Transmission Media in Computer Network ,Categories of Transmission Media, UnGuided Media in Transmission Media, Types of Propagation Modes, Packet switching, Types of waves
PART 2
TRANSMISSION MEDIA :
Unguided Media: This refers to free space, such as air and vacuum.
UNGUIDED MEDIA: WIRELESS
Unguided media refers to communication channels that use wireless signals to transmit data. These
signals travel through the air without any physical conductor. Common types of wireless
transmission include radio waves, microwaves, and infrared.
Wireless communication involves transmitting electromagnetic waves without using physical
conductors like cables or wires. Instead, signals are broadcast through free space, making them
accessible to any device equipped to receive them.
Electromagnetic Spectrum for Wireless Communication :
Wireless communication utilizes a portion of the electromagnetic spectrum, ranging from 3 kHz
to 900 THz, as shown in Figure. This spectrum includes a variety of frequencies that allow
different methods of signal transmission.
 |
| Fig. Electromagnetic spectrum for wireless communication |
Types of Propagation :
Unguided signals, such as radio waves, can travel from the source to the destination in three main
ways, illustrated in Figure below
1. Ground Propagation:
- In this method, low-frequency radio waves travel close to the Earth’s surface, following the curvature of the planet.
- These signals radiate from the transmitting antenna in all directions, and the distance they cover depends on the power of the signal—the higher the power, the farther the signal can travel.
2. Sky Propagation:
- Higher-frequency radio waves are transmitted upward into the ionosphere, where they are reflected back to Earth.
- This method enables long-distance communication with relatively low power.
3. Line-of-Sight Propagation:
- In this method, very high-frequency signals are transmitted directly between antennas in a straight line.
- The antennas must be properly aligned and either tall enough or close enough to avoid being affected by the curvature of the Earth. This method is more complex because radio waves can’t be perfectly focused.
Frequency Bands :
The electromagnetic spectrum for wireless communication is divided into eight different ranges or
"bands." These bands are classified based on frequency and are regulated by government
authorities. The ranges extend from Very Low Frequency (VLF) to Extremely High Frequency
(EHF). Each band has specific propagation characteristics and applications,
Radio Waves :
Frequency Range: Typically, from 3 kHz to 1 GHz.
Characteristics of Radio Waves:
- Omnidirectional Propagation: Radio waves are mostly omnidirectional, meaning they spread out in all directions from the transmitting antenna. The sending and receiving antennas don’t need to be aligned for successful communication, as any receiving antenna in range can pick up the signal. However, this characteristic also leads to a disadvantage: interference. Multiple antennas transmitting on the same frequency or band can interfere with one another.
- Long-Distance Travel: Radio waves, especially those that propagate in the sky mode, can travel long distances, making them ideal for applications like AM radio broadcasting.
- Penetration of Walls: Radio waves, particularly those with low and medium frequencies, can penetrate walls. This is useful because devices like AM radios can receive signals indoors. However, it can also be a disadvantage, as signals cannot be restricted to only the inside or outside of a building, leading to potential signal leakage.
Limitations:
- The radio wave band is relatively narrow, just under 1 GHz. When divided into sub bands, the limited width of these sub bands results in low data rates for digital communications.
- Most of the radio wave spectrum is regulated by government authorities, like the FCC in the United States and Department of Telecommunications (DoT) & Telecom Regulatory Authority of India (TRAI) in India. Any use of this spectrum requires official permission.
- Licensed Frequency Bands: FM band is licensed between 88-108 MHz for radio broadcasting and Unlicensed Frequency Bands: Wi-Fi: 2.4 GHz, 5 GHz band.
Omnidirectional Antenna:
Radio waves typically utilize omnidirectional antennas, which transmit signals in all directions.
These antennas come in various types, depending on factors like wavelength, signal strength,
and the intended purpose of transmission.
Applications of Radio Waves:
Thanks to their omnidirectional nature, radio waves are widely used for multicasting, where one
sender transmits to many receivers. Common examples include:
- AM and FM radio
- Television broadcasting
- Maritime radio
- Cordless phones
- Paging systems
 |
| Fig. Omnidirectional antenna |
Microwaves:
- Unlike circuit-switched networks, packet switching does not reserve any specific resources like bandwidth or processing time for the packets.
- Resources are allocated only when needed, and packets are processed on a first- come, first-served basis.
