Download B227_2001_ASS2SOLUTIONS

Murdoch University
B227 Data Communications
Assignment 2
Solutions
This assignment has twenty-five questions worth in total 150 marks. Each question has its
value indicated. The assignment, as a whole, is worth 10% of the total marks for the
course.
NB. These questions DO NOT require essay answers. 1-2 sentences will suffice in most
cases.
If the students is not quite on track with the answers then apportion marks accordingly,
the answer must be complete to get full marks.
Question 1.
Explain the difference between TCP and UDP
(4 marks).
 Answer must have both abbreviations correct, User Datagram Protocol
 TCP connection oriented, can use virtual circuits, only first packet addressed rest
hops info only
 UDP every packet addressed can go by different routes, it is connectionless
Question 2.
Explain the difference between active repeaters and passive interfaces used on optic fiber
 Passive interface via light therefore very reliable as if a LED breaks only loose 1
computer not complete ring
 Active repeaters light to electrical signal to light again. If this breaks the ring is
broken
Question 3.
List four different types of connection topology
(2 marks).
 Bus, tree, ring, totally connected, star etc. ½ mark each.
(4 marks).
Question 4.
Give a disadvantage that high frequency ( eg > 8 GHz) wireless transmissions have and
the reason why?
(2 marks).
 Can be absorbed in to rain drops therefore it can be affected by climatic conditions
such as rain and fog.
Question 5.
Explain what a constellation pattern is and its use. If a constellation pattern has 16 points
on it how many bits can this pattern represent?
(6 marks).

16 is 2 to the power of 4 therefore it can represent 4 bits. Used to record the
amplitude and phase shifts of a single cycle of a signal. Also it is the means that these
encoding methods are officially registered.
Question 6.
Draw a diagram of three computers connected together on a 100BaseTX LAN, labelling
all the components required
(8 marks)

Basically a 100 Mbs hub with three Cat 5 UTP cables, RJ45 connectors and each
computer having a NIC. Patch panels and patch leads required for full marks
otherwise 6.
Question 7.
Describe any two of the TCP/IP suite of programs supplied by the application layer.
(4 marks).
 FTP, SMTP, TELNET etc
Question 8.
Explain the Flooding routing algorithm and the mechanisms it employs to stop packets
circulating the subnet forever.
(6 marks)
Packet arrives at a router and is copied (broadcast) out all outgoing lines.
 Hop counts are used to stop packets circulating forever.
Question 9.
Explain the difference between congestion control and flow control(4 marks)
Congestion control is concerned with controlling (reducing/eliminating) congestion when
it occurs. To be able to use congestion control, there must first be congestion.
Flow control is concerned with ensuring that the sender is not sending packets at a faster
rate than the receiver can handle.
Question 10.
Explain in simple terms with the aid of a diagram the Shortest Path routing algorithm and
how the deletion or insertion of a node on a subnet would be dealt with in a static routing
algorithm such as the Flow-based routing algorithm.
(8 marks)
The shortest path algorithm is concerned with finding the shortest path through a subnet.
However the shortest path in this case need not be the number of hops or physical
distance from source to destination, it may be any metric, such as bandwidth, cost, mean
queue length, delay, and many other factors. A graph of the subnet is made, whereby
each node on the graph represents a router and each arc of the graph represents a
communication line (link). The arcs on the graph are weighted according to the factors
affecting it, delay, etc. There are a number of algorithms to help determine the shortest
path where it is not simply distance or number of hops.
In a static routing algorithm such as the flooding algorithm, the insertion or deletion of a
node are not dealt with. The changes are only updated when the routing tables for each
router in the subnet are updated. This is usually done offline and the routing tables are
updated periodically.
Question 11.
Explain the two methods that can be used to control congestion and their side effects.
(6 marks)
The primary congestion control methods are admission control, load shedding, and choke
packets. Congestion control is different to congestion prevention in that to be able to use
congestion control there must first be congestion, whereas congestion prevention is
concerned with stopping congestion from every starting. The primary congestion
prevention method is traffic shaping (leaky and token bucket) which are used in ATM
networks. Therefore traffic shaping is not an acceptable answer for this question.
