Virtual Circuit Networks in Computer Network, Important Question About Data Communication, Transport Layer, Network & Network Types, Network Software Components, Transmission Technology of Network , Hardware types

Virtual-Circuit Networks :

virtual-circuit network (VCN) is a hybrid network model that combines features of both circuit-switched and datagram networks. It provides a balance between connection-oriented and connectionless transmission methods.

Key Features:

1. Connection Phases:

A VCN has three distinct phases: setup, data transfer, and teardown

Setup Phase: A path is established between the sender and receiver before data transmission.
Data Transfer Phase: Data is sent along the established path in packets.
Teardown Phase: After data transmission, the connection is terminated, and resources are released.

Fig. Virtual-circuit network

2. Resource Allocation:

Resources can either be allocated during the setup phase (as in a circuit-switched network) or dynamically during data transmission (similar to a datagram network).

3. Packetized Data with Local Addressing:

Data is divided into packets, each of which carries an address. However, unlike in a datagram network, the address is not end-to-end but local, meaning it only tells the next switch where to send the packet.

4. Consistent Path for Packets:

Once a connection is established, all packets follow the same predetermined path. This ensures a predictable route for all the packets between the sender and receiver, akin to a circuit-switched network.

5. Layer of Operation:

Virtual-circuit networks typically operate at the data-link layer, while circuit- switched networks operate at the physical layer, and datagram networks at the network layer.

Addressing in Virtual-Circuit Networks:

Global Addressing:

A global address is used to uniquely identify the source and destination during the setup phase. This address is typically unique within the network or globally if the network is part of a larger system.

Virtual-Circuit Identifier (VCI):

During the data transfer phase, a virtual-circuit identifier (VCI) is used instead of the global address. The VCI is a small number with local scope, meaning it only identifies the path between two adjacent switches.

The VCI changes at each switch, as each switch uses its own set of VCIs to manage the connection. This allows efficient and simple management of packet forwarding.

Fig. Virtual-circuit identifier

Three Phases in a Virtual-Circuit Network :

In a virtual-circuit network, the communication between a source and destination involves three phases: setup, data transfer, and teardown. These phases ensure that a reliable path is established and maintained for the communication session.

Setup Phase:

The source and destination use their global addresses to establish a connection. During this phase, switches along the path create table entries to store information about the virtual circuit. This phase ensures that each switch is prepared to route the data properly.

Data Transfer Phase:

After the setup phase, data is transferred between the source and destination. The switches use the table entries created during the setup phase to route the frames. The switches maintain information like the incoming and outgoing ports and Virtual Circuit Identifiers (VCI). Each frame is processed the same way, with the VCIs changing at each switch to ensure the data follows the correct path. This phase continues until all frames are transferred.

Teardown Phase:

Once the data transfer is complete, the source and destination send signals to the switches to remove the corresponding table entries, effectively ending the virtual circuit.

Fig. Switch and tables in a virtual-circuit network

Data-Transfer Phase :

During data transfer, the key action is switching the frames between the source and destination. Each switch in the path must have a table with entries corresponding to the virtual circuit. A table typically consists of four columns: incoming port, incoming VCI, outgoing port, and outgoing VCI.

When a frame arrives at a switch, the switch looks for the entry that matches the incoming port and VCI.
After identifying the entry, the switch updates the VCI to the new value and forwards the frame to the next switch via the outgoing port.

For example: if a frame arrives at switch 1 with VCI 14 on port 1, the switch finds this entry in its table, updates the VCI to 22, and forwards the frame through port 3. This process repeats at each switch, ensuring the frame reaches its destination.

Setup Phase :

In the setup phase, a virtual circuit is established between the source and destination by creating table entries at each switch. This phase consists of two main steps:

1. Setup Request:

A setup request frame is sent from the source (A) to the destination (B). As the frame passes through each switch, the switch creates an entry in its table.

For example, when the setup frame reaches switch 1, it identifies that the outgoing port for the connection is port 3 and assigns an incoming VCI (14) for the frame coming from port 1. At this point, the outgoing VCI remains unknown.

Fig. Source-to-destination data transfer in a virtual-circuit network

2. Acknowledgment:

Once the setup request reaches the destination, the destination assigns a VCI (e.g., 77) for incoming frames from the source. The acknowledgment is sent back to the source, and each switch updates its table to complete the missing outgoing VCI information.

