The The Open System Interconnection Reference Model is some thing that should be completely learned and memorized.  Pretty much just read the chart a couple of times and try to learn it so that when you are asked you can have a quick response.

Group

#

Layer Name

Key Responsibilities

Data Type Handled

Scope

Common Protocols and Technologies

Lower Layers

1

Physical

Encoding and Signaling; Physical Data Transmission; Hardware Specifications; Topology and Design

Bits

Electrical or light signals sent between local devices

(Physical layers of most of the technologies listed for the data link layer)

2

Data Link

Logical Link Control; Media Access Control; Data Framing; Addressing; Error Detection and Handling; Defining Requirements of Physical Layer

Frames

Low-level data messages between local devices

IEEE 802.2 LLC, Ethernet Family; Token Ring; FDDI and CDDI; IEEE 802.11 (WLAN, Wi-Fi); HomePNA; HomeRF; ATM; SLIP and PPP

3

Network

Logical Addressing; Routing; Datagram Encapsulation; Fragmentation and Reassembly; Error Handling and Diagnostics

Datagrams / Packets

Messages between local or remote devices

IP; IPv6; IP NAT; IPsec; Mobile IP; ICMP; IPX; DLC; PLP; Routing protocols such as RIP and BGP

4

Transport

Process-Level Addressing; Multiplexing/Demultiplexing; Connections; Segmentation and Reassembly;
Acknowledgments and Retransmissions;
Flow Control

Datagrams / Segments

Communication between software processes

TCP and UDP; SPX; NetBEUI/NBF

Upper Layers

5

Session

Session Establishment, Management and Termination

Sessions

Sessions between local or remote devices

NetBIOS, Sockets, Named Pipes, RPC

6

Presentation

Data Translation; Compression and Encryption

Encoded User Data

Application data representations

SSL; Shells and Redirectors; MIME

7

Application

User Application Services

User Data

Application data

DNS; NFS; BOOTP; DHCP; SNMP; RMON; FTP; TFTP; SMTP; POP3; IMAP; NNTP; HTTP; Telnet