Advanced Higher Computing
Data Communications
1
a
b
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
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.
i
ii
iii
c
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.
4
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
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.



2
6
Security
Network failures
Multiple access
6
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 message
transmissions.
a
State whether a connection oriented or a connectionless service is preferable for the
following applications. Justify each answer carefully.
i
Web page retrieval
ii
Electronic mail
iii
Telephony
iv
Remote weather station monitoring
4
1
b
c
3
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 may be lost

Packets may be duplicated

Packets may be resequenced
4
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
The (fictional) republic of Sans Serif holds annual referendums to decide on its policy in areas
of national importance. The government wishes to hold these referendums electronically and
has retained your services as a consultant for the design of the communications network. Sans
Serif consists of four islands, Bodoni, Helvetica, Optima and Palatino, situated in the
Informatica Ocean. A map giving the voting population on each island and distance to other
islands is shown below.
Voting in referendums is compulsory. Every member of the population will vote on the island
on which they live. The votes on each island are collected in a communications centre on that
island, and must all be transmitted to the Referendum Count Centre which is situated on
Optima. The transmission of the votes will not start until the polls have closed, and must be
completed within one hour. In 1998, the volume of data associated with each person's votes is
1000 ASCII characters. The votes on Optima are collected at the Referendum Count Centre
itself.
Two designs have been proposed for linking the communications gateways together.
Design A
In this design, the gateway on Palatino is connected to the Helvetica
gateway using a microwave radio link. The Bodoni gateway is linked to the Helvetica
gateway using a fibre optic cable, known in Sans Serif as Type 1. The link from the Helvetica
gateway to the gateway on Optima is a fibre optic cable known in Sans Serif as Type I I.
Design B
In this design, Type I fibre optic cables are used to connect each island
directly to Optima.
A table giving the speed, purchase price and annual running costs of the hardware components
is given below.
2
Microwave transceiver
Optical fibre interface (Type 1)
Optical fibre interface (Type 11)
Optical fibre (Type I)
Optical fibre (Type 11)
a
Speed
(bits per second)
64000
128000
256000
128000
256000
Purchase Price
(Dollars)
10000
12000
15000
100 per km
150 per km
Annual Cost
(Dollars)
200
600
800
0
0
Compare the suitability of the two designs. Your comparison should refer to the following
characteristics of the design:
i
purchase price;
4
ii
annual running costs;
2
iii
reliability.
4
State any assumptions made in your answer.
b
c
4
In 1999, it is anticipated that the number of questions to be decided by referendum will double
the volume of data to be transmitted. The requirement that all the data should reach the
Referendum Count Centre within one hour will remain. For each design, A and B, outlined
above, show that the design would not be adequate for the new situation. Justify your answer.
For each design, specify an appropriate upgrade, and indicate its cost.
6
For each proposed design, A and B, identify possible security weaknesses and describe how
the design can be made more secure. State any assumptions made in your answer.
4
Communication protocols in the data link layer of the ISO/OST model incorporate mechanisms for
detecting transmission errors.
a
b
Give two examples of mechanisms for detecting errors. Compare the benefits and limitations
of your chosen mechanisms. In each case, identify types of transmission errors that the
mechanism fails to detect.
In a stop and watt protocol, the sending process transmits a frame and then waits for an
acknowledgement of that frame before transmitting the following frame. The receiver
transmits an acknowledgement for any frame which arrives without an error.
i
Given that errors could corrupt the frames or the acknowledgements or both, describe
techniques which could be used to ensure that:


ii
c
d
3
a frame which has not been received correctly is retransmitted by the sender; and
the receiver ignores any retransmitted copies of frames that have already been
received without corruption.
2
Using structured English or otherwise, outline an algorithm for the sending process.
6
Modern local area networks have a very low rate of transmission errors. Using stop and wait
protocol, a fixed frame size may give poor performance for large file transfers. Describe a
protocol that gives better performance than stop and wait. justify your answer fully.
3
Local area networks based on a bus topology are likely to adopt different protocols from those
based on a ring topology.
3
5.
a.
i
Explain why these topologies use different protocols.
2
ii
At which level in the ISO/OSI model do these differences occur?
