Transcript cos440-Intro

Network Structure
• Network
 An interconnection of autonomous computers
• Host
 Autonomous computer running end user programs
• Transmission System
 Collection of links and processor that carry
information between hosts
Two Major Subject Areas
• Data Communications
 Transmission Systems
 Data representation
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Text
Images
Audio
Video
• Computer Networks
 Protocols
Transmission System
• Circuits – channels
 These are the paths over which data is carried
• Point-to-point
• Multipoint
• Broadcast
• Network Processors
 Computers that process and move data over the
channels
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Routers
Bridges
Packet switches
Gateways
Transmission System
• Packet
 Unit of data processed by the network
processors
• Sometimes referred to as a frame
Network Classification
• Local Area Network (LAN)
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Room
Building
Campus
Covers a few feet to a few miles
• Wide Area Network
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State
Country
Global
Covers from 10s to 1000s of miles
Network Classification
• Metropolitan Area Network (MAN)
 City
 Region
Distances between LANS and MANs are often
gray
Optical fiber reduces distances
Why Networks?
• Resource sharing
 Data
• Information
• Visual
• auditory
 Programs
 Equipment – from any location
• Distributed Processing
 Grid computing
• Collaboration
Desirable Characteristics
• Performance
 Capacity - Throughput
 Quality of Service
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Reliability
Robustness
Security
Cost Effective
Network Applications
• Access to databases
 Inquiry
• Schedules
• Market
• Libraries
 Update
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Electronic mail
Bulletin boards
Teleconference
Voice
Directory services
Layered Communications
Network Architecture
• Networking is organized and defined in layers
• Collection of definitions and rules as each layer
is called the protocol
• Between each adjacent layer there must be a
defined interface
• Functions are allocated to specific layers
• The set of layers and their definitions is called
the network architecture
• Set of protocols is called the protocol stack
Simple Protocol Stack
Application
Transport
Network
• Addressing
 Must identify source
computer
 Must identify
destination computer
 Define Service Access
Points (SAP)
Standards
Rules and definitions to allow for interconnections between
vendors
De Facto (by fact) Standards
Standards accepted by widespread use
proprietary
non proprietary
De Jury (by law) Standards
Standards legislated by an officially recognized body
The Data Communications
Industry
• The Data Communications industry has many
stakeholders with complex relationships
Deregulation
• Prior to early 1980s, most communications
provided by a single vendor – AT&T
• 1982 – AT&T judged to be a monopoly and
broken up into a number of small
companies (divestiture)
• Divestiture created long distance carriers
and local carriers called Regional Bell
Operating Companies (RBOCs)
• This created the concept of Local Access
Transport Areas (LATAs)
Standards Organizations
• International Telecommunications Union (ITU)
 ITU-R
 ITU-T
 ITU-D
Radio
Telephony standards
Development
http://www.itu.int
Previously known as International and Telephone
Consultative Committee (CCITT)
Standards Organizations
• International Standards Organization (ISO)
http://www.iso.org
• American National Standards Institute
(ANSI)
http://www.ansi.org
• Electronics Industries Association (EIA)
http://www.eia.org
• Institute of Electrical and Electronics
Engineers (IEEE)
http://www.ieee.org
Standards Organizations
• Various Forums
 Frame Relay
http://www.frforum.com
http://www.frame-relay-resource.com
 ATM Forum
http://www.atmforum.com
Internet Standards Organization
• Internet Society (ISOC)
http://www.isoc.org
• Internet Architecture Board (IAB)
http://www.iab.org
• Internet Engineering Task Force (IETF)
http://www.ietf.org
• This group is responsible to develop and publish Internet
standards
• Internet Research Task Force (IRTF)
http://www.irtf.org
Internet Standards Organization
• Internet Corporation for Assigned Names and
Numbers (ICANN)
http://www.icann.org
 Private, nonprofit, international board supported by
U.S. Government
 Responsible for assignments of all Internet address
space, domain names, protocol numbers
• Internet Assigned Numbers Authority (IANA)
http://www.iana.org
OSI Model
• One organization (ISO) developed a model for
data communications architectures
• Open System Interconnect (OSI model)
• Defines communications issues into 7 layers
• Purpose was to standardize interconnections
• Not restricted to a single topology, single
medium, or single communications technology
Layer1 – Physical Layer
• Function is to transmit raw bits from one point to
adjacent point
• Actual mechanical, electrical, and optical interface is
defined here
• Issues:
 Voltages, representation of bits
 Signals
