Electronic Design

System Breaks The Internet Log Jam Clogging Telco Switches

With an appetite that is seemingly insatiable, the Internet just goes on gobbling up more and more of the critical components of the public switched telephone network (PSTN). In fact, Internet service providers (ISPs) in New Hampshire have actually had their racks unplugged. Telephone companies simply had to make a choice—either local subscribers would have a dial tone, or the ISPs would have a business. This has all come about because the telecommunications industry is running out of switch capacity.

Recognizing this problem, Ariel Corp. of Cranbury, N.J., has jumped in with an interesting solution—a bypass system that augments the vaunted #SS7 out-of-band signaling system. Ariel's BypaSS7 system helps unclog present switch-capacity bottlenecks. The BypaSS7 network access system (NAS) lets carriers offer dial-up 56-kbit/s and ISDN Internet access without tying up crucial components of the PSTN, such as overworked Class 5 switches.

The SS7 is an out-of-band signaling system that provides basic routing information, call set-ups, and call-termination functions. Class 5 switches are central-office (CO) switches that accept toll calls from Class 4 major switching centers. According to Dennis Schneider, president of Ariel, "There are simply not enough switches, and the phone companies can't get them fast enough." This has created what has become known as the Internet offloader switch bypass problem.

Until now, most carriers have delivered subscriber calls to ISPs via T1 and primary rate interface (PRI) lines provisioned from the nearest Class 5 switch (Fig. 1). PRIs are ISDN equivalents of T1 lines. They deliver "23B + D" channels for carrying voice, data, and video information to customer-premise equipment (CPE) in North America. The 23 64-kbit/s channels carry the video, voice, and data signals, while the single 16-kbit/s D channel carries out-of-band signaling information.

The drawback to this approach is that it ties up expensive PSTN resources that were designed originally to handle short-duration voice calls, not long-hold-time Internet calls. It resulted in inefficient use of the PSTN, not to mention large capital expenses.

But Ariel's SS7-enabled NAS bypasses the end office (Class 5) switch by letting carriers connect ISPs to the PSTN via inter-machine trunks (IMTs)—circuits that interconnect automatic switching centers—rather than PRIs (Fig. 2). Carriers can expand their ISP business without having to purchase new switches.

The BypaSS7 combines one or more network access servers (NASs) with an SS7 signaling gateway (SSG). The SSG can handle 115 calls per second, or 414,000 busy-hour call attempts (BHCAs). With this performance, it can support up to 10,752 (16 DS-3) ports.

The SSG uses signaling information from the SS7 network to terminate V.90 and ISDN subscriber calls destined for the NAS. A PCI Linux system, the NAS receives 56-kbit/s and basic rate interface (BRI) ISDN subscriber calls from the PSTN via IMTs. The system can accommodate up to 96 remote-access sessions per server, originating from any combination of V.90-compatible modem or 64-kbit/s BRI ISDN customer-premises equipment.

Together, the NAS and SSG enable a Class 4 switch to behave as if it were a Class 5 switch while performing the task of a telephone-system remote access concentrator (RAC), a server used by ISPs to provide access to their networks. Also, the SSG communicates with the NAS over an IP-based LAN or WAN. The NAS and SSG, then, can be installed at the same facility or distributed over multiple locations. Such flexibility gives the carrier or ISP a variety of deployment options.

By locating multiple NAS units at each CO, all controlled by a single SSG, carriers can pick up incoming calls at the originating switch and bypass not only the end office (Class 5) switch, but the rest of the circuit-switched network as well. This approach also facilitates load balancing by enabling the SSG to route calls from busy sites to alternate sites with available ports, letting the carrier provision ports based on average rather than peak loads.

Relative to traditional Internet-access approaches, this bypass technique provides significant per-port savings. In the past, carriers have had to provision one port on the RAC ($200) and one port on the end-office Class 5 switch ($1600) for each port that they offer. Bypassing now reduces the total cost per port from $1800 to just $200, gateway included. Furthermore, there's no need for a Class 5 switch.

Another way the BypaSS7 cuts capital expenditures is by reducing the number of points of presence (POPs) the carriers must deploy to provide toll-free, local access for their customers. A POP is a long-distance carrier's office in a local community. In the past, this has required smaller ISPs to install POPs everywhere, because these ISPs provide their customers with fixed, local access.

Because of the new approach the BypaSS7 offers, only one POP will be required for every local access and transport area (LATA). A LATA is a local geographic area in the U.S. within which a local telephone company may offer services. Since there are only 193 LATAs in the U.S., there will only be 193 POPs at most, instead of the 700 to 800 local calling areas that are required by the current system. For instance, seven or eight POPs may have been located around Manhattan with the old method. Now, only one is necessary.

"With this system, major carriers and local-exchange carriers will no longer have to tie up expensive Class 5 ports in order to provide Internet access," Schneider concludes. "What's more, ISPs registered as competitive local-exchange carriers will be able to offer Internet access without having to purchase a Class 5 switch at all. This adds up to substantial capital savings and better utilization of the network infrastructure."

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