Datornätverk A – lektion 11

1. Datornätverk A – lektion 11 Kapitel 16: Connecting LAN:s, Backbone Networks and Virtual Lans. (Kapitel 18: Frame Relay and ATM översiktligt)

2. Chapter 16 Connecting LANs,Backbone Networks, and Virtual LANs

3. Limitations of Ethernet Technologies • Distance (the length of the cable) • 200 m in Thin Ethernet (10Base2) • 100 m in twisted pair Ethernet (10BaseT or 100BaseT or Fast Ethernet) • Number of collisions when too many stations are connected to the same segment • The situation is similar in other LAN technologies

4. Figure 16.2Repeater

5. Note: A repeater connects segments of a LAN.

6. Note: A repeater forwards every frame bit-by-bit; it has no packet queues, no filtering capability and no collision detection.

7. Figure 16.3Function of a repeater A repeater is a regenerator

8. Hubs A hub is a multiport repeater used in 10BaseT and Fast Ethernet Hubs give a possibility to have a physical star topology but logical bus topology.

9. Hub’s Limitations • Hubs and repeaters resolve the problem with the distance, but does not resolve the problem with collisions. • A hub network can have lower throughput than several separate networks. • The maximum througput of the three separate networks = 3x10Mbps • The throughput of the connected network = 10Mbps

10. Bridges – A Simple Example • The frame from H1 to H4 is forwarded by the bridge • The frame from H1 to H3 is dropped by the bridge H1 H2 H3 LAN1 H6 H5 H4 P2 B1 P1 LAN2 Traffic within the same group Traffic between the two groups

11. Note: A bridge has a table used in filtering decisions.

12. Figure 16.5Bridge

13. Figure 16.6Learning bridge

14. Figure 16.7Loop problem

15. Cycles in Bridged Network 2. B1 and B2 forward the frame, F1 and F2 are generated 1. host writes frame F to destination which is unknown for B1 and B2 3. B2 receives F1, B1 receives F2 F B1 B2 B1 B2 B1 B2 F2 F1 F1 F2 4. B1 and B2 forward the frames F1 and F2 5. The situation in 3. is repeated and the frames are sent back 6. The frames can circulate in the network for ever F2 F1 F1 F2 B1 B2 B1 B2 B1 B2 F1 F2

16. Figure 16.10Forwarding ports and blocking ports Dotted lines = blocking (non-active redundant) ports. May be used if one of the other bridges or links fails. Continuous black lines = forwarding (active) ports. These constitute a spanning tree (ett spännande träd) without loops.

17. Spanning Tree Algorithm – Definitions • Root Path Cost: For each bridge, the cost of the min-cost path to the root. Costs are assigned to each port or hop count is used, based on for example bandwith, delay or number of hops (1 per port). • Each bridge is assigned a unique identifier: Bridge ID • If not assigned, the lowest MAC addresses of all ports is used as the bridge ID. • Low ID number means high priority. • Each port within a bridge has a unique identifier (port ID). Typically the MAC address of the port is used.

18. The Spanning Tree Algorithm • Elect the root bridge. (The bridge with lowest ID.) • Choose a root port for every bridge. (For lowest cost to the root bridge.) • Chose one designated bridge for each LAN, for minimum cost between the LAN and the root bridge. Mark the corresponding port as a designated port. • If two bridges have the same cost, select the one with lowest ID. • If the min-cost bridge has two or more ports on the LAN, select the port with the lowest identifier • Mark the root ports and designated ports as forwarding (active) ports, the others as blocking (non-active) ports.

19. Figure 16.9Applying spanning tree Root ports: Minimum one star.Designated ports: Two stars. The other ports are blocking ports.

20. 1 B1 2 1 4 3 B2 Spanning Tree - Example The corresponding graph The network B1 • Networks are graph nodes, ports are graph edges • A spanning tree is a connected graph which has no loops (cycles) • The dotted links are the blocked ports on the bridge, in order to prevent loops and duplicated frames Network 1 Network 2 Network 4 Network 3 B2

21. Another example B8 Cost for each port is 1 (hop-count) B3 B5 B7 B2 B1 B6 B4

22. The Root Bridge and the Spanning Tree ** B8 * ** Spanning Tree: B3 ** * B5 B1 ** ** * B7 B2 * * B2 B4 B5 B7 ** ** ** B1 ** ** Root B8 * * B6 ** A spanning tree is a connected graph which has no loops (cycles) B4 **

23. Multiple LANs with Bridges with Costs Assigned L1 4 4 6 LAN 1 B1 B5 B6 2 1 Cost=4 5 B1 Cost=6 LAN 2 Cost=2 L2 L3 B6 3 2 Cost=4 Cost=5 B3 6 Cost=6 Cost=2 B5 6 B3 B2 B4 LAN 3 Cost=1 B2 Cost=3 4 5 Cost=4 L4 Cost=6 The cost of sending from L1 to L4 via B1 and B2 is 6 Only costs for going from a bridge to a LAN are added B4 Cost=5 LAN 4

24. L1 4 4 6 B1 B5 B6 2 1 5 3 L2 2 L3 B3 6 6 B4 B2 4 5 L4 Example: Root Bridge and Root Ports Root • Lowest cost from each bridge to the root bridge are calculated. • The root bridge and root ports are marked in red Cost=3 Cost=6 Cost=2 Cost=8 Cost=6

