# How virtual input switching works
**Topics**
+ [Virtual input switching using asynchronous ESAM messages](vips-asynch.md)
+ [Virtual input switching using SCTE-35 messages](vips-s35.md)
# Virtual input switching using asynchronous ESAM messages
## Input types
Asynchronous input switching works only with the following sources:
+ MPTS programs, from an MPTS input. MPTS programs are typically live sources.
+ File inputs—any type of input that Elemental Live considers to be a file input. See [Supported upstream systems for file inputs](supported-inputs-file-types.md)
If your event doesn't include these types of inputs, you can't set up for asynchronous input switching.
## Setup
You set up the inputs in the Elemental Live event. You also set up the POIS with information about the same set of inputs. In other words, the inputs must be set up in both Elemental Live and the POIS.
You start the event. At any time, the POIS can send an asynchronous ESAM request to Elemental Live.
## Principle of operation
The POIS sends a request to Elemental Live to request a switch away from the currently active input and to another input. The request can be seen as a switch away to another input. In addition, the originator of any potential action is the POIS.
Compare this principle of operation to SCTE-35-based triggers, where Elemental Live probes each non-active input for requests to switch to that non-active input. The request can be seen as a switch to this input. In addition, the originator of any potential action is Elemental Live.
## Types of requests
The POIS can send one or more of the following requests:
+ Prepare input. This request makes Elemental Live start processing the input before you switch to it. This preparation reduces the delay that always occurs between the switch request and the switch happening.
+ Switch input. The POIS sends this request to instruct Elemental Live to switch, either immediately, or at a specific time.
+ Handles SCTE-35 messages for ad avails. The POIS can request that Elemental Live insert an ad avail message. For information about the handling of these types of SCTE-35 messages, see [POIS conditioning](pois-conditioning.md).
+ SCTE-35 messages that instruct Elemental Live whether to include the SCTE-35 in the output or not. The POIS can choose to instruct Elemental Live to include or to remove any of the three types of SCTE-35 messages.
## Timing of input switching requests
The prepare input request should be sent at least 15 seconds before the switch request.
If the POIS sends both a prepare request and a switch request, the switch will occur in less than 4 seconds. After the switch occurs, Elemental Live stops preparing the input.
If the POIS sends a switch request but no prepare request, the input switch might take as long as 12 seconds.
## Result in outputs
If the POIS instructs Elemental Live to include an SCTE-35 message in the output, that message becomes part of the stream. This rule applies to all types of messages – prepare input, switch input, and message relating to ad avails.
For a prepare input or switch input, Elemental Live still performs the action (prepare or switch), even if the POIS instructs Elemental Live not to include the message itself in the output.
Elemental Live includes these messages in output types that support SCTE-35 messages. For example, Elemental Live includes the messages in an HLS output.
For detailed information about how Elemental Live handles ad avail SCTE-35 messages, see [POIS conditioning](pois-conditioning.md).
## No queuing of requests
There is no way to create a queue of requests. Elemental Live can handle only one request. The POIS might send a request to switch to input A at 2:00:00.00. Then, before Elemental Live has performed that request, the POIS might send a request to switch to input B.
Even if the second request has a start time that is later than the first request, the second request will replace the first request in the request list.
## Example
Here's an example of an input switching scenario that illustrates the different input switching capabilities.
The time is 2:00:00. You start the event and Elemental Live starts ingesting the first input, which is a file input.
**Asynchronous request to a file input**
At 2:01:00, the POIS sends an asynchronous request to switch to input B. Input B is the first in a series of 20-second ads.
**Asynchronous request to switch to a live input**
At 2:02:07, while these files are being processed, the POIS sends an asynchronous request to switch to input Y at 2:04:00.00. Elemental Live switches to input Y at the specified start time. Assume that input Y is a live TS input.
**Canceled asynchronous request**
This example shows cancellation of an asynchronous request.
At 2:51:44, the POIS sends a request to switch to input F at 2:52:00.
At 2:51:47, the POIS sends a request to switch to input Y (the input that is currently active) at 2:49:60.00. This request effectively cancels the switch to input F. Elemental Live can only have one request in its buffer, and the most recently received request always replaces any request already waiting.
**Multiple requests in an unexpected order**
At 2:53:48, the POIS sends a request to switch to input D at 2:57:00.
At 2:54:00, the POIS sends a request to switch to input E at 2:55:30.
Elemental Live ignores the request to switch to input D. Elemental Live can only have one request in its buffer, and the most recently received request always replaces any request already waiting.
Therefore at 2:55:30, Elemental Live switches to input E.
# Virtual input switching using SCTE-35 messages
This type of input switching works with SCTE-35 messages that are in a transport stream source. When Elemental Live encounters an SCTE 35 message in the source, it sends the message to the POIS. The POIS might decide to send a request to switch input. Note that there is no sense of a particular type of SCTE 35 message that can trigger an input switch request. The POIS can request a switch in response to any type of SCTE 35 message that it receives.
