PDF of this tutorialWANs are changing on a daily basis to serve the growing demand for advanced voice and data services. DS–1 (T1) physical link connections are running at 1.544 Mbps, and end-loop services such as asymmetrical digital subscriber line (ADSL) and other high-bandwidth service offerings are continually expanding. Indeed, more and more technologies for using these physical DS–1 transport facilities are being developed to improve the efficiency of the bandwidth and to support the integration of voice and data services. However, this rapid Internet access expansion overloads the PSTN. The Telecommunications Act of 1996 greatly increased the number of options for local access services and other service providers. Efficient dial-up access is of paramount importance for customers who are paying both line access and Internet access charges, as well as losing time and money on nonconnected calls. This convergence of service demand with emerging providers is pushing the resources of network facilities beyond the capabilities for which they were originally designed.
Remote Access Server Technologies
The rapid increase in the number of ISPs is made possible by low-cost Internet access products called remote access servers. This type of network equipment is undergoing rapid evolution through the use of 56–kbps digital modems and increased modem-per-system capacity. Product offerings of up to 672 modems per system are common in today's market. ISPs are typical subscribers of telephone service for terminating calls from their dial-up customers; consequently, LECs can sell these ISPs either basic subscriber loops or PRIs, running 23 traffic (B) channels and the one common signaling data (D) channel. PRIs offer the ISP access to advanced services such as 64 and 128–kbps data rates to customers (in available markets) and special routing and switch service functions offered by the LEC. Analog loops provide low-cost access for start-up and rural ISPs and in markets where PRIs are not available. Another alternative offers channelized T1 subscriber access from the LEC in markets where T1s but not PRIs are available. These services allow for in-band signaling like a subscriber loop circuit. The subscriber loop and dial-up channelized T1 interfaces suffer from extended call setup delays, as the dialed number must be presented as in-band tone signals.
The above service offerings from the LEC are derived from features and equipment configurations on the end central-office switching platform. As an ISP operates according to incoming dial-up calls, all calls must come into the one central office serving the ISP (unless larger ISPs have multiple locations for access). An ISP may provide service beyond a local access and transport area (LATA), and, as LECs support inter–LATA calls, utilization of the trunk-side switch ports becomes quite high for ISP calls.
Switching
A 1996 ILEC traffic study concluded that traffic volume for non-Internet (i.e., voice and fax) was typically between 4–5 minutes per line during peak time, while traffic volume for an ISP call usually averaged 17–18 minutes per line. With a 4:1 ratio in the difference between voice and Internet traffic utilization of incoming trunks to the end office, the end office quickly becomes overburdened for other local subscribers and other services carried by that end office. ISPs, with their high subscriber to port ratios, use the trunk ports constantly throughout the day and night, thereby denying them use for other services. LECs have been addressing this problem by adding costly switch-capacity and trunk facilities to their network. While this helps with the existing problem, it does not solve the problems caused by the expansion of ISP providers and customers in the marketplace. It also changes the cost infrastructure model for the LEC in offering local service.
Trunks between central offices are switched via SS7 in over 90 percent of the North American market. This highly efficient, common-channel, message-based signaling system is the foundation for getting call information from the origination exchange to the end terminating office. The SS7 switched network is provided through STPs. Because SS7 is also a data-packet, message-based service, it is ideal for routing voice, data, packet, and video services; the customer is able to select the circuit bandwidth required based on the call, which is an effective means of transporting ISDN service offerings. SS7's messages also contain all information for the call setup, which reduces the post-dialing delay, improving revenue opportunities for service providers.
Because Internet traffic is routed to end offices via trunks between local exchange offices in a LATA, the calls from the ISP subscribers are indeed already routed to the end offices via SS7. This results in a concentration effect at the ISP's end office, and all of this trunk traffic is routed into the end-office switch for the ISP.
