1 LAN/WAN Interconnectivity Chapter 2

2 Learning Objectives Explain the OSI reference model, which sets standards for LAN and WAN communications Discuss communication between OSI stacks when two computers are linked through a network Apply the OSI model to realistic networking situations continued…

3 Learning Objectives Describe the types of networks as represented through LAN topologies Describe major LAN transmission methods, including Ethernet, token ring, and FDDI Explain basic WAN network communications topologies and transmission methods, including telecommunications, cable TV, and satellite technologies

4 LAN/WAN Interconnectivity Intense competition between three sectors: Telecommunications companies Cable TV companies Satellite communications companies

5 OSI Reference Model Foundation that brings continuity to LAN and WAN communications Product of two standards organizations: ISO ANSI Developed in 1974 Set of communication guidelines for hardware and software design

6 OSI Guidelines Specify… How network devices contact each other; how devices using different protocols communicate How a network device knows when to transmit and not transmit data How physical network network devices are arranged and connected continued…

7 OSI Guidelines Specify… Methods to ensure that network transmissions are received correctly How network devices maintain a consistent rate of data flow How electronic data is represented on network media

8 OSI Layers

9 Bottom layers Support for physical connectivity, frame formation, encoding, and signal transmission Middle layers Establish and maintain a communication session between two network nodes Monitor for error conditions Uppermost layers Application/software support for encrypting data and assuring interpretation/presentation of data

10 Physical Layer Functions Provides transfer medium (eg, cable) Translates data into a transmission signal Sends signal along the transfer medium Includes physical layout of network Monitors for transmission errors Determines voltage levels for data signal transmissions and to synchronize transmissions Determines signal type (eg, digital or analog)

11 Analog Signals

12 Digital Signals

13 Data Link Layer Functions Constructs data frames Creates CRC information; checks for errors Retransmits data if there is an error Initiates communications link; makes sure it is not interrupted (ensures node-to-node physical reliability) Examines device addresses Acknowledges receipt of a frame

14 Data Link Layer Data link frame contains fields consisting of address and control information Two important sublayers Logical link control (LLC) Media access control (MAC) Connectionless service versus connection- oriented service

15 Network Layer Functions Determines network path for routing packets Helps reduce network congestion Establishes virtual circuits Routes packets to other networks, resequencing packet transmissions when needed Translates between protocols

16 Transport Layer Functions Ensures reliability of packet transmissions Ensures data is sent and received in the same order Sends acknowledgement when packet is received Monitors for packet transmission errors and resends bad packets Breaks large data units into smaller ones and reconstructs them at the receiving end for networks using different protocols

17 Session Layer Functions Establishes and maintains communications link Determines which node transmits at any point in time Disconnects when communication session is over Translates node addresses

18 Presentation Layer Functions Translates data to a format the receiving node understands (eg, from EBCDIC to ASCII) Performs data encryption Performs data compression

19 Application Layer Functions Enables sharing remote drivers and printers Handles messages Provides file transfer services Provides file management services Provides terminal emulation services

20 Communicating Between Stacks OSI model provides standards for: Communicating on a LAN Communicating between LANs Internetworking between LANs and WANs and between WANs and WANs

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22 Peer Protocols

23 Primitives

24 Layered Communications

25 Applying the OSI Model

26 Types of Networks Three main topologies Bus Ring Star

27 Bus Topology Built by running cable from one PC or file server to the next Terminators signal the physical end to the segment

28 Advantages of Bus Topology Works well for small networks Relatively inexpensive to implement Easy to add to it

29 Disadvantages of Bus Topology Management costs can be high Potential for congestion with network traffic

30 Ring Topology Continuous path for data with no logical beginning or ending point, and thus no terminators

31 Advantages of Ring Topology Easier to manage; easier to locate a defective node or cable problem Well-suited for transmitting signals over long distances on a LAN Handles high-volume network traffic Enables reliable communication

32 Disadvantages of Ring Topology Expensive Requires more cable and network equipment at the start Not used as widely as bus topology Fewer equipment options Fewer options for expansion to high-speed communication

33 Star Topology Oldest and most common network design Multiple nodes attached to a central hub

34 Advantages of Star Topology Good option for modern networks Low startup costs Easy to manage Offers opportunities for expansion Most popular topology in use; wide variety of equipment available

35 Disadvantages of Star Topology Hub is a single point of failure Requires more cable than the bus

36 Bus Networks in a Physical Star Layout No exposed terminators Capability for connecting multiple hubs to expand network in many directions Expansion opportunities for implementing high-speed networking Popular design; wide range of equipment available

37 LAN Transmission Methods Ethernet IEEE specifications Broadest options for expansion and high-speed networking Token ring IEEE specifications FDDI (Fiber Distributed Data Interface) High-speed variation of token ring

38 Ethernet Uses CSMA/CD access method for data transmission on a network Typically implemented in a bus or bus-star topology Carrier sense Collision

39 Ethernet Communications

40 Ethernet II

41 Ethernet Standards

42 Token Ring Developed by IBM in the 1970s; remains a primary LAN technology Employs physical star topology with logic of ring topology Each node connects to a central hub, but the frame travels from node to node as though there were no starting or ending point

43 Token Ring Frame

44 Token Ring Terms Multistation access unit (MAU) Beaconing Broadcast storms

45 FDDI Fiber-optic data transport method capable of a 100-Mbps transfer rate using a dual ring topology Synchronous versus asynchronous communications Nodes monitor network for error conditions Long periods of no activity Long periods where the token is not present Class A and Class B nodes

46 WAN Network Communications Typical providers of WAN network services Telecommunications companies Cable TV companies Satellite providers Newer sources of WAN connectivity Cable television networks Satellite TV companies Wireless WANs Wide use of star topology

47 Telecommunications WANs Earliest source of WAN connectivity Regional telephone companies, also called: Telcos Regional bell operating companies (RBOCs) Long-distance telecommunications companies Plain old telephone service (POTS) or public switched telephone network (PSTN)

48 General Topology Linking LATA and IXC Lines

49 Connecting LANs through a T-Carrier Line

50 T-Carrier Services and Data Rates

51 Cable TV WANs Also called cablecos or multiple system operators (MSOs) Use a distributed architecture that consists of several star-shaped centralized locations Headend is the main focal point in the star

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53 Wireless WANs Use radio, microware, and satellite communications Packet radio communications

54 Topology of a Radio Wave WAN Joining Two LANs

55 WAN Transmission Methods Use different switching techniques to create data paths (channels) for transmitting data Switching Enables multiple nodes to simultaneously transmit and receive data, or Enables data to be transmitted over different routes to achieve maximum efficiency in terms of speed and cost

56 Switching Techniques Time division multiple access (TDMA) Divides channels into distinct time slots Frequency division multiple access (FDMA) Divides channels into frequencies Statistical multiple accessDynamically allocates bandwidth based on application need Circuit switchingUses a dedicated physical circuit Message switchingUses store-and-forward method of data transmission Packet switchingCombines circuit and message switching

57 Chapter Summary Open Systems Interconnection (OSI) model Basic network topologies Key LAN transmission methods WAN communications options WAN transmission methods