Computer Networks Chapter 2 – Network Models

Summer 2006Computer Networks2 Communication Tasks  The tasks given below need to be taken care of in any kind of communication system.  Signal generation  Transmission  Interfacing  Synchronization  Error detection and correction  Flow control  Addressing and routing  Recovery from malfunctioning  Message formating  Security  Network management  Applications

Summer 2006Computer Networks3 Dealing with Communication Tasks  Communication system is complicated and need to solve a large number of tasks  Solution  Use the devide and conqure technique  This technique splits the problem in managable pieces and is therefore convinient for dealing with complex systems

Summer 2006Computer Networks4 Design Principle  Communication tasks are divided into series of layers or levels  Each layer is responsible for particular task and act on them by using one or more protocols  Each layer is built upon one bellow it  The number and name of the layers differ from network to network

Summer 2006Computer Networks5 The Concept of Layers  Layer n on one machine communicates with layer n on the other machine via layer n protocol.  The communication is virtual  Peers are entities comprising the corresponding layers on different machines.  There is an interface between each pair of adjacent layers for communication with the layer above and the layer below.

Summer 2006Computer Networks6 An Example of Five Layers Network Layer 5 Layer 4 Layer 3 Layer 2 Layer 1 Physical medium Layer 5 Layer 4 Layer 3 Layer 2 Layer 1 Layer 5 protocol Layer 4 protocol Layer 3 protocol Layer 2 protocol Layer 1 protocol Layer 4/5 interface Layer 3/4 interface Layer 2/3 interface Layer 1/2 interface Machine 1 Machine 2 The path through which the actual transmission take place

Summer 2006Computer Networks7 Peer-to-peer Processes  The processes on the two machines that communicate at a given layer are called peer-to-peer processes  At the physical layer communication is direct  At the upper layers the communication has to go down through the layers on the sender machine, than to be transmited through the physical layer and than to go back up to the same layer at the receiving machine

Summer 2006Computer Networks8 Protocols Construction versus Reduction H – header (control data added at the front end of the data unit) T – trailer (control data added at the back end of the data unit) Trailers are usually added only at layer 2 Layer 5 Layer 4 Layer 3 Layer 2 Physical BITS H2H2DATA UNITT2T2 H4DATA UNIT H5H5 DATA H3 DATA Construction Reduction Layer 5 Layer 4 Layer 3 Layer 2 Physical

Summer 2006Computer Networks9 Illustration of the Construction and Reduction Process  Observe how headers and trailer are added at the sender and removed at the receiver Annimation of Figure 2.4 in the book

Summer 2006Computer Networks10 Messages and Protocol Stacks  On the sender machine, each layer:  Accepts an outgoing message from the layer above  Adds a header and does other processing  Passes resulting message to next lower layer  On the receiver, each layer:  Receives an incoming message from the layer below  Removes the header for that layer and performs other processing  Passes the resulting message to the next higher layer

Summer 2006Computer Networks11 Drawbacks in the layered architecture  The resources necessary for the user’s data (payload) are also used for transfering information in the headers and trailers.  This cannot be avided.  However, well designed protocols should have as litle as possible additional information carried, yet provide in the best way the service required.

Summer 2006Computer Networks12 Service vs. Protocol  Service  Treat the protocol as a black box. what service is provided to the upper layer  Protocol  How do the peer entities cooperate with each other and the lower layer to provide the service to the upper layer Service Lower layer Interface Upper layer Uper layer Upper layer protocol Lower layer protocol

Summer 2006Computer Networks13 Network Architecture  A set of layers and protocols  Enough information to allow implementers to write a program or build the hardware for each layer, so that it obeys the appropriate protocol  Protocol stack or protocol suit – a list of protocols used by a certain system

Summer 2006Computer Networks14 Protocol Suites  Open Sysytem Interconnection (OSI)  Used mostly as a reference model  Internet (TCP/IP)  Most popular suite today  Xerox Networking Sysytems (XNS)  System Network Architecture (SNA – IBM)  Digital Network Architecture (DNA – DEC)  NetBIOS (Software interface)

Summer 2006Computer Networks15 Open System Interconnection (OSI)  Developed by International Standard Organization (ISO) as a first step towards international standardization  De jure protocol  Deals with interconnecting systems that are open for communication with other systems  Open protocol suite  Good as theoretical model, but not widely implemented in practice

Summer 2006Computer Networks16 The Layers of OSI Model Application Presentation Session Transport Network Data Link Physical Network Data Link Physical Intermediate System End System Application Presentation Session Transport Network Data Link Physical R

Summer 2006Computer Networks17 The OSI layers  Physical layer  Transmission of unstructured bit stream  Deals with the mechanical, electrical, functional and procedural characteristics to access the physical medium  Data link layer  Provides reliable transfer across the physical link between two ends connected via single link  Sends blocks of data (frames) with the necessary synchronization, error control and flow control  Can add header and trailer

Summer 2006Computer Networks18 The OSI layers  Network layer  Provides upper layers with independence from the data transmission and switching technologies accross internetwork  Responsible for source-to-destination delivery, addressing and routing in the internetwork  Transport layer  Provides transparent transport of data between end points that might not be connected via single link  Provides source-to-destination connection, error recovery and flow control

Summer 2006Computer Networks19 The OSI layers  Session layer  Provides the control structure for communication between applications (dialog control)  Establishes, manages and terminate connections (sessions) between cooperating applications  Presentation layer  Provides independence to the application processes from differences in data representation  Application layer  Provides access to the OSI environment for users and provides distributed information services

Summer 2006Computer Networks20 Summary of OSI Layer Functions Application Presentation Session Transport Network Data Link Physical To allow access to network resources To establish, manage and terminate sessions To move packets from source to destination; to provide internetworking To transmit bits over a medium; to provide mechanical and electrical specifications To translate, encrypt and compress data To provide reliable end-to- end message delivery and error recovery To organize bits into frames, to provide node- to-node delivery

Summer 2006Computer Networks21 Illustration of the Source-to-end Delivery at the Network Layer  Observe how data are sent from node to node to reach the final destination. Animation of Figure 2.11 in the book

Summer 2006Computer Networks22 TCP/IP Protocol Suite  De facto (and after that de jure) standards  Open (All modification and newly proposed protocols are published in a form of RFC (Request for Comments)  RFC as well as drafts are published on the Internet  can be found on many URL (one is editor.org) editor.org  RFC becomes a standard when it is:  Stable and well understood  Technically competent  Implemented on multiple independent places

Summer 2006Computer Networks23 The TCP/IP Protocol Suite (Cont.)  Allows computers of many sizes, vendors and operating systems to communicate with each other  History:  Developed as de facto standard before OSI  1960’s: started as goverment financed research project  1990’s: most widely used form of networking  Forms the basis for the Internet (capital “I”) (a WAN that spans the globe)

Summer 2006Computer Networks24 TCP/IP Layers and Relation to OSI Transport Network Application Physical NetworkNetwork SMTPFTP TELNETDNS HTTP Application Presentation Session Transport TCPUDP Network IGMPICMP IP RARPARP Data Link Physical Protocols defined by the underlying networks

Summer 2006Computer Networks25 Some Standard Organizations  ITU: International Telecommunication Union  CCITT: International Telegraph and Telephone Consultative Committee  ISO: International Standards Organization  IEEE: Institute of Electrical and Electronic Engineering  IRTF: Internet Research Task Force  IETF: Internet Engineering Task Force