Transcript CS 352 Internet Technology

Internet Technology
Fundamentals
1
Why Study Networks?
• Integral part of society
• Work, entertainment, community
• Pervasive
• Home, car, office, school, mall …
• Understand what they do, how they work, and
limitations
– Any jobs left?
– What happened to IT?
– Future of the IT industry.
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Impact of the Net on People
• Anytime access to remote information
– HW assignments from my server
• Person-to-person and group communication
– email, blogs, chat, meeting
• Form and strengthen communities
– chat rooms, MUDs, newsgroups
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Impact of the Net on Society
• Huge impact!
– Continuation of technologies that reduce problems
of time & space
• (e.g. railroads,phone,autos,TV)
• Good, bad and ugly
– mirror of society
• Changes still on the horizon
– Commerce, services, entertainment, socializing
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Internet Roles
• Users
• Everyone (mom and pop, kids)
• work, leisure, serious, frivolous
• Designers
• protocol design and implementation
• performance, cost and scale
• Service Providers
• Administrators and ISPs
• Management, revenue, deployment
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What is Internet Technology?
• What is an internet?
– Network of networks
• What is the Internet?
– A global internet based on the IP protocol
• To what does “Internet technology” refer?
– Architecture, protocols and services
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Sample Internet Applications
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Electronic mail
Remote terminal
File transfer
Newsgroups
File sharing
Resource distribution
World Wide Web
Video conferencing
Games
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What is a Network?
• Carrier of information between 2 or more
entities
• Interconnection may be any media capable of
communicating information:
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copper wire
lasers
microwave
satellite link
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Some Definitions
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Network: Collection of interconnected machines
Host: Machine running user application
Channel: Logical line of communication
Media: Physical process used
Protocol: Rules of communication
Router: decide were to send data next
Topology: How network is interconnected
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How Do Computers Communicate?
• With 1’s and 0’s
– Computers only deal with 1’s and 0’s
– So do networks
– Must build all further structures from this basic
representation
• How do we transmit 1’s and 0’s in a network?
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Physical Transmission
A physical quantity (e.g.
voltage), varying over
time represents a
digital 0 or 1
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Concepts for this week
• Layering and encapsulation
– IP Hourglass
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Core and Edge of the Internet
Circuit, message and packet switching
Single link transmission delay
Multi-link transmission delay
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Circuit switching
Message switching
Packet switching
Computing general pipelining delay
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Layering and Encapsulation
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Why Layering?
• Network communication is very complex
• Separation of concerns
– Different vendors and organizations responsible
for different layers
– Testing and maintenance is simplified
– Easy to replace a single layer with a different
version
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Protocol Hierarchy
• Use layers to hide complexity
– Each layer implements a service
• Layer N uses service provided by layer N-1
• layer N-1 provides a service to layer N
– Protocols
• Each layer communicates with its peer by a set of rules
• Interface
– A layers interface specifies the operations
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Protocol Hierarchy (cont’d)
Host A
Layer 7
Layer 6
Layer 5
Layer 4
Layer 3
Layer 2
Layer 1
Host B
Layer 7 Protocol
Layer 6 Protocol
Layer 5 Protocol
Layer 4 Protocol
Layer 3 Protocol
Layer 2 Protocol
Layer 1 Protocol
Physical Medium
Layer 7
Layer 6
Layer 5
Layer 4
Layer 3
Layer 2
Layer 1
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Different Layering Architectures
• ISO OSI 7-Layer Architecture
• TCP/IP 4-Layer Architecture
– + application layer = 5 layers in Kurose
• Novell NetWare IPX/SPX 4-Layer
Architecture
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Standards Making Organizations
ISO = International Standards Organization
ITU = International Teletraffic Union (formerly CCITT)
ANSI = American National Standards Institute
IEEE = Institute of Electrical and Electronic Engineers
IETF = Internet Engineering Task Force
ATM Forum = ATM standards-making body
...and many more
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Why So Many Standards Organizations?
• Multiple technologies
• Different areas of emphasis and history
– Telecommunications/telephones
• ITU,ISO,ATM
– Local area networking/computers
• IETF, IEEE
– System area networks/storage
• ANSI
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ISO OSI Layering Architecture
Host A
Host B
Application
Layer
Application Protocol
Presentation
Layer
Presentation Protocol
Session
Layer
Session Protocol
