Transcript Chapter 1 - UniMAP Portal

Communication
Networks
Chapter 1 : Introduction
to Communication
Networks and Services
1
Communication Services &
Applications
A communication service enables
the exchange of information
between users at different locations.
 Communication services &
applications are everywhere.

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Communication Services &
Applications
E-mail
E-mail
server
Exchange of text messages via servers
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Communication Services &
Applications
Web Browsing
Web server
Retrieval of information from web servers
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Communication Services &
Applications
Instant Messaging
Direct exchange of text messages
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Communication Services &
Applications
Telephone
Real-time bidirectional voice exchange
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Communication Services &
Applications
Cell phone
Real-time voice exchange with mobile users
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Communication Services &
Applications
Short Message Service
Fast delivery of short text messages
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What is a communication network?
Communication
Network

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The equipment (hardware & software) and facilities
that provide the basic communication service
Virtually invisible to the user; Usually represented by
a cloud
 Facilities
Equipment

Routers, servers,
switches, multiplexers,
hubs, modems, …

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Copper wires, coaxial
cables, optical fiber
Ducts, conduits,
telephone poles …
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How are communication networks designed and operated?
Communication Network
Architecture
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Network architecture: the plan that specifies how the network is
built and operated
Architecture is driven by the network services
Overall communication process is complex
Network architecture partitions overall communication process
into separate functional areas called layers
Next we will trace evolution of three network architectures:
telegraph, telephone, and computer networks
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Network Architecture Evolution
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Telegraph Networks
 Message switching & digital transmission
Telephone Networks
 Circuit Switching
 Analog transmission → digital transmission
 Mobile communications
Internet
 Packet switching & computer applications
Next-Generation Internet
 Multiservice packet switching network
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Bell’s Telephone
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Alexander Graham Bell (1875) working on harmonic telegraph
to multiplex telegraph signals
Discovered voice signals can be transmitted directly
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Microphone converts voice pressure variation (sound) into
analogous electrical signal
Loudspeaker converts electrical signal back into sound
Telephone patent granted in 1876
Bell Telephone Company founded in 1877
Signal for “ae” as in cat
Microphone
sound
Loudspeaker
analog
electrical
signal
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sound
Bell’s Sketch of Telephone
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Signaling
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Signaling required to establish a call
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Flashing light and ringing devices to alert the
called party of incoming call
Called party information to operator to establish
calls
Signaling + voice signal transfer
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The N2 Problem
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For N users to be fully connected directly
Requires N(N – 1)/2 connections
Requires too much space for cables
Inefficient & costly since connections not always on
1
N = 1000
N(N – 1)/2 = 499500
2
N
4
3
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Telephone Pole Congestion
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Circuit Switching
Patchcord panel switch invented in 1877
Operators connect users on demand
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Establish circuit to allow electrical current to flow
from inlet to outlet
Only N connections required to central office
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1
N
N–1
3
2
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Manual Switching
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Strowger Switch
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Human operators intelligent & flexible
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Strowger invented automated switch in 1888
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But expensive and not always discreet
Each current pulse advances wiper by 1 position
User dialing controls connection setup
Decimal telephone numbering system
Hierarchical network structure simplifies routing
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Area code, exchange (CO), station number
1st digit
2nd digit
...
0
0
0
.
.
.
.
.
.
.
.
.
9
0
9
9
19
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Strowger Switch
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Hierarchical Network Structure
CO = central office
Toll
Tandem
Tandem
CO
CO
CO
CO
CO
Telephone subscribers connected to local CO (central office)
Tandem & Toll switches connect CO’s
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Three Phases of a Connection
1.
2.
Telephone
network
Pick up phone
Dial tone.
Telephone
network
Connection
set up
Dial number
3.
Telephone
network
Network selects route;
4.
Telephone
network
Sets up connection;
Called party alerted
Information
transfer
Connection
release
5.
Telephone
network
6.
Telephone
network
Exchange voice
signals
Hang up.
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Computer Connection Control
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Coordinate set up of telephone connections
To implement new services such as caller ID, voice mail, . . .
To enable mobility and roaming in cellular networks
“Intelligence” inside the network
A separate signaling network is required
Computer
Signaling
Switch connects
Inlets to Outlets
...
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A computer controls connection in telephone switch
Computers exchange signaling messages to:
...