## Input types
SCTE-35-triggered input switching works only with the following sources:
+ MPTS programs, from one MPTS input. MPTS programs are typically live sources.
## Principle of operation
Elemental Live probes each non-active MPTS program for requests to switch to that non-active input. The request can be seen as a switch to this input. In addition, the originator of any potential action is Elemental Live.
Compare this principle of operation to asynchronous input switching, where the POIS sends a request to Elemental Live to request a switch away from the currently active input and to another input. The request can be seen as a switch away to another input. In addition, the originator of any potential action is the POIS.
## Setup
In the event, you set up the inputs:
+ You identify all the MPTS programs that you want to use, and set up each program as an individual input.
You also set up the POIS with information about the same set of inputs. In other words, the inputs must be set up in both Elemental Live and the POIS.
You start the event. As Elemental Live ingests the first input, it encounters SCTE-35 messages. It sends these messages to the POIS. It sends all SCTE-35 messages, not only switching-related messages. For example, it also sends ad avail messages.
For any switching-related message, the POIS might decide to send a request to Elemental Live to switch to another input at a specific time. The POIS can send this instruction at any time. There is no timeout between receiving the information and sending an instruction. The POIS might also decide not to send an instruction.
## Types of requests
The following are the types of requests:
+ Prepare input. This request makes Elemental Live start processing the input, even before the input is active. This preparation reduces the delay that always occurs between the switch request and the switch happening.
+ Switch input. The POIS sends this request to instruct Elemental Live to switch, either immediately, or at a specific time.
The prepare input request should be sent at least 15 seconds before the switch request.
If the POIS sends a prepare input and a switch input, the switch will occur in less than 4 seconds.
If the POIS doesn't send a prepare input, the input switch can take as long as 12 seconds.
+ SCTE-35 messages for ad avail handling. The POIS can request that Elemental Live insert an ad avail message, or delete or modify an ad avail message that is in the source stream (and that Elemental Live therefore sent to the POIS). For information about the handling of these types of SCTE-35 messages, see [POIS conditioning](pois-conditioning.md).
+ SCTE-35 messages that instruct Elemental Live whether to include the SCTE-35 in the output or not. The POIS can choose to instruct Elemental Live to include or to remove any of the three types of SCTE-35 messages.
## Timing of input switching requests
The prepare input request should be sent at least 15 seconds before the switch request.
If the POIS sends a prepare input and a switch input, the switch will occur in less than 4 seconds. After the switch occurs, Elemental Live stops preparing the input.
If the POIS doesn't send a prepare input, the input switch might take as long as 12 seconds.
## Result in outputs
If the POIS instructs Elemental Live to include an SCTE-35 message in the output, that message becomes part of the stream. This rule applies to all types of messages – prepare input, switch input, and messages relating to ad avails.
For a prepare input or switch input, Elemental Live still performs the action (prepare or switch), even if the POIS instructs Elemental Live not to include the message itself in the output.
Elemental Live includes these messages in output types that support SCTE-35 messages. For example, Elemental Live includes the messages in an HLS output.
For detailed information about how Elemental Live handles ad avail SCTE-35 messages, see [POIS conditioning](pois-conditioning.md).
## Example
Here's an example of an input switching scenario that illustrates the different input switching capabilities.
The time is 2:00:00. You start the event and Elemental Live starts ingesting the first input. At the same time, Elemental Live reads all the other inputs,
**SCTE-35-triggered request**
At 2:17:33, while ingesting input Y, Elemental Live encounters an SCTE-35 message. It sends the message to the POIS. Shortly after, the POIS sends a request to switch to input Z at 2:30:00.
At 2:30:00, Elemental Live switches to input Z (another live input).
**SCTE-35 message that the POIS ignores**
At 2:37:00, Elemental Live encounters an SCTE-35 message. It sends the message to the POIS, but the POIS doesn't send an input switch request. There is no rule that dictates that the POIS must switch on receiving a message.
**Multiple SCTE-35-triggered requests**
At 2:40:02, Elemental Live encounters another SCTE-35 message, and this time the POIS does respond by sending two requests. One request is to switch at 2:45:13 to input B (one of the ads that played previously). The second request is to switch at 2:45:43 back to input Z.
At 2:45:13, Elemental Live switches to input B.
At 2:45:43, Elemental Live switches back to input Z.
**Canceled asynchronous request**
At 2:51:44, the POIS sends a request to switch to input F at 2:52:00.
At 2:51:47, the POIS sends a request to cancel the switch to input F.
This example shows cancellation of an asynchronous request. The POIS can cancel either an asynchronous request of a SCTE-35-triggered request.
**Multiple requests in an unexpected order**
At 2:53:48, the POIS sends a request to switch to input D at 2:57:00.
At 2:54:00, the POIS sends a request to switch to input E at 2:55:30.
At 2:55:30, Elemental Live switches to input E. Then at 2:57:00, Elemental Live switches to input D.
Note that Elemental Live doesn't ignore the request to switch to input D, even though it received the request for input E in the meantime.