Download Part 1 R1 What is the difference between a host and an end system

Part 1
R1
What is the difference between a host and an end system? List the types of end systems? Is a Web server an end system?
A host and an end system are terms that are used interchangeably meaning that a host is an end system.
The types of application programs that run are web browser programs, web server programs, and e-mail
readers and senders. So Yes a web server is an end system.
R3
Why are standards important for protocols?
Protocols are fundamental and necessary to any form of working communication within and
between devices.
Standards are a required knowledge-base. IEEE 802.3 and 802.11 contain info about bandwidth,
frequency, and other insight into the way data moves across these physical mediums. The
evolution of standards is neat to see how good idea's became great idea's; although older
standards that have been completely abandoned wont do you much good to learn, its neat to
see the history of these organizations and companies working together to try and make things
work with a mentality rewarding best idea which could mean a group drops what they have
been working on the last 5 years.
R4
List six access technologies. Classify each one as residential access, company access or mobile access
Cable Modems -> Residential Access
Digital subscriber Line -> Residential Access
Hybrid fiber-coaxial cable -> Residential Access
Ethernet Technologies -> Company Access
Wireless LAN -> Wireless Access
Bluetooth -> Wireless Access
R5
Is HFC transmission rate dedicated or shared among users? Are collisions possible in a downstream HFC channel? Why or Why
not?
HFC transmission is in fact shared among users, it has protocols that are created to limit if not reduce the
possibility of collisions.
R8
What are some physical media that Ethernet can run over?
Ethernet can use twister pair and coaxial.
R9
Dial-up modems, HFC, and DSL are all used for residential access. For each of theses technologies provide a range of transmission
rates and comment on whether the transmission rate is shared or dedicated.
Dial-up is a dedicated transmission that servers 56.6kbps and under. HFC is a shared transmission and the
rate really is dependent on the users on the line. On the other end DSL which is a dedicated transmission
has a range of 768 kbps to 1.5 mbps downstream and 128kbps to 768 kbps upstream.
R11
What advantage does a circuit-switched network have over a packet-switched network? What advantages does TDM have over
FDM in a circuit-switched network?
Circuit-switched networks have a huge overhead that is created to make sure that message is delivered. In
contrast the packet-switched networks are very quick but do not offer the guarantee. TDM allows all the
bandwidth to be used for a short period of time. This is very versatile for burst packet sending.
R12
Why is it said that packet switching employs statistical multiplexing? Contrast statistical multiplexing with the multiplexing that
takes place in TDM.
Statistical Multiplexing is the idea that link capacity is determined dynamically, rather than a
predetermined bandwidth. TDM multiplexing gives all of the bandwidth for a short period of time. This
can be problematic and cause gaps. Statistical Multiplexing fixes this issue by only allowing the time
needed to send each message be used.
Part 2
1.
Other than conventional computers, (i.e. computers such as laptops, personal computers, minicomputers and mainframes and
supercomputers), what other types of devices can appear as nodes in a computer network? (Name at least three).
Three types of conventional computers would be, cell phones, pda’s and according to the book, toasters.
2.
Explain what it means for a communication link to fall into the unguided media category, and list two types of unguided media
links.
In order to be considered unguided media you first cannot be using a solid medium. Two types of
unguided media would be satellite radio waves and terrestrial radio waves.
3.
List the three different types of guided media that are commonly employed as to implement links in computer
networks.
1) Coax wire
2) Optical fiber
3) Twister pair
4.
Define the notions of simplex, half-duplex , and full-duplex transmission.
A simplex transmission is a connection between one sender and one receiver where there is a pair of
wires where the sender has a plus and minus terminals in which data can be transmitted from the sender
to the receiver. In comparison a half duplex allows the sender and receiver to both send and receive data
but only one node can send at a time. Finally the Full-duplex transmission which has two pairs of wires
such that both sides can send and receive messages. The problem with this is that these wires suffer from
Emi so to fix that it would be suggested to use twisted wires instead.
5.
Twisted pairs are used a lot in both data and voice communications
1)
Why does it require a pair of wires to carry a signal from a sender to a receiver?
You use two wires to complete the circuit in order for the current to flow from the sender terminal to
the receiver. Based on a couple factors the receiver can then decode the binary into 1’s and 0’s. The
sender sends information through the negative terminal.
2)
Why are the two wires that carry the signal twister together?
The twisting of wires is done this way to prevent / cut down on the amount of electro-magnetic
interference.
3)
If you pull the telephone wire out of a telephone handset and examine the connect, you will see 4 contacts, suggesting that
the cable to the telephone consists of 4 wires, that is, two twisted pairs. Why are there two twisted pairs instead of one?
