1. Multimedia Applications
Multimedia and QoS

2. Multimedia Applications
Multimedia requirements
Streaming
Recovering from Jitter and Loss
RTP
Multimedia and QoS

3. Application Classes Typically:
sensitive to delay, but
can tolerate packet loss
would cause minor glitches that can be concealed
Data contains audio and video content (“continuous media”), three classes of applications:
Streaming
Unidirectional Real-Time
Interactive Real-Time
Multimedia and QoS

4. Application Classes (more)
Streaming
Clients request audio/video files from servers and pipeline reception over the network and display
Interactive: user can control operation (similar to VCR: pause, resume, fast forward, rewind, etc.)
Delay: from client request until display start can be 1 to 10 seconds
Multimedia and QoS

5. Application Classes (more)
Unidirectional Real-Time:
similar to existing TV and radio stations,
but delivery on the network
Non-interactive, just listen/view
Interactive Real-Time :
Phone conversation or video conference
More stringent delay requirement than Streaming and Unidirectional because of real-time nature
Video: < 150 msec acceptable
Audio: < 150 msec good, <400 msec acceptable
Multimedia and QoS

6. Multimedia Challenges
TCP/UDP/IP suite provides best-effort, no guarantees on expectation or variance of packet delay
Streaming applications delay of 5 to 10 seconds is typical and has been acceptable, but performance deteriorate if links are congested (transoceanic)
Real-Time Interactive requirements on delay and its jitter have been satisfied by over-provisioning (providing plenty of bandwidth), what will happen when the load increases?...
Multimedia and QoS

7. Challenges (more) Most router implementations:
use only First-Come-First-Serve (FCFS)
Limited packet processing and transmission scheduling
To mitigate impact of “best-effort” protocols, we can:
Use UDP to avoid TCP and its slow-start phase…
Buffer content at client and control playback to remedy jitter
Adapt compression level to available bandwidth
Multimedia and QoS

8. Solution Approaches in IP Networks
Just add more bandwidth and enhance caching capabilities (over-provisioning)!
Need major change of the protocols :
Incorporate resource reservation (bandwidth, processing, buffering), and new scheduling policies
Set up service level agreements with applications, monitor and enforce the agreements, charge accordingly
Need moderate changes (“Differentiated Services”):
Use two traffic classes for all packets and differentiate service accordingly
Charge based on class of packets
Network capacity is provided to ensure first class packets incur no significant delay at routers
Multimedia and QoS

9. Encoding and compressing Audio
Pulse Code Modulation (PCM)
Sample at a fixed rate
value is an arbitrary real number
Quantization: map samples to a given set of values
Encoding: encode the values as bits.
Examples:
sample rate 8000/sec. & 256 values : 64Kbps
CD: sample 44,100/sec. & 16 bits : Kbps
Compressions:
Depend on the output rate:
GSM (13 Kbps) G.729 (8 Kbps) G (6.4 or 5.3 Kbps)
MP3 (128 or 112 Kbps)
Multimedia and QoS

10. Video Compression Raw rate: Compression schemes (many):
images: 24 or 30 per second.
Size: 1024 x 1024 = 1 M pixels
pixel encoding 24 bit
Rate: 576 or 900 Mbps !!!
Compression schemes (many):
MPEG 1 (1.5 Mbps) CD quality
MPEG 2 (3-6 Mbps) DVD quality
Motion JPEG
H.261 (for ISDN)
Variable rate compression
Compression is a world of its own
Multimedia and QoS

11. Streaming Important and growing application due to
reduction of storage costs,
increase in high speed net access from homes,
enhancements to caching and
introduction of QoS in IP networks
Audio/Video file is segmented and sent over either TCP or UDP, public segmentation protocol: Real-Time Protocol (RTP)
Multimedia and QoS

12. Streaming
User interactive control is provided, e.g. the public protocol Real Time Streaming Protocol (RTSP)
Helper Application: displays content, which is typically requested via a Web browser; e.g. RealPlayer; typical functions:
Decompression
Jitter removal
Error correction: use redundant packets to be used for reconstruction of original stream
GUI for user control
Multimedia and QoS

13. Streaming From Web Servers: I
Audio: in files sent as HTTP objects
Video (interleaved audio and images in one file, or two separate files and client synchronizes the display) sent as HTTP object(s)
A simple architecture is to have the Browser requests the object(s) and after their reception pass them to the player for display
- No pipelining
Multimedia and QoS

14. Streaming From Web Server: II
Alternative: set up connection between server and player, then download
Web browser requests and receives a Meta File (a file describing the object) instead of receiving the file itself;
Browser launches the appropriate Player and passes it the Meta File;
Player sets up a TCP connection with Web Server and downloads the file
Multimedia and QoS

