Ch02

2.1

Chapter 2

Network Models

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2-1 LAYERED TASKS2-1 LAYERED TASKS

We use the concept of We use the concept of layerslayers in our daily life. As an in our daily life. As an example, let us consider two friends who communicate example, let us consider two friends who communicate through postal mail. The process of sending a letter to a through postal mail. The process of sending a letter to a friend would be complex if there were no services friend would be complex if there were no services available from the post office. available from the post office.

Sender, Receiver, and CarrierHierarchy

Topics discussed in this section:Topics discussed in this section:

2.3

Figure 2.1 Tasks involved in sending a letter

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2-2 THE OSI MODEL2-2 THE OSI MODEL

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

Layered ArchitecturePeer-to-Peer ProcessesEncapsulation

Topics discussed in this section:Topics discussed in this section:

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ISO is the organization.OSI is the model.

Note

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Figure 2.2 Seven layers of the OSI model

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Figure 2.3 The interaction between layers in the OSI model

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Figure 2.4 An exchange using the OSI model

A packet consists of two kinds of data: control information and user data (also known as payload). The control information provides data the network needs to deliver the user data, for example: source and destination addresses, error detection codes like checksums, and sequencing information. Typically, control information is found in packet headers and trailers, with user data in between.

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2-3 LAYERS IN THE OSI MODEL2-3 LAYERS IN THE OSI MODEL

In this section we briefly describe the functions of each In this section we briefly describe the functions of each layer in the OSI model.layer in the OSI model.

Physical LayerData Link LayerNetwork LayerTransport LayerSession LayerPresentation LayerApplication Layer

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Organization of the layers

Layers 1,2, and 3 physical, data link, and network are Network Support layers

Layers 5,6 and 7 are session, presentation, and application are User support layer

Layer 4 transport layer links the 2 subgroups and ensures that what the lower layers have transmitted is in a form that the upper layers can use.

2.11

Common Names For Messages

The most common terms that are used for messages are the following:

Packet: This term is considered by many to most correctly refer to a message sent by protocols operating at the network layer of the OSI Reference Model. So, you will commonly see people refer to “IP packets”. However, this term is commonly also used to refer generically to any type of message, as I mentioned at the start of this topic.

Datagram: This term is basically synonymous with “packet” and is also used to refer to network layer technologies. It is also often used to refer to a message that is sent at a higher level of the OSI Reference Model (more often than “packet” is).

Frame: This term is most commonly associated with messages that travel at low levels of the OSI Reference Model. In particular, it is most commonly seen used in reference to data link layer messages. It is occasionally also used to refer to physical layer messages, when message formatting is performed by a layer one technology. A frame gets its name from the fact that it is created by taking higher-level packets or datagrams and “framing” them with additional header information needed at the lower level.

Cell: Frames and packets, in general, can be of variable length, depending on their contents; in contrast, a cell is most often a message that is fixed in size. For example, the fixed-length, 53-byte messages sent in Asynchronous Transfer Mode (ATM) are called cells. Like frames, cells usually are used by technologies operating at the lower layers of the OSI model.

Protocol Data Unit (PDU) and Service Data Unit (SDU): These are the formal terms used in the OSI Reference to describe protocol messages. A PDU at layer N is a message sent between protocols at layer N. It consists of layer N header information and an encapsulated message from layer N+1, which is called both the layer N SDU and the layer N+1 PDU. After you stop scratching your head, see the topic on OSI model data encapsulation for a discussion of this that may actually make sense. J

2.12

Figure 2.5 Physical layer

The physical layer is responsible for movements ofindividual bits from one hop (node) to the next.

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Physical layer

The physical layer is also concerned with:

Physical characteristics of interfaces and medium.

Representation of the bits ( type of encoding).

Data Rate. The transmission rate

Synchronization of bits.

Line configuration ( point-point, multipoint)

Physical Technology (how devices are connected –network)

Transmission mode (simplex, h-d, f-d

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Figure 2.6 Data link layer

The data link layer is responsible for moving frames from one hop (node) to the next.

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Data link layer

The Data link layer is also concerned with:

Framing (data received from the NW layer into frames)

Physical addressing ( add header to frame to define sendr/reciever)

Flow Control (data flow rate between S/R to avoid overwhelming)

Error Control mechanism to detect and retransmit damaged or lost frames

Access Control when 2 or more devices are connected through the link, it determines which device has control over the link at any given time

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Figure 2.7 Hop-to-hop delivery

Router

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Figure 2.8 Network layer

The network layer is responsible for the delivery of individual packets from

the source host to the destination host.

