Computer networks(OSI layer)

7/31/2019 Computer networks(OSI layer)

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TCP/ I P Prot ocol Suit e 1

Upon completion you will be able to:

The OSI Model and the TCP/IP Protocol Suite

Understand the architecture of the OSI model Understand the layers of the OSI model and their functions Understand the architecture of the TCP/IP Protocol Suite Differentiate between the OSI model and the TCP/IP Suite Differentiate between the three types of Internet addresses

Objectives


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2.1 The OSI Model

Established in 1947, the International Standards Organization (ISO) is a multinational body dedicated to worldwide agreement on international standards. An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model. Itwas first introduced in the late 1970s.

The topics discussed in this section include:

Layered Architecture Peer-to-Peer Processes

Encapsulation


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ISO is the organization.

OSI is the model

Note:


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Figure 2.1 The OSI model


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Figure 2.2 OSI layers


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Figure 2.3 An exchange using the OSI model


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2.2 Layers in the OSI Model

The functions of each layer in the OSI model is briefly described.


Physical Layer Data Link Layer Network LayerTransport LayerSession Layer

Presentation Layer

Application LayerSummary of Layers


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OSI Reference Model (cont)1. Physical layer

Point-to-point transmission of unstructured bit streamDeals with mechanical, electrical, timing interfaces,and the physical transmission medium such ascable,connectors.It gets the data unit from second layer puts into aformat capable of being carried by a communicationlink.


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LAYER 1: PHYSICAL

The physical layer deals with the physical characteristics of thetransmission medium.

It defines the electrical, mechanical, procedural, and functionalspecifications for activating, maintaining, and deactivating thephysical link between end systems.

Such characteristics as voltage levels, timing of voltage changes,physical data rates, maximum transmission distances, physicalconnectors, and other similar attributes are defined by physicallayer specifications.

Examples :- EIA/TIA-232, RJ45, NRZ.


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Figure 2.4 Physical layer


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OSI Reference Model (cont)Task of Physical layer

Physical characteristics of interface and media. Representation of bits Data rate Synchronization of bits Line configuration Topology Transmission mode


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The physical layer is responsible

for the movement of individual bits from one hop (node) to the next.

Note:


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OSI Reference Model (cont)2. Data Link layer

It is responsible for node to node delivery.Reliable error-free transmission on a link, with flow

control and synchronizationOrganizes bits into , adds acknowledgementsand retransmissionsNeeds MAC sublayer to control access to sharedmedium (e.g., in LANs


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LAYER 2: DATA LINK

The data link layer provides access to the networking media andphysical transmission across the media and this enables the datato locate its intended destination on a network.

The data link layer provides reliable transit of data across aphysical link by using the Media Access Control (MAC) addresses.

The data link layer uses the MAC address to define a hardware ordata link address in order for multiple stations to share the samemedium and still uniquely identify each other.

Concerned with network topology, network access, errornotification, ordered delivery of frames, and flow control.

Examples :- Ethernet, Frame Relay, FDDI.


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Figure 2.5 Data link layer


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OSI Reference Model (cont)Dut ies of Data Link layer

o Framingo Physical Addressingo Flow controlo Error controlo Access control


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The data link layer is responsible for

moving frames from one hop (node) to the next.

Note:


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Figure 2.6 Hop-to-hop delivery


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OSI Reference Model (cont)

3. Network layerEstablishes and manages connections throughthe network Carries packets from source to destination byrouting between nodesCongestion control

Guarantee of quality of service (QoS)Including packet loss rate, delay, and jitter


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LAYER 3: NETWORK

Defines end-to-end delivery of packets.Defines logical addressing so that any endpoint can be identified.Defines how routing works and how routes are learned so thatthe packets can be delivered.The network layer also defines how to fragment a packet intosmaller packets to accommodate different media.Routers operate at Layer 3.Examples :- IP, IPX, AppleTalk.


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Figure 2.7 Network layer


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OSI Reference Model (cont)Dut ies of Net w ork Layer

Routing: It determines how packets are routed from source todestination.

Congest ion Contr ol: Many packets in the subnet trying to use thesame route.Internetworking: It allows heterogeneous networks to beinterconnected.

Accounting Funct ion .


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The network layer is responsible for

the delivery of individual packets from the source host to the destination host.

Note:


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Figure 2.8 Source-to-destination delivery


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4. Transport layer A true layerReliable end-to-end transfer of informationbetween usersEstablishes and manages connectionsbetween users

Segments and reassembles informationto/from subnetEnd-to-end error recovery and flow control


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LAYER 4: TRANSPORT

The transport layer regulates information flow to ensure end-to-end connectivity between host applications reliably and accurately.The transport layer segments data from the sending host's systemand reassembles the data into a data stream on the receiving

host's system.The boundary between the transport layer and the session layercan be thought of as the boundary between application protocolsand data-flow protocols. Whereas the application, presentation,and session layers are concerned with application issues, the lowerfour layers are concerned with data transport issues.

Layer 4 protocols include TCP (Transmission Control Protocol) andUDP (User Datagram Protocol).


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Figure 2.9 Transport layer


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OSI Reference Model (cont)Dut ies of Transpor t Layer

Process to process delivery Port addressing Segmentation and reassembly Connection control Flow and error control


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The transport layer is responsible for

the delivery of a message from one process to another.

Note:

i 2 10 R li bl d li f


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Figure 2.10 Reliable process-to-process delivery of a message


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5. Session LayerIt allows users on different machines to establish sessions betweenthem.I nteract ion Management: The data exchange associated with a

dialog may be:Duplex: Two-way simultaneous.Half-Duplex: Two-way alternate.Simplex: One-way.