Fig. Setup acknowledgment in a virtual-circuit network

3. Teardown Phase :

When the communication is finished, the source and destination send a signal to the switches to remove the corresponding table entries, thus ending the virtual circuit. This process frees up the resources for future virtual circuits.

Efficiency in Virtual-Circuit Networks

In a virtual-circuit network, resource allocation can happen either during the setup phase or on demand during the data-transfer phase. When resources are reserved during the setup phase, each packet experiences the same delay. However, if resources are allocated on demand, packet delays may vary.

Even when resource allocation is on demand, a significant advantage of virtual-circuit networks is that the source can check resource availability before data transfer.

In virtual-circuit switching, all packets from the same source to the same destination follow the same path. However, with on-demand resource allocation, packets may arrive with different delays depending on resource availability.

Delay in Virtual-Circuit Networks

In a virtual-circuit network, delays occur during the setup and teardown phases. These are one- time delays. If resources are allocated during setup, there is no additional waiting time for individual packets during data transfer.

The total delay in such a network includes:

Three transmission times (3T): the time taken for the packet to be transmitted across links.

Three propagation times (3τ): the time taken for the packet to travel across physical distances.

Setup delay: includes transmission and propagation in both directions during the setup phase.

Teardown delay: includes transmission and propagation in one direction during the teardown phase.

For simplicity, processing delays at the switches (routers) are ignored in this calculation. Thus, the total delay for the packet is:

T𝑜𝑡𝑎𝑙 𝑑𝑒𝑙𝑎𝑦 = 3𝑇 + 3𝑟 + 𝑠𝑒𝑡𝑢𝑝 𝑑𝑒𝑙𝑎𝑦 + 𝑡𝑒𝑎𝑟𝑑𝑜𝑤𝑛 𝑑𝑒𝑙𝑎𝑦

QUESTIONS :

1. Data Communications :

1. What are the five key components of a data communication system, and what role does each play in ensuring effective communication?

2. Explain the different forms of data representation used in data communications and provide examples for each.

3. Describe the three modes of data flow and provide real-world examples where each mode is used.

2. Networks :

1. What are the key criteria used to evaluate the performance, reliability, and security of a network?

2. Explain the difference between point-to-point and multipoint physical structures in network connections. Provide advantages and disadvantages of each.

3. Network Types :

1. Compare and contrast a Local Area Network (LAN) and a Wide Area Network (WAN) in terms of characteristics, speed, and geographical coverage.

2. Define packet switching and explain the key differences between circuit switching and packet switching.

3. Discuss the role of routers and switches in the structure of the Internet. How do they contribute to efficient data transfer?

4. What are the different ways to access the Internet, and how do factors like speed and coverage differ between these methods?

4. Protocol Layering :

1. Explain the key principles of protocol layering and discuss how these principles ensure efficient communication between two devices over a network.

2. What are logical connections in the context of protocol layering, and how do they facilitate communication between peer layers on different devices?

3.Explain the layered architecture of the TCP/IP protocol suite. How does each layer contribute to overall network communication?

4. Discuss the main functions of the following layers in the TCP/IP protocol suite:

Application Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer

5. Describe the process of encapsulation and decapsulation in the TCP/IP protocol suite. Why are these processes crucial for data communication?

6. Explain the different types of addresses used at each layer of the TCP/IP protocol stack. Why is addressing critical for network communication?

7. What is multiplexing and demultiplexing in the TCP/IP protocol suite? How do these processes ensure that data is correctly sent and received by the right application?

5. Transmission Media :

1. Define guided media and list and explain three types of guided media used in network communications.

2. Explain the difference between unshielded twisted-pair (UTP) and shielded twisted-pair (STP) cables. What are their primary uses?

3. Describe how the twisting of pairs in twisted-pair cables helps to reduce crosstalk.

4. What are the main components of a coaxial cable, and how do they contribute to its performance?

5. Describe the principle of operation of fiber-optic cables and explain why they provide high- speed data transmission.

6. Discuss the advantages and disadvantages of fiber-optic cables compared to coaxial and twisted-pair cables.

7. What are radio waves, and how are they used in wireless communication? Provide an example of a common application.

8. Explain the concept of frequency allocation in radio wave communication and its importance.

9. Discuss the characteristics of microwave transmission and its typical uses in communication systems.

10. Describe how infrared communication works and list two common applications where infrared technology is used.