When two computers communicate over a large network, they are usually not connected by one permanent direct cable. Data may have to pass through several intermediate devices before reaching the destination.
The process used to move communication from one network link to another is called switching. The three classic switching techniques are Circuit Switching, Message Switching and Packet Switching.
Instead of memorizing their definitions, we will understand one main question: “When data reaches an intermediate switching node, what exactly does that node do with it?”
Switching is the method used to transfer data from a source to a destination through intermediate network nodes and communication links.
Circuit Switching establishes a dedicated communication path before data transfer.
Message Switching stores and forwards the complete message at every intermediate node.
Packet Switching divides the message into smaller packets and forwards those packets through the network.
What Is Switching in Computer Networks?
Suppose Computer A wants to communicate with Computer B.
If both computers were directly connected, communication would be simple.
But a large network may contain thousands or millions of devices. Creating a separate physical cable between every pair of computers would be impractical.
Instead, networks contain intermediate devices called switching nodes.
Why Do Computer Networks Need Switching?
Imagine a network containing 100 devices.
If every device needed a separate physical connection to every other device, the number of required links would become enormous.
Switching allows many users to share network infrastructure efficiently.
Shared Network Links
Multiple users can communicate through common intermediate infrastructure.
Scalability
Large numbers of devices can communicate without direct links between every pair.
Path Selection
Communication can travel through suitable intermediate nodes.
Lower Infrastructure Cost
Shared network infrastructure reduces the need for dedicated physical connections between all devices.
What Is a Switching Node?
A switching node is an intermediate network device or system that receives data and forwards it toward another link or destination.
The exact behaviour depends on the switching technique.
| Technique | What the Intermediate Node Handles |
|---|---|
| Circuit Switching | Maintains an already established communication circuit |
| Message Switching | Stores the complete message before forwarding it |
| Packet Switching | Receives and forwards individual packets |
Types of Switching Techniques
Circuit Switching
Circuit switching establishes a dedicated communication path between sender and receiver before actual data transfer begins.
Once the circuit is established, the reserved resources remain associated with that communication session until the connection is released.
Three Phases of Circuit Switching
Circuit Establishment
Before communication begins, the network creates an end-to-end circuit through intermediate switches.
Data Transfer
After the circuit is ready, data travels through the established path.
Circuit Release
When communication finishes, the reserved resources are released so they can be used by other connections.
How Circuit Switching Works
Suppose Device A wants to communicate with Device B.
- A connection request is sent through the switching network.
- Intermediate switches identify an available route.
- Communication resources are reserved along that route.
- An end-to-end circuit is established.
- Data starts flowing through the dedicated circuit.
- After communication finishes, the circuit is released.
Important Characteristic
Data normally follows the same established path throughout the communication session.
Resource Reservation in Circuit Switching
The important feature of circuit switching is resource reservation.
The network may reserve communication capacity for a connection.
This can provide predictable service, but it can also waste capacity when the user is temporarily silent.
How Can Capacity Be Divided?
Traditional circuit-switched systems can divide transmission resources using techniques such as:
- Frequency Division Multiplexing (FDM) – different connections use different frequency ranges.
- Time Division Multiplexing (TDM) – different connections use different repeating time slots.
Advantages of Circuit Switching
Advantages
- Dedicated communication path
- Predictable performance after setup
- Reserved bandwidth or resources
- No per-message routing decision during established communication
- Suitable for continuous real-time communication
- Low variation in delay after circuit establishment
Disadvantages
- Connection setup takes time
- Reserved capacity may remain unused during silence
- Less efficient for bursty computer data
- Circuit failure can interrupt communication
- Resources are unavailable to other users while reserved
Circuit Switching Delay
Circuit switching usually includes a setup delay before useful data can be transmitted.
If a message contains M bits and the dedicated circuit operates at rate R bits/second, the message transmission time is approximately:
Message Switching
Message switching does not establish a dedicated circuit before communication.
Instead, the entire message is treated as one unit.
Each switching node receives the complete message, stores it temporarily, examines its destination information and then forwards it to the next node.
Store-and-Forward in Message Switching
The term store-and-forward is extremely important.
