Notes-for-CAIE

Table of contents

• 3.2.1 Protocols
  • Protocol layers
  • Define communication protocol
  • Protocols that may be used in the transfer of emails
  • Why protocols are essential for communicat ion between computers.
  • Explain how the BitTorrent protocol allows files to be shared
  • Describe the TCP/IP protocol suite
  • How TCP used to transfer files
  • How IP used to transfer files
• 3.2.2 Circuit switching, packet switching and routers
  • Items contained in an IP packet
  • packet switching
  • Describe packet switching
  • Benefits of using packet switching
  • Drawbacks of using packet switching for file transmission
  • Drawbacks of using packet switching for video conferencing
  • Explain the role of routers in sending an email using packet switching
  • Why sending an email message may use packet switching
  • The purpose of a packet header
  • How packet, router, TCP/IP are used in requesting a web page
  • Packet
  • Router
  • TCP/IP
  • circuit switching
  • Explain what is meant by circuit switching
  • How circuit switching overcome problems of packet switching
  • Drawbacks of using circuit switching
• 3.2.3 Local Area Networks (LAN)
  • Items stored in a routing table
  • Bus topology
  • Benefits and drawbacks of using bus topology
  • How to handle collision in bus topology
  • Star topology
  • Benefits and drawbacks of using star topology
  • How star topology solves problem in bus topology
  • Describe CSMA/CD protocol
  • How router used to university LAN
  • How NIC used to university LAN
  • How wireless access point used to university LAN

3.2.1 Protocols

• show understanding of why a protocol is essential for communication between computers
• show understanding of how protocol implementation can be viewed as a stack, where each layer has its own functionality
• show understanding of the function of each layer of the TCP/IP protocol suite
• show understanding of the application of the TCP/IP protocol suite when a message is sent from one host to another on the Internet
• show understanding of how the BitTorrent protocol provides peer-to-peer file sharing
• show an awareness of other protocols (HTTP, FTP, POP3, SMTP) and their purposes

Protocol layers

s18 31 Q3.c [3]
w19 33 Q7 [4]

Layer	Protocol
Physical	Modems
Data link	Ethernet / WiFi
Network	IP
Transport	TCP / UDP
Application	FTP

Remember: DNTA

Define communication protocol

w20 32 Q3 [2]

• Set of rules
• … for transmission of data.

Protocols that may be used in the transfer of emails

w20 32 Q3 [4]

name	purpose
POP3	downloading email
IMAP	downloading email
SMTP	sending email
HTTP	accessing email using browser

Why protocols are essential for communicat ion between computers.

w19 Q3 [2]

• Provides a set of standards for transmission of data
• … that gives a known set of rules for transmitting and receiving data.
• This enables communication between devices
• … from different platforms.

Explain how the BitTorrent protocol allows files to be shared

s18 Q3.c [3]

• BT client software made available
• A computer joins a swarm by using the software to load a BT descriptor file
• File is split into small pieces
• Once a computer has a piece it can become a seed and upload to other computers
• Pieces of file are both downloaded and uploaded
• A server ‘tracker’ keeps records of all computers in the swarm
• The server shares their IP addresses allowing them to connect to each other

Describe the TCP/IP protocol suite

w20 33 Q3 [5]

• should say all the layers
• layers should be in order

• A layered model
• … with 4 layers
• Layers (ordered):
  • Data Link, Network,
  • Transport, Application
• Uses a set of protocols for transmission of data

How TCP used to transfer files

w18 33 Q3 [3]

• Allows applications to exchange data.
• Establishes and maintains a connection,
• … until exchange of data is complete.
• Acknowledges all packets that arrive.
• Detects when a packet has not arrived at destination.
• Reassembles packets into the correct order.

How IP used to transfer files

w18 33 Q3 [2]

• Routes the packets around the network.
• Adds to the IP header a source address for each packet.
• Encapsulates data into datagram.
• Pass datagram to the network layer.

3.2.2 Circuit switching, packet switching and routers

• show understanding of circuit switching and where it is applicable
• show understanding of packet switching
• show understanding of the function of a router in packet switching
• explain how packet switching is used to pass messages across a network, including the Internet

Items contained in an IP packet

s18 32 Q5.d [4]

• Source address
• Destination address
• Sequence number
• Payload
• Data length

packet switching

Describe packet switching

w20 33 Q3 [3]

• File is divided into packets
• … and necessary data added to each packet
• … which are sent independently of each other
• … and takes different routes.
• Packets are reassembled at the destination.
• Missing packets can be resent.

Benefits of using packet switching

s18 32 Q4.d [4]

• Alternative route available
• … in case of network problem
• If packet fails to arrive
• … only that packet has to be resent

Drawbacks of using packet switching for file transmission

w20 33 Q3 [1]

• Packets can be delayed

Drawbacks of using packet switching for video conferencing

s17 31 Q5.b [2]
Don’t write ( )

• Picture and sound not synchronised (Because packets may not arrive in order)
• Interruptions
• Can be degraded by other competing traffic

Explain the role of routers in sending an email using packet switching

w15 31 Q3.b [3]

• Email has been split up into packets.
• Packet has destination address.
• Packets pass through different routers.
• Routers use IP addresses.

