Circuit Switching, Virtual Circuits, and Multiplexing Techniques

Posted on Apr 1, 2025 in Computer Engineering

A circuit-switched network is made of a set of switches connected by physical links, in which each link is divided into n channels. Figure 8.3 illustrates a trivial circuit-switched network. In circuit switching, resources need to be reserved during the setup phase; the resources remain dedicated for the entire duration of data transfer until the teardown phase. 

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Virtual-Circuit Networks

A virtual-circuit network is a cross between a circuit-switched network and a datagram network, exhibiting characteristics of both.

In virtual-circuit switching, all packets belonging to the same source and destination travel the same path. However, the packets may arrive at the destination with different delays if resource allocation is on demand.

Multiplexing Techniques

Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link. The opposite operation, retrieving multiple signals from a single data link, is called Demultiplexing.

  • Multiplexing combines signals from several sources to achieve bandwidth efficiency.

Frequency-Division Multiplexing (FDM)

  • FDM is an analog multiplexing technique that combines analog signals.
  • It can be applied when the bandwidth of a link is greater than the combined bandwidth of all the signals to be transmitted.
  • A number of signals can be carried simultaneously.
  • The bandwidth of a medium is divided into independent frequency channels.
  • Each signal is then translated into different frequency bands using a modulation technique.
  • Finally, the signals are combined into a single composite signal using a multiplexer.
  • Carrier frequencies are sufficiently separated so signals do not overlap (guard bands).
  • Examples: broadcast radio, cable TV

Wavelength-Division Multiplexing (WDM)

Multiplexing and De-multiplexing involves light signals. Multiple beams of light at different frequencies are carried by optical fiber. Each color of light (wavelength) carries a separate data channel.

Operation:

  • Number of sources generating laser beams at different frequencies.
  • The multiplexer consolidates sources for transmission over a single fiber.
  • Optical amplifiers amplify all wavelengths.
  • Demux separates channels at the destination.

Time-Division Multiplexing (TDM)

Time-division multiplexing (TDM) is a digital process that allows several connections to share the high bandwidth of a link.

  • TDM is a digital multiplexing technique for combining several low-rate channels into one high-rate one.
  • Instead of sharing a portion of the bandwidth as in FDM, time is shared.
  • Each connection occupies a portion of time in the link.
  • The link is sectioned by time rather than by frequency.

Synchronization in TDM

  • To ensure that the receiver correctly reads the incoming bits (i.e., knows the incoming bit boundaries to interpret a “1” and a “0”), a known bit pattern is used between the frames.
  • The receiver looks for the anticipated bit and starts counting bits till the end of the frame.
  • Then it starts over again with the reception of another known bit.
  • These bits (or bit patterns) are called synchronization bit(s).
  • They are part of the overhead of transmission.

Synchronous TDM: Time Slots and Frames

  • In synchronous TDM, the data flow of each input connection is divided into units, where each input occupies one input time slot.

A unit can be 1 bit, one character, or one block of data.

Each input unit becomes one output unit and occupies one output time slot.

The duration of an output time slot is n times shorter than the duration of an input time slot. If an input time slot is T s, the output time slot is T/n s, where n is the number of connections.

  • In synchronous TDM, the data rate of the link is n times faster, and the unit duration is n times shorter.

Spread Spectrum (SS)

  • In spread spectrum (SS), we combine signals from different sources to fit into a larger bandwidth, but our goals are to prevent eavesdropping and jamming. To achieve these goals, spread spectrum techniques add redundancy.
  • Wrap message in a protective envelope for a more secure transmission.
  • The expanding must be done independently. There are two types:

Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS)