Cosc244 Final Test

Front 1

What is a computer network?

Back 1

A computer network is an interconnected collection of autonomous computers which are able to exchange information. It may include devices such as printers, faxes, etc.

Front 2

What does LAN stand for? and give a description?

Back 2

Local Area Network – Covers a small geographic area and connects devices in a single building or group of buildings

Front 3

What does WAN stand for? and give a description?

Back 3

Covers a larger area such as a city /municipal
region, country, or the world

Front 4

what is a network topology? and give some examples?

Back 4

A way of organising the physical connections

Examples:

> Star

> Ring

> Bus

> Fully Connected

Front 5

What is a Star Topology? advantages and disadvantages?

Back 5

devices communicate through a central device.

Advantage: focal point for responsibility

Disadvantage: failure of central device takes down network.

Front 6

What is a Ring Topology? advantages and disadvantages?

Back 6

device are connected in a ring, either unidirectional or bidirectional.

Advantages:

> no central coordination

> bidirectional rings need 2 failures to take down the network

Disadvantages:

> unidirectional rings will fail if any device is fails

> more time relaying messages

> all stations between sender and reciever are involved in sending the messages

Front 7

What is Bus Topology? advantages and disadvantages?

Back 7

devices communication through a single bus.

Advantages:

> easy to add or remove a device

Disadvantages:

> needs collision detection

> high collision rate may reduce data rate

Front 8

What is a Fully Connected Topology?

Back 8

Every device is connected to every other device.

Front 9

What is Combined Topology?

Back 9

The de facto situation - combining multiple network topologies in one network.

Front 10

What is a protocol?

Back 10

An agreement about how to do something
– A set of rules governing the exchange or transmission of data electronically between devices.
– This enables computers and software built by different people to be able to communicate in the same language

Front 11

how many layers are there in the OSI model? And What are they?

Back 11

7

Physical

Data Link

Network

Transport

Session

Presentation

Application

Front 12

What does the Physical Layer do?

Back 12

it handles the movement of bits to from one hop to the next via the transmission medium.

signal and encoding of the bits.

Unit: bits

Front 13

What does the Data Link layer do?

Back 13

Hop to Hop communication

it breaks messages up in to frames and send them hop to hop using the Physical Layer.

> Flow control between two adjacent network nodes.
> Error control - deals with damaged, lost, and
duplicate frames

Units: frames

Front 14

what does the Network Layer do?

Back 14

Host to Host communication

The network layer is responsible for the delivery of individual packets from the source host to the destination host.

> Determines routes from source to destination.
> Send messages hop by hop to the destination.
> Control of congestion

> Address mapping
> Interconnection of heterogeneous networks

Units: packets

Front 15

what does the Transport Layer do?

Back 15

Process to Process Communication

> Ensures pieces all arrive correctly at receiver and reassembles them into the original order.
> Flow control between source and destination.

> Connection management

Units: Segments

Front 16

What does the Session Layer do?

Back 16

The session layer provides the mechanism for opening, closing and managing a session between end-user application processes.

> used for syncronisation

> authentification

Front 17

What does the Presentation Layer do?

Back 17

The presentation layer is responsible for translation, compression, and encryption

Front 18

What does the Application Layer do?

Back 18

The application layer is responsible for providing services to the user.

Front 19

How many layers are there in the TCP/IP model? and what are they?

Back 19

4

Application

Transport

Network

Host to Network Layer

Front 20

Why have medium access control?

Back 20

– Shared transmission medium
– Multiple stations access the medium

Front 21

what is Random Access Protocols?

Back 21

– There is no scheduled time for a station to transmit
– Stations compete with one another to access the medium

Front 22

what is the Pure Aloha System?

Back 22

> Each station transmits when it has a frame to transmit.
> When the receiver receives a frame, it sends an acknowledgment to the sender.
> If the sender receives an acknowledgment, it knows its data has been received. Otherwise the sender assumes
a collision and retransmits.
> If a collision occurs, each collided sender waits a random time before retransmission.

Front 23

What is Slotted Aloha Protocol?

Back 23

> Divide time into intervals (slots) of T units each.
> Each station sends only at the beginning of a slot.

Front 24

What does CSMA stand for and how does it work?

Back 24

Carrier Sense Multiple Access

If a station has a frame to send:
– It first check the status of the medium.
– If there is no activity, transmit; otherwise, wait.

Front 25

what is Medium Sensing Methods Non-persistent method?

Back 25

Sense the medium if its not idle wait a random time and sense again.

Front 26

what is Medium Sensing Methods persistent method?

Back 26

> If the medium is not idle, continuously sense the medium.

Front 27

what is p-Persistent CSMA?

Back 27

sendPacket = function (frame, p) {

if (medium.idle) {

medium.send(frame);

} else {

tryToSend = function() {

if(Math.random() <= p) {

medium.send(frame);

} else {

medium.once('TimeSlot', sendPacket);

};

medium.once('idle', tryToSend);

}

Front 28

CSMA/CD. what is the CD and what does it do?

Back 28

CD stands for Collision Detection.

> While transmitting we listen for collisions. If a collision is detected a transmission is stop and a short jamming signal is sent.

> While transmitting if we recieve a jamming signal we stop transmitting

After a collision, it waits a random amount of time according to the Binary Exponential Backoff algorithm and then attempt the whole process again.

Front 29

how does the Binary Exponential Backoff Algorithm work?

Back 29

If a station's frame collides for the first time, wait 0 or 1 time
slot (randomly chosen)
– If it collides a second time, wait 0, 1, 2, or 3 slots (randomly
chosen)
– After a third collision, wait from 0 to 7 slots.
– After n collisions, wait from 0 to 2n-1 slots if n <= 10. If n >
10, wait from 0 to 1024 (2^10) slots
– After 16 collisions, give up and report an error.

Front 30

With the Binary Exponential Backoff Algorithm how many collisions do we wait till be give up?

Back 30

16 collision

Front 31

Why is a frame size that is too big a problem and why is a frame size that is too small be a problem in CSMA/CD?

Back 31

Too Big -> one station could monopolise the medium

Too Small -> collisions may not be detected

Front 32

what is contention?

Back 32

contention is the media access method (random access)

-> first come first served

Front 33

What is IEEE802.3 standard, Ethernet minimum frame size?

Back 33

512 bits