Download Data Communications

ADVANCED HIGHER COMPUTING
DATA COMMUNICATIONS
Revision Questions
Data Communications – Revision Exercise 1
Questions
1.
Networks can include the following devices:
 repeaters
 bridges
 routers
 gateways.
Select three of these devices and, for each device, describe:
2.
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a.
the function of the device;
b.
an example of an appropriate application;
c.
the highest level at which the device operates according to the ISO OSI model.
A large city hospital, consisting of several buildings, is updating its computer system. It is
planning to install a network of computers that will expand in line with future requirements.
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The network installation will be required to fulfil the following purposes:
 administration including registering new patients as they arrive at the hospital,
controlling out-patient appointments, recording bed availability, etc;
 access to appropriate patient details in the buildings housing the wards and laboratories;
 access by hospital staff to patient details held in remote databases located in the offices
of local doctors;
 the facility in all parts of the hospital for appropriate staff to create and amend patient
records;
 access to the databases held in other hospitals to obtain information on available
transplants.
By constructing an annotated diagram, or otherwise, describe a possible layout of the
planned network, taking into account the hospital's requirements. Discuss the design of your
proposed network with particular reference to security, network applications and standards,
bandwidth requirements and cabling, taking account of links between different
buildings.
3.
Accessing world wide networks, such as the Internet, has become more common in recent
times.
a.
Explain why usage of these networks has increased.
2
b.
Identify and describe two factors which still limit the uptake of such networks by
some users.
2
Total
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Data Communications – Revision Exercise 2
Questions
1.
Protocols at all layers in a protocol hierarchy add overhead (additional data) to the data being
transmitted. For each of the following layers, identify one component of this additional data
and describe its purpose. (You should choose a different component at each layer.)
 Data link layer
 Network layer
 Transport layer
2.
The Aberness Bookshop has decided to open a web site to attract new customers. The site is
to be maintained on the bookshop's own server. Speed of response is thought to be very
important to the popularity of the site. Each book will have its own web page, containing
5000 bytes of text and a compressed image of the book's cover, which takes 15 000 bytes.
3.
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a.
The Internet Service Provider (ISP) that the bookshop uses imposes a maximum packet
size of 1040 bytes, of which 40 bytes are reserved for protocol overheads. The
bookshop's server is connected to the ISP by a 64000 bits per second ISDN link. What
is the minimum time that will elapse between a book's web page being requested from
the server for the first time and it being available at the ISP for onward transmission to
the customer? Justify your calculations.
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b.
It is suggested that shorter times may be possible if the bookshop changes its ISP to
one that offers a 96000 bits per second link. This alternative ISP imposes a maximum
packet length of 600 bytes of which 100 bytes are reserved for protocol overheads.
Calculate the minimum time for a book's web page to be available at this ISP.
2
c.
There are many other sources of delay in network communications apart from the time
for a requested page to reach the ISP. Describe measures that the ISP can take to reduce
the delay.
2
A business with a web site wishes to investigate the possibility of on-line ordering of goods
from its web site. Describe what problems there could be with this development and how
you would advise the business to solve them. Your answer should consider the following
factors.
 Security
 Network failures
 Multiple access
Total
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Advanced Higher Computing
Revision Questions
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Data Communications
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Data Communications – Revision Exercise 3
Questions
One way of classifying protocols is to describe them as providing either a connection-oriented service
or a connectionless service. A protocol providing a connection-oriented service is one in which both
processes involved in the communication maintain state information about the progress of the
communication. A protocol providing a connectionless service maintains no such state information and
merely transmits each message without regard to the outcome of earlier transmissions.
1.
State whether a connection-oriented or a connectionless service is preferable for the
following applications. Justify each answer carefully.
a.
Web page retrieval
1
b.
Electronic mail
1
c.
Telephony
1
d.
Remote weather station monitoring
1
2.
Explain how a connection oriented service can be provided by the transport layer in the case
