Networking: Chapter 8 (Data Transmission)

Question 1

What is Asynchronous Data Transmission?

Answer 1

Asynchronous Data transmission is when each byte of data is sent individually with its accompanying start and stop bits.

Question 2

What is Synchronous Data Transmission?

Answer 2

Synchronous Data transmission is when packets of data are sent with its accompanying start frame, which tell the receiving station that a packet is coming.

Question 3

What are 2 types of error checking in data transmission?

Answer 3

• Parity Check

* Cyclic Redundancy Check (CRC)

Question 4

What is a Parity Check?

Answer 4

A Parity Check is a way of checking for errors in data that has been transferred between systems

Question 5

How does a Parity Check work? (4)

Answer 5

• A bit in each byte of data is set aside as the parity bit
• In even parity this bit is set to 1 or 0 to ensure an even number of 1s
• In odd parity this bit is set to 1 or 0 to ensure an odd number of 1s
• If even parity is used and arrives to its destination with an odd number of 1s an error is registered, and vice verse

Question 6

What is a Cyclic Redundancy Check (CRC)?

Answer 6

A Cyclic Redundancy Check is a way of checking for errors in data before it leaves the source system

Question 7

How does a Cyclic Redundancy Check work? (3)

Answer 7

• It divides binary data in the packet by a 16 bit or a 32 bit number and produces a remainder, which is sent along with the data
• When it reaches the receiving system the same calculation is carried out
• The results of these calculations must be the same, and if they aren’t then there has been an error

Question 8

What is Transmission Control Protocol/Internet Protocol (TCP/IP)?

Answer 8

It is a protocol which establishes a communication between nodes on a network before sending data and then routes packets of data to these nodes.

Question 9

What is Carrier Sense Multiple Access/Collision Detection (CSMA/CD)?

Answer 9

It is a protocol which is used to control which node on a network can transmit at any one time on a baseband network

Question 10

Why is CSMA/CD needed? (2)

Answer 10

• Baseband networks only allow one device to transmit at any one time
• If more than one device transmits data at one time a collision will occur, and the more collisions there are the slower the transmission rate across the network

Question 11

How does CSMA/CD work? (3)

Answer 11

• When a node wants to transmit data on the network, the Carrier Sense finds out if another data transfer is taking place and if not then the node will transmit.
• Multiple Access means that more than one node can begin to transmit on the network at the same time.
• If 2 nodes do transmit at the same time, Collision Detection makes the two transmissions wait a random amount of time before trying to re transmit so there isn’t another collision

Question 12

What is circuit switching?

Answer 12

When a fixed connection path is established between two points on a network and lasts as long as it takes to transmit all the data being sent. It is used in dialup and ISDN connections

Question 13

What is packet switching?

Answer 13

When data messages between two points on a network are divided into packets and each individual packet can take a different path through a network instead of a fixed path. The packets are reassembled at their destination. It is used in Internet data transmission

Question 14

What is an advantage of circuit switching?

Answer 14

Once the connection is set up, data can be transmitted directly and steadily to its destination, which is suitable for thing such as transmitting audio/video files

Question 15

What is a disadvantage of circuit switching?

Answer 15

Circuit switched connections take time to establish, which is unsuitable for things such as connecting to web servers

Question 16

What is an advantage of packet switching?

Answer 16

Since packets take different routes it utilises the whole network instead of clogging up one route so congestion is avoided

Question 17

What is a disadvantage of packet switching?

Answer 17

Messages at the receiving end will need to be constructed since the packets of data take different routes at different times

Question 18

What is a Dialup connection? What speed does it operate at? Where would it be used?

Answer 18

• It is an Internet connection that uses modem and telephone lines to transmit data.
• It operates at 56 kbps
• It would be used for light home use

Question 19

What is a Cable modem connection? What speed does it operate at? Where would it be used?

Answer 19

• It is an analogue modem that sends and recieves data through a coaxial cable television network instead of telephone lines, and does not require dialling.
• It operates at 36Mbps downstream and 2 to 10Mbps upstream
• It would be used in residential areas

Question 20

What is a Leased line connection? What speed does it operate at? Where would it be used?

Answer 20

• It is a permanent, dedicated connection from one point to another to allow high speed, always on connection channels
• It operates at 1.54 Mbps
• It would be used for large businesses

Question 21

What is an ISDN connection? What speed does it operate at? Where would it be used?

Answer 21

• It is a digital transmission service which uses telephone lines, but cuts out analogue to digital conversions out of the modems.
• It operates at a rate of 2 x 64Kbps transmission channels and 1 x 16Kbps for control information
• It would be used for small businesses

Question 22

What is an ADSL connection? What speed does it operate at? Where would it be used?

Answer 22

• It uses ordinary phone lines to provide broadband access, and provides high transfer rates, but customers must be within 5500m of the exchange
• It operates at 9Mbps downstream and 1.5Mbps upstream
• It would be used for tele-working

Question 23

What is a Network Interface Card (NIC)?

Answer 23

It is a device that connect a node to a network, and is identified by a unique MAC address.

Question 24

Why is a Network Interface Card used? (4)

Answer 24

• Data Conversion (From binary to whatever type of signalling is used on the network)
• Buffering (Storing the data during data conversion)
• Packaging data into frames (Adding headers to packets of data for error checking)
• Auto-sensing (Sensing the highest speed supported by network hubs and switches)

Question 25

What is a MAC address?

Answer 25

It is a unique 6 byte number used to identify a node to a network, stored in the ROM of a NIC.

Question 26

Why is a MAC address used? (2)

Answer 26

• To receive only intended data for a specific NIC

* Routers use MAC addresses to build routing tables

Question 27

What does TCP do?

Answer 27

• It breaks down data into packets
• It adds a header containing a sequence number
• It reassembles the packets of data when it reaches the destination

Question 28

What does IP do?

Answer 28

• It adds its own header to the packets containing address information
• It routes packets of data to the correct addresses

Question 29

What is a disadvantage of using CSMA/CD?

Answer 29

Time is taken to check if line is free, which reduces network performance

Question 30

Why is Synchronous Transmission better than Asynchronous Transmission?

Answer 30

The data transfer rate is quicker as a start and stop frame is only needed for each packet with synchronous, whereas a start and stop bit is needed for each byte with asynchronous transmission

Question 31

What are Cyclic Redundancy checks more effective than Parity checks?

Answer 31

Parity check will not pick up if two bits are flipped, whereas a Cyclic Redundancy Check can detect this type of error

Question 32

Why would error checking reduce network performance? (2)

Answer 32

• Calculations carried out at each end of transmission

* Extra data is sent (eg. parity bits)

Question 33

State 2 ways in which CSMA/CD increases transmission time

Answer 33

• If collisions occur, random amounts of time are taken for re transmissions to occur
• Before transmission, time is taken to see if the line is free