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to the Application layer. Included in these services, and the ones you need to know, are:
" Data encryption
"
Data compression
" Data formatting
"
Data conversion
Think of the Presentation layer like a United Nations translator: It translates the data it gets from the
Application layer into a format that the receiving nodeÇs Application layer can interpret. The Presentation
layer is also responsible for negotiating data transfer syntax between the application layers. This is the only
layer that changes data. The other layers can put data in different encapsulation forms but the Presentation
layer can change it. An example of this would be from changing the data from ASCII to EBCDIC. The
Presentation layer not only formats and converts text but also audio, video, and graphic data.
Instant Answer On the exam, you may find references to some of the conversion standards defined on the
Presentation layer for data conversion and formatting. Table 7-1 lists the Presentation layer standards by
category. We recommend that you know which category each of the standards falls under.
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Table 7-1: Presentation Layer Standards
Category Standards
Data conversion ASCII, EBCDIC, encryption
Audio/video conversion MIDI, MPEG, QuickTime, AVI
Graphics conversion GIF, JPEG, PICT, TIFF
Applying the Application Layer
The OSI modelÇs Application layer is the layer closest to the user. Remember that the Application layer
doesnÇt define the applications used by the end-user to perform tasks. Rather, it defines the communication
services used by the userÇs applications to transmit data over the network. The Application layer provides
access services to desktop (and notebook, and tower) computer applications.
The Application layer provides three basic services to applications. It makes sure the resources needed to
carry out a session are present, it matches the application to the appropriate communication protocol or
service, and it synchronizes the transmission of data between the application and its protocol.
Remember The Application layer services and protocols you should know are:
" FTP (File Transfer Protocol)
"
E-mail clients
" Web browsers
"
Telnet
"
SNMP (Simple Network Management Protocol)
"
BBS (bulletin board system) servers
"
EDI (Electronic Data Interchange) and other transaction services
A Quick Review of the OSI Model
Time-Saver Table 7-2 summarizes what you know about the layers of the OSI Reference Model, including a
brief description of the functions defined on each layer and some examples of protocols or resources that
operate on each layer. This is a very useful study aid for your last-minute review.
Table 7-2: OSI Reference Model Summary
Number Layer Activities Protocols and Services
7 Application Identifies and establishes FTP, SMTP, Telnet, Web
availability of communication Browsers, CMIP, and Virtual
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partners. Determines whether Terminal Protocol
resources are sufficient for the
communication to occur.
6 Presentation Defines data formats, including MPEG, HTML, MIDI, ASCII,
encryption, data conversion, EBCDIC, TIFF, GIF, PICT
audio/ visual, and graphics.
5 Session Establishes, controls, and ends NFS, SQL, NetBIOS,
sessions. Full- and half-duplex AppleTalk ASP, X Windows,
communications. RPC, DECNet SCP
4 Transport Segments and reassembles data. TCP, UDP, SPX
Defines reliable and unreliable
data transmission, and
connection-oriented and
connectionless protocols.
3 Network Concerned with end-to- end IP, IPX, and AppleTalk DDP
delivery of data, logical
addressing, and routing and
how routes are identified.
2 Data Link Translates PDUs from higher Frame Relay, PPP, HDLC,
layers into data frames. Defines IEEE 802.2, 802.3, 802.5,
the LLC and MAC sublayers. FDDI, ATM, SDLC, LAPB,
ISDN
1 Physical Defines the characteristics of EIA/TIA 232 and 449, RJ45,
the physical medium used to Ethernet, CAT 5, X.21, HSSI
send and receive data bits.
Prep Test
1. A series of related connection-oriented transmissions between two network nodes is a
A. Socket
B. Session
C. Handshake
D. Virtual circuit
2.
The transmission mode thatÇs capable of transmitting in two directions but in only one direction at a
time is
A. Full-duplex
B. Simplex
C. Half-duplex
D. Full-nelson
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3. On half-duplex transmission networks, which access method is used to avoid collisions?
A. CSMA/CA
B. CSMA/CD
C. SDH (semi/demi/hemi)
D. Ethernet
4.
The transmission mode thatÇs capable of transmitting in two directions simultaneously is
A. Full-duplex
B. Simplex
C. Half-duplex
D. Half-nelson
5.
Which of the following is not a bandwidth supported by full-duplex Ethernet networks?
A. 10 Mbps
B. 100 Mbps
C. Gigabit Ethernet
D. 1.54 Mbps
6. Which of the following is not a Session layer protocol or service?
A. NFS
B. SQL
C. FTP
D. RPC
7.
Which of the following is not a Presentation layer data conversion standard or function?
A. ASCII
B. MPEG
C. EBCDIC
D. Encryption
8.
Which of the following is not an Application layer protocol or service?
A. E-mail client
B. SNMP
C. FTP
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D. TCP/IP
9.
Which layer of the OSI model is concerned with end-to-end delivery of data?
A. Session
B. Transport
C. Network
D. Physical
10.
Which of the following lists the layers of the OSI model in correct sequence, starting with Layer 1
and moving up?
A. Data Link, Physical, Network, Session, Transport, Application, Presentation
B. Physical, Network, Data Link, Transport, Session, Presentation, Application
C. Physical, Data Link, Network, Transport, Session, Presentation, Application
D. Application, Presentation, Session, Transport, Network, Data Link, Physical
Answers
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