What are Repeaters and Hubs?

Intermediate devices like Repeaters and Hubs are used at the physical layer to amplify or regenerate the signal over longer distances. As the distance of the telecommunication link or cable increases, the signal strength automatically keeps decreasing due to attenuation (loss of energy due to cable resistance and external noise factors). To overcome this, devices like repeaters and hubs are used between links that span long distances.

A sample Repeater and Hub

Repeater

A repeater is a physical layer device and consists of two ports/interfaces. Its main use is to amplify and regenerate signals. Whenever a repeater receives a signal through one of its ports, it repeats or sends the incoming signal onto the other port.

As a signal travels through a media, not only is its strength lost, but additional line noise gets added to it. So, if a repeater blindly repeats the incoming signal onto the other port, then it may result in the noise too getting amplified. To overcome this factor, repeaters are intelligent and they understand the line coding technique used at the physical layer. Instead of blindly repeating the signals, repeaters decode the incoming digital signal continuously.  Since it understands the line coding technique, it is able to decode each digital symbol coming on the incoming port. After decoding the incoming bit, it regenerates a fresh  signal using the line encoding strategy, thereby helping in filtering out unnecessary noise. For this reason, repeaters are protocol specific (e.g. Ethernet Repeaters, Wireless LAN repeaters etc.).

Hub:

A hub is nothing but a multi-port repeater, with more than 2 ports. Whenever a hub receives a signal on one of its ports, it copies the same signal on each of its other ports. There are both dumb hubs (that blindly copies signal including noise) as well as intelligent hubs (that understand the line encoding technique, decodes the incoming digital signals and regenerates fresh signals on other ports).

 On shared links,  repeaters and hubs cannot perform collision detection, when multple nodes transmit signals simultaneously. Also they cannot  perform  Media Access Control (MAC) functionalities like sending jam signals.

What are Guided and Unguided media?

Telecommunication links  can broadly be classied into two categories, namely, guided media (wired) and unguided media(wireless). Both media are used for short distance (LANs, MANs) and  long distance (WANs) communication.

Guided Media or Wired links:

Examples of Wired Media

As the name indicates, in guided media

• Electrical/Optical signals are passed through a solid medium (different types of cables/wires)
• As the path traversed by the signals is guided by the size, shape and length of the wire, this type of media is called guided media. Also, in guided media, the signals are confined within the wire and do not propogate outside of the wire/media.
• E.g., Copper Unshielded Twisted Pair (UTP), Copper Shielded Twisted Pair (STP), Copper Co-axial cables, Fiber Optic Cables.

Twisted Pair Copper:

•  It is the most widely deployed media type across the world, as the last mile telephone link connecting  every home with the local telephone exchange is made of twisted pair copper. These telephone lines are reused as last mile DSL access links to access the internet from home.
• They are also used in Ethernet LAN cables within homes and offices.
• They support  low to High Data Rates (in order of Giga bits)
• However, they are effective only upto a maximum distance of a few kilometres/miles, as the signal strength is lost significantly beyond this distance.
• They come in two variants, namely UTP (unshielded twisted pair) and STP (shielded twisted pair). Within each variant, there are multiple sub-variants, based on the thickness of the material (like UTP-3, UTP-5, UTP-7 etc.)
• E.g. DSL, 10/100/1000Mbps Ethernet cables

Copper Co-axial Cables

• Co-axial copper cables have an inner copper conductor and an outer copper shield, separated by a di-electric insulating material, to prevent signal losses. 
• It is primarily used in cable TV networks and as trunk lines between telecommunication equipments. 
• It serves as an internet access line from the home. 
•  It supports medium to High Data Rates
•  It has much better immunity to noise and hence signal strength is retained for longer distances than in copper twisted pair media.

Fiber Optic Cables

• Here, information is transmitted by propogation of optical signals (light) through fiber optic cables and not through electrical/electromagnetic signals. Due to this, fiber optics communication supports longer distances as there is no electrical interference.
• As the name indicates, fiber optic cables are made of very thin strands of glass (silica).
• As they support very high data rates, fiber optic lines are used as WAN backbone and trunk lines between data exchange equipments.
• They are also used for accessing internet from home through FTTH (Fiber-To-The-Home) lines.
• Additionally, they are used even for LAN environment with different LAN technologies like Fast Ethernet, Gigabit Ethernet etc. using optical links at the physical layer.
•  
  OC-48, OC-192, FTTC, HFC are examples of Fiber Optical links.

Unguided Wireless Media:

Here information is transmitted by sending electromagnetic signals through free space and hence the name unguided media, as the signals are not guided in any specific direction or inside any specific medium.

All unguided media transmission are classified as wireless transmission.

Wireless transmission can be used as the medium in both LAN and WAN environments, as illustrated in the diagrams below:

Two laptops communicating within a LAN using a wireless Access Points

Two laptops communicating via. a long distance WAN using a WiMax Wireless transmission network

Different forms of wireless communication used in the internet vary mainly based on the following attributes:

• Distance separating the end stations
• Frequency spectrum used by the electromagnetic signals
• Line Encoding technique used

Based on these attributes, a wide variety of wireless PHYs and different types of antennaes are used in wireless communication.
The diagram given below illustrates different types of antennaes typically used in wireless communication

Different Types of Antennaes Used in wireless communication

As illustrated in the diagram, antennaes can be of many sizes and shapes. Some of them are point to point antennaes while others are omni-directional antennaes. Even satellites act as giant antenaes in the sky, by receiving and transmitting signals generated from the earth.

Wi-Fi, Wi-Max. 3G are example wireless networks used for internet communication