PVC's (Permanent Virtual Circuits)
PVCs are usually established for long term use (months, years) between two end points. Once established, no further intervention by the network is required to transfer end-user information between two locations.
Typical VBR-rt and VBR-nrt SCR ranges from 20% and 70% of PCR. SCR and PCR are rounded to the nearest 64 kbps or 1 Mbps increments.
Each PVC is made up of a series of connected virtual paths (VPs) or virtual channels (VCs). Each VP/VC has an address of local significance at the ATM User-Network-Interface (UNI). This address is known as the VPI/VCI address where VPI refers to the virtual path identifier and VCI to the virtual channel identifier in the ATM addressing field of the 5-byte ATM cell header. VPI/VCIs identify the VPs and VCs that ATM cells will take as they are routed across the network.
A VC is a connection between two communicating ATM entities. Virtual paths are a group of VCs carried between two points. While the virtual channels are associated with a VP, they are not unbundled or processed in any way. The cell sequence of each virtual channel is still preserved through the VP (cells are delivered in order), and VCs within a VP must all be of the same service category as the VP.
PVCs have the following characteristics:
· VPI/VCI uniquely identifies a VC within a specific physical interface.
· VPI/VCI addresses are re-used on each physical interface.
· The VPI/VCI is translated at every switch/cross-connect point.
· A series of “connected” VCs make up a virtual channel connection (VCC), which is one form of an end-to-end connection (i.e., PVC) providing ATM end-user to end-user communication.
· A series of "connected" VPs make up a virtual path connection (VPC), which is another form of an end-to-end connection (i.e., PVC) providing ATM end-user to end-user communication.
· The provider’s current service offering is based on PVCs that contain traffic that flows bi-directionally and uni-directionally. For bi-directional PVCs, even though traffic flows in both directions, it is billed on a uni-directional basis.
· PVCs may be either symmetrical, meaning the traffic flowing in each direction is assigned the same information rate, or asymmetrical, meaning the traffic flowing in each direction can be assigned different information rates.
The provider will provision both types of PVCs: VPCs and VCCs. VPCs are the higher order addressing layer (i.e., within each VPC, multiple VCCs can be defined); therefore, they are easier to implement for both The provider and the customer by simplifying the provisioning process. In addition, the customer may use The provider-provided VPs to “tunnel” (i.e., transparently pass) SVCs through the The provider ATM cloud, allowing for even less circuit configuration at each customer end site. “Tunneling” allows customers to connect their two locations over which they control all the virtual circuits. The provider’s ATM Switches will only switch at the VPI level. The VCI only has meaning to the customer end systems.
The provider will assign the VPI/VCI for each PVC. VPI/VCI addresses may be negotiated if the customer wants to use their own VPI/VCI address instead of those assigned by The provider. The provider will ensure that received cells with valid VPI/VCI numbers are switched through the ATM network to the appropriate destination.
In anticipation of offering Switched Virtual Circuit (SVC) capability within the The provider ATM network, VPI 0 is reserved by The provider. Unless specifically requested by the customer, VPI 0 will not be provisioned by The provider for PVC (VPC or VCC) services. Since SVCs will utilize VPI 0 for signaling and VC connections, customers with existing PVCs using VPI 0 will need to reconfigure those PVCs to another VPI prior to implementing SVC capability on the UNI.
VCI numbers 0 through 31 have been reserved by the ATM Forum and the ITU-T, and will not be provisioned by The provider.