Transmission Media

Definition

Transmission media are the means by which a communication signal is carried from one system to another. They form the physical path between the sender and the receiver in a communication system. Broadly, transmission media are classified into two major categories: Wired (Guided) Media and Wireless (Unguided) Media.

Guided Media vs. Unguided Media

Types of Guided Media

Guided media (also called wired or bounded transmission media) use physical links to direct and confine signals. There are three major types:

1. Twisted Pair Cable

A twisted pair cable consists of two separately insulated conductor wires wound about each other. Generally, several such pairs are bundled together in a protective sheath. They are the most widely used transmission media.

Twisting the wires together helps reduce electromagnetic interference (EMI) and crosstalk from neighboring pairs.

Twisted pair cable is further of two types:

a) Unshielded Twisted Pair (UTP)

• Has no additional shielding around the wire pairs.
• Most common and cost-effective.
• Easy to install and widely used in telephone networks and LAN (Ethernet).
• More susceptible to noise and interference compared to STP.
• Speed: Up to 100 Mbps (or higher with Cat6/Cat7 standards).

b) Shielded Twisted Pair (STP)

• Has a metallic foil or braid shield surrounding each pair of insulated conductors.
• The shield reduces electromagnetic interference significantly.
• More expensive and harder to install than UTP.
• Used in environments with high interference (e.g., industrial settings).
• Provides better performance at higher data rates over longer distances.

Advantages of Twisted Pair:

• Inexpensive and easy to install.
• Flexible and lightweight.

Disadvantages:

• Limited bandwidth.
• Susceptible to noise over long distances.

2. Coaxial Cable

A coaxial cable consists of a central solid conductor surrounded by an insulating layer, which is further surrounded by a metallic mesh or foil (outer conductor/shield), and finally enclosed in a plastic outer jacket.

The two conductors share the same geometric axis, hence the name "coaxial."

Types:

• Baseband coaxial cable: Used for digital transmission (e.g., Ethernet).
• Broadband coaxial cable: Used for analog transmission (e.g., cable TV).

Characteristics:

• Higher bandwidth than twisted pair cable.
• Better shielding against electromagnetic interference.
• Can carry signals over longer distances than twisted pair.
• Frequency range: Up to 1 GHz.

Applications:

• Cable television (CATV) networks.
• Earlier Ethernet LAN installations (10Base2, 10Base5).
• Long-distance telephone transmission.

Advantages:

• Higher bandwidth and better noise immunity than twisted pair.
• Can support both analog and digital signals.

Disadvantages:

• More expensive than twisted pair.
• Bulkier and less flexible.

3. Fiber Optic Cable

A fiber optic cable transmits data as pulses of light rather than electrical signals. It consists of:

• A core made of glass or plastic through which light travels.
• A cladding surrounding the core that reflects light back into the core (using total internal reflection).
• A protective outer jacket.

Working Principle: Light signals are transmitted through the core by the principle of total internal reflection, where light bounces along the length of the fiber without escaping.

Types:

• Single-mode fiber: Has a very narrow core; allows only one mode (path) of light. Used for long-distance, high-speed transmission.
• Multi-mode fiber: Has a wider core; allows multiple modes of light. Used for shorter distances.

Characteristics:

• Extremely high bandwidth (in the range of Gbps to Tbps).
• Immune to electromagnetic interference.
• Very low signal attenuation over long distances.
• Highly secure (signals cannot be easily tapped).
• Lightweight and thin.

Applications:

• Backbone of the internet and long-distance telecommunications.
• Submarine cable systems.
• High-speed LAN and data center connections.

Advantages:

• Very high data rate and bandwidth.
• No electromagnetic interference.
• Secure and reliable.

Disadvantages:

• More expensive than copper cables.
• Difficult to install and splice.
• Fragile compared to metal cables.

Summary Table of Guided Media

Conclusion: Guided media provide a physical path for signal transmission and are preferred where high speed, security, and reliability are required over fixed, shorter distances. Among the three types, fiber optic cable offers the best performance but at a higher cost, while twisted pair remains the most economical and widely deployed option.

Flow Control and ARQ Protocols

1. What is Flow Control? (2 marks)

Flow control is a set of procedures that tells the sender how much data or how many frames it can transmit before overwhelming the receiver. It is a technique that allows two stations working and processing at different speeds to communicate with each other effectively.

