CSMA and DNS Flashcards
What are the two types of Multiple Access Protocol?
Carrier Sense Multiple Access Collision Detection for wired networks
Carrier Sense Multiple Access Collision Avoidance for wireless networks
What is collision detection?
If two computers on a CSMA/CD network transmit at the same time, they can sense this as what is on the wire is not what they’re transmitting.
In this instance both computers will stop transmitting, wait for a random interval chosen from an exponentially doubling range and try again.
How does a random interval help in collision detection?
It avoids repeated collisions
Why does an exponentially doubling range help in collision detection?
If the network is busy ( more collisions ) then increasing the time waited every time a collision happens will means computers are more reluctant to send messages and will therefore help with network traffic.
What is collision avoidance?
A computer finding the frequency is busy counts down from a random value while the channel is clear. The countdown is frozen while the channel is busy
What is the hidden station problem?
A computer may be in range of a base station but out of range of another machine. Therefore the other machine won’t be detected when using collision avoidance
Solution to hidden station problem?
The computer must first send a ‘ready to send’ packet to the base station. The base station will then send out a ‘clear to send’ signal to the network. The computer now knows the frequency is clear.
How is DNS scalable?
It achieves scalability by organising a large number of servers in a hierarchical fashion
How is DNS efficient?
It achieves efficiency by local server caching
How is DNS reliable?
There are thirteen root servers spread across the world
How is DNS maintanable?
Authorative domain servers ( such as exeter.ac.uk ) can be registered, updated and deregistered automatically.
How is DNSSEC different?
The sender signs a message by attaching it’s hash encrypted with their private key.
