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.mh.se/~mageri/animations/netbook/anim06_2-csma.mov • www.itm.mh.se/~mageri/animations/bjnil/anim1long.exe Animering som illustrerar kollisionshantering i CSMA/CD: • www.itm.mh.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. 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: Hybrid av många system? Heterogen täckning? COFDM-modulation? Dynamisk kanalallokering? 2010-talet?

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

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

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

18. Figure 13.14Chip sequences

19. Figure 13.15Encoding rules

20. Figure 13.16CDMA multiplexer

21. Figure 13.17CDMA demultiplexer

22. Chapter 14 Local Area Networks:Ethernet

23. 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

24. Figure 14.1Three generations of Ethernet

25. 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)

26. 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

27. Figure 14.2802.3 MAC frame

28. Figure 14.3Minimum and maximum length

29. 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

30. 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

31. Figure 14.5Unicast and multicast addresses

32. Figure 14.10Categories of traditional Ethernet

33. 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

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

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

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

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

38. 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.

39. Figure 14.15Sharing bandwidth

40. Figure 14.16A network with and without a bridge

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

42. Ethernet Evolution • Introducing bridges • Unlike a hub, a bridge is capable of filtering frames • Each port of the bridge is connected to a single segment of LAN • Capable of learning which the stations are connected to which ports • Separates collision domains and therefore increases bandwidth • Introducing switches • Similar function as bridges • Contain bigger number of ports • A single device can be attached to a port

43. Figure 14.18Switched Ethernet

44. Figure 14.19Full-duplex switched Ethernet

45. Bridge F E A B C D Switch Bridged vs. Switched Ethernet

46. 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.

47. 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

48. 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