Lesson 3: Switching — Circuit, Message & Packet (Cell) 🧭

⚙️ What Is Switching?

When networks grow large, devices cannot all be directly connected — they rely on switches to forward data intelligently. Switching is the technique used to decide how data travels from source to destination through intermediate nodes.

There are three major types of switching:

Type Generations Example
Circuit Switching 1st Gen (Analog/Digital) Telephone Networks
Message Switching 2nd Gen (Fully Digital) Early Store-and-Forward Systems
Packet (Cell) Switching 3rd Gen (Digital) Modern Internet (TCP/IP, ATM)

🔌 Circuit Switching

🧩 Concept

A dedicated path (circuit) is established between sender and receiver before communication starts. This circuit remains reserved for the entire duration of the session — like a private lane.

Used originally in landline telephony.

🧠 How It Works

graph TD
    U1(User A) --> S1(Subswitch 1)
    U2(User B) --> S1
    S1 --> S2(Subswitch 2)
    S2 --> S3(Telecom Central Switch)
    S3 --> S4(Remote Subswitch)
    S4 --> U3(User C)
  1. User A dials User C.
  2. The signal passes through local subswitches.
  3. Switches coordinate to reserve a continuous electrical path.
  4. Once connected, analog signals (voice) flow directly until one party hangs up.

🧰 Variants

Type Description
Analog Circuit Switching Physical electrical connections; mechanical relays close circuits.
Digital Circuit Switching Logical switching using electronic boards; no moving parts.

⚠️ Limitations

Issue Explanation
Setup Time A path must be found and reserved before transmission begins — causing delay.
Busy Line If all circuits on a route are occupied, new calls are blocked (no alternative path).
Inefficient Utilization Even when no data is being sent (pauses in conversation), the line remains reserved.
High Cost Each active circuit requires dedicated resources.

🧠 Think of circuit switching as booking a private train track for two cities — even if no train is currently running, no one else can use it.

💬 Message Switching

🌐 Concept

Here, the entire message is treated as one big unit (a “block”). Each node receives the whole message, stores it, and then forwards it — known as store and forward.

No dedicated circuit is established.

🗺️ Simplified Flow

sequenceDiagram
    participant User as Sender
    participant R1 as Router 1
    participant R2 as Router 2
    participant R3 as Router 3
    participant Rec as Receiver
    
    User->>R1: Send full message
    R1-->>User: ACK (Received)
    R1->>R2: Forward after node processing
    R2->>R3: Forward
    R3->>Rec: Deliver full message

🧮 Timing Visualization (as described in class)

Time (ms) Event
0 Sender sends message
5 Router 1 receives it
6 Router 1 forwards it (after processing)
12 Router 2 receives it
14 Router 2 forwards it
… continues until receiver

Each router introduces a small node processing time — for routing decisions and interface selection.

🔁 Acknowledgment Variant

⚙️ Node Processing Time

Every intermediate device (router) needs time to:

  1. Inspect the header.
  2. Decide which output interface to use.
  3. Queue the message for transmission.

This delay ensures correct delivery, but adds latency at each hop.

🚫 Problems with Message Switching

Problem Description
Variable Message Size Routers must store entire messages; if too large, buffers overflow.
High Delay Each hop must completely receive the message before forwarding.
Inefficient Under Load When many large messages exist, routers get congested — some paths become unusable.
Unpredictable Routing Sender and receiver don’t know the exact path; intermediate policy may change routing behavior.

🧩 Message switching is reliable but slow. Think of it like mailing whole letters — each post office must receive and repackage the full letter before passing it on.

📦 Cell (Packet) Switching

🌟 Evolution

To overcome the inefficiencies of message switching, data is divided into small fixed-size units called cells or packets. Each packet travels independently or semi-independently through the network.

Two main models exist:

  1. Virtual Circuit (VC) Switching
  2. Datagram Switching

🔄 Virtual Circuit (VC) Switching

💡 Concept

A temporary logical path (not physical) is established before sending data — similar to a “session.” This is the virtual version of circuit switching.

🧭 How It Works

  1. A VC setup cell (empty) is sent from sender → receiver.
  2. It records the routers and interfaces it passes through.
  3. Upon arrival, it returns back along the same path.
  4. That path information is stored and used by all subsequent data cells.

Each data cell carries:

🔗 Diagram

graph LR
    A(Sender) --> R1[Router 1]
    R1 --> R2[Router 2]
    R2 --> R3[Router 3]
    R3 --> B(Receiver)
    classDef data fill:#eef,stroke:#333;

Once the VC path is set up, all packets follow the same route, ensuring ordered delivery.

✅ Pros

❌ Cons

🗺️ Datagram Switching

💬 Concept

Each packet (cell) is independent. There is no setup phase — every packet carries full destination information and finds its own path.

🔀 How It Works

graph LR
    A(Sender) --> R1[Router 1]
    A --> R2[Router 2]
    A --> R3[Router 3]
    R1 --> B(Receiver)
    R2 --> B
    R3 --> B

Each router decides the best route dynamically based on network conditions.

Packets may:

⚖️ Comparison: Virtual Circuit vs. Datagram

Feature Virtual Circuit Datagram
Connection Setup Required Not required
Path Consistency Same path for all packets Different paths possible
Reliability Higher (ordered delivery) Must reorder at receiver
Flexibility Lower (fixed path) High (adaptive routing)
Typical Use ATM, Frame Relay Internet (IP)

🧠 Virtual Circuit = like a reserved lane. Datagram = like individual cars each finding their own fastest route.

🧮 Summary Table

Switching Type Setup Data Unit Efficiency Delay Example
Circuit Yes (physical) Continuous Low Low (after setup) Telephony
Message No Full message Moderate High Early networks
Packet / Cell (VC) Yes (logical) Fixed packets High Low ATM, MPLS
Packet / Cell (Datagram) No Fixed packets Very High Variable Internet

🧩 Key Takeaways