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Voice
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Digitization of Telephone Network
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Pulse Code Modulation digital voice signal
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Time Division Multiplexing for digital voice
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T-1 multiplexing (1961): 24 voice signals = 1.544x106 bps
Digital Switching (1980s)
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Voice gives 8 bits/sample x 8000 samples/sec = 64x103 bps
Switch TDM signals without conversion to analog form
Digital Cellular Telephony (1990s)
Optical Digital Transmission (1990s)
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One OC-192 optical signal = 10x109 bps
One optical fiber carries 160 OC-192 signals = 1.6x1012 bps!
All digital transmission, switching, and control
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Elements of Telephone Network
Architecture
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Digital transmission & switching
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Circuit switching
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User signals for call setup and tear-down
Route selected during connection setup
End-to-end connection across network
Signaling coordinates connection setup
Hierarchical Network
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Digital voice; Time Division Multiplexing
Decimal numbering system
Hierarchical structure; simplified routing; scalability
Signaling Network
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Intelligence inside the network
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Computer Network Evolution
Overview
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1950s: Telegraph technology adapted to computers
1960s: Dumb terminals access shared host computer
 SABRE airline reservation system
1970s: Computers connect directly to each other
 ARPANET packet switching network
 TCP/IP internet protocols
 Ethernet local area network
1980s & 1990s: New applications and Internet growth
 Commercialization of Internet
 E-mail, file transfer, web, P2P, . . .
 Internet traffic surpasses voice traffic
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What is a protocol?
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Communications between computers requires
very specific unambiguous rules
A protocol is a set of rules that governs how
two or more communicating parties are to
interact
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Internet Protocol (IP)
Transmission Control Protocol (TCP)
HyperText Transfer Protocol (HTTP)
Simple Mail Transfer Protocol (SMTP)
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A familiar protocol ??
Caller
Dials 411
“What city”?
Caller
replies
Caller
replies
“Springfield”
“What name?”
“Simpson”
“Thank you, please hold”
Caller
waits
Caller
replies
Caller
waits
Caller
dials
“Do you have a first name or
street?”
System
replies
System
replies
System
replies
Operator
replies
“Evergreen Terrace”
“Thank you, please hold”
Operator
replies
System
replies with
number
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Terminal-Oriented Networks
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Early computer systems very expensive
Time-sharing methods allowed multiple
terminals to share local computer
Remote access via telephone modems
Terminal
...
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Terminal
Modem
Host computer
Telephone
Network
Modem
Terminal
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Medium Access Control
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Dedicated communication lines were expensive
Terminals generated messages sporadically
Frames carried messages to/from attached terminals
Address in frame header identified terminal
Medium Access Controls for sharing a line were developed
Example: Polling protocol on a multidrop line
Polling frames & output frames
input frames
Terminal
Host computer
Terminal
...
Terminal
Terminals at different locations in a city
Must avoid collisions on inbound line
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Statistical Multiplexing
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Statistical multiplexer allows a line to carry frames that
contain messages to/from multiple terminals
Frames are buffered at multiplexer until line becomes
available, i.e. store-and-forward
Address in frame header identifies terminal
Header carries other control information
Frame
CRC
Information
Terminal
Header
Header
Information
...
