Unit-4

Real-Time Communication

Basic Concepts in Real-time Communication: Real-time communication refers to any form of

communication where there is an immediate exchange of information between two or more parties. Some
common examples of real-time communication include phone calls, video conferencing, instant messaging, and
live streaming.

Here are some basic concepts in real-time communication:

1. Latency:
● Latency refers to the delay between when data is sent and when it is received.
● In real-time communication, low latency is crucial to ensure that communication is smooth and
uninterrupted.
2. Bandwidth:
● Bandwidth refers to the amount of data that can be transmitted over a network at any given time.
● The higher the bandwidth, the more data can be transmitted, which is important in real-time
communication for maintaining a good quality of service.
3. Packet loss:
● Packet loss refers to the loss of data packets during transmission.
● In real-time communication, even a small amount of packet loss can cause disruptions and make the
communication experience less smooth.
4. Jitter:
● Jitter is the variation in latency over time.
● In real-time communication, jitter can cause delays and interruptions, which can lead to a poor user
experience.
5. Quality of Service (QoS):
● Quality of Service is a measure of the level of service that a network provides.
● In real-time communication, QoS is important for ensuring that the communication is of high quality
and that any issues are quickly resolved.
6. Codecs:
● Codecs are used to encode and decode audio and video data for transmission over a network.
● Different codecs have different levels of compression and quality, which can affect the overall quality
of real-time communication.
● Some popular codecs used in real-time communication include H.264 for video and Opus for audio.

Real-time communication (RT) systems can be categorized into two main types: Soft real-time systems
and Hard real-time systems.
Basic concepts of soft Real-time communication (RT) systems:
● Soft real-time communication systems are those that have timing requirements that are not as strict as
hard real-time systems.
● They allow some flexibility in meeting timing deadlines, and missing a deadline may not result in
catastrophic consequences.
● Here are some basic concepts of soft real-time communication systems.

1. Timing Constraints:
● Soft real-time communication systems have timing constraints that must be met to ensure optimal
performance.
● However, there is some flexibility in meeting these timing constraints, and a missed deadline is not
critical.
● This is in contrast to hard real-time systems, where a missed deadline can have serious consequences.

2. Quality of Service (QoS):

● QoS is important in soft real-time communication systems to ensure that data is delivered in a timely
and efficient manner, without causing major disruptions or delays.
● This includes considerations such as bandwidth, latency, packet loss, and jitter.
3. Data Compression:
● Soft real-time communication systems often use data compression techniques to minimize the amount
of data that needs to be transmitted.
● This helps to reduce bandwidth requirements and improve the overall performance of the system.
4. Feedback Control:
● Feedback control mechanisms can be used in soft real-time communication systems to adjust the timing
and QoS parameters as needed to ensure optimal performance.
5. Network Protocols:
● Soft real-time communication systems rely on network protocols to ensure that data is transmitted
reliably and efficiently over the network.
● Some common protocols used in soft real-time communication systems include RTP (Real-time
Transport Protocol) for audio and video streaming and SIP (Session Initiation Protocol) for voice and
video calls.
6. Resource Allocation:
● Soft real-time communication systems must allocate resources, such as CPU time and memory,
efficiently to ensure that the system meets its timing and QoS requirements.
● This may involve using specialized hardware or software to optimize performance.

Basic concepts of Hard Real- time communication (RT) systems:

● Hard real-time communication (RT) systems are those that have strict timing requirements and a missed
deadline can have catastrophic consequences.
Here are some basic concepts of hard real-time communication systems:
1. Timing Constraints:
● Hard real-time communication systems have strict timing constraints that must be met to ensure optimal
performance.
● A missed deadline can result in serious consequences such as system crashes, data loss, or physical harm
to users.
2. Predictability:
● Hard real-time communication systems are designed to be predictable and deterministic, with known
worst-case execution times for all operations.
● This is necessary to ensure that timing constraints can be met reliably and without fail.
3. Resource Reservation:
● In hard real-time communication systems, resources such as CPU time, memory, and network
bandwidth are often reserved in advance to ensure that they are available when needed.
● This may involve using specialized hardware or software to allocate resources efficiently.
4. Fault Tolerance:
● Hard real-time communication systems often incorporate fault-tolerance mechanisms to ensure that the
system continues to operate correctly in the event of hardware or software failures.
● This may involve redundancy, error detection and correction, or failover mechanisms.
5. Priority-Based Scheduling:
● Priority-based scheduling is often used in hard real-time communication systems to ensure that tasks are
executed in the correct order and with the necessary timing constraints.
● Higher-priority tasks are executed before lower-priority tasks, and tasks with critical timing
requirements are given the highest priority.
6. Real-Time Operating Systems (RTOS):
● Real-time operating systems are specialized operating systems designed specifically for hard real-time
communication systems.
● They provide deterministic scheduling, resource management, and other features that are necessary to
ensure that timing constraints can be met reliably.

