Spring 2006Computer Networks1 Chapter 2 Network Models

Spring 2006Computer Networks2 Figure 2.1 Sending a letter

Spring 2006Computer Networks3 Layered Protocols  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

Spring 2006Computer Networks4 Figure 2.17 The OSI seven layer model

Spring 2006Computer Networks5 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

Spring 2006Computer Networks6 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

Spring 2006Computer Networks7 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

Spring 2006Computer Networks8 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

Spring 2006Computer Networks9 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

Spring 2006Computer Networks10 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

Spring 2006Computer Networks11 Protocol Suites  Open System Interconnection (OSI)  Today used mostly as a reference model  Prevously used in X.25 based protocols  Internet (TCP/IP)  Most popular suite today  Xerox Networking Sysytems (XNS)  System Network Architecture (SNA – IBM)  Digital Network Architecture (DNA – DEC)  NetBIOS (Software interface)  AppleTalk

Spring 2006Computer Networks12 The TCP/IP five layer model

Spring 2006Computer Networks13 TCP/IP-modellen TCP, UDP IP Ethernet SMTP, HTTP Exempel:

Spring 2006Computer Networks14 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

Spring 2006Computer Networks15 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)