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January 2000 Communication Networks 2
Communication
People need to communicate with each other.
Why? To exchange information. What? Voice, Sound, Graphics,
Pictures, Text, or Data One way is to use the postal
service.
January 2000 Communication Networks 3
Communication (cont.)
Better, pull wires between each two people. (point-to-point communication)» cost is high and resources are wasted» complexity (mesh, drop and add)
We need to build a shared communication media (links)» Efficient» needs management (data may wait)
January 2000 Communication Networks 4
Communication (cont.)
We build communication networks. These are arrangements of
hardware and software that allow users to exchange information. (computer net)
Thus we have two types of nodes:» terminal nodes» communication nodes
January 2000 Communication Networks 5
Computer Network?
An interconnected collection of autonomous computers and computer resources
Terminal node
communication node
Shared medium
January 2000 Communication Networks 6
Simple Data Communication Model
TransceiverTransceiverTransport System
DigitalAnalog/Digital
Digital
001101
001101
Data Network
Public Telephone Network
January 2000 Communication Networks 7
Network Services
The objective is to provide services to the users.» Information transportation» signaling» billing
A service is the execution of a sequence of basic actions on network resources. (how to place a phone call?)
January 2000 Communication Networks 8
Network Services (cont.)
Scripts are executed by the communicating entities only.
These scripts could be very complex.
It is more efficient to have a modular construction of scripts:» action it performs» interaction to other modules
January 2000 Communication Networks 9
Network Services (Cont.)
Advantages of such modular:» reusability» upgradability» interoperability
The general organization of services into simpler services is called the network architecture.
January 2000 Communication Networks 10
Communication Protocols To provide error-free and convenient
information transfers, the network communication is regulated by a set of rules and conventions called network protocols.
Protocols define connectors, cables, signals, data formats, error control techniques, and algorithms for message preparation, analysis and transfer.
January 2000 Communication Networks 11
Protocol Data Units (PDU)
Protocol entities exchange PDUs» Each PDU must contain two major parts:
– Header: Identifies how the following parts are to be
handled and routed.
– Message: This is the message body itself. This is where the protocol is determined to be
character oriented or bit oriented.
Header Message Trailer
January 2000 Communication Networks 12
OSI Reference Model of ISO
Architecture/structure that defines communication tasks and which would:» Serve as a reference model for international
standards» would facilitate efficient internetworking among
systems from different technologies, manufacturers, administrations, nationalities, and enterprises.
January 2000 Communication Networks 14
Most Important Standards Organizations
ITU-T: International Telecommunication Union (a United Nations specialized agency, was created on March 1, 1993)
ISO: International Organization for Standardization (an international voluntary, nontreaty organization, founded in 1946)
IETF: Internet Engineering Task Force (responsible for publishing RFCs (Requests For Comments))
IEEE: Institute of Electrical and Electronic Engineers (ATM Forum: This organization is not a standard organization. After ITU defined
the ATM concept in Nov 1990, ATM Forum was initiated in October 1991 to accelerate the deployment of ATM products and services. ATM Forum develops implementation agreements and publishes them as “specifications” on its web site: www.atmforum.com)
January 2000 Communication Networks 15
ISO OSI Reference Architecture
The architecture is layered to reduce complexity.» Each layer offers certain services to the
layer immediately above it.» Each layer shields the higher layer from
the details of implementation of how the services are offered.
» Layer "n" on one station carries on a conversation with layer "n" on another network station.
January 2000 Communication Networks 16
7 Application ftp, telnet, email, www, etc.
6 Presentation Data representation
5 Session Negotiation and connection
4 Transport End-to-end delivery
3 Network Addresses and best path (routing)
2 Data Link Access to media (transfer of frames) 1 Physical Binary transmission and cabling
Layer Functions
January 2000 Communication Networks 17
Application Application Presentation Presentation Session Session Transport Transport NetworkNetwork Data Link Data Link Physical Physical
Layer Functions
bits
frames
packets
segments
Host A Host B
January 2000 Communication Networks 18
Data Encapsulation
Router Router
RouterRouter
Frames
Bits
Packets Data
DataSalamsSalams
7. application6. presentation5. session4. transport
January 2000 Communication Networks 19
Data Encapsulation Example
segment data header
network segment data header header
data
Frame Network Segment Data Frame header header header trailer
Data
Segment
Packet
Frame
Bits01111111010101101000100100010110101
Router
January 2000 Communication Networks 20
Summary Internetworking evolved to support current and
future applications The OSI reference model organizes network
functions into seven layers Data flows from upper-level user applications to
lower-level bits transmitted over network media Peer-to-peer functions use encapsulation and de-
encapsulation at layer interfaces Most network manager tasks configure the lower
three layers
January 2000 Communication Networks 21
Interfaces and Services
Each layer in the OSI layered architecture has a function (service) to be provided to the upper layer through some interface (service access point).