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Figure 2.9 Source-to-destination delivery

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Network Layer

The Network layer is also concerned with:

Logical Addressing (data link layer give the data a physical address locally , if the packet passes the network boundary, we need another address to distinguish the source and destination received from the NW layer into frames)

Routing ( when connecting independent networks or a links to creat internetworks or a large network, the network layer provide this mechanism

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Figure 2.10 Transport layer

The transport layer is responsible for the delivery of a message from one process to another.

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Figure 2.11 Reliable process-to-process delivery of a message

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Transport Layer

The Transport layer is also concerned with:

Service Point Addressing delivery of messages can be performed from a process to a process (running program), the transport layer header include a type of address called SPA( port address), the network layer get the packet to the correct computer and the transport layer get the message to the correct process.Segmentation and reassembly: message is segmented, each segment has sequence number.

Connection control ( 2 types 1-connection less 2- connection oriented) Connectionless each segment is treated as independent packetConnection oriented : connection is performed with the destination, packets are transferred, connection is terminated.

Flow ControlError Control

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Figure 2.12 Session layer(network dialogue controller)

The session layer is responsible for dialog control and synchronization.

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Session Layer

The Session layer is also concerned with:It establishes, maintains, and synchronizes, the interaction among communicating system

Dialog Control allows the communication between two processes to take place in either half-duplex or full-duplex.

Synchronization : allows a process to add a check points to a stream of data ( a system sends 2000 page, put a check point every 100 page to ensure their receiving).

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Figure 2.13 Presentation layer

The presentation layer is responsible for translation, compression, and encryption.

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Presentation Layer

The Presentation layer is also concerned with:

Translation Process in different computer are usually exchange information in the form of character, numbers,..the information is changed to a stream of bits before transmitted using different codes, presentation is responsible for the interoperability between different encoding methods.

Encryption: sensitive information, and also to ensure privacy

Compression Data compression reduces the number of bits contained in the information.

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Figure 2.14 Application layer

The application layer is responsible for providing services to the user.

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Application Layer

The application layer is also concerned with: enables the user( human or software) to access the network, support for services ( Email, shared data base management,…).

Network Virtual terminal software version ( created by the application) of a physical terminal, and allows the user to log on to a remote host

File transfer , access, management : allows the user to access files in the remote host.

Mail Services provides distributed data base source and access for global information about various objects and services.

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Figure 2.15 Summary of layers

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2-4 TCP/IP PROTOCOL SUITE2-4 TCP/IP PROTOCOL SUITE

The layers in the The layers in the TCP/IP protocol suiteTCP/IP protocol suite do not exactly do not exactly match those in the OSI model. The original TCP/IP match those in the OSI model. The original TCP/IP protocol suite was defined as having four layers: protocol suite was defined as having four layers: host-to-host-to-networknetwork, , internetinternet, , transporttransport, and , and applicationapplication. However, . However, when TCP/IP is compared to OSI, we can say that the when TCP/IP is compared to OSI, we can say that the TCP/IP protocol suite is made of five layers: TCP/IP protocol suite is made of five layers: physicalphysical, , data linkdata link, , networknetwork, , transporttransport, and , and applicationapplication..

Physical and Data Link LayersNetwork LayerTransport LayerApplication Layer

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Figure 2.16 TCP/IP and OSI model

Internet layer

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2-5 ADDRESSING2-5 ADDRESSING

Four levels of addresses are used in an internet employing Four levels of addresses are used in an internet employing the TCP/IP protocols: the TCP/IP protocols: physicalphysical, , logicallogical, , portport, and , and specificspecific..

Physical AddressesLogical AddressesPort AddressesSpecific Addresses

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Figure 2.17 Addresses in TCP/IP

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Figure 2.18 Relationship of layers and addresses in TCP/IP

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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.

Example 2.1

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Figure 2.19 Physical addresses

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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:

Example 2.2

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

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

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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.

Example 2.3

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Figure 2.20 IP addresses

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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.

Example 2.4

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Figure 2.21 Port addresses

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The physical addresses will change from hop to hop,but the logical addresses usually remain the same.

As we will see in Chapter 23, a port address is a 16-bit address represented by one decimal number as shown.

753A 16-bit port address represented

as one single number.

The physical addresses change from hop to hop,but the logical and port addresses usually remain the same.