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Q and A, Ch 14.6.2, Ch 15

Q and A, Ch 14.6.2, Ch 15. IS 333 2014. 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?

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Q and A, Ch 14.6.2, Ch 15

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  1. Q and A, Ch 14.6.2, Ch 15 IS 333 2014

  2. 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.

  3. 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.

  4. Q: In the process of using CSMA/CD to send a packet while waiting for the packet to be read can other packets be sent on that medium or is the relationship one to one where it will wait how ever long until that specific packet is ready? A: If two NICs send packets that collide, they both back off. But, while both are idle, another machine could jump in and communicate.

  5. Q: Do NICs only work on computer’s bus? What about other topologies? A: (There is a bus within a computer for communication between the motherboard, hard drives, NIC, etc. But that is something different.) An Ethernet NIC assumes it is connected to a bus.

  6. 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.

  7. Twisted pair vs. Thicknet Q: It looks like using twisted pair Ethernet uses much more wire, so what is the advantage to it if it is essentially (logically) the same as Thicknet? A: Twisted pair is cheap. It has been used for phone systems for many years. It is very flexible, too, so it can be routed through walls, around corners, etc. Thicknet can’t do any of these things.

  8. 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).

  9. 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)

  10. 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.

  11. Backward Compatibility Q: How does backward compatible work? A: NICs that support faster ethernet have to be backward compatible with other slower ethernet (AFAIK). They do “auto-sensing”, and downgrade to the slower speed if they detect NICs on the network that only do the slower speeds. (I don’t know how this actually works.)

  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. 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.

  14. 24-bit OUI with LLC/SNAP Q: What is the 24-bit OUI used for in the IEEE LLC/SNAP header? A: I don’t know that these are related… I’m thinking more about this…

  15. CSMA/CD vs /CA Q: Since CSMA/CD and CSMA/CA operate differently, how do networks reconcile these networks? A: They operate only on how they physically (or physics-ally :-) get the packets between machines. But, they both use MAC addresses and both get carry Ethernet packets that carry IP packets.

  16. 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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