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Explain how network topologies influence planning decisions ... to a machine in New York, you are charged for a long distance phone call. If.
Typology: Summaries
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After reading this chapter you will be able to:
(^) Understand the benefits of networking (^) Understand what telephony networking is (^) Identify the layers of the OSI reference model and know what part each one plays in networking (^) Discuss how the Internet communicates (^) Understand the basics of ATM networks (^) Apply knowledge of different networking components (^) Explain how network topologies influence planning decisions
60 CHAPTER 2 Communications Network
In today’s complex world, most companies have networks. In fact, most companies have a Web presence as well. A network can range from simply two computers that are linked together to the complexity of computers that can access data across continents. Networks are used to improve communi- cation between departments, foster customer relationships, and share data throughout the world. As a network administrator, it is your job to manage the network. To do this, you must understand the fundamental networking principles. Having this knowledge will help develop your planning and troubleshooting skills. This chapter provides you with those fundamental principles on which you will build knowledge and experience. It also focuses on the concept of a network, what makes it possible for devices to communicate, and what types of medium are used for communication.
A network is a group of computers that can communicate with each other to share information. This can range, in its simplest form, from two com- puters in a home that are connected by one cable to the most complex net- work that includes many computers, cables, and devices spanning across continents. Before we can explore larger complex networks, we must look at what allows computers to talk to each other. When computers can communicate with each other, they can share resources. These resources can be data (such as documents or spread- sheets), applications (such as Microsoft Word or Microsoft Excel), or hardware (such as modems or printers). What if you want to share a file with a friend who lives down the street? Each of you has your own com- puter, and neither computer is hooked up to any other computer or the Internet. Each of the computers is considered a stand-alone computer. See Figure 2.1. The two of you can share files by transferring them onto a floppy disk and loading them onto each computer. This is also known as a sneakernet. It stems from the fact that you have to physically walk the files on disk back and forth to transfer them. This was the primary method of file transfer before networks became popular. An example of a sneakernet is shown in Figure 2.2.
62 CHAPTER 2 Communications Network
Others
Others
Printer
Server
Figure 2. A server network
A server is a computer that allows its resources to be used by other com- puters on the network. A client is a computer that uses the resources of the server. Figure 2.4 depicts a network with a server and clients. As we discussed earlier in this section, resources come in many different forms. Resources are the files, applications, and hardware shared by the server for the client to access. When a server provides a resource for a client to access, this is referred to as a shared resource. Shared resources are accessed across the network. An important concept to remember is that of shared resources. Sharing allows for access across the network. If I share with you, you can use my resources by traversing the network. Shared resources will come into play fur- ther in Chapter 12 when the management of access and accounts is discussed. Technology is advancing rapidly, and most networks tie into some type of telephone system, whether it is a single analog line used to connect a home computer to the Internet or a high-speed digital connection used in most businesses. In Chapter 1 we explained analog and digital signaling. Now
2.2 Telephony Networks 63
that we’ve introduced what a network is, it’s time to look at how analog and digital signaling are used in the communication of a network.
The telecommunications (Telecom), or Private Branch Exchange (PBX), system is a vital part of an organization’s infrastructure. A PBX is a tele- phone system within an organization that switches calls between users on local lines yet allows all users to share a certain number of external phone lines. The main purpose of a PBX is to save the cost of requiring a line for each user to the telephone company’s central office. The PBX is owned and operated by the organization rather than the telephone company. Origi- nally, private branch exchanges used analog technology, but most now use digital technology. Digital signals are converted to analog for outside calls on the local loop using plain old traditional telephone service. A PBX includes telephone trunk lines, a computer with memory that manages the calls, a network of lines within the PBX, and a console or switchboard. In essence, users of the PBX share a certain number of outside lines for mak- ing telephone calls. Most medium-sized or large companies use a PBX because it’s much less expensive than connecting an external telephone line to every telephone in the organization. In addition, it’s easier to call some- one within a PBX because you simply dial a 3- or 4-digit extension. An example of this is a company with one published phone number yet the employees can answer up to five lines at a time. When a call comes in, the receptionist answers the phone and then transfers it to the requested party by dialing their 3-digit extension.
Not long ago the Internet ran on phone systems, but now many phone sys- tems are running on the Internet. For years, companies carried data traffic on voice networks. During the mid-1990s, advances in technology made it pos- sible to use existing network resources to reduce or eliminate telephony costs. Many companies have moved to Voice over IP (VoIP) to integrate computer telephony, videoconferencing, and document sharing. See Figure 2.5.