1
iii
Briefly discuss the suitability of each topology for a network that controls a number of
industrial robots.
3
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.
i.
Discuss the advantages and disadvantages of each type of network for the applications
des
6
crib
ed.
ii.
What are the implications for workstation linkage and bandwidth requirements if the
age
4
ncy
deci
des
to
inst
all a
clie
nt
serv
er
net
wor
k?
b.
In the ISO OSI model, each layer has its own functions and provides services to the layer above.
4
i.
Describe briefly three functions of the transport layer.
3
ii.
(ii) Describe briefly two services which the transport layer provides to the session layer.
2
c.
In the ISO OSI model, the data link layer is responsible for the reliable transfer of data across the data link being
used. This involves protocols for flow control and error control.
i.
X-on / X-off and window mechanisms are two common flow control schemes.
5
3
Des
crib
e
eac
h of
thes
e
sch
eme
s.
Wh
ich
sch
eme
wo
uld
be
mor
e
app
ropr
iate
to
con
trol
the
flo
w
of
data
whe
n a
co
mp
uter
is
sen
din
g
cha
ract
ers
to a
prin
ter?
Giv
e a
reas
on
for
you
r
ans
wer
.
6
ii.
In continuous repeat request transmission, selective retransmission and go-back-N are two alternative
procedures for error control.
2
Des
crib
e
ho
w
eac
h of
thes
e
pro
ced
ures
wo
uld
han
dle
an
erro
r
whe
re
fra
me
nu
mb
er
K
in a
seq
uen
ce
of
fra
mes
is
corr
upt
ed.
6.
A university student cannot find the answer to a programming problem she has encountered in her
project. She decides to use the international links provided by the university's computer network to
help find the solution. There is a range of network applications available, which includes: directory
services; bulletin boards; remote databases; and file transfer.
a.
i.
Describe briefly the operation of each of the network applications shown in italics
above.
2
ii.
Explain how the student could make use of each application to help overcome her problem.
4
7
iii.
b.
Identify one other network application and explain how it could be used to help the
student.
2
The student decides to retrieve some information from a remote database. Two client
programs are available, either of which could be used to access the database. The programs
operate in different ways as follows.
Program 1: Every time the user issues a command, this program logs on to the remote
database, retrieves the necessary information and logs off.
Program 2: This program remains continuously connected to the remote database unti 'I all
the student's commands have been processed.
Compare these two methods of operation with regard to:
i.
financial cost;
ii.
network traffic;
iii.
the load on the database server;
iv.
the response time experienced by the student.
8
c.
The student would like to investigate the possibility of completing the project from home. She would like to be
able to connect to the university network and, from there, communicate with people and access computer
resources throughout the world. Also, the connection to the university should allow her to run programs on the
university's minicomputer systems. In addition, she hopes to be able to take part in real-time computer
conferencing, perhaps involving the transmission of digitised video.
8
i.
Describe the communications hardware and software required.
ii.
Describe the advantages and disadvantages to the student of completing the project
fro
4
m
ho
me
in
this
way
.
9
Advanced Higher Computing
Data Communications - Solutions
Q
1.
a.
b.
c.
2.
a.
b.
Marking Scheme

data link layer: sequence number, acknowledgement,
control, CRC or frame check sequence numbering is to
detect missing frames acknowledgement give 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: address(es),
virtual circuit number, age

transport layer: sequence numbers, quality of service
parameters, control fields
i.
Each book requires 20,000 bytes, which is 20 packets.
Hence need to transmit 20*1,040 = 20,800 bytes - 166,400
bits. At 64,000 bits per second = 2.6 sees
ii. This ISP requires 40 packets, 40*600 = 24,000 bytes =
192,000 bits
At 96,000 bits per second = 2 sees
iii An option to allow no graphics, but only ASCII text to be
.
downloaded would save a lot of time, and reduced delay.
Caching of web pages at local servers will help too. Other
answers acceptable
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
this problem
Network failures less of a problem if the protocol is stateless,
and only 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 be sure that once an item is shown as available it remains so
until ordered.
i.
connection oriented (probably need many packets to
retrieve a large web page and its associated graphics)
ii. connection oriented, because we need acknowledgements
that the whole message has been received.
iii connection oriented, since it is vital that packets arrive in
.
the correct order, loss of packets is not so important
iv. connectionless is most appropriate, since the stations
probably only send in reports once every hour or day or ...
