overview - basicincpaper.snu.ac.kr/images/9/97/2017_em_basic1_0908.pdf · 2017-09-08 · fdm...
TRANSCRIPT
Overview- Basic
Chong-kwon Kim
Topics of Today’s Lecture Networking Architecture Protocol
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Communication & Network Communication is exchange of information between
users (stations, nodes) at a distance
Network– A system consists of devices (often referred to as nodes) and
links for transportation of entities– Example: roads, railroads, water
Two types of communication network– Voice– Computer networks
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Connectivity Impossible to connect (large) number of users directly
Share resources (links)– Network is a mechanism to make the connectivity easy by sharing
resources
Sharing mechanisms– Multiplexing– Access control
Requires - O(N^2) links- O(N) accesses/user
s1
sn
sis3
s2
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Simplex/Duplex
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Link Types
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Architecture Divide & Conquer
– To solve a large & complex problem, first partition the problem into small pieces
– Solve each partial problem– Combine sub-solutions into a whole solution
Architecture– A set of sub-functions that comprise a larger function
Abstraction– Shield internal implementation details and show only interfaces
Example– Program modules
M2M1
M4M3
M5
8
Layered Architecture Layered architecture
– Keep the interaction simple
Layer 1
Layer N
Layer n+1
Layer n
Layer n-1
Raw
Abstract
Layer n uses serviceprovided by layer n-1, addsits own functions andprovide more abstract service to layer n+1
Q: What functions layer 1provides? And Layer N?
Overview- Protocol
Chong-kwon Kim
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Protocol Communications involve with two or more devices Suppose A and B communicate each other
– Should A and B use the same program?– If A and B use Windows and Linux OS, respectively, how they
communicate?
Note that communication is exchanges of messages Protocol
– Rules that communicating entities should abide to understand and properly process messages received
– Protocol specifies the meaning (semantics) and syntax of messages
– And timing of messages
A B
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Protocol – Example 1 Error detection
– Communication links are not 100 % reliable– Errors may change, add, delete bits in the original message– An Internet bank user C requests to transfer $100 from account
A1 to A2– If the first bit is changed to 1, then you transfer $228
How do you detect errors?– There are many solutions
• Parity bit• One’s complement addition• CRC
01100100 11100100
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Protocol – Example 2 We need to agree
– Use the same method (Algorithm)– How to apply the method– How to represent additional data
Assume we agreed to 1. Even parity bit2. Apply parity to every bytes3. Attach parity bits to the end of the original message as a
byte stream
01001100 11100111 10101100 00010010
0000100001001100 11100111 10101100 00010010
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Protocol – Example 3 Suppose layer n performs error detection
Layer n
Layer n+1
Layer n-1
Layer n
Layer n-1
Layer n+1
Sender Receiver
01001100 11100111
01001100 11100111 00000010
Protocol- Syntax- Semantics- Timing
Today’s News Supplement of Oct. 2 class
– Sept. 11 at 7:00PM
Revised note and problem set will be posted on the class homepage by Sept. 7 noon
Topics– Comm., Networks, Architecture, Protocol(Sept. 4) – PD: 1.3
– Multiplexing, Queueing (Sept. 6, Sept. 11) – PD: 1.2, 1.5– Layer 2
• Data Link (Sept. 11 supp.) – PD: 2.1• Error control (Sept. 13, Sept. 18) – PD: 2.5
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Protocol Principle - 1
Good protocols abide the protocol principle Why is the protocol principle important in designing
protocols?
PROTOCOL PRINCIPLEMessage that layer n generates at the sender
Message that layer n receives at the receiver
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Protocol Principle - 2
Layer n
Layer n+1
Layer n-1
Layer n
Layer n-1
Layer n+1
Sender Receiver
01001100 11100111
01001100 11100111 00000010
01001100 11100111
01001100 11100111 00000010
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Virtual Communication Two layer entities at the same level think they
communicate directly In fact, a message goes down to lower layers at the
sender and then goes up from lower layers at the receiver
Layer n
Layer n+1
Layer n-1
Layer n
Layer n-1
Layer n+1Msg Msg
MsgCtl MsgCtl
Video
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Standard Protocols To communicate, should use the same protocol Proprietary protocols
– Created (usually) by one or more companies– Closed protocol
• Protocol is hidden or the owner may claim IPR
Open protocol– Specifications are open to the public and everyone can use them
free
Standard protocol– Open protocol that many agree to use
Examples of standard protocols– ISO OSI– IEEE LAN, WLAN, …– TCP/IP– …
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TCP/IP
Typical Network Configuration
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Multiplexing Multiplexing
– Set of techniques allowing simultaneous transmission of multiple signals across a single data link
– Channel: portion of a link that carries a transmission between a given pair of sender & receiver
• A link with n channels supports n simultaneous communications
Demultiplexing– Separation of combined signals
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Categories of MuxTechniques
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Frequency-Division Multiplexing (FDM)
Analog communication technique Medium (link) bandwidth (or spectrum) is much larger than a
single station requires– A co-axial cable supports a few Mhz while a voice BW is 20 KHz
Signals generated by each sender modulate different carrier frequency
Guard Band– Unused bandwidth separating channels
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FDM Multiplexing Example
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FDM Demultiplexing Example
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Synchronous TDM (STDM) Digital transmission system Frame
– Time is partitioned into small, equal time duration called (time) slots each of which is dedicated to one sending device
– Frame: One complete cycle of time slots – Multiplexer allocates the time slots of the same position to a
sender– The slots are dedicated to the sender regardless of actual use/idle
Demultiplexing– Based on slot position
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STDM Examle: T-1 Line for Telephony
How to detect the start(or end) of a frame?
