Intro into communication systems

What is ASCII and what is it used for

• American Standard Code for Information Interchange

• It is a table which is used to convert characters into bits

What is Routing

• The process of finding a path between a souce and destination. Uses routers which hold networks(e.g.the internet) together.

Give the difference between packet switching and circuit switching

Circuit switching - The source knows the path the message is going to travel. In addition the netork is made up of switches. The message is sent without stopping at any point.

Packet Switching - The source doesn’t know the path the message will take. The network is made of routers, and the message stops at each router to determine where it next travels.

How can we ensure reliable communication

• We can use encryption to secure messages

• We can use error detection at the destination

Define a “Protocol”

Protocol is set of rules/language that allows two devices to communicate with each other, without it devices cannot communicate with each other

Define a ‘Host’

Either a client(device) or a server

Define ‘frequency division multiplexing’

Where phone lines can carry multiple signals at different frequencies (upstream, downstrean, phone signals)

Define ‘Peer Linking’

Directly connecting routers to each other

What are ‘IXPs’

A third party service consisting of many switches that provide links to connect ISPs to each other

Define ‘Multi-homing’ & which ISPs cannot use multi-homing

Where an ISP can connect to 2 or more provider ISPs. Tier-1 ISPs can’t use it as they have no provider ISP

State a benefit of ‘Multi Homing’

If a connection between two ISPs fail there will be another one for data to still be sent/retrieved.

What is the purpose of the Application layer

To create the message. Puts the message in a format that can be sent

What is the purpose of the Transport layer

Splits the message into packets. It is also responsible for reliable data transfer

What is the purpose of the Network layer

Improves routing, Exists in the routers, source and destination

What is the purpose of the Data Link layer

Reduces the chance of multiple access occuring 

What is the purpose of the Physical layer

Converts sequences of bits into physical signals to be sent

What is the name for the packet in the transport layer & what does its header and payload contain

The segment, the header contains parity bits/error checking bits and the payload is just the message

What is the name for the packet in the network layer & what does its header contain & what is the name of the payload

The datagram. The header contains the IP address of where it should be sent next and the payload is just the segment

What is the name for the packet in the data link layer & what does its header contain & what is the name of the payload

The frame. The header contains bits to prevent multiple access and the payload is just the datagram

How many layers does the OSI model have and the TCP/IP model have

TCP/IP consists of 4 layers
OSI consists of 7 layers

Name the 2 layers in the OSI model which are dealt with in the Application Layer of the TCP/IP model & what they do in the OSI model

Presentation - Finds general solutions to things e.g. encryption, compression etc

Session - Manages connection between end users

Define a ‘process’

Where a program runs in RAM

What 2 conditions are needed to receive a message

• Processes must have identifiers

• The host device must have a unique IPv4 address(32 bits)

What do Identifiers contain that associate with the process on the host

• IP addresses

• Port number

Define a ‘Port Number’

A number given to a process to help identify a process in a host

What is the purpose of a socket

Acts as a door between the transport layer & application layer

What 4 services does the transport layer provide?

• Reliable data transfer

• Timing

• Throughput

• Security

Define ‘Persistent HTTPS’

Multiple objects can be sent over the TCP connection between the client and server

Define ‘Non-Persistent HTTPS’

Only one object can be sent over the TCP connection

Define RTT(Round Trip Time)

The time taken for a small packet to travel from the client to the server and back

Describe the application-layer to transport-layer flow for a persistent HTTP message.

• Message is created & formatted in app layer

• Sent to transport layer via socket

• Sender & receivers transport layer creates a TCP connection

• Message is sent across then pushed up the socket to the app layer

• TCP is shut down

Describe the application-layer to transport-layer flow for a non-persistent HTTP message.

• Message is created & formatted in app layer

• Sent to transport layer via socket

• Sender & receivers transport layer creates a TCP connection

• Message is sent across then pushed up the socket to the app layer (One response per TCP connection)

State a downside to using non-persistent HTTPS compared to persistent HTTPS

non-persistent HTTPS requires 2 RTT per object, where as persistent HTTPS requires only 1.

State the 3 phases of transfer for SMTP

• Handshaking

• Transfer of Messages

• Closure

Which devices are the transport layer protocols only implemented on

End-Systems

How does the transport layer work on end-systems

• The source end-system breaks the message into packets & adds a header to create a segment

• The destination end-system reassemtbles the segment into the original message

Give 2 differences between UDP & TCP

TCP

• Provides reliable data transfer

• Guarantees the message arrives in order

UDP

• Doesn’t guarantee the order of the message will be delivered correctly

• Doesn’t guarantee the message arrives in order

What differentiates UDP from TCP

UDP is connectionless whist TCP isn’t

Give advantages of UDP

• No connection needed to be made in comparison to TCP which could cause delay

• Doesn’t keep a connection state

• Has a smaller header size, so processing time is shorter

Name 3 things that can go wrong when sending packets

• Errors in the packet(bits flipped due to noise)

• Packets may get lost due to overflow

• Packets may get delayed if previous packets are stilll waiting to be transmitted

• Packets arrive out of order

Explain how the parity bit technique works

For any sequence of bits if there’s an even number of 1s in the sequence, the parity bit will equal zero. I (no errors)

If there is an odd number of 1s, the parity bit = 1 (Meaning errors are present)

Explain how parity bits work between two devices

• Calculate the parity bit for the packets payload & store it in the packets header

• Send to destination

• Repeat 1st step & compare the parity bit to the source parity bit

• Match = No errors

• No Match = Errors

Explain how parity bits work between two devices using checksum

• Source breaks packet into equal size words

• performs binary addition

• Places checksum into header

• Receiver checks if checksum = checksum field value

• Match = No Error

• No Match = Error

Define ACK & NAK

ACK(Acknowledgement) - Receiver tells sender tht the pkt has arrived with no errors

NAK(Negative Acknowledgement) - Receiver tells sender tht the pkt has arrived with errors

Explain the stop & wait operation process

• Sender takes all bits packet (size L) into a channel at a rate R bits per second (t=L/R)

• ACK is sent back to sender by receiver

• Time from sending first packet to being ready to send the next [t = RTT + L/R]

• U = Utilisation

• U(sender) = (L/R)/(RTT + L/R)

Define ‘Pipelining’

Where the sender can send multiple packets without waiting for acknowledgement

Give two methods of Congestion Control

• End to End

• Network assisted

GIve features of end to end congestion

• No explicit feed back from the network

• Approach taken by TCP

• End to end systems usually observe losses/delay

Give features of network assisted congestion control

• Routers provide feedback to end systems

• 1 bit indicates congestio

• Explicit rate for sender to send at

• Direct/Indirect feedback from routers

Give features of TCP congestion control

Additive increase - Increases window size by max segment size every RTT until loss is detected

Multiplicative increase - Cuts window size by half after loss