Exam Prep

Question 0

What are the disadvantages of simulation?

Answer 0

Often tempting to use simulation when not the best method of solving the problem
Doesn’t produce optimal results
It can be very costly

Question 1

What are the advantages of simulation?

Answer 1

Easily changed to see how modifications affect behaviour
May add to the understanding of the system
Simulation methods are easier to apply then analytic methods
In some cases the only way to study a system
Simulation data often less costly

Question 2

Give examples of objects, attributes and events?

Answer 2

Objects: Routers, Switches, Packets
Attributes: Processor speed, Packet size
Events: Packet arrives, Packet dropped

Interaction: attributes are linked to different objects and vary giving them realistic properties events occurs when objects experience change

Question 3

Challenges in multimedia traffic?

Answer 3

Compression 
Real time capabilities 
Inter-media sync 
Intra-media continuity 
Delays and jitters

Question 4

Elements,of a multimedia transmitter?

Answer 4

Source device 
Acquisition of data 
Data compression 
Inter media synchronisation 
Integration of streams

Question 5

Elements of multimedia receiver?

Answer 5

Media extractor
Decompression and reconstruction
Play back device

Question 6

What is the role of an extractor?

Answer 6

Received integrated streams and extracts into separate streams, eg audio and video

Question 7

Compare different types of jitter?

Answer 7

Intermedia: Time difference between packets of two streams eg A/V
E.g Audi video out of sync

Intramedia: Time difference between desired presentation and actual presentation of packets . Results in shaky picture or quivering voice because frames arrive early or late.

Question 8

Contrast common compression techniques?

Answer 8

Header: only compresses packet header. Done hop by hop basis. Useful if pdus are smaller than headers.

Payload: whole body of packet. End to end.

Question 9

Endogenous and Exogenous variables.

Answer 9

Endogenous: From within the boundary of the model. E.g, number of users, processor speed and bandwidth adopted.

Exogenous: from outside the boundary of the model. E.g, quantity of incoming data, power shortages.

Question 10

Token bucket

Answer 10

Credits idle host
Token generated per tick of the clock
Preserves burstyness
Uses one token per object call

Question 11

Leaky bucket

Answer 11

Input varies output is constant
Doesn’t allow burstyness
Doesn’t credit idle host
Output is a smooth constant rate

Question 12

Can a leaky bucket be used with a token bucket?

Answer 12

Yes! Leaky must go after token and leaky rate must be greater then that if the token bucket output rate.

Question 13

How is sequencing provided in MM?

Answer 13

RTP runs,on top of UDP to,provide time stamps and sequence numbers which help the receiver play back the packets in the correct order regardless of arrival time.

Question 14

Why is TCP unsuitable for MM?

Answer 14

Never designed for MM or real time traffic
No QoS
Doesn’t provide critical timing
Overhead heavy
Aimed at apps with little sync necessity
No mutlicast

Question 15

Realtime transport protocol (RTP)?

Answer 15

Designed for real time traffic and MM
Runs on top of UDP 
Designed for mutli casting 
May be considered an application later protocol works between 4 and 7  
Audio and video need different streams 
Provides time stamping and sequence numbers 
Secure access via encryption
Payload identifiers

Question 16

Real time control protocol (RTCP)?

Answer 16

Works with RTP
Works between layer 4 and 7 of the OSI model
Sends reports on; packet loss, arrival, delay etc
Used mainly to modify sender rates and for diagnostics
Uses 5% of available BW, spits 75% to receiver and 25% to sender

Question 17

Congestion management concepts, policing and shaping?

Answer 17

Policing: propagates bursts, excess traffic is dropped - saw tooth pattern

Shaping: stores excess In a buffer,output is smooth. Relies on sufficient buffer size.

Question 18

QoS mechanisms (FIFO, PQ and WFQ)?

Answer 18

FIFO: excess packets stored in buffer and sent in order of arrival. No priority!

PQ: priority streams sent first (eg voice). Can starve low priory traffic and if all data is high priority, the high priority queue starts to discard.

WFQ: assigns streams a weight, with highest priory data getting a higher weight. Then uses round robin mechanism to take so many packets ( = to the assigned weight) from each stream as it goes.

Question 19

PCF wireless technique?

Answer 19

Uses Pifs
No contention : polls (pc)
Can be used with DCF in the same BSS

Question 20

How to overcome the WFQ buffer problem?

Answer 20

Move discarded packets into a lower priory queue
Alter weight of queues in adaptive fashion based on queue size
Allow dynamic buffer allocation

Question 21

Random early detection (RED)?

Answer 21

Monitors traffic loads
Randomly discards PDUs if congestion begins to increase
One type is weighted RED (WRED)

Question 22

What is weighted RED (WRED)?

Answer 22

Combined RED with IP precedence
Selectively discards lower priority traffic when interface starts to get contested
Dropped on criteria; queue depth, source etc

Question 23

Types of delay?

Answer 23

Process: time taken for router to receive PDU on out interface examine it and place it on in

Queue: time spent in output queue

Serialisation: time for bits to be put on huge wire

Propagation: time to travel across a link

Question 24

Temporal and spatial compression?

