computer networks 1 (mạng máy tính 1) lectured by: dr. phạm trần vũ
TRANSCRIPT
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Computer Networks 1
(Mạng Máy Tính 1)
Lectured by: Dr. Phạm Trần Vũ
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Lecture 3: Networking Technologies
Reference:
Chapter 4 - “Computer Networks”,
Andrew S. Tanenbaum, 4th Edition, Prentice Hall, 2003.
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Faculty of Computer Science and Engineering - HCMUT
Content
Channel Allocation Problem
Multiple Access Protocols
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Faculty of Computer Science and Engineering - HCMUT
Two types of transmission technology
Point-to-Point Link Broadcast Link
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Faculty of Computer Science and Engineering - HCMUT
Channel AllocationProblem
Who's first?
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Faculty of Computer Science and Engineering - HCMUT
Channel AllocationProblem
Static Method Dynamic Method
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Faculty of Computer Science and Engineering - HCMUT
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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Faculty of Computer Science and Engineering - HCMUT
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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Faculty of Computer Science and Engineering - HCMUT
Multiple Access Protocols
ALOHA
Carrier Sense Multiple Access
Protocols
Collision-Free Protocols
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Faculty of Computer Science and Engineering - HCMUT
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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Faculty of Computer Science and Engineering - HCMUT
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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Faculty of Computer Science and Engineering - HCMUT
Pure ALOHA – vulnerableperiod
t – frame time, to – starting time
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Faculty of Computer Science and Engineering - HCMUT
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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Faculty of Computer Science and Engineering - HCMUT
Slotted ALOHA – who's next?
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Faculty of Computer Science and Engineering - HCMUT
Channel utilization
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Faculty of Computer Science and Engineering - HCMUT
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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Faculty of Computer Science and Engineering - HCMUT
Propagation Delay & CD
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Faculty of Computer Science and Engineering - HCMUT
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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Faculty of Computer Science and Engineering - HCMUT
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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Faculty of Computer Science and Engineering - HCMUT
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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Faculty of Computer Science and Engineering - HCMUT
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
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Ethernet
Ethernet Cabling
Manchester Encoding
The Ethernet MAC Sublayer Protocol
Switched Ethernet
Fast Ethernet
Gigabit Ethernet
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Ethernet Cabling
The most common kinds of Ethernet cabling.
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Ethernet Cabling (2)
Three kinds of Ethernet cabling.
(a) 10Base5, (b) 10Base2, (c) 10Base-T.
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Ethernet Cabling (3)
Cable topologies. (a) Linear, (b) Spine, (c)
Tree, (d) Segmented.
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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
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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
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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)
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Manchester Encoding
(a) Binary encoding, (b) Manchester encoding,
(c) Differential Manchester encoding.
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Ethernet MAC Sublayer Protocol
http://en.wikipedia.org/wiki/MAC_address
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Ethernet MAC Sublayer Protocol
Unicast: 1 receiver
Multicast: group of receivers
Broadcast: all
281,474,976,710,656 possible MAC addresses
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802.3 Ethernet frame structure
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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)
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Switched Ethernet
A simple example of switched Ethernet.
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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
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Fast Ethernet
The original fast Ethernet cabling.
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Gigabit Ethernet
(a) A two-station Ethernet. (b) A multistation Ethernet.
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Gigabit Ethernet (2)
Gigabit Ethernet cabling.
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Wireless LAN Protocols
Hidden station problem
Exposed station problem
A wireless LAN. (a) A transmitting B. (b) B
transmitting A.
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The 802.11 MAC Sublayer
Protocol (1)
The use of virtual channel sensing using CSMA/CA
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The 802.11 MAC Sublayer
Protocol (2)
A fragment burst.
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The 802.11 Frame Structure
The 802.11 data frame.
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802.11 Services
• Association
• Disassociation
• Reassociation
• Distribution
• Integration
Distribution Services
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802.11 Services
• Authentication
• Deauthentication
• Privacy
• Data Delivery
Intracell Services