15. Meta file requests
Multimedia and QoS

16. III: Using a Streaming Server
This gets us around HTTP, allows a choice of UDP vs. TCP and the application layer protocol can be better tailored to Streaming; many enhancements options are possible (see next slide)
Multimedia and QoS

17. Options When Using a Streaming Server
Use UDP, and Server sends at a rate (Compression and Transmission) appropriate for client; to reduce jitter, Player buffers initially for 2-5 seconds, then starts display
Use TCP, and sender sends at maximum possible rate under TCP; retransmit when error is encountered; Player uses a much large buffer to smooth delivery rate of TCP
Multimedia and QoS

18. Real Time Streaming Protocol (RTSP)
For user to control display: rewind, fast forward, pause, resume, etc…
Out-of-band protocol (uses two connections, one for control messages (Port 554) and for media stream)
RFC 2326 permits use of either TCP or UDP for the control messages connection, sometimes called the RTSP Channel
As before, meta file is communicated to web browser which then launches the Player; Player sets up an RTSP connection for control messages in addition to the connection for the streaming media
Multimedia and QoS

19. Meta File Example <title>Twister</title> <session>
<group language=en lipsync>
<switch>
<track type=audio
e="PCMU/8000/1"
src = "rtsp://audio.example.com/twister/audio.en/lofi">
e="DVI4/16000/2" pt="90 DVI4/8000/1"
src="rtsp://audio.example.com/twister/audio.en/hifi">
</switch>
<track type="video/jpeg"
src="rtsp://video.example.com/twister/video">
</group>
</session>
Multimedia and QoS

20. RTSP Operation
Multimedia and QoS

21. Jitter: Definitions Input: t0, …, tn Delay Jitter: Rate Jitter
Delay jitter J
For every k: |t0 + kX - tk |  J
X = (tn - t0) / n
Rate Jitter
Rate Jitter A
Ik = tk - tk-1
For every k and j: |Ij - Ik|  A
Jitter and buffering
delay versus jitter
Multimedia and QoS

22. Fixed Playout Delay (Delay Jitter)
Multimedia and QoS

23. Adaptive Playout Delay (Rate Jitter)
Objective is to use a delay value that tracks the network delay performance as it varies
Modify the inter-packet times
Estimate the inter-arrival time
Similar to estimates of RTT and deviation in TCP
Use the estimate for playout rate
Multimedia and QoS

24. Recovery From Packet Loss
Loss is in a broader sense:
packet never arrives or
arrives later than its scheduled playout time
Retransmission is inappropriate for Real Time applications,
Other methods are used to reduce loss impact.
Multimedia and QoS

25. I: Recovery From Packet Loss - FEC
FEC is Forward Error Correction
Simplest FEC scheme adds a redundant chunk made up of exclusive OR of a group of n chunks;
redundancy is 1/n;
can reconstruct if at most one lost chunk;
playout time schedule assumes a loss per group
More errors ...
Multimedia and QoS

26. II: Recovery From Loss - Mixed Quality
Include two qualities
High quality
Low quality
If high quality lost, play low quality.
Better lower quality than nothing
Send the low quality with the next high quality
Can recover from single losses
Bandwidth blowup:
Depends on the low quality
Multimedia and QoS

27. Piggybacking Lower Quality Stream
Multimedia and QoS

28. III: Recovery From Loss - Interleaving
Divide audio data to smaller units and interleave
Upon loss, the missing data is spread out
Has no redundancy, but can cause delay in playout beyond Real Time requirements
Multimedia and QoS

29. Real-Time Protocol (RTP)
Provides standard packet format for real-time application
Typically runs over UDP
Specifies header fields below
Payload Type: 7 bits, providing 128 possible different types of encoding; eg PCM, MPEG2 video, etc.
Sequence Number: 16 bits; used to detect packet loss
Multimedia and QoS

30. Real-Time Protocol (RTP)
Timestamp: 32 bytes; gives the sampling instant of the first audio/video byte in the packet; used to remove jitter introduced by the network
Synchronization Source identifier (SSRC): 32 bits; an id for the source of a stream; assigned randomly by the source
Multimedia and QoS

31. RTP Control Protocol (RTCP)
Protocol specifies report packets exchanged between sources and destinations of multimedia information
Three reports are defined: Receiver reception, Sender, and Source description
Reports contain statistics such as the number of packets sent, number of packets lost, inter-arrival jitter
Used to modify sender transmission rates and for diagnostics purposes
Multimedia and QoS

