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This appendix provides guidelines for network providers for setting up Cisco 6100 Series system ATM virtual path connection (VPC) and virtual channel connection (VCC) addresses. In this appendix, you are assumed to have a working knowledge of ATM switching from a permanent virtual circuit (PVC) provisioning perspective.
When you are determining the optimal Cisco 6100 Series system ATM address space within a network provider's ATM network, consider the following:
These subjects are discussed in the following sections.
The following address utilization constraints are imposed by the ATM Forum's UNI Version 3.1 specification:
The following ATM address space types (VPC and VCC) and ranges are supported in Release 1.x and 2.x of Cisco 6100 Series system software (see Figure B-1):
Network port:
Subscriber port:
Subtend port:
If ATM UNI 3.1 addressing guidelines are used, which prohibit the use of VCI 0 to VCI 31, the maximum connection space is reduced to
Robust mechanisms have been developed for transporting packet-based traffic (such as IP traffic) through ATM networks. The combination of AAL5 encapsulation and selective cell discard techniques ensure reliable transport and predictable behavior even during periods of
network congestion.
The selective cell discard techniques employed during periods of congestion include Partial Packet Discard (PPD) and Early Packet Discard (EPD). PPD works on the premise that if one cell of a packet must be discarded because of congestion, the entire packet is invalid and the remaining cells can be discarded to avoid further congesting of the network with useless cells. EPD uses a predefined congestion threshold to begin dropping entire packets until the congestion level drops below the threshold. Both of these techniques are critical to obtaining good performance when transporting packets across an ATM network. Without PPD or EPD functionality, good packet throughput, or "goodput," is severely impacted due to cells being dropped randomly during periods of congestion, resulting in a large number of invalid packets being passed through.
The Cisco 6100 Series system employs both EPD and PPD to ensure the best possible packet throughput during periods of congestion. However, EPD and PPD work only on VCCs, where the beginning and end of each packet can be monitored. For VPCs, only the VPI field of each ATM cell is monitored when routing cells, so there is no visibility at the VCI level to determine where individual packets begin and end. For this reason, Cisco recommends that VCCs be used exclusively when routing packetized (IP) traffic to ensure the best possible performance during periods
of congestion.
The service provider must determine expected subscriber demand per SVC. Demand can be met in a variety of Cisco 6100 Series system configurations ranging from a dedicated (64 subscribers per Cisco 6100 Series system) to fully concentrated (400 subscribers per Cisco 6100 Series system).
The network provider should also consider the number of PVCs required per subscriber.
The network provider should consider the point in time when, due to either network design goals or the need for sheer volume connection deployment, SVCs begin to supersede PVCs as a means of connection establishment. One of the biggest arguments for the use of SCVs is the desire to free service providers from the need to discretely map thousands or even millions of POTS splitter chassis (PSC) segments to enable end-to-end ATM connections across an ATM bearer service. The use of SVCs also reduces the strain on the connection space limits imposed by various network elements, because connection space is required only for active connections.
Existing PVCs should be able to be preserved with the coming of SVCs. The signaling stack manages selection of an available VCC for each user call. Customers shifting from PVC service to SVC service would have their PVCs deleted by the network provider. Reservation of VCCs VPIn/VCI 0 to 31 protects appropriate space per VP for signaling purposes. SVC functionality is not available yet, and therefore Cisco makes no guarantee that some PVC address space reprovisioning will not be required. A Cisco goal is to minimize this. Utilities are being investigated that would enable these PVCs to be remapped in an automated fashion.
To simplify API design, do the following:
Posted: Tue Oct 12 08:15:59 PDT 1999
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