. Mesh Topology The mesh topology is the most complex in terms of physical design. In this topology, each device is connected to every other device (see Figure 6.10). This topology is rarely found in wired LANs, mainly because of the complexity of the cabling. If there are x computers, there will be (x × ( x – 1)) ÷ 2 cables in the network. For example, if you have five computers in a mesh network, it will use (5 × (5 – 1)) ÷ 2 = 10 cables. This complexity is compounded when you add another workstation. For example, your 5-computer, 10-cable network will jump to 15 cables if you add just one more computer. Imagine how the person doing the cabling would feel if you told them they had to cable 50 computers in a mesh network—they’d have to come up with (50 × (50 – 1)) ÷ 2 = 1,225 cables! (Not to mention figuring out how to connect them all.) Figure 6.10 The mesh topology 328 Because of its design, the physical mesh topology is expensive to install and maintain. Cables must be run from each device to every other device. The advantage you gain is high fault tolerance. With a mesh topology, there will always be a way to get the data from source to destination. The data may not be able to take the direct route, but it can take an alternate, indirect route. For this reason, the mesh topology is often used to connect multiple sites across WAN links. It uses devices called routers to search multiple routes through the mesh and determine the best path. However, the mesh topology does become inefficient with five or more entities because of the number of connections that need to be maintained. Hybrid Topology The hybrid topology is simply a mix of the other topologies. It would be impossible to illustrate it because there are many combinations. In fact, most networks today are not only hybrid but heterogeneous. (They include a mix of components of different types and brands.) The hybrid network may be more expensive than some types of network topologies, but it takes the best features of all the other topologies and exploits them. Table 6.1, earlier in this chapter, summarizes the advantages and disadvantages of each type of network topology. Rules of Communication Regardless of the type of network you choose to implement, the computers on that network need to know how to talk to each other. To facilitate communication across a network, computers use a common language called a protocol. We’ll cover protocols more in Chapter 7, “Introduction to TCP/IP,” but essentially they are languages much like English is a language. Within each language, there are rules that need to be followed so that all computers understand the right communication behavior. 329 To use a human example, within English there are grammar rules. If you put a bunch of English words together in a way that doesn’t make sense, no one will understand you. If you just decide to omit verbs from your language, you’re going to be challenged to get your point across. And if everyone talks at the same time, the conversation can be hard to follow. Computers need standards to follow to keep their communication clear. Different standards are used to describe the rules that computers need to follow to communicate with each other. The most important communication framework, and the backbone of all networking, is the OSI model. The OSI model is not specifically listed in the CompTIA A+ exam objectives. However, it’s a critical piece of networking knowledge and a framework with which all technicians should be familiar. OSI Model The International Organization for Standardization (ISO) published the Open Systems Interconnection (OSI) model in 1984 to provide a common way of describing network protocols. The ISO put together a seven-layer model providing a relationship between the stages of communication, with each layer adding to the layer above or below it. This OSI model is a theoretical model governing computer communication. Even though at one point an “OSI protocol” was developed, it never gained wide acceptance. You will never find a network that is running the “OSI protocol.” Here’s how the theory behind the OSI model works: As a transmission takes place, the higher layers pass data through the lower layers. As the data passes through a layer, that layer tacks its information (also called a header) onto the beginning of the information being transmitted until it reaches the bottom layer. A layer may also add a trailer to the end of the data. The bottom layer sends the information out on the wire (or in the air, in the case of wireless). At the receiving end, the bottom layer receives and reads the information in the header, removes the header and any associated trailer related to its layer, and then passes the remainder to the next highest layer. This procedure continues until the topmost layer receives the data that the sending computer sent. The OSI model layers are listed here from top to bottom, with descriptions of what each of the layers is responsible for: 7—Application layer The Application layer allows access to network services. This is the layer at which file services, print services, and other applications operate. 330 6—Presentation layer This layer determines the “look,” or format, of the data. The Presentation layer performs protocol conversion and manages data compression, data translation, and encryption. The character set information also is determined at this level. (The character set determines which numbers represent which alphanumeric characters.) 5—Session layer This layer allows applications on different computers to establish, maintain, and end a session. A session is one virtual conversation. For example, all of the procedures needed to transfer a single file make up one session. Once the session is over, a new process begins. This layer enables network procedures, such as identifying passwords, logins, and network monitoring. 