1 / 100

Chapter 9

Chapter 9. Local Area Networks. Introduction. Network- a set of two or more interconnected devices or computers LAN may consist of PCs and/or MACs, mainframes, minicomputers, etc.

Jeffrey
Download Presentation

Chapter 9

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Chapter 9 Local Area Networks

  2. Introduction • Network- a set of two or more interconnected devices or computers • LAN may consist of PCs and/or MACs, mainframes, minicomputers, etc. • Local area is typically within an office, a building, or a group of buildings. The distance among the computers and devices is only one characteristic that makes a network a local area network. • Due to new technologies, LANs now often span much longer distances, up to many miles in some cases. chapter 9

  3. Figure 9.1 a networked arrangement and a single multi-user system chapter 9

  4. Introduction Continued • What features differentiate a LAN from a wide area, WAN? • Generally, WANs are geographically larger than LANs. • Often WANs are used to connect many more computers than LANs • LANs are owned and operated by a single company mostly, and their wiring, systems, and devices are on the company's premises • WAN is often provided by common carriers such as the telephone companies • However, many companies provide their own private WANs by leasing or purchasing transmission equipment. • LANs make use of media access techniques that are different than the physical interfaces to WAN. LANs may use different protocols from WANs • The interconnection of LANs over WANs is very common in corporations today • New networking and switching technologies are blurring the lines between the two types of networks. chapter 9

  5. Motivation for Using a LAN • Two primary reasons: • to share resources such as printers or files • to improve communications between users in a workgroup, office, department, or company • Disk sharing: • provides access to commonly used programs reducing total disk space requirements • provides access to commonly needed data. • With only a shared "copy" of the information, each user is assured that it is always accurate and up-to-date. • allows centralized backup of files chapter 9

  6. Figure 9.2: In addition to saving on hardware, LAN also results in software savings chapter 9

  7. Motivation for Using a LAN Continued • LAN can also offer improved security. • Security features such as passwords are built into the servers • System attached to the LAN could be diskless workstation (no floppy disk drive). • No software or data resides on the system, and none can be copied to a floppy disk and taken away. • Also limits the introduction of viruses or loading unauthorized programs. chapter 9

  8. Motivation for Using a LAN Continued • Improves communications • Electronic mail and workgroup applications • Documents, programs, and data files exchanged as attachments using electronic mail among LAN users. • Workgroup applications allow multiple users to cooperate in performing various tasks chapter 9

  9. Components of Local Area Networks • Often consist of interconnected computers including personal computers, UNIX workstations, multi-user systems, and even large mainframe computers. • Computers containing shared resources are called servers. • The users and applications running on other systems access these servers over the LAN, and are called clients, hence the name client server computing chapter 9

  10. Terminology • Standalone workstation - a workstation that is not connected to a network, but relies on its own hard disk for data storage and applications. • Client - a workstation connected to a network. A person whose workstation is part of a network may also be called a client, or that person may be known more informally as a user. • Servers - store shared data and programs on their hard disks. They can also perform management functions, such as determining which users have access to certain programs. • Client-server - a network that uses a server to enable clients to share data, data storage space, and devices. chapter 9

  11. Figure 9-3: Client Server LAN Configuration chapter 9

  12. Components of Local Area Networks Continued • Print server- this computer accepts print requests from other systems on the LAN, temporarily writes the data to be printed onto its disk storage, then sends the data to the printer. • The print server manages the print streams from several systems concurrently • A directly attached printer acts as a print server. Other systems on the LAN can direct print streams to the printer. chapter 9

  13. Components of Local Area Networks Continued • A LAN adapter is a hardware board (e.g., a PC adapter card) that can be inserted into an expansion slot in the PC. • LAN adapters are commonly called network interface adapters (NIAs) or network interface cards (NICs). • The adapter provides the interface to the PC or device on one side and the network on the other side. • The adapter provides a socket into which the LAN medium, such as a cable, can plug. • The adapter has to translate the signals used within the PC or printer into signals used on the LAN cable. chapter 9

