The physical layer provides a service to the data

The physical layer provides a service to the data link layer, encoding the data-link frame into a
pattern of 1s and 0s (bits) for transmission on the medium (usually a wire) at Layer 1.
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1-58 Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.0 © 2007 Cisco Systems, Inc.
TCP/IP Suite
The TCP/IP suite—whose name is actually a combination of just two individual protocols,
Transmission Control Protocol (TCP) and Internet Protocol (IP)—is divided into layers, each of
which performs specific functions in the data communication process. This topic describes how
the layers of TCP/IP are organized into a stack.
© 2007 Cisco Systems, Inc. All rights reserved. ICND1 v1.0—1-14
Defines four layers
Uses different names for Layers 1
through 3
Combines Layers 5 through 7 into
single application layer
TCP/IP Stack
The TCP/IP suite was developed at approximately the same time as the OSI model. Like the
OSI model, the TCP/IP suite is a means of organizing components in an order that reflects their
functions in relation to one another. The components, or layers, of the TCP/IP stack are as
follows:
Network access layer: This layer covers the same processes as the two lower OSI layers:
— Physical layer: The physical layer defines the electrical, mechanical, procedural,
and functional specifications for activating, maintaining, and deactivating the
physical link between end systems. Characteristics such as voltage levels, timing of
voltage changes, physical data rates, maximum transmission distances, physical
connectors, and other similar attributes are defined by physical layer specifications.
— Data link layer: The data link layer defines how data is formatted for transmission
and how access to the network is controlled.
Internet layer: This layer provides routing of data from the source to the destination by
defining the packet and the addressing scheme, moving data between the data link and
transport layers, routing packets of data to remote hosts, and performing fragmentation and
reassembly of data packets.
Transport layer: The transport layer is the core of the TCP/IP architecture, providing
communication services directly to the application processes running on network hosts.
The PDF files and any printed representation for this material are the property of Cisco Systems, Inc.,
for the sole use by Cisco employees for personal study. The files or printed representations may not be
used in commercial training, and may not be distributed for purposes other than individual self-study.