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Chapter 1: Foundation
Dr. Rocky K. C. Chang
30 January 2004
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1. What is a computer network?
• Many networks:
–
–
–
–
–
–
Telephone networks
Satellite networks
Mobile telephone networks
Cable TV networks
Internet, intranets, and extranets
Virtual private network
• Data networks vs. telecommunications
networks
• Convergence of services
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2. Classifications of computer networks
• According to the network size:
– Interconnection networks: connecting multiple
processors
– System/Storage area networks (SAN): connecting
processors to storage servers
– Local area networks (LAN): connecting a limited
number of hosts in a close proximity
– Metropolitan area networks (MAN): connecting a
limited number of LANs in a close proximity
– Wide area networks (WAN): connecting hosts
situated anywhere
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2. Classifications of computer networks
• According to mobility:
– Fixed networks
– Wireless networks
– Mobile networks
• According to the network speed:
– Dial-up networks (kbps)
– High-speed networks (Mbps)
– Gigabit networks (Gbps)
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2. Classifications of computer networks
• According to the “networking software”:
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–
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Novell NetWare (IPX)
IBM’s SNA
Xerox’s XNS
DEC’s DECnet
Apple’s AppleTalk
Microsoft’s NetBIOS and NetBEUI
TCP/IP
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3. Four basic requirements
•
•
•
•
•
Connectivity
Cost-effective resource sharing
Support for common services
Performance
Other considerations, e.g.,
– Security
– Accounting for resources
– Economical factors
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3.1 Connectivity
• Connectivity includes directly and indirectly
connected nodes.
– Directly connected
(a)
(b)
…
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3.1 Connectivity
– Indirectly connected
• switched networks: packet switching vs. circuit switching
• internetworks
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3.1 Connectivity
• Network connection components:
– Cables: UTP, 10baseT/2/5, coaxial cables, fibers,
free space, etc.
– Modems
– Network interface cards (NIC)
– Hubs (repeaters, layer one)
– Switches/bridges (layer two)
– Routers (layer three)
– Layer-4 switches
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3.2 Cost-effective resource sharing
• Multiplexing: a process of sharing a system
resource among multiple users.
L1
R1
L2
R2
L3
Switch 1
Switch 2
R3
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3.2 Cost-effective resource sharing
• Multiplexing approaches:
– Synchronous time-division multiplexing (STDM)
– Frequency-division multiplexing (FDM)
– Statistical multiplexing
• On demand, rather than basing on a fixed time schedule
or a fixed frequency assignment.
• Packet switching, rather than message switching, and the
packet size is limited to avoid monopoly.
• Need other mechanisms for packet transmission.
– Packet scheduling
– Medium access mechanisms
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3.3 Support for common services
• From an application’s point of view, the
underlying network provides a logical channel
between two application processes.
Host
Host
Application
Host
Channel
Application
Host
Host
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3.3 Support for common services
• Application requirements vary:
– A data file transfer through the FTP
– A webpage transfer through HTTP
– A video streaming through RTP
• Requirements:
– Total reliability
– Timing constraints: delay and delay jitter
• Results for the Internet protocol:
– Provide a best-effort service at IP (connectivity).
– Provide a reliable service on top of IP.
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3.4 Performance
• Network performance measured in two ways:
– Throughput (in bits per second)
– Latency (or delay, in time units)
• Bandwidth and throughput
– Bandwidth of a channel in terms of Hz
– Bandwidth of a channel in terms of bits/second
– Throughput of a channel, in terms of bits/second, is
the maximum data rate realized.
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3.4 Performance
(a)
1 second
(b)
1 second
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3.4 Performance
• Latency measures the time it takes a
message/packet to travel from a source to a
destination.
– Round-trip time (RTT)
• Latency is a sum of
– queueing delays: waiting time for its turn of
transmission
– transmission delays, and
– propagation delay: time for propagating a packet
from a source to a destination
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3.4 Performance
• For example, transfer a 1-MB file in a network
of 10Mbps with 5000m apart.
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–
–
–
Ignoring queueing and node processing delays
Transmission delay: (1x220x8)/10x106 = 0.839s
Propagation delay: 5000/2x108 = 0.025ms
Latency = 0.839s.
• Now change 10Mbps to 10 Gps:
– Transmission delay: (1x220x8)/10x109 = 0.839ms
– Latency = 0.864ms
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3.4 Performance
• Combine data size, bandwidth, and propagation
delay:
10,000
5000
2000
Perceived latency (ms)
1000
500
1-MB object, 1.5-Mbps link
1-MB object, 10-Mbps link
2-KB object, 1.5-Mbps link
2-KB object, 10-Mbps link
200
100
50
1-byte object, 1.5-Mbps link
1-byte object, 10-Mbps link
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10
5
2
1
10
RTT (ms)
100
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3.4 Performance
• Delay-bandwidth product: gives the volume of
the pipe--the maximum number of bits it holds.
• For example, a transcontinental channel with
one way latency of 50 ms and a bandwidth of
45 Mbps, the product is 2.25x106 bits (280 KB).
Delay
Bandw idth
• Short fat pipe, long fat pipe, long thin pipe
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4. Network architecture
• Layered architecture
– Layering decomposes the problem of building a
network into more management components.
– Layering provides a more modular design.
