1. Computer Networks 1 (Mạng Máy Tính 1)
Lectured by: Dr. Phạm Trần Vũ

2. Lecture 3: Networking Technologies
Reference:
Chapter 4 - “Computer Networks”, Andrew S. Tanenbaum, 4th Edition, Prentice Hall, 2003.

3. Channel Allocation Problem Multiple Access Protocols
Content
Channel Allocation Problem
Multiple Access Protocols

4. Two types of transmission technology
Point-to-Point Link
Broadcast Link

5. Channel Allocation Problem
Who's first?

6. Channel Allocation Problem
Static Method
Dynamic Method

7. Static Channel Allocation in LANs and MANs
KEY: Each user is assigned with a
equal-portion of the bandwidth
Minus:
1. Ineffective bandwidth utilization
2. Some users will be denied in high traffic
Plus:
1. Simple
2. No interference

8. Dynamic Channel Allocation in LANs and MANs
Key assumptions
1. Station Model
2. Single Channel Assumptions
3. Collision Assumption
4. Continuous Time vs Slotted Time
5. Carrier Sense vs No Carrier Sense

9. Multiple Access Protocols
ALOHA
Carrier Sense Multiple Access Protocols
Collision-Free Protocols

10. ALOHA Idea: Users can transmit whenever they have data to send Aloha!!
Two types of ALOHA:
Pure ALOHA
Slotted ALOHA
The main difference between them is time and time and time

11. Pure ALOHA continuous time no global time synchronization frame
Send & then wait
for collision
collision?
end no
wait a random
time
yes
continuous time
no global time synchronization

12. Pure ALOHA – vulnerable period
t – frame time, to – starting time

13. Slotted ALOHA 1. Time is divided to equal intervals (slots)
2. Need time synchronization
3. Frames can only be transmitted at starts of time slots

14. Slotted ALOHA – who's next?

15. Channel utilization

16. CSMA with Collision Detection (CSMA/CD)
How?
Idea: A station stop transmitting its frame immediately after a collision is detected to save time and bandwidth
This is the basis of Ethernet LAN
But: half-duplex

17. Propagation Delay & CD

18. Collision-free Protocols The basic bit-map control
Idea: At the contention period, all station announce their needs to transmit. And then at the transmission period all registered stations take turn to send their frames.
Drawback: suffer n bits delay in contention period

19. Collision-Free Protocols The binary countdown protocol
Unique binary station addresses with the same length
Rules:
Compete by “OR” with other bits in the same order
If there is a “0 to 1 transformation” in the result → give up

20. The Adaptive Tree Walk Protocol
Distributed Depth-first tree walk

21. The Adaptive Tree Walk Protocol - improvement
At heavy traffic, skip nodes father nodes 1,2,3,...
Number of nodes at level i: 2^i
Assume that, p is number of ready stations. Then k = p/2^i is number of ready stations per node at level i
Optimal: k = 1 or p/2^i= 1 from this
i = log2(p)

22. Wavelength Division MA Protocols (WDMA)
To allow multiple transmissions the spectrum is divided into channels (wavelength bands)
Total internal reflection:
n1>n2
i1>ilimit

23. WDMA
Each station is assigned two channels: control & data and two transmitters and two receivers
1. A fixed-wavelength receiver for listening to its own control channel.
2. A fixed-wavelength transmitter for outputting data frames.
3. A tunable transmitter for sending on other stations' control channels.
4. A tunable receiver for selecting a data transmitter to listen to.

24. WDMA
Require global time synchronization for both channels

25. Wireless LAN Protocols
CSMA is not appropriate
Interference at the receiver, not at the sender
Hidden station problem
Exposed station problem

26. Wireless LAN Protocols - MACA
A sends RTS to B and begins transmission only after receiving CTS reply from B

27. Wireless LAN Protocols - MACA
1. C doesn't see CTS and thus is free to send its RTS
2. C and B may want to send RTS at the same time so cause collision.

28. Wireless LAN Protocols - MACAW
Improvements:
1. ACK frame after each successful data frame
2. Use CSMA for RTS frame

29. Ethernet Ethernet Cabling Manchester Encoding
The Ethernet MAC Sublayer Protocol
Switched Ethernet
Fast Ethernet
Gigabit Ethernet

30. The most common kinds of Ethernet cabling.

31. Ethernet Cabling (2) Three kinds of Ethernet cabling.
(a) 10Base5, (b) 10Base2, (c) 10Base-T.

32. Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.
Ethernet Cabling (3)
Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.

33. Manchester Encoding Using voltage to encode binary code:
0 volts ~ 0 bit and 5 volts ~ 1 bit = or , because the difference between an idle sender (0 volts) and a 0 bit (0 volts) is ambiguous
-1 and +1? out of synchronization, long sequence of 0 or 1

34. Manchester Encoding Using two voltages to encode one bit:
1 ~ high – low; 0 ~ low - high 010 = (-1,1)(1,-1)(-1,1)
Plus: every bit has a transition in the middle → determine bit boundary & sync
Minus: double bandwidth, 10Mbs ~ 20 mil changes per second

35. Differential Manchester encoding
1 bit ~ absence;0 bit ~ presence of a transition at the start of the interval. In both cases, there is a transition in the middle.
Plus: complex equipment
Minus: better noise immunity. (the token ring)
All Ethernet systems use Manchester encoding due to simplicity.( volts; volts)

36. Manchester Encoding
(a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding.

37. Ethernet MAC Sublayer Protocol

38. Ethernet MAC Sublayer Protocol
Unicast: 1 receiver
Multicast: group of receivers
Broadcast: all
281,474,976,710,656 possible MAC addresses

39. 802.3 Ethernet frame structure

40. 802.3 Ethernet frame structure
Header: source and destination MAC address; Ethertype protocol identifier field; optional IEEE 802.1Q VLAN tag
Frame check sequence: 32-bit CRC (cyclic redundancy check)

41. In addition to there being a maximum frame length, there is also a minimum frame length. While a data field of 0 bytes is sometimes useful, it causes a problem. When a transceiver detects a collision, it truncates the current frame, which means that stray bits and pieces of frames appear on the cable all the time. To make it easier to distinguish valid frames from garbage, Ethernet requires that valid frames must be at least 64 bytes long, from destination address to checksum, including both. If the data portion of a frame is less than 46 bytes, the Pad field is used to fill out the frame to the minimum size.

42. A simple example of switched Ethernet.

43. Fast Ethernet 802.3u vs 802.12 802.3u: Ideas: backward compatible
unforeseen problems
technology change period
Ideas:
keep all the old stuffs
reduce the bit time from 100 nsec to 10 nsec

44. The original fast Ethernet cabling.

45. Gigabit Ethernet
(a) A two-station Ethernet. (b) A multistation Ethernet.

46. Gigabit Ethernet cabling.

47. Wireless LAN Protocols
Hidden station problem
Exposed station problem
A wireless LAN. (a) A transmitting B. (b) B transmitting A.

48. The 802.11 MAC Sublayer Protocol (1)
The use of virtual channel sensing using CSMA/CA

49. The 802.11 MAC Sublayer Protocol (2)
A fragment burst.

50. The Frame Structure
The data frame.

51. 802.11 Services Distribution Services Association Disassociation
Reassociation
Distribution
Integration

52. 802.11 Services Intracell Services Authentication Deauthentication
Privacy
Data Delivery