A switched virtual circuit (SVC) is a connection between two devices that acts like a dedicated point-to-point link between them.
It is called a “virtual” circuit because there is no dedicated physical path between the two devices. Instead, the network routes data packets between the devices as needed.
SVCs provide an on-demand way to establish connections between devices only when needed.
Once established, the connection acts like a dedicated circuit. SVCs are often used in wide area networks (WANs) and by devices connecting to the internet.
In this article, we will explain what is switched virtual circuit is, a compare SVC and PVC, their components, advantages, disadvantages, and SVC implementation considerations.
Some key points about SVCs:
- Virtual: No dedicated physical path, network dynamically sets up a connection
- Circuit: Behaves like a dedicated circuit once established
- Switched: Connections are set up on demand when needed
- Temporary: Only exists for the duration of the communication session
- Variable bandwidth: Can allocate more or less bandwidth as required
SVCs provide an efficient way to share network resources between multiple devices flexibly based on real-time demands.
How does a Switched Virtual Circuit work?
A switched virtual circuit works by using underlying network protocols to establish a logical connection between two devices when needed. Here are the basic steps:
- Device 1 sends a request to set up a connection to Device 2. This is called a “call request”.
- The network protocols create a route between Device 1 and Device 2 using available network paths. This route is the virtual circuit.
- Device 2 accepts the call request and confirms the connection.
- Now a virtual circuit exists between Device 1 and Device 2. They can send data as if a dedicated circuit exists between them.
- When the communication is done, the virtual circuit is terminated and network resources are freed up.
The key benefit is that a virtual circuit is created only when needed and provides the impression of a dedicated link between the devices.
The network takes care of routing data packets through the best available path.
When are Switched Virtual Circuits used?
Here are some common uses of switched virtual circuits:
- Wide Area Networks (WANs): SVCs are commonly used to connect devices over WANs where dedicated point-to-point links are not cost-effective. For example, connecting branch offices to a central office.
- Internet Access: Devices like routers use SVCs to connect to internet service providers to access the internet. The SVC provides a virtual link between the device and the ISP.
- Backup Paths: SVCs can be used to provide backup connectivity between devices in case primary circuits fail. The backup SVC activates only when needed.
- Bandwidth On Demand: SVCs allow bandwidth to be allocated between devices only when it is needed. So bandwidth utilization is highly efficient.
- Temporary Connections: SVCs are used when connections between devices are needed for short durations only. For example, teleconferences between offices.
So in summary, SVCs are perfect for situations where point-to-point connectivity is needed on an on-demand basis instead of full-time.
Comparison between SVC and PVC
The main alternative to SVCs is permanent virtual circuits (PVCs). Let’s compare the two:
| Switched Virtual Circuit | Permanent Virtual Circuit |
| Temporary connection exists only during a communication session | Permanent predefined connection |
| Established on demand when needed | Always available once configured |
| Bandwidth allocated dynamically | Fixed predefined bandwidth |
| Routes can change between connections | The same route is used for all connections |
| More efficient use of resources | Reserved resources even when idle |
| Connections have to be initiated and terminated | Always ready to transfer data |
| More setup overhead | Less overhead to start transfers |
| Suited for bursty, intermittent traffic | Good for constant, predictable traffic |
Components of a Switched Virtual Circuit
The main components that enable an SVC to be established and used are:
- Signaling Protocols – Network protocols like X.25, Frame Relay, and ATM that are used to set up and manage SVCs. They handle call requests, route establishment, bandwidth allocation, and call termination.
- Switching Equipment – Physical routers and switches that route data between devices to establish the virtual circuits. The devices must support the signaling protocols.
- End Devices – The customer premise equipment (CPE) like routers that originate requests for SVCs and terminate connections. They are the endpoints of the circuit.
- Management Tools – Software for monitoring SVCs, gathering statistics, and managing policies and QoS parameters.
So in summary, SVCs require support across network protocols, hardware, software, and tools to provide effective on-demand connectivity.
The key benefit is that this all happens dynamically without pre-provisioning dedicated links.
Advantages of using Switched Virtual Circuits
Here are some key advantages of using switched virtual circuits:
- Efficient use of resources – Bandwidth and network paths are used only when needed. No pre-provisioning of fixed bandwidth links.
- Flexibility – Virtual circuits provide flexibility to dynamically connect devices on demand. Easy to modify topologies.
- Cost savings – No need for expensive dedicated leased lines between all endpoints. Saves on operational costs.
- Scalability – SVCs can easily scale to support new endpoints as needed. No need to provision more fixed PVC links.
- Resiliency – Easy to set up backup SVCs if primary links fail. Temporary routes help overcome outages.
- Bursty traffic support – SVCs are ideal for traffic that is bursty and intermittent instead of constant.
