Question

Suppose BLAST runs over a 10-Mbps Ethernet, sending \(32 \mathrm{~K}\) messages. (a) If the Ethernet packets can hold 1500 bytes of data, and optionless IP headers are used as well as BLAST headers, how many Ethernet packets are required per message? (b) Calculate the delay due to sending a \(32 \mathrm{~K}\) message over Ethernet (i) directly (ii) broken into pieces as in (a), with one bridge Ignore propagation delays, headers, collisions, and interpacket gaps.

Short answer

22 Ethernet packets are required per message. The delay for direct transmission is 26.2144 ms. For broken pieces with one bridge, the delay is 27.6 ms.

Step-by-step solution

Convert message size to bytes

Given the message size is 32 KB, convert this size to bytes. 1 KB = 1024 bytes. Therefore, 32 KB = 32 * 1024 bytes = 32768 bytes.

Determine number of packets per message

Given each Ethernet packet can hold 1500 bytes, calculate the number of packets required to send the 32768-byte message. Divide the message size by the packet size: \( \left\lceil \frac{32768}{1500} \right\rceil \). This results in approximately 21.85, which means 22 packets are required (since the number of packets must be an integer).

Calculate transmission time for the entire message (direct transmission)

First, determine the total size in bits. The message size is 32768 bytes, so the total size is \(32768 \text{ bytes} \times 8 \text{ bits per byte} = 262144 \text{ bits} \). Ethernet speed is 10 Mbps, so the time to send the message is: \( \frac{262144 \text{ bits}}{10 \text{ Mbps}} = 26.2144 \text{ ms} \).

Calculate transmission time per packet

Each Ethernet packet is 1500 bytes, which equals \( 1500 \text{ bytes} \times 8 \text{ bits per byte} = 12000 \text{ bits} \). The time to send one packet is \( \frac{12000 \text{ bits}}{10 \text{ Mbps}} = 1.2 \text{ ms} \).

Calculate total delay with bridge

For each packet, the delay is 1.2 ms. Since there are 22 packets, the total delay for sending all packets is \( 1.2 \text{ ms per packet} \times 22 = 26.4 \text{ ms} \). With one bridge, each piece has an additional delay of 1.2 ms. The total delay with one bridge is \( 26.4 \text{ ms} + 1.2 \text{ ms} = 27.6 \text{ ms} \).

Key concepts

Ethernet

Ethernet is a widely used technology in computer networks, particularly for local area networks (LANs). It standardizes how data is transmitted over a network. Ethernet frames are the units of data sent over Ethernet; they contain both the data being transmitted and various headers that include addressing information.
In our exercise, we're dealing with a 10 Mbps Ethernet network. Mbps stands for megabits per second and indicates the speed at which data is transmitted. So, a 10 Mbps Ethernet can transmit 10 million bits each second.
Ethernet packets in this context can hold up to 1500 bytes of data. This size is commonly known as the Maximum Transmission Unit (MTU). Since 1 byte = 8 bits, one packet can contain up to 12000 bits of data. Understanding these basic Ethernet structures and parameters is important in analyzing how data flows through network systems.

Packet Transmission

Packet transmission involves breaking down large data messages into smaller packets, which are then sent over the network individually. In our exercise, a 32 KB message is divided into packets because Ethernet packets can only hold 1500 bytes of data each.
Breaking down an entire message into packets not only makes it easier to handle and route the data over the network but also improves reliability. If one packet encounters an error or gets lost, only a small part of the message is affected, not the entire message.
In the exercise, a total of 22 Ethernet packets are required to send a 32 KB message. Calculating this involves converting 32 KB to bytes (32768 bytes) and then dividing by the packet size (1500 bytes): \(\frac{32768}{1500} \approx 21.85\). Since you can't have a fraction of a packet, we round up to 22 packets.
Each packet, wholly independent, can navigate through the network until it reaches the destination, where the original message is reassembled from these packets.

Network Delay Analysis

Understanding network delay is crucial for optimizing network performance. Network delay refers to the time it takes for data to travel from the source to the destination.
In the context of the exercise, delays include transmission time and any additional delays caused by network devices like bridges. The transmission time for a whole message is calculated by converting the message size from bytes to bits (32768 bytes \(\times 8 \text{ bits per byte} = 262144 \text{ bits}\)) and then dividing by the Ethernet speed (10 Mbps). Thus, the transmission time is approximately 26.2144 ms.
When the message is broken into 22 packets, each packet's transmission time is calculated similarly. Since one packet is 1500 bytes, the bit size is 12000 bits\(1500 \text{ bytes} \times 8). Given the network speed, each packet takes 1.2 ms to transmit \(\frac{12000 \text{ bits}}{10 \text{ Mbps}}\).
Finally, we sum up the delays for all packets (1.2 ms per packet \times 22 = 26.4 ms\) and add any additional delays incurred through bridges. In our case, one bridge adds a delay of 1.2 ms, making the total delay 27.6 ms. Therefore, it's clear how packet transmission and network structures contribute to overall network delay.