Question

Consider a 1000 MIPS computer than can execute one instruction per nanosecond. Suppose that it takes 50 instructions to process a packet header, independent of the payload size and 10 instructions for each 8 bytes of payload. How many packets per second can it process if the packets are (a) 128 bytes and (b) 1024 bytes? What is the goodput in bytes/sec in both cases?

Short answer

The computer processes 5,000,000 128-byte packets/sec (goodput: 600,000,000 bytes/sec) and 757,576 1024-byte packets/sec (goodput: 769,694,016 bytes/sec).

Step-by-step solution

Understand the Problem Statement

We need to determine how many packets per second a computer can process, given that it executes 1000 MIPS, where 50 instructions are required to process a packet header and 10 instructions are needed for each 8 bytes of payload. The payload sizes for the two cases are 128 bytes and 1024 bytes, respectively.

Calculate Total Instructions for 128-Byte Packet

The fixed header requires 50 instructions. The payload is 120 bytes (since 128 bytes include the header). For 120 bytes of payload, it takes \( \frac{120}{8} \times 10 = 150 \) instructions. Thus, the total is 50 (header) + 150 (payload) = 200 instructions.

Calculate Execution Time for 128-Byte Packet

Since the computer executes one instruction per nanosecond, 200 instructions require 200 nanoseconds to process one 128-byte packet. Thus, packets per second is \( \frac{10^9}{200} = 5,000,000 \) packets.

Calculate Goodput for 128-Byte Packet

Goodput is the number of payload bytes processed per second. For a 128-byte packet with 120 byte payload, we have \( 5,000,000 \times 120 = 600,000,000 \) bytes per second.

Calculate Total Instructions for 1024-Byte Packet

Similarly, a fixed header requires 50 instructions and 1016 bytes of payload requires \( \frac{1016}{8} \times 10 = 1270 \) instructions. Therefore, the total is 50 (header) + 1270 (payload) = 1320 instructions.

Calculate Execution Time for 1024-Byte Packet

This requires 1320 nanoseconds per packet. Therefore, packets per second is \( \frac{10^9}{1320} \approx 757,576 \) packets.

Calculate Goodput for 1024-Byte Packet

With 1016 bytes of payload per packet, the goodput is \( 757,576 \times 1016 = 769,694,016 \) bytes per second.

Key concepts

MIPS (Million Instructions Per Second)

Understanding MIPS, which stands for Million Instructions Per Second, is essential in computer performance analysis. MIPS is a measure of a computer's processor speed. It tells us how many million instructions a computer can handle in one second. MIPS can vary significantly based on the specific workload and instruction set being used.

• 1000 MIPS: In our exercise, the computer operates at 1000 MIPS, meaning it can execute one billion instructions every second. This is crucial as it helps determine how quickly tasks can be processed by the machine's CPU.
• Instruction Count: Knowing the MIPS allows us to calculate how long it takes to perform a given number of instructions. For example, if executing 200 instructions is necessary for processing a packet, it takes this computer 200 nanoseconds.

It's important to note that MIPS may not always reflect the actual performance due to differences in processor efficiency and workload nature.

Packet Header Processing

Packet header processing is a key task in computer networking. Every data packet moving through a network contains a header, which is essential for routing and data integrity. Understanding this is critical for grasping the workload in network-based computer applications.

• Header Instructions: The exercise specifies that it takes 50 instructions to process a packet header. This is a fixed number that does not change with payload size, showing that headers require a consistent processing effort.
• Payload Processing: In contrast to the header, payload processing varies by size. For every 8 bytes of payload, 10 instructions are necessary. This variability emphasizes the need to account for different payload sizes when analyzing network performance.

Effective packet header processing is crucial for maintaining speed and efficiency in networks, as every packet must undergo this operation before moving to its destination.

Goodput Calculation

Goodput is the rate of successful data transfer, excluding protocol overhead. It measures the amount of usable data transmitted per second, making it a key metric in assessing network performance.

• Goodput vs. Throughput: Unlike throughput, which measures all bits transferred, goodput focuses only on the actual data payload. It neglects headers and retransmissions, which don't contribute to user data.
• Calculation for 128-Byte Packets: In the problem, a packet with a 128-byte total size has only 120 bytes of payload. With 5,000,000 packets processed per second, the goodput is the payload per packet multiplied by packets per second, resulting in 600,000,000 bytes per second.
• Calculation for 1024-Byte Packets: Similarly, a 1024-byte packet with a payload of 1016 bytes results in a goodput of 769,694,016 bytes per second when 757,576 packets are processed each second.

By focusing on goodput, network performance can be effectively optimized by maximizing the actual data transmitted rather than total traffic volume.