- Simplified management – SVCs are created automatically as needed. Less manual intervention is required.
The dynamic on-demand nature of SVCs makes them very versatile for interconnecting devices flexibly over WANs and the internet.
Disadvantages SVC’s
SVCs also come with some disadvantages compared to permanent circuits:
- Setup latency – It takes time to initialize and establish a new SVC which adds delay. Not suited for real-time traffic.
- No guarantees – SVCs may not be available if network resources are fully utilized. No guarantees like PVCs.
- Overhead – Signaling, setup and tear down of SVCs has protocol overhead.
- No visibility – Harder to monitor and trace a specific SVC, unlike permanent links.
- Lower security – SVCs share resources so more vulnerable compared to dedicated links.
- Limited long-term planning – SVCs are dynamic so harder to predict capacity needs and traffic engineering.
So PVCs may be preferable for real-time, latency-sensitive traffic that needs guaranteed availability at all times.
Switched Virtual Circuit Protocols
Some key protocols that provide SVC capabilities:
- X.25 – One of the earliest WAN protocols (from the 1970s) that allowed SVCs. Defined signaling for call setup and teardown.
- Frame Relay – Widely adopted successor to X.25. Provided faster SVCs compared to X.25.
- ATM (Asynchronous Transfer Mode) – High-speed cell-based switching protocol from the 1990s that relied on SVCs.
- MPLS (Multiprotocol Label Switching) – MPLS VPNs use SVCs to connect sites. MPLS tags are used to establish SVCs.
So from the traditional X.25 protocol to MPLS today, various protocols have built on the fundamentals of SVCs and enhanced their capabilities over the decades.
SVC over IP Networks
Modern networks increasingly use MPLS or IP directly to offer SVC capabilities:
- MPLS Layer 3 VPNs – MPLS L3VPNs allow SVCs to be created between customer sites. MPLS tags are used to identify SVCs.
- VPWS (Virtual Private Wire Service) – Layer 2 SVCs created over MPLS networks. Provide point-to-point connectivity.
- IPSec VPNs – Some IPSec VPN implementations allow SVCs to be created on demand instead of permanent tunnels.
- SD-WAN (Software-Defined WAN) – SD-WAN platforms allow SVCs to be created across underlying IP networks.
So SVCs continue to adapt to new underlying network technologies like IP while retaining their core benefits of flexibility and efficiency.
SVC Implementation Considerations
Some aspects to consider when implementing SVCs:
- Signaling protocols – Ensure network elements like routers support protocols like MPLS to establish SVCs.
- Addressing and identifiers – Endpoints need addressing schemes like IP addresses or MPLS labels to identify SVCs.
- Call admission control – Mechanism to allow or reject SVC requests depending on resource availability. Ensures QoS.
- Traffic engineering – Design optimal network paths taking into account topology, bandwidth, and latency.
- QoS parameters – Configure QoS policies like traffic priority and bandwidth reservation for SVCs.
- Resiliency – Provide backup paths in case of network outages so SVCs can re-route.
- Security – Use encryption and access controls to protect SVCs from interception or misuse.
- Monitoring & management – Track SVC connections, resource usage, routing, and faults to ensure performance.
A well-designed SVC implementation provides the right balance of flexibility and management.
Conclusion
SVCs provide efficient and flexible connectivity on demand between devices without having dedicated pre-provisioned links.
MPLS has become the dominant modern protocol for providing SVCs over IP networks.
With the right implementation, SVCs can provide significant benefits over permanent virtual circuits.
Frequently Asked Questions (FAQ)
Ques 1. What protocols are used for SVCs?
Ans. Some key protocols that support SVCs are X.25, Frame Relay, ATM, MPLS, IPSec VPNs, and SD-WAN solutions.
MPLS is one of the most widely used technologies today for SVCs over IP networks.
Ques 2. When is an SVC established?
Ans. An SVC is established dynamically only when a device needs to communicate with another device.
The network signaling protocols handle automatically setting up the SVC when required.
Ques 3. Do SVCs provide guaranteed connectivity?
Ans. No, SVCs do not provide guaranteed connectivity at all times since they are created on demand.
Network resource availability can impact SVC setup requests. So there are no guarantees.
Ques 4. Is bandwidth reserved for an SVC?
Ans. SVCs can reserve a specific amount of bandwidth when setting up the connection.
This provides QoS by ensuring a minimum amount of bandwidth is available for that SVC. Bandwidth reservation is configurable.
Ques 5. How are SVCs identified uniquely?
Ans. Protocols like MPLS use special identifiers like labels to identify each SVC.
Devices attach these labels to data packets so the network can forward them through the correct virtual circuits. Addressing schemes identify SVCs.
