- •Introduction
- •Applications of Real-Time Systems
- •Voltage
- •Figure 7: Conversion of an Analog Signal to a 16 bit Binary Number
- •Figure 11: Schematic Representation of tmr
- •It is relatively simple to design a hardware equipment to be fault-tolerant. The following are two methods that are popularly used to achieve hardware fault-tolerance:
- •Software Fault-Tolerance Techniques
- •Types of Real-Time Tasks
- •Timing Constraints
- •Events in a Real-Time System
- •Figure 16: Delay Constraint Between Two Events el and e2
- •Examples of Different Types of Timing Constraints
- •Figure 19: Classification of Timing Constraints
- •Real-Time Task Scheduling
- •Figure 1: Relative and Absolute Deadlines of a Task
- •Figure 2: Precedence Relation Among Tasks
- •Types of Real-Time Tasks and Their Characteristics
- •Classification of Real-Time Task Scheduling Algorithms
- •Figure 5: An Example Schedule Table for a Cyclic Scheduler
- •Figure 6: Major and Minor Cycles in a Cyclic Scheduler
- •Comparison of Cyclic with Table-Driven Scheduling
- •Hybrid Schedulers
- •Event-driven Scheduling
- •Is edf Really a Dynamic Priority Scheduling Algorithm?
- •Implementation of edf
- •Figure 10: Priority Assignment to Tasks in rma
- •We now illustrate the applicability of the rma schodulability criteria through a few examples.
- •Deadline Monotonic Algorithm (dma)
- •Handling Aperiodic and Sporadic Tasks
- •Dealing With Task Jitter
- •W Good real-time task scheduling algorithms ensure fairness to real-time tasks while scheduling.
- •State whether the following assertions are True or False. Write one or two sentences to justify your choice in each case.
- •Figure 2: Unbounded Priority Inversion
- •Highest Locker Protocol(hlp)
- •Priority Ceiling Protocol (pcp)
- •Comparison of Resource Sharing Protocols
- •Handling Task Dependencies
- •Fault-Tolerant Scheduling of Tasks
- •Clocks in Distributed Real-Time Systems
- •Clock Synchronization
- •Figure 1: Centralized synchronization system
- •Cn Slave clocks
- •Commercial Real-Time Operating Systems
- •Time Services
- •Clock Interrupt Processing
- •Providing High Clock Resolution
- •Figure 2: Use of a Watchdog Tinier
- •Unix as a Real-Time Operating System
- •In Unix, dynamic priority computations cause I/o intensive tasks to migrate to higher and higher priority levels, whereas cpu-intensive tasks are made to seek lower priority levels.
- •Host-Target Approach
- •Preemption Point Approach
- •Self-Host Systems
- •Windows As a Real-Time Operating System
- •Figure 9: Task Priorities in Windows nt
- •Open Software
- •Genesis of posix
- •Overview of posix
- •Real-Time posix Standard
- •Rt Linux
- •7.8 Windows ce
- •Benchmarking Real-Time Systems
- •Figure 13: Task Switching Time Among Equal Priority Tasks
- •Real-Time Communication
- •Figure 2: a Bus Architecture
- •Figure 4: Logical Ring in a Token Bus
- •Soft Real-Time Communication in a lan
- •Figure 6: Priority Arbitration Example
- •Figure 8: Problem in Virtual Time Protocol
- •Figure 9: Structure of a Token in ieee 802.5
- •Figure 10: Frames in the Window-based Protocol
- •Performance Comparison
- •A Basic Service Model
- •Traffic Characterization
- •Figure 16: Constant Bit-Rato Traffic
- •Routing Algorithms
- •Resource Reservation
- •Resource Reservation Protocol (rsvp)
- •Traffic Shaping and Policing
- •Traffic Distortion
- •Traffic Scheduling Disciplines
- •Figure 20: Packet Service in Jittor-edd
- •Differentiated Services
- •Functional Elements of DiffServ Architecture
- •Real Time Databases
- •Isolation: Transactions are executed concurrently as long as they do not interfere in each other’s computations.
- •Real-Time Databases
- •Real-Time Database Application Design Issues
- •Temporal Consistency
- •Concurrency Control in Real-Time Databases
- •It can bo shown that pcp is doadlock froo and single blocking. Rocolloct that single blocking moans that once a transaction starts executing after being blocked, it may not block again.
