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This appendix provides guidelines that you as a network provider need to set up Cisco 6100 Series system ATM virtual path connection (VPC) and virtual channel connection (VCC) addresses. This appendix is oriented from the viewpoint of a network provider representative who is provisioning subscribers to permanent virtual channels and has a working knowledge of ATM switching.
When you are determining the optimal Cisco 6100 Series system ATM address space within the ATM network, keep the following considerations in mind:
These topics 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:
Because ATM UNI 3.1 addressing guidelines prohibit the use of VPI 0 in combination with VCIs 0 to 31, the maximum connection space is reduced to the following settings:
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 that are 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 congesting the network further 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 packet-based (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 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 when, due to either network design goals or the need for high volume connection deployment, SVCs begin to supersede PVCs as a means of connection establishment. A strong motivator for using SVCs is to prevent service providers from having to discretely map thousands or even millions of POTS splitter chassis segments in order to enable end-to-end ATM connections across an ATM bearer service. The use of SVCs can also reduce the strain on the connection space limits that various network elements impose, because only active connections require connection space.
SVCs should be able to preserve existing PVCs. The signaling stack manages the selection of an available VCC for each user call. A network provider would delete the PVCs of customers who shift from PVC service to SVC service. Reservation of VCCs VPI/VCI 0 to 31 protects appropriate space per virtual path for signaling purposes. SVC functionality is not available yet, and therefore Cisco cannot guarantee that you will not have to provision some PVC address space again. Research and development teams are investigating utilities that would enable these PVCs to be remapped in an automated fashion, but these utilities are not currently available.
To simplify API design, do the following tasks:
Posted: Mon Feb 14 16:36:32 PST 2000
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