PDF of this tutorialIn the beginning, the ISP connection to the PSTN was a dedicated high-speed line from the egress switch to ISP–owned modem banks utilizing remote access server (RAS) technology. These huge modem banks proved to be very expensive for the ISP because the more successful the Internet became, the more dial-up modems needed to be purchased and operated in order to meet demand. Meanwhile, in terms of traffic management, because the egress Class-5 switch now had to handle local voice traffic and 100 percent of the ISP’s traffic, the egress switch became the network hot spot as dial-up Internet traffic grew.
Figure 4. Remote Access Server (RAS) and Egress Switch Congestion
Carriers were motivated to respond to ISPs with the concept of CO–based RAS (COBRAS) service. In this scenario, the modem pool evolves from strategic technology that the ISP must own, to a service purchased from the lowest-cost provider. From a carrier perspective, as RAS is housed within the egress central office, COBRAS provides much greater control over modem traffic engineering and eliminates mandated reciprocal compensation. But despite the advantages to this approach, the network hot spot remains the egress Class-5 switch as Internet usage heats up.
Figure 5. COBRAS and Continued Egress Switch Congestion
Egress switch congestion has driven carriers to look at alternative RAS connection architectures, including direct RAS connection to tandem switches. This tandem-based approach effectively off-loaded the egress Class-5 switch, but the burden just moved “upstream” to the tandem switch. Juggling the demands of its normal voice-oriented trunk switching and handling direct RAS connections, the Class-4 tandem switch now experienced capacity problems—it had become the network facility being overrun by dial-up Internet traffic instead of the Class-5 switch. In the end, direct RAS connections to tandem switches merely served to aggravate tandem exhaust problems—a new hotspot but a hot spot just the same.
Figure 6. Tandem-Based Off-Load Moves Hot Spot to Class-4 Facilities
The tandem exhaust issues have driven carriers to directly connect RAS to ingress Class-5 switches, which offload both egress Class-5 switches and Class-4 tandem switches. As major ISPs have begun demanding their own direct trunks to COBRAS, capacity issues arise again at the ingress Class-5 switch. The tandem exhaust problem is solved but not the overall problem—the hot spot is simply being moved around.
Figure 7. Direct Trunk Offload and Resulting Ingress Switch Congestion
More recently, a fresh offload approach has emerged with the advent of the trunking gateway (TGW). Deploying a TGW within a tandem application allows trunks to be shared across multiple RASs/ISPs and provides a secondary benefit of simplifying trunk management. Furthermore, the gateway can parse voice and data traffic, totally off-loading the egress Class-5 switch of ISP traffic.
Figure 8. Trunk Consolidation/Off-Load via Trunking Gateway (TGW) and Continued Ingress Switch Congestion
However, the TGW does nothing to mitigate the traffic problems at the ingress Class-5 switches—they remain hot spots. In other words, no matter what solution is employed on the trunk side, the hot spot inevitably moves upstream to a Class-5 switch or Class-4 tandem switch.
