PDF of this tutorialLECs have developed an SS7–based switched network between local telephone end offices, tandem offices that serve interexchange carriers (IXCs) with switched access services, and dedicated trunks for business customers requiring larger bandwidth extensions between their locations. Technologies including PSTN–switched voice traffic, frame relay, other data services, ATM, and higher bandwidth data services are available to the business and IXC environments.
Residential and business consumers using powerful computing platforms drive the great demand for Internet access. These platforms are now equipped with 400-MHz CPUs or greater, 3-D graphics, multimedia capabilities, and e-mail facilities and are able to use all 56 kbps of technology available over local loops. While people continue to analyze, debate, test, and install ISDN and ADSL technologies in limited urban environments, dial-up services are available in both the analog and digital domains of ILEC deployments. See Figure 1 for an illustration of a typical ILEC connection for providing residential user access to the ISP by using PRI ports.
Figure 1. A Typical ILEC Connection for Providing the Residential User Access to the ISP by Using PRI Ports
The connection for the ISP customer served by the same central office is a simple route inside the switch from the PRI subscriber port to the end-user line port. The case is more complicated for the ISP's customers not served by the same central switch. The calls are routed through the ILEC's LATA via interoffice facility (IOF) trunks from the switch that originates the call and sends it to the switch PRI port for the ISP. These same IOF trunks also serve end users wanting connections to their long-distance carriers as well as voice calling or dedicated direct-dial data services in the LATA. See Figure 2 for an illustration of a CLEC with collocated switching services serving unbundled loops and ISP customers. The CLEC has the option of installing its own switch or purchasing switched local-loop services from the ILEC for the CLEC's customers. In Figure 2, the CLEC installs its own switch for competitive service offerings and can connect directly to the ISPs via the ILEC's digital crossconnect facility (DSX) as well as purchase the unbundled local loops from the ILEC.
Figure 2. A CLEC with Collocated Switching Services Serving Unbundled Loops and ISP Customers
IOF trunks are required between the CLEC's switch and the ILEC's switch at the same central office as well as to other ILEC/CLEC locations in the LATA. ISPs have a competitive choice in that they may purchase PRIs from either the CLEC or the ILEC and other service providers under the configuration seen in Figure 2. Because CLECs can purchase many different types of unbundled services, a switch is not necessarily needed by the CLEC for local services. Alternatively a CLEC might purchase a digital PBX for specialized local services, use unbundled trunks with line-side features from the ILEC, and offer PRIs directly from the ILEC to ISPs. Any of the above cases still require that IOF trunks be used through the other LATA end offices to gain access to the ISPs, thereby becoming a congestion point in the network at the end office servicing the ISP. Furthermore, CLECs must purchase line-side PRIs for its ISPs at a cost premium, versus switching services using the IOF ILEC trunk. Figure 3 demonstrates a case in which the CLEC and ISP are the same or the CLEC offers resold ISP facility access to ISPs.
Figure 3. A Case in Which the CLEC and ISP Are the Same or the CLEC Offers Resold ISP Facility Access to ISPs
Collocated ISP and CLEC combinations allow for direct PRI connections to the CLEC's switch, or if virtual switch services are offered by the ILEC, the CLEC and ISP remote-access equipment can connect directly to the central office DSX facility, eliminating the need to purchase DS–1 or higher transport links, as shown in Figures 1 and 2. Again, with line-side treatment of the PRI facility, routing through the CLEC's exchange or the ILEC's end office is still necessary, and the IOF trunks are dedicated for service for ISP functions.
Realizing that the PSTN cannot handle the ISP points of presence (POPs) traffic, some ILECs have begun to implement Internet bypass facilities for large POPs and congestion regions. See Figure 4 for an example of this capability.
Figure 4. An Internet Bypass Facility for Large POPs and Congestion Regions
Based on the previously mentioned traffic study, an ILEC could install remote-access servers in hub locations to terminate local dial-up traffic into remote-access servers and pass this data via an overlay frame-relay network to the ISP. The ISP would subscribe to the ILEC's switched frame-relay–network services to terminate this data traffic, requiring dedicated trunk facilities to the ILEC for the amount of bandwidth desired. This offers the ILEC an advantage: the ILEC may either offer ISP services or selectively route high-density ISP traffic away from IOF trunks and into the frame-relay network. This facility addresses large POP locations but does require a significant up-front investment in high-capacity remote-access-server equipment. Recurring dedicated trunk and frame-relay–service charges also must be considered for the ISP.
Smaller-market CLECs face a significant expense to overlay frame-relay–network access for a limited or initial customer base, which might not pay the added facility access charges. CLECs with multiple locations would be confronted with purchasing leased dedicated trunks and either overlaying their own frame-relay network or purchasing switched frame-relay–network services from the ILEC. This might be desirable for the large corporate environments with high-data transport demands, but for small-to-medium sized competitive ISP startups and limited-access rural markets, the cost may be prohibitive.
Recent product offerings of remote access server platforms using SS7 signaling gateways have the same bypass issues as the frame-relay servers. Dedicated IOF trunks are required to move switched traffic to the gateway location, thus increasing recurring access costs. In addition, distributed placement of SS7 gateways is expensive as a result of the initial computing platform requirement costs at each location.