Other answers possible, so long as well argued
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. When a packet is sent a
timer is set, and if it expires before an acknowledgement is
received, the packet 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. The receiver can either insist on
receiving packets in order, and ignore those that do not have the
correct sequence number, or it can store those packets that
arrive before their predecessors, and deliver them all to the user
when the missing packet(s) arrive.
Comments
1 mark for name of
component, 1 for
purpose at each level;
6 marks
Outcome
2 a,
c, d
4 marks
2d
2 marks
2d
up to 2 marks, either
or both answers
acceptable
2d
6 marks
2
1 a,
2 a,
d
1 mark each, mostly
for justification
sequence numbers,
acknowledgements,
timers
retransmissions,
buffering of packets
arriving out of order,
2 marks each total: 8
marks
2
10
c.
3.
a.
Encoding: Split the file up into n packets, 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.
The traffic generated by each island is 8*P* 100013600 where p
is the voting population of the island in question, and 8 bits are
assumed for ASCII characters. If 7 bit ASCII is used, all
figures need to be reduced by 7/8!
This gives 125,000 bps from Bodoni, 50.000 from Palatino,
25,000 from Helvetica, and 8,333 from Optima
i.
System A Cost is 2 microwave transceivers, + two 128K
optical units + 5Okm of 128K fibre + two 256K optical
units and 25 km of type II fibre Capital
20,000+24,000+50* 100+30,000+25* 150 = 82750
splitting file,
initialisation,
identification of parts,
timeouts,
retransmission of
missing parts, 2 marks
each up to total of 8
marks
ii.
1 mark each system
system A Running 2*200+2*600+2*800 = 3,200 per
annum system B Running 6*600=3,600
iii System A. The bandwith used on the links from Bodoni
. and Palatino to Helvetica will be the same as the amount
of traffic generated by Bodoni and Palatino (125,000 and
50,000), which is within the capacity of the links proposed.
Ale fibre optic cable from Helvetica to Optima will use
720,000,000/3,600 = 200,000 bits per second. Doesn't
leave much room for expansion of the questions, and the
whole system is cut off if the fibre optic cable fails
2a
Optima is not needed
1 mark for correct
formula, 1 for working,
less 0.5 for each
arithmetic error
2 marks for single
point of failure
identification, 2 marks
for differing reliability
of fibre and
microwave, 2 marks
for bandwidth
limitations, - 1 if no
assumptions made
System B. This uses 125,000 bps on the Bodoni/Optima
link. 50,000
Palatino/Optima and 25,000 Heivetica/Optima fibre optics
cables are used to connect each island directly to Optima.
Each cable has a bandwith of 128,000 bits per second.
Overall, the system is more reliable, since there is no
single point of failure, but the bandwidth of the Bodoni
link is very close to capacity.
11
b.
c.
4.
a.
b.
Design A is not adequate, since it already carries 200,000 bps
and the capacity of the link Helvetica/Optima is only 256,000.
This will need to be augmented with a second 256,000 bps fibre
optic link. Likewise, Bodoni/Helvetica will need to be
upgraded to 256K. Palatino/Helvetica will need upgraded to a
128K link too.
Capital cost of system is 6*256K transmitters, 2*128K
transmitters, 10Okm of 256K fibre and 3Skm of 128K fibre =
6*15,000+2*12,000+100*150+35*100=132,500
Other possible upgrades to design A would include adding a
direct type II link from Bodoni to Optima and dropping the link
between Bodoni and Helvetica, or having two parallel
microwave links between Palatino and Helvetica instead of a
type 1 fibre.
Design B is not adequate, since the Bodoni/Optima link is
almost saturated. It will need to be upgraded to 256K. Other
links are adequate, so cost is 2*256K+4* 128K+70km 256K
fibre + 75km 128K fibre 96,000.