Suppose a 10 MB message reaches Switch A.
Switch A does not immediately forward the first few bytes while the rest are still arriving.
In classic message switching:
- The complete message arrives.
- The message is stored in the switch's memory.
- The destination address is examined.
- The next path is selected.
- The complete message is transmitted to the next switching node.
No Dedicated Path
Unlike circuit switching, message switching does not reserve a complete end-to-end path in advance.
A message can wait at an intermediate node until an outgoing link becomes available.
Advantages of Message Switching
- No dedicated circuit required
- Network links can be shared efficiently
- Messages can wait when the next link is busy
- Routing decisions can be made at intermediate nodes
- Priority handling can be implemented
- Large storage is required
- Delay can be high
- Not suitable for interactive real-time communication
- The complete message must arrive before forwarding
- Large messages can occupy buffers for a long time
Message Switching Delay
Suppose a complete message of M bits passes through N equal links.
Each link has transmission rate R bits/second.
Ignoring propagation, processing and queueing delays:
Why?
Because every switching node must receive the entire message before sending it across the next link.
Packet Switching
Packet switching solves one major weakness of message switching.
Instead of treating a complete large message as one unit, the message is divided into smaller units called packets.
Switch A
Switch B
What Is Packetization?
Packetization is the process of dividing a large message or data stream into smaller packets before transmission.
Why Divide Data Into Packets?
- Smaller units are easier to store and forward.
- Network resources can be shared between many users.
- Packets from different users can be interleaved.
- Packets can begin moving before the full original message has reached an intermediate node.
- If data is lost, a smaller unit may need retransmission instead of the entire message, depending on the protocol.
How Packet Switching Works
Data Is Divided
The original data is divided into smaller packets.
Headers Are Added
Each packet receives control information required by the communication protocol.
Packets Enter the Network
Packets are transmitted through intermediate packet-switching devices.
Packets Are Forwarded
Each switching node forwards packets according to network forwarding information.
Destination Receives Packets
The receiving system processes the packets and higher-layer protocols may reconstruct the original data.
What Does a Packet Contain?
A packet is not made only of user data.
It also needs control information.
Packet Structure
Headers may contain addresses, identifiers, sequence-related information and other protocol fields depending on the protocol.
Why Headers Are Needed
Network devices need control information to know how the packet should be handled and delivered.
However, header bits also create overhead because they are not part of the original application data.
Datagram Packet Switching
In datagram packet switching, each packet is treated independently.
A complete end-to-end path does not have to be established before packets are sent.
Each packet carries enough addressing information for the network to forward it.
Important Characteristics
- No connection setup is required at the packet-switching level.
- Each packet is forwarded independently.
- Packets may follow different routes.
- Packets may experience different delays.
- Packets can potentially arrive out of order.
Virtual Circuit Packet Switching
Virtual circuit packet switching combines some ideas of circuit-oriented communication with packet switching.
Before data transfer, a logical path is established through the packet network.
Packets then follow that logical path.
Virtual Circuit Phases
- Setup phase
- Packet transfer phase
- Connection release phase
Packets still share network links with other traffic. The virtual circuit provides a logical path, not necessarily exclusive physical resources like traditional circuit switching.
Datagram vs Virtual Circuit Packet Switching
| Datagram | Virtual Circuit |
|---|---|
| No connection setup required at packet-switching level | Logical path setup occurs before data transfer |
| Packets are handled independently | Packets follow the established logical path |
| Packets may take different routes | Packets normally follow the same virtual circuit |
| Packets may arrive out of order | Ordering is easier to preserve within the chosen path |
| Routing/forwarding decision can be based on destination address | Packets may use a shorter virtual-circuit identifier after setup |
| Example concept: IP datagram forwarding | Seen in virtual-circuit-oriented packet technologies |
Packet Pipelining
One important advantage of dividing data into packets is pipelining.
An intermediate node does not need to wait for the entire original message before forwarding the first packet.
Once Packet 1 has been received and forwarded, the next packet can follow behind it.
Packet Switching Delay
Suppose:
- Message is divided into P equal packets.
- Each packet contains L bits.
- The path contains N links.
- Every link has transmission rate R.
