Descriptive Questions Flashcards
Help of a diagram explain LAN, MAN, WAN
LAN
Used for connecting groups of computers or network devices
Distance: covers a small area, a floor, a building or a few buildings
Speed: 10Mbps-40Gbps
Uses switches to form LANs along with different cables
MAN
Used to connect LANs and BNs within a city, distance approximately (5km to 50km)
Speed: 64 Kbps-10Gbps
Mostly public data networks used for some organisations have their own MANs but it is expensive
WAN
Used to connect MANs between cities or countries
Distance: hundreds to thousands of KMs
Speed: 64 Kbps-10Gbps
A WAN can be a number of LANs and MANs linked together
Fibre, Microwave, Satellite
List the advantages and disadvantages of satellite communication and 3 types
Advantages:
- broadcast capability (point to multi-point)
- wireless (good for remote areas, islands, travellers)
- backup of existing links
- universal service access (access from anywhere e.g mountain top, sea etc)
Disadvantages:
- less secure compared to fibre
- limited bandwidth
- more noise
- less availability (approx 99% available, 88Hrs/yr down)
Geo Stationary Earth Orbit (GEO)
Medium Earth Orbit (MEO)
Low Earth Orbit (LEO)
Discuss and compare token passing and CSMA/CD
Token Passing = Token Ring = Controlled access
-also known as controlled access MAC, commonly used in Token Ring
LAN
-mainframe controls the circuit and determines which clients can access
media at what time
-good for high traffic, slow, all get a chance
Carrier Sense Multiple Access with Collision Detection.
CSMA/CD = Ethernet = Contention
-also known as contention-based MAC, commonly used in Ethernet LAN
-Computer waits till circuit is free, and then transmits whenever they have data
-good for low traffic, fast, cheap, can be monopolised
Describe the advantages and disadvantages of Ring, Star, Partial and Full Mesh topologies [week 10]
-Ring topology-routers in a WAN are in a closed loop, each computer linked to the next.
Advantage if failure in any part does not stop its function as messages can be rerouted around the failed link but reduce in network performance.
Disadvantage is if ring is too big, messages need to pass many nodes before reaching destination.
-Star topology-most common in computer networking, connects clients to the switch.
Advantage if a failure of one link, doesn’t affect the rest, not expensive, easier to add links/nodes to the network.
Disadvantage if central fails, whole network is down, no alternative link if one fails.
Partial Mesh:
+networks combine performance benefits of both ring and star
+if one link fails, there are alternative routes
+not too expensive
-higher delay than full mesh
-less link diversity than full-mesh
Full Mesh:
+have point-point link between all cities (low delay)
+if one link fails, several alternative links
-too much cable, expensive
Describe the differences between stop and wait, ARQ and continuous ARQ with diagram
Stop-and-Wait ARQ:
- half duplex
- sender first sends packet, then waits to hear from receiver
- if packet received without error, ACK is sent back by receiver, next packet sent
- if receiver detects error in packet, receiver sends NAK and sender resends packet
Continuous ARQ:
- full duplex
- sender can keep sending packets without waiting for receiver to acknowledge if packet has been received correctly
- acknowledgements still sent back by receiver once processed and includes information on which packet was acknowledged
- if a packet received in error, the receiver sends back NAK for a specific packet to be resent
Describe the difference between Datagram Packet Switching and Virtual Circuit Switching
Datagram (connection less):
- This is connection less
- different packets might travel different routes
- ordering problem at the end
- each packet contains a full destination address
- Packets can take any practical route
- each packet is treated independently
Virtual circuit (connection oriented):
- Connection Oriented
- pre planned route established before any packet sent
- Call setup establishes the route for data communication
- All packets in connection follow the established route
- Label Switching.
- PVC(Permanent), faster, uses same path, allow more users to use the network
- SVC(Switched), a little delay, uses different path
Discuss and compare the characters of a switch and a hub
Switch
- Data Link Layer
- Switches have Mac addresses
- Switches are more suitable in centre of the network
- More efficient and more expensive than hubs
- Increase average (max) throughput (traffic)
Hub:
- Physical layer
- does not store MAC addresses and work with raw data (0s 1s)
- Hubs used in corners of network
- pass traffic to all connected
- can act as repeater
Describe error detection and prevention in data communication and what is LRC error detection technique and when does it not work
Detection:
Sender must send extra data with each message based on mathematical calculation.
Receiver performs same mathematical calculation on the message it receives.
If it matches the error detection data sent, then message is assumed correct and vice-versa
- Parity checking
- Longitudinal redundancy checking (LRC)
- Polynomial checking (checksum and CRC)
Prevention:
- shielding, protect wire by insulating coating, can prevent impulse noise, cross talk
- move cable from noise sources e.g. heavy machinery, electrical cables, power sources
LRC:
- improvement over parity checks
- adds an additional character of parity checks (BCC) to each block of data
- BCC parity calculations are based in addition on the 1st bit, 2nd bit, positions etc. in a block
- only catches 98% errors
LRC fails to detect errors that occur in an “even rectangular form” and other forms harder to describe as long as there are even number of errors in each row and column
What is DSL technology and cover the features of ADSL in your discussion and where does it find most suitable applications? [week 10]
Digital subscriber line (DSL):
- increases data rates over traditional telephone line.
- voice telephone circuits (local loop) was constrained by 4KHz bandwidth and thus limited data communication capacity
- most local loop telephone lines actually have much higher bandwidth and can carry data at higher rates if signal is sent down the line.
Asymmetric DSL:
- provides different data rates to telephone switch (upload = 1Mbps, download =8Mbps)
- lower rates as distance increases (less than 3km)
- includes analogue channel for voice
E.g
- Telecom Broadband (ADSL)
- good for internet, can download more
- can use TP in local loop
- solves local loop problem to some extent for data transfer
- uses PSTN
Describe the local loop and the limitation [week 5]
- line from phone to first phone switch is analogue signal (local loop/last mile)
- converted to digital by codec in switch using PCM or PAM
- local loop for voice only (analogue), computer signal is digital
- bandwidth limitations (0-4KHz). Limits Baud and bit rate
- distance to telephone exchange
- quality of line, S/N ratio
- TP not best media
Solution:
- use modem to change Digital to Analogue
- use wireless
- use Fiber