In analog connectivity, a plain old telephone system (POTS) is used. This is also referred to as a public switched telephone network (PSTN). A modem converts the signals from digital to analog to be used over the phone lines and then back to digital for the computer to understand. For example, you and a friend install modems in your computers so that you
2.2 Telephony Networks 65
circuit switching and discussed in further detail in Chapter 5, we’ll just go over some basics.
Here’s how circuit switching works: When a call is made, a dedicated con- nection is opened and maintained between the parties for the duration of the call. No other calls can use those resources until the call is terminated. In contrast, packet switching does not require a dedicated circuit. Data packets are routed over any circuit that is available at any given point, and they don’t travel over a fixed path. In other words, numerous users share the same circuit simultaneously because the circuits are available to all users.
A good example of packet switching would be similar to sending an e-mail. You compose an e-mail message and send it to your friend. The message you send is broken down into small pieces called packets or frames. This is done because large packets take up a lot of bandwidth, preventing other computers from communicating. After the packets hit the network, they are forwarded from computer to computer until they reach their final des- tination. All the packets can travel the same route or each can take a differ- ent route depending on how busy the network is. When the packets reach their final destination, they are assembled back into the original message and your friend reads it.
Integrating voice and data communications can be very cost effective. Let’s look at an example of this. If you install a modem and use POTS to connect to a machine in New York, you are charged for a long distance phone call. If you use the Internet to make the connection, you connect via a local num- ber and then use that connection to make contact with the machine in New York, saving the cost of a long-distance phone number.
Because IP telephony networks make better use of available bandwidth, a VoIP network carries voice traffic for less cost than a switched circuit tele- phone network does. In a public-switched telephone network, a dedicated end-to-end circuit is allocated for each call. In a VoIP network, data is much more compressed and carried in packets. Using the same bandwidth, a VoIP network can carry many times the number of voice calls as a switched circuit network and with better voice quality. Now that we have learned how analog and digital communications affect networking, we will go one step further and look at what allows devices to communicate over the network.
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Figure 2.7 1. Physical layer The OSI reference model
As networking became the norm for businesses, the need arose for busi- nesses to be able to connect with each other even though their equipment and systems were different. In 1978, the International Organization for Standardization (ISO) developed an architecture that would allow the devices of different manufacturers to work together to communicate with different operating systems. In 1984, the ISO architecture became an inter- national standard known as the Open Systems Interconnection (OSI) ref- erence model. This architecture determines how hardware, software, topologies, and protocols exist and operate on a network. The OSI model is based on seven layers, as shown in Figure 2.7. Each layer adds functionality to the previous layer and communicates with the layers directly above and below it. Because each layer of the OSI model handles a different part of the
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Header
Data
Figure 2.8 Trailer The structure of a packet
networking is possible, network medium can also describe the type of wire- less communications used to permit computers to exchange data via some wireless transmission frequency. A protocol implements the functions of one or more of the OSI layers. A wide variety of communication protocols exist, and many of them rely on others for operation. This concept of building upon the layers already in existence is the foundation of the OSI model. The OSI layers use various forms of control information to communicate with their equal layers in other computer systems. This information consists of specific requests and instructions that are exchanged between equal OSI layers. The data to be exchanged is broken down into packets. We briefly mentioned packets earlier in this chapter; let’s now look at the parts of a packet. All packets consist of a header, data and a trailer. (See Figure 2.8.) The header contains the source and destination addresses, clocking information, and an alert signal. The data section contains the actual data or payload. The trailer contains information to verify that the contents of the packet are valid.
Control information is then added to the packets. Control information typ- ically takes one of two forms: headers and trailers. Header and trailer infor- mation is added or removed as the data passes from layer to layer. An OSI layer may or may not attach a header or a trailer to data from upper layers. Error checking determines whether transmitted data has become corrupt or damaged while traveling from the source to the destination. Error check-
2.3 The OSI Reference Model 69
ing is implemented at several of the OSI layers. A common error-checking method is the cyclic redundancy check (CRC) that detects and discards corrupted data. A CRC value is generated by a calculation that is performed at the source device. The destination device compares this value to its own calculation to determine whether errors occurred during transmission. If the values are equal, the packet is considered valid. If the values are unequal, the packet contains errors and is discarded.
Flow control prevents network congestion by ensuring that transmitting devices do not flood receiving devices with data. A high-speed modem may generate traffic faster than the phone lines can transfer it, or faster than the destination modem can receive and process it. The three commonly used methods for handling network congestion are windowing, buffering, and transmitting source-quench messages.