  14. Network Interface Cards (NICs) Types of NICs • Industry Standard Architecture (ISA) • MicroChannel Architecture (MCA) • Extended Industry Standard Architecture (EISA) • Peripheral Component Interconnect (PCI) FIGURE: Four primary bus architectures

  15. Network Interface Cards (NICs) FIGURE : Three kinds of bus connections on the same board

  16. Network Interface Cards (NICs) • NICs may connect to interfaces other than a PC’s bus • For laptop computers, Personal Computer Memory Card International Association slots may be used to connect NICs • PCMCIA • Also called PC card • Developed in the early 1990s to provide a standard interface for connecting any type of device to a portable computer

  17. Network Interface Cards (NICs) FIGURE: Typical PCMCIA NIC FIGURE : Parallel port NIC

  18. Network Interface Cards (NICs) FIGURE : Wireless NIC and transceiver FIGURE : Ethernet NICs for printers

  19. Basics of Local Area Networks • IEEE 802-series of standards is focused on LAN interconnection and operation • The IEEE LAN standards are specific set of standards that conform to the lower layers of the OSI reference Model • The most popular types of LAN physical media are coaxial cable, twisted-pair wire, and fiber optic cable chapter 9

  20. Coax in LANs • The original type of coaxial cable used in Ethernet LANs is known as thick coax, approximately 1/2 inch thick and is relatively inflexible and difficult to install. • Main advantage of coaxial cable - less susceptible to interference than twisted-pair wire, and supports relatively high rates of data transmission over greater distances. • The main disadvantage - more expensive that other media, and in the case of thick coax, its size and inflexibility makes it more difficult to install. • Typical data rates over coaxial cable LANs is 10 Mbps with distances ranging from 100's to 1000's of feet. • "thin" coax came into widespread use because it is less expensive than thick coax, and easier to work with. • It is approximately 1/4 inch (0.63 cm) thick and very similar to the coax used for cable TV. • But, this coax supports shorter distances than the thick coax. chapter 9

  21. Twisted-pair Cables in LANs • Unshielded twisted-pair (UTP) is the standard telephone cable • Main advantages of UTP wiring - inexpensive, flexible, easy to install and available • In the past, only relatively low speed transmissions were possible over UTP but newer techniques are now in use that will support UTP speed in the 100 megabits per second range. • Disadvantages: • more susceptible to electrical interference. Not a huge problem in offices, but can be a problem in factories where electrical machinery is in use. • Another problem is that signals lose their strength as they are transmitted over UTP, high attenuation. chapter 9

  22. Twisted-pair Cables in LANs Continued shielded twisted-pair, STP: • shield provides protection from noise, thus eliminating somewhat the problems associated with susceptibility to noise that plagues UTP • shield also helps keep signals from emanating out of the wire, important in certain environments. • A negative of STP though, is that the shield increases the cost of the wire so it is typically more expensive than UTP chapter 9

  23. Fiber Optic cabling in LANs • Advantages: • high transmission rates, up to 100 Mbps and therefore greater bandwidth • immune to electrical interference because they use light rather than electricity • very thin and flexible. Thus, a fiber optic cable is easy to install, and ideal for bundling many fibers together to create a cable that carries very large amount of traffic. • Less attenuation than copper wiring (longer length of a fiber optic cable before repeaters are required). • Very secure. Any tap into the cable interrupts the flow of light and is easily detected. • Disadvantages: cables and adapters more expensive • Commonly used for high speed backbones chapter 9

  24. Transmission Techniques • Baseband: • Signals placed directly onto the transmission medium (not modulated by a carrier) • A stream of such digital pulses represents the information being transmitted. • The signal takes up the entire bandwidth of the transmission medium. • Signals from multiple sources can be transmitted via the technique known as Time Division Multiplexing or TDM as seen in Chapter 2. • Relatively inexpensive. No special equipment is required to modify the signals. • One problem -signals lose strength as they are carried over longer distances and must be regenerated. chapter 9

