Physical Layer

Physical layer dose two things: it sends bits and receives bits. The physical layer communicates with directly with the various types of actual communication media. Different kinds of media represent these bits values in different ways. Some use audio tones, while others employ state transmissions-changes in voltage from high and low to high.

The physical layer specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, procedure, and functional requirements for activating, maintaining, and deactivating a physical link between end systems. Physical layers describe the electrical or optical signals used for communication.

The electrical or optical signals characteristics determine:

-What physical medium options can be used.

–How many volts/db should be used to represent a given signal state, using a given  physical medium.

The mechanical characteristics associated determine:

–Connector type

–Cable type & size

–Cable Length

Mechanical characteristics explain about cables and connectors used for communications.

It also describes about:

–Media  :Guided (bounded) or unguided(unbounded).


Atlenuation: weakening of signal while traveling from source to destination.


Distortion: change in the shape of signals.


Bandwidth: maximum capacity of channels.

Throughput: How fast data can transmits.

latency: Delay of signals reaching destination.


Coaxial cables: It is of two types: Thicknet and Thinnet. Thicknet cable runs up to 500m whereas Thinnet cable runs up to 185m. Thicknet cable use vamp wiretap.

Connectors and Thinnet cables use BNC Connectors.

Twisted pair:  It is also of two types: Shielded and Unshielded Shielded twisted pair has extra shield in its outer cover whereas unshielded don’t have any outer cover.

Unshielded twisted pair has many types:

o   cat 1: used in telephone and has one pair of wires.

o   cat 2: used in telephone and has two pair wires.

o   cat 3:used in telephone and has 4 pairs wire.

o   cat 4: used in networks and has 4 pairs of wires. It uses token ring technology and goes up to 16 mbps speed.

o   cat 5: used in networks and has 4 pairs. it is used in Ethernet technology and goes upto 100mbps speed.

o   cat 6:It goes upto 10gbps speed.

Connectors used by twisted pair are: RJ11(2-pins), RJ12(4-pins), RJ45(8-pins).

Fiber optics: In fiber optics light signals are carried from one end to another.

In this we have two types:

Single mode fiber: in this only one light signal is passed through source to destination. Here laser is use because light signals has to travel a long distance.

o  Multimode fiber: It uses multiple light signals to pass through source to destination. It uses LED.

Connectors used here is SC (subscriber channel) and ST (straight tip

Procedural characteristics:

When the signal is send what could be the outcome is described in this.

Action to reaction of a pair is explained by procedural characteristics.

Bits to signal transformation: Conversion of bits to signal and signal to bits is carried out on this functions.

Bit synchronization: It talks about how to synchronize the clocks between sender and receiver to reorganize the bits.

Bit rate control: It talks about the maximum rate of bits passes between source to destination.

Transmission types: This is of two types:

Baseband: It is used for digital signals and uses only one signal.

Broadband: One channel is divided into different channels. Multiple signal is transferred.

Transmission mode: This is of two types:

Parallel: Several bytes will be transferred from one end to another. It will calculate bytes/sec.

Serial: One wire can carry one bit at a time from one device to another.



CCNA ,Routing and switching guide Book.

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