Transcript Networks and Services

Questions about networks?
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What is /what kinds?
Purposes?
Requirements?
Functions?
How ? Implementation and usage?
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Networks and Services
• Communication Networks
• Information Services
• Network Evolution
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What is a communication network?
• Infrastructure consisting of equipment & facilities
to transfer information between/among users in
different locations (telephony, television, cellular,
Internet)
• Like railroads, highways, & airline infrastructures
which transport people & goods across space
• Communications networks transport information
across space, but
– at the speed of light and at very high rates
• The exchange of information enables interaction at
a distance nearly instantaneously
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Communication Networks VS.
Transportation Networks
Flow of people & goods  Flow of information
Flexible interconnectivity&various services (both)
Roads & highways  transmission lines
Intersections & interchanges  switches/routers
Deciding the route (which exit in an interchange)
routing
• Moving along the route forwarding
• Traffic lights & stop signs flow control
• Road maintenance, detour, …
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Networks enable services
• Transportation systems enable one basic service
– the transfer of objects, then (built on it)
– postal / passenger / freight service
• Communications networks typically provide a
basic information service
– telephony: two-way exchange of voice signals
– voice messaging, fax, modem
– Internet: transfer of information packets
– e-mail, web browsing, e-commerce ...
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Typical Network Interactions
• Gathering/ Concentration
– this is the traditional interaction model such as
– bill payments / registration
– polling, voting, auctions
• Distribution
– electronic newspaper delivery
– television & radio broadcasting
– mass e- mail.
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Typical Network Interactions (cont.)
• Request/ Reply (typical client/server model)
– catalog ordering
– Web browsing
• Two & Multi-way Interactive
– telephone (& conferencing)
– video-on-demand
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Typical services and Requirements
• Radio & television broadcasting, telephone
service, cellular telephone service, E-mail,
web-browsing, video-on-demand, audiovideo-conferencing
• Delay, reliability of service, accuracy of
transmission, volume of information,
security and privacy, other specific
requirements.
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Two Examples of Networks & Services
• Telephone Networks were developed solely to
provide two-way voice communications
• Internet was developed to provide a basic service
(transfer of packets of information) which would
provide a platform for developing many services
–e- mail
– web- browsing
– video streaming
– many variants of e- commerce
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Telephone
Office
The caller picks up the phone triggering the flow of current in wires
that connect to the telephone office.
Telephone
Office
The current is detected and a dial tone is transmitted by the
telephone office to indicate that it is ready to receive the
destination number.
Telephone
Office
The caller sends this number by pushing the keys on the telephone set.
Each key generates a pair of tones that specify a number. (In the older
phone sets the user dials a number which in turn generates a
corresponding number of pulses.)
Telephone
Office
The equipment in the telephone office then uses the telephone
network to attempt a connection. If the destination telephone busy,
then a busy tone is returned to the caller. If the destination telephone
is idle, then ringing signals are sent to both the originating and
destination telephones.
Telephone
Office
The ringing signals are discontinued when the destination
phone is picked up and communication can then proceed.
Telephone
Office
Either of the users terminate the call by putting down a
receiver.
Telephone set up and communication
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Figure 1.1
Requirements for telephone service
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Two-way communication
Real-time, not much delay
Reliable connection-oriented
Voice quality reduces a little (not exact),OK
Others: connection all the time,
security&privacy sometime, 800 calls,
caller ID, voice mail, call return, etc.
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Email service and requirements
Given receiver’s email address, input subject and
message, then Click send.
NO
NO
YES
•Delivery confirmation and security and
privacy (sometime)
Real time?
Connection-oriented?
Accuracy?
•Others: email reply, email forward,
email list, etc
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Retrieving a web page
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The user clicks on a link to indicate which document is to be
retrieved.
The browser must determine the address that contains the
document. It does this by sending a query to its local name
server.
Once the address is known the browser establishes a connection to the
specified machine, usually a TCP connection. In order for the
connection to be successful, the specified machine must be ready to
accept TCP connections.
The browser runs a client version of HTTP, which issues a request
specifying both the name of the document and the possible document
formats it can handle.
The machine that contains the requested document runs a server
version of HTTP. It reacts to the HTTP request by sending an
HTTP response which contains the desired document in the
appropriate format.
The TCP connection is then closed and the user may view
the document.
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Figure 1.4
WWW service & requirements
• Not real time, but can not delay too long
• Not only text, but also audio, images
• Dynamic content, links to anywhere
Concepts: HTTP protocol (HyperText Transfer
Protocol)
Client/server model (http client, http server),
URL (Uniform Resource Locator):
(protocol://host:port/file path and file name)
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Network design and evolution
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Network functions and network topology
Telegraph network—message switching
Telephone network—circuit switching
Computer networks and the Internet--packet switching
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Network Design
• Applications impose requirements on the services
provided by a network
– delay; reliability; accuracy
– volume and rate
– cost & convenience, even security.
• The network design
– meets these requirements cost- effectively
– must address a common set of functions (what?)
– may take fundamentally different approaches
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Essential Network Functions
• Basic user service
--Voice connections or packet transfer.