- Since there is no dedicated path or reserved resources, packets might experience delays. For instance, if a switch is busy processing other packets, newly arrived packets must wait their turn, which can increase transmission time.
Frequency Range: From 1 GHz to 300 GHz.
Characteristics:
Microwaves require line-of-sight transmission, meaning the transmitter and receiver must be directly visible to each other. They are less effective in penetrating obstacles like buildings.
 |
| Fig. Unidirectional antennas |
Applications:
Satellite communications, radar systems, and microwave ovens. In networking, microwaves are used for point-to-point communication links and cellular networks.
Infrared :
Frequency Range: From 300 GHz to 400 THz.
Characteristics:
Infrared signals are used for short-range communication and do not penetrate walls, making them suitable for indoor use. They are highly directional and require line-of-sight transmission.
Applications:
Remote controls, short-range data transmission (such as between computers and peripherals), and infrared sensors for detecting heat in security systems or medical devices.
PACKET SWITCHING :
In data communication, when a message needs to be sent from one end system to another through a packet-switched network, it must be divided into smaller units called packets. These packets can be of either fixed or variable sizes, depending on the network and the protocol being used.
reKey Features of Packet Switching:
1. No Resource Allocation:
2. Possible Delays:
Types of Packet-Switched Networks:
- Datagram Networks: In these networks, each packet is treated independently, and it may take different routes to reach the destination.
- Virtual Circuit Networks: These networks establish a pre-determined path before any data packets are sent, ensuring all packets follow the same route.
Packet switching is an efficient way to transfer data, especially in systems where multiple users
need to share the same network resources.
Datagram Networks :
In a datagram network, each packet is handled independently, even if it's part of a larger
transmission. The network treats each packet as if it stands alone. These individual packets are
known as datagrams.
Key Features of Datagram Networks:
Packet Independence: Each packet in a datagram network can take a different path to its
destination, and the network doesn't maintain any connection state between sender and
receiver.
Routing: Packet routing is typically done at the network layer, where packets are
forwarded based on their destination address. The devices that manage packet routing are
called routers.
No Fixed Path: Since packets may travel along different routes, they might reach their
destination out of order or with varying delays. Some packets could even be dropped if the
network runs out of resources.
Connectionless: A datagram network is often referred to as a connectionless network
because it doesn’t require a setup phase (like circuit-switched networks). No information
about the connection is saved, and each packet is routed independently.
How Datagram Networks Work:
 |
| Fig. A datagram network with four switches (routers) |
In Figure four packets are sent from station A to station X using the datagram approach. Here,
switches are called routers, and they are depicted with a different symbol. Although these four
packets belong to the same message, they might take different paths to reach their destination due
to varying link capacities and network congestion. This can lead to packets arriving out of order,
with differing delays, or even being lost or dropped due to insufficient resources. Upper-layer
protocols typically handle the reordering and retransmission of lost packets before delivering them
to the application.
Routing Table :
In a datagram network, each switch uses a routing table based on destination addresses to forward
packets. These tables are dynamic and updated regularly.
 |
| Fig. Routing table in a datagram network |
The routing table records destination addresses and the corresponding output ports. This differs
from circuit-switched networks, where entries are created during the setup phase and removed
during teardown.
Destination :
Address
Every packet in a datagram network has a header containing a destination address. Upon receiving
a packet, the switch checks this address and uses the routing table to determine the appropriate
forwarding port. This destination address remains unchanged throughout the packet's journey.
Delay :
Despite their efficiency, datagram networks can experience higher delays compared to virtual-
circuit networks. Although there are no setup or teardown phases, each packet may encounter
waiting times at switches. Additionally, since packets from the same message may travel through
different routes, delays are not uniform.
Figure, illustrates the delay for a packet traveling
through two switches, including transmission times (3T), propagation delays (3τ), and waiting
times (w1 + w2). The total delay is given by:
T𝑜𝑡𝑎𝑙 𝑑𝑒𝑙𝑎𝑦 = 3𝑇 + 3𝑟 + 𝑤1 + 𝑤2
- Transmission Time: The time to send a packet from one point to another
- Propagation Delay: The time it takes for the signal to travel through the medium.
- Waiting Time: Time spent at routers before being forwarded.
 |
| Fig. Delay in a datagram network |
Advantages:
Efficiency: Datagram networks can be more efficient than circuit-switched networks.
Resources like bandwidth are allocated only when packets are being transmitted, allowing
for better utilization of network resources.
Tags:
Computer network