Synchronization: For lengthy transaction, the user may choose to

establish synchronization points associated with the transfer. If afault develops during a transaction, the dialog may be restarted atan agreed synchronization point.


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LAYER 5: SESSION

The session layer defines how to start, control and end conversations(called sessions) between applications.This includes the control and management of multiple bi-directionalmessages using dialogue control.

It also synchronizes dialogue between two hosts' presentation layersand manages their data exchange.The session layer offers provisions for efficient data transfer.

Examples :- SQL, ASP(AppleTalk Session Protocol).

Fi 2 11 S i l


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Figure 2.11 Session layer


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OSI Reference Model (cont)6. Presentation Layer

Changes syntax (data format) and semantics (theirmeanings) of information, as necessary

E.g., conversion between EBCDIC between ASCII codes

It converts from a code used by a computer to a standard network code. Different computers have different codes for representingcharacters (e.g., ASCII and EBCDIC), integers (e.g., onescomplement and twos complements), and so on.Duties:Dat a Compression and ExpansionData Encrypt ion and Decrypt ionTranslation


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LAYER 6: PRESENTATION

The presentation layer ensures that the information that theapplication layer of one system sends out is readable by theapplication layer of another system.

If necessary, the presentation layer translates between multipledata formats by using a common format.Provides encryption and compression of data.Examples :- JPEG, MPEG, ASCII, EBCDIC, HTML.

Figure 2 12 Presentation layer


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Figure 2.12 Presentation layer


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7. Application Layer

User program that generates or uses dataWhat the user sees

The application layer contains a variety of protocols that are commonly needed.

It handles the incompatibilities on thedifferent machines


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LAYER 7: APPLICATION

The application layer is the OSI layer that is closest to the user.

It provides network services to the users applications.

It differs from the other layers in that it does not provide services

to any other OSI layer, but rather, only to applications outside theOSI model.

Examples of such applications are spreadsheet programs, wordprocessing programs, and bank terminal programs.

The application layer establishes the availability of intended

communication partners, synchronizes and establishes agreementon procedures for error recovery and control of data integrity.

OS f d l ( )


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OSI Reference Model (cont)Dut ies of Appl icat ion Layer

N/w virtual terminal Mail services Directory services

Figure 2 13 Application layer


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Figure 2.13 Application layer

Figure 2 14 Summary of layers


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Figure 2.14 Summary of layers


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2.3 TCP/IP Protocol Suite

The TCP/IP protocol suite is made of five layers: physical, data link, network, transport, and application. The first four layers provide physical standards, network interface, internetworking, and transport functions that correspond to the first four layers of the OSI model. The three topmost layers in the OSI model, however, are represented in TCP/IP by

a single layer called the application layer.


Physical and Data Link Layers

Network LayerTransport Layer Application Layer

Figure 2 15 TCP/IP and OSI model


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Figure 2.15 TCP/IP and OSI model


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2.4 Addressing

Three different levels of addresses are used in an internet using theTCP/IP protocols: physical (link) address , logical (IP) address , and

port address .


Physical Address Logical Address Port Address

Figure 2.16 Addresses in TCP/IP


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Figure 2.16 Addresses in TCP/IP

Figure 2.17 Relationship of layers and addresses in TCP/IP


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g . p y


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In Figure 2.18 a node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link. At the data link level

this frame contains physical (link) addresses in the header. These are the only addresses needed. The rest of the header contains other information needed at this level. The trailer usually contains extra bits

needed for error detection.

See Next Slide

Figure 2.18 Physical addresses


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g y


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As we will see in Chapter 3, most local area networksuse a 48-bit (6 bytes) physical address written as 12

hexadecimal digits, with every 2 bytes separated by a

colon as shown below:

07:01:02:01:2C:4B

A 6-byte (12 hexadecimal digits) physical address.


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In Figure 2.19 we want to send data from a node with network address A and physical address 10, located on one LAN, to a node with a network address P and physical address 95, located on another LAN. Because the two devices are located on

different networks, we cannot use link addresses only; the link addresses have only local jurisdiction. What we need here areuniversal addresses that can pass through the LAN

boundaries. The network (logical) addresses have this

characteristic.

See Next Slide


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The packet at the network layer contains the logical addresses,which remain the same from the original source to the final

destination ( A and P , respectively, in the figure). They will not change when we go from network to network. However, the

physical addresses will change as the packet moves from one network to another. The boxes labeled routers areinternetworking devices, which we will discuss in Chapter 3.

(Con t i n u ed )

See Next Slide

Figure 2.19 IP addresses


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As we will see in Chapter 4, an Internet address (in IPv4) is 32 bits in length, normally written as four decimal numbers, with each number representing 1

byte. The numbers are separated by a dot. Below is anexample of such an address.

132.24.75.9

An internet address in IPv4 in decimal numbers


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Figure 2.20 shows an example of transport layer communication. Data coming from the upperlayers have port addresses j and k ( j is the address of the

sending process, and k is the address of the receiving process). Since the data size is larger than the networklayer can handle, the data are split into two packets,each packet retaining the service-point addresses ( j

and k). Then in the network layer, network addresses(A and P) are added to each packet.

See Next Slide


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The packets can travel on different paths and arrive at the destination either in order or out of order. The two packets are delivered to the destination transport

layer, which is responsible for removing the networklayer headers and combining the two pieces of data for delivery to the upper layers.

See Next Slide

Figure 2.20 Port addresses


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As we will see in Chapters 11, 12, and 13, a port address is a 16-bit address represented by one decimal number as shown below.

753 A 16-bit port address represented as one single number.


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2.5 IP Versions

IP became the official protocol for the Internet in 1983. As the Internet has evolved, so has IP. There have been six versions since its inception.We look at the latter three versions here.


Version 4Version 5Version 6

Computer networks(OSI layer)

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