This is a long article, but it is worth it. I have highlighted the juicy parts. The article was written in May issue of magazine.
tcs-prod.weblabs.com
Switched Virtual Circuits Find a Home with DSL
Broadband Access
Users may not have embraced switched virtual circuits in ATM networks, but they are now providing the automatic connection processing necessary to deploy DSL services rapidly and efficiently.
From the outset, switched virtual circuits (SVCs) have been integral to ATM specifications, and most ATM switches and customer premises equipment (CPE) support SVCs. Because SVCs are established dynamically and remain in place only for the duration of the tasks for which they are established, they have always been viewed as an improvement on permanent virtual circuits (PVCs), which generally require committed bandwidth whether or not a task is being performed. The dynamic connection set-up and tear down of SVCs results in efficient utilization of network resources because bandwidth is not dedicated to a particular application. When a connection is terminated, bandwidth is available for other applications.
For carriers, SVCs are less expensive and easier to implement. Using PVCs requires the configuration of multiple PVCs to complete the circuit path between the customer and the destination network address. Through exchange of signaling information, SVCs are automatically set up and torn down. The automatic call set-up process relieves carriers from identifying available bandwidth in the ATM network to build a connection. It also eliminates the possibility of configuration errors associated with PVC deployment.
Since 1997 carriers such as AT&T and Sprint have offered SVCs to customers as part of their ATM service offerings. While PVC pricing is flat-rate, SVC pricing and billing are usage-based. But users have not embraced ATM SVCs, perhaps because the pricing structure for SVC connections may vary depending on a number of parameters, including the connection?s cell rate (bandwidth), quality of service (QoS), connection holding time, and number of cells transmitted. Because users have not embraced SVCs, carriers have not been able to realize the benefits of SVC deployment, such as lower infrastructure and network management costs. It appeared that SVCs were a great idea that would never have significant impact in the real world, but they are receiving new life from an unanticipated source: DSL networks.
DSL networks present unique deployment problems that SVCs are singularly capable of solving. Perhaps the biggest challenge facing DSL providers is managing network growth. For instance, how does a carrier scale a DSL network from a few hundred subscribers to several thousand? Using PVCs to provision thousands of DSL subscribers across an ATM transport network may be theoretically possible, but most would agree it is an inefficient and costly way to deploy a large-scale DSL network. SVCs provide the automatic connection processing necessary to do the job rapidly and efficiently.
PVCs and DSL Networks
To date, providers of DSL services have implemented PVCs to establish paths between a subscriber?s terminal equipment and a content provider such as an ISP or corporate LAN. Use of PVCs to provide connections between end users and Internet and corporate gateways requires that a network administrator configure multiple PVCs throughout a service provider?s network. A PVC is first configured from the subscriber premises to a DSL access multiplexer (DSLAM) in a central office (CO). The network manager must then configure a PVC from the DSLAM to an ATM switch in the transport network and establish a PVC between each ATM switch in the transport network required to complete the path between the user and the ISP or corporate network (see Figure 1).
Because most DSL providers have focused on the business market to provide an alternative to T1 and frame relay services, growth has been slow. According to Telechoice, at the end of 1999 CLECs had installed only about 110,000 DSL lines. Compared to the millions of businesses and residences using analog modems for Internet access, this number is small indeed. Recognizing the tremendous potential of the residential marketplace, CLECs have started to offer DSL services to the home user. G.lite and retail distribution are proving to be the keys to deploying DSL to the millions of consumers currently using analog modems.
With a small number of DSL subscribers, a PVC-based network can be managed effectively, but as DSL networks grow to several thousand users, the establishment of several thousand PVCs becomes costly and difficult, if not impossible, to deploy and administer (see Figure 2). The likelihood of configuration errors increases as the number of subscribers on a DSL access network grows. Allocation of network management resources to troubleshoot and repair configuration errors results in increased administrative costs. Subscribers requesting changes to their destination ISP or corporate network, which requires network managers to reconfigure PVCs, further increase complexity and cost.
Network Scalability
SVCs provide the scalability necessary to deliver DSL to the consumer market. They eliminate the manual set-up required with PVCs. They also eliminate the possibility of configuration errors associated with PVC deployment. With SVCs, available network bandwidth is dynamically assigned to a connection. An SVC path is set up across switches that are able to provide the connection?s required bandwidth and support any associated QoS parameters. This automatic call set-up relieves network managers from identifying available bandwidth to build a connection. SVCs also provide fault recovery capability for DSL access networks. In the event of a switch failure, the SVC is rerouted around the outage and remains available to the customer (see Figure 3).
SVCs are on-demand connections initiated by the user through User-Network Interface (UNI) and Private Network-Network Interface (PNNI) signaling. They connect and disconnect under customer control just like analog modem dial-up calls. To implement end-to-end SVCs from the DSL modem in the consumer?s home to an ISP router, the ATM switches, DSLAMs and subscriber modems that comprise the DSL network must support SVC signaling. Today, G.lite modems support only PVCs. Vendors are currently developing G.lite modems that will support SVCs, but until these become available, service providers must manually configure a PVC from the customer?s modem to the DSLAM. After configuring the PVC, the benefits of SVCs can be realized in the ATM transport network, provided the DSLAM and ATM switches comprising the network support PNNI signaling.
The ITU G.992.2 standard provides that G.lite modems will be capable of powering down when a user is idle. This sleep mode reduces the amount of power dissipation at both the customer premises and the CO. SVC connections are terminated when a modem goes into sleep mode and are rebuilt when the modem powers up and transmits data. This dynamic call set-up and tear-down frees up bandwidth allocated to idle users for use by active users.
Finding a Home
At the time SVCs were being incorporated into ATM standards, few could have predicted that the deployment of DSL to the home would be the catalyst for their widespread implementation. Modeled on the dynamic connection capabilities of circuit-switched voice networks, SVCs were intended to give business users the ability to use and pay for bandwidth on an as-needed basis. Additionally, it was thought that the automatic call set-up and tear-down of SVCs, would greatly streamline carriers? network operations. As it turned out, business users never warmed up to SVCs so carriers had no compelling reason to deploy them. With the advent of residential broadband enabled by consumer DSL and its deployment challenges, carriers are rediscovering the advantages of SVCs.
Tom Mitchell is executive director of residential DSL products for Promatory Communications. (tmitchell@promatory.com)
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