The upper layer can request a service from the lower layer through SAP.
January 2000 Communication Networks 22
Interface and services (cont.)
When layer n+1 requests a service from layer n, Layer n will encapsulate the PDU of layer n+1 into a new PDU to be handed to layer n-1.
Services provided by each layer:» Connection-oriented (telephone)» Connectionless (post)
January 2000 Communication Networks 23
Service Primitives
Layer services are specified by a set of operations available to the layer above it.
These operations either:» Ask for the service» report the status of the service
January 2000 Communication Networks 24
Service Primitives (cont.)
Layer n+1
Layer n
Request
Indication Response
Confirm
January 2000 Communication Networks 25
Services and Protocols
Services are set of operations provided to upper layers or user.
Protocols are sets of rules governing how the same peers should be communicating.
Protocols are transparent from user and may change without changing services.
January 2000 Communication Networks 26
Protocols and Services
Session
Transport
Network
Session
Transport
NetworkServ
ices
Protocols
January 2000 Communication Networks 28
Application Layer
Computer Applications
Word Processing Presentation Graphics Spreadsheet Database Design/Manufacturing Project Planning Others
Network Applications
Electronic mail File Transfer Remote Access Client/Server Process Information Location Network Management Others
January 2000 Communication Networks 29
Application Layer (cont.)
Network Applications
(For enterprise communication)
Electronic mail File Transfer Remote Access Client/Server Process Information Location Network Management Others
Internetwork Applications(Extend beyond the enterprise)
Electronic Data Interchange World Wide Web E-mail Gateways Special-Interest Bulletin Boards Financial Transaction Services Internet Navigation Utilities Conferencing (Video, Voice,
Data)
January 2000 Communication Networks 30
Presentation Layer Text Data
» ASCII» EBCDIC» Encrypted
Sound Video
» MIDI (Musical Instrument Digital Interface)» MPEG (Motion Picture Experts Group)» QuickTime
January 2000 Communication Networks 31
Presentation Layer
Graphics Visual Images
» PICT(format to transfer QuickDraw graphics between Macintosh or PowerPC programs)
» TIFF (Tagged Image File Format)» JPEG (Joint Photographic Experts Group)» GIF
Provides code formatting and conversion for applications
January 2000 Communication Networks 32
Session Layer
Coordinates applications as they interact on different hosts (dialogue control and synchronization)
Service Request
Service Reply
January 2000 Communication Networks 33
Session Layer (contd.)
Network File System (NFS)– Allows transparent access to remote network resources
Structured Query Language (SQL) Remote-Procedure Call (RPC)
– RPC procedures are built on clients and executed on servers
X Window System– Allows intelligent terminals to communicate with remote
UNIX machines AppleTalk Session Protocol (ASP)
– Establishes and maintains sessions between an AppleTalk client and server
DNA Session Control Protocol (SCP)
January 2000 Communication Networks 35
Transport Layer Overview
Segments upper-layer PDUs Establishes an end-to-end connection Sends segments from one end host to
another Ensures end-to-end data reliability
January 2000 Communication Networks 36
Segment Upper-Layer PDUs
Transport segments share traffic stream
Application Data Application Data port port
Electronic mail
File transfer
Terminal session
ApplicationPresentationSession
Transport
January 2000 Communication Networks 37
Establishes Connection
sender
receiver
synchronize
Negotiate connection
synchronize
Acknowledge
Connection established
Data transfer (send segments)
January 2000 Communication Networks 38
Establishes Connection
sender
receiver
transmit
not ready
ready
Resume Transmission
Buffer full process segments
Buffer OK
January 2000 Communication Networks 39
Reliability with Windowing
In the most basic form of reliable connection-oriented transfer, data segments must be delivered to the recipient in the same sequence that they were transmitted.
Windowing is a method to control the amount of information transferred end-to-end. Some protocols measure information in terms of number of packets
January 2000 Communication Networks 40
Reliability with Windowing
sender
receiver
sender
receiver
Send 1
Send 2
Send 1
Send 2
Send 3
Window size 1
Window size 3
Receive 1
Receive 2
Receive 3
Receive 1
Receive 2
ACK 2
ACK 3
ACK 4
Send 4
January 2000 Communication Networks 41
PAR Technique Reliable delivery guarantees that a stream of
data sent from one machine will be delivered through a functioning data link to another machine without duplication or data loss. Positive acknowledgement with retransmission is one technique that guarantees reliable delivery of data streams.
