Exam Prep Flashcards
What are the disadvantages of simulation?
Often tempting to use simulation when not the best method of solving the problem
Doesn’t produce optimal results
It can be very costly
What are the advantages of simulation?
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
Give examples of objects, attributes and events?
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
Challenges in multimedia traffic?
Compression Real time capabilities Inter-media sync Intra-media continuity Delays and jitters
Elements,of a multimedia transmitter?
Source device Acquisition of data Data compression Inter media synchronisation Integration of streams
Elements of multimedia receiver?
Media extractor
Decompression and reconstruction
Play back device
What is the role of an extractor?
Received integrated streams and extracts into separate streams, eg audio and video
Compare different types of jitter?
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.
Contrast common compression techniques?
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.
Endogenous and Exogenous variables.
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.
Token bucket
Credits idle host
Token generated per tick of the clock
Preserves burstyness
Uses one token per object call
Leaky bucket
Input varies output is constant
Doesn’t allow burstyness
Doesn’t credit idle host
Output is a smooth constant rate
Can a leaky bucket be used with a token bucket?
Yes! Leaky must go after token and leaky rate must be greater then that if the token bucket output rate.
How is sequencing provided in MM?
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.
Why is TCP unsuitable for MM?
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
Realtime transport protocol (RTP)?
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
Real time control protocol (RTCP)?
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
Congestion management concepts, policing and shaping?
Policing: propagates bursts, excess traffic is dropped - saw tooth pattern
Shaping: stores excess In a buffer,output is smooth. Relies on sufficient buffer size.
QoS mechanisms (FIFO, PQ and WFQ)?
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.
PCF wireless technique?
Uses Pifs
No contention : polls (pc)
Can be used with DCF in the same BSS
How to overcome the WFQ buffer problem?
Move discarded packets into a lower priory queue
Alter weight of queues in adaptive fashion based on queue size
Allow dynamic buffer allocation
Random early detection (RED)?
Monitors traffic loads
Randomly discards PDUs if congestion begins to increase
One type is weighted RED (WRED)
What is weighted RED (WRED)?
Combined RED with IP precedence
Selectively discards lower priority traffic when interface starts to get contested
Dropped on criteria; queue depth, source etc
Types of delay?
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
Temporal and spatial compression?
Temporal: removes redundancy between consecutive frames as they are almost the same
Spatial: done independently inside each frame by removing redundancy from the frame.
802.11e?
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
Enhanced distributed channel access (EDCA)
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)
Advantages of EDCA?
Voice and video have priority over data
Works well if network has light load
Disadvantages of EDCA?
Lower priority traffic starvation
Streams of same priory contest
Not able to guarantee access based on latency, BW etc
Distributed coordination function (DCF)?
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
What are virtual collisions?
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
Difference between peak rate and burst size?
Peak rate: measured in B/Sec. Max data rate of traffic
Burst Size: measured in sec. Time traffic is generated at peak rate
EDCA access categories?
In order of increasing priority:
Background traffic
Best effort
Video
Voice
Which conditions allow a frame to be sent immediately?
If queue is empty and edit has been idle longer than the AIFS
Calculation involving video?
Frame size X bit rate X frame rate
/BW of link in B/Sec
Calculate the bandwidth needed and compression ratio
Frame size X bitrate X frame rate
/ compression ratio
Round up to allow excess
What does RTSP provide?
Used in multimedia to provide extra Streaming functionalities like play and pause
Controls multimedia communication
Out of band
Describe echo
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.
Stages of methodological approach
Defining the problem Modelling Programming Experimentation Implementation and monitoring
Defining the problem
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
Modelling
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
Programming
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
Experimentation
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
Implementation and monitoring
Small amounts of info to be given regularly to those involved
Don’t try to please everyone
802.11e HCF introduces two possible modes of operation
EDCA: like DIFS but with priority levels
HCCA: is a CSMA/CA polling based method like PCF but without the shortcomings of PCF
Five principles of model building
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
Iterative nature of the 5 phase methodology
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.
How is time stamping provided by UDP?
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
