Packet Switching & Wireless Networks: Techniques, Protocols & Real-World Use

TAKEAWAYS:

• Packet switching divides data into packets for efficient routing, unlike circuit switching’s dedicated paths.
• ALOHA protocols (pure/slotted) manage collisions in wireless networks, with throughput calculated via frame length, bandwidth, and frame arrival rate.
• Wireless networks (satellite, cellular, Wi-Fi) use different topologies (star, mesh) and access methods (FDMA, TDMA, CDMA).
• Packet switching phases include fragmentation, routing, queuing, and reassembly, each critical for reliable data delivery.
• Wireless networks face unique challenges like interference, latency, and bandwidth limitations, addressed via modulation and error control.
• Real-world applications include eSewa’s secure packet routing, Ncell’s cellular TDMA, and Daraz’s load-balanced servers.

1. Packet Switching: The Backbone of Modern Networks

Packet switching is a store-and-forward technique where data is divided into small units called packets, each containing:

• Header (source/destination, sequence number, error-checking bits).
• Payload (actual data).
• Trailer (optional error-checking bits like CRC).

How Packet Switching Works

1. Fragmentation: Large data is split into packets (e.g., a 2000-byte file → 5 packets of 400 bytes each).
2. Routing: Packets take independent paths (unlike circuit switching’s fixed path).
3. Queuing: Packets wait in buffers at routers/switches (FIFO or priority-based).
4. Reassembly: Packets are reassembled at the destination using sequence numbers.

stateDiagram-v2
    [*] --> Fragmentation: Data Split
    Fragmentation --> Routing: Independent Paths
    Routing --> Queuing: Buffers at Nodes
    Queuing --> Reassembly: Sequence Check
    Reassembly --> [*]

Packet Switching vs. Circuit Switching

2. ALOHA Protocols: Managing Collisions in Wireless Networks

ALOHA protocols are contention-based methods for wireless networks where multiple devices share the same channel. Collisions occur when two devices transmit simultaneously.

Types of ALOHA

1. Pure ALOHA

   • Devices transmit whenever ready.
   • Throughput (S) formula: $$S=G\cdot e^{-2G}$$ where $G=\frac{\text{Frame arrival rate (frames/sec)}}{\text{Channel bandwidth (bits/sec)}}\times \text{Frame length (bits)}$.

2. Slotted ALOHA

   • Time is divided into slots (equal to frame transmission time).
   • Devices transmit only at slot boundaries, reducing collisions by 50%.

Worked Example: Pure ALOHA Throughput Calculation

Problem: A pure ALOHA network transmits 200-bit frames on a 200-kbps channel. If 1000 frames/sec are generated, what is the throughput? Solution:

1. Calculate $G$: $$G=\frac{1000\text{ frames/sec}\times 200\text{ bits/frame}}{200,000\text{ bits/sec}}=1$$
2. Throughput $S$: S = 1 \cdot e^{-2 \times 1} = 0.135 \text{ (or 13.5%)} Interpretation: Only 13.5% of channel capacity is used due to collisions.

3. Wireless Network Technologies

Wireless networks use electromagnetic waves (radio, microwave, infrared) and are classified by:

• Range: Personal Area Network (PAN), Local Area Network (LAN), Wide Area Network (WAN).
• Topology: Star (Wi-Fi), Mesh (ad-hoc networks), Point-to-Point (satellite links).
• Access Method: FDMA, TDMA, CDMA (used in cellular networks).

Key Wireless Technologies

Satellite Networks

• Geostationary (GEO): Fixed orbit (36,000 km), high latency (~270 ms).
• Low Earth Orbit (LEO): Low latency (~10 ms), used by Starlink.
• Applications:
  • Nepal: NTC’s satellite links for rural internet.
  • Global: GPS (navigation), TV broadcasting.

4. Medium Access Control (MAC) in Wireless Networks

MAC protocols regulate how devices share the wireless medium:

1. CSMA (Carrier Sense Multiple Access)

   • Devices listen before transmitting (reduces collisions).
   • CSMA/CD (used in Ethernet): Stops transmitting if collision detected.
   • CSMA/CA (used in Wi-Fi): Uses ACK/NACK to confirm reception.

2. FDMA (Frequency Division)

   • Divides bandwidth into frequencies (e.g., FM radio).
   • Used in 2G cellular networks.

3. TDMA (Time Division)

   • Divides time into slots (e.g., GSM in 2G/3G).
   • Example: Ncell’s TDMA for voice calls.

