2.2LAYERED TASKSLAYERED TASKSWe use the concept ofWe use the concept of layerslayers in our daily life. As anin our daily life. As anexample, let us consider two friends who communicateexample, let us consider two friends who communicatethrough postal mail. The process of sending a letter to athrough postal mail. The process of sending a letter to afriend would be complex if there were no servicesfriend would be complex if there were no servicesavailable from the post office.available from the post office.
2.4THE OSI MODELTHE OSI MODELEstablished in 1947, the International StandardsEstablished in 1947, the International StandardsOrganization (Organization (ISOISO) is a multinational body dedicated to) is a multinational body dedicated toworldwide agreement on international standards. An ISOworldwide agreement on international standards. An ISOstandard that covers all aspects of networkstandard that covers all aspects of networkcommunications is the Open Systems Interconnectioncommunications is the Open Systems Interconnection((OSIOSI) model. It was first introduced in the late 1970s and) model. It was first introduced in the late 1970s andpublished in 1984.published in 1984.
2.5ISO is the organization.OSI is the model.Note
2.6Seven layers of the OSI modelA seven-layer logical breakdown of network interactionto facilitate communicationstandardsEach layer deals with aparticular aspect of networkcommunication"All people seem to need data processing."
2.7The interaction between layers in the OSI modelHierarchy_
2.8An exchange using the OSI modelEncapsulation_
2.9LAYERS IN THE OSI MODELLAYERS IN THE OSI MODELLets briefly describe the functions of each layer in theLets briefly describe the functions of each layer in theOSI model.OSI model.
2.10Physical layerThe physical layer is responsible for movement ofindividual bits from one hop (node) to the next. It deals with themech & electrical specs of the interface & transmission medium,as well as the procedures & functions they have to perform fortransmission to occur.
2.23Presentation layerThe presentation layer is responsible for translation, compression,and encryption. It is concerned with the syntax and semantics ofthe information exchanged between the two systems.
2.28TCP/IP PROTOCOL SUITETCP/IP PROTOCOL SUITE• It was first introduced in 1974 and v4 was publishedIt was first introduced in 1974 and v4 was publishedin 1979.in 1979.• The layers in theThe layers in the TCP/IP protocol suiteTCP/IP protocol suite do notdo notexactly match those in the OSI model. The originalexactly match those in the OSI model. The originalTCP/IP protocol suite was defined as having fourTCP/IP protocol suite was defined as having fourlayers:layers: host-to-networkhost-to-network,, internetinternet,, transporttransport, and, andapplicationapplication..• However, when TCP/IP is compared to OSI, weHowever, when TCP/IP is compared to OSI, wecan say that the TCP/IP protocol suite is made of fivecan say that the TCP/IP protocol suite is made of fivelayers:layers: physicalphysical,, data linkdata link,, networknetwork,, transporttransport, and, andapplicationapplication..• We can call this the Hybrid model.We can call this the Hybrid model.
2.33ADDRESSINGADDRESSINGFour levels of addresses are used in an internet employingFour levels of addresses are used in an internet employingthe TCP/IP protocols:the TCP/IP protocols:• physicalphysical,,• logicallogical,,• portport, and, and• specificspecific..
2.34Relationship of layers and addresses in TCP/IP
2.37As we will see later, most local-area networks use a 48-bit (6-byte) physical address written as 12 hexadecimaldigits; every byte (2 hexadecimal digits) is separated by acolon, as shown below:Example07:01:02:01:2C:4BA 6-byte (12 hexadecimal digits) physical address.
Ethernet (?) network card showing its MAC address
2.39An IPv4 address is a 32-bit address that uniquely and universally defines theconnection of a device (for example, a computer or a router) to the Internet.Dotted-decimal notation and binary notation for an IPv4 address:
2.40Next Figure shows a part of an internet with two routersconnecting three LANs. Each device (computer orrouter) has a pair of addresses (logical and physical) foreach connection. In this case, each computer isconnected to only one link and therefore has only onepair of addresses. Each router, however, is connected tothree networks (only two are shown in the figure). Soeach router has three pairs of addresses, one for eachconnection.Example
IP addressesRouting Table used to find the next hop logical address & ARP gives the physical address.
2.43Figure shows two computers communicating via theInternet. The sending computer is running threeprocesses at this time with port addresses a, b, and c. Thereceiving computer is running two processes at this timewith port addresses j and k. Process a in the sendingcomputer needs to communicate with process j in thereceiving computer. Note that although physicaladdresses change from hop to hop, logical and portaddresses remain the same from the source todestination.Example
2.45ExampleAs we will see later, a port address is a 16-bit addressrepresented by one decimal number as shown.107A 16-bit port address representedas one single number.
2.46What is a socket?is the combination of an IP address (the location of the computer)and a port (which is mapped to the application program process)into a single identity, much like one end of a telephone connectionis the combination of a phone number and a particular extension.