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Physical Layer (Layer 1)
(Page 1 of 2)
The lowest layer of the OSI Reference
Model is layer 1, the physical layer; it is commonly abbreviated
PHY. The physical layer is special compared to the other
layers of the model, because it is the only one where data is physically
moved across the network interface. All of the other layers perform
useful functions to create messages to be sent, but they must all be
transmitted down the protocol stack to the physical layer, where they
are actually sent out over the network.
Note: The physical layer is also special in that it is the only layer that really does not apply specifically to TCP/IP. Even in studying TCP/IP, however, it is still important to understand its significance and role in relation to the other layers where TCP/IP protocols reside.
Understanding the Role of the Physical Layer
The name physical layer
can be a bit problematic. Because of that name, and because of what
I just said about the physical layer actually transmitting data, many
people who study networking get the impression that the physical layer
is only about actual network hardware. Some people may say the physical
layer is the network interface cards and cables. This is
not actually the case, however. The physical layer defines a number
of network functions, not just hardware cables and cards.
A related notion is that all
network hardware belongs to the physical layer. Again, this isn't
strictly accurate. All hardware must have some relation
to the physical layer in order to send data over the network, but hardware
devices generally implement multiple layers of the OSI model, including
the physical layer but also others. For example, an Ethernet network
interface card performs functions at both the physical layer and the
data link layer.
Physical Layer Functions
The following are the main responsibilities
of the physical layer in the OSI Reference Model:
- Definition of Hardware Specifications:
The details of operation of cables, connectors, wireless radio transceivers,
network interface cards and other hardware devices are generally a function
of the physical layer (although also partially the data link layer;
- Encoding and Signaling: The physical layer
is responsible for various encoding and signaling functions that transform
the data from bits that reside within a computer or other device into
signals that can be sent over the network.
- Data Transmission and Reception: After
encoding the data appropriately, the physical layer actually transmits
the data, and of course, receives it. Note that this applies equally
to wired and wireless networks, even if there is no tangible cable in
a wireless network!
- Topology and Physical Network Design:
The physical layer is also considered the domain of many hardware-related
network design issues, such as LAN and WAN topology.
In general, then, physical layer
technologies are ones that are at the very lowest level and deal with
the actual ones and zeroes that are sent over the network. For example,
when considering network interconnection devices, the simplest ones
operate at the physical layer: repeaters, conventional hubs and transceivers.
These devices have absolutely no knowledge of the contents of a message.
They just take input bits and send them as output. Devices like switches
and routers operate at higher layers and look at the data they receive
as being more than voltage or light pulses that represent one or zero.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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