The sender keeps the record of each segment it sends and waits for an acknowledgement.
The sender also starts a timer when it sends a segment, and it retransmits a segment it the timer expires before an acknowledgement arrives.
January 2000 Communication Networks 42
PAR Technique (contd.)
send 1 send 2 send 3 Ack 4 send 4 send 5
send 6Ack 5
send 5Ack 7
sender receiver
1 2 3 4 5 61 2 3 4 5 6
X
January 2000 Communication Networks 43
Transport to Network Layer
End-to-end segments
Routed packetsRouter
Router
Router
RouterRouter
January 2000 Communication Networks 44
Network Layer
Controlling the operation of the network.
How to route data from source to destination? » Static routing tables» Dynamic routing tables
Datagram is similar to a postal letter.
January 2000 Communication Networks 45
Summary Presentation layer formats and converts network
application data to represent text, graphics, images, video, and audio.
Session-layer functions coordinate communication interactions between applications.
Reliable transport-layer functions include» Multiplexing» Flow control» Error recovery» Reliability through windowing
January 2000 Communication Networks 47
Physical and Data-Link Standards
The data link layer provides data transport across a physical link. To do so, the data link layer handles physical addressing, network topology, line discipline, error notification, orderly delivery of frames , and optional flow control.
The physical layer specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating the physical link between end systems.
These requirements and characteristics are codified into standards.
January 2000 Communication Networks 48
LAN Data-Link Sublayers
Logical Link Control
Media Access Control
MAC Frame 802.2 LLC Packet or datagram
Network
Physical
Data Link
LLC
MAC
January 2000 Communication Networks 49
LAN Data-Link Sublayers
LLC refers upward to higher-layer software functions.
MAC refers downward to lower-layer hardware functions.
LAN protocols occupy the bottom two layers of OSI reference model: the physical layer and data link layer.
January 2000 Communication Networks 50
LAN Data-Link Sublayers
The IEEE 802 committee subdivided the data link layer into two sublayers: » The logical link control (LLC) sublayer» The media access control (MAC) sublayer
The LLC sublayer provides for environments that need connectionless or connection-oriented services and the data link layer.
The MAC sublayer provides access to the LAN medium in an orderly manner.
January 2000 Communication Networks 51
LLC Sublayer Functions
Enable upper layers to gain independence over LAN media access.
Allow service access points (SAPs) from interface sublayers to upper-layer functions.
Provide optional connection, flow control, and sequencing services.
January 2000 Communication Networks 53
Client Server Model
Client-Server paradigm is the primary pattern of interactions among cooperating applications.
This model constitutes the foundation on which distributed algorithms are built.
January 2000 Communication Networks 54
What is the Client-Server Paradigm?
The paradigm divides communicating applications into 2 broad categories, depending on whether the application waits for communication or initiates it.» An application that initiates a
communication is called a client.» End users usually invoke a client software
when they use a network service.
January 2000 Communication Networks 55
Client Server Model (cont.)
Server: Any program that offers a service reachable over the network» If a machine’s primary purpose is to
support a particular server program, the term server is usually applied to both, the machine and the server program
Client: An executing program becomes a client when it sends a request to a server and waits for a response
January 2000 Communication Networks 56
Client Server Model (cont.)
A server is any program that waits for incoming communication requests from a client.» Each time a client application needs to contact a
server, it sends a request and awaits a response.
» The server receives a client’s request, performs the necessary computation, and returns the result to the client.
» When the response arrives at the client, the client continues processing.
January 2000 Communication Networks 57
Client Server Model (cont.)
Request
Reply
Machine Running Client Application
Machine Running Server Application
ClientProgram
ServerProgram
January 2000 Communication Networks 58
Client Server Model (cont.)
A Misconception:» Technically, a server is a program and
not a piece of hardware.» However, computer users frequently
(mis)apply the term to the computer responsible for running a particular server program.– For example, Web Server, is usually a
computer running the http server program.
January 2000 Communication Networks 59
Summary Internetworking evolved to support current and
future applications. The OSI reference model organizes network
functions into seven layers. Data flows from upper-level user applications to
lower-level bits transmitted over network media. Peer-to-peer functions use encapsulation and de-
encapsulation at layer interfaces. Client-Server paradigm constitutes the foundation
on which distributed algorithms are built.
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