Networking Guide : The OSI Reference Model
Even though most computer network technologies do not follow strictly to the
OSI Model in that they combine several OSI layers functions in one
protocol, the OSI Model is still used as a
reference and a guideline in network design. Understanding the OSI Model will help you
understand how a network works. The OSI
Model protocol stack is explained in the following table:
| Layer |
Layer Name |
Functions |
Examples |
| Layer 7 |
Application Layer |
application support |
HTTP,
FTP,
Telnet,
SMTP,
SNMP,
POP3,
IMAP4 |
| Layer 6 |
Presentation Layer |
data format conversion, data compression, and encryption |
|
| Layer 5 |
Session Layer |
user identification; establishing, maintaining, and terminating a session |
SIP |
| Layer 4 |
Transport Layer |
end-to-end transport |
TCP,
UDP,
RTP,
RTCP |
| Layer 3 |
Network Layer |
addressing, routing |
IP,
IPSec,
IPX,
NetBEUI,
AppleTalk,
ICMP |
| Layer 2 |
Data Link Layer |
medium access control, error detection, retransmission |
Ethernet,
Wi-Fi,
HomePNA,
HomePlug,
PPP,
PPTP,
L2TP,
ATM,
Frame Relay,
Token Ring,
FDDI |
| Layer 1 |
Physical Layer |
electrical/optical signaling, cabling, connector pin assignment |
RF,
UTP,
STP, coax, fiber optic, connectors, signaling, voltages |
Due to its complex functions, the Data Link Layer is divided into two sublayers, that is
Media Access Control (MAC) sublayer and
Logical Link Control (LLC)
sublayer. MAC sublayer is the lower part, closer to the Physical
Layer. MAC sublayer controls access to the physical medium. LLC
sublayer is the upper part that interfaces with the Network Layer.
Session Layer, Presentation Layer, and Application Layer are often
referred to as Upper Layers. These layers basically handle user
connection and data formatting. In most network technologies,
such as TCP/IP, the differences between the three layers are blurred
and their functions are often handled by one protocol.
Physical Layer, Data Link Layer, Network Layer, and Transport
Layer are referred to as Lower Layers. The lower layers generally
concern with how data is transported across the network.
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