CS 414 - Spring 2011 CS 414 – Multimedia Systems Design Lecture 16 – Multimedia Transport (Part 2) Klara Nahrstedt Spring 2011CS 414 - Spring 2011 Administrative HW1: due on Wednesday, March 2 Midterm review session: Friday, March 4, in class Midterm: Monday March 7, in class Class canceled on Friday, March 11 due to EOH Android Tutorial Mar 1st, 7 – 8:30 pm, 2405 Mar 8th, 8 – 9:30 pm, 2405Outline Data Streaming/Transmission Phase Traffic Shaping Isochronous Traffic Shaping Shaping Bursty Traffic Rate Control Error Control CS 414 - Spring 2011QoS Enforcement – Traffic Shaping In Packet Network, admission control, reservation is not sufficient to provide QoS guarantees Need traffic shaping at the entry to network and within network Traffic shaping Decides how packets will be sent into the network , hence regulates traffic Decides whether to accept a flow’s data Polices flows CS 414 - Spring 2011Purpose of Traffic Shaping Traffic shape A way of a flow to describe its traffic to the network Based on traffic shape, network manager (s) can determine if flow should be admitted into the network Given traffic shape, network manager(s) can periodically monitor flow’s traffic CS 414 - Spring 2011Example If we want to transmit data of 100 Mbps, Traffic Shape A: Do we take 1 packet size of size 100 Mbit and send it once a second, or Traffic Shape B: Do we take 1 packet of size 1 Kbit and send it every 10 microseconds? CS 414 - Spring 2011 1 Mbit 1 Mbit Kth second K+1 th second A BFlow’s Traffic Shape Parameters (Network QoS) Traffic Envelope Peak rate Average rate Burst length Burst duration Service Envelope Maximum tolerable delay Desired delay jitter others CS 414 - Spring 2011Source Classification Classification of sources : Data – bursty, weakly periodic, strongly regular Audio – continuous, strong periodic, strong regular Video – continuous, bursty due to compression, strong periodic, weakly regular Classification of sources into two classes: Constant Bit Rate (CBR) – audio Variable Bit rate (VBR) – video, data CS 414 - Spring 2011Bandwidth Allocation CBR traffic (shape defined by peak rate) CBR source needs peak rate allocation of bandwidth for congestion-free transmission VBR traffic (shape defined by average and peak rate) average rate can be small fraction of peak rate underutilization of resources can occur if pessimistic allocation (peak rate allocation) is applied Losses can occur if optimistic allocation (average rate allocation) is applied CS 414 - Spring 2011Isochronous Traffic Shaping (Simple Leaky Bucket Traffic Shaper) CS 414 - Spring 2011 Developed by Jon Turner, 1986 (Washington University, St. Louis)Example Consider for audio flow, size of the bucket β = 16 Kbytes Packet size = 1 Kbytes (one can accumulate burst up to 16 packets in the bucket) Regulator’s rate ρ = 8 packets per second, or 8KBps or 64Kbps Consider video flow, size of bucket β = 400 Kbytes Packet size = 40 Kbytes (burst of 10 packets) Regulator’s rate ρ = 5 packets per second, 200 KBps, 1600Kbps CS 414 - Spring 2011Isochronous Traffic Shaping (r,T)-smooth Traffic Shaper Developed by Golestani, 1990 Part of stop-and-go queuing/scheduling algorithm Traffic divided into T-bits frames, where T is fixed r-bits packet size per flow is considered, where r varies on a per flow basis CS 414 - Spring 2011(r,T) Traffic Shaper CS 414 - Spring 2011 Time line T-bits frames, sent every T-bit times r-bits packets • Flow is permitted to inject no more than r bits of data into the network frame in any T bit times • if the sender wants to send more than one packet of r-bits, it must wait for next T-bit frame. • A flow that obeys this rule has (r,T)-smooth traffic shape. r ≤ TComparison • It is relaxed from the simple leaky bucket traffic shaper because • Rather than sending one packet of size c every 1/ρ time units, (in simple leaky bucket ) • The flow can send c*k bits every 1/ρ time units , where k is T-bits times within the period 1/ρ CS 414 - Spring 2011 1/ρ K=2Limitations of Isochronous Traffic Shaping In case of (r,T)-smooth traffic shaping, one cannot send packet larger than r bits long, i.e., unless T is very long, the maximum packet size may be very small. The range of behaviors is limited to fixed rate flows Variable flows must request data rate equal to peak rate which is wasteful CS 414 - Spring 2011Isochronous Traffic Shaping with Priorities Idea: if a flow exceeds its rate, excess packets are given lower priority If network is heavily loaded, packets will be preferentially dropped Decision place to assign priority At the sender Application marks its own packets since application knows best which packets are less important In the network (policing) Network marks overflow packets with lower priority CS 414 - Spring 2011Shaping Bursty Traffic Patterns (Token Bucket) CS 414 - Spring 2011Token Bucket The effect of TB is different than Leaky Bucket (LB) Consider sending packet of size b tokens (b<β): Token bucket is full – packet is sent and b tokens are removed from bucket Token bucket is empty – packet must wait until b tokens drip into bucket, at which time it is sent Bucket is partially full – let’s consider B tokens in bucket; if b ≤ B then packet is sent immediately, Else wait for remaining b-B tokens before being sent. CS 414 - Spring 2011Comparison between TB and LB Token Bucket Simple Leaky Bucket TB permits burstiness, but bounds it LB forces bursty traffic to smooth out Burstiness is bounded as follows: - Flow never sends more than β+τ*ρ tokens worth of data in interval τ and - Long-term transmission rate will not exceed ρ Flow never sends faster than ρ worth of packets per second TB does not have discard or priority policy LB has priority policy TB more flexible LB is rigid TB is easy to implement - Each flow needs counter to count tokens, -
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