Computing | Networking 
Layer One of OSI Model
Physical Layer

In the last article I explained in depth about the OSI and TCP/IP Models. Now in this layered protocol structure, we have to see what each layer does. Basically in the OSI Model, we have two layers:

1) Upper Layer – Comprising of Sessions, Presentation and Applications Layers.

This layer basically deals with the application issues (basically software). Application Layer is the closest to the end user. For eg: Applications like Microsoft Word, Powerpoint on which we work can be said to be closer to the end user like us.

2) Lower Layer – Comprising of the Physical, Data Link, Network and Transport Layers.

This layer deals with the data transport issues. The Physical and Data Link layers basically deal with hardware and software. Other Lower Layers are generally implemented using software.

Lets have a look at some of the Lower Layer Protocols in detail :

1) Physical Layer:

The Physical Layer defines the Mechanical, Electrical, Procedural and Functional specifications for activating, maintaining and deactivating the physical link between communication network systems.  These specifications define characteristics such as:

• Voltage levels and timings of voltage changes.
• Physical data rates and maximum transmission distances
• Physical Connectors.

Devices in this layer copy frames or data from one network to another, depending upon the device in operation like a Hub or a Repeater. This layer basically deals with the network components that you can touch. Any device or protocol that operates at the Physical Layer deals with the physical components of the network. It deals with medium like Copper Wires, UTP cables, Coaxial cables or Fiber Optic Cables. When a protocol at the physical layer receives information from the upper layers, it translates all the data into signals that can be transmitted on a transmission medium. This is called Signal Encoding. That is the ones and zeros of data are converted into electrical on –off’s by the physical layer protocols.

Physical Layer also specifies how much of the media will be used during the data transmission.

1. Base band Signaling: Technology in which a network uses all available signal frequencies or the entire bandwidth. Eg: Most LAN technologies like Ethernet.

2. Broad band Signaling: Technology in which a network uses only one frequency or a part of the entire bandwidth i.e multiple signals can be transmitted over a media simultaneously.Eg: TV signals, where you have various channels like CNBC, ZEE, BBC, each on a different frequency and hence each occupies a part of the bandwidth.

Physical layer also specifies the topology or layout of the transmission media. There are four possible kinds of topologies:

• Bus
• Star
• Ring
• Mesh

Devices used in the Physical Layer are

• Network Interface Cards (NIC)
• Transceivers
• Repeaters
• Hubs
• Multi Station Access Units (MAU’s)

Implementations in the Physical Layer :

• LAN specifications and
• WAN specifications

Remember that the physical layer is the same for TCP/IP Model so all the above applies to TCP/IP Model too. I will cover all the subtopics like the devices, topologies, implementations and transmission media in the forthcoming articles. In the next article, lets have a look at the Data Link Layer and also some part of the Network Layer.   

– Deepak Chandrasekaran
September 3, 2002

Articles on Computer Networking

Introduction  
Computer Network - Basics  
OSI Model and TCP/IP Model  
Layer One of OSI Model - Physical Layer 

Are you Safe on Network?    
Bluetooth   
GPS : Tension Free Driving  
Robots in Human Life   
Router : An Introduction 
Secure Socket Layer : Online Data Security  
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VoIP : Technology to Cut Phone Bills 
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