Q and A, Ch 14.6.2, Ch 15
IS 333
Victor Norman
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.)
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.
Old Slides
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.
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.
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).
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)
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.
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.