Alternatively, candidates having identified that neither design is
adequate, could specify that data compression might be used to
bring the data transmission rate down. Here, they will still
receive 0,5 marks for each overloaded link, up to 2 in total. 'Re
other marks will be gained from a discussion of data
compression. eg compression ratios are not guaranteed, but
depend on the data being transmitted. ASCII characters are
likely to give a good compression rate. The cost of this option
would be the costs of implementing the software
Microwave links in particular are susceptible to eavesdropping.
It is much more difficult to tap into fibre optics, but probably
still possible. This can be prevented by the use of cryptography.
Votes can be encrypted before the details are transmitted over
the network The major security weakness in both designs is
probably outwith the network we are
designing. Possibility of eavesdropping, and insertion of votes
which are not properly registered at the islands. Best solution to
all these problems is a high integrity cryptographic system.
Parity bit, simple to calculate, but fails to catch errors @vhich
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
sending process should time out, and retransmit the frame
frames should be sequence numbered
ii. sender
seq_no:=1;
get next packet
do forever:
set sequence number in frame from seq_no
send frame and set time out
wait for next event
timeout:
retransmit same frame
set time out
acknowledgement:
if acknowledge = seq_no then
get next packet
seq_no := seq_no+ 1
½ mark for identifying
each overloaded link
maximum 2 marks
2 marks for design of
new system 2 marks
for new costs)
1 mark for
compression, 1 for
discussion and
realisation that its not a
guaranteed solution, 2
marks for costs
1 mark for
eavesdropping, 1 for
cryptography, 1 for
security weaknesses
probably outside
current network. Extra
mark for full answer.
Other relevant
considerations can gain
marks too.
½ mark for each
technique, 2 marks for
discussion
2c
1 mark each
2a
1 mark for
initialisation, 1 mark
for timeout, 1 for
checking acknowledge
seq_no, 1 mark for
advancing seq_no, 2
marks for general
structure
2a
2a
12
c.
A protocol which ought to give better throughput is a blast
protocol. The name of the protocol is not required (they get
marks for the description, even if they have don't know the
name). All frames are transmitted without waiting for
acknowledgements. On completion, the receiver sends an
acknowledgement frame which will identify any frames that had
transmission errors and need retransmission. If error rates are
low, will only have to retransmit a few frames, and hence will
only have to incur the wait for the acknowledgement a few
times.
d.
i.
ii.
iii
.
5.
a.
i.
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.
Layer 2, data link layer
Robots need to be controlled in real time. Must be able to
guarantee timely communication. This can be ensured by a
priority mechanism. Bus based local area networks such
as Ethemet do not support any priority mechanism. Ring
based systems such as token ring or FDDI do support
priorities using token passing. Credit for bus based
network should only be given if it is made clear that it will
support prioritised traffic, eg Token Bus , or possibly
DQDB. Ethemet or CSMAICD do not support a priority
scheme.
Advantages
Disadvantages
Client
centralised network
entering passwords
server
management,
etc adds to startup
maintenance and
overhead; loading up
backup; easier
software from remote
control of printing
server adds startup
resources;
tiine; requires
network manager;
requires more
security through
software for network
passwords and
restricted user access;
2 marks for description,
1 for explanation as to
why better
Candidates might
suggest a sliding
window protocol. 'Mis
would be OK in a
normal full duplex link,
but on a LAN the link
is half duplex, so will
not work.
Give 2 marks if sliding
window fully explained
(ie send up to n frwnes
before waiting for
acknowledgement, n
chosen so that
acknowledgement for a
frame should arrive just
before it is necessary to
stop transmission
1 mark for each
characteristic
2c
accept Media access
layer as well, ½ mark
for physical
1 mark for idea of time
critical communication,
1 for priorities, 1 for
token mechanism
2d
6 x ½ marks for client
server advantages /
disadvantages;
6 x ½ marks for peerto-peer advantages /
disadvantages.
For each network type,
both advantages and
disadvantages must be
mentioned and
maximum of 4 x ½
marks if not related to
application.
