SUBJECT D14)ATM signalling and SVC
SUBJECT D14-1)Question:What is the difference between a PVC, Soft PVC, and SVC?
Answer: First lets define the three terms, PVC, Soft PVC, and SVC.
A PVC in the usual meaning is a VC that is not signaled by the end points. Both of the endpoint (user) VC values are manually provisioned. The link-by-link route through the network is also manually provisioned. If any equipment fails, the PVC is down, unless the underlying physical network (sonet, for example) can re-route below ATM. So a PVC is a VC which is statically mapped at every point in the ATM network. A failure of any link that a PVC crosses results in the failure of the PVC.
A Soft PVC also has manually provisioned endpoint (user) VC values (which as defined above do not change), but the route through the network can be automatically revised if there is a failure. Historically this feature pretty much required a single-vendor network. A vendor may employ signaling (invisibly to the endpoints) within the network, or may just have a workstation somewhere sending proprietary configuration commands when it detects a failure. However, the PNNI 1.0 spec defines a standard way of doing this which does not require a vendor proprietary solution. So a Soft PVC is a VC that is programmed to be present at all times (like a PVC), but does not use static routes to determine its path through the ATM network. Failure of a link causes a Soft PVC to route around the outage and remain available.
A SVC is established by UNI signalling methods. So an SVC is a demand connection initiated by the user. If a switch in the path fails, the SVC is broken and would have to be reconnected.
Summarizing, the difference between a PVC and a Soft PVC is that a Soft PVC will be automatically rerouted if a switch or link in the path fails. From that perspective a Soft PVC is considered more robust that a simple PVC.
The difference between a SVC and a Soft PVC is that a SVC is established on an "as needed" basis through user signalling. With a Soft PVC the called party cannot drop the connection.
Answer: Actually, it's not the AAL layer that originates the request for a connection (although if one were a strict believer in network layering, one might assume so :-). AAL just defines how information of a given type is packaged for transporting over the ATM network. There is a signalling protocol (which, by the way, uses AAL5) which defines a protocol which includes the end stations, plus any relevant ATM switches along the path.
There are various entities above AAL that could determine a connection is needed, including the LAN Emulation Client, an IP-ATM end station, a direct video-over-ATM application, or a human network operator. If the connection is set up via Switched Virtual Circuits (SVC's), then the protocol used is most likely Q.2931, previously called Q93B, most commonly referenced via the ATM Forum's specs:
Answer:
Answer:
Answer: Proxy signaling capability allows a user, called the Proxy Signaling Agent (PSA), to perform signaling for one or more users that do not support Signaling. This enables End Systems that do not understand ATM Signaling to set up signal and set up SVC calls with the help of the Proxy Sig Agent.
PSA uses VPI/VPCI mapping to determine the destination User (that does not support signaling).
| VPCI | UNI interface ID | VPI |
| 1 | 10 | 0 |
| 2 | 11 | 0 |
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