Datornätverk A – lektion 10

1. Datornätverk A – lektion 10 Kapitel 13: Multiple access control. Local Are Networks. (CSMA/CD,Token Bus, Token Ring, Logical Link Control) Kapitel 14: Ethernet (Kapitel 15: Wireless LANs översiktligt.)

2. Chapter 13 MultipleAccess

3. Figure 13.1Multiple-access protocols

4. Figure 13.2Evolution of random-access methods

5. Evolution of Contention Protocols Aloha • Developed in 1970 to be used on radio LAN on Hawaiian islands. The access to the channel is random • Improvement to Aloha: Start transmission only at fixed time slots • Carrier Sense Multiple Access: Start transmission only if no transmission is ongoing • CD=Collision Detection: Stop ongoing transmission if collision is detected SlottedAloha CSMA CSMA/CD

6. Figure 13.5Collision in CSMA

7. Animeringar Animeringar som illustrerar tystnadsdetektering i CSMA: • www.itm.miun.se/~mageri/animations/netbook/anim06_2-csma.mov • www.itm.miun.se/~mageri/animations/bjnil/anim1long.exe Animering som illustrerar kollisionshantering i CSMA/CD: • www.itm.miun.se/~mageri/animations/bjnil/anim1.exe

8. Figure 13.6Persistence strategies

9. 13.7CSMA/CD procedure

10. Figure 13.8CSMA/CA procedure

11. 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

12. 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

13. 13.3 Channelization FDMA TDMA CDMA

14. Note: In FDMA, the bandwidth is divided into channels.

15. Note: In TDMA, the bandwidth is just one channel that is timeshared.

16. Note: In CDMA, one channel carries all transmissions simultaneously.

17. Figure 13.14Chip sequences

18. Figure 13.15Encoding rules

19. Figure 13.16CDMA multiplexer

20. Figure 13.17CDMA demultiplexer

21. Trådlös transmission

22. Mobiltelesystemens generationer • 1G: Analog modulation – FDMA. T.ex. NMT. 80-talet. • 2G: Digital modulation, TDMA + FDMA. T.ex. GSM. 90-talet. • 2.5G: GPRS, dvs paketförmedling. • 3G: Edge (8PSK) eller WCDMA (spread spectrum). 2000-talet. • 3.5G: All-IP-infrastructure, inkl IP-telefoni istället för kretskopplad telefoni? Asymmetrisk. HSDPA. • 4G: OFDMA (OFDM-baserad) nedlänk. 2010-talet • 5G: Massive MIMO. Gigahertzfrekvenser.

23. Nästa generations mobilsystem? Samverkan mellan olika system

24. Spektrum • Gråmarkerade frekvenser är i huvudsak upptagna • Högre frekvenser ger dyr utrustning/kort räckvidd Radiovågor Mikrovågor IR UV Röntgen Synligt ljus Mobiltelefoni

25. Våglängd och frekvens Ju högre frekvens desto kortare våglängd.

26. Vågutbredning av radio- och mikrovågor • Exempel: Radio-LAN använder ofta frekvensen 2.4GHz,dvs våglängden 300/2400 =0.125m. • Radioskugga kan uppstå bakom föremål med storlek några våglängder (några dm i vårt exempel). • Radiovågor dämpas kraftigt av metallnät, t.ex. armeringsjärn, med mindre hål än en halv våglängd (ca 6 cm i vårt fall). Metallnätet utgör då Faradays bur. • Avståndsberoende dämpning. I vakuum avtar signalen kvadratiskt med avståndet, dvs 6 dB dämpning per dubblering av avståndet. I stadsbebyggelse är dämpningen ca 9 – 12 dB per dubblering av avståndet.

