Chapter 2 Network Models PDF

Chapter 2 Network Models 2.1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  2-1 LAYERED TASKS We use the concept of layers in our daily life. As an example, let us consider two friends who communicate through postal mail. The proc...

Chapter 2 Network Models 2.1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  2-1 LAYERED TASKS We use the concept of layers in our daily life. As an example, let us consider two friends who communicate through postal mail. The process of sending a letter to a friend would be complex if there were no services available from the post office. Topics discussed in this section: Sender, Receiver, and Carrier Hierarchy 2.2  Figure 2.1 Tasks involved in sending a letter 2.3  2-2 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. It was first introduced in the late 1970s. Topics discussed in this section: Layered Architecture Peer-to-Peer Processes Encapsulation 2.4  Note ISO is the organization. OSI is the model. 2.5  Figure 2.2 Seven layers of the OSI model 2.6  OSI Model Application Application (Upper) Presentation Layers Session Transport Network Data Flow Layers Data-Link Physical 7  Figure 2.3 The interaction between layers in the OSI model 2.8  Figure 2.4 An exchange using the OSI model 2.9  2-3 LAYERS IN THE OSI MODEL In this section we briefly describe the functions of each layer in the OSI model. Topics discussed in this section: Physical Layer Data Link Layer Network Layer Transport Layer Session Layer Presentation Layer Application Layer 2.10  Figure 2.5 Physical layer 2.11 Layer 1 - The Physical Layer 7 Application This is the physical media 6 Presentation through which the data, 5 Session represented as electronic signals, is sent from the 4 Transport source host to the destination host. 3 Network 2 Data Link Move bits between devices Encoding 1 Physical PDU - Bits 2.12  Note The physical layer is responsible for movements of individual bits from one hop (node) to the next. 2.13  Figure 2.6 Data link layer 2.14  Layer 2 - The Data Link Layer 7 Application Performs Physical Addressing This layer provides reliable transit of data across a physical 6 Presentation link. Combines bits into bytes and 5 Session bytes into frames Access to media using MAC address 4 Transport Error detection, not correction LLC and MAC 3 Network Logical Link Control performs Link establishment 2 Data Link MAC Performs Access method 1 Physical PDU - Frames Preamble DMAC SMAC Data length DATA FCS 2.15  Note The data link layer is responsible for moving frames from one hop (node) to the next. 2.16  Figure 2.7 Hop-to-hop delivery 2.17  Figure 2.8 Network layer 2.18 Layer 3 - The Network Layer 7 Application Sometimes referred to as the “Cisco Layer”. End to End Delivery 6 Presentation Provide logical addressing that routers use for path determination 5 Session Segments are encapsulated Internetwork Communication 4 Transport Packet forwarding Packet Filtering 3 Network Makes “Best Path Determination” Fragmentation 2 Data Link PDU – Packets – IP/IPX 1 Physical 2.19  Note The network layer is responsible for the delivery of individual packets from the source host to the destination host. 2.20  Figure 2.9 Source-to-destination delivery 2.21  Figure 2.10 Transport layer 2.22 Layer 4 - The Transport Layer 7 Application This layer breaks up the data from the sending host and then 6 Presentation reassembles it in the receiver. 5 Session It also is used to insure reliable data transport across the network. 4 Transport Can be reliable or unreliable Sequencing 3 Network Acknowledgment Retransmission 2 Data Link Flow Control 1 Physical PDU - Segments 2.23  Note The transport layer is responsible for the delivery of a message from one process to another. 2.24  Figure 2.11 Reliable process-to-process delivery of a message 2.25 Figure 2.12 Session layer 2.26 Layer 5 - The Session Layer 7 Application This layer establishes, manages, and terminates sessions between two 6 Presentation communicating hosts. Creates Virtual Circuit Coordinates communication between 5 Session systems Organize their communication by 4 Transport offering three different modes Simplex 3 Network Half Duplex Full Duplex 2 Data Link 1 Physical Example:  Client Software ( Used for logging in) 2.27  Note The session layer is responsible for dialog control and synchronization. 2.28  Figure 2.13 Presentation layer 2.29 Layer 6 - The Presentation Layer 7 Application This layer is responsible for presenting the data in 6 Presentation the required format which 5 Session may include: Code Formatting 4 Transport Encryption 3 Network Compression 2 Data Link PDU - Formatted Data 1 Physical 2.30  Note The presentation layer is responsible for translation, compression, and encryption. 2.31  Figure 2.14 Application layer 2.32 Layer 7 - The Application Layer 7 Application This layer deal with networking 6 Presentation applications. 5 Session 4 Transport Examples:  Email 3 Network  Web browsers 2 Data Link PDU - User Data 1 Physical Each of the layers have Protocol Data Unit (PDU) 2.33  Note The application layer is responsible for providing services to the user. 2.34  Figure 2.15 Summary of layers 2.35 2.36  2-4 TCP/IP PROTOCOL SUITE The layers in the TCP/IP protocol suite do not exactly match those in the OSI model. The original TCP/IP protocol suite was defined as having four layers: host-to- network, internet, transport, and application. However, when TCP/IP is compared to OSI, we can say that the TCP/IP protocol suite is made of five layers: physical, data link, network, transport, and application. Topics discussed in this section: Physical and Data Link Layers Network Layer Transport Layer Application Layer 2.37  Figure 2.16 TCP/IP and OSI model 2.38  Figure 2.12: TCP/IP and OSI model 2.39  2-5 ADDRESSING Four levels of addresses are used in an internet employing the TCP/IP protocols: physical, logical, port, and specific. Topics discussed in this section: Physical Addresses Logical Addresses Port Addresses Specific Addresses 2.40  Figure 2.17 Addresses in TCP/IP 2.41  Figure 2.18 Relationship of layers and addresses in TCP/IP 2.42  Example 2.1 In Figure 2.19 a node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link (bus topology LAN). As the figure shows, the computer with physical address 10 is the sender, and the computer with physical address 87 is the receiver. 2.43  Figure 2.19 Physical addresses 2.44  Example 2.2 Most local-area networks use a 48-bit (6-byte) physical address written as 12 hexadecimal digits; every byte (2 hexadecimal digits) is separated by a colon, as shown below: 07:01:02:01:2C:4B A 6-byte (12 hexadecimal digits) physical address. 2.45  Example 2.3 Figure 2.20 shows a part of an internet with two routers connecting three LANs. Each device (computer or router) has a pair of addresses (logical and physical) for each connection. In this case, each computer is connected to only one link and therefore has only one pair of addresses. Each router, however, is connected to three networks (only two are shown in the figure). So each router has three pairs of addresses, one for each connection. 2.46  Figure 2.20 IP addresses 2.47  Example 2.4 Figure 2.21 shows two computers communicating via the Internet. The sending computer is running three processes at this time with port addresses a, b, and c. The receiving computer is running two processes at this time with port addresses j and k. Process a in the sending computer needs to communicate with process j in the receiving computer. Note that although physical addresses change from hop to hop, logical and port addresses remain the same from the source to destination. 2.48  Figure 2.21 Port addresses 2.49  Note The physical addresses will change from hop to hop, but the logical addresses usually remain the same. 2.50  Example 2.5 A port address is a 16-bit address represented by one decimal number as shown. 753 A 16-bit port address represented as one single number. 2.51