32. Quality of Service Support
Multimedia and QoS

33. QoS in IP Networks
IETF groups are working on proposals to provide QoS control in IP networks, i.e., going beyond best effort to provide some assurance for QoS
Work in Progress includes RSVP, Differentiated Services, and Integrated Services
Simple model for sharing and congestion studies:
Multimedia and QoS

34. Principles for QoS Guarantees
Consider a phone application at 1Mbps and an FTP application sharing a 1.5 Mbps link.
bursts of FTP can congest the router and cause audio packets to be dropped.
want to give priority to audio over FTP
PRINCIPLE 1: Marking of packets is needed for router to distinguish between different classes; and new router policy to treat packets accordingly
Multimedia and QoS

35. Principles for QOS Guarantees (more)
Applications misbehave (audio sends packets at a rate higher than 1Mbps assumed above);
PRINCIPLE 2: provide protection (isolation) for one class from other classes
Require Policing Mechanisms to ensure sources adhere to bandwidth requirements; Marking and Policing need to be done at the edges:
Multimedia and QoS

36. Principles for QOS Guarantees (more)
Alternative to Marking and Policing: allocate a set portion of bandwidth to each application flow; can lead to inefficient use of bandwidth if one of the flows does not use its allocation
PRINCIPLE 3: While providing isolation, it is desirable to use resources as efficiently as possible
Multimedia and QoS

37. Principles for QOS Guarantees (more)
Cannot support traffic beyond link capacity
Two phone calls each requests 1 Mbps
PRINCIPLE 4: Need a Call Admission Process; application flow declares its needs, network may block call if it cannot satisfy the needs
Multimedia and QoS

38. Summary
Multimedia and QoS

39. Scheduling And Policing Mechanisms
Scheduling: choosing the next packet for transmission
FIFO
Priority Queue
Round Robin
Weighted Fair Queuing
We had a lecture on that!
Multimedia and QoS

40. Multimedia and QoS

41. Policing Mechanisms (Long term) Average Rate Peak Rate:
100 packets per sec or 6000 packets per min??
crucial aspect is the interval length
Peak Rate:
e.g., 6000 p p minute Avg and 1500 p p sec Peak
(Max.) Burst Size:
Max. number of packets sent consecutively, ie over a short period of time
Units of measurement
Packets versus bits
Multimedia and QoS

42. Policing Mechanisms
Token Bucket mechanism, provides a means for limiting input to specified Burst Size and Average Rate.
Bucket can hold b tokens;
tokens are generated at a rate of r token/sec
unless bucket is full of tokens.
Over an interval of length t, the number of packets that are admitted is less than or equal to (r t + b).
Multimedia and QoS

43. Integrated Services
An architecture for providing QOS guarantees in IP networks for individual application sessions
relies on resource reservation, and routers need to maintain state info (Virtual Circuit??), maintaining records of allocated resources and responding to new Call setup requests on that basis
Multimedia and QoS

44. Call Admission
Session must first declare its QoS requirement and characterize the traffic it will send through the network
R-spec: defines the QoS being requested
T-spec: defines the traffic characteristics
A signaling protocol is needed to carry the R-spec and T-spec to the routers where reservation is required;
RSVP is a leading candidate for such signaling protocol
Multimedia and QoS

45. Call Admission
Call Admission: routers will admit calls based on their R-spec and T-spec and base on the current resource allocated at the routers to other calls.
Multimedia and QoS

46. Integrated Services: Classes
Guaranteed QoS: this class is provided with firm bounds on queuing delay at a router; envisioned for hard real-time applications that are highly sensitive to end-to-end delay expectation and variance
Controlled Load: this class is provided a QoS closely approximating that provided by an unloaded router; envisioned for today’s IP network real-time applications which perform well in an unloaded network
Multimedia and QoS

47. QoS Routing: Multiple constraints
A request specifies the desired QoS requirements
e.g., BW, Delay, Jitter, packet loss, path reliability etc
Two type of constraints:
Additive: e.g., delay
Maximum (or Minimum): e.g., Bandwidth
Task
Find a (min cost) path which satisfies the constraints
if no feasible path found, reject the connection
Multimedia and QoS

48. Constraints: Delay (D) < 25, Available Bandwidth (BW) > 30
Example of QoS Routing
D = 24, BW = 55
D = 30, BW = 20 A
D = 5, BW = 90 B
D = 14, BW = 90
D = 5, BW = 90
D = 5, BW = 90
D = 7, BW = 90
D = 10, BW = 90
D = 5, BW = 90
D = 3, BW = 105
Constraints: Delay (D) < 25, Available Bandwidth (BW) > 30
Multimedia and QoS