Transport
Layer
Transport Protocol
Application
Layer
Presentation
Layer
Session
Layer
Transport
Layer
Network
Layer
Network
Layer
Network
Layer
Network
Layer
Data Link
Layer
Data Link
Layer
Data Link
Layer
Data Link
Layer
Physical
Layer
Physical
Layer
Physical
Layer
Physical
Layer
Router
Router
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ISO’s Design Principles
• A layer should be created where a different level of
abstraction is needed
• Each layer should perform a well-defined function
• The layer boundaries should be chosen to minimize
information flow across the interfaces
• The number of layers should be large enough that
distinct functions need not be thrown together in the
same layer out of necessity, and small enough that
the architecture does not become unwieldy
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Layer 1: Physical Layer
• Functions:
– Transmission of a raw bit stream
– Forms the physical interface between devices
• Issues:
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Which modulation technique (bits to pulse)?
How long will a bit last?
Bit-serial or parallel transmission?
Half- or Full-duplex transmission?
How many pins does the network connector have?
How is a connection set up or torn down?
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Layer 2: Data Link Layer
• Functions:
– Provides reliable transfer of information between
two adjacent nodes
– Creates frames, or packets, from bits and vice
versa
– Provides frame-level error control
– Provides flow control
• In summary, the data link layer provides the
network layer with what appears to be an
error-free link for packets
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Layer 3: Network Layer
• Functions:
– Responsible for routing decisions
• Dynamic routing
• Fixed routing
– Performs congestion control
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Layer 4: Transport Layer
• Functions:
– Hide the details of the network from the session layer
• Example: If we want replace a point-to-point link with a
satellite link, this change should not affect the behavior of
the upper layers
– Provides reliable end-to-end communication
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Transport Layer (cont’d)
Host A
first
end-to-end
layer
Host B
Application
Layer
Application Protocol
Presentation
Layer
Presentation Protocol
Session
Layer
Session Protocol
Transport
Layer
Transport Protocol
Application
Layer
Presentation
Layer
Session
Layer
Transport
Layer
Network
Layer
Network
Layer
Network
Layer
Network
Layer
Data Link
Layer
Data Link
Layer
Data Link
Layer
Data Link
Layer
Physical
Layer
Physical
Layer
Physical
Layer
Physical
Layer
Router
Router
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Transport Layer (cont’d)
• Functions (cont’d):
– Perform end-to-end flow control
– Perform packet retransmission when packets are
lost by the network
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Layer 5: Session Layer
• May perform synchronization between
several communicating applications or logical
transmissions
• Groups several user-level connections into a
single “session”
• Examples:
– Banking session
– Network meetings
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Layer 6: Presentation Layer
• Performs specific functions that are requested
regularly by applications
• Examples:
– encryption
– ASCII to Unicode, Unicode to ASCII
– LSB-first representations to MSB-first
representations
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Layer 7: Application Layer
• Application layer protocols are applicationdependent
• Implements communication between two
applications of the same type
• Examples:
– FTP
– Quake
– SMTP (email)
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Encapsulation
Treat the neighboring layer’s information as a
“black box”, can’t look inside or break
message
• Sending: add information needed by the
current layer “around” the higher layers’ data
– headers in front
– trailers in back
• Receiving: Strip off headers and trailers
before handing up the stack
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Encapsulation
Data
Application
Layer
AH
Presentation
Layer
PH
Session
Layer
SH
Transport
Layer
TH
Network
Layer
NH
Data Link
Layer
DH
Physical
Layer
PH
Data
Data
• Headers
Data
Data
• Trailer
Data
Data
Data
DT
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Internet “Hourglass” Architecture
• Defined by Internet Engineering Task Force (IETF)
• “Hourglass” Design
FTP
HTTP
RTP
TFTP
UDP
TCP
IP
Ethernet
CAT-5
802.11
Single-Mode
Fiber
…
PPP
RS-232
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Internet Design Principles
– Scale
• Protocols should work in networks of all sizes and
distances
– Incremental deployment
• New protocols need to be deployed gradually
– Heterogeneity
• Different technologies, autonomous organizations
– End-to-end argument
• Some functions can only be correctly implemented at the
end hosts; the network should not provided these.
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TCP/IP Layering Architecture
Application
Transport
Internet/Network
Host-to-Net
• A simplified model
• The network layer
– Hosts drop packets into
this layer, layer routes
towards destination- only
promise- try my best
• The transport layer
– reliable byte-oriented
stream
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TCP/IP Layering Architecture (cont’d)
Host A
Host B
Application Protocol
Application
Layer
Transport Protocol (TCP)
Transport
Layer
Network
Layer
Host-toNet Layer
