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2a. Introduction to Data Communications and Networking
1. Communication Link
2. General Definition
3. Example of Computer Communication
Systems
4. Networking
a. Telephone Network
b. Computer Networks
c. Cable Television
d. Wireless Networks.
5. Communication Standards
a. System Interconnection
6. OSI/RM
7. Layer Descriptions
8. The TCP/IP Reference Model
a. Protocol Hierarchies
b. Internet Layer
c. Transport Layer
d. Application Layer
e. Host-to-Network Layer
9. Packet Switching and Circuit Switching
10.Connect-oriented and Conn.less Services
(T. pg. 1-84)
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1. Communication Link
Data Communication Link
1
2
3
4
5
6
Input
Information
Input Data
or Signal
Transmitted
Signal
Received
Signal
Output Data
or Signal
Output
Information
1
Input
Device
2
Transmitter
Source System
3 Transmission 4
Medium
Receiver
5 Output
6
Device
Destination System
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2.
General Definitions
• Information is the meaning that a human being assigns to
data by means of the conventions applied to those data.
• Data is a representation of facts, concepts, or instructions in a
formalized manner suitable for communications.
• Signals are the physical encoding of data, electric, or
electromagnetic means.
Signals can be:
• Analog (continuous in time and amplitude),
• Discrete (discrete in time, continuous in amplitude), or
• Digital. A digital signal (discrete in time and amplitude),it is a
sequence of digital values which changes once every interval.
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3. Example of a Computer Communication Systems
m
Message
Generator
gd (t)
Message
Encoder
Sd (t)
Error
Control
Encoder
Se (t)
Computer
Modulator
Digital Signal
Sm (t)
Encoded Digital Signal
Modulated Signal
Noise
Analog
Transmission
Line
?m (t)
Altered Modulated Signal
Analog Signal
Demodulator
Regenerated Encoded Digital Signal
(with possible errors)
Regenerated Digital Signal
(errors detected and corrected)
?a (t)
Regenerator
Remote Device
?e (t)
m
User
gd (t)
Message
Decoder
Sd (t)
Error
Control
Decoder
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4. Networking
• Communication networks enable many users to transfer
information in different form of voice, video, electronic mail, and
computer files.
• a. In Telephone Network:
Circuit switching. “Circuit" reefers-one telephone conversation
along one link.
• Circuit switching occurs at the beginning of new telephone
call.
• An electronic interface, coder/decoder (codec) in the switch
converts the analog signal traveling on the link from the
telephone set to the switch into digital signal-a bit stream.
• Since the 1980s the transmission links of the telephone network
have been changing to the SONET, or Synchronous Optical
Network, standard. SONET rates are arranged in the
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Synchronous Transfer Signal (STS).
Phone connection to digital network
Analog signal
Codec
Switch
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•
•
•
•
•
•
•
•
•
•
•
Carrier
OC-1
OC-3
OC-9
OC-12
OC-18
OC-24
OC-36
OC-48
OC-192
OC-768
Signal
Rate in Mbps
STS-1
51,840
STS-3
155,520
STS-9
466,560
STS-12
622,080
STS-18
933,120
STS-24
1244,160
STS-36
1866,240
STS-48
2488,320
STS-192
9853,280
STS-768 39,413,120
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• b. Computer or Data Communication Networks
• Data packets
• Packet switching + Rules of operations (protocols)
=The ARPANET -single packet format and addressing scheme.
Through the ARPANET was evolved into the Internet.
ARPANET architecture was formalized layered model of OSI
• The packet switching technique in networks is based on
Multiplexing and/or Multiple Access methods of computer
interconnections.
• Multiplexing:
TDM
FDM (WDM / DWDM)
• Multiple Access: Ethernet Network.
Token Ring Network.
Fiber Distributed Data Interface FDDI-Timed-token mechanism-fixed time of the arrivals token
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Model of the information system
Server
Clients
Network
Broadcast links.
Point-to-point (unicast) links
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LAN
Topology of the:
Ethernet (a) and Token Ring networks (b).
