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.
