Q and A for Ch. 17

1. Q and A for Ch. 17 IS 333, Spring 2014

2. Fiber modems Q: Do fiber modems convert/translate Ethernet packets to optical fiber packets via encapsulation? A: I don’t think so. I think they just forward them over the fiber using the fiber’s physical layer 1 specs. (But, a fiber modem might be a layer 2 device – I’m not sure!)

3. Common LAN extension Q: What is the most common form of LAN extension. Is there a most common form or is it based on circumstances? A: I think the most common form is a switch with twisted pair ports and one fiber port. So you have twisted pair in a building and fiber between buildings.

4. Repeaters Q: So do the repeater make it so that the signal can stay stronger for a longer distance? Does it give the signal as extra push? A: Yes, that is correct. It boosts the signal. Q: How many repeaters can you have before errors can occur? A: I don’t know the answer to that… Hardware stuff…

5. Analog? Q: What exactly is “analog” (what kinds of devices use it) and how does it amplify in repeater devices for long distances? And why do repeaters now use infrared receivers? A: Analog is voltages on a wire – not interpreted into bits or frames. It means it is layer 1. Repeaters simply operate at layer 1. (I don’t think Ethernet repeaters use infrared. The book just uses that as an example.)

6. LAN Segment Q: The book says that a bridge is a mechanism that connects two LAN segments. What is meant by "LAN segments"? Are they together part of one LAN or are they both different LANS? A: LAN segment is collision domain. That is, it is all machines that when they send a packet, no other packet can send at the same time, or there will be a collision. E.g., a bus or hub with many computers connected.

7. Bridge with > 2 ports? Q: Could there exist a bridge that forwards between more than just two hubs? A: Yes! It is called a “switch”. It can have connected to it as many LAN segments as it has ports.

8. Do we have bridges/switches? Q: In our lab, where are these "bridging" devices seen? A: Over in the corner. All the yellow cables go to one switch and all the green/blue cables go to another. Each forms a separate LAN. (We also have a drawer full of hubs/switches, for this Friday’s lab.)

9. Offline computers? Q: A bridge sounds kind of like what we did for the activity we did in class with the switch. How then does a bridge know if a computer is offline? Or is it exactly what we talked about in class since a bridge is associated with a switch? A: A bridge/switch does not know if a computer is offline. But, if it does not hear from a computer for a while, it removes its entry from its forwarding table.

10. Figure 17.4 Q: Can you explain the process in Figure 17.4 (pg.297)? (Why is the bridge involved when packets/frames are being sent within each segment?) A: There is a mistake in the table. The heading “Frame Sent” should be “Frame Seen”.

11. Advantages of a hub? Q: The chapter also seems to paint switches in a very good light. Are there any advantages to using a hub instead of a switch? A: The only advantage I know of is that a hub repeats everything everywhere, so you can use it to snoop packets between other computers.

12. Building connectivity Q: And how are buildings connected by bridges (Is there a third building that hosts the bridging device?)? A: First, “no one” uses bridges anymore (bridges have only 2 ports). Buildings are connected by having a switch in each building and a switch/router in one main building (CIT) to forward packets between all. Or they may have multiple connections, and then use DST.

13. Broadcasting with switches Q: Can you explain how broadcasting with bridges works a little bit more? A: When a bridge/switch receives a broadcast packet, it sends to all (other) ports. (A switch also sends to all other ports when it receives a packet for a host it does not know about.) This behavior allows the switch to emulate the same broadcast domain as a bus/hub.

14. Simultaneous transmission Q: Could you please elaborate on this "A bridge permits simultaneous transmission on each segment" ? A: On a bus/hub topology, only one computer can talk at a time, or you have a collision. With a switch, you only have collision domains on each segment connected to the switch’s ports.

15. Queue overflow Q: Can the queue within a switch grow to be too long and therefore drop packets? A: Yes! This is how you get congestion, increased latency (delay), and dropped packets.

16. Switch vs. VLAN switch Q: When would you want to use a switch rather than a VLAN switch? A: A regular old plain-Jane switch is going to be cheaper than a VLAN switch. A VLAN switch has to be a “managed” switch – something you can log in to and configure. Many cheapo switches aren’t manageable.

17. Repeater speeds Q: It says in the book that a repeater can actually speed up a network by allowing bidirectional communication. However, since the repeater is a another node that a given message needs to pass through, doesn't it slow down each direction it transmits? A: Theoretically, I suppose it does. But, a repeater, at layer 1, is very fast… I might be able to keep up (“operate at line speed”).

18. DST/STP Q: Could you explain the Distributed Spanning Tree? A: Spanning Tree Protocol (STP) is 802.1d. A spanning tree is the set of links that connect all devices, without any loops. Each switch is configured (at the factory) to belong to a special multicast MAC group. The switches send many messages to each other to determine who is connected to whom, and which ports should be put in “backup” state so that a loop is broken.