A
a
B
IEEE 802.3
b
IEEE 802.5
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Metropolitan Area Networks (MAN)
MAN represents as a interconnected LANs by pointto-point communication links.
The interconnection is controlled by switches,
Wide Area Network (WAN)
Subnet
Links
Router (Switch)
Client
Host (Server)
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Sprint US backbone network
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• Cable Television, originally known as Community Antenna
Television or CATV. In CATV the signal from one master
antenna distributed over a large area using coaxial cable and
amplifiers. The key innovations in cable TV are optical fiber
links, digital compression techniques, and service integration.
• Today cable TV uses frequency-division multiplexing to transmit
up to 69 analog TV channels, each 4.5 MHz wide. Transmission
is over coaxial cables arranged as a unidirectional tree.
• Amplifiers used to compensate for the attenuation of the cable
signal. The number of TV a channels is limited by the
bandwidth of coaxial cables.
• Optical fibers are used to transmit the TV signals over longer
distance. Transmission over the fiber is still analog. The signal
is fed into the coaxial cable network at various points, where
the optical signal is converted into electrical signals. This hybrid
fiber/coaxial cable distribution system has a longer span and
better signal quality than a coaxial cable network. This network
called fiber-to-the-curb (FTTC) network.
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• To increase the number of channels, digital transmission
technology.
• Before transmitting the TV signals, the CATV company uses a
TV codec that converts each signal into a bit stream.
• Using Motion Pictures Expert Group (MPEG) algorithms,
the codec compresses the bit stream to reduce its rate.
• The bit streams are transmitted over fibers to the curb and then
distributed by the neighborhood coaxial network.
• The compression gain now allows-transmit about 500 TV
channels. MPEG1 standard, TV signal is encoded-1.5 Mbps
bit stream, which can be modulated in a signal that has a
bandwidth of about 600 kHz.
• Set-up boxes at the user residence perform the decompression.
This CATV network is still unidirectional. Video on demand,
Internet access, and telephony, the CATV industry is
organizing bidirectional networks. Such a network connects
video servers to users by means of control messages.
• The user choices these messages to select the video program,
and the video program is sent over the network to the user. 14
Residential access: cable modems
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Diagram: http://www.cabledatacomnews.com/cmic/diagram.html
Cable Network Architecture: Overview
Typically 500 to 5,000 homes
cable headend
cable distribution
network (simplified)
home
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Cable Network Architecture: Overview
cable headend
cable distribution
network (simplified)
home
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Cable Network Architecture: Overview
server(s)
cable headend
cable distribution
network
home
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c. Cable Network Architecture:
Overview
FDM:
V
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O
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A
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C
O
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T
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O
L
1
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Channels
cable headend
cable distribution
network
home
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d. Wireless Networks.
The first packet-switched wireless network- in 1971- Alohanet,
interconnected computers on four islands in a star topology:
A first approximation wireless network- three main categories:
1. Components interconnection. Short-range radio. Bluetooth network.
2. Wireless LANs. Wireless LAN permitting per-to-per communications
networks. LANs called IEEE 802.11, Wireless LAN can operate at bit rates
up to about 50 Mbps over distances of tens of meters.
3. Wireless WANs. The radio network used for cellular telephones is an
example of a low-bandwidth wireless wide area system. This system has
already gone through three generations:
a. The first- analog and voice only.
b. The second- digital and for voice only.
c. The third- digital and is for both voice and data.
Cellular systems operate below 1 Mbps, but the distances between the base
station and the computer or telephone is measured in kilometers.
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Wireless access networks
• Shared wireless access
network connects end system
to router
– via base station “access point”
• wireless LANs:
- 802.11b:
- 50 Mbps, tens of meters
• wireless WAN
- Cellular systems
- < 1Mbps, several km
router
base
station
mobile
hosts
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Telephone
Networks
Computer
Networks
Cable TV
Wireless
Networks
Circuit
switching,
separation of
call
control
from
voice
transfer.
ISDN
and
service
integration.
Optical links.
