The Physical layer

Ok so in this post i am going to be talking about the physical layer and the various configurations and types of medium used at this layer. I'll also be outlining the limitations, advantages and disadvantages of each of the main types of physical medium in use today.

So as I have said in my previous post the physical layer is the cabling and or wireless signals that carries the data from source to destination. Within this layer we have two broad categories of media which are Guided Media and Unguided Media. Guided media is where the message or data flows through a physical medium such as wiring or cables. On the other hand then when the message or data is broadcast through a medium, such as over the air or through a vacuum. we call this Unguided Media. The key issues in the selection of one medium over another are data rate (bps) and distance.

Guided Media:

• TWP (Twisted Wire Pair)
• Co-Axial
• Optical Fiber

TWP - Twisted Wire Pair:

Twisted wire pair consists of two copper wires(or other conductors) twisted together and is the most common medium in use today. 

TWP is widely used in the telephone network and local area networks in various specifications.

There are two main types of TWP available which are Unshielded Twisted Pair and Shielded Twisted Pair although the shielded variety has become commonplace due to its metal braid or sheathing which increases its performance. TWP is available in many different specifications or categories(Cat 3, Cat 5, Cat 5E etc.) to cater for different uses and network requirements.

The advantages of TWP:

• Cheap
• Fast over short range
• Easy to work with 

Disadvantages of TWP:

• Low data rates
• Susceptible to interference
• Short range between amplifiers

Co-Axail:

Co-Axial cabling is more expensive than TWP but less susceptible to interference and can carry much more data. Mainly used for the connection of video equipment due to the high data transfer rates the most common use today is the connection of many cable tv providers set-top boxes to the antennae or satellite dishes.

Advantages of Co-Axial:

• Supports multiple channels
• Greater data rates
• Lower error rates
• Greater range between amplifiers

Disadvantages of Co-Axial:

• More expensive than TWP
• Harder to work with

Fibre Optics:

An optical fibre cable contains one or more optical fibres that are used to transmit converted data in the form of light pulses. Fibre Optics allow for much greater point to point transmission speeds than any of the other media available today with data rates of up to 1 Pbs(Petabits per second) with is the equivalent of 125,000,000 Mbps. 

The core itself transfers data at the speed of light in a vacuum but due to the effect of what is called total internal refraction (shown in diagram below) over extended distances this continuous refraction can slow the signal down. This calls for the use of repeaters or extenders like in TWP and Co-Axial however with a far greater distance between them.

Advantages of Fibre Optics:

• Very fast data rates
• Greatly increased bandwidth and capacity
• Lower signal loss
• Immune to electromagnetic and radio frequency interference
• Overall system economy

Disadvantages of Fibre Optics:

• Fibre optic components are very expensive
• Very specialized and expensive to repair

Unguided Media:

• Radio-waves
• Microwave 

Both of these methods of transmission provide a means of transmitting data over a network without the use of physical means to define the path it takes. Some examples of unguided media include the cellular 3G/4G networks, Wi-Fi and Wi-Max.

All of us are very familiar with Wi-Fi at this stage as this particular unguided medium has become very widespread as it allows the connection of many different users on varying devices to connect to a network simultaneously and wirelessly.

Advantages of Unguided Media:

• Ease of access
• Multiple users and devices
• Wireless connection

Disadvantages of Unguided Media:

• Congestion (more users one connection point)
• Signal absorption, reflection and obstruction.