Virtual Circuit (VC) is a connection-oriented data transmission method that operates similarly to circuit switching, but instead of using a physical path, it creates a logical or "virtual" path through the network. This virtual path, known as a virtual circuit, is established before any data is sent and remains active during the communication session.
In a virtual circuit setup, the physical medium is divided into multiple subchannels, referred to as logical channels (LC). These subchannels are linked in sequence to form a complete virtual circuit. Each virtual circuit is dedicated to carrying data for a specific user or application, ensuring that data flows along the same path throughout the transmission.
The process of using a virtual circuit involves three main stages:
1. **Virtual Circuit Establishment**: Before any data is transmitted, the sender sends a control message, such as a call packet, containing address information. As this message passes through intermediate nodes, each node allocates a logical channel and updates its mapping tables. The logical channels across all intermediate nodes are then connected in series to create the virtual circuit.
2. **Data Transmission**: Once the virtual circuit is established, all data packets from the sender travel along the same path. This ensures that the order of data arrival at the destination matches the order of sending, making it easier for the receiving end to process the data.
3. **Virtual Circuit Release**: After the data transfer is complete, a release message is sent to terminate the virtual circuit. Either the sender or the receiver can initiate this process. However, because setting up and tearing down a virtual circuit takes time, this mode is not ideal for applications that require frequent, short connections—such as interactive communications.
There are two types of virtual circuits: **Permanent Virtual Circuits (PVCs)** and **Switched Virtual Circuits (SVCs)**. A PVC is pre-established between two sites, similar to a dedicated line, allowing communication to occur anytime without additional setup. An SVC, on the other hand, is dynamically created and released by the user as needed, offering more flexibility.
A schematic diagram of the virtual circuit structure and data transmission process would typically show how logical channels are interconnected to form a path for data flow.
When comparing virtual circuits with datagrams, there are several key differences:
- **Datagrams** operate in a connectionless manner, meaning no prior setup is required. Each packet contains full addressing information, which increases overhead. Since packets may take different routes, they can arrive out of order, requiring more complex processing at the destination. However, if a node or link fails, alternative paths can be used, making the system more reliable. Datagrams are well-suited for small data transfers.
- **Virtual Circuits**, on the other hand, require a connection to be established and released before data can be sent. Data packets contain minimal address information (just the logical channel number), and since they follow a fixed path, they arrive in order. However, if any part of the virtual circuit fails, the entire connection is disrupted. This makes virtual circuits more efficient for large-scale data transfers.
Overall, the choice between virtual circuits and datagrams depends on the specific needs of the application, including data size, reliability requirements, and the frequency of communication.
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