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GCSE · Computer Systems - Networks

P1.5 Networks and Topologies

OCR J277 · Calculators not allowed · about 105 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

A local college has one building in the centre of Sheffield. All of its computers are connected together using cabling installed inside that building.

(a)Define the term Local Area Network (LAN).(1)

(b)Define the term Wide Area Network (WAN).(1)

(c)The college later joins its Sheffield building's network with a second campus in Rotherham, 10 km away, so staff at both sites can share the same files. State, giving a reason, whether the combined network is now a LAN or a WAN.(2)

(Total for Question 1 is 4 marks)

State the piece of network hardware described in each case.

(a)Connects a LAN to another network (such as the internet) and directs data packets between the two networks based on IP address.(1)

(b)Connects multiple wired devices within a LAN and forwards each data frame only to the specific device it is addressed to, using the device's MAC address.(1)

(c)Allows wireless devices to join a wired network by transmitting and receiving data using radio waves.(1)

(d)Hardware, built into or fitted inside a device, that allows the device to connect to a network and is given a unique MAC address by its manufacturer.(1)

(e)Converts digital data from a computer into a signal suitable for transmission over a telephone/broadband line, and converts the incoming signal back into digital data.(1)

(Total for Question 2 is 5 marks)

The diagram below shows part of a small office network.

(a)Identify the network topology shown in the diagram.(1)

(b)Give two advantages of using a star topology rather than a mesh topology to connect the computers in a small office.(2)

(c)Give two disadvantages of using a star topology rather than a mesh topology.(2)

(Total for Question 3 is 5 marks)

A network administrator is connecting 6 computers together in a server room.

(a)Calculate the number of cables needed to connect all 6 computers in a full mesh topology, where every computer has a direct connection to every other computer. Show your working.(2)

(b)State the number of cables needed to connect the same 6 computers using a star topology with one central switch.(1)

(c)Give one reason, other than the cost of cabling, why a school computer network is more likely to use a star topology rather than a full mesh topology.(2)

(Total for Question 4 is 5 marks)

A cafe offers free WiFi to its customers using a single wireless access point (WAP).

(a)State one factor, other than bandwidth, that can affect the performance of this wireless network.(1)

(b)Explain how having many customers' devices connected to the same wireless access point at the same time can affect network performance.(2)

(c)Modern WAPs can broadcast on the 2.4 GHz or the 5 GHz frequency band. Give one advantage of using the 5 GHz band rather than the 2.4 GHz band.(2)

(Total for Question 5 is 5 marks)

A small graphic design studio in Bristol, run by Priya, has 4 employees who each need to share files between their own computers.

(a)Define the term client-server network.(1)

(b)Define the term peer-to-peer network.(1)

(c)State one advantage of a client-server network over a peer-to-peer network.(1)

(d)Recommend, with a justification linked to the studio, which network model Priya's studio should use.(2)

(Total for Question 6 is 5 marks)

A student uploads a photo of size 45 MB to their school's cloud storage over a connection with an upload bandwidth of 9 Mbps. Assume 1 MB = 8 Mb (megabits).

(a)Calculate the size of the photo in megabits (Mb).(2)

(b)Calculate how many seconds the upload will take, assuming the connection runs at the full bandwidth of 9 Mbps.(2)

(Total for Question 7 is 4 marks)

Every device on a network is given an IP address, which is stored and processed in binary.

(a)Convert the 8-bit binary number 11010110 to denary. Show your working.(2)

(b)Convert your answer to part (a) into hexadecimal.(2)

(c)The final octet of an IPv4 address has the denary value 47. Convert 47 to 8-bit binary.(2)

(Total for Question 8 is 6 marks)

Every network interface card is manufactured with a unique MAC address.

(a)State what a MAC address is used for.(1)

(b)Give two ways in which a MAC address is different from an IP address.(2)

(c)A MAC address is 48 bits long and is usually written in hexadecimal. Calculate how many hexadecimal digits are needed to represent 48 bits, and explain your answer.(2)

(Total for Question 9 is 5 marks)

Name the protocol described in each case.