Model of Real-Time Communication: Real-time communication (RTC) is a model of communication that

involves sending and receiving data in real time.
The model typically includes the following components:

1. Sender:
● The sender is the entity that initiates the communication by sending data to the receiver.
● In RTC, the sender may be a person using a device such as a phone, computer, or video conferencing
system.

2. Network:
● The network refers to the infrastructure that carries the data between the sender and the receiver.
● This may include wired or wireless networks, such as the internet, cellular networks, or local area
networks.
3. Protocol:
● The protocol is the set of rules that governs how the data is transmitted between the sender and the
receiver.
● In RTC, the protocol must be designed to handle real-time constraints, such as low latency and low
packet loss, to ensure that the communication is timely and efficient.
4. Receiver:
● The receiver is the entity that receives the data from the sender.
● In RTC, the receiver may be a person using a device such as a phone, computer, or video conferencing
system.
5. Media:
● The media refers to the type of data being transmitted between the sender and receiver.
● This may include audio, video, or other types of data.

6. Codec:
● The codec is the software or hardware that compresses and decompresses the media to reduce the amount
of data that needs to be transmitted.
● In RTC, codecs are designed to be efficient and fast to minimise latency and ensure real-time
performance.
7. Application:
● The application is the software or system that enables real-time communication between the sender and
receiver.
● This may include communication platforms, video conferencing systems, or messaging applications.

Overall, the model of real-time communication involves a sender sending data over a network using a protocol
that is optimized for real-time performance, with the receiver receiving the data and using an application to
enable communication. This model is essential for many applications, such as video conferencing, online
gaming, and real-time collaboration, where timely and efficient communication is critical.

Priority-Based Service Disciplines for Switched Networks:

● Priority-based service disciplines are used in switched networks to provide different levels of service to
different types of traffic based on their priority.
● This allows high-priority traffic, such as real-time communication traffic, to receive preferential
treatment over lower-priority traffic, such as file transfers or email.

There are several types of priority-based service disciplines used in switched networks, including:
1. Priority Queuing (PQ):
● In PQ, traffic is divided into multiple priority queues, with each queue assigned a different priority level.
 ● Higher-priority traffic is transmitted before lower-priority traffic, regardless of how much traffic is
waiting in each queue.
● This ensures that high-priority traffic receives preferential treatment.

2. Weighted Fair Queuing (WFQ):

● In WFQ, traffic is divided into multiple flows, with each flow assigned a different weight.
● Traffic is then transmitted in a round-robin fashion, with each flow receiving a proportional amount of
bandwidth based on its weight.
● This ensures that high-priority flows receive more bandwidth than lower-priority flows.

3. Class-Based Queuing (CBQ):

● In CBQ, traffic is divided into different classes based on its priority, with each class assigned a different
bandwidth allocation.
● Higher-priority traffic is allocated more bandwidth than lower-priority traffic, which ensures that high-
priority traffic receives preferential treatment.

4. Hierarchical Fair Service Curve (HFSC):

● In HFSC, traffic is divided into different classes based on its priority, with each class assigned a service
curve that specifies its bandwidth allocation over time.
● Traffic is then transmitted based on its service curve, with higher-priority traffic receiving more
bandwidth than lower-priority traffic.
Weighted Round-Robin Service Disciplines for Switched Networks:
● Weighted Round-Robin (WRR) is a service discipline used in switched networks to allocate bandwidth
among multiple queues.
● It is a variation of the round-robin discipline, where each queue is allocated a weight proportional to its
priority level or the desired amount of bandwidth.
● In a WRR service discipline, each queue is assigned a weight that determines the fraction of bandwidth
that should be allocated to that queue.
● The scheduler then selects packets from each queue in a round-robin fashion, taking into account the
weight assigned to each queue.

For Example:
● Consider a switched network with two queues.
● Queue 1 is assigned a weight of 3, and Queue 2 is assigned a weight of 1.
● This means that Queue 1 should receive three times as much bandwidth as Queue 2.
● The scheduler would select 3 packets from Queue 1 for every 1 packet selected from Queue 2.

WRR is often used in situations where multiple queues need to share a limited amount of bandwidth, such as in
a network with multiple classes of service or in a network with limited bandwidth. By allocating bandwidth
based on the weight of each queue, WRR ensures that each queue receives a fair share of the available bandwidth
while also allowing higher-priority queues to receive a larger share if needed.
Overall, the WRR service discipline is an effective way to allocate bandwidth among multiple queues in a
switched network, ensuring that packets are transmitted in a fair and efficient manner.