Windowing is a flow-control scheme in which the source device requires an acknowledgment from the destination after a certain number of packets have been transmitted. If the destination does not receive one or more of the packets for some reason, it does not receive enough packets to send an acknowledgment. The source then retransmits the packets at a reduced transmission rate. Buffering is used to temporarily store bursts of excess data in memory by network devices until they can be processed. Receiving devices use source-quench messages to help prevent their buffers from overflowing.
The seven layers of the OSI reference model can be divided into two cate- gories: upper layers and lower layers. The upper layers of the OSI model deal with application issues and generally are implemented only in soft- ware. The lower layer s of the OSI model handle data transport issues. The physical layer and the data link layer are implemented in hardware and software. Now we are ready to delve into the different layers and the devices that operate at those layers.
The Physical layer (Layer 1) handles the mechanical and electrical communications. In other words, it translates bits (1s and 0s) into data that can be transmitted. Layer 1 specifications determine the shape, size, and pin-out of connectors; what voltages and currents are used; and how the physical media and electrical components work together.
2.3 The OSI Reference Model 71
Logical Link Control (LLC)
Media Access Control (MAC) Figure 2. The sublayers of the Data Link layer
addresses enable multiple devices to uniquely identify one another. These unique addresses are assigned at the manufacturer. Because it only under- stands the MAC address, this layer cannot route to other networks—it can only pass on packets in its own segment. Devices that operate at this layer are bridges, switches, and routers.
The Network layer (Layer 3) manages the routing of packets that are to be forwarded on to different networks. The Network layer relies on the use of routable protocols to deliver packets to distant networks. The Network layer defines the network address, which is different from the MAC address. The MAC address is considered the physical address, and the net- work address is considered the logical address. Because this layer defines the logical network layout, routers can use it to determine how to forward
72 CHAPTER 2 Communications Network
Layer 1 devices
Layer 2 devices
Layer 3 devices
Repeater
Bridge
Router
Hub
Switch
Figure 2. Devices that operate in the various lower levels of the OSI model
packets. Because routers function at this layer, much of the design and con- figuration of a network is done here. See Figure 2.10 for examples of the devices that operate on the first three layers of the OSI model.
The Transport layer (Layer 4) manages the connection between the source and the destination to ensure that the data has reliable delivery. The Trans- port layer accepts data and segments it for transport across the network. Generally, the Transport layer is responsible for making sure that the data is delivered error free and in the proper sequence. Flow control generally occurs at the Transport layer. Flow control manages data transmission between devices so that the transmitting device does not send more data than the receiving device can process. Reliable delivery involves error checking and recovery. Error checking involves detecting transmission errors, while error recovery involves acting to resolve any errors that occur. Transport protocols can be characterized as being either connection-ori- ented or connectionless. Connection-oriented services must first establish a connection with the desired service before passing any data. A connec- tionless service can send the data without any need to establish a connec-
74 CHAPTER 2 Communications Network
not typically associated with a particular protocol stack. Some well-known standards include Motion Picture Experts Group (MPEG), Graphics Inter- change Format (GIF), Joint Photographic Experts Group (JPEG), and Tagged Image File Format (TIFF).
The Application layer (Layer 7) provides the user interface for communi- cation. The Application layer is the OSI layer closest to the end user, which means that both the OSI Application layer and the user interact directly with the software application. This layer interacts with software applica- tions. Application layer functions typically include file transfer, file man- agement, message handling, and database query functions. The Application layer also determines the availability of an application with data to trans- mit, and decides whether sufficient network resources for the communica- tion exist. The Application layer is not application itself that is communicating. It is a service layer that provides these services. Some examples of Application layer implementations include Telnet, File Trans- fer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP). Let’s summarize what we have learned. Each layer of the OSI model per- forms a particular function. This type of organization allows each layer to communicate only with its surrounding layers within a given host. Applica- tions use layers 5 through 7 to communicate with another machine using the same protocol. Layers 3 and 4 define how data delivery is set up and defined on computers that use the same protocol. Layers 1 and 2 define the physical and electrical signal characteristics. Between hosts, each layer com- municates with its corresponding layer on the other machine, but only through the lower layers on both machines. Information being transferred from a software application in one computer system to a software applica- tion in another must pass through all of the OSI layers. For example, if a software application in your system has information to pass to a software application in a coworker’s system, the application program in your system will pass its information to the Application layer. The Application layer then passes the information to the Presentation layer, which relays the data to the Session layer, and so on, until it reaches the Physical layer. At the Physical layer, the information is placed on the physical network medium and is sent across the medium to the coworker’s system. The Physical layer
2.4 The Internet 75
of that system removes the information from the physical medium, and then its physical information is passed up to the Data Link layer, which passes it to the Network layer, and so on, until it reaches the Application layer. The Application layer of the system then passes the information to the application program to complete the communication process. See Fig- ure 2.11 for an overview of data flowing through the OSI model layers.