4. CDMA (Code Division)

   • Uses unique codes for simultaneous transmission (e.g., 4G LTE).
   • Advantage: Higher capacity, used by NTC’s 4G.

sequenceDiagram
    participant Device1
    participant Device2
    participant Channel
    Device1->>Channel: Sense idle
    Device2->>Channel: Sense idle
    Device1->>Channel: Transmit (collision)
    Device2->>Channel: Collision detected
    Device1->>Channel: Backoff (random delay)
    Device2->>Channel: Retransmit

5. Challenges in Wireless Networks

In the Real World

1. eSewa’s Packet Switching

   • When you pay a bill via eSewa, your transaction data is split into packets, routed dynamically, and reassembled at the bank’s server. Packet switching ensures efficiency even during peak hours (e.g., Dashain).

2. Ncell’s TDMA in 2G/3G

   • Ncell divides its 2G bandwidth into time slots (TDMA) to allow multiple calls simultaneously. If you’re on a call while someone else texts, TDMA ensures both work without interference.

3. Daraz’s Load Balancing

   • Daraz’s servers use packet switching to distribute orders across multiple data centers. During sales (e.g., Dashain), packets take the fastest route, reducing delays.

4. Satellite Internet in Rural Nepal

   • NTC’s VSAT (Very Small Aperture Terminal) uses geostationary satellites to provide internet in remote areas like Dolpa. The high latency (~270 ms) is a trade-off for connectivity where fiber isn’t feasible.

5. Wi-Fi Collisions in Kathmandu Traffic

   • In crowded areas like Thamel, multiple Wi-Fi routers (hotels, cafes) use CSMA/CA. If two devices transmit at once, collisions occur, and the protocol forces a random backoff, slowing speeds during peak hours (5–8 PM).

Exam Tip

1. Throughput Calculations

   • Memorize the Pure ALOHA and Slotted ALOHA throughput formulas: $$S_{\text{pure}}=G{e}^{-2G},S_{\text{slotted}}=G{e}^{-G}$$
   • Always convert units (frames/sec → bits/sec) before plugging into $G$.

2. Packet Switching Phases

   • Exams often ask for steps in packet switching. Use the acronym FRQR:
     • Fragmentation → Routing → Queuing → Reassembly.

3. Wireless Topologies

   • Draw star (Wi-Fi), mesh (ad-hoc), and point-to-point (satellite) topologies. Label access points, nodes, and collisions.

4. ALOHA vs. CSMA

   • ALOHA: No listening before transmit (high collisions).
   • CSMA/CA: Listens before transmit (used in Wi-Fi).

5. Real-World Applications

   • Link eSewa (packet switching), Ncell (TDMA), and Daraz (load balancing) to theoretical concepts. Examiners love Nepali examples!

6. Short Notes

   • For questions like "Explain satellite networks", structure your answer as:
     1. Types (GEO, LEO, MEO).
     2. Applications (NTC, GPS, TV).
     3. Challenges (latency, cost).

Practice Questions (Based on Past Exams)

1. A slotted ALOHA network has a 1 Mbps channel and transmits 1000-bit frames. If 500 frames/sec are generated, calculate the throughput. (Hint: First find $G$)
2. Differentiate between CSMA/CD (Ethernet) and CSMA/CA (Wi-Fi) with a table.
3. Explain how packet switching reduces the cost of long-distance communication compared to circuit switching. (Use eSewa as an example.)
4. Draw a mermaid sequence diagram for a successful Wi-Fi transmission using CSMA/CA.
5. Why is LEO satellite preferred for low-latency applications like online gaming, while GEO satellites are used for TV broadcasting? (Hint: Latency vs. coverage.)

Key Formulas to Remember

Final Visual Summary

mindmap
  root((Packet Switching & Wireless Networks))
    Packet Switching
      Fragmentation
      Routing (Independent Paths)
      Queuing (Buffers)
      Reassembly (Sequence Numbers)
    ALOHA Protocols
      Pure ALOHA (High Collision)
      Slotted ALOHA (Slotted Time)
    Wireless Technologies
      Wi-Fi (CSMA/CA)
      Cellular (FDMA/TDMA/CDMA)
      Satellite (GEO/LEO)
    Challenges
      Interference
      Latency
      Bandwidth Limitations
    Real-World Examples
      eSewa (Packet Switching)
      Ncell (TDMA)
      Daraz (Load Balancing)