畢業(yè)設(shè)計(jì)英文翻譯.doc

英文原版文獻(xiàn)：Internetworking Basics Networks and networking have grown exponentially over the last 15 yearsunderstandably so. Theyve had to evolve at light speed just to keep up with huge increases in basic missioncritical user needs such as sharing data and printers, as well as more advanced demands such as video conferencing. Unless everyone who needs to share network resources is located in the same office area (an increasingly uncommon situation), the challenge is to connect the sometimes many relevant networks together so all users can share the networks wealth. Its also likely that at some point, youll have to break up one large network into a number of smaller ones because user response has dwindled to a trickle as the network grew and grew and LAN traffic congestion reached overwhelming proportions. Breaking up a larger network into a number of smaller ones is called network segmentation, and its accomplished using routers, switches, and bridges. Possible causes of LAN traffic congestion are (1)Too many hosts in a broadcast domain (2)Broadcast storms (3)Multicasting (4)Low bandwidth (5)Adding hubs for connectivity to the network (6)A large amount of ARP or IPX traffic (IPX is a Novell routing protocol that is like IP, but really, really chatty) Routers are used to connect networks together and route packets of data from one network to another. Cisco became the de facto standard of routers because of their high-quality router products, great selection, and fantastic service. Routers, by default, break up a broadcast domain, which is the set of all devices on a network segment that hear all broadcasts sent on that segment. Breaking up a broadcast domain is important because when a host or server sends a network broadcast, every device on the network must read and process that broadcastunless youve got a router. When the routers interface receives this broadcast, it can respond by basically saying“Thanks, but no thanks,” and discard the broadcast without forwarding it on to other networks. Even though routers are known for breaking up broadcast domains by default, its important to remember that they break up collision domains as well. Two advantages of using routers in your network are (1)They dont forward broadcasts by default. (2)They can filter the network based on layer 3 (Network layer) information (i.e., IP address). Four router functions in your network can be listed as (1)Packet switching (2)Packet filtering (3)Internetwork communication (4)Path selection Remember that routers are really switches, but theyre actually what we call layer 3 switches (well talk about layers later in this chapter). Unlike layer 2 switches that forward or filter frames, routers (layer 3 switches) use logical addressing and provide what is called packet switching. Routers can also provide packet filtering by using access-lists (discussed in Chapter 10), and when routers connect two or more networks together and use logical addressing (IP), this is called an internetwork. Lastly, routers use a routing table (map of the internetwork) to make path selections and to forward packets to remote networks. Conversely, switches arent used to create internetworks, theyre employed to add functionality to an internetwork LAN. The main purpose of a switch is to make a LAN work betterto optimize its performanceproviding more bandwidth for the LANs users. And switches dont forward packets to other networks as routers do. Instead, they only “switch” frames from one port to another within the switched network. Okay, you may be thinking, “Wait a minute, what are frames and packets?” Ill tell you all about them later in this chapter, I promise! By default, switches break up collision domains. This is an Ethernet term used to describe a network scenario wherein one particular device sends a packet on a network segment, forcing every other device on that same segment to pay attention to it. At the same time, a different device tries to transmit, leading to a collision, after which both devices must retransmit, one at a time. Not very efficient! This situation is typically found in a hub environment where each host segment connects to a hub that represents only one collision domain and only one broadcast domain. By contrast, each and every port on a switch represents its own collision domain.Note：Switches create separate collision domains, but a single broadcast domain.Routers provide a separate broadcast domain for each interface. The term bridging was introduced before routers and hubs were implemented, so its pretty common to hear people referring to bridges as “switches.” Thats because bridges and switches basically do the same thingbreak up collision domains on a LAN. So what this means is that a switch is basically just a multiple-port bridge with more brainpower, right? Well, pretty much, but there are differences. Switches do provide this function, but they do so with greatly enhanced management ability and features. Plus, most of the time, bridges only had two or four ports. Yes, you could get your hands on a bridge with up to 16 ports, but thats nothing compared to the hundreds available on some switches!Note：You would use a bridge in a network to reduce collisions within broadcast domains and to increase the number of collision domains in your network. Doing this provides more bandwidth for users. And keep in mind that using hubs in your network can contribute to congestion on your Ethernet network. As always, plan your network design carefully! Figure 1.1 shows how a network would look with all these internetwork devices in place. Remember that the router will not only break up broadcast domains for every LAN interface, but break up collision domains as well.When you looked at Figure 1.1, did you notice that the router is found at center stage, and that it connects each physical