Application
Layer
IP
Network
Layer
Host-toNet Layer
IP
Network
Layer
Host-toNet Layer
Transport
Layer
IP
Network
Layer
Host-toNet Layer
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Internet Topology
• Current structure divides network into the
“core” and “edge” networks
• Core ISP’s “tiers”
– Tier 1: Biggest ISPs
• E.g. MCI, Sprint, AT&T
– Tier 2 and 3: Regional and very small.
• Edges:
– Companies, organizations with a “default route”
• E.g. Rutgers
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Edge Networks
Company A
Internet Service Provider 1
ISP 2
Company B
Edge router
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Core Networks
Internet Service Provider 1
Company A
ISP 2
Company B
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Single link Network Performance
A Brief Introduction
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Why Study Network Performance
• Networks cost $
– OC-3 line ~= $10,000/month
– Cable modem: $40/month
– Are you getting your $/worth?
• Why is the network “slow”?
• Approach:
– Build abstract models of network performance
– Observe where real networks deviate from model
– Simple Models: Tells us average/best/worse cases->useful,
practical
– Complex Models: Hard to understand -> useless
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Digression: Units
•
Bits are the units used to describe an amount of data in a network
– 1 kilobit (Kbit)
– 1 megabit (Mbit)
– 1 gigabit (Gbit)
•
Seconds are the units used to measure time
– 1 millisecond (msec)
– 1 microsecond (msec)
– 1 nanosecond (nsec)
•
= 1 x 103 bits = 1,000 bits
= 1 x 106 bits = 1,000,000 bits
= 1 x 109 bits = 1,000,000,000 bits
= 1 x 10-3 seconds = 0.001 seconds
= 1 x 10-6 seconds = 0.000001 seconds
= 1 x 10-9 seconds = 0.000000001 seconds
Bits per second are the units used to measure channel
capacity/bandwidth and throughput
– bit per second (bps)
– kilobits per second (Kbps)
– megabits per second (Mbps)
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Types of Delay
• Processing
– Time to execute protocol code
• Queuing
– Time waiting in queue to be processed
• Transmission
– Time to “get bits on wires”
• Propagation
– Time for bits to “move across wires”
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Some Definitions
• Packet length: size of a packet (units = bits or bytes)
• Channel speed: How fast the channel can transmit bits (units =
bits/second)
• Packet transmission time: amount of time to transmit an entire
packet (units = seconds)
• Propagation delay: Delay imposed by the properties of the link.
Depends on the link’s distance (units = seconds)
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Transmission vs. Prop. delay
A single transmission link as a water pipe
1.
2.
The thicker the pipe, the more water it can carry from one end
to the other
Water is carried from one end of the pipe to the other at
constant speed, no matter how thick the pipe is
Water = Data bits
Thickness of the pipe = Channel capacity
Speed of water through the pipe = Propagation delay
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Transmission vs. Prop. Delay (cont)
pipe
1.
2.
Propagation delay is how long takes to cross
the pipe, irrespective of volume
Transmission (bandwidth delay) is related to
how much water can be pushed in through
the openning per unit time
Assumption that inserting water and propagation overlap
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Transmission Time
How long does it take A to transmit an entire packet onto the link?
Relevant information: packet length = 1500 bytes
channel capacity = 100 Mbps
Another way to ask this question:
If the link can transmit 10 million bits in a second, how
many seconds does it take to transmit 1500 bytes (8x1500
bits)?
100 Mbits
1 sec
=
1500 x 8 bits
t
Solving for t…
t = 0.00012 sec (or 120 msec)
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Propagation Delay
How long does it take a single bit to travel on the link from A to B?
Relevant information: link distance = 500 m
prop. delay factor = 5 msec/km
Another way to ask this question:
If it takes a signal 5 msec to travel 1 kilometer, then how
long does it take a signal to travel 500 meters?
5 msec
1000 m
=
t
500 m
Solving for t…
t = 2.5 msec
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Processing Delay
Stylized format required to send data
Analogy: adding and removing envelopes to letters
Host
Host
Application
Layer
Application
Layer
Transport
Layer
Transport
Layer
Router
Network
Layer
Network
Layer
Network
Layer
Host-toNet Layer
Host-toNet Layer
Host-toNet Layer
How long does it take
to execute all these
layers?
Why is this time
important?
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Example
A
B
500 m
Protocol Processing Time = 40 msec
packet length = 1500 bytes
channel capacity = 100 Mbps
propagation delay factor = 5 msec/km
1.
2.
3.
How long to format the data?
How long does it take a single bit to travel on the link
from A to B?
How long does it take A to transmit an entire packet
onto the link?
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Timeline Method
Host A
40
1st bit
Host B
Protocol Delay
2.5
Propagation delay
120
Transmission time
Time
last bit
40
Protocol Delay
Total time: 40+120+2.5+40 = 202.5 msec
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Queuing Delay
Router
Network
Layer
Host-toNet Layer
Packets waiting
processing
at input
ports
2 0
3
1 0
0
Router
0 0
1
2 2
3
Packets waiting
transmission at
output ports
Packets arriving faster than
