Real-Time Database

January 5, 2018 | Author: Anonymous | Category: Engineering & Technology, Electrical Engineering, Telecommunications
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Real-Time Communication

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 Sistem Waktu Nyata (CS4613)

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

Sistem Waktu Nyata (CS4613)

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 Sistem Waktu Nyata (CS4613)

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

Sistem Waktu Nyata (CS4613)

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 Sistem Waktu Nyata (CS4613)

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

Sistem Waktu Nyata (CS4613)

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

Sistem Waktu Nyata (CS4613)

Traffic characteristics may change Class 1

n

Rate

Class 2

Class 2 Class 1 0

a

c b Time

Sistem Waktu Nyata (CS4613)

 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

Communications media  Three most important media: • Electrical • Optical • Wireless

 Each medium has a distinct set of properties: • Bandwidth • Distance • Fault / interference etc. Sistem Waktu Nyata (CS4613)

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

Sistem Waktu Nyata (CS4613)

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

Sistem Waktu Nyata (CS4613)

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 Sistem Waktu Nyata (CS4613)

Protocols  Contention-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

Sistem Waktu Nyata (CS4613)

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? Sistem Waktu Nyata (CS4613)

Exploiting CSMA  Facts: • 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

Sistem Waktu Nyata (CS4613)

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

Sistem Waktu Nyata (CS4613)

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

Sistem Waktu Nyata (CS4613)

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 Sistem Waktu Nyata (CS4613)

RC & VC ( = 2 ) t4'

VC

t3'

channel busy

t2'

channel idle t1'

0

t1

t3

t2

RC Sistem Waktu Nyata (CS4613)

t4

VTCSMA algorithm

No Is the channel idle?

Yes Initialize the VC and start it running at the prescribed rate

Discard any message that have missed their transmission deadlines Yes

Is there M such that VSX(M)≤VC?

No

No Is the channel idle?

Freeze VC

Yes Transmit the highest priority M at that node

Yes Collision? No

Sistem Waktu Nyata (CS4613)

Retransmit immediately with probability p, or (with probability 1-p) modify VSX(M)

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 -  Sistem Waktu Nyata (CS4613)

VSX(M)  Virtual time to start transmission of message M  AM T  M VSX ( M )    LM  DM

Sistem Waktu Nyata (CS4613)

for VT CSMA - A for VT CSMA - T for VT CSMA - L for VT CSMA - D

When collision occur…  Retransmits M immediately, with probability p  Modify VSX(M): random number drawn from the interval I (current VC, L M ) (0, T )  M I  (current RC, L M ) (current RC, D M ) Sistem Waktu Nyata (CS4613)

for VT CSMA - A for VT CSMA - T for VT CSMA - L for VT CSMA - D

When the channel switches state from busy to idle…  VC is initialized as follows: no change for VT CSMA - A  VC  0 for VT CSMA - T RC for VT CSMA - L and VT CSMA - D 

Sistem Waktu Nyata (CS4613)

Example  VTCSMA-L;  = 2  TM = 15;  = 1  Messages: Node

M

RC at arrival

DM

LM

1

1

0

32

16

2

2

10

36

20

3

3

20

56

40

4

4

20

72

56

Sistem Waktu Nyata (CS4613)

Example (cont’d) 64 M4 transmission

56 48 M3 transmission

VC

40 32 24 16

M1 transmission

M2 discarded

8 0

8

16

Sistem Waktu Nyata (CS4613)

24

32

40

48

56

64

72

RC

List of protocols Protocol

DL guarantees?

Network

VTCSMA Window Timed token IEEE 802.5

No No Yes Yes

Broadcast Broadcast Ring Ring

Stop-and-Go Polled bus Hierarchical round-robin

Yes No Yes

Point-to-point Bus Point-to-point

Deadline-based

No

Point-to-point

Sistem Waktu Nyata (CS4613)

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 Sistem Waktu Nyata (CS4613)

LiveConcert

Main problem: DELAY (
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