Ignoring propagation, queueing, processing and header overhead, store-and-forward packet switching with pipelining requires approximately:
Why N + P − 1?
The first packet requires transmission over all N links.
After the pipeline is full, every additional packet arrives approximately one packet-transmission time later.
Solved Switching Delay Example
Suppose:
- Total message = 12,000 bits
- Number of links = 3
- Link rate = 1 Mbps
- Ignore propagation, processing and queueing delay
Case 1: Message Switching
= 0.036 seconds
= 36 ms
Case 2: Packet Switching
Divide the message into 3 packets of 4,000 bits each.
P = 3
N = 3
L/R = 4,000 / 1,000,000
= 4 ms
= 5 × 4 ms
= 20 ms
Case 3: Circuit Switching
Assume circuit setup time = 10 ms.
Message transmission time:
12,000 / 1,000,000 = 12 ms
Effect of Packet Size on Packet Switching
Students often assume that making packets as small as possible is always better.
It is not that simple.
Very Large Packets
- Lower relative header overhead
- Longer transmission time per packet
- One packet may occupy a link for longer
- Retransmission can waste more data if a large packet is lost
Very Small Packets
- Better fine-grained sharing of links
- Smaller individual transmission times
- More packets are needed
- Header overhead becomes larger relative to useful data
Advantages of Packet Switching
- Efficient use of network links
- No dedicated physical circuit required
- Suitable for bursty computer traffic
- Multiple users can share the same links
- Packets can be dynamically forwarded
- Alternative paths may be possible
- Supports scalable data networks
Limitations
- Queueing delay can vary
- Packets may be lost during congestion
- Datagrams can arrive out of order
- Every packet requires control information
- Delay variation can affect real-time applications
- Network devices need buffers and forwarding logic
Circuit vs Message vs Packet Switching
| Feature | Circuit Switching | Message Switching | Packet Switching |
|---|---|---|---|
| Dedicated Path | Yes, logical/physical circuit resources reserved | No | No dedicated physical circuit in normal packet switching |
| Setup Required | Yes | No | Datagram: No Virtual Circuit: Yes |
| Data Unit | Continuous data stream / circuit | Complete message | Packet |
| Store-and-Forward | Not for the whole data unit in the same way | Entire message | Usually individual packet |
| Resource Reservation | Yes | No | Generally shared; depends on network technology |
| Buffer Requirement | Relatively low for complete-message storage | High | Smaller packet buffers |
| Delay | Setup delay first, predictable afterward | Can be very high | Variable due to queueing |
| Link Utilization | Can be inefficient during idle periods | Better sharing | Efficient statistical sharing |
| Real-Time Suitability | Traditionally good | Poor | Good when network and protocols control delay/QoS properly |
| Typical Historical Example | Traditional telephone network | Store-and-forward telegraph/message systems | Modern computer data networks |
Connection-Oriented vs Connectionless Switching Idea
Switching techniques are often easier to understand when we connect them with connection-oriented and connectionless communication.
Path or Connection Established
A logical relationship is established before normal data transfer.
Circuit switching and virtual-circuit packet switching have connection-oriented characteristics.
Packets Sent Independently
No dedicated connection setup is required before each datagram is sent.
Datagram packet switching follows this model.
How Network Resources Are Used
Reserved Capacity
Resources remain associated with a connection even when no useful data is being transmitted.
Whole-Message Sharing
Links are shared, but the entire message occupies storage and transmission resources at each stage.
Statistical Sharing
Packets from different users can share communication links dynamically according to traffic demand.
Switching vs Routing
Students sometimes use switching and routing as if they mean exactly the same thing.
They are related but not identical.
| Switching | Routing |
|---|---|
| General process of moving data between links/nodes | Process of determining paths through interconnected networks |
| Can refer to circuit, message or packet switching | Mainly associated with network-layer packet forwarding decisions |
| May operate at different communication levels | Uses network topology and routing information |
| Broad networking concept | Specific path-selection and forwarding concept |
What Happens If a Link Fails?
Circuit Switching
If a link forming the established circuit fails, the connection may be interrupted and a new circuit may need to be established.