Terminal
CRC
Terminal
Host computer
Multiplexer
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Error Control Protocol
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Communication lines introduced errors
Error checking codes used on frames
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“Cyclic Redundancy Check” (CRC) calculated based on
frame header and information payload, and appended
Header also carries ACK/NAK control information
Retransmission requested when errors detected
CRC
Information
Header
Terminal
Header
Information
CRC
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Computer-to-Computer Networks
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As cost of computing dropped, terminal-oriented networks viewed as
too inflexible and costly
Need to develop flexible computer networks
 Interconnect computers as required
Support many applications
Application Examples
 File transfer between arbitrary computers
 Execution of a program on another computer
 Multiprocess operation over multiple computers
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Packet Switching
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Network should support multiple applications
 Transfer arbitrary message size
Low delay for interactive applications
 But in store-and-forward operation, long messages induce high
delay on interactive messages
Packet switching introduced
 Network transfers packets using store-and-forward
 Packets have maximum length
 Break long messages into multiple packets
ARPANET testbed led to many innovations
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ARPANET Packet Switching
Host generates message
Source packet switch converts message to packet(s)
Packets transferred independently across network
Destination packet switch reasembles message
Destination packet switch delivers message
Packet
Switch
Message
Packet 2
Packet
Switch
Packet 1
Packet 2
Message
Packet
Switch
Packet
Packet 1
Packet
Switch Packet 1
Switch
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ARPANET
Routing
Routing is highly nontrivial in mesh networks
No connection setup prior to packet transmission
Packets header includes source & destination addresses
Packet switches have table with next hop per destination
Routing tables calculated by packet
switches using distributed algorithm
Packet
Switch
Hdr Packet
Packet
Switch
Packet
Switch
Dest: Next Hop:
Packet
Switch
Packet
Switch
xyz
abc
wvr
edf
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OtherError
ARPANET
Protocols
control between adjacent packet switches
Congestion control between source & destination
packet switches limit number of packets in transit
Flow control between host computers
prevents buffer overflow
Packet
Switch
Packet
Switch
Error
Control
Congestion
Control
Packet
Switch
Packet
Switch
Packet
Switch
Flow
Control
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ARPANET Applications
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ARPANET introduced many new applications
Email, remote login, file transfer, …
Intelligence at the edge
AMES
McCLELLAN
UTAH
BOULDER
GWC
CASE
RADC
ILL
CARN
LINC
USC
AMES
MIT
MITRE
UCSB
STAN
SCD
ETAC
UCLA
RAND
TINKER
BBN
HARV
NBS
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Ethernet Local Area Network
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In 1980s, affordable workstations available
Need for low-cost, high-speed networks
 To interconnect local workstations
 To access local shared resources (printers, storage,
servers)
Low cost, high-speed communications with low error rate
possible using coaxial cable
Ethernet is the standard for high-speed wired access to
computer networks
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Ethernet Medium Access Control
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Network interface card (NIC) connects workstation to LAN
Each NIC has globally unique address
Frames are broadcast into coaxial cable
NICs listen to medium for frames with their address
Transmitting NICs listen for collisions with other stations, and
abort and reschedule retransmissions
Transceivers
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The Internet
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Different network types emerged for data
transfer between computers
ARPA also explored packet switching using
satellite and packet radio networks
Each network has its protocols and is
possibly built on different technologies
Internetworking protocols required to enable
communications between computers
attached to different networks
Internet: a network of networks
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Internet Protocol (IP)
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Routers (gateways) interconnect different networks
Host computers prepare IP packets and transmit
them over their attached network
Routers forward IP packets across networks
Best-effort IP transfer service, no retransmission
Router
Net 1
Net 2
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Addressing & Routing
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Hierarchical address: Net ID + Host ID
IP packets routed according to Net ID
Routers compute routing tables using
distributed algorithm
H
H
Net 3
G
Net 1
G
G
G
H
Net 2
Net 5
G
Net 4
G
H
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Transport Protocols
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Host computers run two transport protocols on top of IP
to enable process-to-process communications
User Datagram Protocol (UDP) enables best-effort
transfer of individual block of information
Transmission Control Protocol (TCP) enables reliable
transfer of a stream of bytes
Transport
Protocol
Internet
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Names and IP Addresses
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Routing is done based on 32-bit IP addresses
Dotted-decimal notation
 128.100.11.1
Hosts are also identified by name
 Easier to remember
Hierarchical name structure
 tesla.comm.utoronto.edu
Domain Name System (DNS) provided conversion between
names and addresses
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Internet Applications
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All Internet applications run on TCP or UDP
TCP: HTTP (web); SMTP (e-mail); FTP (file
transfer; telnet (remote terminal)
UDP: DNS, RTP (voice & multimedia)
TCP & UDP incorporated into computer
operating systems
Any application designed to operate over
TCP or UDP will run over the Internet!!!
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Elements of Computer Network
Architecture
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Digital transmission
Exchange of frames between adjacent equipment
 Framing and error control
Medium access control regulates sharing of
broadcast medium.
Addresses identify attachment to network or
internet.
Transfer of packets across a packet network
Distributed calculation of routing tables
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Elements of Computer Network
Architecture
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Congestion control inside the network
Internetworking across multiple networks using routers
Segmentation and reassembly of messages into
packets at the ingress to and egress from a network or
internetwork
End-to-end transport protocols for process-to-process
communications
Applications that build on the transfer of messages
between computers.
Intelligence is at the edge of the network.
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Trends in Network Evolution
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It’s all about services
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Building networks involves huge expenditures
Services that generate revenues drive the
network architecture
Current trends
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Packet switching vs. circuit switching
Multimedia applications
More versatile signaling
End of trust
Many service providers and overlay networks
Networking is a business
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Packet vs. Circuit Switching
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Architectures appear and disappear over time
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Telegraph (message switching)
Telephone (circuit switching)
Internet (packet switching)
Trend towards packet switching at the edge
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IP enables rapid introduction of new applications
New cellular voice networks packet-based
Soon IP will support real-time voice and telephone network will
gradually be replaced
However, large packet flows easier to manage by circuit-like
methods
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Optical Circuit Switching
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Optical signal transmission over fiber can carry huge volumes of
information (Tbps)
Optical signal processing very limited
 Optical logic circuits bulky and costly
 Optical packet switching will not happen soon
Optical-to-Electronic conversion is expensive
 Maximum electronic speeds << Tbps
 Parallel electronic processing & high expense
Thus trend towards optical circuit switching in the core
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Standards
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New technologies very costly and risky
Standards allow players to share risk and
benefits of a new market
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Reduced cost of entry
Interoperability and network effect
Compete on innovation
Completing the value chain
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Chips, systems, equipment vendors, service providers
Example
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802.11 wireless LAN products
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Standards Bodies
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Internet Engineering Task Force
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International Telecommunications Union
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International telecom standards
IEEE 802 Committee
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Internet standards development
Request for Comments (RFCs): www.ietf.org
Local area and metropolitan area network standards
Industry Organizations
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MPLS Forum, WiFi Alliance, World Wide Web Consortium
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Exercise 1
The introduction of a new services usually impacts other
services through substitution. Describe how substitution takes
place in the following cases.
a)
b)
c)
d)
E-mail, facsimile and postal mail
E-mail, local and long distance phone service
Cell phone, local and long distance phone service
Peer-to-peer file exchange and commercial CD recording
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Solution Ex 1.1
a) E-mail, facsimile and postal mail
E-mail is used for most of the correspondence previously handled by
postal mail. Documents sent by facsimile are also transferred using
E-mail as attachments. Hardcopies can be scanned for electronic
transmission
b) E-mail, local and long distance phone service
E-mail is an inexpensice and convenient alternative for most of the
communication in which real time interaction is not essential.
Instant-messaging is faster than e-mail and more closely
approaches the real-time experience of the telephone
c) Cell phone, local and long distance service
cell phone is used for local and long distance calls mostly because
users can be reached even if they are not in a specific location such
as home or office. The steep drop in the cost of long distance
relative to the cost of cellular airtime has enable ‘call anywhere’
cellular service offering
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Exercise 1.2
Describe step-by-step procedure that is involved from the time
you deposit a letter in a mailbox to the time the letter is delivered
to its destination.
What role do names, addresses and mail codes play?
How might the letter be routed to its destination?
To what extent can be the process be automated?
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Solution Ex 1.2
The steps involved in mailing a letter are
1. The letter is deposited in mailbox
2. The letter is picked up by postal employee and placed in a
sack
3. The letter is taken to a sorting station, where it is sorted
according to destination, as determined by the mail code and
grouped with other letters with the same destination mail
code. ( or by largest geographical unit / country/ city)
4. The letter is shipped to the post office that handles the mail
for the specific mail code
5. The letter is then sorted by street address
6. The letter is picked up at the post office by the postal worker
responsible for delivering to the specified address
7. The letter is delivered according to the number and street
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Exercise 1.3
Suppose that a letter is sent by fax. Is this mode of
communications connectionless or connection-oriented? Realtime or non-real time?
SOLUTION:
In order to send a letter by fax, a telephone connection must first
be established. Therefore, the mode of communication is
connection oriented. The transfer of information across the
network accurs in real time.
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Exercise 1.4
Two musician is located in different cities wish to have a jam session
over a communications network. Find the maximum possible distance
between the musicians if the are to interact in real time, in the sense of
experiencing the same delay in hearing each other as if they were 10
meters apart. The speed of sound is approximately 330 m/s. Assume
that the network transmits the sound at the speed of light in cable, 2.3 x
108 m/s.
SOLUTION:
Find the delay for the sound when the musician 10 m apart:
t10 = 10/ (330 m/s) = 30.30 milliseconds
The maximum distance is the time required for a real time ‘experience’
times the cable speed
d = (2.3 x 108 m/s) x (30.30 x 10-3 s) = 6 969 km
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