of a network layer that offers only a connectionless service. Your answer should take
account of the following possibilities.
 packets being lost
 packets being duplicated
 packets being resequenced
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3.
A file transfer protocol is to be developed. Outline the design of such a protocol using a
connectionless transport service. Your answer should make reference to:
 encoding
 flow control
 error recovery
8
Total
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Revision Questions
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Data Communications – Revision Exercise 4
Questions
1.
An advertising agency has decided to install a computer network to allow its artists to share
designs created with an art package and to share a colour graphics printer. In addition to
workstations used for art work, other workstations on the network will be used by secretarial
staff to produce word processed documents which will be printed using a shared laser
printer.
The agency must choose between a client server network and a peer-to-peer network. With
the client server network, all the software and files would be stored on a central hard disk
attached to the file server while the workstations would have no disk storage. With the peerto-peer network, there would be no central disk storage but each workstation would have its
own hard disk storage which could be accessed by any other workstation on the network.
With either type of network, all workstations would be able to access the colour graphics
printer and the laser printer.
a.
b.
2.
3.
Discuss the advantages and disadvantages of each type of network for the
applications described.
4
What are the implications for workstation linkage and bandwidth requirements if the
agency decides to install a client server network?
4
In the ISO OSI model, each layer has its own functions and provides services to the layer
above.
a.
Describe briefly two functions of the transport layer.
2
b.
Describe briefly two services which the transport layer provides to the session layer.
2
A university student cannot find the answer to a programming problem she has encountered
in her project. She decides to use the Internet through her university's computer network to
help find the solution. There is a range of network applications available, which includes:
 Information access
 File transfer
 Electronic mail and
 Remote access
a.
b.
Describe briefly the operation of each of the network applications shown in italics
above.
2
Explain how the student could make use of each application to help overcome her
problem.
4
Total
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Data Communications – Revision Exercise 5
Questions
1.
2.
3.
4.
A large architectural practice, which will use CAD software, anticipates that its architects
will wish to exchange files in which their designs are stored. The architects are based in a
number of offices in different cities. The files may be large. Describe, in detail, how the
transfer could be accomplished using each of the following methods. Your answer should
discuss the advantages and disadvantages of each method and should include reference to
any relevant international standards.
a.
Information access (World Wide Web)
b.
Standard file transfer protocols (FTP)
c.
Electronic mail
d.
Physical transfer of floppy disks.
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If architects in different cities wish to work on the same files simultaneously, none of the
methods in question 1 is entirely satisfactory. Discuss a further method of interconnecting
the offices which will allow simultaneous access to files.
2
Communication protocols in the data link layer of the ISO/OSI model incorporate
mechanisms for detecting transmission errors. Give two examples of mechanisms for
detecting errors. Compare the benefits and limitations of your chosen mechanisms.
2
Local area networks based on a bus topology are likely to adopt different protocols from
those based on a ring topology.
a.
Explain why these topologies use different protocols.
2
b.
At which level in the ISO/OSI model do these differences occur?
1
c.
Briefly discuss the suitability of each topology for a network that controls a number
of industrial robots.
3
Total
22
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Revision Questions
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Data Communications
Data Communications – Revision Exercise 6
Questions
1.
2.
Domestic users of the Internet normally connect their home computers to the network using
a modem and the public telephone network.
a.
Describe two benefits and two limitations of this form of access.
2
b.
Describe an alternative method of connection to the Internet that is appropriate to
domestic or small business users. Describe two advantages of this alternative
method compared to modem access.
4
A large corporation uses a wide area network to link its offices in different cities. The
network is used to transfer files and send electronic mail.
Describe how an electronic mail message might be transferred between offices. Your answer
should explain how the message is transformed as it is passed through each of the following
layers of a protocol hierarchy.

