OSI Layers

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The OSI model consists of seven layers, each of which can (and typically does) have several sublayers. Cisco requires that CCNAs demonstrate an understanding of each layer as well as the protocols that correspond to each OSI layer. The names of the OSI model layers and their main functions are simply good things to memorize. And frankly, if you want to pursue your Cisco certifications beyond CCNA, these names and functional areas will come up continually.

The upper layers of the OSI model (application, presentation, and session—Layers 7, 6, and 5) are oriented more toward services to the applications. The lower four layers (transport, network, data link, and physical—Layers 4, 3, 2, and 1) are oriented more toward the flows of data from end to end through the network. CCNAs work mostly with issues in the lower layers, in particular with Layer 2, upon which switching is based, and Layer 3, upon which routing is based. Table 3-2 diagrams the seven OSI layers, with a thorough description and a list of example protocols.

Table 3-2 OSI Reference Model

Layer Name Functional Description

Examples

Application An application that communicates with other computers (Layer 7) is implementing OSI application layer concepts. The application layer refers to communications 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).

Presentation This layer's main purpose is defining data formats, such (Layer 6) 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.

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

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

continues

Table 3-2

OSI Reference Model (Continued)

Layer Name Functional Description

Examples

Session The session layer defines how to start, control, and end RPC, SQL, NFS,

(Layer 5) conversations (called sessions). This includes the control NetBios names, and management of multiple bidirectional messages so AppleTalk ASP, DECnet that the application can be notified if only some of a SCP

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 checking 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.

Transport Layer 4 includes the choice of protocols that either do or

(Layer 4) 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.

Network This layer defines end-to-end delivery of packets. To

(Layer 3) accomplish this, the network layer defines logical addressing so that any endpoint 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 OSI defines most of the details that a Cisco router considers when routing. 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.

IP, IPX, AppleTalk DDP

Table 3-2

OSI Reference Model (Continued)

Layer Name Functional Description

Examples

Data link The data link (Layer 2) specifications are concerned with

(Layer 2) 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, OSI often does not create any original specification 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.

Physical These physical layer (Layer 1) specifications, which are

(Layer 1) 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 currents, encoding, and light modulation are all part of different physical layer specifications. Multiple specifications are sometimes used to complete all details of the physical layer. For example, RJ-45 defines the shape of the connector and the number of wires or pins in the cable. Ethernet and 802.3 define the use of wires or pins 1, 2, 3, and 6. So, to use a category 5 cable, with an RJ-45 connector for an Ethernet connection, Ethernet and RJ-45 physical layer specifications are used.

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

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

Some protocols define details of multiple layers. For example, because the TCP/IP application layer correlates to OSI Layers 5 through 7, the Network File System (NFS) implements elements matching all three layers. Likewise, the 802.3, 802.5, and Ethernet standards define details for the data link and physical layers.

CCNAs deal with many aspects of Layers 1 through 4 on a daily basis. However, the upper layers are not as important to CCNAs. In addition, most networking people know what the OSI model is but do not need to memorize everything about it. Table 3-2 shows plenty of detail and explanation for a more in-depth idea of the OSI model components. If you are daunted by the task of memorizing all the examples in Table 3-2, you can refer to Table 3-3, which offers a more condensed description of the layer characteristics and examples. This table is taken directly from Cisco's ICND course, so if you are just not willing to try and remember all of Table 3-2, the information in Table 3-3 is a good compromise. (ICND is the instructor-led course in the official CCNA training path.)

Table 3-3 OSI Reference Model (Condensed Information)

OSI Layer Name

Functional Description

Examples

Application (Layer 7)

User interface

Telnet, HTTP

Presentation (Layer 6)

How data is presented

Special processing, such as encryption

JPEG, ASCII, EBCDIC

Session (Layer 5)

Keeping data separate from different applications

Operating systems and application access scheduling

Transport (Layer 4)

Reliable or unreliable delivery

Multiplexing

TCP, UDP, SPX

Network (Layer 3)

Logical addressing, which routers use for path determination

IP, IPX

Data link (Layer 2)

Combination of bits into bytes, and bytes into frames

Access to the media using MAC address Error detection and error recovery

802.3/802.2, HDLC

Physical (Layer 1)

Moving of bits between devices

Specification of voltage, wire speed, and cable pin-outs

EIA/TIA-232, V.35

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