What are transmission and propagation delay?

Network delay is defined as how much time it takes for data to travel from one node to another node in a network. Understanding network delay is crucial for optimizing network performance and predicting data transfer times.

Network delay can be divided into the following components:

• Transmission delay − Time to push all packet bits onto the transmission medium
• Propagation delay − Time for bits to travel through the physical medium to reach the destination

Transmission Delay

Transmission delay is the time required to push all of a packet's bits onto the transmission medium (wired or wireless). This delay depends on the packet size and the bandwidth of the network.

The formula for transmission delay is:

Transmission delay = Packet size (bits) / Bandwidth (bps)

Examples

Example 1: A 12,000-bit Ethernet packet is transmitted on a 100 Mbps link.

Transmission delay = 12,000 bits / (100 × 10? bps) = 0.12 milliseconds

Example 2: A 10-bit packet on a 1 bps link.

Transmission delay = 10 bits / 1 bps = 10 seconds

Propagation Delay

Propagation delay is the time required for bits to travel from the source to the destination through the physical medium. This delay depends on the distance between nodes and the propagation speed of the medium.

The formula for propagation delay is:

Propagation delay = Distance (meters) / Propagation speed (m/s)

Propagation delay depends on:

• Distance between sender and receiver
• Propagation speed of the transmission medium (typically 2/3 the speed of light for copper and fiber)

Examples

Example 1: Data transmission over 5,000 km of copper wire or optical fiber.

Propagation speed = 2 × 10? m/s (2/3 speed of light)
Distance = 5 × 10? meters
Propagation delay = (5 × 10?) / (2 × 10?) = 0.025 seconds = 25 milliseconds

Example 2: Optical fiber network over 2.1 km distance.

Speed = 3 × 10? × 70% = 2.1 × 10? m/s
Distance = 2.1 × 10³ meters
Propagation delay = (2.1 × 10³) / (2.1 × 10?) = 10?? seconds = 0.01 milliseconds

Transmission vs Propagation Delay

Conclusion

Transmission delay depends on packet size and bandwidth, while propagation delay depends on distance and medium speed. Both delays contribute to total network latency and must be considered for accurate network performance analysis.