  25. Figure 9.5: Baseband Transmission chapter 9

  26. Transmission Techniques Continued • Broadband: • signals modulated to different frequency ranges typically provided using radio frequency modems. • Frequency Division Multiplexing (FDM) is used where each signal occupies a different frequency range. • The different frequency ranges called logical channels. • Therefore, multiple channels are available using broadband transmission. (technique used in standard cable TV). • Each logical FDM channel could also be shared by multiple applications by using TDM within the channel. chapter 9

  27. Transmission Techniques Continued • Broadband provides: • greater bandwidth than baseband and is, therefore, able to carry more information and support greater distances than baseband. • ability to carry voice, data, and video at the same time. • Disadvantages: • Typically more expensive than baseband transmission. • more difficult to configure and costly to modify. • Best suited to large installations chapter 9

  28. Figure 9.6: Broadband Transmission chapter 9

  29. Media Access Methods • All devices attached to a LAN share the transmission medium. • What happens if multiple devices attempt to send data onto the LAN at the same time? • A media access control (MAC) method determines how multiple devices share the transmission medium. chapter 9

  30. Media Access Methods Continued • Carrier Sense Multiple Access with Collision Detection (CSMA/CD) • Devices must sense the presence of a carrier signal (presence of a carrier signal indicates that information is currently being transmitted) • If a carrier signal is not detected, it is an indication that the LAN is free and the device can attempt to send. • If another device had sent information at the same time, though, the listening devices would detect a collision. This is where the "collision detection" part of the name comes from. • When such a collision is detected, the devices stop transmitting and wait for some period of time before attempting to transmit again. chapter 9

  31. Media Access Methods CSMA/CD Continued • CSMA/CD is the access method used in Ethernet and 802.3 LANs and became an international standard when IEEE 802.3 was approved. • Traffic increases may result in much more frequent collisions degrading network performance. • Rule of thumb: CSMA/CD LANs (Ethernet) work fine with constant traffic load of 30 percent of capacity and traffic bursts of about 60 percent of capacity; • otherwise divide the Ethernet into additional segments to reduce the number of stations that are sharing a given segment's bandwidth. chapter 9

  32. CSMA/CD continue • CSMA/CD is a probabilistic access control method (the opportunity to transmit is not guaranteed). • The possibility that a device might not gain access to the network at a critical time is unacceptable. • This is one reason why IBM invented the Token-Ring network. • Token passing is the media access technique used in token ring LANs: • A token ring operates as a logical ring. • The transmitter of each device is connected to the receiver of the next device in the ring enabling devices to pass messages around the ring. chapter 9

  33. Figure 9.8: Token Passing chapter 9

  34. Token Ring continued • Token: special type of data frame that circulates around the ring. • A device can transmit only when it is in possession of the token • After a data frame is transmitted, the device releases the token to the network so that other devices can transmit. • Apparently simple method but: • How is the loss of a token detected and how is a new token created? • What steps should a station take if it stops receiving data? • What if a station that was to receive a frame goes off the network? • How is the network to identify frames that have circulated the network too many times? chapter 9

  35. Token Ring continued • Ring error monitor (REM) can regenerate lost tokens and remove bad frames from the network. • Additionally, each device is capable of signaling certain problems by transmitting a beacon signal. • Error detection and diagnostic tools available on a token ring are quite extensive. By contrast, no such tools are built into Ethernet (CSMA/CD) networks. • The mechanism that controls a token ring network is much more involved and expensive than that required for Ethernet. • Token ring is deterministic (every device is guaranteed a chance to transmit each time the token circulates the ring ) while Ethernet is probabilistic. chapter 9

  36. Token Ring vs. CSMA/CD • Token passing may suffer less performance degradation than CSMA/CD in very large LANs. • Because contention for the transmission medium is more orderly than with CSMA/CD, eliminating collisions, timeouts, and subsequent retries. • Token passing allows stations to transmit whenever a free token is available, but they may have to wait a while to get a token. • A potential problem is that stations that get a token can "hog" the LAN. • Implementations attempt to minimize this by placing a limit on the amount of time a single station can send before passing on the token. chapter 9

