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  Figure 2.3 The interaction between layers in the OSI model 2.7  Figure 2.4 An exchange using the OSI model 2.8  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 2.9  Figure 2.5 Physical layer 2.10  Note The physical layer is responsible for movements of individual bits from one hop (node) to the next. 2.11  Physical Layer Physical characteristics of interfaces and medium Interface between the devices and the transmission medium. Defines the type of transmission medium. Representation of bits The type of encoding (how 0’s and 1’s are changed to signals). Data rate The number of bits sent each second. Synchronization of bits The sender and the receiver clocks must be synchronized. Line configuration Connection of devices to the medium (point to point or multipoint configuration). Physical topology How devices are connected to make a network. Transmission mode Simplex, half-duplex, or full-duplex. 2.12  Figure 2.6 Data link layer 2.13  Note The data link layer is responsible for moving frames from one hop (node) to the next. 2.14  Data Link Layer Framing Dividing the stream of bits received from the network layer into manageable data units called frames. Physical Addressing Defining the sender and/or the receiver of the frame. Station on the network or the device that connects the network to the next one. Flow Control Controlling the transmission speed of the sender (across a single link). Error Control Detecting and retransmitting damaged or lost frames (across a single link). Recognizing duplicate frames. Access Control Which device has control over the link at any given time. 2.15  Figure 2.7 Hop-to-hop delivery 2.16  Figure 2.8 Network layer 2.17  Note The network layer is responsible for the delivery of individual packets from the source host to the destination host. 2.18  Network Layer Logical Addressing Adding logical addresses of the sender and receiver. Routing Routing or switching the packets to their final destination 2.19 2.20  Figure 2.9 Source-to-destination delivery 2.21  Figure 2.10 Transport layer 2.22 2.23  Note The transport layer is responsible for the delivery of a message from one process to another. 2.24  Transport Layer Service-Point Addressing The address of the process (running program) on a computer. Segmentation and Reassembly Division of a message into segments. Adding sequence numbers to reassemble the message correctly at the destination. Connection Control Connectionless or connection-oriented. Flow Control End to end flow control. Error Control End to end error control. 2.25  Figure 2.11 Reliable process-to-process delivery of a message 2.26 2.27 Figure 2.12 Session layer 2.28  Note The session layer is responsible for dialog control and synchronization. 2.29  Session layer Dialog control Half-duplex or full-duplex. Synchronization Adding checkpoints synchronization points to a stream of data. 2.30 2.31  Figure 2.13 Presentation layer 2.32  Note The presentation layer is responsible for translation, compression, and encryption. 2.33  Presentation layer Translation Changing information Sender-dependent format -> common format -> receiver dependent format. Encryption Sensitive information. Transforming the original information to another form and sending the resulting message out over the network. Decryption: Transforming the message back to its original form. Compression Reducing the number of bits contained in the information. Important in the transmission of multimedia. 2.34 2.35  Figure 2.14 Application layer 2.36  Note The application layer is responsible for providing services to the user. 2.37  Application Layer Network Virtual Terminal Allows the user to log on to a remote host. File Transfer, Access and Management Allows the user to access files in a remote host. Mail Services Provides the basis for email forwarding and storage. Directory Services Provides distributed database sources and access for global information. 2.38 2.39 2.40  Figure 2.15 Summary of layers 2.41  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 2.42  Figure 2.16 TCP/IP and OSI model 2.43  Network Layer Internetworking Protocol (IP) Unreliable and connectionless protocol. Best effort delivery (it tries to get a transmission through to its destination but with no guarantees. Transports data in packets called datagrams. 2.44  Four supporting protocols: Address Resolution Protocol (ARP). Is used to find the physical address of the node when its internet address is known. Reverse Address Resolution Protocol (RARP) Allows a host to discover its Internet address when it knows only its physical address. Internet Control Message Protocol (ICMP) Sends query and error reporting messages. Internet Group Message Protocol (IGMP) Facilitates the simultaneous transmission of a message to group of recipients. 2.45  Transport Layer User Datagram Protocol (UDP) Unreliable delivery. Connectionless. Transmission Control Protocol (TCP) Reliable delivery. Connection-oriented. Stream Control Transmission Protocol (SCTP) Provides support for newer applications such as VOIP. Combines the best features of UDP and TCP. 2.46  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 2.47  Figure 2.17 Addresses in TCP/IP 2.48  Figure 2.18 Relationship of layers and addresses in TCP/IP 2.49  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. 2.50  Figure 2.19 Physical addresses 2.51  Example 2.2 As we will see in Chapter 13, 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. 2.52  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. 2.53  Figure 2.20 IP addresses 2.54  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. 2.55  Figure 2.21 Port addresses 2.56  Note The physical addresses will change from hop to hop, but the logical addresses usually remain the same. 2.57  Example 2.5 As we will see in Chapter 23, 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. 2.58  Note The physical addresses change from hop to hop, but the logical and port addresses usually remain the same. 2.59  Specific address User-friendly addresses. Email address (e.g. [email protected]). Universal Resource Locator (URL) (e.g. www.mhhe.com) Gets changed to the corresponding port and logical addresses by the sending computer. 2.60