Bonded ADSL – How Does It Work?

Don’t confuse bonded ADSL with load balanced ADSL. Bonding involves splitting data up at the packet level, and distributing it across all the available lines on site. The data is then cleverly put back together again before being sent out to the internet, or to another site within the same network.

This works both ways, for uploading and downloading. Load balancing ADSL lines means individual data sessions are put over separate lines. You will never get the top speed of a bonded ADSL connection, and each line will have a different IP address, making hosting and resiliency nigh on impossible.
A Familiar Setup

A Familiar Setup

A regular single line connection using ADSL involves an ADSL modem connected to a BT phone line. Then, in order to share this connection across your network, you will have a router or firewall device that sits with one cable connecting it to the modem, and another to your network.This kind of setup works for most single line connections and is a familiar setup.

A Bonded ADSL Setup

A Bonded ADSL Setup

Increasing the number of lines and then bonding them together involves installing more than 1 BT phone line with broadband on it and then installing an additional device to do the actual bonding work on site.So, like the single line setup, you have a series of modems all connected to their respective lines, but then instead of having a firewall on each, or all connected into the same firewall on different ports, with different IPs, they all get connected to a bonder. Like a load balancer, but much better, this device, coupled with a carrier grade ISP server, does all the work for you, so that your network doesn’t know its dealing with more than one line.The bonder then behaves like your “default gateway”, which used to be the ADSL modem. Your firewall gets an IP address, or a block of them, and sends all its internet requests to the bonder, through one single ethernet cable.

How Do Bonded Connections Work?

How Do Bonded Connections Work?

Data is split between the lines and sent simultaneously

Bonding works by splitting data up at a packet level by an on-site bonding device, and distributing it over multiple internet connections. This data is then put back together in a central device of some kind, in an ISP’s network, before being sent onto the internet in a single stream.Because you have a central bonding server (or appliance) near the internet, and another bonding device near your own local network, neither side knows that bonding is taken place.It is the job of the bonder and the central bonding server to transparently transmit data to and from the internet, without your computers or the servers on the internet knowing anything is different at all.Your computers and network then get the benefits of increased uptime and more bandwidth, without having to send data in any specifically different way to normal.What Happens When A Line Goes Down?

What Happens When A Line Goes Down?

Both the bonder on site, and the central bonder at the ISP constantly monitor the lines with tiny packets of data. When they detect a problem, such as a line stopping responding, they stop the traffic flow on that line.But the traffic remains flowing along the other bonded lines, and you won’t notice!Of course, you need an ISP that cares when lines go down, and that takes the headache away from you of dealing with BT for broadband faults, and whether lines need reporting or replacing.With constant monitoring, you are informed when you need to be. Some problems can be fixed without the customer even knowing about them, and others are often legitimately problems the customer has on site with their equipment.Power outages are a very common issue, and you want an ISP that cares enough to call you if they see your connection is down, just to make sure everything is ok.Bonding is Not Load Balancing

Bonding is Not Load Balancing

It’s important to note that bonding is not just load balancing, where only individual sessions are split per line. In that set up, there are different IPs for each circuit, and no single user download or upload will be able to use more bandwidth than a single line.The IP address difference is quite important, because if you are hosting anything on site yourself, even allowing remote access to a server, which is quite common, then load balancing won’t give you any resilience.Bonding allows you to have a single IP address for the entire connection, regardless of how many lines are up or down. Load balancing doesn’t have this, and each IP is tied to a single line. If that line goes down, you lose that IP and any incoming traffic that is using it.

How Does IP Addressing Work With Bonded ADSL?

With a proper bonded ADSL connection, you can have a lot of flexibility with IPs, so you can tailor the connection to your business requirements. Fully routed IPs, NAT IPs, private IPs, and any combination of them are available.
For example, you have a VPN, so you need a fully routed IP on your firewall, but you also have a WiFi network that you don’t want to infringe the security of your LAN, but you do want to share your internet connection with. It’s no trouble to have that fully routed IP, but also have a private range for your WiFi router, and have them both connected to the bonder sharing the same bonded ADSL connectivity.
Or you may just have a normal server setup for your small business, where you have a mail server, maybe an intranet or other web facilities for your staff. You might have an FTP server, or Citrix or Remote Desktop services. For this, a public routed NAT IP is perfect. You can forward all of this traffic to its different servers through your firewall, but have the most efficient setup possible, by using only a single IP address. With the growing shortage of IPv4 addresses, you are doing your bit for internet efficiency!

Hardware Required for Bonding

Hardware Required for Bonding

This can vary between bonding providers, and will depend on whether you are managing your own broadband or not.Each circuit will need to terminate on a modem. This is true for ADSL, ADSL 2+ and FTTC. When it comes to EFM and Ethernet, they tend to terminate on a different kind of device, but it’s still a powered unit that will need some space in a cabinet, or on a wall, and a power socket.Each DSL modem will then be plugged into a bonder as they act as the default gateway for your router/firewall to access the internet.Some bonding providers don’t have a single dedicated bonding device, preferring to have a kind of master/slave arrangement between integrated modem/routers.This way of working tends to be with consumer-grade routers, altered from their standard manufacturer’s firmware. In this situation, you don’t actually have a single Ethernet port to plug in to so would need to add to those modem/routers a separate switch, which again would require the space and power it needs.Other providers can also have limits on the amount of throughput an individual device can take. For those with even a mediocre connection, it means another additional hardware unit for each line, so that you have a modem and router for each line, and then a switch. For a 4x bonded ADSL service, this would mean 9 powered devices, and all of their cables, before you plug into your firewall.You might also come across providers that have a minimum speed limit for each ADSL line. This is not good news for those with the slowest connections – under 2 mbps each. It’s important to get a bonding supplier that doesn’t have these limits, so you can benefit when you need to, and rest assured that you are future proofed if your location does get upgraded to things like FTTC.You wouldn’t want the bonding hardware or software to hold you back from these increased speeds.