If you've ever looked at the cost of consumer ADSL and wished you could get business connectivity at that price, you are not alone. It's not just the capacity that's nice though - it's the ability to buy in lines from multiple ISPs, in the hope that if one goes down you will still be able to use the others.

This is a technique called multi-homing, and in the past it has relied on technology such as BGP, the Border Gateway Protocol, which is how routers tell each other about link failures so incoming packets can be re-routed onto the active link.

However, where BGP focused on leased line connectivity and required powerful routers, a range of suppliers can now aggregate bandwidth from broadband connections instead.

One such is FatPipe, which can aggregate and load balance up to 20 lines, creating a single virtual connection.

"Lots of companies have two lines, one sitting there doing nothing, and when a problem occurs it can take two or three hours to move over. I can use both lines at the same time," says Jesse Carillo, FatPipe's international franchise manager. "We allow a customer to bond lines from different companies and aggregate them through one router.

The lines do not have to be the same capacity or type - an important consideration in the UK where two supposedly separate DSL suppliers will most likely both be using BT for the final mile.

Combining technologies
"We are talking to some WISPs in Ireland, they can offer satellite, wireless and cable in parts of Dublin, so now there's lots more infrastructure you'd have to knock out to kill all the connectivity," Carillo adds. "Or in the Gulf of Mexico there's an oil rig that has both a line of sight wireless link and satellite, and when cruise liners dock they block the line of sight."

"The concept of bandwidth aggregation is not new," says Simon Lok, the founder of Lok Technology, which recently added bandwidth aggregation to its AirLok gateway device - it calls the technology link control.

Lok points out that 64k ISDN technology has been able to provide 128k for ages, by merging two lines into one in a process called channel bonding. "What's interesting is we can aggregate lines from different ISPs without the ISPs having to participate," he says. "Channel bonding requires equipment at both ends but we only have a single device, on the client side."

If all you want is redundancy for fail-over, these solutions might be overkill. There are a number of Far Eastern companies offering inexpensive two-port boxes designed to aggregate say a DSL line with a cable modem line. However, this will only provide fail-over for outbound traffic, and even then some Web-based applications may fail and need to be restarted, warns Jesse Carillo.

Inbound vs outbound
He adds that while most solutions can load-balance outbound traffic across the available lines, only a few can do the same for inbound connections without requiring the use of BGP.

"Inbound is only an issue if you're hosting your own servers or sites, but that's the opportunity for us," he says. He explains that FatPipe builds a local nameserver into its boxes, based on BIND 8.3 and called Smart DNS.

"We become the authoritative DNS server, that's how we do the inbound load balancing," he says. "We don't need ISP co-operation, unlike BGP routing. You just have to notify them that you are now the authoritative DNS."

Office Web browsing
Of course, for many organisations, a multi-homed leased line is still going to be essential for inbound traffic. But outbound traffic is also growing fast - and like most home users, office Web and email users are asymmetrical in their Internet usage, downloading far more than they upload.

Some organisations have therefore used broadband aggregation in connection with Internet access tools such as Microsoft's ISA Server, says Rieko Sato, product management director at Rainfinity whose RainConnect device was a relatively early entrant in the bandwidth aggregation business.

According to Sato, educating potential users remains a big issue, but is a lot better than it was. She adds that it was not unusual in the past to hear from users who had simply plugged two ISP connections into one server and then wondered why they weren't getting the full bandwidth of both.

"People are better educated now," she adds. "RainConnect used to be mostly SMBs, now it's enterprises, especially on ISA Server - they were just using it as a proxy server, now they're turning on its firewall features and starting to realise it's actually quite powerful, so they're adding NAT and so on."

Bandwidth aggregation is becoming more complex too. Jesse Carillo says FatPipe recently added QoS support, with the ability to allocate high priority traffic to the connection with the lowest latency, as part of the load balancing process.

The next step - QoS
"VOIP is becoming a huge aspect so we added support for that," he says. "You can route on IP address, traffic type, user address, etc. You could put all the voice traffic down one line first, then have a second preference and so on. You can put quality rules in - that's very powerful for VOIP."

Simon Lok agrees: "What I think people miss out on is that aggregation alone is not enough - once you want to aggregate four lines or replace a T3, you really need to do bandwidth management too," he says. "That needs a bandwidth shaper, and per-user bandwidth management, with the concept of allocating lines to tasks.

"You need think about what your load looks like and figure out which of your applications are latency-sensitive, or are bandwidth hogs, and provision for those. Instead of six DSL lines replacing one T3, it might be five DSLs and a T1. That's still cheaper - around a third of the cost of a T3, and you send the latency-sensitive traffic over the T1."

He adds that bandwidth agreggation is an area where two plus two doesn't necessarily equal four - sometimes it's three, and sometimes it's five...