The OSI Reference Model

The Open System Interconnection (OSI) reference model describes how information from a software application in one computer moves through a network medium to a software application in another computer. The OSI model divides the tasks involved with moving information between networked computers into seven smaller, more manageable task groups. This enables the solutions offered by one layer to be updated without affecting the other layers.

The application layer (Layer 7) is the OSI layer that is closest to the user; it provides network services to the user’s applications. It differs from the other layers in that it does not provide services to any other OSI layer, but rather, only to applications outside the OSI model. Examples of such applications are spreadsheet programs, word processing programs, and bank terminal programs. The application layer establishes the availability of intended communication partners, synchronizes and establishes agreement on procedures for error recovery and control of data integrity.

The presentation layer (Layer 6) ensures that the information that the application layer of one system sends out is readable by the application layer of another system. If necessary, the presentation layer translates between multiple data formats by using a common format.

As its name implies, the session layer (Layer 5) establishes, manages, and terminates sessions between two communicating hosts. The session layer provides its services to the presentation layer. It also synchronizes dialogue between the two hosts' presentation layers and manages their data exchange. In addition to session regulation, the session layer offers provisions for efficient data transfer, class of service, and exception reporting of session layer, presentation layer, and application layer problems.

The transport layer (Layer 4) segments data from the sending host's system and reassembles the data into a data stream on the receiving host's system. The transport layer attempts to provide a data transport service that shields the upper layers from transport implementation details. In providing communication service, the transport layer establishes, maintains, and properly terminates virtual circuits. In providing reliable service, transport error detection-and-recovery and information flow control are used.

The network layer (Layer 3) is a complex layer that provides connectivity and path selection between two host systems that may be located on geographically separated networks.

The data link layer (Layer 2) provides reliable transit of data across a physical link. In so doing, the data link layer is concerned with physical (as opposed to logical) addressing, network topology, network access, error notification, ordered delivery of frames, and flow control.

The physical layer (Layer 1) defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other, similar, attributes are defined by physical layer specifications.

(From CISCO Networking Academy)

A handy way to remember the seven layers of the OSI reference model is the sentence All people seem to need data processing. The beginning letter of each word corresponds to a layer.

• All Application layer

• People Presentation layer

• Seem Session layer

• To Transport layer

• Need Network layer

• Data Data link layer

• Processing Physical layer

Task 12. Read the text and choose the correct words.

Bank transfer: payment can be made (1) by / from ordering a home bank to transfer money to an overseas account. The Society for Worldwide Interbank Financial Communications (SWIFT) offers facilities for a 24-hour transfer of money (2) in / to a beneficiary on its computer systems.

Bill of exchange: the seller prepares a bill of exchange in the name of the buyer. The bill states (3) that / what the buyer will pay the seller an amount within a stated time, e.g. 30 days. The bill is sent to the buyer either by post, or through a bank, and the buyer signs (accepts) the bill before the goods are sent. Shipping documents usually accompany bills when the bank acts (4) as / how an intermediary in overseas transactions.

Letter of credit (L/C): letters of credit (or documentary credits, as banks refer to them) have to be applied for from the buyer's bank, by filling out a form giving details of the type of credit (i.e. revocable, which can be cancelled, or irrevocable, which cannot be cancelled), the beneficiary (the person receiving the money), the amount, (5) when / how long the credit will be available for (i.e. valid until...), the documents involved (bill of lading, insurance, invoice, etc.), and a description of the goods. The money will be credited to the supplier's account as soon as confirmation of shipment is made. This is done (6) when / if the documents are lodged with the customer's bank.

Notes:

beneficiary (N) – бенефициар (получатель платежа)

bill of exchange (N) – переводной вексель, тратта

bill of lading (N) – коносамент, накладная на груз

insurance (N) – страховка invoice (N) – счет-фактура

letter of credit (N) – аккредитив

lodge (V) – помещать, класть (в банк) overseas (A) - зарубежный

revocable (A) – отзывный, irrevocable (A) – безотзывный

shipment (N) – отправка товаров, поставка

Task 13. Read the text and fill in the table.

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