49. Constraints: Delay (D) < 25, Available Bandwidth (BW) > 30
Example of QoS Routing
D = 24, BW = 55
D = 30, BW = 20 A
D = 5, BW = 90 B
D = 5, BW = 90
D = 14, BW = 90
D = 5, BW = 90
D = 7, BW = 90
D = 10, BW = 90
D = 5, BW = 90
D = 3, BW = 105
Constraints: Delay (D) < 25, Available Bandwidth (BW) > 30
Multimedia and QoS

50. Differentiated Services
Intended to address the following difficulties with Intserv and RSVP;
Scalability: maintaining states by routers in high speed networks is difficult sue to the very large number of flows
Flexible Service Models: Intserv has only two classes, want to provide more qualitative service classes; want to provide ‘relative’ service distinction (Platinum, Gold, Silver, …)
Simpler signaling: (than RSVP) many applications and users may only want to specify a more qualitative notion of service
Multimedia and QoS

51. Differentiated Services
Approach:
Only simple functions in the core, and relatively complex functions at edge routers (or hosts)
Do not define service classes, instead provides functional components with which service classes can be built
Multimedia and QoS

52. Edge Functions at DiffServ (DS)
At DS-capable host or first DS-capable router
Classification: edge node marks packets according to classification rules to be specified (manually by admin, or by some TBD protocol)
Traffic Conditioning: edge node may delay and then forward or may discard
Multimedia and QoS

53. No state info to be maintained by routers!
Core Functions
Forwarding: according to “Per-Hop-Behavior” or PHB specified for the particular packet class; such PHB is strictly based on class marking (no other header fields can be used to influence PHB)
BIG ADVANTAGE:
No state info to be maintained by routers!
Multimedia and QoS

54. Classification and Conditioning
Packet is marked in the Type of Service (TOS) in IPv4, and Traffic Class in IPv6
6 bits used for Differentiated Service Code Point (DSCP) and determine PHB that the packet will receive
2 bits are currently unused
Multimedia and QoS

55. Classification and Conditioning
It may be desirable to limit traffic injection rate of some class; user declares traffic profile (eg, rate and burst size); traffic is metered and shaped if non-conforming
Multimedia and QoS

56. Forwarding (PHB)
PHB result in a different observable (measurable) forwarding performance behavior
PHB does not specify what mechanisms to use to ensure required PHB performance behavior
Examples:
Class A gets x% of outgoing link bandwidth over time intervals of a specified length
Class A packets leave first before packets from class B
Multimedia and QoS

57. Forwarding (PHB) PHBs under consideration:
Expedited Forwarding: departure rate of packets from a class equals or exceeds a specified rate (logical link with a minimum guaranteed rate)
Assured Forwarding: 4 classes, each guaranteed a minimum amount of bandwidth and buffering; each with three drop preference partitions
Multimedia and QoS

58. Differentiated Services Issues
AF and EF are not even in a standard track yet… research ongoing
“Virtual Leased lines” and “Olympic” services are being discussed
Impact of crossing multiple ASs and routers that are not DS-capable
Multimedia and QoS

59. DiffServ Routers DiffServ Edge Router DiffServ Core Router Classifier
Marker
Meter
Policer
DiffServ Core Router
PHB
Select PHB
PHB
Local
conditions
PHB
DiffServ edge router utilises these functions in order to perform its task: - Classification (based on for example application, user or load) - Marking (of the DiffServ packets) - Metering (for policing or marking purposes) - Policing (handling of non-conformant traffic)
DiffServ core router will treat an IP packet based on 1) DS codepoint 2) Local conditions
DS codepoint is actually an index to a table that contains so called Per Hop Behaviors (PHB).
Per Hop Behavior is the set of rules how the packet (or actually the aggregate flow that the packet belongs to) is treated.
The local conditions can be, for example, the occupancy level(s) of the buffer(s) of the router.
Based on these two pieces of information, the edge router treats the packet.
For example, the packet may be discarded or accepted for buffering.
PHB
Extract
DSCP
Packet
treatment
Multimedia and QoS

60. IntServ vs. DiffServ DSCP IP IP IntServ network DiffServ network
This figure illustrates the basic principles of Integrated Services and Differentiated Services methods.
With Integrated Services -method the network creates "bit pipes" through the IP network.
If there is congestion, the user does not get such a bit pipe.
With Differentiated Services method the idea is to equip the IP packet with some priorities. Effectively, these provide IP packets with different capabilities to "fight" their way through the congestion points of the network.
If there is congestion, the packets with poor priorities will be discarded first. However, the user may always get at least something through.
"Call blocking" approach
"Prioritization" approach
Multimedia and QoS

61. Comparison of Intserv & Diffserv Architectures
Multimedia and QoS

62. Comparison of Intserv & Diffserv Architectures
Multimedia and QoS