SONET.
ATM.
Packetswitched
networks.
Multipleaccess
Networks.
Layered
architecture,
ARPANET.
Internet.
OSI model
Integrated
services.
ATM.
Digitization
and
compression
using signal
processing
techniques.
Fiber-to-thecurb network.
Two-way
links.
Service
integration.
Radio and
TV
Broadcast.
Cellular.
Telephones.
Wireless
LANs.
Voice, data
Integration.
Bluetooth.
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5.
System interconnection
• An important concept in data communications is the
interconnections between the communication system
components. The interconnection could be done if:
• Physical characteristics of the interconnected
equipment are fitted to each other.
• It allows manufacturers of different systems to
interconnect their equipment through standard
interfaces.
• It also allows software and hardware to integrate well
and be portable on differing systems.
• So, standards of hardware and software for
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interconnections in systems are necessary.
Communication Standards
• Computer communication uses different standard for different
approach.
• The RS-232-C standard is used for the serial port of
computer devices. This standard is for low bit rate
transmissions (up to 38 Kbps) over short distances (less than
30 m). Transmissions take place over untwisted wires.
• A serial link is often used to attach a computer to a modem. A
modem transmits data by converting bits into tones that can
be transported by the telephone network. The receiving
modem then converts these tones back into bits, thus
enabling two computers with compatible modems to
communicate over the telephone network as if they were
directly connected by a serial link. Modems conforming to
new V.90 standard can transmit 56,000 bps.
• The Synchronous Transmission Standard increases the
transmission rate. These standards are known as
Synchronous Data Link Control (SDLC). The main idea of
SDLC is to avoid the time wasting by RS-232-C.
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• SDLC groups many data bits into packets.
• With an open system of standards any company can
manufacture equipment or write software. Companies must
cooperate on standards.
• Standard organizations create and administer standards. Often
competing companies will form a committee to create a
standard acceptable to all interested parties. Then the
companies will ask a standard organization for formal
recognition of that standard.
• An example: Ethernet, a Local Area Network (LAN) system
created by Xerox, Intel, and Digital Equipment Corporation.
These companies asked the Institute of Electrical and
Electronics Engineers (IEEE) to formalize Ethernet, and this
becomes standard IEEE 802.3.
• United States major standards from industry are:
The American National Standard Institute (ANSI), the IEEE,
and the Electronic Industries Association (EIA). The major
governmental standards organization is the National institute
of Standards and Technology (NIST).
NIST major standards concerns are the standard Volt,
standard Ampere, time, and dimensions for manufactures.
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Open System Interconnection Reference Model
Data
Data
Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
Each layer is a kind of
virtual machine, offering
certain services to the
layer above
User application, process
And management functions
Data interpretation, format
And control transformation
Administration and control
Of session between two nodes
Network transparent data transfer and transmission control
Routing, switching and flow
Control over a network
Maintain and release data:
Link, error and flow control
Electrical and mechanical
characteristics
Actual Data Flow
Communication subnet
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Protocol Hierarchies
•
The philosopher-translator-secretary architecture.
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Why layering?
• Each layer is a kind of virtual machine, offering certain
services to the layer above it.
• Layer n on one machine carries on a conversation with layer
n on another machine. The rules and conversations are
known as the layer n protocol.
• A protocol is an agreement between the communicating
parties on how communication is to proceed.
Dealing with complex systems:
• Clear structure allows identification, relationship of complex
system’s pieces
• modularization eases maintenance, updating of system
change of implementation of layer’s service transparent to
rest of system
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• The function of
layer: what task the layer is perform, but
not how the layer performs its task.
• The function of interface: how a layer will communicate
with the layer above it and the layer below it.
• For software interfaces, information may be passed in a
manner similar to parameter passing. The information must be
in a particular format (a. length, b. the order in which individual
fields appear within a frame, c. the bit order within individual
frames).
• The hardware interfaces (physical level) may be: a. voltages,
b. impedance, and c. mechanical dimensions.