Application

Transport

Network

Data link

Physical
3.
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The corporation mentioned in Q2 above wishes to achieve a high level of security for its
electronic mail. It already ensures physical security and has password control.
Describe an additional technique that provides a high level of security. Include in your
answer the extra information needed by the technique.
4
Total
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Revision Questions
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ADVANCED HIGHER COMPUTING
DATA COMMUNICATIONS
Revision Questions
Answers
Advanced Higher Computing
Revision Questions
8
Data Communications
Data Communications – Revision Exercise 1
Answers
The following are indicators of the level of answer required at advanced higher, where “level” refers to
the concepts, not necessarily the language used.
1.
2 marks for each device, up to 6 marks.
Repeaters
a.
As a signal is propagated along a transmission medium its amplitude decreases as the length
of the cable increases. Normally a limit is set on the length of cable to ensure a reliable
signal. One way to lengthen the usable cable is to use repeaters, inserted at intervals along
the cable to restore the signal to the original level.
b.
Expansion of a network
c.
Physical layer
Bridges
a.
These are simple devices which bridge the gap between two remote LANs. The distance
between the two LANs depends on the bridge's capabilities (many are now able to utilise
public circuits, either using dial-up services or permanent private circuits). The intelligence
of the bridge can vary but usually a bridge will examine the address of each packet of
information on the network. Any locally addressed packets are ignored and all packets with
addresses remotely related to that bridge will be passed across to the remote bridge which
will place them on its network.
b.
LANs in two offices within the one building.
c.
A bridge operates at the data link level of the LAN. It can work without needing to know or
understand any of the higher layers.
Routers
a.
Highly intelligent devices designed to provide single or multiple connections between
WANs and LANs. The types of LAN can be dissimilar and a selective choice can be made
dynamically based on the cheapest and fastest way to route a data packet.
b.
Needed when two networks use the same transport layer but have different network layers.
c.
Network layer.
Gateways
a.
It is a specialised form of access device. It is designed to create access between networked
systems or environments running different protocols. The gateway is concerned with
conversion of incompatible protocols and network applications.
b.
For example a gateway may connect an Ethernet LAN operating- TCP/IP to an X.25 network
c.
A gateway operates at a high level in the OSI model - usually above layer 3 - Application
layer.
Advanced Higher Computing
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Data Communications
2.
4 marks for annotated diagram or description
Example:
Server
Gateway
Admin
Lab
Intensive Care
Wards
2 marks for each of the following:
Software and standards:
The hospital would require e-mail software, network software, all the usual application packages;
 admin - WP ( appointment letters, doctor referrals), database ( patient records);
 lab - spreadsheet(for results), database(labels), graphics package.
It would be sensible for the hospital to purchase equipment from manufacturers that adhere to
recognised standards so that the equipment can be interchangeable with equipment from any other
manufacturer that complies with standards. The international standards bodies concerned with
computer manufacturers are ISO and IEEE. The other major standards body is CCITT, which is
concerned with connecting the equipment to the public network.
The hospital could adopt the TCP/IP protocol suite for the network environment.
The three types of international standards associated with LAN - CSMA/CD bus, token ring and
token bus - mentioned in the standards documents.
Cabling and bandwidth :
The cabling between buildings would normally be fibre optics to link the hub each building hub to a
main central hub. The rest of the cabling could also be fibre optic, since it provides fast and secure
transmission, but this would be expensive and not usual.
The cables used inside the buildings would be dependent on the speed required and the transmitting
distances. Twisted pair would not be very sensible for the hospital because low transmission rates.
Co-axial cable would be the better option for the hospital. Thin co-ax could be used to interconnect
the workstations in the lab and the wards and thick co-ax (which is more rigid) could be used along
the corridors of the hospital.
ISDN could be used via a gateway for WAN communications.
Security
Discuss security aspects - physical and otherwise - taking into account the various staff that will be
using the network. Examples could include firewall software to stop users accessing sensitive data
via WAN access.
Advanced Higher Computing
Revision Questions
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Data Communications
3.a.
1 mark each for any two of the following:
 Economic factors – the cost of equipment required to access the Internet has reduced
dramatically. ISPs now provide low cost access, etc.
 Technical factors – improvements in communications technology such as the speed of modems or
improvements in the quality of cabling in the telephone network have made communications
more easy. Software uses GUI which makes access much more straight forward for the novice
user.
 Social factors – Public acceptability of the use of networks for shopping, banking, etc.
b.
Any two factors, 1 mark for each.
 Organising the information into easily accessible categories can be difficult and users have to
learn a number of procedures before they can search the database successfully. Search engines
give far too many results to be useful.
 There are quite high costs involved - telephone charges, charges by ISP, the cost of the hardware
and software etc.
Total 20
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Revision Questions
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Data Communications
Data Communications – Revision Exercise 2
Answers
The following are indicators of the level of answer required at advanced higher, where “level” refers to
the concepts, not necessarily the language used.
1.
1 mark for name of component, 1 for purpose at each level; up to 6 marks
data link layer: sequence number, acknowledgement, control, CRC or frame check. Sequence
numbering is to detect missing frames; acknowledgement indicates sequence
numbers successfully received at other end; control allows sequence numbers to be
initialised, protocol parameters negotiated, etc; CRC or frame check is used to
detect transmission errors in frame.
network layer: addresses, virtual circuit number
transport layer: sequence numbers, quality of service parameters
2.a.
1 mark for each part, up to 4 marks
Each book requires 20,000 bytes
Hence need to transmit
20*1,040
At 64,000 bits per second
b.
2 marks
This ISP requires 40 packets, 40*600
At 96,000 bits per second
=
=
=
=
20 packets
20,800 bytes
166,400 bits.
2.6 sees
=
=
=
24,000 bytes
192,000 bits
2 seconds
c.
2 marks
An option to allow no graphics, but only ASCII text to be downloaded would save a lot of time and
reduce delays. Caching of web pages at local servers will help too.
3.
2 marks for each, up to 6 marks.
 Security of order, both from point of view of encrypting credit card numbers and of ensuring that
all are genuine is important. Use of encryption or Secure Sockets Layer(SSL) should solve the
problems.
 Network failures are less of a problem if the protocol is stateless, and accumulates orders until
committed. One might wish to confirm orders by e-mail.
 Multiple access is a problem if on-line ordering is updating the stock records of the business –
need to use lockout. Need also to ensure that orders are only taken for items in stock.
Total 20
Advanced Higher Computing
Revision Questions
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Data Communications
Data Communications – Revision Exercise 3
Answers
The following are indicators of the level of answer required at advanced higher, where “level” refers to
the concepts, not necessarily the language used.
1.a.
1 mark
Connection-oriented: web pages with the associated graphics will tend to require a number of packets
of data.
b.
1 mark
Connection-oriented: once the message has been sent it is important that an acknowledgement is
received.
c.
1 mark
Connection-oriented: it is vital that packets arrive in the correct order
d.
1 mark
Connectionless: the stations probably only send in reports once every hour or day.
2.
2 marks for each bullet point.
 Each packet sent by the transport protocol through the network layer should contain sequence
numbers to allow the detection of missing packets at the receiving end.
3.

