Offload Where it Counts

Line-side offload means a return to the good old days, where traffic jams on the information highway are minimized

Remember when the only time you ever heard, “We’re sorry, all circuits are busy” was when you were dialing Aunt Martha and Uncle Charlie on Christmas morning? The minor inconvenience was simply part of the holiday, with so many people calling distant family members. Usually, though, you only heard this message once; on the second try, you got connected and wouldn’t hear the message again for another year.

Fast forward to today. At peak times in many cities, these same “busy-out” messages are as regular as rush-hour traffic jams. The cause is well understood: nailed-up modem calls to Internet service providers (ISPs), with circuit-hogging Web sessions that can last for hours and bring Class 5 switches to their knees.

While the cause of the problem is clear, the solution hasn’t been obvious. The last great hope was DSL technology, which delivers ISP traffic without involving Class 5 switches. But DSL hasn’t gained nearly the traction originally hoped for it, and the good old dial-up link still is the runaway connection of choice for many Web surfers–even as average hold times and associated “We’re sorry…” messages continue to increase.

Eager to relieve network hot spots without endlessly adding expensive Class 5 switches, network operators have pursued various forms of trunk-side and tandem-switch offload techniques. But because these techniques alter traffic patterns without actually reducing dial-up Internet traffic within the PSTN, they merely relocate these problems to other network points, rather than eliminating them.

Detour: Congested Area Ahead

In the early days of Internet access, ISPs terminated calls at their own facilities via ISP-owned remote access servers (RASs). As the number of connections multiplied, network hot spots appeared at the egress Class 5 switches feeding traffic to those RASs. The first solution carriers came up with was to move the RASs to the central office, with the advent of the central office RAS.

As Internet access began to enter the mainstream, time division multiplexing (TDM) network congestion forced network operators to move their RASs closer to their ISP customers via tandem switch connections. Tandem exhaust problems quickly grew to the point that the tandem switches were now the hot spots. The next solution? Connect the RASs to the trunk side of Class 5 ingress switches, offloading both the tandem and egress switches (Figure 1).

When the popularity of the Internet skyrocketed, rapid growth in the number of ISPs and amount of data traffic meant rapid growth in the number of RAS platforms and the quantity of direct trunks. As a result, every Class 5 switch and tandem switch became a potential hot spot, depending on the data traffic patterns of the moment. Now what? Try IMT-PRI (inter-machine trunk to primary rate interface) offload to bypass the egress Class 5 switch. 

Gateways that convert IMTs to PRIs allow carriers to bypass the egress Class 5 and connect directly to the ingress Class 5 or tandem switches.  This approach allows shared trunking between the egress and ingress offices and eliminates the need for dedicated trunking for each RAS platform in the egress office.  Deploy next-generation network tandems so that offload trunks can be shared across multiple RASs/ISPs (Figure 2).

This is where we are today, with solutions that succeed in simplifying trunk management but fail to ease congestion, simply rearranging hot spots rather than eliminating them.

Imagine a city with severe traffic problems at key intersections. Trying to manage that traffic by temporarily detouring it to less clogged intersections would simply move the problems to the next set of vulnerable areas. Proper traffic planning in such a situation would typically include the building of an “outer loop” that moves substantial traffic completely around the city completely, eliminating altogether a portion of the traffic causing the problem in the first place.

While trunk-side and tandem offload techniques provide some relief, they’re not a cure for circuit overload. The key is to eliminate as much of the circuit-switched data traffic as possible.

And so it is with congestion on the public switched telephone network (PSTN). While trunk-side and tandem offload techniques provide some relief, they’re not a cure for circuit overload. The key is to eliminate as much of the circuit-switched data traffic as possible. And that requires offloading ingress switches–on the line side, before the traffic hits the first Class 5 switch, just as an outer loop expressway routes vehicle traffic completely around a city.

A Major Step Toward NGN

By offloading Internet traffic before it ever reaches a Class 5 switch, line-side offload delivers PSTN relief at the earliest possible point in the network. The key component enabling this relief is an access gateway (AGW).

The access gateway’s job is to separate Internet traffic from voice traffic, allowing narrowband voice traffic to advance to the Class 5 switch as always, while intercepting modem calls and routing them around the Class 5 switch (and the PSTN) directly to the ISP. The logistics involved are actually relatively simple, given that the vast majority of ISP calls at any central office go to a relatively small, known set of phone numbers.

Dial-up calls made to ISPs such as AOL, EarthLink and MSN can easily be identified in AGW look-up tables, then routed around Class 5 switches to eliminate an enormous amount of network congestion. With that accomplished, there would be infinitely less concern about how many of those calls were placed–or how long the connections lasted. Class 5 switches would be free to carry the voice traffic they were designed to carry. And carriers could effectively cap their ongoing expenditures on legacy PSTN infrastructure (Figure 3).

In addition to separating Internet traffic from voice traffic, the AGW packetizes Internet traffic to carry it across the public data network (PDN). This pre-switch packetization is key to total PSTN offload, and paves the way for further packetization of voice as well as data, via voice over ATM and voice over IP–effectively accelerating the move toward next-generation voice/data convergence.

Fork in the Road

For carriers, addressing dial-up Internet traffic congestion comes down to a choice between two options:

• continue to add Class 5 switch capacity and hope to stay ahead of the problem, or

• implement line-side, pre-switch offload.

It is also a choice between maintaining a defensive, reactive posture on traffic management versus taking aggressive, preemptive action to move the network forward.

...addressing dial-up       Internet traffic loading by expanding Class 5 switching capacity alone results in a cost up to five times more than pre-switch offload or packetization solutions.

From a financial standpoint, the difference in approaches becomes even clearer. For an incumbent local exchange carrier (ILEC), studies show that addressing dial-up Internet traffic loading by expanding Class 5 switching capacity alone results in a cost up to five times more than pre-switch offload or packetization solutions. While trunk-side efforts net modest gains over merely expanding switch capacity, line-side solutions are up to three times more effective from a cost perspective.

These cost differentials derive from various factors, including the relative expense of the substantially higher-price of a Class 5 switch as compared to an access gateway subscriber-side offload solution. Relative effectiveness in offloading traffic from the entire network is another important factor.  Consistent with this analysis, pre-switch packetization is the most effective in offloading the entire network and also is the least expensive option.

Advantage: Line-Side

Access gateways are real and so are the advantages of subscriber-side offload. What’s more, both the carrier and their customers share the rewards. For the carrier, line-side offload redirects PSTN-clogging, dial-up Internet calls around the Class 5 switch for an express trip to the ISP. This approach also simplifies and modernizes the network while laying the foundation for next generation voice/data convergence.

At the same time, it caps carrier expenditure on legacy infrastructure and is much less expensive for carriers to implement than continuing to add Class 5 switch capacity–hoping to stay ahead of Internet growth and popularity. Ultimately for customers, line-side offload means a return to the good old days, where the “We’re sorry…” message is yet again relegated to Christmas morning.
Fred Ellefson is vice president of marketing for Rapid5 Networks. He can be reached at fred@rapid5.com.

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