Answer 24

Temporal: removes redundancy between consecutive frames as they are almost the same Spatial: done independently inside each frame by removing redundancy from the frame.

Question 25

802.11e?

Answer 25

Built on 802.11 MAC Introduces new hybrid coordination function (HCF) Two types: EDCA: like DCF but introduces different priorities for services HCCA: like PCF but with priority

Question 26

Enhanced distributed channel access (EDCA)

Answer 26

``` Divides traffic into different priority queues -access categories (AC) Each queue gets different: - minimum contention window - maximum contention window - arbitrary inter-frame space (AIFS) - transmission opportunity (TXOP) ```

Question 27

Advantages of EDCA?

Answer 27

Voice and video have priority over data | Works well if network has light load

Question 28

Disadvantages of EDCA?

Answer 28

Lower priority traffic starvation Streams of same priory contest Not able to guarantee access based on latency, BW etc

Question 29

Distributed coordination function (DCF)?

Answer 29

``` Uses basic CSMA/CA mechanism Random back off - binary exponential Uses inter frame space: -SIFS: shortest, used for CTS/RTS messages -PIFS: medium, used for PCF -DIFS: normal (longest), used for data ```

Question 30

What are virtual collisions?

Answer 30

If the backoff counters of two or more parallel ACs in one QSTA reach zero at the same time, a scheduler inside the QSTA will avoid the virtual collision by granting the EDCA-TXOP to the highest priority AC. At the same time, the other colliding ACs will enter a backoff process as if there is an external collision

Question 31

Difference between peak rate and burst size?

Answer 31

Peak rate: measured in B/Sec. Max data rate of traffic Burst Size: measured in sec. Time traffic is generated at peak rate

Question 32

EDCA access categories?

Answer 32

In order of increasing priority: Background traffic Best effort Video Voice

Question 33

Which conditions allow a frame to be sent immediately?

Answer 33

If queue is empty and edit has been idle longer than the AIFS

Question 34

Calculation involving video?

Answer 34

Frame size X bit rate X frame rate /BW of link in B/Sec

Question 35

Calculate the bandwidth needed and compression ratio

Answer 35

Frame size X bitrate X frame rate / compression ratio Round up to allow excess

Question 36

What does RTSP provide?

Answer 36

Used in multimedia to provide extra Streaming functionalities like play and pause Controls multimedia communication Out of band

Question 37

Describe echo

Answer 37

Annoying effect you here on a phone call when you hear your own voice playing back after a period of time. Echo if a few milliseconds is bearable but a few 100 milliseconds disrupts the call severely. For echo to be noticeable it has to be loud and delayed. Delay is present in PSTN but worse in converged VoIP networks as they are more prone to delay. PSTN has less than 10 milliseconds delay where as VoIP can have 400 milliseconds.

Question 38

Stages of methodological approach

Answer 38

``` Defining the problem Modelling Programming Experimentation Implementation and monitoring ```

Question 39

Defining the problem

Answer 39

``` The problem must be defined What is the object of the exercise? What level of accuracy is required? The grater he accurate the more data required and the higher the cost What data must be collected? What form should the solution take? Is simulation the correct approach? Considerations of safety is paramount Carry out cost benefit analysis at regular intervals ```

Question 40

Modelling

Answer 40

Can involve compromises and decisions Who will do the modelling? What level of detail is needed? If opnet is being used an in screen model can be created. If alternative simulation used activity cycle diagram must be constructed. Where possible data collected by modeller should be used. The speed the model and results are to be provided to management? Time scales must be dynamic as more info is available Verification takes place here to ensure management accept model as appropriate and correct

Question 41

Programming

Answer 41

Programming involves the representation of the model, in a computer language. Choice of language used decided at formulation stage but can be changed. In general special purpose languages are used Coded program has to be verified Must be documented in detail Validation occurs here, to ensure model is accurate representation of the system etc

Question 42

Experimentation

Answer 42

Define the base run of the model. Run the media in a steady state and transient states and collect Consider sensitivity of model Discuss output with management

Question 43

Implementation and monitoring

Answer 43

Small amounts of info to be given regularly to those involved Don't try to please everyone

Question 44

802.11e HCF introduces two possible modes of operation

Answer 44

EDCA: like DIFS but with priority levels HCCA: is a CSMA/CA polling based method like PCF but without the shortcomings of PCF

Question 45

Five principles of model building

Answer 45

Relevance: should only include relevant aspects if the system Clarity: clear statement of objectives and assumptions Accuracy: how accurate diss it have to be? How much data? Available data: data must be readily available Cost: limited funds, very important factor, weak model that is complete is better than an incomplete stronger model

Question 46

Iterative nature of the 5 phase methodology

Answer 46

Each phase on its own is iterative S questions arise which may require you to go back to the start if the phase Iteration occurs between phases where progression may not be linear and several stages may be repeated Worst aces scenario all stages have to be done again if solution is not appropriate.

Question 47

How is time stamping provided by UDP?

Answer 47

Each packet has a time stamp that shows the time it was produced relative to the first or last packet, the receiver can add this time to the playback time to separate arrival time from playback time. UDP has no time stamping but RTP runs on top to provide this