When a packet is sent a timer is set, and if it expires before an acknowledgement is received, the
packet is identified as lost and must be retransmitted.

The receiver should reply with an acknowledgement of the sequence number just received. If a
packet is lost, it is possible for packets to be received in the wrong order. They then need to be
re-sequenced into the correct order by the receiving end.

The receiver can insist on receiving packets in order, and ignore those that do not have the correct
sequence number. This means that some packets will be duplicated. Alternately it can store
those packets that arrive before their predecessors, and deliver them all to the user when other
packets arrive.
2 marks for each of splitting file, identification of parts, retransmission of missing parts and
timeouts, up to 8 marks.
Encoding: Split the file up into n packets, where packets should be as large as possible. Could also
mention encoding in the sense of character sets used by different machines.
Flow control: Send a header packet that indicates that n packets are to follow, and wait for it to be
acknowledged (if it is not, the receiver will not be expecting packets). If no acknowledgement comes,
try again. Once the initial message is acknowledged, send each packet, individually, with an identifier
which states where in the file it belongs.
Error recovery: After an interval, the receiver can time out and send an acknowledgement of the
packets that have already arrived. On the basis of this, the missing packets can be retransmitted.
Total 20
Advanced Higher Computing
Revision Questions
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Data Communications
Data Communications – Revision Exercise 4
Answers
The following are indicators of the level of answer required at advanced higher, where “level” refers to
the concepts, not necessarily the language used.
1.
½ mark for each advantage /disadvantage related to the application, up to 4 marks
Art - client server



WP - client server
Art - peer-to-peer



WP - peer-to-peer
b.

