Multimedie- och kommunikationssystem, lektion 9

1. Kapitel 8: LAN. Multiple-access. CSMA/CD. The spanning tree algorithm. Multimedie- och kommunikationssystem, lektion 9

2. MultipleAccess

3. Figure 13.1Multiple-access protocols

4. Evolution of random access protocols • Aloha • “Try and error”. Developed in 1970 to be used on radio- LAN on Hawaiian islands. The access to the channel is random. • Slotted Aloha • Improvement to Aloha: Start transmission only at fixed time slots • Carrier Sense Multiple Access (CSMA) • Start transmission only if no transmission is ongoing • CSMA/CA=Collision Avoidance • Used in today’s WLAN:s • CSMA/CD=Collision Detection • Stop ongoing transmission if collision is detected • Used in the Ethernet protocol

5. Figure 13.5Collision in CSMA

6. Animeringar Animeringar som illustrerar tystnadsdetektering i CSMA: • www.itm.mh.se/~mageri/animations/netbook/anim06_2-csma.mov • www.itm.mh.se/~mageri/animations/bjnil/anim1long.exe

7. Figure 13.8CSMA/CA procedure

8. CSMA/CD • Sense for carrier. • If carrier present, wait until carrier ends. • Send packet and sense for collision. • If no collision detected, consider packet delivered. • Otherwise, abort immediately, perform “exponential back off” and send packet again. • CSMA/CD is used in traditional Ethernet LAN Animering som illustrerar kollisionshantering i CSMA/CD: • www.itm.mh.se/~mageri/animations/bjnil/anim1.exe

9. Figure 8.4 CSMA/CD MAC sublayer operation: (a) transmit;

10. Exponential Back-off • When a sender detects a collision, it sends a “jam signal”. • Jam signal is necessary to make sure that all nodes are aware of the collision • Length of the jam signal 48 bits • When collision is detected, the sender resends the signal after a random time • The random time is picked from an interval of 0 to 2N x maximum propagation time • N is the number of attempted retransmission • Length of the interval increases with every retransmission

11. Figure 8.4 CSMA/CD MAC sublayer operation: (b) Receive.

12. Figure 8.31 LAN protocols: (a) protocol framework;

13. IEEE standards for LANs and similar technologies. IEEE 802.1 Station management 802.1d Transparent bridges 802.2 Logical link control (LLC) IEEE 802.3 CSMA/CD (Ethernet) bus IEEE 802.3u Fast Ethernet IEEE 802.3x Hop-by-hop switch flow control IEEE 802.3z Gigabit Ethernet IEEE 802.5 Token ring IEEE 802.11 Wireless LANs IEEE 802.15 Wireless Personal Area Networks (PANs) IEEE 802.16 Broadband Wireless Access (”WiMAX”) IEEE 802.20 Mobile Broadband Wireless Access

14. Traditional Ethernet • Work started back in 1973 by Bob Metcalfe and David Boggs from Xerox Palo Alto Research Center, as an improvement of the ALOHA • Experimental Ethernet implemented in 1975. • Cooperative effort between Digital, Intel, and Xerox produced Ethernet Version 1.0 in 1980. • Ethernet was adopted with modifications by the standards committees IEEE 802.3 and ANSI 8802/3. • Structure of Ethernet frame (Length)

15. Structure of Ethernet Frame • Preamble: • 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 • Used to synchronize receiver, sender clock rates • Addresses: 6 bytes, the frame is received by all adapters on a LAN and dropped if address does not match • Type: 2 bytes, is actually a length field in 802.3 • CRC: 4 bytes, checked at receiver, if error is detected, the frame is simply dropped • Data payload: maximum 1500 bytes, minimum 46 bytes. If data is less than 46 bytes, pad with zeros to 46 bytes

16. Figure 14.2802.3 MAC frame

17. Figure 14.3Minimum and maximum length

18. Figure 14.10Categories of traditional Ethernet

19. Figure 14.12Connection of stations to the medium using 10Base2

20. Reflektioner Animering: Se www.itm.mh.se/~mageri/animations/ledningsreflex/

21. Classic 10Mbps Ethernet • Four different implementation at the physical layer for the baseband 10Mbps Ethernet • Thick Ethernet (10base5) – obsolete • Thick coaxial cable (0.5” diameter) • 500meter max length, bus physical topology • Thin Ethernet (10base2 802.3a) - obsolete • RG58 coaxial cable • 185 meter max length, bus physical topology • Twisted Pair Ethernet (10baseT 802.3i) • 4 pair UTP (unshielded twisted pair) cable • 100 meter max length, star physical topology • Fiber-link Ethernet (10Base-FL) • Fiber cable connected to external transceiver • Star topology is used

22. Fast Ethernet • Go from 10mbit/s to 100mbit/s • 3 competing standards: • 100Base-TX • 100Base-T4 • 100VG-Anylan • 100Base-T4 and 100VG-Anylan are the losers (were not very well accepted). • 100Base TX is the winner. It is almost a standard everywhere.