2
2d
2d
13
Art - client
server
WP client
server
Peer-topeer
Art - peerto-peer
WP - peerto-peer
ii.
b.
i.
same software, data
files and library of
shapes available to
all users regardless
of station; easier to
access other designs
and send designs to
odiers;
cheap hardware
requirements as
lower data transfer
rate does not affect
performance as with
graphics;
faster loading of
software and data
files; users
determine public
files at each station;
faster transmission
of files and printing
as lower network
traffic; 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;
slower to transmit
large files of graphics
to the file server or
the colour printer;
expensive hardware
required for the high
data transmission
volumes;
applications must be
headed from server;
multiple access to
application will slow
down network;
data and software
integrity lost as each
station is
independent;
resources available to
a user depend on the
station used; expense
on hardware (eg hard
disks at each station);
network management
is in the hands of all
the users;
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
Candidates should answer in terms of the size of the
graphics files relative to the WP files and suitable network
topology. Justification to include discussion of bandwidth
and physical connection.
1 mark for size of files,
1 mark for topology, 1
mark for bandwidth, 1
mark for physical
connection (4 marks).
2
Responses must be in the context of client server.
Esta
blis
hme
nt
of
the
link
-
14
net
wor
k
con
nect
ion,
mul
tiple
xing
or
split
ting
,
sele
ct
best
tran
spor
t
prot
ocol
for
data
size
,
map
tran
spor
t
addr
esse
s
onto
net
wor
k
addr
esse
s;
Dat
a
tran
sfer
pha
se seq
uen
cing
,
bloc
king
,
flo
w
cont
rol,
erro
r
dete
ctio
15
n
and
reco
very
,
con
nect
ion
iden
tific
atio
n;
Release the data link - notifying release.
1 2d
m
ar
k
fo
r
ea
c
h
d
es
cr
ip
ti
o
n
(3
m
ar
ks
).
ii. Two from:
establish the transport connections for the session entities;
provide the data transfer agreed;
release the connection when the session is complete.
c. i.
Xon/
Xoff the
rece
iver
sen
ds a
spec
ial
char
acte
r
(Xoff)
to
the
tran
smit
ter
1 mark for each
description (2 marks).
2d
16
to
ceas
e
tran
smi
ssio
n
unti
l an
over
load
con
diti
on
is
clea
red;
whe
n
the
over
load
con
diti
on
is
clea
red
the
rece
iver
then
sen
ds a
spec
ial
char
acte
r
(Xon)
to
the
tran
smit
ter
to
resu
me
tran
smi
ssio
n.
Win
dow
mec
hani
sms
-
17
the
tran
smit
ter
sen
ds
win
dow
s
(fra
mes
)
and
rece
ives
ack
now
ledg
eme
nts
fro
m
the
rece
iver
; if
an
over
load
con
diti
on
occ
urs
then
ack
now
ledg
eme
nts
are
not
sent
and
the
tran
smit
ter
stop
s
tran
smi
ssio
n
onc
e a
cert
ain
num
ber
18
of
una
ckn
owl
edg
ed
win
dow
s
buil
ds
up;
onc
e
the
over
load
clea
rs
ack
now
ledg
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nts
resu
me
and
the
tran
smit
ter
resu
mes
tran
smi
ssio
n.
X-on/X-off would be more appropriate as the printer cannot sustain the speed of output of the computer and the
controlling device within the printer sends X-on and X-off signals as required. Also characters are indicated
rather than blocks of output which would be grouped as a certain window size.
19
1
m
ar
k
fo
r
d
es
cr
ip
ti
o
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of
X
o
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X
of
f,
1
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cr
ip
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o
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of
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o
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ifi
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2a
20
ii.
4.
a.
i.
ii.
iii
With selective retransmission, the receiver acknowledges
each correct frame; the sender detects that no
acknowledgement has been received for frame K and
retransmits that frame, subsequent frames (K + 1, K + 2
etc) which have already been received are not
retransmitted
With go-back-N, the receiver receives frame K+ 1 out of
sequence and returns a negative acknowledgement (NAK)
indicating that the last correct frame was K-1, it then
discards frame K + 1 and all other frames K + 2 etc
received until frame K is received correctly.
The
transmitter re-transmits all the frames from the last
correctly received ie K - 1
main feature is to allow users to look
Directory
up names based on attributes and map
services
these names onto a network address.
