CS 313 Introduction to Computer Networking & Telecommunication Introduction

Chi-Cheng Lin, Winona State University

Topics        Introduction Metric Units Network Hardware Network Software Reference Models Example Networks Standards and Standards Organizations 2

Network Software   Old computer networks:  HW main concern  SW afterthought  Not working now!

Network SW is now highly structured  Approach: Protocol Hierarchies 3

Protocol Hierarchies   What is protocol?

 Agreement between communication parties on HOW communication is processed Layered architecture  Reduce design complexity: Lower layer offers service to higher layer  Hiding implementation details  Layer n another on one machine talks to layer n  Rules and conventions used in layer talk: Layer n protocol n ’s on 4

Protocol Hierarchies   Peers  Entities comprising corresponding layers on different machines  Virtual communication using protocol  Peer process abstraction make network design becomes that of individual layers Physical communication  Sender: Data and control passed to layer below  Data transmitted via physical media  Receiver: Data and control passed to layer above 5

Layers, Protocols, and Interfaces

Virtual Communication Physical Communication

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Protocol Hierarchies  Interface between two adjacent layers  Defines primitive operations and services a lower layer offers to the upper one  Minimizes amount of information passed between two layers  Simplifies replacement of implementation  E.g., telephone lines  satellite channels 7

Protocol Hierarchies  

Network architecture

 Set of layers and protocols  Implementation and interface specification not included

Protocol stack

 A list of protocols used by a certain system, one protocol per layer 8

Multilayer Communication - Example  Philosopher-translator-secretary architecture  It is ok if  Dutch is replaced by Finnish  fax is replaced by email 9

Information Flow - Example  Virtual communication for layer 5  Header: control information 00011100011100001110 … Layer 1 protocol 00011100011100001110 … 10

Key Design Issues for the Layers  Reliability  Error control  Error-detecting  Error-correcting  Routing  Selecting the best path for sending a packet from one point to another 11

Key Design Issues for the Layers    Sender/receiver identification mechanism  Addressing/naming Sequencing Message disassembling, transmitting, reassembling 12

Key Design Issues for the Layers      Resource allocation 

Multiplexing

 The process of combining signals from multiple sources for transmission across a single data link  Multiple connections can share the link Flow control  Needed for fast sender, slow receiver Congestion control Quality of service Security 13

Connection-Oriented and Connectionless Services    Two basic types of services  Connection-oriented  Connectionless Consider reliability  Reliable … Connection-oriented Unreliable Connectionless Note that: Connection  Reliability 14

Connection-Oriented Service    A connection is established first, then used, and then released when done.

Works like a pipe:  Sender pushes data in at one end  Receiver takes them out, often in the same order, at the other end Analogy  Telephone system 15

Connectionless Service    No need to set up a connection first Each message carrying full destination address is routed independently of others  No guarantees on the order Analogy  Postal system 16

Service Primitives   Service is formally specified by a set of primitives (e.g., OS’s system calls) available to users or entities Five service primitives for implementing a simple connection-oriented service.

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Service Primitives  Packets sent in a simple client-server interaction on a connection-oriented network.

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Relationship of Services to Protocols   Service  Set of primitives a layer provides to the layer above it  Define WHAT not HOW operations implemented Protocol  Set of rules governing format and meaning of message exchanged by peer entities within a layer  Used by entities to implement service definitions 19

Services to Protocols Relationship  The relationship between a service and a protocol.

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Relationship of Services to Protocols  Analogy: object-oriented languages  Service :: ADT or Object  Users do not know the implementation of a service  Protocol :: Implementation  The protocol of the service is invisible to users  Do you have to understand http (hypertext transport protocol) before you can surf the Internet?

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Reference Models  Two reference models will be discussed  OSI reference model  TCP/IP model 22

OSI Reference Model   ISO/OSI (Open Systems Interconnection) Reference Model NOT a network architecture itself  Exact services and protocols are not specified  Just " what should be done" in each layer  However, standards are produced for all layers 23

  OSI Reference Model Seven layers  Layer 7: application layer  Layer 6: presentation layer  Layer 5: session layer  Layer 4: transport layer  Layer 3: network layer  Layer 2: data link layer  Layer 1: physical layer (lowest) Diagram of OSI reference model  Note: this is one of the most important figures in the whole book!!

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Physical medium 25

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End-to-End Host A Point-to-Point Point-to-Point Subnet Physical medium Point-to-Point Host B 27

Physical Layer   Transmitting raw bits (0s and 1s) over communication channel Design issues  Representation of bits  How is 0/1 represented?

 Data rate: number of bits sent per second  How long does a bit last?

