1. Real-Time Communication

2. In embedded systems… Data flows :
from the sensors and control panels to the central cluster of processors
between processors in the central cluster
from processors to the actuators and output displays
Communication overhead adds to the computer response time
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3. In hard & soft RTS…
Hard: use communication protocols that allow the communication overhead to be bounded
Soft: (exp: MulMed & video conf.)
excessive delays in message delivery can significantly degrade the quality of service
occasional failure to meet message-delivery is not fatal
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4. Comm. key performance Traditional  system throughput
How much data can be transferred over the network in one unit time from source to destination
RTS  probability of delivering a message by a certain deadline
Lost message = infinite delivery time
Measures:
the speed with which messages are delivered
the probability of losing messages
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5. Overheads causing delay
Formatting and / or packetizing the message
Queueing the message, as it waits for access to the communication medium
Sending the message from the source to the destination
Deformatting the message
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6. Real-Time Traffic Typically classified by:
Its deadline
Arrival pattern
Priority
In hard RTS (exp: embedded appl.): the deadline of the traffic is related to the deadline of the task to which that communication belongs
In soft RTS (exp: MulMed appl.): the deadline is related directly to the application
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7. Real-Time Traffic (cont’d)
Priority is based on the importance of that message class to the application
If there is an overload traffic, message priority can be used to determine which message are dropped to ensure that the more important traffic is delivered in a timely fashion
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8. RT Traffic Rates
Constant rate: fixed-size packets are generated at periodic intervals
Many sensors produce such traffic
Smooth and not bursty  easy to handle, small buffer
Variable rate: fixed-size packet being generated at irregular intervals (exp: voice –talkspurt–) or variable-sized packet being generated at regular intervals (exp: video)
Bursty traffic  greater demands on buffer space
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9. Traffic characteristics may change
Class 1 has priority at node n
Class 2 will piled up over the intervals [0,a] and [b,c]
The output of class 2 from node n will also bursty
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10. Communications media Three most important media:
Electrical
Optical
Wireless
Each medium has a distinct set of properties:
Bandwidth
Distance
Fault / interference etc.
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11. Network Topologies
Must be carefully chosen, since it affects the system response time and reliability
Broadly classified: point-to-point & shared
Popular topologies: bus, ring, dual-ring, star, n-dimensional hypercube, multistage network
Physical topology vs logical topology
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12. Important features of topology
Diameter: max distance (number of hops) between any two nodes
Node degree: number of edges adjacent to each node  determine the number of I/O port per node and the number of links in the system
Fault-tolerance: measure the extent to which the network can withstand the failure of individual links and nodes while still remaining functional
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13. Sending message Packet switching Circuit switching
Wormhole routing  require less node buffer
Pipelining packet transmission in a multihop network
Each packet is broken down into a train of flits, each about one or two bytes long
The sender transmits one flit per unit time, and the flits are forwarded from node to node until they reach their destination
Only the header flit in a train has the destination information; each node is simply forwards the next flit to the same node that it sent the previous flit to in the train
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14. Protocols Contention-based protocols Token-based protocols
Virtual-Time Carrier-Sensed Multiple Access (VTCSMA)
Window protocol
Token-based protocols
Timed-token protocol
Token-Ring protocol (IEEE 802.5)
Stop-and-Go Multihop protocol
Polled bus protocol
Hierarchical round-robin protocol
Deadline-based protocols
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15. CSMA
CSMA is an efficient communication scheme when the end-to-end transmission delay is much less than the average time to transmit a packet and when the load is not very high
CSMA is a truly distributed algorithm: each node deciding when it will transmit
Q: Is it possible to implement a priority algorithm using CSMA?
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16. Exploiting CSMA Facts: Each node has information about:
Nodes do see a consistent time if their clocks are synchronized
Nodes observe the same channel
Each node has information about:
The state of the channel
The priorities of the packets waiting in its transmission buffer to be transmitted over the network
The time according to the synchronized clock
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17. Exploiting CSMA (cont’d)
Node does not have any idea of the priorities of any packets that may be awaiting transmission at the other nodes
Simply using the state of the channel and the priorities of its packets is not sufficient; the time information must also be used
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18. Road to VTCSMA
The key to VTCSMA: if the priority of the packet can be computed as a function of the current time, as well as of some other parameter, it may be possible to use the time information to implicitly arrive at some global ordering of priorities.
Exp: we can strive to serve packets according to their deadlines, arrival times, and laxities
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19. Clocks in VTCSMA Uses two clocks at each node: The real clock (RC)
tells the ‘real time’
is synchronized with the clocks at the other nodes
The virtual clock (VC)
when the channel is busy, the VC freeze
when the channel become free, the VC is reset and then runs at the rate  >1
VC runs faster than RC when the channel is free, and not at all when it is busy
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20. RC & VC ( = 2 )
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21. VTCSMA algorithm
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22. Some notations
: Propagation time from one end of the network to the other
AM: Arrival time of message M
TM: Time required to transmit message M
DM: Deadline by which message M must be delivered to its destination
LM: Latest time by which the message must be sent to be able to meet the deadline; LM = DM – TM - 
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23. VSX(M) Virtual time to start transmission of message M
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24. When collision occur… Retransmits M immediately, with probability p
Modify VSX(M): random number drawn from the interval I
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25. When the channel switches state from busy to idle…
VC is initialized as follows:
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26. Example VTCSMA-L;  = 2 TM = 15;  = 1 Messages: Node M RC at arrivalDM LM 1 32 16 2 10 36 20 3 56 40 4 72
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27. Example (cont’d)
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28. List of protocols Protocol DL guarantees? Network VTCSMA No Broadcast
Window
Timed token
Yes
Ring
IEEE 802.5
Stop-and-Go
Point-to-point
Polled bus
Bus
Hierarchical round-robin
Deadline-based
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29. Project:
Is a community-oriented environment for distributed live musical performances
It allows people in different cities to:
connect musical instruments and video cameras to computers
find other musicians
organize virtual music bands (up to four musicians)
finally, play together over broadband Internet with high quality sound and see the video picture of all the members, as if they were in the same room
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30. LiveConcert Main problem: DELAY (<50 ms)
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31. Sistem Waktu Nyata (CS4613)

32. omniMusic technical overview
For RT audio & video transmission the smart clients use peer-to-peer approach and multicast technology
omniMusic commits to deliver high quality sound and ultra-low latency (<50 ms)
Fast audio capturing with ASIO drivers
Fast audio rendering to network with self-developed kernel-mode driver using Kernel Streaming technology
Fast audio compression with self-developed codec
Fast audio transmission with Real-Time Transfer Protocol, on top of MSR Conference platform RTP implementation
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33. Sistem Waktu Nyata (CS4613)

34. omniMusic won the first prize in the Software Design Invitational
Russia:
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35. ITU H.323
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36. RTP & RTCP RTP: Real-Time Transport Protocol
RTCP: RTP Control Protocol
H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, RTP: A Transport Protocol for Real-Time Applications, IETF RFC 1889
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37. QoS support
Two levels of congestion control: global internetwork level and operates at router level
Two schemes:
Integrated Services (IntServ)
Resource Reservation Protocol (RSVP)
RFC
Differentiated Services (DiffServ)
RFC 2474, 2475
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