Load shedding – essentially let all packets in, if the buffers become full then start
dumping packets. Can be configured to understand priorities and selective packet
dumping. Problem: Packets that are dumped must be retransmitted, which can prolong
the overall period of congestion or cause the congestion to spread to alternate routers, etc.
Admission Control – used in virtual circuits. When congestion is signaled no more virtual
circuits can be setup until congestion has cleared. Similar to an overload in the telephone
systems around Christmas, whereby when a switch becomes full it gives no dial tone.
Method 2: allow new virtual circuits but route around congested host. Better method, but
Problem: can spread congestion to surrounding routers. Method 3: an agreement
between host and subnet that specifies the resources required and reserves these resources
on a per-host basis. Problem: wastes bandwidth.
Choke packets – addressed in question 20.
Question 12.
Explain the Exponential Back-off algorithm giving the upper limits of the number of
choices and attempts.
(8 marks)
The exponential back-off algorithm is used in 802.2 LAN’s to control collisions. It uses
1-persistent CSMA/CD, therefore it continually checks the channel until idle, and when
idle it sends the packet immediately. If the packet collides the sender immediately sends a
jamming packet to inform other stations of the collision, thus informing them to abort
transmission. The stations that collided then wait for a period of 2 intervals, i.e. – 0 or 1,
thereby having a 50% probability that it will have to wait 0 or 1 interval. The desired
affect is that one station will transmit immediately (0 periods) and the other will transmit
at interval 1. If the packets collide again the stations must wait 0,1,2,3 intervals before
transmitting again. The general algorithm is (2t – 1), where t is the number of collisions.
After 10 collisions the interval is fixed at 1023 and there is a maximum number of 16
collisions before the process abandons the operation and passes an error message to the
upper layer.
Question 13.
Discuss the “Channel Allocation Problem” mentioning the five assumptions that can be
made and where the IEEE 802.3 standard sits in relation to this model.
(8 marks)
The channel allocation problem describes the problem of allocating a single broadcast
channel among competing users. With only a single channel, it is important to maximize
throughput and distribute access among competing users as fairly and equally as possible.
There are a number of protocols for allocating a single broadcast channel, e.g. – Aloha,
CSMA protocols, etc.
The 5 key assumptions of the channel allocation problem are: 1. Station Model: each station (host) is independent of each other, i.e. No
coordination exists between hosts
2. Single Channel Assumption: a single channel is used for both transmitting and
receiving (unlike Distributed Queue Dual Bus standard for MANs which has a
bus for transmitting and a bus for receiving, or a token ring where transmission is
strictly controlled by a rota)
3. Collision Assumption: if any part of two signals occupy the wire at the same
time a collision has occurred. Only possible errors are caused by collisions and all
stations are capable of detecting collisions
4. Time Assumptions:
a) Continuous Time: No master clock and transmission can begin at any
time.
b) Slotted Time: time divided into discrete portions - slots. Frame
transmission begins at the beginning of a slot. A slot can carry three
different loads
1. frame = idle
2. frame = successful transmission
3. frame = collision
5. Carrier sensing: if a host has the ability to sense if the line is busy or free so
transmission can occur. Two types a) Sense carrier:
b) Can’t sense carrier:
IEEE 802.3 is based on the Collision and Carrier Sensing assumptions.
Question 14.
There is no optimal solution. This solution minimises the distance between any 2 hubs
and any machine and hub, whilst grouping hubs into the same room where applicable to
aid in maintenance. The solutions uses switched hubs throughout, assuming there were no
budgetary constraints. There are sufficient unused ports on each hub for spares, printers,
and expansion.




Central Hub- located in room 15, 75m from the start of the building. 100baseTX
switched hub 8-16port.
Accounting Hub- located in room 4 with the server, 20m from start. 100baseTX
switched hub 16pt.
Sales Hub 1 – located in room 15, 75m from start. 100baseTX switched hub 24pt.
Sales Hub 2 – located in room 28, 140m from start. 100baseTX switched hub
16pt.
Question 15.