• Bottom three layers - Communications Subnet. They are:
1. the Physical Layer, (is hardware)
2. the Data Link Layer (DLL), (can be a mixture of hardware
and software).
3. the Network Layer.
The Communication Subnet is one of the major subjects of
CS 117 and CS M 171L classes to study.
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Communications Subnet.
1. the Physical Layer, is hardware The
Physical is the actual medium that conveys
the bit stream. This connects the networks
together and carries the "ones" and "zeros"
(voltage or light pulses). Typical questions
here are how many volts should be used to
represent a “1” and how many for “0”. How
many nanoseconds a bit lists,
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• Layer 1: Physical Layer. Transmitted signals are
modulated electromagnetic waves that propagate through
medium.
• The medium can be fiber optics, twisted pair copper wire,
coaxial cable, microwaves, satellite, laser beams, or radio
waves. Layer 1 also includes the antennas, cables, satellites,
and connectors.
• The transmitter converts the bits into signals, and the physical
layer in the receiver converts the signals back into bits. The
receiver must be synchronized to be able to recover the arrival
bits. To assist the synchronization, the transmitter inserts a
specific bit pattern, called a preamble, at the beginning of the
packet.
• The physical layer transmits bits by converting them into
electrical, electromagnetic waves, or optical signal.
• Generally, wireless links are slower than copper links,
and copper links are slower than optical links.
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Physical Media: coax, fiber
Coaxial cable:
Fiber optic cable:
• copper conductors
• bidirectional
• baseband:
– single channel on
cable
• glass fiber carrying light
pulses, each pulse a bit
• high-speed operation:
– high-speed point-to-point
transmission (e.g., 5 Gps)
• low error rate: repeaters
• broadband:
spaced far apart ; immune
to electromagnetic noise
– multiple channel on
cable
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Physical media: radio
• signal carried in
electromagnetic
spectrum
• no physical “wire”
• bidirectional
• propagation
environment effects:
– reflection
– obstruction by objects
– interference
Radio link types:
• terrestrial microwave:
– e.g. up to 45 Mbps channels
• LAN (e.g., WaveLAN)
– 2Mbps, 11Mbps
• wide-area (e.g., cellular)
– e.g. 3G: hundreds of kbps
• Satellite:
– up to 50Mbps channel (or
multiple smaller channels)
– 270 msec end-end dela
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Communications Subnet (cont)
2. the Data Link Layer (DLL): 1. Error control; 2.
Flow control; 3 Synchronizes the receiver to the
incoming bit stream; 4. Decodes the bit stream.
• Sublayer 2a: Media Access Control (MAC).
regulate the access to that shared link
• Sublayer 2b: Logical Link Control (LLC).
Implement error detection or reliable packet
transmission between computers attached to
a shared link.
• The MAC and LLC together constitute the data link
layer to implement a packet transmission service with
error detection or a reliable packet transmission
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service
Communications Subnet (cont)
• Layer 3: Network Layer
•
•
•
•
•
is concerned with routing
the frame. The three steps of routing are: 1. Establishing the
connection, 2. Maintaining the connection, 3. Terminating the
connection after the data transfer is complete.
Routing is the function to find the path the packets must
follow.
The network layer appends unique network addresses of the
source and destination computers.
Addressing scheme in packet-switched networks is that used
by the Internet.
Circuit-switch networks, like the telephone network, use
different addressing schemes.
The network layer uses the transmission over point-to-point
links provided by the data link layer to transmit packets
between any two computers attached in a network.
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The network edge:
• End systems (hosts):
– Run application programs
– Web, email
– at “edge of network”
• Client/server model
– Client host requests, receives
service from always-on server
– Web browser/server; email
client/server
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The Network Core
• Mesh of interconnected
routers
• the fundamental
question: how is data
transferred through
network?
