OSI Layers

The OSI reference model consists of seven layers. Each layer defines a set of typical networking functions. When OSI was in active development in the 1980s and 1990s, the OSI committees created new protocols and specifications to implement the functions specified by each layer. In other cases, the OSI committees did not create new protocols or standards, but instead referenced other protocols that were already defined. For instance, the IEEE defines Ethernet standards, so the OSI committees did not waste time specifying a new type of Ethernet; it simply referred to the IEEE Ethernet standards.

Today the OSI model can be used as a standard of comparison to other networking models. Figure 2-8 shows OSI, as compared with TCP/IP and Novell NetWare.

Figure 2-8 Comparing OSI, TCP/IP, and NetWare

OSI

TCP/IP

NetWare

Application

Presentation

Application

SAP, NCP

Session

Transport

Transport

SPX

Network

Internetwork

IPX

Data Link

Network

Mac

Physical

Interface

Protocols

Because OSI does have a very well-defined set of functions associated with each of its seven layers, you can examine any networking protocol or specification and make some determination of whether it most closely matches OSI Layer 1, 2, or 3, and so on. For instance, TCP/IP's internetworking layer, as implemented by IP, equates most directly to the OSI network layer. So, most people say that IP is a network layer, or Layer 3, protocol, using OSI terminology and numbers for the layer. Of course, if you numbered the TCP/IP model, starting at the bottom, IP would be in Layer 2—but, by convention, everyone uses the OSI standard when describing other protocols. So, using this convention, IP is a network layer protocol.

Cisco requires that CCNAs demonstrate an understanding of the functions defined by OSI for each layer, as well as some example protocols that correspond to each OSI layer. The names of the OSI reference model layers, a few of the typical protocols at each layer, and the functions of each layer are simply good things to memorize for the INTRO exam. 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 reference model (application, presentation, and session—Layers 7, 6, and 5) define functions focused on the application. The lower four layers (transport, network, data link, and physical—Layers 4, 3, 2, and 1) define functions focused on end-to-end delivery of the data. Both CCNA exams focus on issues in the lower layers—in particular, with Layer 2, upon which switching is based, and Layer 3, upon which routing is based. Table 2-4 defines the functions of the seven layers, and Table 2-5 lists typical protocols considered to be comparable to the OSI layers.

Table 2-4 OSI Reference Model Layer Definitions

Layer

Functional Description

7

Layer 7 defines the interface between the communications software and any applications that need to communicate outside the computer on which the application resides. For example, a web browser is an application on a computer. The browser needs to get the contents of a web page; OSI Layer 7 defines the protocols used on behalf of the application to get the web page.

6

This layer's main purpose is to define data formats, such as ASCII text, EBCDIC text, binary, BCD, and JPEG. Encryption also is 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.

5

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 notified 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 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.

4

Layer 4 protocols provide a large number of services, as seen in Chapter 5 of this book. Although Layers 5 through 7 focus on issues related to the application, Layer 4 focuses on issues related to data delivery to the other computer—for instance, error recovery, segmentation of large application data blocks into smaller ones for transmission, and reassembly of those blocks of data on the receiving computer.

3

This layer defines end-to-end delivery of packets. To 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. Chapter 4 of this book examines Layer 3 concepts in detail. 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.

2

The data link layer (Layer 2) specifications deliver data across one particular link or medium. These protocols are necessarily concerned with the type of media in question; for example, 802.3 and 802.2 define Ethernet for the IEEE, which are referenced by OSI as valid data link layer (Layer 2) protocols. 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.

continues continues

Table 2-4 OSI Reference Model Layer Definitions (Continued)

Layer

Functional Description

1

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 currents, encoding, and light modulation are all part of different physical layer specifications. Multiple specifications sometimes are 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.

Table 2-5 OSI Reference Model—Example Protocols

Layer Name

Examples

Application (Layer 7)

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

Presentation (Layer 6)

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

Session (Layer 5)

RPC, SQL, NFS, NetBIOS names, AppleTalk ASP, DECnet SCP

Transport (Layer 4)

TCP, UDP, SPX

Network (Layer 3)

IP, IPX, AppleTalk DDP

Data link (Layer 2)

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

Physical (Layer 1)

EIA/TIA-232, V.35, EIA/TIA-449, RJ-45, Ethernet, 802.3, 802.5, B8ZS

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