Q and A for Chapter 7, 13

1. Q and A for Chapter 7, 13 CS-332 Victor Norman

2. Coaxial Cable Q: What exactly is at the center of a coaxial cable? Is it just one wire in the middle? Is there a circuit? A: (I don’t really know…) But, I think the mesh shielding completes the circuit, besides shielding from radiation. (I don’t know what the book means when it says it is “symmetrical”.)

3. Coaxial Cable (2) Q: Will coaxial cables protect against all noise? What if you literally have a fluorescent light right next to it? Or some type of motor? Could this be a physical way to attack someone--just hide a bunch of fluorescent lights around their network cables? A: I don’t know that much about this. I think they are very good.

4. Twisted Pair Types Q: What differentiates between, say, CAT1 and CAT2? They're both listed as being “Unshielded twisted pair”. A: The different types of cables have different costs. I think they are made of different quality copper, etc. The higher quality has less “attenuation” – the signal degrades less.

5. Copper vs. fiber Q: Which is the most practical/efficient copper wire or fiber optics? A: Copper is much cheaper and much easier to work with – you can splice it and cut it and fix it, etc. You can walk on it. Fiber is more expensive and you have to be careful about kinking it, breaking it, walking on it, etc. But fiber has higher bandwidth, lower signal degradation.

6. Circuit-switching Q: How does multiplexing for circuit switching work? Specifically, how can multiplexing occur if circuit switching is supposed to guarantee an “isolated” point-to-point communication? A: A cable can support only a fixed # of channels. The endpoints manage the allocation of the channels. Each channel gets a frequency (FDM) or time slot (TDM). End-to-end channels form a circuit. Delivery guarantees can be made because the circuit is established before any data is sent.

7. Adv/Disadv of Circuit Switching • fixed # of “channels” and only max are assigned – so when data is available on a channel it can be sent without delay. • Can use high-priority circuits that get multiple channels – so more bandwidth, etc. • If channel is assigned and no data is sent, the bandwidth is unused/wasted. • Uses set-up phase, transmission, and tear-down phase.

8. Packet Switching Q: What exactly is packet-switching aside from just using packets to send the information? A: Packet switching “chunks” up the data, but doesn’t assign channels to packets. The packets are sent when they get to the front of the queue. So, less bandwidth is wasted, but there are not hard guarantees on delays of packets. (In fact, packets can be dropped if way too many arrive at the same time.)

9. Statistical Multiplexing Q: What is statistical about “statistical multiplexing”? A: Statistically speaking, if you have a reasonable amount of traffic on a network, your data will get through “pretty quickly.”

10. Network Topologies Q: What is the best way to determine the network topology to use? Does it depend more on the number of computers or the type of traffic on the network? A: In practical terms for “regular” networks, the star is always used – with some redundant links, often. (For supercomputers, backplane networks, etc. you might use mesh,etc.)

11. More topology questions Q: How do you tell which type of LAN you have? A: Very hard to tell – probably impossible to tell. Q: Could you have multiple topologies in a LAN? E.g., could you have a bus topology for every computer, but then have half the computers be connected in a mesh topology? A: You could, but I’ve never seen that done.

12. Bus topology Q: The book seemed to describe the main disadvantage of BUS technology was that if a wire was cut, the network would go down. Is that the only disadvantage, or are there more disadvantages? A: Another disadvantage is that when you send out a packet, no other machine can send a packet at the same time. (You share a “collision domain”.) And, theoretically any device connected to the wire can see your packet.

13. Star Topology Q: What are the problems with star topologies? A: You have to have a device in the middle to duplicate packets – intelligently or not… That is something else that can go wrong. And, it costs money.

14. Important Point about Ethernet • Ethernet was originally a bus topology – plug multiple machines into one single wire. • Ethernet protocols still assume it is a bus topology – so sending a packet out on an Ethernet means multiple machines could see it. • The protocols do not assume if you send a packet out it goes to one machine at the other end of the wire (star topology).

15. Collisions Q: How do computers on a bus network coordinate to ensure that only one computer sends a signal at a time? What happens if two send at the same time? A: All this and more will be revealed in an upcoming episode.

16. MAC addresses Q: There will never be two identical MAC addresses? A: Correct. All MAC addresses in this universe are unique. Q: Are all MAC addresses the same length? A: Yes: 6 octets (48 bits), written: xx:xx:xx:xx:xx:xx or xx-xx-xx-xx-xx-xx where each x is a hexadecimal digit (representing 4 bits)

17. MAC addresses (2) Q: Is there an organization that knows all the MAC addresses, and thereby would be able to track where each device is? A: Yes. See http://standards.ieee.org/develop/regauth/oui/oui.txt Q: How are these addresses handed out? A: You apply for a range and it is assigned to you… I think.

18. MAC addresses (3) Q: What are the important characteristics and types of MAC addresses? A: There is the broadcast address: all 1s (written with all f’s). And there are some bits in the address that mean stuff, but I don’t care about that stuff.

19. Ethernet Frame Formats • Ethernet contains a header and a payload. • The header is just there to help get the data through. • Header contains dest MAC address, source MAC address, type of data in the payload (but no payload length?) • Data is sent with voltages so certain patterns are reserved to indicate a start of header (SOH), end of transmission (EOT), etc.

20. Byte and bit stuffing Q: Why would you use bit/byte stuffing rather than the header/footer method? A: You use both. Because a transmission is “couched” in these special patterns (SOH, EOT), you have to “escape” these patterns if they appear in the transmission.

21. How are bits sent over a wire/fiber? See 6.16 – Manchester Encoding.