processing or transmission
delay
=> queuing (I.e. waiting in line)
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Switching Schemes
How much “state” about the connection
between two hosts does each node/router
along a path through the network maintain?
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Switching Schemes
(1) Circuit Switching
(2) Message Switching (Store-and-Forward)
(3) Packet Switching (Store-and-Forward)
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Circuit Switching
• Provides service by setting up the total path
of connected lines hop-by-hop from the origin
to the destination
• Example: Telephone network
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Circuit Switching (cont’d)
1. Control message sets up a path from origin
to destination
2. Return signal informs source that data
transmission may proceed
3. Data transmission begins
4. Entire path remains allocated to the
transmission (whether used or not)
5. When transmission is complete, source
releases the circuit
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Circuit Switching (cont’d)
Call request signal
Time
Propagation Delay
Transmission
Delay
Call accept signal
Data
Transmission
Time
Data
A
B
C
D
Routers/Switches
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Message Switching
• Each message is addressed to a destination
• When the entire message is received at a router, the
next step in its journey is selected; if this selected
channel is busy, the message waits in a queue until
the channel becomes free
• Thus, the message “hops” from node to node through
a network while allocating only one channel at a time
• Analogy: Postal service
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Message Switching (cont’d)
Header
Time
Msg
Transmission
Delay
Msg
Queueing
Delay
Msg
A
B
C
D
Routers/switches
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Packet Switching
• Messages are split into smaller pieces called
packets
• These packets are numbered and addressed and
sent through the network one at a time
• Allows Pipelining
– Overlap sending and receiving of packets on multiple links
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Packet Switching (cont’d)
Pkt 1
Time
Header
Pkt 2
Pkt 1
Pkt 3
Transmission
Delay
Pkt 2
Pkt 1
Pkt 3
Pkt 2
Pkt 3
A
B
C
D
IMPs
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Comparisons
(1) Header Overhead
Circuit < Message < Packet
(2) Transmission Delay
Short Bursty Messages:
Packet < Message < Circuit
Long Continuous Messages:
Circuit < Message < Packet
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Analytic Comparison
• Given choice of 2 switching schemes, how
would you compare their performance?
– What would you need to know?
– What are the independent variables?
– What is the dependent variable?
• Could you come up with a closed form
expression based on your choices?
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Example: Circuit Switching vs. Packet
Switching
• Goal: Determine which is faster
– Formal definition: Least time to move a fixed
amount of data
• Approach:
– Compute time where circuit switching and packet
switching are equal based on all possible factors
– A factor moving in one direction or the other will tip
the balance in favor of one or the other
– We’ll ignore wire-line propagation delay in this
example
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Factors:
•
•
•
•
•
•
Number of bytes in the message: N
Time to set up circuit:
c
Per-link bandwidth:
B
Size of the packet:
p
Size of the header:
h
Number of switches:
s
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Circuit Switching Time
• Time to send N bytes using circuit switching
• = Set-up cost + bandwidth delay
N
C
B
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Pipelining “Parallelogram”
for packet switching
Host A
Switch 1 Switch 2 Host B
Packet 1
Packet 2
Time
Packet 3
Bandwidth
Delay
Packet 4
Propagation
Delay
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Note on Pipelining
• The above analysis is very general:
– Packets in a computer network
• Messages/packets are the unit of work.
– Instructions in a processor
• Instructions are the unit of work.
– Jobs through a batch Q in an operating system.
• Processes are the unit of work.
• Pipelining speeds up work over time.
– How?
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Packet Switching Time
Delay = Transmission + “Propagation” delays
Transmission delay (also bandwidth delay):
Time to push all the packets into the network
+ “Propagation” delay:
Time for the last packet to cross
- not really prop. delay in the traditional sense
N ( p  h)
( p  h) 
 S  1




P  B 
 B 

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Packet Switching Time
Transmission delay
Number of packets
“Propagation” delay
Number of links/hops
N ( p  h)
( p  h) 
 S  1




P  B 
 B 

Time for each packet

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
Equilibrium Point

N p  h N 
C   
  S  1


B  B P 
Assuming all other factors equal, solve for C
Q: Can you add link propagation delay to this example?

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Homework Questions
• If we use message switching, how does the
time increase as we scale s?
• How does packet switching reduce the impact
of increasing s?
• Show, using an equation, how reducing the
packet size and packet switching reduces the
impact of increasing s.
• Where does the approach of reducing packet
size fail to give any benefit?
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