(a)Transfers web page data between a web server and a browser, without encryption.(1)

(b)Transfers web page data between a web server and a browser, using encryption.(1)

(c)Used to transfer files between a client and a server.(1)

(d)Used to send an email from a client to a mail server, or between mail servers.(1)

(e)Used by an email client to download messages and store a copy locally, normally removing them from the server.(1)

(f)Used by an email client to view and synchronise messages that remain stored on the mail server, allowing access from several devices.(1)

(g)The suite of protocols that governs how data is broken into packets, addressed, transmitted, routed and reassembled across the internet.(1)

(Total for Question 10 is 7 marks)

The TCP/IP model organises network communication into four layers.

(a)Starting with the layer closest to the user's application, list the four layers of the TCP/IP model in order.(2)

(b)State the function of the Transport layer.(2)

(c)State the function of the Link layer (also called the Network Access layer).(2)

(Total for Question 11 is 6 marks)

A large video file is sent from a source device to a destination device across the internet.

(a)Describe how packet switching is used to send the file across the internet.(4)

(b)State one advantage of packet switching over sending the whole file as a single continuous, unformatted stream of data.(1)

(Total for Question 12 is 5 marks)

A network technician wants to demonstrate a very simple encryption method before explaining why modern networks do not rely on it.

(a)Encrypt the plaintext ROUTER using a Caesar cipher with a shift of +4 (each letter is replaced by the letter 4 places later in the alphabet).(2)

(b)Explain one weakness of the Caesar cipher as a method of encrypting data sent across a network.(2)

(c)State one advantage of using a modern symmetric encryption algorithm rather than a Caesar cipher to protect data sent over a network.(1)

(Total for Question 13 is 5 marks)

For each scenario, name the type of attack or malware being described.

(a)An employee receives a convincing email, apparently from their bank, asking them to click a link and enter their password on a fake but realistic-looking website.(1)

(b)A school's network becomes extremely slow, then stops responding, after being flooded with an overwhelming number of fake requests sent simultaneously from thousands of different devices.(1)

(c)A user unknowingly installs software that secretly records every key they press, including their online banking password.(1)

(d)State two ways a school could reduce the risk of the attack described in part (b).(2)

(Total for Question 14 is 5 marks)

A secondary school in Leicester is planning a new computer network across its main teaching building, which will connect around 60 classroom computers. The IT manager is deciding between a star topology and a full mesh topology. Discuss the factors the school should consider, and recommend which topology would be more suitable. Justify your recommendation.

(Total for Question 15 is 6 marks)

A user wants to download a game update file of size 3 GB over a broadband connection with a bandwidth of 80 Mbps. Assume 1 GB = 1000 MB and 1 MB = 8 Mb.

(a)Calculate the size of the file in megabits (Mb).(2)

(b)Calculate the minimum time, in seconds, needed to download the file at the full bandwidth of 80 Mbps.(2)

(c)Give the download time found in part (b) in minutes and seconds.(1)

(Total for Question 16 is 5 marks)

A network technician writes the following algorithm, using OCR Exam Reference Language, to count how many port numbers in a list are valid (a valid port number is in the range 0 to 65535 inclusive).

ports <- [21, 80, 443, 3306, 8080, 70000]
count <- 0
FOR i <- 0 TO 5
   IF ports[i] >= 0 AND ports[i] <= 65535 THEN
      count <- count + 1
   ENDIF
NEXT i
OUTPUT count

Complete the trace table for this algorithm. The rows for i = 0 and i = 1 have been completed for you.

(a)Complete the row for i = 2 (ports[i] = 443).(1)

(b)Complete the row for i = 3 (ports[i] = 3306).(1)

(c)Complete the row for i = 4 (ports[i] = 8080).(1)

(d)Complete the row for i = 5 (ports[i] = 70000).(1)

(e)State the value output by the algorithm.(1)

(Total for Question 17 is 5 marks)

An IP address is stored as an array of four integers, each representing one octet, for example octet <- [192, 168, 1, 15].

Write an algorithm, using OCR Exam Reference Language, that:
- uses a loop to check every one of the four octets
- determines whether all four octets are within the valid range 0 to 255 inclusive
- outputs the message "Valid IP" if every octet is within range, or "Invalid IP" if at least one octet is not within range.

