2.3.1 Smoothing out the Bursts

Application data is often bursty (e.g., a web browser downloading a 5MB image). Traffic shaping smooths this data into a steady stream to prevent network spikes.

Study Deep: Token Bucket vs. Leaky Bucket

The key difference is flexibility:

• Leaky Bucket: Rigid. No matter what, output is constant. Good for voice.
• Token Bucket: Allows for Bursts. If a user hasn't sent data for 10 seconds, they "save up" tokens and can burst at line speed for a short duration. This is how modern ISPs manage "Speed Boost" features.

2.3.2 The Leaky Bucket Algorithm

Imagine a bucket with a small hole at the bottom. No matter how fast you pour water in, it leaks out at a constant rate.

• Input: Any bursty rate.
• Output: Constant rate.
• Overflow: If the bucket fills up, any new water is "spilled" (dropped).
• Usage: Ideal for CBR (Constant Bit Rate) traffic.

2.3.3 The Token Bucket Algorithm

The Token Bucket allows for some "burstiness" while still maintaining an average rate.

• Tokens: A "token" is added to the bucket periodically (e.g., every 10ms).
• Sending: To send a packet, the sender must "spend" a token.
• The Advantage: If the sender hasn't sent anything for a while, the bucket fills with tokens. This allow the sender to send a "burst" of packets all at once until the tokens are gone.
• Usage: The industry standard for modern Internet QoS.

2.3.4 Policing vs. Shaping

• Traffic Shaping: Buffers excess packets and sends them later. (Adds delay but avoids loss).
• Traffic Policing: Drops excess packets immediately or marks them with a low priority bit (Tagging). (No added delay, but causes loss).

2.3.5 Implementation in ATM: GCRA

In ATM networks, the Leaky Bucket is implemented as the GCRA (Generic Cell Rate Algorithm).

• It checks every incoming cell against a Theoretical Arrival Time (TAT).
• If a cell arrives too early (violating the PCR - Peak Cell Rate), it is marked as non-compliant.