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Synchronous TDM Disadvantage of Synchronous TDM
– Waste transmission resources
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Asynchronous TDM On-demand resource allocation
– Dynamically allocate resources to a station that needs the resources
Demultiplexing is the problem– Should specify
• The start and end of each frame• Destinations of frames
Addressing and Overhead– Framing bits– Specify the destination ID (address, port, ..) in a head– In addition, layer 2/3 header usually contains
• Message size• Source address• …
Basic Queueing Theory
Chong-kwon Kim
Ref: Gallager’s book pp.149-170
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Queueing System & Theory Queueing system
– Any systems that are shared by multiple users– Example: Bus, bank, …
Server
DepartureArrival
Customer(Job)
Queue
Queueing theory analyzes the performance of queueing systems
Performance = Delay, Blocking prob.
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Queueing Theory
Parameters– Arrival Rate,
• The number of customers arrived at the system per time unit
• Inter-arrival time– Service Rate,
• The number of service completions given that the server is busy at all times
• Service time– Memoryless property
• The next (Arrival, departure) events is independent of the previous events
• Exponential distribution– Utilization Factor,
• Measure of how busy is the system
/
Little’s Theorem (N=λT)
Queue
Server
A1 A2
A1
A3
A2 A3
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Queueing Theory M/M/1 Queueing System
– Exponential (Memoryless process) Inter-arrival and Service time
– One server– p : Steady-state prob. that n customers are in the
system
– Balance equations• , n = 0,1,2,...
Pnn ( )1
Pii
0
1
P Pn n 1
n
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Queueing Theory
M/M/1 Queueing System– N : Average number of customers in the system
– T : Average system time experienced by a customer– Little’s Theorem
T 1/( )
N T
N n Pn /( )1
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Queueing Theory Two different systems
– System 1 : Three slow servers each of which is dedicated to one source
– System 2 : A fast server that is three times faster than a slow server. Arrivals from three sources are aggregated and use one queue
Server 1
Server 2
Server 3
Source 1
Source 2
Source 3
System 1
System 2
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Queueing Theory Comparison of FDM (TDM), ATDM
– System 1 : FDM that separates a link into three channels– System 2 : ATDM that is shared dynamically by three users
2 1
2 1
1 2
3
3
3
T T
Performance Performance of a transmission link
– How long does it take to transfer a message from one end to the other end?
Depends on three factors– Bandwidth (or data rate) of a link– Message size– Length of a link
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Bandwidth & Transmit Time Bandwidth (Data Rate)
– Bit transmission (receiving) rate• How fast a transmitter feeds bits into the medium• bps (bits per second)
Transmit time– Time to completely transmitting a message– S/B where S is message size and B is bandwidth
1 Mbps
2 Mbps
1 μs
0.5 μs
ExampleHow many seconds will it take to transmit a 12 Kbit TCP segment over a network of 1 Mbps bandwidth?
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Propagation Delay (Time) Propagation delay
– Time required for a signal travels from a transmitter to areceiver
• The speed of EM signal is about 65~80% of the speed of light
– d/L where• d: distance between the transmitter to the receiver• L: Speed of EM signal
Example– What is the propagation time if the distance between the
two points is 12,000 km? Assume the propagation speed to be 2.4 × 10 m/s in cable
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Latency Time gap between the start of first bit transmission
from a sender to the reception of the last bit at a receiver
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First bit
Last bit
Perceived Latency
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Large message Bandwidth is importantSmall message Propagation is important
RTT & Throughput Usually, a message is sent after a request or a receiver
sent an Acknowledgement (ACK) when it receives a message successfully
RTT (Round Trip Time)– Two times of
propagation time
Transfer time= RTT + Transmit time
Throughput= Transfer size / Transfer time
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First bit
Last bit
Delay-Bandwidth Product Consider a link as a pipe
Delay is the length of the pipe and Bandwidth is the width of the pipe
Delay-bandwidth product– Maximum number of bits in the pipe– Example: Delay = 50 ms, Bandwidth = 45 Mbps
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High Speed Network Network with large bandwidth
– Decrease transmission delay– Not propagation speed
Multimedia communications & High Speed Network
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