Any correct logic will be awarded; ignore minor syntax errors.

(Total for Question 18 is 5 marks)

Some schools use network monitoring software to log and inspect the websites, files and messages accessed by students and staff on the school network. Discuss the security, ethical and legal issues associated with this kind of network monitoring, and evaluate whether you think schools are justified in using it.

(Total for Question 19 is 6 marks)

A section of a company's network design reserves 10 bits to represent the host part of each device's address within a subnet.

(a)Calculate the maximum number of different binary patterns (addresses) that can be represented using 10 bits. Show your working.(2)

(b)State how many of these 1024 addresses can actually be assigned to individual host devices, given that one address identifies the network itself and one is reserved as the broadcast address.(1)

(c)Explain why the network address and the broadcast address cannot be assigned to an individual host device.(2)

(Total for Question 20 is 5 marks)

Mark scheme · P1.5 Networks and Topologies

Question 1

(a) B1 network of devices connected together within a small/single geographical area (e.g. one building or site), usually owned and controlled by one organisation
(a) Answer: A network of devices connected together in one building or a small site, usually owned/managed by a single organisation.
(b) B1 network connecting devices/LANs across a large geographical area, often using infrastructure (e.g. telecoms lines) owned by a third party
(b) Answer: A network that connects devices or LANs over a large geographical area, often using third-party infrastructure.
(c) B1 identifies WAN
(c) B1 valid reason, e.g. it now spans a large geographical area / connects two separate sites using external telecoms infrastructure rather than internal cabling (oe)
(c) Answer: WAN, because it now connects two separate sites over a large geographical area using external infrastructure rather than in-building cabling.

Question 2

(a) B1 router
(a) Answer: Router
(b) B1 switch
(b) Answer: Switch
(c) B1 wireless access point (WAP)
(c) Answer: Wireless Access Point (WAP)
(d) B1 network interface card / controller (NIC)
(d) Answer: Network Interface Card / Controller (NIC)
(e) B1 modem
(e) Answer: Modem

Question 3

(a) B1 star topology
(a) Answer: Star topology
(b) B1 requires far less cabling (one cable per device to the switch), so is cheaper and simpler to install (oe)
(b) B1 easier to add a new device - only one new cable is needed to the switch, rather than a new cable to every existing device (oe)
(b) Answer: Less cabling needed (cheaper/simpler); easier to add new devices.
(c) B1 the network relies on the central switch - if it fails, no devices can communicate with each other (single point of failure)
(c) B1 there is only one path between any two devices (via the switch), so a faulty cable/switch port breaks that connection, whereas a mesh offers alternative routes/redundancy
(c) Answer: Central switch is a single point of failure; only one route between devices, so no redundancy.

Question 4

(a) M1 correct method shown, e.g. 6 x 5 / 2 or a complete systematic count of pairs
(a) A1 15 cao
(a) Answer: 15
(b) B1 6
(b) Answer: 6
(c) B1 valid point, e.g. it is much easier to add, remove or trace a faulty device
(c) B1 developed with detail, e.g. because each device connects independently to the switch, a fault can be traced to a single cable/device without needing to check every connection between every pair of devices as would be needed in a mesh
(c) Answer: Easier to install/manage and to diagnose faults, since each device has one independent connection to the switch rather than many interlinked connections.

Question 5

(a) B1 any one of: interference from other wireless devices/appliances; number of devices connected at once; distance from the access point; physical obstructions such as walls (any one, oe)
(a) Answer: Interference from other devices (or: distance from the WAP, or: number of connected devices, or: physical obstructions).
(b) B1 the available bandwidth is shared between all connected devices
(b) B1 so each device receives a smaller share of the bandwidth, causing slower data transfer/increased latency for every user
(b) Answer: Bandwidth is shared among all connected devices, so as more devices connect, each gets a smaller share, slowing data transfer for everyone.
(c) B1 fewer devices/networks/appliances use the 5 GHz band, so there is less interference/congestion
(c) B1 so a faster and more reliable connection can usually be achieved (though typically over a shorter range than 2.4 GHz)
(c) Answer: Less congestion/interference on 5 GHz, giving a faster, more reliable connection (though with a shorter range).

