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
Multiple Access

3. Figure 13.1 Multiple-access protocols

4. Figure 13.2 Evolution 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
Slotted Aloha
CSMA
CSMA/CD

6. Figure 13.5 Collision in CSMA

7. Animeringar Animeringar som illustrerar tystnadsdetektering i CSMA:
Animering som illustrerar kollisionshantering i CSMA/CD:

8. Figure 13.6 Persistence strategies

9. CSMA/CD procedure

10. Figure 13.8 CSMA/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. 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) 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. In FDMA, the bandwidth is divided into channels.
Note:
In FDMA, the bandwidth is divided into channels.

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

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

18. Figure Chip sequences

19. Figure Encoding rules

20. Figure 13.16 CDMA multiplexer

21. Figure 13.17 CDMA demultiplexer

22. Local Area Networks: Ethernet
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 14.1 Three 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 and ANSI 8802/3.
Structure of Ethernet frame
(Length)

26. Structure of Ethernet Frame
Preamble:
7 bytes with pattern followed by one byte with pattern
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 MAC frame

28. Figure 14.3 Minimum 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
C-4D-1B (example address)
NIC for a laptop

30. Ethernet Addressing
Each station recognizes three classes of addresses.
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 14.5 Unicast and multicast addresses

32. Figure 14.10 Categories 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 (10base a) - obsolete
RG58 coaxial cable
185 meter max length, bus physical topology
Twisted Pair Ethernet (10baseT i)
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 14.11 Connection of a station to the medium using 10Base5

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

36. Reflektioner Animering:Se

37. Figure 14.13 Connection 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 broadcast channel with collisions detected by receiving nodes.

39. Figure 14.15 Sharing bandwidth

40. Figure 14.16 A network with and without a bridge

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

42. Ethernet Evolution Introducing bridges Introducing switches
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 14.18 Switched Ethernet

44. Figure 14.19 Full-duplex switched Ethernet

45. Bridged vs. Switched EthernetA B C D E F
Switch

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