Packet Switching Exam Questions

• A Destination IP address (1)
• The only correct answer is A. A router reads the destination IP address in a packet header to forward the packet towards its destination.
• B is not correct because a MAC address identifies a device on a local network and is not used to route packets across the internet
• C is not correct because the packet number is used at the destination to put the packets back into the correct order
• D is not correct because the source IP address shows where the packet came from, not where it is going

• C To reassemble the packets in the correct order (1)
• The only correct answer is C. The correct answer is To reassemble the packets in the correct order.
• A is not correct because error checking uses the error checking data in the packet, not the packet number
• B is not correct because the destination IP address identifies the destination device, not the packet number
• D is not correct because routers use the destination IP address to choose the route, not the packet number; routers may briefly buffer packets but do not use the packet number to store them

• B Router (1)
• The only correct answer is B. The correct answer is Router.
• A is not correct because a hub sends the data it receives to every device on the network
• C is not correct because a server provides services, such as web pages, to client computers
• D is not correct because a switch forwards data between the devices on one network

• B Sends it on to the next router (1)
• The only correct answer is B. The correct answer is sends it on to the next router.
• A is not correct because a packet is given its number by the sending computer before it is sent
• C is not correct because packets are put back into order by the receiving computer
• D is not correct because the original file is rebuilt by the receiving computer once all the packets arrive

Award one mark for:

• So the packets can be put/reassembled back into the correct order (at the receiving computer/recipient) (1)

Allow 'to show the order of the packets' / 'so the packets can be reordered (by the receiving computer)'

Do not accept 'to number the packets' without a purpose

Do not accept 'to count the packets'

Do not accept 'to identify the packet' on its own

Do not accept 'so packets can be checked/resent' unless linked to putting the packets in order

Do not accept answers about choosing a router or the route

Award one mark for:

• Router (1)

Allow 'wireless router', 'home router', 'border router'.

Allow minor misspellings, e.g. 'routor', 'rooter'.

Do not accept 'gateway' on its own.

Do not accept 'switch', 'hub' or 'modem'.

Award one mark for any of the following up to a maximum of two marks:

• A block/unit/chunk of data (1)
• A piece of a file/message (1)
• Sent/transmitted across a network/the internet (1)
• Contains a header with the destination/source IP address (as well as the data) (1)
• Can be routed/forwarded by routers using the destination IP address (1)

Allow 'unit' on its own for the first mark

Do not accept 'data' on its own

Do not accept a header/destination IP address on its own without identifying a unit/block of data

Do not accept 'a connection/cable between computers'

Do not accept MAC address for the addressing point

Award one mark for any of the following up to a maximum of two marks:

• A unique number/identifier assigned to a device on a network/the internet (1)
• Identifies the destination (or source) of data packets (1)
• Used by routers to forward/route packets towards their destination (1)

Allow 'so that data can be sent to the device' for the second or third mark, but award it once only.

Do not accept MAC address/hardware address (it identifies the network interface, not a device's location on the internet)

Award up to two marks for a linked description such as:

• The header contains the destination IP address (1) so routers know where to send/forward the packet (1)
• The header contains the source IP address (1) so the destination computer knows which device sent the packet/can reply to it (1)
• The header contains the packet number/sequence number (1) so the packets can be put back into the correct order/reassembled at the destination (1)
• The header contains error checking data (1) so the destination computer can check whether the packet was corrupted (1)

Allow 'order number' for packet number.

Allow 'so the file/data can be reassembled' as an alternative to putting the packets back into the correct order.

Do not accept 'the address' without saying that it is an IP address or whether it is the destination/source address.

Do not accept MAC addresses in place of IP addresses.

Do not award credit for descriptions of the data/payload or of the file being sent as a single unit.

Award up to two marks for a linked description such as:

• The packets are put back into the correct order and joined together (1) using the packet/sequence numbers in their headers (1)
• Each packet is checked for errors (1) using the error checking data in its header (1)
• Corrupt/damaged packets are requested to be resent (1) so that the original file can be rebuilt correctly (1)

Allow 'header/packet header' and 'sequence/packet number'.

Allow 'reassembled/rebuilt/combined' as other ways of saying the packets are put back together.

Award the second mark only for a development that says how this is done or what it leads to; 'checked for errors' on its own earns one mark only.

Do not award a second mark for an answer that only repeats the first point, e.g. 'the packets are put back together to make the file'.

Do not accept responses that say a router/the network/the internet reorders, checks or reassembles the packets.