 Timing, bit rates
• Mostly done in hardware
• Bits received at physical layer are passed on to layer 2
• String of bits to be transmitted received from layer 2
Common Physical Layer
Layer 2
Data Link Layer
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Receives raw bits from physical layer
Sends raw bits to layer 1
May collect data into groups called frames
Addressing information may be added at this
layer
• May provide acknowledgement frames for data
received
• May include error detection
• May provide for access control
 Who sends, when
Data Link Layer (con’t)
• May provide for flow control
 Prevent a fast sending from ‘drowning’ a slow
receiver
• May provide for synchronization of events
and timing
• Responsible for delivery to a next
(neighbor) node
Layer 3
Network Layer
• Responsible to determine paths (routes) to get
packets of data from source to ultimate
destination
• Frames received from Data Link Layer
 Network Layer determines if it is the destination
 If not destination, this layer determines where to send
frame next
• This routing may be static or dynamic
• Accounting may be done at this level
• The layer is concerned with end-to-end delivery
Source to Destination
Layer 3
Network Layer
• May move frames from one network
architecture to another
 This may involve addressing issues
 Physical addresses may vary between
architectures
 Data frame sizes may vary between
architectures
Network Subnet
• First three layers often called the Subnet
• They constitute the actual delivery system
• These layers may not even reside in a
host device
• Many implementations have these three
layers in specialized computers (routers)
• Many vendors have provided the services
of these three layers, for a fee
Layer 4
Transport Layer
• Sometimes viewed as the host to host or end to end
delivery of an entire message
• May required ordering of arriving packets to properly reconstruct an entire message
• May break a message into segments and reassemble
them at destination
• May provide different levels of types of transport
services
• May provide buffering of data
• May provide some flow control (end to end)
• May support multiple concurrent streams of logical
connections
 Must provide for connection identification (addressing)
• This layer is not concerned with the details of the subnet
Layer 5
Session Layer
• Establishes, maintains, and synchronizes
communications sessions between
systems
• May apply access rights
• May provide dialog control between
systems
• May provide for session synchronization
 Example: Applying checkpoints in a large file
transfer
• Graceful closing of sessions
Layer 6
Presentation Layer
• May perform various data manipulation functions
• Text Compression
 Condensing strings of repeated characters
 Coding commonly used words or strings
• Data encryption
• Code conversion between different character
sets
 ASCII – EBCDIC
 Between incompatible devices (printers, displays)
• Conversion between different file systems
Layer 7
Application Layer
• This is what it is all about – the application
• The ultimate function we are trying to perform
• Actual functions depend on what the application
is
• Examples
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File transfer
E-mail
Logging on to a remote system as a virtual terminal
Real time application – watching video steam or
listening to audio stream
• Directory services
OSI Reference Model
OSI Reference Model
Other Protocol Stacks
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IBM – Systems Network Architecture (SNA)
Digital Equipment – DECNET
Novell Corporation – Netware-IPX
Apple Computer – AppleTalk
Public Networks
 X.25
 Frame Relay
 ATM
• Internet – TCP/IP
Summary of OSI Model
• Brought out clear distinctions between
 Services
 Interfaces
 Protocols
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Organized functions into layers
Not widely accepted
Not widely implemented
WHY not?
Failure of OSI Model
Bad Timing
Apocalypse of two elephants
Failure of OSI Model
• Bad Technology
 Too many layers, some with few functions, some overloaded with
function
 Very complex
 Did not include connectionless service protocols
• Bad implementation
 Large, unwieldy, slow
 Image of poor quality
• Bad politics
 UNIX popular in academe and TCP/IP viewed as part of UNIX
 OSI thought to be the creation of the European
telecommunication community and later US Government
Internet Model
TCP/IP
• Has only 4 layers
• Subnet is based on a single protocol
called the Internet Protocol (IP)
 This is in fact the network layer
• Transport layer has two protocols
 Transmission Control Protocol (TCP)
 User Datagram Protocol (UDP)
Comparison of OSI and TCP/IP
Hybrid Model
TCP/IP Addresses
Network Programming
Client
Server
Make Contact
(connect)
Send
Listen for connection
Receive
Send
End
(Disconnect)
Close
Receive
Networking paradigm
Ping
response
Internet today
Network Tools
• Cyberkit
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Ping
Traceroute
Name lookup
Time
• Located on cos440 web site as Zip file