Advantages
same software,
data files and library of shapes
available to all users regardless
of station;
easier to exchange designs;


Disadvantages
slower to transmit large files of
graphics to / from the file
server and to the colour printer;
expensive hardware required
for the high volume data
transmission;
applications must be headed
delivered from server;
multiple access to application
will slow down network;
cheap hardware requirements as
lower data transfer rate does not
affect performance as with
graphics;

faster transmission of files and
printing as network traffic is
reduced (hard discs);
easier to implement virtual
memory facilities using the
local hard disk to allow very
large graphic layouts;
no real advantages except
secure e-mail facilities;

large graphics files require
large local disk storage;

more difficult to access
resources such as document
files, templates and printing
resources as these are scattered
through the other stations.

1 mark for size of files
Graphics files are far larger than WP files. Typically, graphics files range from a few hundred
kilobytes to many megabytes depending on the resolution and bit depth. WP files are small made up
of ASCII codes and formatting details.
1 mark for topology
A number of network topologies are applicable to a client-server network. The most commonly used
on an Ethernet system will tend to be a star topology.
1 mark for bandwidth
Bandwidth is more important for the graphics files due to their size.
1 mark for physical connection
The physical connection required will be dependent on the distance between nodes and any electrical
interference. For example co-axial cables can be used over longer distances and are less susceptible
to electrical interference than twisted pair.
Advanced Higher Computing
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Data Communications
2.a.
1 mark for each bullet point, up to 2 marks
 establishment of the link
 data transfer phase
 release the data link.
b.
1 mark for each description, up to 2 marks
 establish the transport connections for the session entities;
 provide the data transfer agreed;
 release the connection when the session is complete.
3.a.
½ mark for each
Information access
b.
Large collection of information which can be searched from a machine which
is not permanently linked; information is often formatted and allows linking
from page to page.
File transfer
Involves moving files from one machine to another. The machines are
usually remote from one another and the files are transferred over a network
rather than by exchanging disks.
Electronic mail
Allows the exchange of messages between people. Messages are stored on a
server until the users log on and download their messages.
Remote access
It is often possible to access an intranet remotely to allow access to
information or even to peripherals such as printers.
1 mark for each
Information access
there may be a ready-made solution on a remote web site or newsgroup that
could then be copied and implemented.
File transfer
there may be a ready-made solution on a remote file server - perhaps a set of
library routines or even a full solution.
Electronic mail
Other students may have already completed the task and may be able to
email a solution.
Remote access
While working at home she could access the university server and find some
help files or contact her tutor.
Total 18
Advanced Higher Computing
Revision Questions
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Data Communications
Data Communications – Revision Exercise 5
Answers
The following are indicators of the level of answer required at advanced higher, where “level” refers to
the concepts, not necessarily the language used.
1.
1 mark for description of how transfer is accomplished in each method.
1 mark each for an advantage and disadvantage for each. Maximum of 12 marks.
Information Access - Originator of file must log in to web site space (probably an ISP), and then
upload the file to the site. The availability of the file would then be made known. Those wishing to
use the file then have to access the web site and download their own copy.
Advantages:
simple to arrange,
easy for anybody to take own copy,
no hardware compatibility problems.
Disadvantages: difficult to restrict file to subset of users.
File Transfer Protocols: Originator of file must publicise the availability of file, giving appropriate
passwords to intended recipients. The file remains on the originator's machine. Users must use an
FTP program to log in to the originator's machine, with the user name and passwords supplied, and
transfer their own copy.
Advantages:
simple to arrange,
good for limited circulation list.
Disadvantages: requires recipient to explicitly request file;
may need separate communication of passwords for security purposes;
may be difficult to have different sets of files available for distinct sets of users.
Electronic mail: Using e-mail system, send intended recipients a copy of the file.
Advantages:
secure system, only intended recipients will receive it.
Disadvantages: inefficient for multiple recipients;
older e-mail systems may have difficulty with binary data such as graphics.
Physical transfer of floppy disks: Copy files to floppy disks and send through post.
Advantages:
simple to organise,
very high data accuracy.