Question 6

(a) B1 a network in which one or more dedicated server(s) provide services/resources (e.g. files, printing) to other devices, called clients, which request them
(a) Answer: A network where dedicated server(s) provide services/resources that other devices (clients) request and use.
(b) B1 a network where all devices (peers) have equal status and can each act as both client and server, sharing resources/files directly without a dedicated central server
(b) Answer: A network where all connected devices have equal status and share resources directly, without a dedicated server.
(c) B1 any one of: centralised management of security/user accounts/backups; easier to keep data consistent as it is stored in one place; easier to scale to many users (oe)
(c) Answer: Centralised management (e.g. of user accounts, security and backups) is possible from one server.
(d) B1 correct recommendation: peer-to-peer
(d) B1 justification linked to context, e.g. cheaper as no dedicated server/server software/specialist IT staff are needed, and with only 4 computers it is small enough to manage without central control
(d) Answer: Peer-to-peer, because with only 4 computers it is cheap and simple to set up without needing a dedicated server or IT specialist.

Question 7

(a) M1 correct method, 45 x 8
(a) A1 360 Mb cao
(a) Answer: 360 Mb
(b) M1 ft their (a) / 9
(b) A1 40 seconds cao
(b) Answer: 40 seconds

Question 8

(a) M1 correct place-value method shown, e.g. 128+64+16+4+2
(a) A1 214 cao
(a) Answer: 214
(b) M1 ft their (a) divided by 16, showing quotient and remainder
(b) A1 D6 cao (ft)
(b) Answer: D6
(c) M1 correct method shown, e.g. 32+8+4+2+1
(c) A1 00101111 cao (8 bits shown)
(c) Answer: 00101111

Question 9

(a) B1 a unique identifier assigned to a device's network hardware, used to address data to that specific physical device on a local network
(a) Answer: To uniquely identify a device's network hardware so data can be addressed to that specific device on a local network.
(b) B1 a MAC address is fixed/permanent, set by the manufacturer, whereas an IP address can change/be reassigned (e.g. by DHCP)
(b) B1 a MAC address identifies a specific physical device, whereas an IP address identifies a device's logical location/position on a network
(b) Answer: MAC address is permanent and set by the manufacturer, identifying the physical device; IP address can change and identifies the device's logical network location.
(c) M1 correct method, 48 / 4, since each hex digit represents 4 bits
(c) A1 12 hex digits cao
(c) Answer: 12 hex digits

Question 10

(a) B1 HTTP
(a) Answer: HTTP
(b) B1 HTTPS
(b) Answer: HTTPS
(c) B1 FTP
(c) Answer: FTP
(d) B1 SMTP
(d) Answer: SMTP
(e) B1 POP3
(e) Answer: POP3
(f) B1 IMAP
(f) Answer: IMAP
(g) B1 TCP/IP
(g) Answer: TCP/IP

Question 11

(a) B1 all four correct layer names given: Application, Transport, Network (Internet), Link
(a) B1 listed in the correct order (Application first, Link last)
(a) Answer: Application, Transport, Network (Internet), Link
(b) B1 splits data into packets/segments and adds a port number so data reaches the correct application (and reassembles data at the destination)
(b) B1 manages reliable delivery, e.g. checking for errors and re-sending lost packets (TCP), or provides faster unacknowledged delivery (UDP)
(b) Answer: Splits data into packets, adds port numbers, and manages reliable delivery/reassembly of the data (via TCP or UDP).
(c) B1 handles the physical/hardware connection to the network, i.e. how data is actually transmitted over the transmission medium (cable or radio waves)
(c) B1 adds the MAC address so data can be delivered to the correct device on the local network
(c) Answer: Handles the physical transmission of data over the network medium and adds MAC addresses to deliver data to the correct local device.

