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EngineeringNotes
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Chapter 04

Switching &
Multiplexing

The mechanisms that efficiently share and route data through the network.

01

Multiplexing

Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link. The goal is to maximize the utilization of the medium's capacity.

TDM Packet Transfer Visualization

A
Source 1
B
Source 2
C
Source 3
MUX
C1
B1
A1
C2
B2
A2
DEMUX
A
Dest 1
B
Dest 2
C
Dest 3

Data from sources A, B, and C is interleaved into Frames. Each frame contains one time slot for each source.

FDM

Frequency Division Multiplexing. Assigns different frequency bands to different signals.

Analog Signals (Radio)

TDM

Time Division Multiplexing. Assigns different time slots to different signals.

Digital Signals

WDM

Wavelength Division Multiplexing. Uses different light colors (wavelengths) over fiber.

Optical Fiber
02

Switching Principles

A network consists of many switching devices (nodes). Usage of these switches to connect multiple devices is called switching. It is more efficient than a mesh topology for large networks.

Circuit Switching

Working Principle

A dedicated physical path is established between the sender and receiver before any data is transmitted. The entire bandwidth of the path is reserved for this communication session.

Phase 1: Setup: Request signal travels to destination, allocating resources.

Phase 2: Data Transfer: Continuous stream of data flows (no headers needed).

Phase 3: Teardown: Release signal frees the resources.

S1
S2
S3
Advantages• Constant data rate (Guaranteed Quality of Service)
• No out-of-order packets
• Best for voice calls
Disadvantages• Connection Setup delay
• Inefficient (bandwidth wasted during silence)
• Single point of failure (link break kills call)

Packet Switching

Working Principle

Data is broken into small chunks called packets. Each packet contains a "Header" with source and destination IP. Packets travel independently through the network, possibly taking different paths.

Datagram Approach: Each packet treated independently. No setup. (UDP/IP)

Virtual Circuit: Logic path setup first, but resources not reserved physically. (Frame Relay)

Advantages• Efficient Bandwidth (shared dynamically)
• Fault tolerant (packets reroute)
• Pipelining works well
Disadvantages• Variable delay (Jitter)
• Packets can arrive out of order
• Header overhead

Message Switching

Known as Store and Forward. The entire message is sent to the next node, stored completely in its disk/memory, checked for errors, and then forwarded to the next node.

History: Telegraph SystemsModern: Email (conceptually)
03

Network Devices

Hardware devices that connect computers, printers, and other electronic devices to a network.

Modem

Modulator-Demodulator.

  • Sender: Converts Digital data to Analog signals (for phone lines).
  • Receiver: Converts Analog signals back to Digital data.

Repeater

Physical Layer Device (Regenerator).

  • Receives a weak signal and regenerates it.
  • Used to extend the physical length of a network.
  • Not an amplifier (amplifiers amplify noise too).

Hub

Physical Layer (Multi-port Repeater).

  • Connects multiple devices in a star topology.
  • Broadcasting: Incoming data is sent to ALL ports.
  • Shared bandwidth (high collision risk). Dumb device.

Switch

Data Link Layer (Intelligent).

  • Uses MAC Addresses to filter and forward data.
  • Unicasting: Sends data ONLY to the intended recipient.
  • Full bandwidth per port. No collisions.

Router

Network Layer (The Traffic Cop).

  • Connects two or more different networks (e.g., LAN to WAN).
  • Uses IP Addresses to route packets.
  • Maintains a routing table to find the best path.

Gateway

Protocol Converter.

  • Operates at any layer (mostly Application).
  • Connects dissimilar networks (e.g., TCP/IP to SNA).
  • Translates data formats and protocols.