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Course 4

• Standard architectures: OSI and TCP/IP. Layers • Information exchange • Information Formats • Multiplexing • Technics of multiple access: OFDM and CDMA • Internetworking • How Network Differ? • How Networks Can Be Connected?

– Concatenated Virtual Circuits – Connectionless Internetworking

• Tunneling 1/26

Layered architectures

• The term “layered” highlights the fact that each layer uses the services provided by the layers below it.

• Two models are widely used: – Open System Interconnection (OSI); – Transmission Control Protocol /Internet Protocol (TCP/IP).

• OSI is a generic frame for the architecture developed by ISO (International Organization for Standardization) in 1984

• OSI describes how information from software application in one computer is transmitted to another software application in another.

• TCP/IP dominates the practical implementations.

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OSI MODEL •Application: the layer at which applications (file transfer, e-mails, database access…) access network services. • Presentation: translates data from the application layer into a network format, performs data compression and encryption. •Session: session setup, control and tear down. •Transport: reliable transfer data between the communicating end points. Error recovery and flow control. •Network: addresses messages and translates logical addresses and names into physical addresses. It also determines the route from the source to the destination. •Data Link: packages raw bits from the Physical layer into frames (logical, structured packets). It also implements acknowledgement mechanisms. •Physical: transmission over the physical medium.

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The upper layers of the OSI model deal with application issues and generally are implemented only in software. -The highest layer (application) is closest to the end user. -Both users and application layer processes interact with applications software that contain a communications component. The lower layers of the OSI model handle data transport issues. -The physical layer and the data link layer are implemented in hardware and software. -The lowest layer, the physical layer, is closest to the physical network medium (e.g. network cabling) and is responsible for actually placing information on the medium.

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OSI Model and Communication Between Systems

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OSI Layer Services

• One OSI layer communicates with another layer to make use of the services provided by the second layer.

• The services provided by adjacent layers help a given OSI layer communicate with its peer layer in other computer systems.

• Three basic elements are involved in layer services: – the service user- the OSI layer that requests services from an

adjacent OSI layer; – the service provider- the OSI layer that provides services to

service users; – the service access point (SAP) - is a conceptual location at which

one OSI layer can request the services of another OSI layer.

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Service Users, Providers, and SAPs Interact at the Network and Data Link

Layers

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Information exchange • Control information typically takes one of two forms:

– headers – trailers.

• Headers are prepended to data that has been passed down from upper layers.

• Trailers are appended to data that has been passed down from upper layers.

• Headers, trailers, and data are relative concepts, depending on the layer that analyzes the information unit.

• At the network layer, for example, an information unit consists of a Layer 3 header and data.

• At the data link layer, however, all the information passed down by the network layer (the Layer 3 header and the data) is treated as data.

• The data portion of an information unit at a given OSI layer potentially can contain headers, trailers, and data from all the higher layers. This is known as encapsulation.

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Headers and Data Can Be Encapsulated During Information Exchange

Application Presentation

Session

Transport

Network

Data Link

Physical

Application

Presentation

Session

Transport

Network

Data Link

Physical

Data

Physical

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Peer-to-peer processes

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The physical OSI layer

• defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems.

• define characteristics such as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and physical connectors.

• Physical layer implementations can be categorized as either LAN or WAN specifications. Figure illustrates some

common LAN and WAN physical layer implementations. 12/26

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Physical Layer Implementations Can Be LAN or WAN Specifications

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OSI Model Data Link Layer (2)

– provides reliable transfer of data; – breaks data (packets) into frames; – adds bits for error detection/correction; – manages access to and uses the channel; – solves problems caused by lost, damaged, and

duplicate frames; – sends acknowledgments; – adds flags to indicate beginning and end of message; – connectionless or connection oriented services.

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The Institute of Electrical and Electronics Engineers (IEEE) has subdivided the data link layer into two sublayers:

• Logical Link Control sublayer of the data link layer manages communications between devices over a single link of a network defined in IEEE 802.2 specification: – supports both connectionless and connection-oriented services used by

higher-layer protocols. – IEEE 802.2 defines a number of fields in data link layer frames that

enable multiple higher-layer protocols to share a single physical data link.

• Media Access Control sublayer of the data link layer manages protocol access to the physical network medium. – MAC specification defines MAC addresses, which enable multiple

devices to uniquely identify one another at the data link layer.

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OSI Model Network Layer (3)

- defines the network address which is different from MAC address: – determines how packets are routed; – divides transport messages into packets and

reassembles them; – performs congestion control, flow control; – provides virtual circuit or datagram services; – recognizes message priorities; – sends messages in proper order; – handles internetworking.

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OSI Model Transport Layer (4) – establishes reliable end-to-end transport session (makes error detection and recovery), once path has been established; – fragmentation of message into packets (if not handled

by layer 3); – multiplexing enables data from several applications to

be sent over a single physical link; – creates distinct network connections; – monitors quality of service; – disassembles and assembles session messages; – flow control (if not done by layer 3); – The transport protocols used on the Internet are TCP

and UDP. 17/26

OSI Model Session Layer (5) - Establishes, manages and terminates

communication sessions. - Communication sessions consists of service requests

and responses between applications from different devices of the network.

- They are coordinated by protocols - authentication of users; - controls dialogue, organizes and synchronizes.

- Some examples of session-layer implementations include:

- Zone Information Protocol (ZIP), the AppleTalk protocol that coordinates the name binding process; and Session Control Protocol (SCP), the DECnet Phase IV session layer protocol.

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OSI Model Presentation Layer (6) – data encryption, security, compression and code

conversion; – make sure data is encoded in standard form; – handles pass-through of services from session to

application layer; – Presentation layer implementations are not typically

associated with a particular protocol stack. Some well-known standards for video include:

• QuickTime (Apple Computer specification for video and audio);

• Motion Picture Experts Group (MPEG) standard for video compression and coding;

• Graphics Interchange Format (GIF) compressing and coding graphic images);

• Joint Photographic Experts Group (JPEG) also graphic images Tagged Image File Format (TIFF).

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OSI Model Application Layer (7) – login, password check; – agreement on semantics for information exchange; – file transfer, access and management; – message handling, email; – job transfer and manipulation; – directory service; – system management; – industry protocols; – database access and management; – virtual terminals. – Some examples of application layer implementations

include Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).

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TCP/IP model Application Layer: all of the processes that

involve user interaction. Examples:Simple Mail Transfer Protocol (SMTP)

and Post Office Protocol (POP) for e-mail, Hyper Text Transfer Protocol (HTTP) for the World- Wide-Web, and File Transfer Protocol (FTP).

Transport Layer: two Transport Layer protocols. The Transmission Control Protocol (TCP) is safer but slower. The User Datagram Protocol (UDP) performs no end-to-end reliability checks, but is more rapid.

Internet Layer: the network layer of the TCP/IP model. Data is transmitted in variable size IP packets. Addresses the two ends of the communcation, routes the data between source and destination.

Network Access Layer: TCP/IP makes use of the existing PHY and Data Link Layer standard.

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Some Protocols in TCP/IP Suite

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OSI vs TCP/IP

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Most networks todays are Internet model networks which have 4 or 5 Layers

Some differences ... one example -Session Layer permits two parties to hold ongoing communications called a session -TCP/IP doesn't have this. This is done by the TCP protocol in the transport layer.

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Information Formats

• Data and control information that is transmitted through internetworks takes a variety of forms.

• Common information formats include: – messages- above network layer; – segments-transport layer; – packets – network layer; – datagrams –network layer; – frames – data link layer; – cells-data link layer; – data units generic term – for a variety of information

units.

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