: "Flow control is basically a technique that gives permission to two stations that are working and processing at different speeds to communicate with another. It is a set of procedures that explains how much data or frames it can transfer before data overwhelms the receiver."

Why it is needed:

• The sender may be faster than the receiver
• Without flow control, the receiver's buffer may overflow
• Data corruption or loss can occur if data rate is not matched on both sides

2. Stop-and-Wait ARQ (4 marks)

Definition

Stop-and-Wait ARQ (Automatic Repeat reQuest) is an error control and flow control protocol in which:

• The sender transmits one frame at a time
• The sender stops and waits for an acknowledgement (ACK) from the receiver before sending the next frame
• If the ACK is not received within a timeout period, the sender retransmits the same frame

Working Mechanism

Cases in Stop-and-Wait ARQ

Case 1: Normal Operation (No Error)

Sender                          Receiver
  |------- Frame 0 ------------>|
  |<------- ACK 1 --------------|
  |------- Frame 1 ------------>|
  |<------- ACK 0 --------------|

Case 2: Lost Frame

Sender                          Receiver
  |------- Frame 0 ---X         |   (Frame lost)
  |  [Timer expires]            |
  |------- Frame 0 ------------>|   (Retransmit)
  |<------- ACK 1 --------------|

Case 3: Lost ACK

Sender                          Receiver
  |------- Frame 0 ------------>|
  |         ACK 1 ---X          |   (ACK lost)
  |  [Timer expires]            |
  |------- Frame 0 ------------>|   (Retransmit)
  |<------- ACK 1 --------------|

Example

Suppose a sender wants to send 4 frames: F0, F1, F2, F3.

Sender          Channel         Receiver
  |----F0-------->              |
  |               ----F0------->|
  |               <---ACK1------|
  |<----ACK1------|              |
  |----F1-------->              |
  |               ----F1------->|
  |               <---ACK0------|
  |<----ACK0------|              |
  ... and so on

• Sender sends F0, waits for ACK
• Receiver gets F0, sends ACK1 (ready for frame 1)
• Sender gets ACK1, sends F1
• This continues until all frames are sent

Limitation

: "Stop-and-Wait ARQ is inefficient compared to other ARQs, because the time between packets is twice the transit time. The throughput on the channel is a fraction of what it could be."

3. Go-Back-N ARQ (3 marks)

Definition

Go-Back-N ARQ is a sliding window protocol where:

• The sender can transmit multiple frames (up to a window size N) without waiting for individual ACKs
• If an error occurs in frame i, the sender must go back and retransmit frame i and all subsequent frames (i, i+1, i+2, ...) even if those subsequent frames were received correctly

Working Mechanism

Sender                              Receiver
  |---F0--->                        |
  |---F1--->                        |
  |---F2--->  (Error in F2)         |
  |---F3--->                        |
  |<---ACK0, ACK1, NAK2-------------|
  |---F2--->  (Retransmit F2)       |
  |---F3--->  (Retransmit F3)       |
  |<---ACK2, ACK3-------------------|

4. Difference: Stop-and-Wait ARQ vs Go-Back-N ARQ (1 mark)

: "To solve this problem (inefficiency of Stop-and-Wait), one can send more than one packet at a time. This is what is done in Go-Back-N ARQ and the Selective Repeat mechanisms."

Summary

• Flow control ensures the sender does not overwhelm the receiver
• Stop-and-Wait ARQ is simple but inefficient -- only one frame is in transit at a time
• Go-Back-N ARQ improves efficiency by allowing a window of N frames to be sent, but on error, all frames from the error point onward must be retransmitted

Routing Table: Static vs Dynamic Routing

Routing Table

A routing table is a data structure stored in a router (or networked device) that lists the routes to particular network destinations. Routers use this table to make decisions on how to forward data packets based on their IP addresses. The routing table contains information such as destination network, next hop address, and interface to use. Routers normally connect LANs and WANs together and have a routing table based on which they make decisions on routing the data packets.

Static Routing Table

A static routing table is one in which routes are manually added and configured by a network administrator. The routes remain fixed unless manually updated or reconfigured.

