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

2. FDM, TDM, StM Here is what I’d like you to know: If multiple stations share a cable, they have to cooperate. A pair can be assigned a frequency (Frequency Division Multiplexing). – Max # of frequencies, so that receiver can distinguish. Time can be divided into slots and a pair can be assigned a slot. (Time Division Multiplexing) – Max # of slots. – If a slot is not used, it cannot be used for anything else. Statistical Multiplexing: Packets of min/max size queued up and sent when the cable is idle. – No setup phase; no fixed # of pairs; – But, unpredictable delays and possibility of dropping pkts.

3. CSMA/CD CS: carrier sense: listen before you transmit. MA: multi-access: multiple machines share the medium. CD: collision detection: listen while you transmit. – There is also collision avoidance (wifi uses it). Exponential Backoff: how to recover from a collision.

4. Carrier sense Q: How is carrier sense actually implemented? A: A carrier wave or carrier signal (or just carrier) is a waveform (usually sinusoidal) that is modulated with an input signal for the purpose of conveying information. So, the NICs “read” this wave to determine if it is carrying bits of information, and do not transmit if it is.

5. Exponential Backoff Q: Why does it choose to use randomness and doubling delay time to recover from collisions? A: By doubling the max backoff 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.

6. Thicknet, etc. Q: Is thicknet used today? A: No. Q: Why not? A: It is coax cable. It is more expensive and harder to snake through walls, etc.

7. Cables and speeds Q: How does a device know what type of cable is being used (CAT 5 or 6, etc.)? A: NICs nowadays do auto-negotiation. For 10/100 Mbps, only 4 of the 8 individual wires (two pair) in a Cat5e/6 cable are needed. 1000Mbps requires all 8 wires (four pair) to establish a connection. So, if you have the wrong kind of cable hooked up, 1000Mbps will fail. If a flaky/bad cable is used to connect the devices, you usually end up seeing poor performance due to the inability of the cable to correctly carry the electrical pulses. The endpoints will either auto-negotiate to a lower speed or will simply drop many frames.

8. Continued Some switches and devices are getting smart when it comes to detecting these conditions. Some switches will detect frame drop rates, and be able to isolate which wires/pins are having issues (1,2,3,6 or 4,5,7,8). If it can detect issues in this way, the switch may drop carrier and only allow carrier to be auto-negotiated to a slower speed. Wires 1,2,3,6 are required for 10/100Mbps connections, while pins 4,5,7,8 are used with 1000Mbps/PoE. (Thanks to Chris Wieringa for all of this…)

9. 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.https://en.wikipedia.org/wiki/IEEE_802.3

10. 802.3 packet formats Various questions about the IEEE 802.3 formats, etc… An answer from the Intertubes: The whole thing is a terribly complex way to specify the 'next protocol' which is essentially the protocol contained in the ethernet frame. In most networks nowadays the type/length field in the ethernet header directly specifies the next protocol, such as 0x0800 (IP) or 0x0806 (arp). If the type/length field specifies the length instead of the type, then an 802.2 LLC header follows. 802.2 LLC can do many interesting things but basically it is the job of the 802.2 LLC header to then specify the next protocol. But wait, the 802.2 LLC header only specifies IEEE protocols. What if you want to run something non-IEEE? Then the 802.2 LLC header specifies the next protocol is SNAP, and a SNAP header follows the 802.2 LLC header. The SNAP header then FINALLY specifies the 'next protocol'. SNAP is basically an augmentation of 802.2 LLC to allow it to specify many many more 'next protocols' than it is capable of doing otherwise. Like I said, 802.2 LLC can do many different things, including connection-oriented service with acknowledgement at layer 2 (yep! Go look THAT up!). Also, when talking 802.2 LLC, the OSI model data link layer is essentially divided into two sub-layers (LLC on top and MAC below) which you NEVER see in any training or books nowadays, but for the most part it boils down to a complex way to specify next protocol.

11. 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.

12. VLANs and Physical vs. Logical A Virtual LAN really separates the physical topology from the logical topology. Can put machines on one physical LAN (wire) into multiple VLANs. Can put machines on multiple physical LANs into one VLAN. – These physical LANs could even be very far apart geographically.

13. Configuring a VLAN Assigning a machine’s interface to a VLAN depends on physical topology. – Bus: have to configure the machine itself. – Switched (star): Port-based VLANs: assign ports to VLANs and forward only to those in the same VLAN. Or, MAC-based VLANs: assign MACs to VLANs. (Compare adv/disadv. of MAC-based vs. Port-based.) And/or can alter the Ethernet frame, inserting the 802.1q header with the assigned VLAN before forwarding it. (trunking).

14. 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.

15. Straight vs. Crossover Cables. Q: Why are their straight cables and crossover cables? A: There are two kinds of ports: ports on computers and ports on forwarding devices. A computer sends on pins 1,2 and a forwarding device expects to receive on 1,2. (etc). If you connect computer to computer or hub to hub (or switch to switch), you need a crossover. But, most devices auto-detect all this and adapt.

16. Old Slides

17. 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.

18. 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.

19. 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.

20. 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.

21. 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.

22. 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.)

23. 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.

24. 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.

25. 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.

26. 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.

27. 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.

28. 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 demultiplexes its 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).

29. 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)http://en.wikipedia.org/wiki/EtherType

30. 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.

31. 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.)

32. 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.