1

Traffic Shaping and BW Allocation Papalexidis Nikos

30/3/2001

2

Traffic Shaping-Motivation

Prevent probable exceed of the Qos contract negotiated at the admission control (due to burstiness)

Reduce the bandwidth requirements for the shaped streams

3

Traffic Shapers Shapes the traffic so that a source may not

violate the traffic envelope negotiated If the traffic generated by the source does not

conform to the traffic envelope enforced,the shaper canDrop the violating cellsTag them as a lower priority trafficHold them in a reshaping buffer

4

Desirable Properties

The traffic envelope it enforces on a source should be easy to describe

Simple to implement

Able to capture a wide range of traffic characteristics

5

Traffic Shapers-Schemes

Leaky Bucket Window shapers

Jumping windowMoving window

Composite ShapersComposite Leaky Bucket

Dual LBTriple LB

Composite windows

6

Leaky Bucket

Data buffer

Peak rate

λp

Average rate

λa

Token

buffer

λt token rate

d

b

7

Leaky BucketSimple to implementThe size of the bucket imposes an upper

bound on the burst length The tolerance against a long burst depends on

the size of the bucket and the leaky rate

Worst case:A burst at t=0 equal to the bucket size followed by bit rate equal to the rate of the token generation

8

Leaky Bucket

Peak rate easy to police

Average rate not so easy (Cells arrive at peak rate during bursts)

Leaky rate > Mean bit rate for stability and achieving the required QoS

9

Leaky Bucket

Actual bit rate=Y x Negotiated mean bit rate

10

Leaky Bucket

Mean bit rate entering the network(leaky rate)=E x Negotiated mean bit rate

11

Leaky Bucket

The larger the LB size the better the control but the reaction time also increases.That is the time between the increase in average bit rate and its detection becomes large.LB size decreases as the leak rate increases for the same Qos.This provides a fast reaction to big jumps in the mean rate.

Trade off between LB size and the leaky rateIdeal behavior as M increases and l decreases.Low M and high l (near peak) provides almost no

control

12

Leaky BucketLB often introduces excessive access delays thereby

making it incapable of regulating real-time traffic

A policy which is less stringent on short-term burstiness while bounding long-term behavior with a LB-bound would be better suited for time critical traffic

AlternativeAllow the violating cells to enter the network with low

priority and discard them at the congested nodes (increase of the utilization/probable congestion)

13

Different LB typesOne LB is not effective for policing both peak and

mean rate.Dual LB is proposedFirst LB should control the peak rate.

Leaky rate set to the peak rateCell which do not confirm are simply dropped

Second LB controls the mean rateAs stated there is a trade-off between high reaction time and

high sensitivity in detection of violating cells.If both are required a mechanism consisting of a LB to control the peak connected in series with two LB in parallel to control the mean bit rate is the ideal solution.(Triple LB).Cell that passed the 1st LB is discarded whenever one of the parallel buckets (controlling the mean rate) overflows .

14

Triple LB

15

Time window shapersJumping window: Divides the time into fixed-size windows of length w

and limits the number of cells accepted within a window to a maximum number m Worst case: Burst length of 2m

Moving window: Similar restrictions but the window can slide on the time axis

Worst case: Burst length of m

• Traffic generated is smoother• More complexity in implementation

16

Composite window shapers

Composite jumping windowTime windows are encapsulated

Composite moving windowExtra complexity

17

Shaping and BW allocationBW allocated to the shaped stream depends

on the shaper parameters

Traffic policing imposes access delay avoiding overflow delays into the network

Trade-off between the access delay(stringent policy) and the network delay(lenient policy) at the switching nodes due to buffer overflows

18

Bandwidth Allocation The viability and economical feasibility of

packet video are largely contingent on the ability to reduce its BW requirements:Several approaches have been proposed that rely on one or more of:

Statistical multiplexingTemporal smoothingMulticasting

19

Statistical MultiplexingThe goal: Reduce the BW requirements of bursty sources.