network together? We have to use this layout because of the older technologies involvedbridges and hubs. Once we have only switches in our network, things change a lot! The LAN switches would then be placed at the center of the network world and the routers would be found connecting only logical networks together. If Ive implemented this kind of setup, Ive created virtual LANs (VLANs). Again, dont stressIll go over VLANs thoroughly with you in Chapter 8, “Virtual LANs (VLANs).”O(jiān)n the top network in Figure 1.1, youll notice that a bridge was used to connect the hubs to a router. The bridge breaks up collision domains, but all the hosts connected to both hubs are still crammed into the same broadcast domain. Also, the bridge only created two collision domains, so each device connected to a hub is in the same collision domain as every other device connected to that same hub. This is pretty lame, but its still better than having one collision domain for all hosts.Notice something else: the three hubs at the bottom that are connected also connect to the router, creating one humongous collision domain and one humongous broadcast domain. This makes the bridged network look much better indeed!Note：Although bridges are used to segment networks, they will not isolate broadcast or multicast packets.The best network connected to the router is the LAN switch network on the left. Why? Because each port on that switch breaks up collision domains. But its not all goodall devices are still in the same broadcast domain. Do you remember why this can be a really bad thing? Because all devices must listen to all broadcasts transmitted, thats why. And if your broadcast domains are too large, the users have less bandwidth and are required to process more broadcasts, and network response time will slow to a level that could cause office riots.Obviously, the best network is one thats correctly configured to meet the business requirements of the company it serves. LAN switches with routers, correctly placed in the network, are the best network design. This book will help you understand the basics of routers and switches so you can make tight, informed decisions on a case-by-case basis.Lets go back to Figure 1.1 again. Looking at the figure, how many collision domains and broadcast domains are in this network? Hopefully, you answered nine collision domains and three broadcast domains! The broadcast domains are definitely the easiest to see because only routers break up broadcast domains by default. And since there are three connections, that gives you three broadcast domains. But do you see the nine collision domains? Just in case thats a No, Ill explain. The all-hub network is one collision domain, the bridge network equals three collision domains. Add in the switch network of five collision domainsone for each switch portand youve got a total of nine.Internetworking Models When networks first came into being, computers could typically communicate only with computers from the same manufacturer. For example, companies ran either a complete DECnet solution or an IBM solutionnot both together. In the late 1970s, the Open Systems Interconnection (OSI) reference model was created by the International Organization for Standardization (ISO) to break this barrier.The OSI model was meant to help vendors create interoperable network devices and software in the form of protocols so that different vendor networks could work with each other. Like world peace, itll probably never happen completely, but its still a great goal.The OSI model is the primary architectural model for networks. It describes how data and network information are communicated from an application on one computer, through the network media, to an application on another computer. The OSI reference model breaks this approach into layers.The Layered Approach A reference model is a conceptual blueprint of how communications should take place. It addresses all the processes required for effective communication and divides these processes into logical groupings called layers. When a communication system is designed in this manner, its known as layered architecture.Think of it like this: You and some friends want to start a company. One of the first things youll do is sit down and think through what tasks must be done, who will do them, what order they will be done in, and how they relate to each other. Ultimately, you might group these tasks into departments. Lets say you decide to have an order-taking department, an inventory department, and a shipping department. Each of your departments has its own unique tasks, keeping its staff members busy and requiring them to focus on only their own duties.In this scenario, Im using departments as a metaphor for the layers in a communication system. For things to run smoothly, the staff of each department will have to trust and rely heavily upon the others to do their jobs and competently handle their unique responsibilities. In your planning sessions, you would probably take notes, recording the entire process to facilitate later discussions about standards of operation that will serve as your business blueprint, or reference model.Once your business is launched, your department heads, armed with the part of the blueprint relating to their department, will need to develop practical methods to implement their assigned tasks. These practical methods, or protocols, will need to be compiled into a standard operating procedures manual and followed closely. Each of the various procedures in your manual will have been included for different reasons and have varying degrees of importance and implementation. If you form a partnership or acquire another company, it will be imperative that its business protocols its business blueprintmatch yours (or at least be compatible with it).Similarly, software developers can