Question 12

(a) B1 the data is split into small units called packets
(a) B1 each packet is given a header containing information such as the source and destination IP address, and a sequence/packet number and total packet count
(a) B1 packets can travel independently, via different routes, and are directed towards the destination by routers
(a) B1 at the destination, the packets are checked (for errors) and reordered using the sequence number, then reassembled into the original file (any missing/corrupted packets may be re-requested)
(a) Answer: The file is split into packets, each with a header (source/destination IP, sequence number). Packets travel independently via routers, possibly by different routes, and are reordered and reassembled at the destination.
(b) B1 makes efficient use of the network / avoids congestion, as packets can take different available routes, and if one route/link fails packets can be rerouted, making the transfer more resilient
(b) Answer: Packets can be routed around congestion or failed links, making the transfer more efficient and resilient.

Question 13

(a) M1 correct method applied, shifting each letter by 4 places, with at least 4 of the 6 letters correct
(a) A1 VSYXIV cao
(a) Answer: VSYXIV
(b) B1 identifies a weakness, e.g. there are only 25 possible shifts
(b) B1 develops the point, e.g. so an attacker could try every shift very quickly (brute force), or use frequency analysis of letters, to break the cipher without needing the key
(b) Answer: There are only 25 possible shift values, so the cipher can be broken almost instantly by trying every shift (brute force) or by frequency analysis.
(c) B1 uses a much larger key space/more complex key and mathematical process, making it far harder/impractical to break by brute force
(c) Answer: It uses a much larger, more complex key, making brute-force attacks impractical.

Question 14

(a) B1 phishing
(a) Answer: Phishing
(b) B1 distributed denial of service (DDoS) attack
(b) Answer: Distributed Denial of Service (DDoS) attack
(c) B1 keylogger (spyware)
(c) Answer: Keylogger (a type of spyware)
(d) B1 use a firewall to filter/block suspicious incoming traffic
(d) B1 use traffic monitoring, anti-DDoS software, or a third-party filtering/proxy service to detect and absorb unusual spikes in traffic
(d) Answer: Use a firewall to filter suspicious traffic, and use traffic-monitoring/anti-DDoS protection to detect and absorb abnormal traffic spikes.

Question 15

Level 1 (1-2): Basic, mostly one-sided answer. Identifies at most one or two relevant factors (e.g. cost or reliability) with little or no development. Any recommendation given is unsupported or not linked to the school context. Little or no use of correct technical terminology.
Level 2 (3-4): Sound answer that identifies several relevant factors (e.g. cost, cabling, reliability, ease of adding devices, fault-finding) and discusses at least one topology in some detail, though the discussion may be somewhat one-sided or the recommendation only partly justified. Mostly correct use of technical terminology.
Level 3 (5-6): Detailed, balanced discussion comparing star and mesh topologies across multiple relevant factors, with a clear recommendation that is well justified and explicitly linked to the school's context (large number of devices, budget, need for reliability/expansion). Correct and consistent use of technical terminology throughout.
Indicative content:
- Cost/cabling: a star topology needs far less cable (one run per device to a switch) than a full mesh, which would need an impractical number of cables for 60 devices (60x59/2 = 1770 connections), making mesh very expensive and impractical at this scale.
- Reliability/fault tolerance: mesh offers multiple alternative routes between devices, so it can tolerate a cable or device failure without losing the whole network, whereas a star network fails completely if the central switch fails (single point of failure), although a single faulty device/cable in a star only affects that one device.
- Ease of installation and maintenance: a star topology is much simpler and quicker to install, and faults are easier to trace to a single cable/device; a mesh is very complex to wire and maintain, especially as more devices are added.
- Scalability: adding a new device to a star only needs one new cable to the switch, while adding a device to a mesh requires a new cable to every existing device, which becomes impractical very quickly as the network grows.
- Performance/data collisions: a star (with a switch) reduces data collisions since each device has a dedicated link to the switch.
- A balanced answer should weigh these factors against the specific context (60 classroom computers, likely a limited school ICT budget, and the practical impossibility of mesh cabling at this scale) and conclude that a star topology is the more suitable choice, with mesh being impractical for a network of this size.