- •Speculative Concurrency Control
- •Comparison of Concurrency Control Protocols
- •Commercial Real-Time Databases
- •Figure 16: Uniform Priority Assignment to Tasks of Example 15
- •Version 2 cse, iit Kharagpur
Traffic Characterization
Traffic characterization is a model of the data generation characteristics of a source. Normally a traffic is characterized by bounding its volume. A bound on tho volume of tho traffic can bo used to bound tho amount of network resources that may have to bo reserved to provide tho required quality of services. During connection establishment, resources are provided based on tho traffic characterization and rosourco requirements. There are many specification models proposed in tho literature for traffic characterization. Wo discuss a few important ones.
(Xw/n i$max) model:
Tho (Xmin,Smax) model bounds a traffic source with a peak rate. A connection satisfies (Xmin,Smax) model, iff tho intor-arrival times botwoon two packets is always loss than Xmin, and size of tho largest packet is bound by Smax. Both tho peak and average rates of traffic in this model is given by .
Figure 16: Constant Bit-Rato Traffic
This model provides a tight bound for CBR traffic. From Fig. 16 it can be soon that for CBR traffic both tho worst case and tho average packet arrival times are tho same. However, for bursty traffic use of this model to specify traffic characteristic results in very conservative rosourco reservation, loading to low utilization of tho reserved resources.
(r,T) model:
In this model tho timo axis is divided into intervals of length T oach, called a frame. A connection satisfies (r,T) model, if it generates no more than r.T bits of traffic in any interval T. In this model, r is tho upper bound on tho average rate over tho averaging interval T. This model is in somo sense similar to tho (Xmin,Smax) model. This model also provides a tight bound for CBR traffic. However, for bursty traffic it results in very conservative rosourco reservation, loading to low utilization of tho reserved resources.
(XminjXaVg *S,nax ,1) model:
In this model, Xm,in specifies tho minimum intor-arrival timo botwoon two packets. Smax specifies tho maximum packet size and I is a certain interval over which tho observations are valid. Xave is tho average intor-arrival timo botwoon packet over any I interval. So a connection satisfies this model if it satisfies (Xmin,Smax) model and tho average intor-arrival timo of consecutive packets is larger than Xave during any interval of length I. Hero in this model tho peak rate and tho average rate of tho traffic are Smax /Xm,in and Smax/Xave. Notes that this model provides bounds for both peak rate and average rate of tho traffic. This model provides bettor characterization of VBR traffic compared to both (Xmin,Smax) and (r,T) models.
(a,p) model:
In this model a is tho maximum burst size and p is tho long term average rate of tho source traffic, respectively. Average traffic is calculated by (number of packets generated over largo duration)/(largo duration). A connection satisfies ((T,p), if during any interval of length t tho number of bits generated by tho connection in that interval is loss than a + pt. Burst may come over t duration or not because it is very infrequent so no of bits generated is loss than or equal a + pt. This model can be satisfactorily be used to model bursty traffic sources.
Multiple rate bounding: A more accurate approximation for bursty traffic sources can be obtained by characterizing the traffic with multiple bounding average rates, each over different averaging intervals. A traffic would satisfy {(rl,Tl)(r2,T2)...}, if И < Г2 < T3..., and over any interval I the number of bits generated is bounded by ri*Ti if Ti — 1 < I < Ti. As the averaging interval gets longer, a source is bounded by a rate lower than its peak rate and closer to its long term average rate.
Routing
Once a traffic source has specified its traffic characteristics and its QoS requirements to the network, it waits for the network to accept the request before it can start its transmissions. The network on its part checks if adequate resource is available along any path from source to destination to meet the demand. Route selection (unicast or multicast) for a session during the connection establishment phase.
QoS Routing
Current Internet protocols use routing algorithms such as the shortest path routing in which the routing can be optimized for some metric without taking into account whether the required resource is actually available. Consequently, the flows might be routed over the paths that are unable to support their requirements while other paths might exist which could have satisfied the specified requirements. This would lead to breach of committed QoS guarantees to connections. Researchers have now proposed QoS routing or constraint-based routing which overcome these problem. The primary goals of QoS routing are:
To select routes that can meet certain QoS requirements,
To increase the utilization of the network.
While determining a route, QoS routing schemes not only consider the topology of a network, but also the requirement of the flow, the resource availability of the links, and other policies that may be specified by the network
administrators. Therefore, QoS routing would be able to find a longer and lightly-loaded path rather than the short
est path that may be heavily-loaded. QoS routing schemes are therefore expected to be more successful than the traditional routing schemes in meeting QoS guarantees.