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BIT3114 lecture6 WAN.ppt
- 1. Lecture 6 Introduction to Metropolitan Area Networks and Wide Area Networks Data Communications and Computer Networks:
- 2. After reading this chapter, you should be able to: • Distinguish local area networks, metropolitan area networks, and wide area networks from each other • Identify the characteristics of metropolitan area networks and compare to LANs and WANs • Describe how circuit-switched, datagram packet- switched, and virtual circuit packet-switched networks work • Identify the differences between connection- oriented and connectionless networks 2
- 3. After reading this chapter, you should be able to: • Describe the differences between centralized and distributed routing • Describe the differences between static and adaptive routing • Document the main characteristics of flooding and use hop count and hop limit in a simple example • Discuss the basic concepts of network congestion, including quality of service 3
- 4. Introduction • As we have seen, a local area network covers a room, a building or a campus. • A metropolitan area network (MAN) covers a city or a region of a city. • A wide area network (WAN) covers multiple cities, states, countries, and even the solar system. 4
- 5. Metropolitan Area Network Basics • MANs borrow technologies from LANs and WANs. • MANs support high-speed disaster recovery systems, real-time transaction backup systems, interconnections between corporate data centers and Internet service providers, and government, business, medicine, and education high-speed interconnections. • Almost exclusively fiber optic systems 5
- 6. Metropolitan Area Network Basics • MANs have very high transfer speeds • MANs can recover from network faults very quickly (failover time) • MANs are very often a ring topology (not a star- wired ring) • Some MANs can be provisioned dynamically 6
- 7. Metropolitan Area Network Basics 7
- 8. SONET versus Ethernet MANs • Most MANs are SONET network built of multiple rings (for failover purposes) • SONET is well-proven but complex, fairly expensive, and cannot be provisioned dynamically. • SONET is based upon T-1 rates and does not fit nicely into 1 Mbps, 10 Mbps, 100 Mbps, 1000 Mbps chunks, like Ethernet systems do. • Ethernet MANs generally have high failover times 8
- 9. SONET versus Ethernet MANs 9
- 10. SONET versus Ethernet MANs 10
- 11. Metro Ethernet • One of the latest forms of the metropolitan area network is metro Ethernet • Metro Ethernet is a service in which the provider creates a door-to-door Ethernet connection between two locations • For example, you may connect your business with a second business using a point-to-point Ethernet connection (Figure 9-4a) 11
- 13. Metro Ethernet • You may also connect your business with multiple businesses using a connection similar to a large local area network (Figure 9-4b) • Thus, by simply sending out one packet, multiple companies may receive the data • Neat thing about metro Ethernet is the way it seamlessly connects with a company’s internal Ethernet network(s) 13
- 15. Wide Area Network Basics • WANs used to be characterized with slow, noisy lines. • Today WANs are very high speed with very low error rates. • WANs usually follow a mesh topology. 15
- 16. Wide Area Network Basics 16
- 17. Wide Area Network Basics • A station is a device that interfaces a user to a network. • A node is a device that allows one or more stations to access the physical network and is a transfer point for passing information through a network. • A node is often a computer, a router, or a telephone switch. • The sub-network or physical network is the underlying connection of nodes and telecommunication links. 17
- 18. Wide Area Network Basics 18
- 19. Types of Network Structures • Circuit switched network - a sub-network in which a dedicated circuit is established between sender and receiver and all data passes over this circuit. • The telephone system is a common example. • The connection is dedicated until one party or another terminates the connection. • AT&T announced end of 2009 that they will begin phasing out their switched networks 19
- 20. Types of Network Structures 20
- 21. Types of Network Structures • Packet switched network - a network in which all data messages are transmitted using fixed-sized packages, called packets. • More efficient use of a telecommunications line since packets from multiple sources can share the medium. • One form of packet switched network is the datagram. With a datagram, each packet is on its own and may follow its own path. • Virtual circuit packet switched network create a logical path through the subnet and all packets from one connection follow this path. 21