4—Transport layer The Transport layer controls the data flow and troubleshoots any problems with transmitting or receiving datagrams. It also takes large messages and segments them into smaller ones and takes smaller segments and combines them into a single, larger message, depending on which way the traffic is flowing. Finally, the TCP protocol (one of the two options at this layer) has the important job of verifying that the destination host has received all packets, providing error checking and reliable end-to-end communications. 3—Network layer The Network layer is responsible for logical addressing of messages. At this layer, the data is organized into chunks called packets. The Network layer is something like the traffic cop. It is able to judge the best network path for the data based on network conditions, priority, and other variables. This layer manages traffic through packet switching, routing, and controlling congestion of data. 2—Data Link layer This layer arranges data into chunks called frames. Included in these chunks is control information indicating the beginning and end of the datastream. The Data Link layer is very important because it makes transmission easier and more manageable, and it allows for error checking within the data frames. The Data Link layer also describes the unique physical address (also known as the MAC address) for each NIC. The Data Link layer is actually subdivided into two sections: Media Access Control (MAC) and Logical Link Control (LLC). 1—Physical layer The Physical layer describes how the data gets transmitted over a communication medium. This layer defines how long each piece of data is and the translation of each into the electrical pulses or light impulses that are sent over the wires, or the radio waves that are sent through the air. It decides whether data travels unidirectionally or bidirectionally across the hardware. It also relates electrical, optical, mechanical, and functional interfaces to the cable. Figure 6.11 shows the complete OSI model. Note the relationship of each layer to the others and the function of each layer. Figure 6.11 The OSI model 331 A helpful mnemonic device to remember the OSI layers in order is “All People Seem To Need Data Processing.” IEEE 802 Standards Continuing with our theme of communication, it’s time to introduce one final group of standards. You’ve already learned that a protocol is like a language; think of the IEEE 802 standards as syntax, or the rules that govern who communicates, when they do it, and how they do it. 332 The Institute of Electrical and Electronics Engineers (IEEE) formed a subcommittee to create standards for network types. These standards specify certain types of networks, although not every network protocol is covered by the IEEE 802 committee specifications. This model contains several standards. The ones commonly in use today are 802.3 CSMA/CD (Ethernet) LAN and 802.11 Wireless networks. The IEEE 802 standards were designed primarily for enhancements to the bottom three layers of the OSI model. The IEEE 802 standard breaks the Data Link layer into two sublayers: a Logical Link Control (LLC) sublayer and a Media Access Control (MAC) sublayer. The Logical Link Control sublayer manages data link communications. The Media Access Control sublayer watches out for data collisions and manages physical addresses, also referred to as MAC addresses. You’ve most likely heard of 802.11ac or 802.11n wireless networking. The rules for communicating with all versions of 802.11 are defined by the IEEE standard. Another very well-known standard is 802.3 CSMA/CD. You might know it by its more popular name, Ethernet. The original 802.3 CSMA/CD standard defines a bus topology network that uses a 50-ohm coaxial baseband cable and carries transmissions at 10 Mbps. This standard groups data bits into frames and uses the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) cable access method to put data on the cable. Currently, the 802.3 standard has been amended to include speeds up to 10 Gbps. Breaking the CSMA/CD acronym apart may help illustrate how it works. CS First, there is the Carrier Sense (CS) part, which means that computers on the network are listening to the wire at all times. MA Multiple Access (MA) means that multiple computers have access to the line at the same time. This is analogous to having five people on a conference call. Everyone is listening, and everyone in theory can try to talk at the same time. Of course, when more than one person talks at once, there is a communication error. In CSMA/CD, when two machines transmit at the same time, a data collision takes place and the intended recipients receive none of the data. CD This is where the Collision Detection (CD) portion of the acronym comes in; the collision is detected and each sender knows they need to send again. Each sender then waits for a short, random period of time and tries to transmit again. This process repeats until transmission takes place successfully. The CSMA/CD technology is considered a contention-based access method. The only major downside to 802.3 is that with large networks (more than 100 computers on the same segment), the number of collisions increases to the point where more collisions than transmissions are taking place. Other examples of contention methods exist, such as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). Whereas CSMA/CD tries to fix collisions after they happen, CSMA/CA tries to avoid them in the first place by actively listening and only transmitting when the channel is clear. Wireless Ethernet uses CSMA/CA.

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