Key characteristics:

• Routes are configured manually by the administrator
• Used when there are very few devices to configure
• Suitable when routes are known and are unlikely to change
• Does not handle network failures well, because any lost connectivity must be manually repaired or reconfigured
• No routing protocol is needed

Dynamic Routing Table

A dynamic routing table is one in which routes are automatically adjusted according to the current state of the network. It uses routing protocols to discover network destinations and the best routes to reach them.

Key characteristics:

• Routes are updated automatically based on current network conditions
• Uses routing protocols such as RIP, OSPF, and BGP to discover routes
• If one route goes down, automatic adjustment is made to find an alternate route
• Suitable for large and complex networks
• Requires more processing and bandwidth due to protocol overhead

Difference Between Static and Dynamic Routing Table

Summary: Static routing is simple but inflexible, best for small stable networks. Dynamic routing is flexible and self-managing, best for large networks where topology may change frequently.

Approaches for Multimedia Streaming

Definition of Multimedia Streaming

Multimedia streaming is a technique for transferring audio, video, and other multimedia data over a network so that the data can be processed as a continuous and steady stream, allowing playback to begin before the entire file is downloaded.

Different Approaches for Multimedia Streaming

There are three main approaches for multimedia streaming:

1. Downloading (Progressive Download / HTTP Streaming)

• In this approach, the multimedia file is downloaded from a server to the client using standard protocols like HTTP.
• Playback can begin after a sufficient portion of the file has been buffered.
• The file is stored on the client side during or after playback.
• Simple to implement but not efficient for live content.
• Example: Downloading a video from a web server and playing it.

Limitation: The entire file must be available on the server; not suitable for live broadcasts.

2. Real-Time Streaming (True Streaming)

• In this approach, multimedia data is delivered continuously in real time from the server to the client.
• Uses specialized streaming protocols such as:
  • RTSP (Real-Time Streaming Protocol) - controls the delivery
  • RTP (Real-Time Transport Protocol) - carries the actual media data
  • RTCP (Real-Time Transport Control Protocol) - monitors quality
• The data is not stored on the client; it is played as it arrives.
• Suitable for both live and on-demand content.
• Requires a dedicated streaming server.

Advantage: Supports live streaming, better bandwidth control.

3. Adaptive Streaming (Adaptive Bitrate Streaming)

• In this approach, the multimedia content is encoded at multiple quality levels (bitrates).
• The client continuously monitors network bandwidth and buffer status.
• Based on current network conditions, the client automatically switches between different quality versions.
• Examples: MPEG-DASH, Apple HLS (HTTP Live Streaming)

Advantage: Provides the best possible quality without buffering interruptions, adapts to changing network conditions.

Comparison Table

Summary

The three approaches -- downloading, real-time streaming, and adaptive streaming -- each serve different use cases. Modern multimedia applications predominantly use adaptive streaming because it provides smooth playback across varying network conditions, making it the most practical approach for today's internet-based multimedia delivery.

Subnetting Class B Network 150.10.0.0 into 4 Subnets

Step 1: Extract Given Data

• Network: 150.10.0.0 to 150.10.255.255
• Network Address: 150.10.0.0
• Class B default mask: 255.255.0.0 (/16)
• Host bits: 16 (last two octets)
• Required subnets: 4

Step 2: Bits to Borrow

$$2^n \geq 4 \implies n = 2 \text{ bits}$$

Borrow 2 bits from the host portion (third octet).

Step 3: New Subnet Mask

New prefix = $16 + 2 = /18$

Third octet: $11000000_2 = 192$

$$\boxed{\text{New Subnet Mask} = 255.255.192.0 ;(/18)}$$

Step 4: Block Size

$$\text{Block Size} = 256 - 192 = 64$$

Subnets increment by 64 in the third octet.

Hosts per subnet: $2^{14} - 2 = 16382$ usable hosts.

Step 5: Subnet Details

Summary

• New Subnet Mask: 255.255.192.0 (/18)
• Subnet IDs: 150.10.0.0, 150.10.64.0, 150.10.128.0, 150.10.192.0
• Broadcast Addresses: 150.10.63.255, 150.10.127.255, 150.10.191.255, 150.10.255.255