Q and A for Ch. 7, 13
CS 332
Shielded vs. Unshielded
Q: I'm not sure I understand why you would
leave a twisted pair unshielded. Can you
explain?
A: $
Q: Do you understand the picture on page 116?
A: Yes!
Circuit switching?
Q: Is circuit switching obsolete? If not, where is it
used? (p.222)
A: Telephone uses circuit switching still (I think), or
at least, virtual circuit switching. Circuit switching is
still used: DSL uses it! On my DSL modem, the
“WAN” side shows the VPI/VCI for my connection:
which identifies the virtual path/virtual channel ids
used for the circuit.
Companies can also buy MPLS service from
providers, and this is circuit-switched.
Circuit-switching
Q: How does multiplexing for circuit switching work?
Specifically, how can multiplexing occur if circuit
switching is supposed to guarantee an “isolated” pointto-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). Endto-end channels form a circuit. Delivery guarantees can
be made because the circuit is established before any
data is sent.
See pages 182-188.
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 offer circuits that get multiple channels –
thus, having more bandwidth.
• If channel is assigned and no data is sent, the
bandwidth is unused/wasted.
• Uses set-up phase, transmission, and teardown phase.
Packet Switching Speed
Q: Does the overall speed decrease when
multiple transfers from multiple computers are
trying to send out packets?
A: Yes: because packets get queued up for
processing and forwarding at “intersections” –
routers, switches, etc.
P2P vs. Multi-access
Q: What is the use in distinguishing between
point-to-point and multi-access networks?
A: Point-to-point implies that whatever you
“send down the pipe” gets sent to the other
endpoint, and only that endpoint. So, you don’t
have to label it, contend with others, encrypt it,
etc. Multi-access means you do have to do this
stuff. But, point-to-point is more expensive.
Byte Stuffing
Q: If the payload length is arbitrary, how do you
know that the payload is over and you are
actually seeing the EOT rather than an example
of byte stuffing?
A: EOT will be sent in “plain text”. If the EOT
sequence is in the packet, it will have been
converted.
Mesh topology?
Q: Why would anyone use a mesh topology
network if it's so impractical? Is it more of just a
concept?
A: A mesh is still used within a supercomputer
or similar architecture.
Comparing Topologies
• Bus: can easily plug in new machines, as long as they aren’t
too close together. Only need one wire. But, if wire is cut,
whole network is down. All machines get all transmissions.
Only one machine can talk at once.
• Mesh: very expensive – lots of wire. Lots of interfaces on
each machine. But, contend only with the machine on
other end.
• Star: Contend only with one machine on other end. Affords
more privacy. Requires a device in the middle to forward
packets/signals. Can’t be expanded easily. Can use more
cable.
• Ring: not used much, afaik. Requires 2 interfaces per
machine. If any two links go down, a section is isolated.
Calvin’s network topology
Q: What kind(s) of topology does Calvin use?
A: Calvin uses a star topology. There is only one
main router, and everything else radiates off of
that. And, each of those sub-networks is
completely switched (not routed).
Most companies use some kind of star or web
topology – multiple connections for redundancy.
Hub vs. Switch
Q: In the star topology, is the hub the same
thing as a switch?
A: As you’ll learn in a few weeks, yes, and no. A
hub and switch are not the same. Hubs actually
aren’t used anymore, for the most part. But,
they both do provide essentially the same
functionality.
Kinds of MAC addresses
Q: Are Unicast, Broadcast and Multicast
addresses all types of MAC addresses?
A: Unicast, broadcast, and multicast are all
concepts in sending information. But, yes,
Ethernet MACs have each type of address. But, I
don’t think multicast addressing is used much, if
at all, at the Ethernet layer.
MAC addresses
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)
Note that the broadcast address is all 1s, so is
written:
ff:ff:ff:ff:ff:ff (12 hex digits, where each digit is 4
bits, all 1s.)
MAC addresses (2)
Q: What are the other bits in the “most significant
byte” of an IEEE MAC address (besides the multicast
and global/local bits), and why are they read this
way? Also, why 48 bits/6 bytes?
A: I don’t know if the other bits have significance.
And, multicasting is rarely used, afaik. I don’t know
why they chose 48 bits, but it does make
(theoretically) 248 addresses, which is a lot.
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.
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).
Manchester Encoding
Chapter 6.16.
I’ll explain…
Old Slides
Router <-> ISP
Q: Is the connection between a Router and the
ISP a p2p network?
A: It is going to be a point-to-point link. The ISP
run a fiber/cable to a business and you plug
your/their router into it.
Different cable types
Q: Why don’t people employ CAT7 cables versus
the CAT5 cables that seem so common? Is it
cost?
A: $
Fiber in America
Q: Could you discuss what we are doing (if
anything) in America right now to switch over to
optical fiber?
A: Fiber is what carries most data on the core of the
Internet. Fiber is being pushed out closer and
closer to the home. AT&T Uverse gets fiber to the
neighborhood (FTTN). Verizon in some places offers
Fiber to the Home (FTTH). See Figure 12.8 (page
208).
Coaxial cables
Q: Are coaxial cables a type of shielded twisted
pair cables?
A: See picture in the book. There is one wire
down the middle with a wire mess around it.
Copper vs. fiber
Q: Which is the most practical/efficient copper wire
or fiber optics? Which is mostly used generally and
is preferred, the optical fiber or the copper wire?"
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.
Copper vs. fiber (2)
Q: How widely are fiber optic cables used
compared to copper wires? It seems to me like
most of the time copper wires are used, but I
know that fiber optic cables are used but have
never heard for what besides going very long
distances.
A: Fiber is used when higher bandwidth is
needed – like between buildings on campus.
Communicating with light
Q: How does light carry information across an
optical fiber?
A: Lasers/LEDs turn the light on and off and the
receivers detect it. See 6.16 for an example of
encoding.
Q: Can optical fibers end up with mixed up
signals, since the reflection alters the speed and
energy of the light traveling and eventually
received?
A: Signals do degrade, just as with copper. But, I
think light over fiber degrades much less…
There is lots of physics involved.
Network Topologies
Q: Can you compare network topologies – their
advantages and disadvantages?
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.)
Comparing Topologies
• Bus: can easily plug in new machines, as long as they aren’t
too close together. Only need one wire. But, if wire is cut,
whole network is down. All machines get all transmissions.
Only one machine can talk at once.
• Mesh: very expensive – lots of wire. Lots of interfaces on
each machine. But, contend only with the machine on
other end.
• Star: Contend only with one machine on other end. Affords
more privacy. Requires a device in the middle to forward
packets/signals. Can’t be expanded easily. Can use more
cable.
• Ring: not used much, afaik. Requires 2 interfaces per
machine. If any two links go down, a section is isolated.
New wiring/energy?
Q: What are some other forms of wiring and
energy being explored today?
A: Most everything seems to be all about
wireless – making it faster, traveling farther, or
restricting its reach. Also, satellite internet is
big, in some areas/applications.
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.)
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.”
Byte and bit stuffing (2)
Q: I am confused with how the byte stuffing
works. How is it that by turning SOH into ESC A,
EOT into ESC B, and ESC into ESC C making it
smaller? It seems like it would be bigger because
you are replacing one thing with two now.
A: You must have misread the text. It does make
it bigger. But, that’s the price you pay for clarity.