Explain what traffic shaping is and what mechanism(s) may be employed to achieve it. (4
marks)
Traffic shaping is a congestion prevention or management mechanism used in ATM
networks. Traffic shaping is used to even out bursty traffic and force packets to be
transmitted at an even rate. Traffic shaping is an attempt to produce an output that is
different from what goes in. Packets tend to arrive in bursts and are output at a constant
and steady rate. The mechanisms used to achieve traffic shaping are the leaky bucket and
token bucket algorithms.
Question 16.
Explain the differences between a bridge and a router. Give an example of where you
might use each of them.
(6 marks)
Bridges are devices that connect 2 or more matching network segments (or LAN’s)
together. It can also be used to split a large LAN into a number of smaller manageable
segments. Bridges operate in the data-link layer and therefore only understand MAC
addresses, not IP addresses. Bridges are store and forward devices, when a packet arrives
its address is examined, it is regenerated at full strength and forwarded to the correct
segment. Any lost packets are retransmitted from the bridge and not the original sender.
Routers are devices that route packets between multiple interconnected networks. They
operate in the network layer and therefore understand IP addresses and differing network
technologies. Routers are store and forward devices that have much more capacity and
capabilities over bridges. However, because they run at a higher level and do more they
generally run at slower speeds. An example of usage would be in a subnet.
Question 17.
What is a virtual circuit? What is the major difference between the use of virtual circuits
and datagrams with respect to addressing?
(8 Marks)
A virtual circuit refers to a connection in a subnet. It is analogous a physical connection
in the telephone network in that the path is fixed for the duration of the call. Virtual
circuits are used primarily in subnets, where the service is connection-oriented. When a
virtual circuit is established, the route between source and destination is chosen as part of
the connection setup and remembered. All traffic flowing over the connection follows
this route. When the connection is released, the virtual circuit is terminated.
Conversely, in datagram subnets no routes are worked out in advance, even for
connection oriented services. All packets are routed independently and may follow
completely different routes.
In terms of addressing, virtual circuits use only sequence numbers to differentiate
packets. There is no need for packets on a virtual circuit to contain either the source or
destination address as the route is predetermined. The drawback is that if a router goes
down on the VC, then the VC is terminated and another VC needs to be setup and the
entire sequence of packets must be resent.
Datagrams contain the full source and destination address of the sender and receiver. This
information is needed as each datagram is routed independently of its predecessors. If the
service is connection oriented and a router goes down, then only unacknowledged
packets need to be resent.
Question 18.
Describe an ARP process and discuss where this process would be used in data
communications.
(8 marks)
Address Resolution Protocol (ARP) is used in LAN’s to resolve MAC addresses from IP
addresses. To send information from one machine to another on the same network we
need both the IP address and the MAC address. This requirement is due to the fact that
Ethernet’s must speak in the native language “Ethernet”. When and IP packet is sent over
an Ethernet, the IP packet is encapsulated in the Ethernet packet until it reaches the
destination, where the IP packet is stripped from the Ethernet packet and passed up to the
IP layer. Ethernet does not understand what IP addresses are and has it’s own addressing
mechanisms, therefore it is essential that the MAC address for the IP address is known.
The MAC address is a unique 48bit number that is contained in the ROM of the NIC.
Given an IP address, ARP is used to resolve the MAC address. ARP uses the
broadcasting feature of the LAN to broadcast an ARP request, asking which machine has
the MAC address for the given IP address. All machines on the LAN hear the request,
and the machine that has the IP address sends back its IP address and MAC address. The
IP address and MAC address are placed in the ARP cache of the machine that made the
request and the machine now has all the information it needs to construct the IP packet,
which is then encapsulated into the Ethernet frame and passed onto the LAN.
Question 19.
Explain how the address [email protected] is resolved to the machine
address.
(6 marks)
Assuming the question is not concerned with the operation of email, we can ignore MX
records and the like. The address babbage.murdoch.edu.au is used as the basis of a DNS
query to first determine the IP address of the host babbage.murdoch.edu.au. An ARP
request is then performed on the IP address (e.g. 134.115.64.18) to obtain the MAC
address of babbage.murdoch.edu.au.
