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[Computer Networking] 02. Network Core: Packet Switching vs Circuit Switching

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This post is based on the textbook Computer Networking: A Top-Down Approach (6th Edition) by James Kurose and Keith Ross.

1. The Network Core

The network core is a mesh of packet switches and links that interconnect end systems. There are two fundamental approaches to data transfer:

  • Packet Switching
  • Circuit Switching

2. Packet Switching

2.1 Basic Concept

Application-layer messages are divided into smaller chunks of data called packets for transmission. Each packet travels through communication links and packet switches (routers, link-layer switches) to reach its destination.

2.2 Store-and-Forward Transmission

Most packet switches use store-and-forward transmission.

The switch must receive the entire packet before it can begin transmitting the first bit of the packet on the outbound link.

Source                Router              Destination
   |                   |                   |
   |==== Packet 1 ====>|                   |
   |                   |==== Packet 1 ====>|
   |==== Packet 2 ====>|                   |
   |                   |==== Packet 2 ====>|

Transmission Delay Calculation

When the packet size is L bits and the transmission rate is R bps:

  • Time to traverse one link: L/R seconds
  • Total transmission delay across N links: N * L/R seconds (due to store-and-forward)

Example: Packet size L = 10,000 bits, transmission rate R = 2 Mbps, 2 links

Total delay = 2 * (10,000 / 2,000,000) = 0.01 seconds = 10ms

2.3 Queuing Delay and Packet Loss

Each packet switch has multiple links connected to it, and maintains an output buffer (or output queue) for each link.

          +----------------------------+
Input --->|   Output Queue (Buffer)     |----> Output Link
Link      |  [pkt3][pkt2][pkt1] --->   |     (R bps)
          +----------------------------+
              ^ Queuing delay occurs!

Queuing Delay

The time a packet waits in the queue before being transmitted on the output link. It varies depending on network congestion.

Packet Loss

Since buffer space is finite, a packet arriving at a full queue will be dropped. This is packet loss.

Scenario: Buffer size = 3 packets

Time 1: [pkt1][pkt2][pkt3] -> Buffer full
Time 2: pkt4 arrives -> Buffer overflow -> pkt4 dropped!

2.4 Forwarding Tables and Routing

For a packet to reach its destination, routers must select the appropriate output link.

  1. Each end system has an IP address
  2. The sender includes the destination IP address in the packet header
  3. The router consults its forwarding table to determine the output link
  4. Forwarding tables are automatically configured by routing protocols
Packet header: Destination IP = 121.7.106.83

Router forwarding table:
+------------------+-------------+
| Address Range    | Output Link |
+------------------+-------------+
| 121.7.0.0/16     | Link 2      |
| 200.23.0.0/16    | Link 3      |
| Default          | Link 0      |
+------------------+-------------+
-> Forward to Link 2

3. Circuit Switching

3.1 Basic Concept

Before communication begins, a dedicated circuit is established between the sender and receiver.

The traditional telephone network is a classic example of a circuit-switched network.

Circuit switching process:
1. Connection setup (circuit reservation)
2. Data transfer (using dedicated resources)
3. Connection teardown (circuit release)

Key Characteristics

  • Guarantees a constant transmission rate for the duration of the connection
  • Resources are reserved, so there is no queuing delay
  • Resources cannot be used by other connections even when idle (possible resource waste)

3.2 FDM (Frequency Division Multiplexing)

The frequency band is divided into multiple frequency bands, each assigned to a connection.

Frequency
  ^
  |  +--------+
  |  | User 4 |  Band 4
  |  +--------+
  |  | User 3 |  Band 3
  |  +--------+
  |  | User 2 |  Band 2
  |  +--------+
  |  | User 1 |  Band 1
  |  +--------+
  +--------------------> Time
  • Each connection uses only its assigned frequency band
  • FM radio stations are a good example of FDM
  • Typical bandwidth: 4 kHz (telephone networks)

3.3 TDM (Time Division Multiplexing)

Time is divided into frames of fixed length, and each frame is divided into a fixed number of time slots, each assigned to a connection.