• --circuit switching:
dedicated circuit per
call: telephone net
– packet-switching:
data sent thru net in
packets
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Circuit Switching
End-to-end
resources
reserved for “call”
• link bandwidth,
switch capacity
• dedicated
resources: no
sharing
• circuit-like
(guaranteed)
performance
• call setup required
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Packet Switching
Each end-end data stream Resource contention:
divided into packets
• aggregate resource
• users share network
demand can exceed
resources dynamically
amount available
bandwidth
• each packet uses full link
bandwidth
• congestion: packets
queue, wait for link
• resources used as
use
needed
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Packet Switching: Statistical
Multiplexing
10 Mbs
Ethernet
A
B
statistical multiplexing
C
1.5 Mbs
queue of packets
waiting for output
link
D
E
Sequence of A & B packets does not have
fixed pattern  statistical multiplexing.
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What’s the Internet:
• Millions of connected
computing devices:
hosts, end-systems
– PCs workstations, servers;
running network applcts
• communication links
router
server
mobile
local ISP
– fiber, copper, radio,
satellite
– transmission rate =
bandwidth
• Routers (gateways):
forward packets (chunks
of data)
workstation
regional ISP
company
network
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What’s a protocol?
Hi
TCP connection
req
Hi
TCP connection
response
Got the
time?
2:00
<file>
time
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What’s a protocol?
human protocols:
• “what’s the time?”
• “I have a question”
• introductions
… specific msgs sent
… specific actions taken
when msgs received,
or other events
network protocols:
• machines rather than
humans
• all communication
activity in Internet
governed by protocols
protocols define format,
order of msgs sent and
received among network
entities, and actions
taken on msg
transmission, receipt 43
Protocol “Layers”
Networks are
complex!
• many “pieces”:
– hosts
– routers
– links of various
media
– applications
– protocols
– hardware,
software
44
A closer look at network structure:
• Network edge:
applications and hosts
• Network core:
– routers
– network of networks
• Access networks,
Physical media:
communication links
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Internet protocol stack
• application: supporting network
applications
– FTP, SMTP, STTP
• transport: host-host data transfer
-TCP, UDP (user datagram protocol)
• Network: routing of datagrams
from source to destination
– IP, routing protocols
• Data link: data transfer between
neighboring network elements
application
transport
network
link
physical
– PPP, Ethernet
• Physical: bits “on the wire”
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Network Software
Protocol Hierarchies
•
Layers, protocols, and interface services.47
The relationship between
a service and a protocol
Layer k+1
Layer k+1
Service provided by layer k
Layer k
Layer k
Protocol
Layer k-1
Layer k-1
A set of layers and protocols is called network architecture.
A list of protocols used by a certain system, one protocol per
layer, is called a protocol stack.
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Correspondence of OSI and TCP/IP
Reference models
7
Application
6
Presentation
5
Session
4
Transport
Transport
3
Network
Internet
2
Data link
Host-to-network
1
Physical
OSI
Application
Not presented
in the model
TCP/IP
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Protocols and networks in the TCP/IP
model initially
TELNET
Protocols
FTP
SMTP
TCP
DNS
Transport
UDP
Network
IP
Networks
ARPANET
SATNET
Packet
radio
Application
LAN
Physical+
data link
Layer (OSI)
names
50
Mixed OSI and TCP/IP layers
Each layer takes data from above
• adds header information to create new data unit
• passes new data unit to layer below
source
M
Ht M
Hn Ht M
Hl Hn Ht M
application
transport
network
link
physical
destination
application
Ht
transport
Hn Ht
network
Hl Hn Ht
link
physical
M
message
M
segment
M
M
datagram
frame
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Layering: physical communication
data
application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
application
transport
network
link
physical
data
application
transport
network
link
physical
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• The Internet Layer is the glue that holds the whole
architecture together. Its job is to permit hosts to
inject packets into any network and have them travel
independently to the destination (potentially on a
different network). They may even arrive in different
order than they were sent, in which case it is the job
of higher layers to rearrange them, if in-order
delivery is desired.
• The internet layer defines an official packet format
and protocol called IP (Internet Protocol). The job of
the internet layer is to deliver IP packets where they
are supposed to go. Packet routing is clearly the
major issue here, as is avoiding congestion. For
these reason, it is possible to say that:
the internet layer is similar in functionality to
the OSI network layer.
