Q and A for Ch. 14.6.2, 15

Q and A for Ch. 14.6.2, 15 CS-332, 2014, and Victor Norman

Carrier Sense Q: In regard to the “carrier sense”, the chapter states that it “prevents the most obvious collision problems, and substantially improves network utilization” (248-9). It is not stated what the obvious collisions are, so I am wondering if it is just that the station doesn't interrupt a transmission that is in progress? A: That’s all I can think of.

Exponential backoff Q: Does Binary Exponential Backoff occur only when there is a collision between two devices, or also when a device senses network traffic? A: Only when there is a collision. Q: Does the delay a computer waits after a collision always double? A: Possibly. The delay maximum is doubled.

Physical vs. Logical Topology Q: I understand that there is a difference between logical and physical topology in an Ethernet, but why should networking care about physical topology if we can’t tell anything about it from the computer’s standpoint and it only exists in the physical world? A: Physical topology does matter. Consider PPP (point-to-point protocol). If you know you are only connected to one endpoint and that is all you can talk to, you don’t have to label your packets – they will always go to that destination.

Physical vs. Logical Topology (2) Q: Also, in reference to the logical and physical topology of ethernet, I have a question. I get the distinction that the physical topology is considered a bus because the hub emulates a physical cable and so is like a cable that all the computers on the network attach to. But to make it a star topology it just needs a central point of connection, which the hub seems to provide. So it seems that it is physically a star topology as well. A: With a hub/switch, it is a star topology physically. But, it acts like a bus: everything sent to the hub goes out to everything else that is connected.

Physical vs. Logical Topology (3) Q: What is required for a logical star topology? Does the central point need to be a switch and not a hub? A: No, a switch must replicate the behavior of a hub – the Ethernet protocol dictates this. To have a true star logical topology, you need a router. (More on this, in due course.)

NIC vs. LAN Q: What is the relation between a NIC and a LAN? What level does the NIC work on? 2? 3? A: A NIC is what is in the computer that you plug the ethernet cable into. The LAN is all the computers that are in the “local network” – the local “broadcast domain.” (More on this, in due course.) The NIC implements layers 1 and 2.

Straight vs. Crossover Cable Q: Why is there a different type of cable needed to communicated between two switches as opposed to using a straight cable for communication between the computer and the switch? A: Grrr…. I don’t know! I don’t understand! Grrr…. http://docwiki.cisco.com/wiki/Ethernet_Technologies#Network_Cabling-Link_Crossover_Requirements

Straight vs. Crossover Cable (2) Q: Straight vs. Crossed ethernet cables: is this something we need to pay attention to if we plan to build even small networks in the future? How easy is it to tell which is which? A: Theoretically, you do have to pay attention to this. Practically, switches auto-detect for straight/crossover. Or, there is a button to push to fix it if it isn’t working.

CRC Q: What is a cycle redundancy check? A: It is a 32-bit field that follows the ethernet payload. It is there to detect errors. It is computed from the octets of the entire packet.

Demultiplexing Q: At what layer of the network stack does the demultiplexing of the Ethernet frame occur? A: Layer 2 – if we are talking about demultiplexing the different kinds of data being carried by the Ethernet frame.

Demultiplexing (2) Q: A packet has the type field in the header that is used for multiplexing and demultiplexing. Why does a packet need to do those things? I thought those processes were only for combining and separating whole packets. A: An ethernet frame carries a whole packet. The type indicates what layer 3 protocol the payload is.

802.3 (DIX) vs. IEEE Q: In what application is the IEEE’s version of the Ethernet standard used and why is it "better"? A: The IEEE version allows for VLANs – virtual LANs – which allow packets to be tagged as belonging to one or another virtual LANs which might share the same physical LAN.

802.3 (DIX) vs. IEEE (2) Q: The internet claims that neither 802.3 nor DIX spec what to do with an ethernet frame whose type/size field is in the range 1501-1535, which the book doesn't mention. Comment? A: If the packet size is bigger than the spec allows, I think the packet can just be dropped by the NIC…

Hubs and Switches Q: Does a modern switch have some sort of buffer that allows a faster connection to talk to a slower one? Say computer A is connected with a 10baseT and CAT5 and B is connected with a 1000baseT and CAT6. Can the switch cache the message coming from B and transmit it to A or does B just need to talk slower? A: Yes. Switches have buffers. Hubs do not.

Hub Layer Q: What layer/range of layers is the hub considered part of? A: A hub operates at layer 1. But, more on this, in due course…

Backward compatibility Q: In what ways is ethernet limited by backwards compatibility? Is backwards compatibility the reason why ethernet still uses bus logical topology? A: I think it is somewhat limited. E.g., extra kinds of packets have to be allowed for auto-negotiation, etc. But, I would guess that, yes, backward compatibility is the reason why ethernet still uses a logical bus topology.

Circuit switched ethernet? Q: Is Ethernet is only used for packet switching, or can it be used for circuit switching? A: What do you think?

Q and A for Ch. 14.6.2, 15