Dial-on-Demand Routing: Analog and ISDN Dialup
State-of-the-art digital modems and ISP remote-access platforms support the new ITU V.92 and V.44 standards with features such as better compression, modem-on-hold, quick-connect, and improved upstream performance. However, only time will tell whether ISPs will rush to migrate to these new standards given the development of alternative access technologies with better margin, the risk of introducing instabilities in stable access networks, and the questionable commercial feasibility of upgrading existing equipment, firmware, or software. All modern modems support at least V.90/V.34/V.42bis.
ISDN is available in the form of Basic Rate Interfaces (BRIs) and Primary Rate Interfaces (PRIs) with the capabilities of channel bundling. ISDN is often used for backup scenarios (dial-on-demand routing) and call aggregation, whereas analog modems are often deployed for remote management of network equipment.
In spite of the positive aspects, ISDN backup scenarios are often plagued by the following problems and restrictions:
Backup for at least 50 percent of business bandwidth is not commercially feasible and essentially means deployment of an access server with one or more PRIs.
Deployment results in complicated policy routing configurations. For example, what kind of traffic triggers a dial connect? What is the definition of timeouts, of thresholds? What condition triggers teardown of a dial line? Flapping interfaces, Network Address Translation (NAT) scenarios, and IP Security (IPSec) backups further complicate the matter.
To carriers and service providers, AAA (authentication, authorization and accounting) is a fundamental issue. The effort regarding authentication, IP address allocation, and assignment is considerable for backup scenarios.
Experience has proven that often carrier DLLs as well as the PSTN use the same trunks originating in the same central office. This defeats the purpose of dial backups and presents a treacherous picture of safety, especially at the edge of these infrastructures, where redundant trunks are rare.
Businesses tend to favor dual-homed Internet access with their own autonomous system (AS), provider-independent (PI) address block, and flexible Border Gateway Protocol (BGP) routing. This makes ISDN backup scenarios obsolete to a large extent. However, with the maturity of DSL, wireless, and cable networks, these technologies could be used for backup scenarios as well, even in concert with BGP routing. One has to consider slightly different service-level agreements (SLAs) and the issue of availability in the region, though.
Analog modem adapters, ISDN cards, single-chip solutions, and PRI/channelized PCI or ISA adapters are available in many variants. For a small number of PRI adapters, proprietary UNIX drivers are available and complemented by some open-driver initiatives. A discussion of channelized interfaces goes beyond the scope of this book.