The OSI reference model is only a guideline. An actual protocol or device may or may not assume all responsibilities of one particular OSI layer. It may also take on functions that span several layers. However, we use the OSI model to help us understand and classify the functions that make up a particular implementation.
There are several ways to remember the layers of the OSI model.You can make up a sentence of your own but two common ones are,“Please Do Not Throw Sausage Pizza Away” and from the top down,“All People Seem To Need Data Processing.”
The ISO created the OSI model and the IEEE further defined the lower lay- ers of the OSI model. Table 2.1 lists those specifications.
The Internet can be considered the largest network in the world. This net- work is made up of computers used by many different types of businesses, educational institutions, governments, and individuals located around the world. Each network operates independently but can connect to other net- works through routers, which are covered in the “Routers” section later in this chapter. Before we discuss how the Internet works, we will go over some of the history behind the Internet.
The name “Internet” refers to the global interconnection of networks made possible by the protocols devised in the 1970s that are still in use today. The Internet was originally called ARPANET (short for Advanced Research Project Agency). It was developed by the Department of Defense to provide a way to connect networks. The ARPANET grew from four nodes in 1969 to about a hundred by 1975. By mid-1975, it was determined that the ARPANET was stable enough to be turned over to a separate agency for
2.4 The Internet 77
TABLE 2.1 IEEE 802 Standards
Standard Name Description
Internetworking
Logical Link Control
Ethernet LAN Token Bus LAN Token Ring LAN Metropolitan Area Network (MAN) Broadband Technical Advisory Group
Fiber-Optic Technical Advisory Group
Integrated Voice/Data Networks
Network Security Technology Advisory Group Wireless Networks
Demand Priority
Cable Modems
Defines internetworking communications,routing, bridging,and switching Provides addressing,error checking,and flow control for data frames Defines all forms of Ethernet interfaces and media Defines all forms of Token Bus interfaces and media Defines all forms of Token Ring interfaces and media Defines MAN services,technologies,and addressing Specifies physical,electrical,and mechanical features of broadband cable Provides technical direction for the use of fiber-optic technology and media Defines integration of voice,video,and data on IEEE LANs Develops a security model for diverse networks that cov- ers authentication and encryption Defines standards for wireless networks that cover a wide range of frequencies Defines the demand priority access method for 100VG- AnyLAN Creates standards for transmission of data over cable television networks
operational management, so responsibility was transferred to the Defense Communications Agency.
In 1973, the Defense Advanced Research Projects Agency (DARPA) began a series of research programs to extend packet switching to ground mobile units and ships at sea through the use of ground mobile packet radio and synchro- nous satellites. This process became known as Internetting. It was intended to
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solve the problem of linking different kinds of packet networks together with- out requiring the users or their computers to know much about how packets traveled. About the same time, DARPA provided additional funding for a research project that began in the late 1960s to explore the use of radio for a packet-switched network. This effort, at the University of Hawaii, led to new mobile packet radio ideas and to the design of what is now Ethernet. The Eth- ernet concept arose when a researcher realized that the random-access radio system could be operated on a coaxial (coax) cable at data rates thousands of times faster than could then be supported over the air. These efforts came together in 1977 when a four-network demonstration was conducted. Also in the early 1970s, researchers at Stanford began to design a new set of computer communication protocols that would allow multiple packet net- works to be interconnected in a flexible and dynamic way. The first phase of this work was successfully completed in July 1977. This success led to an effort to implement robust versions of the two main Internet protocols— Transmission Control Protocol (TCP) and Internet Protocol (IP). By 1980 a serious effort was mounted to require all computers on the ARPANET to adopt the Transmission Control Protocol /Internet Proto- col (TCP/IP) suite. This was accomplished in January 1983. As DARPA was preparing to convert the organizations they supported to TCP/IP, the National Science Foundation started an effort to interconnect the nation’s computer science departments through the use of dial-up connections. The result was “phone-mail,” the capability for electronic mail exchange among computers that were not on ARPANET, which pio- neered the use of TCP/IP over the X.25 protocol standard. NSF’s interest in high bandwidth was heightened in 1986 through its sponsorship of NSFNET, which eventually replaced ARPANET when it was retired in
