ppt ch2 DCN.pp

  • View
    95

  • Download
    3

Embed Size (px)

Text of ppt ch2 DCN.pp

Chapter 2NETWORK MODELS

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

Layered TasksEx. A person sends a letter Components: Sender, Receiver, carrier

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

2-1 PROTOCOL LAYERSIn Chapter 1, we discussed that a protocol is required when two entities need to communicate. When communication is not simple, we may divide the complex task of communication into several layers. In this case, we may need several protocols, one for each layer. Let us use a scenario in communication in which the role of protocol layering may be better understood. We use two examples. In the first example, communication is so simple that it can occur in only one layer.McGraw-Hill The McGraw-Hill Companies, Inc., 2004

Example 2.1

Assume Maria and Ann are neighbors with a lot of common ideas. However, Maria speaks only Spanish, and Ann speaks only English. Since both have learned the sign language in their childhood, they enjoy meeting in a cafe a couple of days per week and exchange their ideas using signs. Occasionally, they also use a bilingual dictionary. Communication is face to face and Happens in one layer as shown in Figure 2.1.

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

Figure 2.1 Example 2.1

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

Example 2.2

Now assume that Ann has to move to another town because of her job. Before she moves, the two meet for the last time in the same cafe. Although both are sad, Maria surprises Ann when she opens a packet that contains two small machines. The first machine can scan and transform a letter in English to a secret code or vice versa. The other machine can scan and translate a letter in Spanish to the same secret code or vice versa. Ann takes the first machine; Maria keeps the second one. The two friends can still communicate using the secret code, as shown in Figure 2.2.

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

Figure 2.2 Example 2.2

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

2-2 THE OSI MODELEstablished in 1947, the International Standards Organization (ISO) is a multinational body dedicated to worldwide agreement on international standards. Almost three-fourths of countries in the world are represented in the ISO. 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.

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

Figure 2.3 The OSI model

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

Figure 2.6 Summary of OSI Layers

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

7 Layers of the OSI ModelLayer 7.) Application 6.) Presentation 5.) Session 4.) Transport 3.) Network 2.) Data Link 1.) PhysicalMcGraw-Hill

Responsible For: Provides Services to User Apps Data Representation Communication Between Hosts, session management Multiplexing, Segmenting the data Routing Synchronization, Error Detection/Correction Medium, Interfaces, Puts Bits on Med.The McGraw-Hill Companies, Inc., 2004

ExamplesLayer 7.) Application 6.) Presentation 5.) Session 4.) Transport 3.) Network 2.) Data Link 1.) PhysicalMcGraw-Hill

Example HTTP, FTP, SMTP ASCII, JPEG, PGP BOOTP, NetBIOS, DHCP, DNS TCP, UDP, SPX IP, IPX, ICMP Ethernet, Token Ring, Frame Relay Bits, Interfaces, HubsThe McGraw-Hill Companies, Inc., 2004

Mnemonics(A)ll (P)eople (S)eem (T)o (N)eed (D)ata (P)rocessingMcGraw-Hill

7.) (A)pplication 6.) (P)resentation 5.) (S)ession 4.) (T)ransport 3.) (N)etwork 2.) (D)ata Link 1.) (P)hysical

(A)way (P)izza (S)ausage (T)hrow (N)ot (D)o (P)leaseThe McGraw-Hill Companies, Inc., 2004

Layer 1: The Physical Layer

Defines physical medium and interfaces Determines how bits are represented Controls transmission rate & bit synchronization Controls transmission mode: simplex, halfduplex, & full duplex Protocol Data Unit (PDU): Bits Devices: hubs, cables, connectors, etcThe McGraw-Hill Companies, Inc., 2004

McGraw-Hill

Layer 2: The Data Link Layer

PDU: Frames Keeps Link alive & provides connection for upper layer protocols Based on physical address space Flow control and error detection/correction at the frame level is done. Ex: Ethernet, Token Ring, ISDN Devices: switches, bridges, NICsThe McGraw-Hill Companies, Inc., 2004

McGraw-Hill

Layer 3: The Network Layer

PDU: Packet End to end delivery of packets Creates logical paths Path determination (routing) Hides the lower layers making things hardware independent Devices: routers, firewalls

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

Layer 4: The Transport Layer

PDU: Segment It reassemble segments into data using sequence numbers Can use either connectionless or connection oriented sessions Uses acknowledgements & retransmission for error correction Example: TCP (used by things like telnet, http)The McGraw-Hill Companies, Inc., 2004

McGraw-Hill

The Transport Layer (cont.)

Connection oriented sessions require the sender to first request a connection, the receiver to acknowledge the connection, and that they negotiate how much data can be sent/received before its reception is acknowledged.

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

Layer 5: The Session Layer

PDU: Data The Session Layer provides the mechanism for opening, closing and managing a session between end-user application processes, i.e. a semi-permanent dialogue. Communication sessions consist of requests and responses that occur between applications. Session Layer services are commonly used in application environments that make use of remote procedure calls (RPCs).The McGraw-Hill Companies, Inc., 2004

McGraw-Hill

The Session Layer (cont.)

In case of a connection loss this protocol may try to recover the connection. If a connection is not used for a long period, the Session Layer Protocol may close it and re-open it. It provides for either full duplex or half-duplex operation and provides synchronization points

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

Layer 6: The Presentation Layer

The Presentation Layer is responsible for the delivery and formatting of information to the application layer for further processing or display. An example of a presentation service would be the conversion of an EBCDIC-coded text file to an ASCII-coded file.

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

Layer 7 : Application Layer

The Application Layer contains all protocols and methods that fall into the realm of process-to-process communications via an Internet Protocol (IP) network. The common application layer services provide semantic conversion between associated application processes.

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

Function of Each Layer

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

Layer 1

The Physical Layer

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

Transmission MediaTransmission Media and Physical Layer

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

Transmission Media

Transmission medium is the physical path between the transmitter and receiver. It is the Transmission medium through which information usually moves from one network device to another. In some cases, a network will utilize only one type of cable, other networks will use a variety of cable types.

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

Transmission media media are located below the physical layer. Computers use signals to represent data. Signals are transmitted in form of electromagnetic energy.McGraw-Hill The McGraw-Hill Companies, Inc., 2004

Transmission

Factors to Select Transmission Media

Data Rate and Bandwidth (BPS and Hz) Distance and Attenuation (meters, dB/km) Interference Characteristics Number of receivers (broadcast vs. point to point) Cost

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

Transmission Impairments

Impairments exist in all forms of data transmission media

Analog signal impairments result in random modifications that impair signal quality Digital signal impairments result in bit errors (1s and 0s transposed)

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

Types of MediaTwo major classes Conducted or guided media use a conductor such as a wire or a fiber optic cable to move the signal from sender to receiver. Energy is confined to the medium and guided by it Wireless or unguided media use radio waves of different frequencies and do not need a wire or cable conductor to transmit signals Energy spreads out and is not confined

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

Classes of transmission media

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

Layer 2 Data Link Layer

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

Functions of the Data Link Layer (2)Relationship between packets and frames.

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

Data Link Layer

Providing services to the Network Layer Framing Error Control Flow Control

McGraw-Hill

The McGraw-Hill Companies, Inc., 2004

Functions of the Data Link Layer

Provide service interface to the network layer Dealing with transmission errors Regulating data flow

Slow receivers not swamped by fast senders

McGraw-Hill