Medium Access Control Protocols for Broadcast Networks:

● Medium Access Control (MAC) protocols are used to control access to the shared communication
medium in broadcast networks.
● In broadcast networks, multiple devices share a common communication medium, and MAC protocols
are necessary to avoid collisions and ensure efficient use of the shared medium.

Here are some examples of MAC protocols commonly used in broadcast networks:

1. Carrier Sense Multiple Access/Collision Detection (CSMA/CD):

● This is a popular MAC protocol used in wired networks, such as Ethernet.
● In CSMA/CD, a device listens to the communication medium before transmitting to ensure that it is not
already in use.
● If the medium is idle, the device transmits its packet.
● If there is a collision with another packet, the devices involved back of for a random time before
attempting to retransmit.

2. Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA):

● This MAC protocol is commonly used in wireless networks, such as Wi-Fi.
● In CSMA/CA, devices use a random back off time before attempting to transmit to avoid collisions.
● Devices also use a request-to-send (RTS)/clear-to-send (CTS) mechanism to reserve the communication
medium before transmitting, reducing the chance of collisions.

3. Token Passing:
● This MAC protocol uses a token that is passed from device to device in a predetermined order.
● The device holding the token is allowed to transmit its packet.
● Once the transmission is complete, the token is passed to the next device in the predetermined order.

4. Reservation-Based MAC protocols:

● These MAC protocols allow devices to reserve the communication medium in advance, reducing the
chance of collisions.
● In reservation-based protocols, devices send a request to reserve the communication medium before
transmitting.
● The reservation is granted by the network, and the device can then transmit its packet.
Internet Reservation Protocols:
● Internet Reservation Protocols (IRPs) are a family of protocols that are used to reserve resources in
computer networks.
● The goal of IRPs is to provide efficient and reliable resource reservation mechanisms that can be used
in various applications, such as multimedia streaming, real-time communications, and cloud computing.

There are several types of IRPs, including:

1. Resource Reservation Protocol (RSVP):
● RSVP is a signalling protocol used to reserve network resources for specific flows.
● It is commonly used in real-time multimedia applications and is designed to provide QoS guarantees for
such applications.
2. Common Open Policy Service (COPS):
● COPS is a protocol that allows network devices to exchange information about resource availability and
to make resource reservations.
● COPS can be used to provide QoS guarantees for various applications, including multimedia streaming
and real-time communications.
3. Next Steps in Signaling (NSIS):
● NSIS is a protocol suite that provides a framework for signalling and resource reservation in IP
networks.
● NSIS includes several protocols, including the NSIS signalling protocol (NSLP) and the NSIS transport
layer protocol (NTLP).
● NSIS is designed to provide QoS guarantees for various applications, including multimedia streaming
and real-time communications.
4. Differentiated Services (DiffServ):
● DiffServ is a QoS mechanism that allows network devices to prioritize traffic based on predefined
service levels.
● DiffServ is often used in conjunction with other IRPs, such as RSVP, to provide QoS guarantees for
specific applications.

Overall, IRPs are an essential part of modern computer networks, providing efficient and reliable mechanisms
for resource reservation and QoS guarantees. The choice of IRP depends on the specific application
requirements and the network topology.

Resource Reservation Protocols:

● Resource Reservation Protocols (RRPs) play a critical role in real-time systems, as they provide
mechanisms to guarantee the availability of system resources during the execution of real-time tasks.

● RRPs are used to reserve system resources, such as CPU time, memory, and network bandwidth, to
ensure that real-time tasks meet their timing and performance requirements.
Here are some examples of RRPs used in real-time systems:

1. Time Division Multiple Access (TDMA):

● TDMA is an RRP used in communication networks to allocate time slots to different devices to transmit
their data.
● TDMA ensures that each device has a reserved time slot during which it can transmit its data without
interference from other devices.

2. Resource Reservation Protocol (RSVP):

● RSVP is an RRP used in IP networks to reserve network resources for specific flows.
● RSVP is designed to provide Quality of Service (QoS) guarantees for real-time multimedia applications
by ensuring that network resources are reserved for these applications.

3. Priority Inheritance Protocol (PIP):

● PIP is an RRP used in real-time operating systems to prevent priority inversion, a situation where a high-
priority task is blocked by a lower-priority task holding a shared resource.
● PIP ensures that a task inherits the priority of the highest-priority task waiting for the resource it holds.

4. Earliest Deadline First (EDF):

● EDF is a scheduling algorithm used in real-time systems to schedule tasks based on their deadline.
● EDF ensures that the task with the earliest deadline is scheduled first, ensuring that all tasks meet their
deadlines.
Overall, RRPs are essential for ensuring the timely and reliable execution of real-time tasks in real-time
systems. The choice of RRP depends on the specific requirements of the system, including the types of resources
being reserved and the timing and performance requirements of the real-time tasks.