Do not award credit for descriptions of how the packets are routed to the destination, or for other contents of a header such as the destination IP address.

Award up to two marks for a linked description such as:

• Each IP address is unique (1) so data is sent to the correct device and not to any other device (1)
• The IP address identifies the device on the network (1) so that routers can forward packets towards it (1)
• The IP address shows which network a device is on (1) so routers can direct packets towards that network (1)
• A device needs an IP address (1) to send and receive data across the internet (1)

Do not accept MAC addresses or 'physical address'.

Do not award credit for responses about how an IP address is assigned.

Award up to two marks for a linked description such as:

• Routers connect/join different networks together (1) so packets can pass from one network to another on the way to the destination (1)
• A router receives a packet (1) and forwards it to the next router on the route to the destination (1)
• Routers read the destination IP address in the packet header (1) to decide where to send the packet next (1)
• Routers use a routing table (1) to choose the best/least congested route for each packet (1)
• Packets are passed from router to router (1) until they reach the network of the destination device (1)

Allow 'uses a routing table' on its own for (1) only.

Do not award a mark for vague statements such as 'routers send/direct data' or 'routers connect devices to the internet' unless they are developed to say how the packet is forwarded.

Do not award the second mark for 'finds the fastest way' unless the route choice is linked to the packet, e.g. choosing the route for each packet.

Routers assigning IP addresses to devices (DHCP/IP allocation) is a different role from reading a destination IP address; do not credit it.

Do not award credit for descriptions of splitting data into packets or reassembling packets.

Award up to two marks for a linked explanation such as:

• If a packet is damaged/lost only that packet has to be resent (1) so the rest of the video does not need to be sent again (1)
• The video can still be delivered if one router/route fails (1) because routers can send the packets by an alternative route (1)
• Packets from the video can share the network with other users' data (1) because routers send packets from many users along the same connections (1)

For both marks, the expansion must follow/associate with the statement.

Do not accept 'it is faster/quicker' on its own without saying why it is faster, e.g. because packets can take a less busy route.

Do not accept 'it is more secure/safer' on its own without expansion as to why it is more secure.

Do not accept 'the video is made smaller/compressed' - splitting a file into packets does not reduce the total amount of data.

Award up to two marks for a linked explanation such as:

• The receiver can put the packets back into the correct order (1) because packets may arrive out of order/take different routes (1)
• The receiver can see if a packet is missing (1) because the sequence of numbers shows a gap (1)
• The receiver knows when the whole file has arrived (1) because the total number of packets/last packet number is known (1)

For both marks, the expansion must follow/associate with the statement.

Allow 'so the packets can be put back in the correct order/reassembled' for the first mark, but the second mark needs the reason, i.e. packets may arrive out of order/take different routes.

Do not accept 'so the packets can be identified' without stating what the number is used for, e.g. reordering or spotting a missing packet.

Do not accept 'so the router knows where to send it' - the router uses the destination IP address, not the packet number.

Award up to two marks for a linked explanation such as:

• The router reads the destination IP address (1) so it knows which network/device the packet is for (1)
• The router looks up the destination IP address in its routing table (1) to find the next router/route for the packet (1)
• The router forwards the packet to the next router (1) that leads towards that IP address / is on the best route (1)

For both marks, the expansion must follow/associate with the statement.

Do not accept MAC address - routers use the IP address to route packets between networks.

Do not accept 'it finds the exact device' without saying how the router uses the address, e.g. routing table/next router.

Award up to two marks for a linked explanation such as:

• Each router chooses the best route at the time (1) so when a route becomes busy/congested or fails, later packets are forwarded along a different path (1)
• Routers forward each packet separately (1) so the path a packet takes depends on the traffic conditions when that packet arrives (1)
• A router may send a packet to a different next router (1) if its routing table shows a better/less congested route for that packet (1)
• Traffic on the internet changes (1) so no single path is kept for the file and each packet can use whichever route is best when it is sent (1)

For both marks, the expansion must follow/associate with the statement.

Allow reference to a routing table, or to how a router decides the route, as part of the linked reason.

Do not accept 'the packets have different IP addresses' because all packets from the same file have the same destination IP address, even though their routes can differ.

Do not accept 'the internet is busy' or 'there is a lot of traffic' on its own without linking this to the router choosing a different route.

Do not accept 'different routers send the packets' on its own without a reason why a different router/route is chosen.

Do not accept an answer that only says the packets take different routes without explaining why.