27. Störningar vid trådlös kommunikation • Brus och elektriska störningar: • ”Gaussiskt vitt brus”, innebär att en gaussiskt fördelad (dvs normalfördelad) slumpmässig spänning som innehåller läggs till signalen. Dess spektrum har lika stark energi vid alla frekvenser. • Samkanalsstörningar (co-channel interference) • Långsam skuggfädning: • Log-normal fördelning av dämpningen. • Flervägsutbredning ger upphov till: • Ekon och tidsspridning av signalen, vilket ger inter-symbol-interferens (ISI) • Snabb fädning. Denna kan vara flat eller frekvensselektiv dämpning. Vid frekvensselektiv dämpning blir symbolen distorderad. • Rayleigh-fördelad om direktvåg saknas • Rician-fördelad dämpning vid line-of-sight. • Fasvridning. • Skurfel • Dopplerskift • M.m

28. Diversitet • Tidsdiversitet genom bit-interleaving (omkastning av bitarna i tid, så att inte skurfel drabbar samma paket) • Rumsdiversitet (flera antenner) • Frekvensdiversitet (frekvenshopp, spread spectrum eller COFDM dvs många smalbandiga bärvågor)

29. Förenklad modell av dämpningen

30. Mobiltelefoni • Cell = täckningsområde för en basstationsantenn. • En basstationssite har ofta tre antennriktningar, dvs tre celler. • Handover = byte av cell eller kanal under samtalet • Roaming = byte av trafikområde i väntan på samtal. • Paging = sökning av mobil över hela trafikområdet vid inkommande samtal.

31. Dynamic resource management with channel reuse factor 1 Channel 1 Channel 2 Channel 3 Channel 4 Radio resource management Traditional static handover Example: Channel reuse factor 4 Channel 1 Channel 2 Channel 3 Channel 4

32. 3 3 2 2 1 Återanvändningav kanaler • Dämpningen möjliggör återanvändning av kanaler • Fler celler som täcker samma yta ger högre ytkapacitet [Mobiler / km2] 3 3 3 Celler med samma siffra använder samma kanaler. I figuren är antalet kanalgrupper tre. 2 2 1 1 1 2 1

33. Channel 2 Channel 1 Channel 3 Handover map Fixed Channel Allocation with static handover Cellerna definieras av handovergränserna, och är (i teorin) hexagonala.

34. Chapter 14 Local Area Networks:Ethernet

35. Local Area Networks (LANs) • A computer network in a limited geographical area, a single building or several close to each other buildings • LANs are privately owned and built by the companies • Generally less expensive than WAN for comparable speed • LAN technologies use multiple access channels • Ethernet is the most common LAN technology

36. Figure 14.1Three generations of Ethernet

37. 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)

38. 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

39. Figure 14.2802.3 MAC frame

40. Figure 14.3Minimum and maximum length

41. Network Interface Card (NIC) NIC for a desktop • Each device on Ethernet network has its own interface card (NIC) to connect to the network • The NIC is usually plugged into the device and has a 6 bytes (48 bits) physical address • The physical address is normally written in hexadecimal notation • 02-11-02-2C-4D-1B (example address) NIC for a laptop

42. Ethernet Addressing • Each station recognizes three classes ofaddresses. • Own address • Broadcast address (all 1's) • Optionally, one or more multicast addresses • Major reason for broadcast is address discovery. Brodcast Ethernet address is all 1s, or in hexadecimal • FF : FF : FF : FF : FF :FF • Multicast addresses are used for specialized link • layer functions. • Ethernet addresses are unique • First three bytes assigned to manufacturer by IEEE, the other three bytes assigned by the manufacturer

43. Figure 14.5Unicast and multicast addresses

44. Figure 14.10Categories of traditional Ethernet

45. 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

46. Figure 14.11Connection of a station to the medium using 10Base5

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

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

49. Figure 14.13Connection of stations to the medium using 10Base-T

50. Hub Concept • Separate transmit and receive pair of wires. • The hub retransmits the signal received on any input pair onto all output pairs. • Essentially the hub emulates a broadcastchannel with collisions detected by receiving nodes.