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Ethernet: Randomness and Doubling Delay Time for Collision Recovery

Learn why Ethernet uses randomness and doubling delay time to recover from collisions, and the potential for long delays and solutions.

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Ethernet: Randomness and Doubling Delay Time for Collision Recovery

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  1. Q and A, Ch 14.6.2, Ch 15 IS 333 Victor Norman

  2. Exponential Backoff Q: Why does it choose to use randomness and doubling delay time to recover from collisions? A: By doubling the maxbackoff time, the chances of the 2 machines sending again at the same time are lesser and lesser after each collision. Q: Is there a chance that the delay will be very long? (Since the range will be doubled if collision appears.) If there is, is there a solution? A: There is a chance, but it becomes a very very small chance. I don’t know if there is a general solution, or if a solution is required.

  3. Figure 15.2 Q: Can you explain Figure 15.2? A: This illustrates demultiplexing. • An Ethernet packet is received at a NIC. • The NIC accepts the packet because it has the proper destination MAC address in it. • The NIC passes it up to the operating system. • The OS needs to handle it – it could be an IP packet or ARP packet or … • The OS looks at the type field in the Ethernet header and then hands the packet to the proper module to handle the IP or ARP or ... packet.

  4. Logical vs. Physical Topology Q: Can you explain Physical vs. Logical Topology more? A: Sure! As I’ve said, Ethernet assumes all machines are on a bus (coax cable was used first). But, most Ethernet is on twisted pair or fiber now – which does not support the bus paradigm. So, we need devices to make these point-to-point subnetworks act like a single bus network. Hubs and switches do that.

  5. Max/min payload size Q: Can a payload be less than or greater than 46 and 1000 bytes? A: No. The Ethernet packet must be at least 46 bytes and only up to 1500 bytes. Q (from Prof. Norman): How is this enforced? A: The device driver code pads out the payload to 46 bytes, and, (I think), returns an error to the caller if the payload is > 1500 bytes.

  6. Hub / switch Q: Is a “hub” the same as a “switch”? A: No. A hub is a layer 1 device. • Repeats voltages from one port on all the others. A switch is a layer 2 device: • Knows about packets, queues them, retransmits them when there is a collision, learns where hosts are, etc.

  7. Detecting collision Q: The only thing I'm confused about right now is with CSMA/CD. I'm confused how it can actually detect that the signal was interrupted when there is a collision. A: How do you detect when you are interrupted? You hear someone else talking while you are talking. The NIC does the same.

  8. 802.3 today? Q: What networks would use 802.3 Ethernet today? A: The split between IEEE 802.3 and DIX formatting was resolved with 802.3x. From then on, we call all Ethernet 802.3. See https://en.wikipedia.org/wiki/IEEE_802.3.

  9. Payload contents Q: I’m still confused about what is contained in the “payload” of an Ethernet frame? A: It is the data to be carried by the Ethernet frame to the destination machine. For us, almost always IPv4 packets.

  10. Purpose of CRC Q: What is the purpose of the CRC? I read the footnote but am still a little confused. A: CRC is a “hash” of the bits of the message. The sender computes it and appends it to the message. The receiver computes it based on the contents and then compares to the one from the sender. If they disagree, the packet is garbage.

  11. Preamble Q: What is the point of the Ethernet preamble? (64 bits of alternating 0s and 1s). A: I don’t know for sure, but it must be to allow the 2 machines to sync their clocks. (We didn’t look at how the bits are sent, but it has to do with voltages going up and down, which means both endpoints must be in sync.)

  12. Straight vs. Crossover Cables. Q: Why do Ethernet cables have to be crossed when going between switches, and straight when going from a switch to a computer? A: An excellent question… and I wish I understood the answer… There are DTEs (Data Transmission Equipment) and DCEs (Data Communication Equipment). For DTE – DTE and DCE – DCE you have to use crossover.

  13. Old Slides

  14. CSMA/CD Q: In CSMA/CD, when a new computer joins the network, how does it know that CSMA/CD is being used and does it have to wait until the wire is idle to ask for an IP address? A: An ethernet NIC only speaks ethernet on the wire, and ethernet uses CSMA/CD techniques. So, it waits until the “wire” is idle before sending anything.

  15. Collisions Q: When a collision occurs in ethernet is part of the transmission lost? A: The whole transmission is essentially garbage, so, yes, the whole transmission is lost.

  16. Demultiplexing Q: Does the 'type' field in the frame's header indicate whether to multiplex or demultiplex, or is the packet already demultiplexed and the header 'type' indicate which module to use (IP or ARP)? A: The type field is how layer 2 demultiplexesits packets. By checking the type field, it figures out which protocol handler it should call to handle the layer 2 payload (which is the layer 3 header and payload).

  17. Demultiplexing (2) Q: Could you possibly give more detail on how the type field works? I don't understand how it allows multiple simultaneous protocols via multiplexing. A: When a NIC receives a packet for itself, it passes the packet up to software that handles layer 2. That software looks at the type field, so that it knows how the data in the payload (i.e., layer 3) is formatted. According to the value, it knows how to handle the packet. (http://en.wikipedia.org/wiki/EtherType)

  18. Cables for different Ethernets Q: What are the differences in the cables used for 10BaseT, 100BaseT, and 1000BaseT? A: See p. 118 for more info on various cabling types. Some are cheapo wires. Some are shielded better. Some have more wires per connection, etc.

  19. FDM and TDM Qs Q: FDM Qs: 1. How is a sender and receiver pair assigned a particular carrier frequency? How are these tracked? 2. What happens when the light frequencies or sound wavelengths are very close? How does it get filtered? TDM & Statistical TDM 3. What is the round robin order? A: Pairs of nodes communicate on a reserved channel and have a protocol for assigning “slots”. The protocols make sure that frequencies cannot be so close that endpoints can’t distinguish between them. I don’t know anything about TDM/Stat TDM… Sorry.

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