use a reference model to understand computer communication processes and see what types of functions need to be accomplished on any one layer. If they are developing a protocol for a certain layer, all they need to concern themselves with is the specific layers functions, not those of any other layer. Another layer and protocol will handle the other functions. The technical term for this idea is binding. The communication processes that are related to each other are bound, or grouped together, at a particular layer.Advantages of Reference ModelsThe OSI model is hierarchical, and the same benefits and advantages can apply to any layered model. The primary purpose of all such models, especially the OSI model, is to allow different vendors networks to interoperate.Advantages of using the OSI layered model include, but are not limited to, the following:(1)It divides the network communication process into smaller and simpler components, thus aiding component development, design, and troubleshooting.(2)It allows multiple-vendor development through standardization of network components.(3)It encourages industry standardization by defining what functions occur at each layer of the model.(4)It allows various types of network hardware and software to communicate.(5)It prevents changes in one layer from affecting other layers, so it does not hamper development.The OSI Reference Model One of the greatest functions of the OSI specifications is to assist in data transfer between disparate hostsmeaning, for example, that they enable us to transfer data between a Unix host and a PC or a Mac. The OSI isnt a physical model, though. Rather, its a set of guidelines that application developers can use to create and implement applications that run on a network. It also provides a framework for creating and implementing networking standards, devices, and internetworking schemes.The OSI has seven different layers, divided into two groups. The top three layers define how the applications within the end stations will communicate with each other and with users. The bottom four layers define how data is transmitted end-to-end. Figure 1.2 shows the three upper layers and their functions, and Figure 1.3 shows the four lower layers and their functions.When you study Figure 1.2, understand that the user interfaces with the computer at the Application layer, and also that the upper layers are responsible for applications communicating between hosts. Remember that none of the upper layers knows anything about networking or network addresses. Thats the responsibility of the four bottom layers.In Figure 1.3, you can see that its the four bottom layers that define how data is transferred through a physical wire or through switches and routers. These bottom layers also determine how to rebuild a data stream from a transmitting host to a destination hosts application.Network devices that operate at all seven layers of the OSI model include(1)Network management stations (NMS)(2)Web and application servers(3)Gateways (not default gateways)(4)Network hostsBasically, the ISO is pretty much the Emily Post of the network protocol world. Just like Ms.Post, who wrote the book setting the standardsor protocolsfor human social interaction, the ISO developed the OSI reference model as the precedent and guide for an open network protocol set. Defining the etiquette of communication models, it remains today the most popular means of comparison for protocol suites.The OSI reference model has seven layers:(1)Application layer (layer 7)(2)Presentation layer (layer 6)(3)Session layer (layer 5)(4)Transport layer (layer 4)(5)Network layer (layer 3)(6)Data Link layer (layer 2)(7)Physical layer (layer 1) The Application LayerThe Application layer of the OSI model marks the spot where users actually communicate to the computer. This layer only comes into play when its apparent that access to the network is going to be needed soon. Take the case of Internet Explorer (IE). You could uninstall every trace of networking components from a system, such as TCP/IP, NIC card, etc., and you could still use IE to view a local HTML documentno problem. But things would definitely get messy if you tried to do something like view an HTML document that must be retrieved using HTTP, or nab a file with FTP. Thats because IE will respond to requests such as those by attempting to access the Application layer. And whats happening is that the Application layer is acting as an interface between the actual application programwhich isnt at all a part of the layered structureand the next layer down, by providing ways for the application to send information down through the protocol stack. In other words, IE doesnt truly reside within the Application layerit interfaces with Application-layer protocols when it needs to deal with remote resources.The Application layer is also responsible for identifying and establishing the availability of the intended communication partner, and determining whether sufficient resources for the intended communication exist.The Presentation LayerThe Presentation layer gets its name from its purpose: It presents data to the Application layer and is responsible for data translation and code formatting.This layer is essentially a translator and provides coding and conversion functions. A successful data-transfer technique is to adapt the data into a standard format before transmission. Computers are configured to receive this generically formatted data and then convert the data back into its native format for actual reading (for example, EBCDIC to ASCII). By providing translation services, the Presentation layer ensures that data transferred from the Application layer of one system can be read by the Application layer of another one.The Session LayerThe Session layer is responsible for setting up, managing, and then tearing down sessions bet