Question 16

(a) M1 correct method, 3 x 1000 x 8
(a) A1 24000 Mb cao
(a) Answer: 24000 Mb
(b) M1 ft their (a) / 80
(b) A1 300 seconds cao (ft)
(b) Answer: 300 seconds
(c) B1 5 minutes 0 seconds (ft from (b))
(c) Answer: 5 minutes 0 seconds

Question 17

(a) B1 condition = True/T, count = 3
(a) Answer: condition: True, count: 3
(b) B1 condition = True/T, count = 4
(b) Answer: condition: True, count: 4
(c) B1 condition = True/T, count = 5
(c) Answer: condition: True, count: 5
(d) B1 condition = False/F, count = 5 (unchanged)
(d) Answer: condition: False, count: 5
(e) B1 5 (ft from trace)
(e) Answer: 5

Question 18

P1 correctly initialises a flag/boolean variable to True before the loop (e.g. valid <- True); any correct logic, ignore minor syntax
P1 uses a FOR loop that iterates through all four octets (e.g. FOR i <- 0 TO 3); any correct logic, ignore minor syntax
P1 uses a correct IF condition to test whether an octet is out of range, e.g. IF octet[i] < 0 OR octet[i] > 255; any correct logic, ignore minor syntax
P1 sets the flag to False inside the IF when an octet is out of range, with correctly closed ENDIF/NEXT; any correct logic, ignore minor syntax
P1 uses IF/ELSE/ENDIF after the loop to output "Valid IP" when the flag is still True, else outputs "Invalid IP"; any correct logic, ignore minor syntax

Answer:

valid <- True
FOR i <- 0 TO 3
   IF octet[i] < 0 OR octet[i] > 255 THEN
      valid <- False
   ENDIF
NEXT i
IF valid == True THEN
   OUTPUT "Valid IP"
ELSE
   OUTPUT "Invalid IP"
ENDIF

Question 19

Level 1 (1-2): Basic answer that identifies at most one or two relevant issues (e.g. safety or privacy) with little development, and gives no clear or unsupported evaluation. Limited use of relevant terminology.
Level 2 (3-4): Sound answer identifying several relevant security, ethical and/or legal issues, with some development, though the discussion may be one-sided or the final evaluation not clearly linked back to the points raised. Mostly appropriate use of terminology.
Level 3 (5-6): Detailed, balanced discussion covering security, ethical and legal issues, with clear development of each point and a well-reasoned evaluation/conclusion that directly weighs the benefits against the concerns raised. Consistent, appropriate use of terminology throughout.
Indicative content:
- Security/safeguarding benefits: monitoring can help detect cyberbullying, grooming/radicalisation, access to inappropriate content, or malware infections, allowing staff to intervene and protect students.
- Legal considerations: under UK data protection law (Data Protection Act/UK GDPR), any monitoring must be necessary and proportionate, data collected must be relevant, kept securely, and not retained longer than needed; schools usually must inform students/parents that monitoring takes place.
- Ethical/privacy concerns: students and staff have a reasonable expectation of some privacy; constant monitoring could feel intrusive or create a climate of distrust between students and the school.
- Risk of misuse: logged data could be viewed by unauthorised staff, used for purposes beyond safeguarding, or leaked/breached if not stored securely, causing harm if sensitive personal information is exposed.
- Practical limitations: monitoring generates large volumes of data and may produce false positives, requiring careful review, and cannot fully prevent students using personal/unmonitored devices or networks.
- Balanced conclusion: a well-reasoned answer weighs the safeguarding benefits against the privacy/ethical costs, concluding that monitoring can be justified provided it is proportionate, transparent, legally compliant, and data is securely and appropriately handled.

Question 20

(a) M1 correct method, 2¹⁰
(a) A1 1024 cao
(a) Answer: 1024
(b) B1 1022 (ft their (a) - 2)
(b) Answer: 1022
(c) B1 the network address identifies the subnet/network as a whole, not any single device on it, so cannot represent one specific host
(c) B1 the broadcast address is used to send data to every device on the subnet at once, so it cannot also represent one specific individual device
(c) Answer: The network address represents the subnet as a whole rather than a specific device, and the broadcast address is used to send data to all devices at once, so neither can identify a single host.

Networks and Topologies - Worksheets, Questions and Revision