Why sending an email message may use packet switching

w15 31 Q3.c [2]
s20 33 Q3 [2]

MS varies, write them all.

• Packets arriving out of order is not an issue.
• Time delay due to packet lost is not significant / Real time communication is not required.
• Sending packets by different routes is not an issue.
• No requirement for a continuous circuit.
• No requirement for full bandwidth.

The purpose of a packet header

w19 33 Q7 [2]

• To store data about packet
• … to ensure that it reaches its destination
• … and that message can be properly reconstructed.

How packet, router, TCP/IP are used in requesting a web page

s17 31 Q5.a [2]x3

Packet

As MS varies, always write these 3:

• Both request and web page are split into packets (required by all MS)
• Each packet is sent on different routes from device to device
• Client reassembles packets to rebuild web page

Router

• Transmits packets
• Contains connections to many other routers
• When packets arrive at router, router decides where next to send packet

TCP/IP

• Is the protocol
• Rules for communication between web server and browser

circuit switching

Explain what is meant by circuit switching

s17 31 Q5.b [2]
w15 32 Q3.a [2] s20 33 Q3 [3]

Always write:

• Dedicated communication channel between the two communicating devices
• … which lasts for duration of connection.
• Established prior to start of communication.
• Removed when transmission completed.

How circuit switching overcome problems of packet switching

s17 31 Q5.b [3]
w15 31 Q3.e [3]
w15 32 Q3.b [3]
s20 33 Q3 [2]

Always write:

• No lost of packets / No loss of synchronization.
• Dedicated circuit has full bandwidth.
• In circuit switching, packets will arrive in order
• … so quality of service guaranteed.
• No delay.
• Two-way real time conversation.

If forgotten, write also:

• In packet switching, packets may not arrive in order
• … as packets can take different routes.

Make full use of spaces writing basics on packets e.g. splitting, contains destination IP

Drawbacks of using circuit switching

w20 32 Q3 [2]

• Bandwidth not available to other users.
• Conversation not available without restarting the conversation.
• Failure

3.2.3 Local Area Networks (LAN)

• show understanding of a bus topology network and the implications of how packets are transmitted between two hosts
• show understanding of a star topology network and the implications of how packets are transmitted between two hosts
• show understanding of a wireless network
• explain how hardware is used to support a LAN: switch, router, servers, Network Interface Cards (NICs), wireless access points
• show understanding of Ethernet and how collision detection and avoidance (such as CSMA/CD) works

Items stored in a routing table

s18 32 Q5.d [2]

• IP address of network destination
• IP address of possible next hop
• Data to decide best route
• Interface

Bus topology

s16 31 Q1 [2]

• A line connecting all devices
• One terminator at each end

Benefits and drawbacks of using bus topology

s18 31 Q3.a [2]

Benefits:

• Easier to set up
• Fewer cables required

Drawbacks:

• Collisions
• Security lower because several computers receive the data

How to handle collision in bus topology

s16 31 Q1.c [3]

• Both stop transmitting
• Each uses a random time
• Wait for time period
• Check for bus status
• Attempt to re-transmit

Star topology

Benefits and drawbacks of using star topology

s18 31 Q3.a [1]
s19 33 Q2 [4]

Benefits:

• Secure
• … since signals only go to destination.
• A new device can be easily added to the network
• … as only one connection direct to server needs to set up.
• If one node or link fails
• … the other computers can continue working.

Drawbacks:

• If central device fails then whole network goes down.
• Performance dependent on central device

How star topology solves problem in bus topology

s16 31 Q1.d [4]

a switch is used.

• A switch has a number of ports
• Each connects to a single device
• Switch provides direct transmission from device to device
• Collisions are no longer possible
• Question-specific examples

Describe CSMA/CD protocol

s19 33 Q2 [3]

• Carrier Sense Multiple Access with Collision Detection.
• Before transmitting a device checks if the channel is busy.
• … if so, the device waits.
• When transmission begins the device listens for other devices also beginning transmission.
• If there is a collision, transmission is aborted.
• Both devices wait a random time then try again.

How router used to university LAN

s19 33 Q2 [2]

• Allows internal connections between the university LANs.
• Allows external connections from the main LAN.

How NIC used to university LAN

s19 33 Q2 [2]

• Provides device with a MAC address
• … to uniquely identify it on the network.
• Allows each individual device to connect to the network.

How wireless access point used to university LAN

s19 33 Q2 [2]

• Allowing devices to connect to the LAN via radio communication
• … instead of using a cable.
• Easy to move device to different locations.