Message Switching
Because no dedicated path is fixed, a stored message may potentially be forwarded through another available route if the network supports such routing.
Packet Switching
Datagram networks can often route future packets through another available path when routing information changes.
Switching and Network Congestion
Packet and message switching share links among many users.
If packets arrive at a switching node faster than the outgoing link can transmit them, they must wait in buffers.
This creates queueing delay.
If buffers become full, packets may be discarded.
Real-Life Applications and Examples
Traditional Telephone Network
Classic telephone systems are a standard example used to explain circuit switching.
Historical Store-and-Forward Systems
Message switching concepts were used in older communication systems in which complete messages were stored before forwarding.
Internet Data
Modern Internet communication is based primarily on packet-switched networking using IP packets.
Messaging Applications
Application messages ultimately travel through packet-switched IP networks.
Video Streaming
Video data is transmitted as packets across shared IP networks rather than using one exclusive traditional physical circuit.
Online Gaming
Game updates and real-time communication are carried through packet-switched networks.
Which Switching Technique Should Be Used?
No switching technique is best for every possible communication system.
| Requirement | Suitable Concept | Reason |
|---|---|---|
| Dedicated predictable communication resources | Circuit Switching | Resources are reserved before transfer |
| Complete-message store-and-forward | Message Switching | Entire message is stored at intermediate nodes |
| Efficient shared computer-data network | Packet Switching | Users dynamically share links |
| No packet-network setup before sending | Datagram Switching | Packets are independently forwarded |
| Logical path before packet transfer | Virtual Circuit | Packets follow a pre-established logical route |
Best Real-Life Analogy for All Three Switching Techniques
Reserved Railway Track
Imagine reserving an entire route for your train before the journey starts. Once reserved, the route remains dedicated to your journey.
One Large Parcel
A huge parcel reaches one warehouse completely, waits there, and then the complete parcel is sent to the next warehouse.
Many Small Parcels
One shipment is divided into smaller parcels. Different parcels can move through the delivery network efficiently and may even use different routes.
Common Student Mistakes
Traditional circuit switching first establishes a dedicated route and communication follows that established circuit.
Message switching treats the complete message as the store-and-forward unit.
Datagram packet switching does not require a virtual-circuit setup. Virtual-circuit packet switching does.
A virtual circuit establishes a logical packet path, but packets may still share physical links with other traffic.
Datagram packets may follow different paths, but virtual-circuit packets normally follow the established logical route.
Packets can experience processing, queueing, transmission and propagation delay.
Smaller packets increase the number of headers and therefore increase protocol overhead.
A circuit requires setup time. Whether it gives lower total delay depends on session length, data amount, path characteristics and network conditions.
Easy Memory Tricks
Switching Techniques Quick Revision Notes
- Switching helps data move through intermediate network nodes.
- The three classic switching techniques are circuit, message and packet switching.
- Circuit switching establishes a dedicated path before data transfer.
- Circuit switching has setup, data-transfer and release phases.
- Resources are reserved during a circuit-switched connection.
- Traditional telephone systems are common circuit-switching examples.
- Message switching does not reserve a dedicated path.
- Message switching stores the complete message at each intermediate node.
- Message switching therefore requires large storage buffers.
- Packet switching divides data into packets.
- Each packet contains control information plus payload.
- Packet switching allows network links to be shared dynamically.
- Datagram switching handles each packet independently.
- Datagram packets may follow different paths.
- Virtual circuit switching establishes a logical path first.
- A virtual circuit is not the same as a dedicated physical circuit.
- Packet pipelining improves transmission efficiency across multiple links.
- Smaller packets reduce individual transmission time but increase header overhead.
- Large packets have lower relative header overhead but occupy links for longer periods.
- Message switching has higher store-and-forward delay for large messages.
- Packet switching is well suited to bursty computer-data communication.
- The Internet is fundamentally packet switched.
- Queueing delay can occur when packet traffic exceeds immediate link capacity.
- Switching and routing are related but different concepts.