In order to communicate bilaterally, the ability for the headset to sender and receive messages you
will need two twisted pairs.
4)
The telephone wiring in residential homes is made up of four pairs of twisted pair, even though a telephone only uses two
of those pairs. What are the two extra twister pairs for?
This allows the residential area to install another phone line without having to rip up the current
line.
6.
What is the difference between RJ-11 and RJ-45 cable?
The difference between these two cables is the amount of twist over a set length. The RJ-45 has more
twists per length then the RJ-11.
7.
Expand the following network acronyms, and state briefly how the concepts denoted by the acronyms differ from each other?
1) LAN – Local Area Network. This is used in residential homes and on school campus to provide
internet/networking services.
2)
WAN – Wide Area Network. This network ranges over a large region, state, country, continent or even
planet.
8.
3) PAN – Personal Area Network. This network ranges a
Explain what Bluetooth is?
very short distance.
Bluetooth is a wireless PAN.
9.
Explain the concept of a network topology, and list four topologies more frequently used in a computer network.
A topology when in context to networking is a pattern of interconnecting nodes. The four types that
popularly used are ring, star, mesh, and bus.
10. Explain what is meant when we say that a link is point to point
Point to point means that messages are sent from point to point until it reaches the destination. In
contrast to broadcast, which is sent to every node in the network.
11. How does a computer network differ from an internetwork?
A computer network is used t connect single computers to each other. In comparison an internetwork
connects various computer networks to one another.
12. What do we mean when we say that a machine is multi-homed?
A machine is considered to be multi-homed if it is a member of more than one network
Term
What is the difference between a host and an end
system? List the types of end systems. Is a web
server an End system?
Term
What is a client program? What is a server program?
Does a server program request and receive services
from a client program?
Term
List 6 access technologies. Classify each one a
residential access, company access, or mobile access.
Definition
There is no difference. Throughout this text, the
words “host” and “end system” are used
interchangeably. End systems include PCs,
workstations, Web servers, mail servers, Internetconnected PDAs, WebTVs, etc.
Definition
A networking program usually has two programs,
each running on a different host, communicating
with each other. The program that initiates the
communication is the client. Typically, the client
program requests and receives services from the
server program.
Definition
1.
Dial-up modem over telephone line:
residential;
2.
DSL over telephone line:
1.
residential or small office;
2.
Cable to HFC: residential;
3.
100 Mbps switched
4.
Ethernet: company;
5.
Wireless LAN: mobile;
6.
Cellular mobile access (for example, WAP):
mobile
Term
Is HFC transmission rate dedicated or shared among
users? Are collisions possible in a downstream HFC
channel? Why?
Term
List the available residential access technologies in
your city. For each type of access provide the
advertised downstream rate, upstream rate and
monthly price.
Term
What is the transmission rate of Ethernet LANs? For
a given transmission rate can each user on the LAN
continuously transmit at that rate?
Term
What are some of the physical media and Ethernet
can run over?
Term
Dial-up modems, HFC, DSL, and FTTH are all user for
redisential access. For each of these access
technologies, provide a range of transmission rate
and comment on whether the transmission rate is
shared or dedicated.
Term
Definition
HFC bandwidth is shared among the users. On the
downstream channel, all packets emanate from a
single source, namely, the head end. Thus, there are
no collisions in the downstream channel.
Definition
1.
2.
3.
4.
dial-up;
DSL;
cable modem;
fiber-to-the-home.
Definition
Ethernet LANs have transmission rates of 10 Mbps,
100 Mbps, 1 Gbps and 10 Gbps. For an X Mbps
Ethernet (where X = 10, 100, 1,000 or 10,000), a user
can continuously transmit at the rate X Mbps if that
user is the only person sending data. If there are
more than one active user, then each user cannot
continuously transmit at X Mbps
Definition
Ethernet most commonly runs over twisted-pair
copper wire and “thin” coaxial cable. It also can run
over fibers optic links and thick coaxial cable.
Definition
Dial up modems: up to 56 Kbps, bandwidth is
dedicated; ISDN: up to 128 kbps, bandwidth is
dedicated; ADSL: downstream channel is .5-8 Mbps,
upstream channel is up to 1 Mbps, bandwidth is
dedicated; HFC, downstream channel is 10-30 Mbps
and upstream channel is usually less than a few
Mbps, bandwidth is shared. FTTH: 2-10Mbps upload;
10-20 Mbps download; bandwidth is not shared.
Definition
Describe the most popular wireless Internet access
technologies today. Compare and contrast them.
Term
What advantage does a circuit-switched network
have over a packet-switched network? What
advantages does TDM have over FDM in a circuitswitched network?
