Information Technology Reference
In-Depth Information
OSI Layer
Name
Layer
Number
7
Application
6
Presentation
5
Session
4
Transport
3
Network
2
Data Link
1
Physical
Figure C-1
Seven-Layer OSI Model
Layered implementations of internetworking technologies do not necessarily map directly
to the OSI model. For example, the TCP/IP architecture model describes only four layers,
with the upper layer mapping to the three upper layers of the OSI model (application,
presentation, and session). The development of IP predates the OSI model. For a more
thorough discussion of the TCP/IP model, see Chapter 8, “Internet Protocol Version 4.”
The following sections describe and provide sample protocols for each OSI layer.
Physical Layer (OSI Layer 1)
The physical layer describes the transportation of raw bits over physical media. It defines
signaling specifications and media types and interfaces. It also describes voltage levels,
physical data rates, and maximum transmission distances. In summary, it deals with the
electrical, mechanical, functional, and procedural specifications for links between net-
worked systems.
Examples of physical layer specifications are
EIA/TIA-232 (Electronic Industries Association/ Telecommunications Industry
Association)
■
EIA/TIA-449
■
V. 3 5
■
IEEE 802 LAN and metropolitan-area network (MAN) standards
■
Physical layer (PHY) groups Synchronous Optical Network/Synchronous Digital Hi-
erarchy (SONET/SDH)
■
Maximum cable distances of the Ethernet standards
■