Application programs
Process-to-process channels
Host-to-host connectivity
Hardware
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4. Network architecture
– Each layer provides a service to the upper layer.
– Often, there are multiple services provided by a
layer.
Application programs
Reliable channel
Unreliable channel
Host-to-host connectivity
Hardware
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4.1 Internet architecture
• A four-layer architecture (a protocol graph)
Application
Transport
FTP
HTTP
TFTP
UDP
TCP
Network
Data-link
NV
IP
NET1
NET2
…
NETn
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4.1 Internet architecture
• Layers:
– Data-link layer provides framing and address
resolution services.
– Network layer provides connectivity service.
– Transport layer provides reliability or unreliable
service, and flow control service.
– Application layer provides application-specific
services.
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4.2 Protocols
• The service provided by each layer is
implemented by protocols at that layer.
– A protocol is a series of steps, involving two or
more parties, designed to accomplish a task.
– A protocol for batch textbook purchase
• Each protocol defines two interfaces:
– Service interface defines the operations that local
objects can perform on the protocol.
– Peer interface defines the form and meaning of
messages exchanged between protocol peers to
implement the communication service.
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4.2 Protocols
Students
Rocky
ts
studen
y
r
e
u
(2) Q
(3) Stu
d
ents re
s
for ba
pond t
chase
tch pur
Bookshop
(1) Ma
k
e an in
quiry a
nd a
textboo sk to reserv
e 30
ks
o the q
uery
(4) Co
nfirm o
r
der an
d
make p
aymen
t
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textbo
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v
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l
(5) De
s
ooks to
b
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x
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liver
(6) De
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tudent
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•
•
•
•
•
•
4.2 Protocols
Involve three parties (students, Rocky, and the
Bookshop).
Involve a series of steps, which must be
executed in a pre-determined order (steps 1-6).
Accomplish a task (purchasing textbooks)
Every party involved in a protocol must know
the protocol and must follow all steps.
The protocol must be unambiguous and each
step is well defined.
The protocol must be complete.
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4.2 Protocols
Batch textbook purchase
Batch textbook purchase
"Email datalink"
"Email data-link"
Email
"Physical
contact datalink"
Batch textbook purchase
"Physical contact data-link"
Physical contact
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4.2 Protocols
• FTP/TCP/IP/Ethernet and FDDI protocols
H1
H2
FTP protocol
FTP
FTP
TCP
TCP
TCP
IP
IP
ETH
ETH
R1
FDDI
IP
FDDI protocol
IP
FDDI
R2
IP
ETH
ETH
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4.3 Protocols at different layers
• Encapsulation
– A header is attached to a message passed down
from the upper layer.
– The message to be encapsulated is called payload or
protocol data unit (PDU).
– A low-level protocol does not interpret the message
it is given by some high-level protocol.
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4.3 Protocols at different layers
• FTP/TCP/IP/Ethernet
user input
FTP
Appl. data
TCP
TCP hdr
Appl. data
IP hdr
TCP hdr
Appl. data
IP hdr
TCP hdr
Appl. data
IP
ETH
Eth. hdr
Send out to the network interface
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4.3 Protocols at different layers
• Demultiplexing: A reverse process of
encapsulation
Other TCP-based application
processes
Appl. data
UDP-based applications
Other nonIP network
protocols
Eth. hdr
IP hdr
IP hdr
TCP hdr
Appl. data
TCP hdr
Appl. data
TCP hdr
Received from the network interface
Appl. data
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4.4 The 7-layer OSI architecture
• The Open Systems Interconnection architecture
serves as a reference model.
• Except for DECnet, the OSI was not
implemented as a working product, because
– The upper three layers were not generally agreed
upon.
– A very complex protocol architecture; a large
number of layers implies inefficiency.
– Significant implementation effort
– TCP/IP was readily available then.
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4.4 The 7-layer OSI architecture
End host
End host
Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Network
Network
Data link
Data link
Data link
Data link
Physical
Physical
Physical
Physical
One or more nodes
within the network
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5. Network programming
• Two most common network APIs: Sockets and
X/Open Transport Interface (XTI), a slight
modification of AT&T’s Transport Layer
Interface (TLI).
• The APIs allows programmers to easily make
“connections” with another application process,
without knowing how the underlying network
operates.
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5.1 Basic socket calls for a client
obtain a socket
socket
sockaddr_in{}
establish a connection
to the peer
connect
receive and send data
recv
send
peer
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5.2 Basic socket calls for a server
obtain a socket
socket
sockaddr_in{}
bind the server’s IP
address and port to the
socket
bind
mark the socket as a
listening socket
listen
local
sockaddr_in{}
accept new
connections
receive and send data
accept
peer
recv
send
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6. Organization of the textbook
• Host-to-host communications
– Chapter 2: On a directly connected network (a single
LAN segment, data-link layer)
– Chapter 3: On multiple non-directly connected
networks of the same type (multiple LAN segments,
data-link layer)
– Chapter 4: On multiple non-directly connected
networks of different types (global Internet, network
layer)
• Process-to-process communications
– Chapter 5: Transport layer
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6. Organization of the textbook
• Other issues:
–
–
–
–
Chapter 6 (network congestion)
Chapter 7 (presentation formatting)
Chapter 8 (security)
Chapter 9 (application protocols)
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