Term
What is it said that packet switching employs
statistical multiplexing? Contrast statistical
multiplexing with the multiplexing that takes places
in TDM.
Term
Suppose there is exactly one packet switch between
a sending host and a receiving host. The
transmission rate between the sending host and the
switch and between the switch and the receiving
host are R1 and R2, respectively. Assuming that the
switch uses store-and-forward packet switching,
what is the end-to-end delay to send a Packet of
length L?
There are two most popular wireless Internet access
technologies today:
a) Wireless LAN: In a wireless LAN, wireless users
transmit/receive packets to/from a base station
(wireless access point) within a radius of few tens of
meters. The base station is typically connected to
the wired Internet and thus serves to connect
wireless users to the wired network.
b) Wide-area wireless access network: In these
systems, packets are transmitted over the same
wireless infrastructure used for cellular telephony,
with the base station thus being managed by a
telecommunications provider. This provides wireless
access to users within a radius of tens of kilometers
of the base station.
Definition
A circuit-switched network can guarantee a certain
amount of end-to-end bandwidth for the duration of
a call. Most packet-switched networks today
(including the Internet) cannot make any end-to-end
guarantees for bandwidth.
Definition
In a packet switched network, the packets from
different sources flowing on a link do not follow any
fixed, pre-defined pattern. In TDM circuit switching,
each host gets the same slot in a revolving TDM
frame.
Definition
At time t0 the sending host begins to transmit. At
time t1 = L/R1, the sending host completes
transmission and the entire packet is received at the
router (no propagation delay). Because the router
has the entire packet at time t1, it can begin to
transmit the packet to the receiving host at time t1.
At time t2 = t1 + L/R2, the router completes
transmission and the entire packet is received at the
receiving host (again, no propagation delay). Thus,
the end-to-end delay is L/R1 + L/R2.
Term
What is the key distinguishing difference between a
tier-1 ISP and a tier-2 ISP?
Term
Suppose users share a 2 Mbps link. Also suppose
each user transmits continuosly at 1 Mbps when
transmitting, but each user transmits only 20
percent of the time.
Definition
A tier-1 ISP connects to all other tier-1 ISPs; a tier-2
ISP connects to only a few of the tier-1 ISPs. Also, a
tier-2 ISP is a customer of one or more tier-1.
Definition
a.Whe na cirvuit switching is used, how many users
can be supported?
2 users can be supported because each user requires
half of the link bandwidth
b.For the remainer of this problem suppose packet
switching is used. Wher will there be essentially no
queuing delay before the link if two or fewer users
transmit at the same time? What will there be a
queuing delay if three users transmit at the same
time?
Since each user requires 1Mbps when
transmitting, if two or fewer users transmit
simultaneously, a maximum of 2Mbps will be
required. Since the available bandwidth of the
shared link is 2Mbps, there will be no queuing delay
before the link. Whereas, if three users transmit
simultaneously, the bandwidth required will be
3Mbps which is more than the available bandwidth
of the shared link. In this case, there will be queuing
delay before the link.
c. Find the probability that a given user is
transmitting
Probability that a given user is transmitting
= 0.2
d. Suppose now there are three users. Find
the probability that at any given time, all three users
are transmitting simultaneously. Find the fraction of
time during which the queue grows.
Probability that all three users are transmitting
simultaneously = (3/3)p^3(1-p)^(3-3) = (0.2)^3 =
0.008. Since the queue grows when all the users are
transmitting, the fraction of time during which the
queue grows (which is equal to the probability that
all three users are transmitting simultaneously) is
0.008.
Term
R16. Consider sending a packet from a source host to
a destination host over a fixed route. List the delay
components in the end-to-end delay. Which of these
delays are constant and which are variable?
Term
R17. Visit the Transmission Versus Propagation
Delay applet at the companion Web site. Among the
rates, propagation delay, and packet sizes available,
finds a combination for which the sender finishes
transmitting before the first bit of the packet
reaches the receiver. Find another combination for
which the first bit of the packet reaches the receiver
before the sender finishes transmitting.
Term
R18. How long does it take a packet of length 1,000
bytes to propagate over a link of distance 2,500 km,
propagation speed 2.5 · 108 m/s, and transmission
rate 2 Mbps? More generally, how long does it take
a packet of length L to propagate over a link of
distance d, propagation speed s, and transmission
rate R bps? Does this delay depend on packet
length? Does this delay depend on transmission
rate?
Term
R19. Suppose Host A wants to send a large file to
Host B. The path from Host A to Host B has three
links, of rates R1 = 500 kbps, R2 = 2 Mbps, and R3 = 1
Mbps.
a.