Bulletin boards allow communications between groups
of people. Public messages can be left
in areas which are accessible to a group
of people who share a common interest.
usually large collections of information
Remote
which can be searched from a machine
databases
which is not permanently linked.
involves moving files from one
File transfer
machine to another. The machines are
usually remote from one another and
the files are transferred over a network
rather than by exchanging disks.
student knows the name of someone
Directory
working in a similar field at a different
services
university but doesn't know his/her email address.
Bulletin boards can post a question to a conference /
news group/Special Interest Group
which carries information on the
relevant topic.
look for research articles or textbooks
Remote
on appropriate topic. Perhaps someone
databases
has already solved the problem and
written it up.
there may be a ready made solution on
File transfer
a remote file server - perhaps a set of
library routines or a full solution.
Electronic mail allows the exchange of information
between people. Many e-mail systems
work by storing to a user until that user
next logs on. at which point the
messages could be transferred. He/she
could send a question to someone
knowledgeable in the appropriate area
but who lives in a different time zone.
E-mail is cheaper than phone, faster
than letter and the ‘store until read'
nature of e-mail makes it easier to cope
with different time zones.
Or
1 mark
2a
1 mark
4 x ½ marks for a good
description of each.
1a,
b, d
NOTE FOR AH THE
RANGE IS NOT THE
SAME AS CSYS SO
THIS QUESTION
OUGHT TO BE
ALTERED
AH : Information
access, file transfer,
email, remote access
4 x 1 mark for
explaining how the
student could use the
application.
1a,
b, d
SEE AH NOTE
ABOVE
Other applications, for
example, could be
video conferencing and
mailing lists.
1a,
b, d
½ mark for identifying
another network
application and 1 ½
marks for good
explanation of how it
would help.
21
Chat
application
b.
c.
i.
Financial cost
ii.
Network traffic
iii
Load on server
iv.
Response time
i.
Hardware
Software
could discuss problem on a real time
chat application. Perhaps with fellow
students working on the same problem
or perhaps there is a group of
developers who use the chat application
to discuss problems and swap ideas.
Working collaboratively on the
problem may help the programmer.
if user is paying 'on-line charges' or
telephone charges program 1 will be
cheaper to run. (Note: [most] university
systems work on a fixed charge regardless of the duration of
connections. If any pupil answers in
these terms they should be given
appropriate credit, but should still be
aware of the telephone charges the
student faces.)
will probably be greater with program 1
due to increased overheads of logging
on and off. Also more clients able to
use remote database simultaneously
(see (iii) below) so again more traffic
generated by program 1.
program 1 is only using the server as
required. Resources not tied up during
users 'thinking' and 'typing' time. Server
is therefore used more efficiently
server can deal with more clients in a
given time.
user may notice performance delay
with program 1 compared to
continuous connection due to increased
overheads (see (ii) above). May also
find server prevents log-on if becomes
overloaded between one instruction
from the user and the next.
for basic e-mail / bulletin board access
a modem and standard telephone would
be adequate. Even running remote
programs would be possible with a
slow link. However, for conferencing
(especially if video involved) an ISDN
line and ISDN card for his/her PC
would
probably
be
necessary.
(Dedicated line is unlikely in this
scenario).
again, for e-mail a straightforward
comms package would do.
To run
remote programs you would probably
need some sort of terminal application
and software which would allow the PC
to use the correct protocols (eg TCP/IP,
WINSOCK, SLIP, PPP etc). Also
include network application software
such as the conferencing software.
4 x 2 marks for each
valid comparison
explained in detail.
1
1 mark for description
of hardware and 1 mark
for description of
software.
½ mark available for
poor description.
2c,
1b
22
ii.
Advantages
Disadvantages
student can choose to work at times
which suit him/her best. Save on the
cost and inconvenience of travelling to
and from university. The anonymity of
communicating through e-mail, ie the
student could ask a question and find
that one of the leading experts in the
field answers whereas face to face they
might not take the time.
running cost could be too high for a
student (especially ISDN line). Could
lead
to
feeling
of
isolation.
Teleworking
can lead to people
working longer than they might
otherwise, or if there are too many
distractions (eg family) could make
doing any work difficult.
4 x ½ mark for each
valid and distinct
advantage/disadvantage
. Must be at least one
advantage and one
disadvantage.
2 c,
1b
23