- 22. Types of Network Structures • Broadcast network - a network typically found in local area networks but occasionally found in wide area networks. • A workstation transmits its data and all other workstations “connected” to the network hear the data. Only the workstation(s) with the proper address will accept the data. 22
- 23. Summary of Network Structures 23
- 24. Connection-oriented versus Connectionless • The network structure is the underlying physical component of a network. What about the software or application that uses the network? • A network application can be either connection- oriented or connectionless. 24
- 25. Connection-oriented versus Connectionless • A connection-oriented application requires both sender and receiver to create a connection before any data is transferred. • Applications such as large file transfers and sensitive transactions such as banking and business are typically connection-oriented. • A connectionless application does not create a connection first but simply sends the data. Electronic mail is a common example. 25
- 28. Connection-oriented versus Connectionless • A connection-oriented application can operate over both a circuit switched network or a packet switched network. • A connectionless application can also operate over both a circuit switched network or a packet switched network but a packet switched network may be more efficient. 28
- 29. Routing • Each node in a WAN is a router that accepts an input packet, examines the destination address, and forwards the packet on to a particular telecommunications line. • How does a router decide which line to transmit on? • A router must select the one transmission line that will best provide a path to the destination and in an optimal manner. • Often many possible routes exist between sender and receiver. 29
- 30. Routing 30
- 31. Routing • The communications network with its nodes and telecommunication links is essentially a weighted network graph. • The edges, or telecommunication links, between nodes, have a cost associated with them. • The cost could be a delay cost, a queue size cost, a limiting speed, or simply a dollar amount for using that link. 31
- 32. Routing 32
- 33. Routing • The routing method, or algorithm, chosen to move packets through a network should be: • Optimal, so the least cost can be found • Fair, so all packets are treated equally • Robust, in case link or node failures occur and the network has to reroute traffic. • Not too robust so that the chosen paths do not oscillate too quickly between troubled spots. 33
- 34. Least Cost Routing Algorithm • Dijkstra’s least cost algorithm finds all possible paths between two locations. • By identifying all possible paths, it also identifies the least cost path. • The algorithm can be applied to determine the least cost path between any pair of nodes. 34
- 35. Least Cost Routing Algorithm 35
- 36. Flooding Routing • When a packet arrives at a node, the node sends a copy of the packet out every link except the link the packet arrived on. • Traffic grows very quickly when every node floods the packet. • To limit uncontrolled growth, each packet has a hop count. Every time a packet hops, its hop count is incremented. When a packet’s hop count equals a global hop limit, the packet is discarded. 36
- 39. Centralized Routing • One routing table is kept at a “central” node. • Whenever a node needs a routing decision, the central node is consulted. • To survive central node failure, the routing table should be kept at a backup location. • The central node should be designed to support a high amount of traffic consisting of routing requests. 39
- 41. Distributed Routing • Each node maintains its own routing table. • No central site holds a global table. • Somehow each node has to share information with other nodes so that the individual routing tables can be created. • Possible problem with individual routing tables holding inaccurate information. 41
- 43. Adaptive Routing versus Static Routing • With adaptive routing, routing tables can change to reflect changes in the network • Static routing does not allow the routing tables to change. • Static routing is simpler but does not adapt to network congestion or failures. 43
- 44. Routing Examples - RIP • Routing Information Protocol (RIP) - First routing protocol used on the Internet. • A form of distance vector routing. It was adaptive and distributed • Each node kept its own table and exchanged routing information with its neighbors. 44
- 45. Routing Examples - RIP • Suppose that Router A has connections to four networks (123, 234, 345, and 789) and has the following current routing table: • Network Hop Cost Next Router • 123 8 B • 234 5 C • 345 6 C • 789 10 D 45