Assuming no other traffic in the network, what
is the throughput for the file transfer.
a.
Suppose the file is 4 million bytes. Dividing the
file size by the through put, roughly how long will it
take to transfer the file to Host B?
a.
Repeat (a) and (b), but now with R2 reduced to
100 kbps.
Definition
The delay components are processing delays,
transmission delays, propagation delays, and
queuing delays. All of these delays are fixed, except
for the queuing delays, which are variable.
Definition
Java Applet
Definition
10msec; d/s; no; no
Definition
a. 500 kbps
b. 64 seconds
c. 100 kbps; 320 seconds
Term
Suppose end system A wants to send a large file to
end system B. At a very high level, describe how end
system A creates packets from the file. When one of
these packets arrives to a packet switch, what
information in the packet does the switch use to
determine the link onto which the packet is
forwarded? Why is packet switching in the Internet
analogous to driving from one city to another and
asking directions along the way?
Term
R22. List five tasks that a layer can perform. Is it
possible that one (or more) of these tasks could be
performed by two (or more) layers?
Term
R23. What are the five layers in the Internet protocol
stack? What are the principal responsibilities of each
of these layers?
Term
R24. What is an application-layer message? A
transport-layer segment? A network-layer
datagram? A link-layer frame?
Term
R25. Which layers in the Internet protocol stack does
a router process? Which layers does a link-layer
Definition
End system A breaks the large file into chunks. To
each chunk, it adds header generating multiple
packets from the file. The header in each packet
includes the address of the destination: end system
B. The packet switch uses the destination address to
determine the outgoing link. Asking which road to
take is analogous to a packet asking which outgoing
link it should be forwarded on, given the packet’s
address.
Definition
Five generic tasks are error control, flow control,
segmentation and reassembly, multiplexing, and
connection setup. Yes, these tasks can be duplicated
at different layers. For example, error control is
often provided at more than one layer.
Definition
The five layers in the Internet protocol stack are –
from top to bottom – the application layer, the
transport layer, the network layer, the link layer,
and the physical layer. The principal responsibilities
are outlined in Section 1.5.1.
Definition
Application-layer message: data which an
application wants to send and passed onto the
transport layer; transport-layer segment: generated
by the transport layer and encapsulates applicationlayer message with transport layer header; networklayer datagram: encapsulates transport-layer
segment with a network-layer header; linklayer
frame: encapsulates network layer datagram with a
link-layer header.
Definition
Routers process layers 1 through 3. (This is a little
bit of a white lie, as modern routers sometimes act
switch process? Which layers does a host process?
Term
R26. What is the difference between a virus, a
worm, and a Trojan horse?
Term
R27. Describe how a botnet can be created, and how
it can be used for a DDoS attack.
Term
R28. Suppose Alice and Bob are sending packets to
each other over a computer network. Suppose Trudy
positions herself in the network so that she can
capture all the packets sent by Alice and send
whatever she wants to Bob; she can also capture all
as firewalls or caching components, and process
layer four as well.) Link layer switches process layers
1 through 2. Hosts process all five layers.
Definition
a) Virus: Requires some form of human interaction
to spread. Classic example: E-mail viruses.
b) Worms: No user replication needed. Worm in
infected host scans IP addresses and port numbers,
looking for vulnerable processes to infect.
c) Trojan horse: Hidden, devious part of some
otherwise useful software
Definition
Creation of a botnet requires an attacker to find
vulnerability in some application or system (e.g.
exploiting the buffer overflow vulnerability that
might exist in an application). After finding the
vulnerability, the attacker needs to scan for hosts
that are vulnerable. The target is basically to
compromise a series of systems by exploiting that
particular vulnerability. Any system that is part of
the botnet can automatically scan its environment
and propagate by exploiting the vulnerability. An
important property of such botnets is that the
originator of the botnet can remotely control and
issue commands to all the nodes in the botnet.
Hence, it becomes possible for the attacker to issue
a command to all the nodes, that target a single
node (for example, all nodes in the botnet might
be commanded by the attacker to send a TCP SYN
message to the target, which might result in a TCP
SYN flood attack at the target).
Definition
Trudy can pretend to be Bob to Alice (and vice-versa)
and partially or completely modify the message(s)
being sent from Bob to Alice. For example, she can
easily change the phrase “Alice, I owe you $1000” to
“Alice, I owe you $10,000”. Furthermore, Trudy can
the packets sent by Bob and send whatever she
wants to Alice. List some of the malicious things
Trudy can do from this position.
even drop the packets that are being sent by Bob to
Alice (and vise-versa), even if the packets from Bob
to Alice are encrypted.