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Understanding the OSI 7 Layer Model

by MD0MDI
OSI 7 Layer Model

7Application
6Presentation
5Session
4Transport
3Network
2Data Link
1Physical

Layer 7 : Application

Examples:

Telnet, HTTP, FTP, WWW Browsers, NFS, SMTP Gateways (Eudora, CC:mail), SNMP, X.400 mail, FTAM

Functional Description:

An application that communicates with other computers is implementing OSI application layer concepts. The application layer refers to communication services to applications. For example, a word processor that lacks communications capabilities would not implement code for communications, and word processor programmers would not be concerned about OSI layer 7. However, if an option for transferring a file were added, then the word processor would need to implement OSI Layer 7 (or the equivalent layer in another protocol specification).

Layer 6 : Presentation

Examples:

JPEG, ASCII, EBCDIC, TIFF, GIF, PICT, Encryption, MPEG, MIDI

Functional Description:

This layer’s main purpose is defining data formats, such as ASCII text, EBCDIC text, binary, BCD, and JPEG. Encryption is also defined by OSI as a presentation layer service. For example, FTP enables you to choose binary or ASCII transfer. If binary is selected, the sender and receiver do not modify the contents of the file. If ASCII is chosen, the sender translates the text from the sender’s character set to a standard ASCII and sends the data. The receiver translates back from the standard ASCII to the character set used on the receiving computer.

Layer 5 : Session

Examples:

RPC, SQL, NFS, Net Bios Names, Appletalk ASP, DEC net, SCP

Functional Description:

The session layer defines how to start, control, and end conversations (called sessions). This includes the control and management of multiple bidirectional messages so that the application can be modified if only some of a series of messages are completed. This allows the presentation layer to have a seamless view of an incoming stream of data. The presentation layer can be presented with data if all flows occur in some cases. For example, an automated teller machine transaction in which you withdraw cash from your bank account should not debit your account, and then fail, before handing you the cash, recording the transaction even though you did not receive money.

The session layer creates ways to imply which flows are part of the same session and which flows must complete before any are considered complete.

Layer 4 : Transport

Examples:

TCP, UDP, SPX

Functional Description:

Layer 4 includes the choice of protocols that either do or do not provide error recovery. Multiplexing of incoming data for different flows to applications on the same host (for example, TCP sockets) is also performed. Reordering of the incoming data stream when packets arrive out of order is included.

Layer 3 : Network

Examples:

IP, IPX, Appletalk DDP

Functional Description:

This layer defines end-to-end delivery of packets. To accomplish this, the network layer defines logical addressing so that any end-point can be identified. It also defines how routing works and how routes are learned so that the packets can be delivered. The network layer also defines how to fragment a packet into smaller packets to accommodate media with smaller maximum transmission unit sizes. (Note: Not all Layer 3 protocols use fragmentation). The network layer of the OSI defines most of the details . For example, IP running in a Cisco router is responsible for examining the destination IP address of a packet, comparing that address to the IP routing table, fragmenting the packet if the outgoing interface requires smaller packets, and queuing the packet to be sent out to the interface.

Layer 2 : Data Link

Examples:

IEEE 802.3/802.2, HDLC, Frame Relay, PPP, FDDI, ATM, IEEE 802.5/802.2

Functional Description:

The data link (layer 2) specifications are concerned with getting data across one particular link or medium. The data link protocols define delivery across an individual link. These protocols are necessarily concerned with the type of media in question; for example 802.3 and 802.2 are specifications from the IEEE, which are referenced by OSI as valid data link (layer 2) protocols. These specifications define how Ethernet works. Other protocols, such as High-Level Data Link Control (HDLC) for a point-to-point WAN link, deal with the different details of a WAN link. As with other protocol specifications for the data link layer but instead relies on other standards bodies such as IEEE to create new standards for the data link layer and the physical layer.

Layer 1 : Physical

Examples:

EIA/TIA-232, V.35, EIA/TIA-449, V.24, RJ45, Ethernet, 802.3, 802.5, FDDI, NRZI, NRZ, B8ZS

Functional Description:

These physical layer (layer 1) specifications, which are also typically standards from other organizations that are referred to by OSI, deal with the physical characteristics of the transmission medium. Connectors, Pins, use of pins, electrical current, encoding, and light modulation are all part of different physical layer specifications. Multiple specifications are sometimes used to complete all details that the shape of the connector and the number of wires or pins in the cable. Ethernet and 802.3 defines the use of wires or pins 1, 2, 3 and 6. so, to use a category 5 cable, with an RJ45 connector for an Ethernet connection, Ethernet and RJ45 physical layer specifications are used.

Protocol Data UnitOSI NumberOSI Layer DescriptionExamples of Layer use.
(S) Segment7ApplicationTelnet, HTTP, FTP, WWW Browsers, NFS, SMTP Gateways (Eudora, CC:mail), SNMP, X.400 mail, FTAM
(P) Packet6PresentationJPEG, ASCII, EBCDIC, TIFF, GIF, PICT, Encryption, MPEG, MIDI
(F) Frame5SessionRPC, SQL, NFS, Net Bios Names, Appletalk ASP, DEC net, SCP
(B) Bits4TransportTCP, UDP, SPX
3NetworkIP, IPX, Appletalk DDP
2Data LinkIEEE 802.3/802.2, HDLC, Frame Relay, PPP, FDDI, ATM, IEEE 802.5/802.2
1PhysicalEIA/TIA-232, V.35, EIA/TIA-449, V.24, RJ45, Ethernet, 802.3, 802.5, FDDI, NRZI, NRZ, B8ZS

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