- 46. Routing Examples - RIP • Now suppose Router D sends out the following routing information (note that Router D did not send Next Router information, since each router will determine that information for itself): • Network Hop Cost • 123 4 • 345 5 • 567 7 • 789 10 46
- 47. Routing Examples - RIP • Router A will look at each entry in Router D’s table and make the following decisions: • 1. Router D says Network 123 is 4 hops away (from Router D). Since Router D is 1 hop away from Router A, Network 123 is actually 5 hops away from Router A. That is better than the current entry of 8 hops in Router A’s table, so Router A will update the entry for Network 123. • 2. Router D says Network 345 is 5 hops away. Add one hop to get to Router D and Network 345 is 6 hops away. That is currently the same hop count as shown in Router A’s table for Network 345, so Router A will not update its table. 47
- 48. Routing Examples - RIP • Router A will look at each entry in Router D’s table and make the following decisions: • 3. Router D says Network 567 is 7 hops away. Add 1 hop to get to Router D, giving 8 hops. Since Router A has no information about Network 567, Router A will add this entry to its table. And since the information is coming from Router D, Router A’s Next Router entry for network 567 is set to D. • 4. Router D says Network 789 is 10 hops away. Add 1 hop to get to Router D. The value of 11 hops is worse than the value currently in Router A’s table. Since Router A currently has information from Router D, and Router D is now saying it takes more hops to get to Network 789, then Router A has to use this information. 48
- 49. Routing Examples - RIP • Router A’s updated routing table will thus look like the following: • Network Hop Cost Next Router • 123 5 D • 234 5 C • 345 6 C • 567 8 D • 789 11 D 49
- 50. Routing Examples - OSPF • Open Shortest Path First (OSPF) - Second routing protocol used on the Internet • A form of link state routing • It too was adaptive and distributed but more complicated than RIP and performed much better 50
- 51. Network Congestion • When a network or a part of a network becomes so saturated with data packets that packet transfer is noticeably impeded, network congestion occurs. • What can cause network congestion? Node and link failures; high amounts of traffic; improper network planning. • When serious congestion occurs buffers overflow and packets are lost. 51
- 52. Network Congestion • What can we do to reduce or eliminate network congestion? • An application can observe its own traffic and notice if packets are disappearing. If so, there may be congestion. This is called implicit congestion control. • The network can inform its applications that congestion has occurred and the applications can take action. This is called explicit congestion control. 52
- 53. Congestion Avoidance • Before making a connection, user requests how much bandwidth is needed, or if connection needs to be real-time • Network checks to see if it can satisfy user request • If user request can be satisfied, connection is established • If a user does not need a high bandwidth or real-time, a simpler, cheaper connection is created • This is often called connection admission control • Asynchronous transfer mode is a very good example of this (Chapter Eleven) 53
- 54. WANs In Action: The Smartphone • The network structure that support cell phones and smartphones is growing more complex every day • All phones within transmission distance of a cell tower are participating in a broadcast network 54
- 55. WANs In Action: The Smartphone 55
- 56. WANs In Action: The Smartphone • Once your data/signal reaches the cell tower, the data becomes part of a packet-switched network within the telephone company • If the data moves from the telephone company into the Internet, then your data is passed from one packet switched network to another 56
- 57. Summary • A metropolitan area network is fast, fiber-based, has very small failover times, and is often dynamically provisional • Early MANs were SONET-based, but Ethernet-based MANs are becoming very popular • SONET-based MANs are rings, while Ethernet-based MANs are meshes • Metro Ethernet is a popular form of MAN 57
- 58. Summary (continued) • Wide area networks cover states, countries, the world • User connects to a station and the station interfaces to a network node • A WAN cloud is based upon nodes (routers/switches) and high-speed links • WANs can be circuit-switched (fading away) or packet switched (datagram and virtual circuit) • RIP and OSPF are two routing protocols 58
Editor's Notes
- #10: SONET – Synchronous Optical Networking