The name part (cole) is only of important to email and has no purpose in determining the
MAC address.
Question 20.
Explain what a choke packet is and how it would be used. Does the hop distance have
any bearing on the efficiency of choke packet use. (6 marks)
Choke packets are a method of congestion control used in both virtual circuit and
datagram subnets. Routers monitor the level of congestion, when present they send choke
packets to the sender that say "slow down". Routers can measure or estimate the level of
congestion by estimating the percentage of buffers in use, line utilisation, or average
queue lengths.
Usage-When a host receives a choke packet, it is required to reduce its traffic to the
destination host by a certain percentage X. Because the host may have already sent more
packets to the destination host prior to receiving the choke packet, the host should ignore
additional choke packets from the destination host for a fixed time interval (long enough
for all sent packets to arrive at the destination host). If after this period another choke
packet arrives then the destination host is still congested, so the host reduces flow until no
more choke packets arrive.
Hop distance does affect the efficiency of choke packet use in the reaction to choke
packets in hosts that are long distances apart is very slow.
Question 21.
What is the purpose of a DNS server? How are they used in a communication process
(6 marks)
The purpose of a DNS server is resolve IP addresses from name addresses (i.e.
murdoch.edu.au). IP is addressing scheme used in TCP/IP, however is far too difficult for
the average human to remember IP addresses, so meaningful name conventions are used
instead. However, in order to communicate over TCP/IP the IP address for the
corresponding name address must first be obtained.
Question 22.
Explain what a collision domain is. How does a switched hub affect the boundaries of a
collision domain.
(8 marks)
A collision domain is a segment of network in which if a collision occurs it may be
heard. It comprises machines, unswitched hubs and any repeaters used.
Figure 22.1: simple network with two collision domains
All 802.3 LAN’s when used without switched hubs (10 and 100base star) are subject to
collisions. Keeping cable between machines and hubs, and hubs to hubs will help to
reduce collisions, as will reducing the amount of traffic on a particular collision domain.
In general, it is important to make collision domains as small as possible, and if
economical use switched hubs throughout (see below).
Switched hubs eliminate broadcasting through internal circuitry that examines the
incoming packet’s destination and sends the packet out on a port that will lead directly to
the destination machine. They also store incoming packets until an acknowledgment is
received from the intended receiver. The ability to store incoming packets means that
switched hubs essentially define collision domains, as any collision that occurs further on
will only require retransmission from switched hub, not the originating machine.
Question 23.
Explain the basic functions of a multi-protocol router. At what layer does it operate?
(6 marks)
Multiprotocol routers (often referred to as gateways) are devices that perform packet
switching between interconnected networks. Multiprotocol routers operate in the network
layer. They are quite often used to connect organisations, i.e. the lines coming in to them
are often owned by different entities. These “Gateways” often are found at political
boundaries and on company LAN boundaries
Operating at the network level gives Multi Protocol Routers increased flexibility
compared to bridges in terms of
1.
Translating addresses between dissimilar networks.
2.
Fragmenting large packets for transmission across networks that carry only small
maximum packet lengths.
3.
Selecting an appropriate path through the subnet.
4.
Enforcing policies (e.g., don't forward any local packets off of this network).
Question 24.
What is an Intranet? How and where would one be normally used? (4 marks)
An intranet is a TCP/IP network that is run on top of an organizations LAN, providing
access to the full range of TCP/IP applications and benefits. The intranet is an interorganisational network that provides TCP/IP services only to machines within the
organizational network protecting it and segregating the network from the public Internet.
Intranets are setup like any other TCP/IP network, after choosing the appropriate class of
network and allocating IP addresses to machines within the intranet. As the network is
not connected to the Internet, IP addresses can be freely allocated to machines in the
network without having to buy IP addresses.
Intranets provide the benefits and features of TCP/IP to the organisation, including the
broad range of TCP/IP applications, the ability to seamlessly share information and files,
the ability to provide organisational information using WWW, video, etc. They paper
waste and reduce overall organizational costs.
Question 25.
This is exactly the same as question 15.