OSI

Question 1

Application Layer (Layer 7)

Answer 1

The Application Layer (Layer 7) refers to communications services to applications and is the
interface between the network and the application. Examples include: Telnet, HTTP, FTP, Internet
browsers, NFS, SMTP gateways, SNMP, X.400 mail, and FTAM.

Question 2

Presentation Layer (Layer 6)

Answer 2

The Presentation Layer (Layer 6) defining data formats, such as ASCII text, EBCDIC text, binary,
BCD, and JPEG. Encryption also is defined as a presentation layer service. Examples include:
JPEG, ASCII, EBCDIC, TIFF, GIF, PICT, encryption, MPEG, and MIDI.

Question 3

Session Layer (Layer 5)

Answer 3

The Session Layer (Layer 5) defines how to start, control, and end communication 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. Examples include: RPC, SQL, NFS, NetBios
names, AppleTalk ASP, and DECnet SCP

Question 4

Transport Layer (Layer 4)

Answer 4

The Transport Layer (Layer 4) defines several functions, including the choice of protocols. The
most important Layer 4 functions are error recovery and flow control. The transport layer may
provide for retransmission, i.e., error recovery, and may use flow control to prevent unnecessary
congestion by attempting to send data at a rate that the network can accommodate, or it might not,
depending on the choice of protocols. Multiplexing of incoming data for different flows to
applications on the same host is also performed. Reordering of the incoming data stream when
packets arrive out of order is included. Examples include: TCP, UDP, and SPX.

Question 5

Network Layer (Layer 3)

Answer 5

The Network Layer (Layer 3) defines end-to-end delivery of packets and defines logical
addressing to accomplish this. It also defines how routing works and how routes are learned; and
how to fragment a packet into smaller packets to accommodate media with smaller maximum
transmission unit sizes. Examples include: IP, IPX, AppleTalk DDP, and ICMP. Both IP and IPX
define logical addressing, routing, the learning of routing information, and end-to-end delivery
rules. The IP and IPX protocols most closely match the OSI network layer (Layer 3) and are called
Layer 3 protocols because their functions most closely match OSI’s Layer 3.

Question 6

Data Link Layer (Layer 2)

Answer 6

The Data Link Layer (Layer 2) is 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 use. Examples include: IEEE 802.3/802.2, HDLC, Frame
Relay, PPP, FDDI, ATM, and IEEE 802.5/802.2.

Question 7

QUESTION NO: 52
Which layer of the OSI reference model uses the hardware address of a device to ensure
message delivery to the proper host on a LAN?
A. physical
B. data link
C. network
D. transport

Answer 7

Answer: B

Question 8

QUESTION NO: 53
Which layer of the OSI reference model uses flow control, sequencing, and acknowledgments to
ensure that reliable networking occurs?
A. data link
B. network
C. transport
D. presentation
E. physical

Answer 8

Answer: C