Question

Ethernet frames must be at least 64 bytes long to ensure that the transmiitter is still going in the event of a collision at the far end of the cable. Fast Ethernet has the same 64-byte minimum frame size but can get the bits out ten times faster. How is it possible to maintain the same minimum frame size?

Short answer

Fast Ethernet uses the same 64-byte minimum frame size to ensure collision detection, despite higher speeds.

Step-by-step solution

Understanding Ethernet Frame Length

The minimum frame length in Ethernet is set to 64 bytes to ensure that the frame is long enough to detect collisions. This length includes both the header and payload portions of the frame.

Explanation of Transmission Speed

In Fast Ethernet, the transmission speed is ten times faster than the original standard Ethernet. This increase in speed means that bits travel the wire more quickly, completing transmission of a single bit in 1/10th the time compared to standard Ethernet.

Reason for Maintaining Minimum Frame Size

Fast Ethernet maintains the same minimum frame size because even though bits are sent faster, they still need to ensure that a signal is present long enough to detect collisions. By keeping the frame size at 64 bytes, Fast Ethernet ensures that a collision will be detected while the frame is still being transmitted.

Analyzing Impact of Cable Length

Considering that Fast Ethernet transmits bits ten times faster, the effect of cable length becomes crucial. Longer cables mean more time for collision detection; however, maintaining the frame at 64 bytes allows cables to stay within the standardized length limits while ensuring collision detection within a frame's transmission time.

Key concepts

Collision Detection

In Ethernet networking, collision detection is a crucial process that helps ensure data is transmitted and received accurately. It refers to the ability of the network to identify when two devices attempt to send data over the same channel simultaneously. This can lead to data packets interfering with one another, known as a collision.
To avoid data corruption and ensure efficiency:

• Networks utilize a method called Carrier Sense Multiple Access with Collision Detection (CSMA/CD).
• This method detects collisions by monitoring the data signal on the network cable.
• When a collision is detected, devices will typically stop transmitting, wait for a random amount of time, and then attempt to retransmit the data.

This process ensures that the network can recover quickly and maintain data integrity. By having an adequate frame length, the system ensures effective collision detection, even at increased transmission speeds.

Fast Ethernet

Fast Ethernet is a version of Ethernet networking that supports faster data transmission rates. Introduced in the mid-1990s, it increased the standard Ethernet speed from 10 megabits per second (Mbps) to 100 Mbps.
This tenfold increase was made possible without altering the basic structure of an Ethernet frame. Key features of Fast Ethernet include:

• Increased speed by increasing the clock rate of the network interface cards and switches.
• Compatibility with the original Ethernet to allow for smooth integration within existing networks.
• Maintaining the same frame size (64 bytes minimum) to ensure interoperability and collision detection.

Fast Ethernet is commonly used because it manages to achieve higher speeds while maintaining backward compatibility with older systems.

Transmission Speed

Transmission speed is a vital concept to understand in the context of Ethernet networks. It refers to how fast data is sent over a network cable.
In Fast Ethernet:

• The transmission speed is increased, allowing data bits to travel the wire much more quickly.
• This is achieved by operating at a frequency ten times higher than that of original Ethernet.
• At this heightened speed, each bit of data completes its journey across the wire much faster.

Despite these changes, the minimum frame size remains crucial. It ensures that the frame's length is sufficient for detecting collisions, even as data travels more quickly.

Minimum Frame Size

The minimum frame size for Ethernet, set at 64 bytes, is a fundamental requirement. This size includes both the Ethernet header and the payload of the frame.
Here's why it is crucial:

• It ensures that frames are long enough for reliable collision detection.
• This requirement stays consistent even when the transmission speed increases, such as in Fast Ethernet.
• Maintaining a frame size of 64 bytes provides enough data time over the wire to ensure that if a collision occurs, it will be detected while the frame is being transmitted.

Thus, this fixed frame size is integral to ensuring the network's collision detection capabilities, regardless of how quickly bits can be transmitted across the network.