• Terminal – the end system that connects to the network,
telephone or computer.
• Transmission system – the means for transmitting
information across a physical medium: copper telephone
wires, coaxial TV cable, optical fiber.
• Information representation – the format of the
information handled by the network, voice or bits.
• Addressing – the means for identifying points of
connection to the network: telephone number or IP address
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Essential Network Functions (cont.)
• Routing – the means for determining the path across
the network
• Switching/forward approach – the means of
transferring information flows between
communication lines.
• Multiplexing – the means for connecting multiple
information flows into shared connection lines.
• Flow & congest control.
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A Network transfers information among users
A single block of information
t1
t0
Network
or a stream of information.
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Figure 1.6
Network Topology: How a Network Grows
• A network involves the interconnection of
transmission lines using switches to convey
information among users
• The growth of a network from a few users in close
proximity to a very large community over a wide
geographic region leads to a hierarchical network
structure
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(a) A switch provides the network to a cluster of users
n lines
n*(n-1)/2 lines
Network
Access network
(b) A multiplexer connects two access networks
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Figure 1.7
1*
a
(a)
2
b
4
3
A
c
Metropolitan network A
consists of access
subnetworks a, b, c, d.
d
Metropolitan
(b)
A
a
b
g
National network
consists of regional
subnetworks a, b, g.
Metropolitan network
A is part of regional
subnetwork a.
Hierarchical network topology
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Figure 1.8
Approaches to Network Design
Function
Telegraph
Network
Telephone
Network
Basic User
Service
Transmission
of telegrams
Bi-directional realtime transfer of
voice signals
Datagram&reliable
stream service
between computers
Switching
Approach
Message
switching
Circuit switching
Connectionless
packet-switching
Terminal
Telegraph,
Teletype
Telephone, modem Computer
Information
Morse, Baudot, Analog voice or
representation ASCII
PCM digital voice
Internet
Any binary
information
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Approaches to Network Design (cont.)
Function
Telegraph
Network
Telephone
Network
Transmission
System
Digital over
various media
Analog and digital
over various media
Digital over various
media
Addressing
Geographical
addresses
Hierarchical
numbering plan
Hierarchical
address space
Routing
Manual routing Route selected
during call setup
Each packet routed
independently
Multiplexing
Character and
Message
multiplexing
Packet multiplexing
Circuit multiplexing
Internet
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Telegraphy Networks & message switching
Samuel B. Morse invented telegraph in 1837
– binary communications using dots & dashes
– Morse code maps alphabet to sequences of dots/ dashes
– text messages transmitted hop- by- hop across a
network of relay stations
– Manual routing, routing decision is done by an operator
– Operator stores a message, finds next station according
to the destination and forwards it, so message switching
–Multiplexing, the symbols from several operators into
the same communication line. (20bits/sec120bits/sec)
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Morse code
(See Table 1.1 at page 3 or 15)
Letter
A
B
C
D
E
…
T
Code
.-…
-.-.
-..
.
Prob.
0.081
0.014
0.027
0.039
0.131
Number
1
2
3
4
5
Code
.---..--…-….…..
Prob.
….
-
0.104
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Telephone networks & circuit switching
1875, Alexander G. Bell invented telephone
• Voice (analog) signal, telephone terminal is very simple,
anybody can use it
• Switches were introduced because of the vast cost of
dedicated lines (recall n*(n-1) lines to n lines)
• Human operator performs switching, manual routing
• Connection-oriented, dedicated end-to-end connection
(dedicated lines and switches), therefore, circuit switching
• Routing decision at setting up of the connection
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(a) A switch in the form of an operator with a patch cord panel (not shown)
(b) Cords interconnect user sockets providing end-to-end connection
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Figure 1.10
Advances of telephone networks
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Digital (0,1) transmission, PCM technique
Multiplexing: 20 voice calls, 1.5Mbps
Digital switches
Hierarchical telephone networks: local/ tandem/toll switches
Hierarchical numbering (addressing) system: area code + exchange
number + phone number
• Automatic connection by computer
• A separate singling network for management
• Enhanced telephone services:credit-card call, long distance
call, 800, caller ID, voice mail, cellular phone (mobility).
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CO: Center Office switch
Tandem switch
Toll switch
Toll
Tandem
Tandem
CO
CO
CO
CO
CO
Hierarchical telephone network structure
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Figure 1.11
Topics discussed (a brief summary)
What is a communication network?
An infrastructure to transfer information over space
What is the purpose of it?
Provides services
What are the main requirements?
Delay, reliability, accuracy
What are four typical interactions?
Gathering, distribution, request/reply, two& multiple way interaction
What does connection-oriented mean?
A connection between two parties must be set up ahead of information transmission
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Topics discussed (a brief summary—cont.)
What are the main network functions?
Switching, information representing, addressing, routing, multiplexing
What are approaches implementing the functions?
In telegraph networks:
Message switching, Morse code, geographical address, manual routing,
char&message multiplexing
In telephone networks:
Circuit switching, analog or digital, hierarchical numbering,
call setup manually/automatically, circuit multiplexing
What is the generic network topology?
Hierarchical network structure
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