  37. LAN Topologies • Topology of the LAN: actual physical layout of a LAN or the arrangement in which devices are interconnected. • Most widely used topologies: • bus • star • ring chapter 9

  38. Bus Topology • All stations (systems and devices) directly connected to the same transmission medium, usually a physical cable. • simple, and as a result, often inexpensive. • very common on LANs • type of topology originally specified for Ethernet LANs. • Information on bus-based LANs is broadcast to all connected stations. • Transmissions go in both directions along the bus or cable. • All stations, thus, receive all transmissions. • Each station has a unique address assigned to it. • Station address included in the data frames that carry information on the bus. chapter 9

  39. Figure 9.9: LAN BUS topology chapter 9

  40. Star Topology • Each station connected to a central piece of equipment commonly called a hub. • All communications go through the hub which amplifies and retransmits signals providing connectivity among stations • Individual stations are not directly connected to one another but indirectly through the central hub. • One problem with a star is that if the central hub fails, the network is inoperable. • Redundancy features are often built into hubs making them very reliable. Also, hubs can be configured so that a bypass is possible in the event that a component fails. chapter 9

  41. Figure 9.10: LAN STAR topology chapter 9

  42. Ring Topology • Stations directly connected to other stations form what looks like a ring. • Unlike a star, adjacent stations on the ring are directly cabled to one another. • No central hub. Information flows in one direction around a ring. Each station receives all signals from the adjacent station, regenerates and retransmits frames to the next station on the ring. • Eliminates problem of depending on a central switch but dependent on each individual station on the ring. • If a station fails, or the link between stations fails, the ring can become inoperable • There are solutions to this problem such as redundant links and ways to bypass failed stations. chapter 9

  43. Ring Topology Continued • As with other LAN topologies, each station on a ring has a unique address. • A station look at the destination address in a frame to determine if a frame is intended for it. • If so, the frame is pulled off the ring. • If not, the frame is retransmitted to the next station. • Token passing schemes are often used on ring-base LANs. • For example, IBM's token-Ring LAN uses a ring topology on which a token passing media access control method is used. chapter 9

  44. Figure 9.11: LAN RING topology chapter 9

  45. Ethernet • Jointly by Xerox, DEC, and Intel • One of the most popular types of LAN in use today. • Original Ethernet specification called for coaxial cable as the transmission medium. • Today, Ethernet LANs make use of other types of cabling such as twisted-pair wire as well. • Ethernet uses a bus topology • The main advantage of Ethernet is its relatively low cost. • Inexpensive Ethernet adapters are available for most PCs and Ethernet interfaces are supported in a wide range of computer equipment. • Multivendor support of Ethernet makes it a popular choice in many cases chapter 9

  46. Figure 9.12: Ethernet uses LAN BUS topology chapter 9

  47. Ethernet Continued • Ethernet LAN uses a CSMA/CD contention protocol (defined as part of IEEE 802.3 standard). • Ethernet and IEEE 802.3 specifications are similar, but not identical. • One difference: Ethernet frame headers include a type field that indicates the higher layer protocol in use. • For example, the type field could indicate that either TCP/IP or XNS (Xerox Network System) protocols were used across the Ethernet LAN. • Instead of a type field, the header of an 802.3 frame includes a length field indicating the length of the data contained in the information portion of the frame. chapter 9

  48. IEEE Networking Specifications TABLE IEEE 802 standards

  49. Figure 9.13: Ethernet frame compared to IEEE 802.3 frame chapter 9

  50. Ethernet Continued • Originally designed for transmission rates of 10 Mbps • Work going on for Ethernet-like LANs at higher speeds (fast Ethernet at 100 Mbps). • Ethernet LANs with large number of users and heavy traffic demands may result in performance problems • Primarily due to the characteristics of the CSMA/CD (collisions, forcing stations to wait before retrying their transmission). • An upper limit of 10 Mbps also becomes a problem: • when transmitting information requiring high bandwidth such as video images. • It also makes Ethernet less attractive as a backbone LAN to interconnect other LANs. • But, multiple Ethernet LANs can be easily interconnected by devices called bridges forming large logical LANs. chapter 9

More Related