Computer Networks 1 (Mạng Máy Tính 1) Lectured by: Dr. Phạm Trần Vũ.
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Transcript of Computer Networks 1 (Mạng Máy Tính 1) Lectured by: Dr. Phạm Trần Vũ.
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.
Faculty of Computer Science and Engineering - HCMUT
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
Faculty of Computer Science and Engineering - HCMUT
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)
Faculty of Computer Science and Engineering - HCMUT
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