23. 100Base - TX • 100 Mbps over 2 pairs of wire (just like 10base-T) • Requires Category 5 UTP wiring or STP • De facto standard today • Very small price difference with 10Mbps-only equipment • Has clearly won over 100baseT4 and 100VG-Anylan by now

24. 100Base-FX • Fast Ethernet with fiber optic cables • Uses two optical fibers, one for transmission and one for reception

25. Gigabit Ethernet • Provides speeds of 1000 Mbps (i.e., one billion bits per second capacity) for half-duplex and full-duplex operation. • Uses Ethernet frame format and MAC technology • CSMA/CD access method • Backward compatible with 10Base-T,100Base-T and 100BaseTX • Can be shared (hub) or switched

26. Gigabit Ethernet Implementations • Fiber • 1000 Base – SX • Short wavelengths, two fiber-optic cables • 1000 Base – LX • Long wavelengths, two fiber-optic cables • Copper • 1000 Base – CX • Uses shielded twisted pair copper jumpers • 1000 Base – TX • Uses category 5 twisted pair copper cable

27. 1000Base - T • Four pairs of Category 5 UTP • IEEE 802.3ab ratified in June 1999. • Category 5, 6 and 7 copper up to 100 meters • Uses encoding scheme 4D-PAM5 • Five level of pulse amplitude modulation are used • Complicated technique

28. 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

29. Devices that Extend Local Networks • Physical layer devices (Repeaters and hubs) • MAC layer devices (Bridges and two-layer switches) • Network layer devices (Routers and three layers switches)

30. Figure 16.2Repeater A repeater connects segments of a LAN. A repeater forwards every frame bit-by-bit; it has no packet queues, no filtering capability and no collision detection.

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

32. 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.

33. 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

34. Bridges – A Simple Example • A frame from H1 to H4 is forwarded by the bridge • A frame from H1 to H3 is dropped by the bridge LAN segment 1 H1 H2 H3 H6 H5 H4 P2 B1 P1 LAN segment 2

35. Figure 8.12 Bridge filtering A bridge has a table used for filtering

36. Figure 16.5Bridge A bridge has a table used in filtering decisions

37. Learning (transparent) bridge

38. Figure 8.13 Effect of dual paths on learning algorithm

39. 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.

40. The Spanning Tree Algorithm • Assign costs to each port, based on for example delay, distance, bandwith, or number of hops (1 per port). • Elect a root bridge. (The bridge with lowest ID number.) • Calculate the Root Path Cost for each bridge, i.e. the cost of the min-cost path (the “nearest” path) to the root. • Choose a root port for every bridge for minimum root path cost. • Chose a designated bridge for each LAN, for minimum cost between the LAN and the root bridge. Mark the corresponding port as a designated port. • Mark the root ports and designated ports as forwarding (active) ports, and the others as blocking (non-active) ports.

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

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

43. 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 **

44. Figure 8.14 Active topology derivation example: (a) LAN topology.

45. (b) Root port selection. PC = Port cost. RPC = Root Path Cost. RP = Root Port. Dashed lines are non-root ports.

46. (c) Designated port selection. DPC = Designated Port Cost.

47. (d) Active topology. DP = Designated ports. RP = Root ports. The rest (dashed lines) are non-active

48. Example 8.2: Spanning Tree To illustrate how the various elements of the spanning tree algorithm work, consider the bridged LAN shown in Figure 8.14(a). The unique identifier of each bridge is shown inside the box representing the bridge together with the port numbers in the inner boxes connecting the bridge to each segment. Typically, the additional bridges on each segment are added to improve reliability in the event of a bridge failure. Also, assume that the LAN is just being brought into service, all bridges have equal priority, and all segments have the same designated cost (bit rate) associated with them. Determine the active (spanning tree) topology.

50. Figure 14.17Collision domains in a nonbridged and bridged network