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• The Transport Layer is designed to allow the source and
destination hosts to carry on a conversation, just as in the OSI
transport layer.
• Two end-to-end transport protocols are:
• 1. TCP (Transport Control Protocol), is a reliable
connection-oriented protocol that allows a byte stream
originating on one machine to be delivered within error on any
other machine in the Internet. It fragments the incoming byte
stream into discrete messages and passes each one on the
internet layer. At the destination, the receiving TCP process
reassembles the received messages into the output. TCP also
handles flow control.
• 2. UDP (User Datagram Protocol), is an unreliable,
connectionless protocol for application that do not want TCP’s
sequencing or flow control and wish to provide their own. It is
also widely used for one-shot, client-server-type request-reply
queries and applications in which prompt delivery is more
important than accurate delivery, such as transmitting speech
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or video.
Connectionless and ConnectionOriented Services
• Internet, generally TCP/IP network provide
two types of services to its applications:
1. connectionless services;
2. connection-oriented services
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Connection-oriented service
Goal: data transfer
TCP service [RFC 793]
between end systems
• handshaking: setup
(prepare for) data
transfer ahead of time
• reliable, in-order bytestream data transfer
– Hello, hello back
human protocol
– set up “state” in two
communicating hosts
• TCP - Transmission
Control Protocol
– Internet’s connectionoriented service
– loss: acknowledgements
and retransmissions
• flow control:
– sender won’t overwhelm
receiver
• congestion control:
– senders “slow down
sending rate” when
network congested
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Connectionless service
Goal: data transfer
between end systems
– same as before!
• UDP - User Datagram
Protocol:
• Internet’s
connectionless service
– unreliable data
transfer
– no flow control
– no congestion control
App’s using TCP:
• HTTP (Web), FTP (file
transfer), Telnet
(remote login), SMTP
(email)
App’s using UDP:
• streaming media,
teleconferencing,
Internet telephony
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• The Application Layer
• The TCP/IP model does not have session or presentation
layers.
• On top of the transport layer is the application layer. It contains
all the higher-level protocols. The early ones included virtual
terminal (TELNET), file transfer (FTP), and electronic mail
(SMTP). The virtual terminal protocol allows a user on one
machine to log onto a distant machine and. Electronic mail
was originally just a kind of file transfer, but later a specialized
protocol (SMTP) was developed for it. Many other protocols
have been added to these over the years, the Domain Name
System (DNS) for mapping host names onto their network
addresses, NNTP, the protocol for moving USENET news
articles around, and HTTP, the protocol for fetching pages on
the World Wide Web, and many others.
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Networking Technologies
Circuit Switching
Static
SDH/SONET
Dynamic
DTM
Packet Switching
ConnectionOriented
ConnectionLess
ATM
Gigabit
Ethernet
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Circuit Switching: FDMA and TDMA
Example:
FDMA
4 users
frequency
time
TDMA
frequency
time
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Packet Switching
each end-end data stream
divided into packets
• user A, B packets share
network resources
• each packet uses full link
bandwidth
• resources used as needed
Bandwidth division into
“pieces”
Dedicated allocation
Resource reservation
resource contention:
• aggregate resource
demand can exceed
amount available
• congestion: packets
queue, wait for link
use
• store and forward:
packets move one
hop at a time
– transmit over link
– wait turn at next
61
link
Layering: logical communication
transport
• take data from
application layer
• add addressing,
reliability check
info to form
“datagram”
• send datagram
to peer
• wait for peer to
ack receipt
• analogy: post
office
data
application
transport
transport
network
link
physical
application
transport
network
link
physical
ack
data
network
link
physical
application
transport
network
link
physical
data
application
transport
transport
network
link
physical
62
Layering: logical communication
Each layer:
• distributed
• “entities”
implement
layer
functions at
each node
• entities
perform
actions,
exchange
messages
with peers
application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
application
transport
network
link
physical
application
transport
network
link
physical
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