Computer Networks 1

(Mạng Máy Tính 1)

Lectured by: Dr. Phạm Trần Vũ

Lecture 3: Networking Technologies

Reference:

Chapter 4 - “Computer Networks”,

Andrew S. Tanenbaum, 4th Edition, Prentice Hall, 2003.

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Content

Channel Allocation Problem

Multiple Access Protocols

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Two types of transmission technology

Point-to-Point Link Broadcast Link

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Channel AllocationProblem

Who's first?

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Channel AllocationProblem

Static Method Dynamic Method

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Static Channel Allocation in LANs and MANs

KEY: Each user is assigned with a equal-portion of the bandwidth

Plus:1. Simple2. No interference

Minus:1. Ineffective bandwidth utilization 2. Some users will be denied in high traffic

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Dynamic Channel Allocation in LANs and MANs

1. Station Model2. Single Channel Assumptions3. Collision Assumption4. Continuous Time vs Slotted Time5. Carrier Sense vs No Carrier Sense

Key assumptions

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Multiple Access Protocols

ALOHA

Carrier Sense Multiple Access

Protocols

Collision-Free Protocols

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ALOHA

Idea: Users can transmit whenever they have data to send

Two types of ALOHA:Pure ALOHASlotted ALOHA

The main difference between them is time and time and time

Aloha!!

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Pure ALOHA

continuous timeno global time synchronization

frame

Send & then wait for collision

collision?

end

no

wait a random time

yes

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Pure ALOHA – vulnerableperiod

t – frame time, to – starting time

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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

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Slotted ALOHA – who's next?

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Channel utilization

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CSMA with Collision Detection (CSMA/CD)

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

How?

But: half-duplex

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Propagation Delay & CD

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Collision-free ProtocolsThe 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

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Collision-Free ProtocolsThe 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

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The Adaptive Tree Walk Protocol

Distributed Depth-first tree walk

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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)

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Wavelength Division MA Protocols (WDMA)

To allow multiple transmissions the spectrum is divided into channels (wavelength bands)

Total internal reflection:

n1>n2i1>ilimit

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WDMA

Each station is assigned two channels: control & data and two transmitters and two receivers1. 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.

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WDMA

Require global time synchronization for both channels

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Wireless LAN Protocols

CSMA is not appropriate

Interference at the receiver, not at the sender

Hidden station problem

Exposed station problem

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Wireless LAN Protocols - MACA

A sends RTS to B and begins transmission only after receiving CTS reply from B

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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.

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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

Ethernet Cabling

The most common kinds of Ethernet cabling.

31

Ethernet Cabling (2)

Three kinds of Ethernet cabling.

(a) 10Base5, (b) 10Base2, (c) 10Base-T.

32

Ethernet Cabling (3)

Cable topologies. (a) Linear, (b) Spine, (c)

Tree, (d) Segmented.

33

Using voltage to encode binary code: 0 volts ~ 0 bit and 5 volts ~ 1 bit

0001000 = 10000000 or 01000000,

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

Manchester Encoding

34

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

Manchester Encoding

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 802.5 token ring)

All Ethernet systems use Manchester encoding

due to simplicity.(+ 0.85 volts; - 0.85 volts)

36

Manchester Encoding

(a) Binary encoding, (b) Manchester encoding,

(c) Differential Manchester encoding.

37

Ethernet MAC Sublayer Protocol

http://en.wikipedia.org/wiki/MAC_address

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)

42

Switched Ethernet

A simple example of switched Ethernet.

43

Fast Ethernet

802.3u vs 802.12

802.3u: backward compatible unforeseen problems technology change period

Ideas: keep all the old stuffs

reduce the bit time from 100 nsec to 10 nsec

44

Fast Ethernet

The original fast Ethernet cabling.

45

Gigabit Ethernet

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

46

Gigabit Ethernet (2)

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 802.11 Frame Structure

The 802.11 data frame.

51

802.11 Services

• Association

• Disassociation

• Reassociation

• Distribution

• Integration

Distribution Services

52

802.11 Services

• Authentication

• Deauthentication

• Privacy

• Data Delivery

Intracell Services