 Transmission mode (bi-directional?)  Mechanical, electrical, timing interfaces  Underlying physical transmission medium 28

Data Link Layer   Takes a raw transmission facility and transforms it into a line ( link ) that appears free of undetected transmission errors to network layer Basic function  Breaks up input data to data frames  Transmits data frames sequentially  Processes acknowledgement frames sent back from receiver for reliable transmission 29

Data Link Layer  Responsibilities  Physical addressing (e.g. MAC address)  Framing  Creating and recognizing frame boundaries  Error control (adjacent nodes, node-to-node)  Errors: damaged, lost, duplicate  Flow control (adjacent nodes, node-to-node)  Traffic regulation between fast sender and slow receiver  Medium access control  Shared channel access control in broadcast networks 30

Network Layer   Subnet operation control Responsibilities  Logical addressing (e.g., IP address)  Routing  Static tables  Determined at the start of conversation  Dynamic  Congestion control  Quality of service  Accounting  Heterogeneous network interconnection 31

Transport Layer   End-to-end layer  Talk to destination machine directly (virtually)  Layers 4 through 7 are end-to-end  Layers 1 through 3 are node-to-node (chained) Basic function  Split data from session layer into smaller units  Pass units to network layer  Ensure units arrive correctly at the other end 32

Transport Layer  Determine services provided to session layer (and ultimately to users)  Error-free point-to-point channel that delivers messages in the order in which they were sent  Transport of isolated messages w/o guarantee about order  Broadcasting 33

Transport Layer  Responsibilities include  Service-point addressing (e.g., port number)  Which message belong to which connection (application):  (End-to-end) Flow control Compare to the  (End-to-end) Error control Data Link layer 34

Session layer   Session establishment between users on different machines Responsibilities  Dialogue control  Deciding who sends, and when  Token management  Control of same critical operation not to be performed at the same time  Synchronization  Inserting checkpoints (checkpointing) 35

Presentation Layer   Syntax/semantics of information transmitted Responsibilities  Make communication between computers with different internal data representations possible  Approach: standard encoding  Convert from data representation used in one host to the standard abstract data structure and back 36

Application Layer   Provides interface and support for services to users (human, software, robots) Examples  File transfer  Email  Network news  Hypertext transfer 37

TCP/IP Reference Model   Goals  Internetworking  Fault tolerance  Flexible architecture Four layers of TCP/IP Reference Model  Host-to-network layer  Internet layer  Transport layer  Application layer 38

Internet Layer     Packet-switching, connectionless Packets injected to network  Independent travel  Out-of-order arrival Analogy  Mail system IP (Internet Protocol)  Packet routing  Congestion control 39

Transport Layer   Two end-to-end protocols  UDP (User Datagram Protocol)  TCP (Transmission Control Protocol) UDP (User Datagram Protocol)  Unreliable, connectionless  Widely used for  client-server type request-reply queries  speech, video 40

Transport Layer  TCP  Reliable connection-oriented  Incoming byte stream (form application layer) is fragmented into discrete messages and passed onto internet layer  Message is reassembled at destination  Flow control  Analogy

A B Pipe

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  Applications and Host-to-Network Layers Application layer  No session and presentation layers  TELNET, FTP, SMTP, DNS, NNTP, HTTP Link layer  Defines what links must do to meet the need of connectionless internet layer 42

TCP/IP Protocols 43

OSI and TCP/IP Models  Correspondence 44

OSI and TCP/IP Models  Similarities  Stack of independent protocols  Layer functionality  Transport layer  Application layer 45

 Differences between OSI and TCP/IP Models OSI  Distinction between services, interfaces, and protocols (perhaps the biggest contribution)  Better Protocol-Hidden  Model first, then protocols  Pro: No bias, more general  Con: Designers did not have   much experience with the subject a good idea of which functionality to put in which layer  No thought given to internetworking  7 layers  Communication  Connection-Oriented and connectionless in network layer  Only connection-oriented in transport layer 46

Differences between OSI and TCP/IP Models  TCP/IP:  No clear distinction between services, interfaces, and protocols  Worse protocol-hidden  Protocol first, then model  Pro: Protocols fit model perfectly  Con: Model does not fit any other protocol stacks (not general)  4 layers  Communication  Connectionless in network layer  Both in transport layer (good for request-response protocols) 47

Summary of Reference Models    OSI  OSI model exceptionally useful for discussing computer networks  OSI protocols not popular TCP/IP  TCP/IP model practically nonexistent  TCP/IP protocols widely used Modified framework is used in the text 48

Summary of Reference Models  Modified framework is used in the text 49

Example Networks  The Internet Overview of the Internet architecture 50

Example Networks    3G mobile networks Wireless LANs: 802.11

RFID and sensor networks 51

Standards and Standards Organizations  Why standards?

  Categories  de facto  de jure Organizations  ITU-T (formerly CCITT)  ISO  ANSI  IEEE  IETF  ATM Forum 52