Important Switching Techniques Exam Questions
Very Short Answer Questions
Short Answer Questions
Long Answer Questions
Numerical / Analytical Questions
Switching Techniques MCQs With Answers
A. Message Switching
B. Circuit Switching
C. Datagram Switching
D. Broadcasting
A. Packet loss
B. Circuit establishment
C. Message fragmentation
D. Queue deletion
A. Message Switching
B. Circuit Switching
C. Frequency Switching
D. Optical Switching Only
A. Store-and-forward
B. Dedicated circuit only
C. No storage
D. No addresses
A. Circuits
B. Packets
C. Frequencies only
D. Telephone lines
A. Packet Switching
B. Permanent Circuit Only
C. Message Switching Only
D. None
A. Datagram
B. Circuit
C. TDM
D. Message
A. Datagram
B. Virtual Circuit
C. Broadcast
D. Bus
A. Must always follow the same path
B. May follow different paths
C. Need a dedicated physical circuit
D. Have no destination information
A. Always a dedicated physical cable
B. A logical packet path
C. A hard disk partition
D. A wireless antenna
A. Circuit Switching
B. Datagram Switching
C. Packet Switching only
D. Message Switching only
A. Message Switching
B. Circuit Switching
C. Datagram Routing
D. Physical Switching
A. Control information
B. Monitor display
C. Keyboard input only
D. Power supply
A. Multiple links can work on different packets at the same time
B. Every packet waits for the whole message
C. A circuit is always reserved
D. Packets are never stored
A. Traditional Telephone Network
B. Modern Ethernet LAN only
C. Database
D. File System
A. Packet Switching
B. Message Switching only
C. Dedicated telephone circuits only
D. No switching
A. Datagram Switching
B. Virtual Circuit
C. Circuit Switching
D. None
A. Queueing delay
B. Infinite reserved capacity
C. Zero packet headers
D. No sharing
A. More relative header overhead
B. No headers
C. Dedicated circuits
D. Infinite bandwidth
A. Message Switching
B. Packet Switching
C. Circuit Setup only
D. Frequency Division only
Frequently Asked Questions
What is switching in computer networks?
Switching is the process used to move data from a source to a destination through intermediate network nodes and communication links.
What are the three main switching techniques?
The three classic switching techniques are Circuit Switching, Message Switching and Packet Switching.
What is circuit switching?
Circuit switching establishes and reserves an end-to-end communication path before data transmission begins.
What is message switching?
Message switching stores the complete message at each intermediate node before forwarding it toward the destination.
What is packet switching?
Packet switching divides data into smaller packets that are forwarded through shared network links.
What is the difference between message switching and packet switching?
Message switching stores and forwards the entire message as one unit, while packet switching divides the message into smaller packets that can be forwarded separately.
What are the two types of packet switching?
The two major forms are datagram packet switching and virtual-circuit packet switching.
What is datagram switching?
Datagram switching treats each packet independently, so different packets may follow different routes through the network.
What is virtual circuit switching?
Virtual-circuit switching establishes a logical path before packet transfer, after which packets normally follow that logical path.
Why is packet switching used for the Internet?
Packet switching allows many users to share network links efficiently, handles bursty traffic well and supports large interconnected networks.
Which switching technique is best?
The appropriate technique depends on the communication requirements. Circuit switching provides reserved resources, message switching stores whole messages, while packet switching provides efficient link sharing for modern data networks.
Conclusion
Switching techniques explain how information travels through a network containing intermediate communication nodes.
In Circuit Switching, an end-to-end path is established before useful data transmission begins. Resources remain reserved until the communication ends.
In Message Switching, there is no dedicated circuit. Instead, the entire message is stored at every intermediate switching node and then forwarded to the next node.
In Packet Switching, the original data is divided into smaller packets. This makes it possible for multiple users to share the same network links efficiently and enables packet pipelining.
Packet switching has two important forms:
Datagram = packets are handled independently.
Virtual Circuit = a logical route is established first.
The easiest way to remember the complete chapter is:
Circuit = Reserve Path
Message = Store Whole Message
Packet = Divide and Share
Once switching becomes clear, concepts such as routing, congestion, TCP/IP and Internet communication become much easier to understand.
Next Chapter: Chapter 8 – ISDN in Computer Networks: Architecture, Channels, Services & Interfaces Explained
