Virtual path connection
Virtual path connections (VPC) consolidate a group of channels with the same destination. VPCs are used within a service provider’s ATM network to reduce the amount of switch-related processing and, thus, nodal delay. Virtual channels destined for the same switch are placed in the same VPC. All intermediate switches process only the virtual path identifier and ignore the virtual channel identifier.
From a customer’s point of view, VPCs offer bulk connectivity between customer-owned ATM edge devices (typically switches). The ATM network looks like a “tie line” between the customer devices. The customer’s edge device is responsible for assigning VCCs to the VPC. The contracted service class is assigned to the VPC as an umbrella policy for all carried VCCs.
Private ATM switch to private ATM switch connectivity
By far, the most beneficial use of VPCs is the interconnection of two private ATM switches. Private ATM networks offer more advanced ATM services than their public counterparts. With VPC connectivity, the full features of these switches can be realized over the public network.
The most common use of VPCs by ATM customers is to connect private (customer-owned and maintained) ATM switches. Two or more private switches are interconnected with VPCs, which allow communication between users connected to the switches.
As with PBXs, the features of private ATM switches are more robust than those of public ATM switches. One example is the availability of switched virtual circuits. Although this on-demand capability has been a standard feature of private switches for over five years, public carriers have only recently begun offering SVC service. If a company wanted advanced features, it had to set up a private ATM network, which did not allow use of the public ATM network for transport.
One way to both use the rich features and receive connectivity from carriers is by using virtual path connections. With a VPC, the carrier provides only transport services at the path level between private switches. The private switches are responsible for creating the channel connection that uses the path. A typical scenario is the establishment of a signaling virtual channel, as well as a link management channel (ILMI), between private switches. These channels establish on-demand or permanent virtual channels for user traffic between the switches. A company could contract for an overall level of service for the VPC. The upper bar would be monitored when each VCC is established, and the switches would ensure that the aggregate of all the channels established did not exceed the overall contract.
Frame relay network connectivity
Using ATM as a backbone transport for frame relay service is an industry standard. Using VPCs between the frame/ATM interworking unit (IWU) provides the following benefits:
- Decreased latency due to reduced nodal processing at the ATM switches
- Decreased provisioning time for frame relay circuits. Each new frame relay is assigned only at the frame switches and IWUs, not at the intermediate ATM switches
- Dynamic frame relay assignment. Most frame relay switches support some type of soft PVC assignment. By using a VPC, the ATM portion of the PVC is transparent to frame relay switches
- Simplified traffic management for the ATM channel. Only the peak cell rate needs to be calculated for each added DLCI. Each additional DLCI deducts from the VPC traffic contract
DSL access multiplexer and ISP router connectivity
Another internal use of VPCs is connectivity between DSL access multiplexers and ISP routers. ATM adds a switching component to DSL environments: ISP services can be changed by adjusting ATM channels instead of physical connections. Using VPCs increases this flexibility by allowing users to be added to a path connection without provisioning each switch in the ATM connection.
The visual shows a typical scenario using ATM in a DSL deployment. The DSL modem contains ATM components. The DSL access multiplexer (DSLAM) acts as an ATM concentrator, mapping the virtual channels from the DSL modems onto a virtual path connection. A VPC is initiated between each DSLAM and ISP router.
When a user is added or a user changes ISPs, only the DSLAM and ISP routers need to be provisioned for the user—the intermediate ATM switches are unaffected. Another advantage of using VPCs is bookkeeping simplification. If a global type addressing plan is used to assign virtual path identifiers, an ISP could be known by its VPC at all associated DSLAMs, and vice versa. This simple detail could save hours when provisioning thousands of users between a number of ISPs.