25. L1 4 4 6 B1 B5 B6 2 1 5 3 L2 2 L3 B3 6 6 B4 B2 4 5 L4 Example: Designated Ports and the Spanning Tree * * Root L1 L2 • Lowest cost from each LAN to the root bridge are calculated (= the cost from an adjacent bridge.) • The designated ports are marked “*”. Cost=3 Cost=6 * * Cost=2 L3 Cost=8 Cost=6 L4 *

26. Example: Designated Ports and the Spanning Tree L1 The rest of the ports areblocked. This results in a spanning tree. 4 B1 B5 B6 2 3 L2 2 L3 B3 6 B4 B2 4 L4

27. Figure 16.13Connecting remote LANs

28. LAN Switches H2 H3 H1 • LAN switching provides dedicated, collision-free communication between network devices, withsupport for multiple simultaneous conversations. • LAN switches are designed to switch data frames at high speeds. • LAN switches can interconnect a 10-Mbps and a 100-Mbps Ethernet LAN. H1 H3 H2

29. A LAN Switch • The computer has a segment to itself – the segment is busy only when a frame is being transfered to or from the computer • As a result, as many as one-half of the computers connected to a switch can send data at the same time

30. Figure 16.12Star backbone

31. 16.3 Virtual LANs Membership Configuration IEEE Standard Advantages

32. Figure 16.15A switch using VLAN software

33. Note: VLANs create broadcast domains.

34. Figure 16.16Two switches in a backbone using VLAN software

35. Chapter 18 Virtual Circuit Switching:Frame Relayand ATM

36. Two Approaches to Packet Switching • Datagram networks (For example IP) • Analogous to the postal service • The inteligence is in the end devices (computers), the network should not be trusted • Each packet carries the destination address • Destination addresses are global internationally • Virtual circuit networks (For example X.25, Frame Relay and ATM) • Analogous to the telephone service • The network should take all the responsibility, the end devices should be as simple as posible • The path that the packets follow is determined at the beginning of the transmission, but store and forward switching is used.

37. Characteristics of WANs Virtual Circuit Circuit Datagram Dedicated path No dedicated path No dedicated path Continuous data Packets Packets transmission No data storage Store and forward Store and forward Connection Route established Route established established for for every packet for every packet entire conversation Call setup delay; Packet transmission Call setup delay; low transmission delay Packet transmission delay delay Busy signal Possible notification Notification of of no/bad deliveries connection denial Blocking at network Delay at network Blocking/delay at overload overload network overload Fixed bandwidth Dynamic bandwidth Dynamic bandwidth No overhead/data Overhead/packet Overhead/packet

38. Figure 18.1Virtual circuit wide area network

39. Figure 18.3VCI phases

40. Virtual Circuit Network • Three Phases • Setup phase • Network protocol establishes a logical path called virtual circuit (VC). The path remains the same during transmission (all packets use it) • Data transfer phase • Each packet carries “tag” or “label” (virtual circuit id, VCI), which determines next hop (the link to which the packet should be forwarded). • At each node, the forwarding is done by inspecting the input line, the VCI and consulting the forwarding table at the switches. • Teardown phase • All switches remove the entries about the VCI from their tables

41. Figure 18.2VCI

42. Figure 18.4Switch and table

43. X.25 Networks • Developed in 1970s in European countries under the auspices of ITU • Public packet-switched networks • Uses virtual circuit connections • Switched virtual circuits – analog to dial-up in circuit switching • Permanent virtual circuits – analog to leased lines in circuit switching. • Operates on the three lowest layers (physical, data-link and network layer) • Performs error-contol and flow-control on the node-to-node basis • Work at speed up to 64Kbps • Nowadays it is obsolete

44. Frame Relay • X.25 data rates were not stisfactory for users looking for higher data rates and lower costs • Checking frames for error at every node is inefficient • Only one fourth of traffic is message traffic, the rest is overhead (necessary for transmission media that are more error prone) • Frame relay – public data network that have improved performance • Developed having in mind new transmission media that have much lower probability of error • Does not provide error checking and acknowledgement at both, the data-link layer and the network layer

45. Data Data Data Data Data Data Data Data Frame ack Frame ack Frame ack Frame ack Ack Ack Ack Ack X.25 versus Frame Relay switch switch switch X.25 traffic (ACKs at both data-link and transport layer) Frame relay traffic (ACKs are required at the transport layer only)

46. Frame Relay in the Internet • The virtual circuits in frame-relay are called DLCI (Data Link Connection Identifier)

47. Figure 18.8Frame Relay network

48. Note: Frame Relay operates only at the physical and data link layers.

49. Note: Frame Relay does not provide flow or error control; they must be provided by the upper-layer protocols.

50. ATM – Basic Idea • Uses small fixed-size packets called cells • The cells are 53 bytes long (48 bytes payload + 5 bytes header) • The length of the cell compromise between American and European telephone companies (average of 32 and 64) • Uses packet switching • Connection oriented (uses virtual circuits) • Speeds of 155 Mbps or 622 Mbps are achieved over SONET • Was heavily promoted by telephone companies as BISDN (Broadband Integrated Services Digital Network) technology.