      Frame 1          Frame 2          Frame 3
  +--+--+--+--+   +--+--+--+--+   +--+--+--+--+
  |S1|S2|S3|S4|   |S1|S2|S3|S4|   |S1|S2|S3|S4|
  +--+--+--+--+   +--+--+--+--+   +--+--+--+--+
  ---------------------------------------------> Time

  S1: User 1's slot, S2: User 2's slot, ...
  • Each connection transmits data only during its time slot in each frame
  • Transmission rate: If the link rate is R and the number of slots is N, each connection gets R/N

4. Packet Switching vs Circuit Switching Comparison

4.1 Quantitative Comparison Example

Link capacity: 1 Mbps

MethodSimultaneous Users
Circuit switching (100 kbps each)10
Packet switching (avg. 10% active)35 or more

Why Packet Switching Can Accommodate More Users

Assume each user generates data at 100 kbps but is active only 10% of the time.

  • Circuit switching: Reserves 100 kbps per user regardless of activity -> max 10 users
  • Packet switching: The probability that more than 10 of 35 users are simultaneously active is less than 0.0004
Statistical Multiplexing in Packet Switching:

  35 users, each active with 10% probability
  Expected number of simultaneously active users = 35 * 0.1 = 3.5
  -> 1 Mbps link can easily handle this

4.2 Advantages and Disadvantages

CriterionPacket SwitchingCircuit Switching
Resource efficiencyHigh (statistical multiplexing)Low (resource reservation)
ImplementationSimpleComplex (connection setup needed)
Delay guaranteeNo guaranteeConstant delay guaranteed
Under congestionPacket loss, increased delayConnection rejection
Bursty trafficHandles wellInefficient

4.3 Conclusion

Today's Internet uses packet switching.

  • More efficient for bursty traffic
  • Simpler and more cost-effective
  • However, QoS guarantees for real-time services (voice, video) remain a challenge

5. A Network of Networks

5.1 Evolution of the ISP Hierarchy

End systems connect to the Internet through access ISPs. But how do access ISPs connect to each other?

Structure 1: Connect All Access ISPs to a Single Global ISP

Access ISP --+
Access ISP --+-- Global ISP
Access ISP --+

Unrealistic: One ISP would need to cover the entire world

Structure 2: Multiple Global ISPs Competing

Access ISP -- Global ISP A -- IXP -- Global ISP B -- Access ISP
  • IXP (Internet Exchange Point): A point where ISPs exchange traffic with each other

Structure 3: Multi-Tier Hierarchy

Access ISP -- Regional ISP -- Tier-1 ISP -- Regional ISP -- Access ISP

5.2 Modern Internet Structure

                    +---------------------+
                    |     Tier-1 ISP      |
                    |  (AT&T, NTT, etc.)  |
                    +------+------+-------+
                           | IXP  |
              +------------+      +------------+
              |            |      |            |
         +----+----+  +---+---+  +----+----+
         |Regional |  |Regional|  |Regional |
         |  ISP    |  |  ISP   |  |  ISP    |
         +----+----+  +---+---+  +----+----+
              |            |            |
         +----+----+  +---+---+  +----+----+
         |Access   |  |Access  |  |Access   |
         |  ISP    |  |  ISP   |  |  ISP    |
         +---------+  +-------+  +---------+

Key Components

  • Tier-1 ISP: Top-level ISPs with worldwide coverage (approximately 12)
  • IXP: Direct traffic exchange points between ISPs (over 600 worldwide)
  • Peering: Settlement-free traffic exchange between ISPs at the same tier
  • Content Provider Network: Companies like Google and Microsoft build their own networks
Google's network strategy:
  +--------------------------------+
  |  Google data centers (global)  |
  |  Connected via private network |
  |  Direct connections to Tier-1  |
  |  and IXPs                      |
  |  -> Reduces intermediate ISP   |
  |     costs                      |
  |  -> Direct control over        |
  |     service quality            |
  +--------------------------------+

6. Summary

Network Core Key Comparison:

Packet Switching              Circuit Switching
----------------              -----------------
Store-and-forward             Dedicated circuit setup
Queuing delay occurs          No queuing delay
Statistical multiplexing      FDM / TDM
Packet loss possible          Resource waste possible
Today's Internet              Traditional telephone network

7. Review Questions

Q1. What is Store-and-Forward?

A transmission method where the packet switch must receive all bits of a packet before transmitting the first bit on the outbound link. This adds a transmission delay of L/R seconds at each link traversal.

Q2. Why does packet loss occur?

Because the output buffer (queue) of a packet switch has finite space. When the arrival rate exceeds the transmission rate of the output link, packets queue up, and when the queue is full, newly arriving packets are dropped.

Q3. Why is packet switching more efficient than circuit switching?

Thanks to statistical multiplexing. Since not all users transmit data simultaneously, packet switching does not reserve resources in advance but uses links only when needed. This allows the same resources to accommodate more users.

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