Disadvantages: may need large number of disks;
long turnaround time.
2.
2 marks for a correct and full answer.
If simultaneous access to files by multiple users at distributed locations is necessary, a simple network
will need to be set up. This will probably involve leased communication links or ISDN connections,
and a file server or file servers. Workstations can then mount appropriate files on their local file
system.
3.
1 mark each for the following:
 Parity bit, simple to calculate, but fails to catch errors which change an even number of bits.
 CRC check (checksum), slightly more complex to calculate, but much more robust. Catches all
errors shorter than check sequence, and typically 97% of those longer errors. Only errors which
exactly divide the check sequence are missed.
4.a.
1 mark for each characteristic.
 In bus based systems, the signal travels to all stations simultaneously, in both directions.
 Ring based systems have unidirectional signal transmission, and the signal visits each station in
turn, before returning to its sender. This return-to-sender means that it must take active steps to
remove its signal.
b.
1 mark
Layer 2, data link layer
Advanced Higher Computing
Revision Questions
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Data Communications
c.
3 marks
Robots need to be controlled in real time (1). Must be able to guarantee timely communication. This
can be ensured by a priority mechanism (1). Bus based local area networks such as Ethernet do not
support any priority mechanism. Ring based systems such as token ring or FDDI do support priorities
using token passing (1).
Total 22
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Revision Questions
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Data Communications
Data Communications – Revision Exercise 6
Answers
The following are indicators of the level of answer required at advanced higher, where “level” refers to
the concepts, not necessarily the language used.
1.a.
½ mark for each benefit or limitation up to a total of 2
Benefits:
most houses have a telephone, and all Internet service providers allow access using
dial-up connections.
ISPs provide local call numbers.
Limitations:
Bandwidth is limited to that accepted by voice grade circuits over telephone
network (about 56K at maximum, nearer 33K in practice).
Voice grade circuits are very noisy, leading to reduced throughput.
Bandwidth may not be enough to support fast graphic updates or real time video.
b.
2 marks for ISDN and description and 1 mark each for 2 advantages.
Integrated Services Digital Network connection is all digital connection giving two 64K digital
channels, and a 16K signaling channel.
Benefits:
No need for modem and error prone digital-to-analogue conversion.
Higher bandwidth available.
2.
2 marks per layer
Electronic mail is usually transferred from a user to a mail transfer agent (MTA) and then via a mail
transfer system (MTS) to the mail transfer agent on the remote system.
Application layer:
In order to transfer a mail message, the MTA will add header information to the text of the message.
This usually takes the form of text lines with From and To addresses, plus other extra fields such as
time, date, CC line etc. These are usually text lines.
Transport layer:
The MTS opens a connection over the network to the remote MTA through the transport layer. The
transport layer is responsible for taking the message, and splitting it up into packets. Packets have a
large maximum size and can travel through the network independently. Each packet has a header
indicating which transport connection it is part of. At the destination end, the transport layer is
responsible for reassembling the packets to form the original message. If any packets are missing,
their retransmission must be arranged.
Network layer:
Responsible for routing packets through the network. For each packet, the protocol decides at each
intermediate packet switch, to which other packet switch it should be forwarded.
Data link layer:
Responsible for the delivery of packets over a single communications link in the network. To this end
it adds sequence numbers and acknowledgements in a header, and a CRC to check for transmission
errors at the end.
Physical layer:
Needs no headers or extra information. All data passed as a stream of bits, with no delimiters.
Standards needed to agree on signaling conventions.
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Data Communications
3.
4 marks for either method well explained.
Cryptography is an appropriate technique. Each office has access to the same secret key (a code used
for the encryption which is often a 48 or 128 bit number) which is used to transform message.
Without key, the message cannot be transformed back to original text. Needs an appreciation that
headers will still need to be in plain text.
Alternative is to use public key scheme (e.g. PGP) which has public key known to the world at large,
and a secret key known only to the receiving office. Transmitting office uses the receiving office's
public key, and knows that no one can read the message except the correct recipient. The most
important requirement of the use of cryptography is the distribution of keys which must not be
compromised.
Total 20
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Data Communications