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Ethernets use Manchester encoding. Assuming that hosts sharing the Ethernet are not perfectly synchronized, why does this allow collisions to be detected soon after they occur, without waiting for the CRC at the end of the packet?

Short Answer

Expert verified
Manchester encoding ensures immediate detection of collisions by monitoring for irregular transitions, avoiding the delay of waiting for a CRC.

Step by step solution

01

- Understand Manchester Encoding

Manchester encoding is a method of synchronizing data transmission by encoding the data into a series of transitions. Each bit is represented by a transition, ensuring that there is at least one transition per bit which helps in synchronizing the clocks of the sender and receiver.
02

- Recognize the Purpose of Collision Detection

Collision detection on Ethernet allows the network to identify when two devices are transmitting simultaneously, which causes interference and loss of data. Detecting collisions early helps in minimizing packet loss and ensuring network efficiency.
03

- Manchester Encoding and Collision Detection

Since Manchester encoding ensures transitions are present for every bit, it allows the detection of any irregularities quickly. When a collision occurs, the expected transitions are disrupted, which can be detected almost immediately.
04

- Immediate Detection vs. Waiting for CRC

Waiting for the CRC would mean waiting until the end of the packet to confirm its integrity, which would delay the detection of a collision. Manchester encoding’s constant transitions help in identifying these disruptions early on without needing to wait for the entire packet to be transmitted.
05

- Conclusion

Manchester encoding allows collisions to be detected promptly because any disruption in the expected pattern of transitions indicates a collision. This early detection prevents the need to wait for the CRC at the end of the packet to determine that an error has occurred.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Collision Detection
In an Ethernet network, two devices transmitting data at the same time results in a collision. When a collision occurs, the signals interfere with each other, causing data corruption. Ethernet uses a technique known as Carrier Sense Multiple Access with Collision Detection (CSMA/CD) to manage these collisions.
Manchester encoding plays a key role in collision detection. Since Manchester encoding involves a transition in the signal for every bit, any irregularity in these transitions can be immediately spotted. This helps in identifying collisions as soon as they occur, without needing to wait until the end of the packet. Early collision detection helps in managing the network effectively by initiating a retransmission of data immediately.
Data Synchronization
Data synchronization is essential for ensuring that both the sender and receiver are on the same timeline. Manchester encoding aids in this process by encoding each bit with a transition. This transition occurs in the middle of each bit period, which helps in aligning the clocks of the sender and receiver.
Because there is a transition for every bit, the receiver can correct any timing discrepancies quickly. This constant adjustment helps maintain synchronization and ensures that data is transmitted and received correctly. This is particularly important in environments like Ethernet networks where multiple devices are communicating simultaneously.
Network Efficiency
Efficient use of network resources is crucial for maintaining high performance. Early collision detection, facilitated by Manchester encoding, plays a significant role in this. By detecting collisions as soon as they occur, the network can minimize the time wasted in transmitting corrupted data.
Once a collision is detected, the devices involved can stop their transmission immediately, freeing up the network for other communications. This reduces lag and increases the overall efficiency of the network. Additionally, since Manchester encoding keeps the sender and receiver synchronized, it further contributes to smooth and efficient data transmissions with fewer errors.
Packet Transmission
Packet transmission involves the sending of data in structured units called packets. Each packet contains not only the data being sent but also headers and trailers, which include information for verifying the integrity of the data.
In the context of Ethernet, Manchester encoding ensures that the data within these packets is transmitted reliably. The regular transitions help in instantly identifying any discrepancies, and therefore, detecting collisions early. This means that the integrity of packet transmission is maintained without needing to rely solely on the Cyclic Redundancy Check (CRC) at the end of the packet.
By managing packet transmission efficiently, Manchester encoding helps in maintaining consistent data flow, reducing errors, and ensuring reliable communication across the network.

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Most popular questions from this chapter

What kinds of problems can arise when two hosts on the same Ethernet share the same hardware address? Describe what happens and why that behavior is a problem.

With 1 parity bit we can detect all 1-bit errors. Show that at least one generalization fails, as follows: (a) Show that if messages \(m\) are 8 bits long, then there is no error detection code \(e=e(m)\) of size 2 bits that can detect all 2-bit errors. Hint: Consider the set \(M\) of all 8-bit messages with a single 1 bit; note that any message from \(M\) can be transmuted into any other with a 2 -bit error, and show that some pair of messages \(m_{1}\) and \(m_{2}\) in \(M\) must have the same error code \(e\). (b) Find an \(N\) (not necessarily minimal) such that no 32 -bit error detection code applied to N-bit blocks can detect all errors altering up to 8 bits.

Suppose we want to transmit the message 11001001 and protect it from errors using the CRC polynomial \(x^{3}+1\) (a) Use polynomial long division to determine the message that should be transmitted. (b) Suppose the leftmost bit of the message is inverted due to noise on the transmission link. What is the result of the receiver's CRC calculation? How does the receiver know that an error has occurred?

Suppose that \(N\) Ethernet stations, all trying to send at the same time, require \(N / 2\) slot times to sort out who transmits next. Assuming the average packet size is 5 slot times, express the available bandwidth as a function of \(N\).

Suppose we want to transmit the message 1011001001001011 and protect it from errors using the CRC-8 polynomial \(x^{8}+x^{2}+x^{1}+1\). (a) Use polynomial long division to determine the message that should be transmitted. (b) Suppose the leftmost bit of the message is inverted due to noise on the transmission link. What is the result of the receiver's CRC calculation? How does the receiver know that an error has occurred?

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