Aggregation mechanism by which several individual streams are asynchronously superposed and transported over the same channel

BW is allocated to the aggregated traffic, resulting in a reduction in the per-stream allocated BW

20

SM with statistical guaranteesRelies on stochastic models.Useful in

determining the required resources for real-time video whose traffic profile is unknown

Obstacles:Difficulty in characterizing the departure traffic from a

statistical multiplexerRestricted applicability of the modelsAccurate video models often require specifying more

parameters than what is currently supported by the standards

21

SM with deterministic guarantees

MPEG sources can be statistically multiplexed at the video server and transported to a switch with minimal bounded delay and no losses.

The server allocates BW to the multiplexed traffic based on the peak value of the aggregate envelope.By manipulating the phase shifts between the MPEG sources the allocated BW can be less than the sum of the source peak rates

22

SM with deterministic guarantees

PSAB (Per Stream Allocation BW)Depends on the relative starting

times of the MPEG streams u :vector of the relative phases Specifies the synchronization structure of the

MPEG streams with respect to their GOPs u vector can be optimized by allowing the server

to control the starting times of new streams in order to minimize the PSAB at the cost of adding delay

Maximum delay:GOP period

23

SM with deterministic guarantees

MRP scheme (Minimum Rate Phase)A new stream is scheduled for multiplexing

in a phase for which the aggregate bit rate is minimal

Even if no scheduling is performed,some bandwidth gain can still be realized by multiplexing MPEG streams.In this case u has an arbitrary structure,not controlled by the server

24

Temporal smoothing General idea is to introduce a buffer in the path of

the stream,either at the sender or at the receiver to reduce the variability in trafficSender : Real-time video

stringent relay requirements and delay jitter guarantees

Receiver : Archive video

buffer placed inside the client set Smoothing buffer acts as low pass filter by

averaging the bit rate over a time window whose length is determined by the size of the buffer and its drain rate

25

Temporal smoothing For archived video the availability of the traffic

profile makes it possible to combine video smoothing with pre-fetching.

Video frames are transported to the client prior to their playback times

The client maintains a buffer that temporally store and smoothes out the received frames

Research for a transmission schedule which ensures that underflow and overflow will not occur at the client’s buffer

26

Temporal smoothing-JSQ pre-fetching JSQ(Join the Shortest Queue)

approach for pre-fetching archive video Assumes :

A single shared link between server-clientsBuffer in the client’s set

Principal idea:Keep track of the number of frames that have been

sent to each client.Sending different numbers of frames to each client,the server exploits the VBR nature of video

Favoring clients with shortest queues Up to 100% utilization of the link

27

Smoothing/SM

Investigate how video smoothing affects the statistical characteristics of video streams

Investigate the effect of smoothed

independent/correlated video streams on network resource control and management(impact on SM gains)

28

29

Unsmoothed: Periodic correlationSmoothed: Strong correlation(depending on the buffer size)Reducing of fast-time scale rate variability has implications on network resource management, especially buffer allocation.

30

Smoothing/SM gain

Independent streams:Video streams arriving at the multiplexer are randomly displaced.The starting frame is likely to be any one of the frames

Correlated streams:many users may start watching videos with a short time span,thus producing correlated video streams.

Multiplexing gain: (1-Total BW /Total peak rate)x100%

31

Unsmoothed:70%-80%Smoothed :10%-60%Still significant multiplexing gains to be exploited by VBR when individual streams are smoothed,especially when client buffers are relatively small

Independent smoothed streams-SM gain

32

Correlated video streams-SM gain

Correlated video streams have an enormous impact on aggregation of homogeneous sources,leaving almost no gain.There is much less severe impact when heterogeneous streams are aggregated

33

Comparison Statistical multiplexing:

Suitable for real-time video in which multiple distinct streams are to be transported over the same path

Omnidirectional video where several collocated cameras generate distinct video streams which are sent to the same direction

Temporal smoothing: Preferred for archive video whose traffic profile is

known a priori. Multicasting:

Best suited when a single video stream is to be transmitted to multiple destinations sharing portions of the path

34

CISCO Proposals Transmit priority: 4 levels Bandwidth allocation:

Call Admission Control Insured RateMaximum Rate

35

CISCO Proposals Traffic Policing: at the edges of the network

Maximum Burst Insured Burst

36

CISCO Proposals LB algorithm-Dual LB