With continued growth in Internet traffic, not all businesses can access fibre broadband services to improve throughputs, and dedicated leased lines or MPLS connections can be very expensive. Bonded ADSL (also known as ADSL bonding) can be a cost-effective solution for businesses that have outgrown the capabilities of a single ADSL or ADSL2+ broadband connection. For organisations dependent upon Internet connectivity, ADSL bonding offers much-valued resilience and offers distinct advantages over load balancing.
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How does bonded ADSL work?
True bonded ADSL is where multiple ADSL lines are effectively combined into a single aggregated connection to deliver greater download and upload speeds. ADSL bonding involves the aggregation of two or more ADSL or ADSL2+ connections, as shown in the figure below.
While the concept of ADSL bonding is deceptively simple (i.e. combining multiple ADSL connections together), the practical implementation of bonded ADSL is relatively involved and requires significant processing in the splitting and recombination of datastreams.
Consider a video stream (e.g. BBC iPlayer or YouTube) being viewed on a PC. With bonding, the original streamed video datastream has to be split up into multiple streams, each of which is sent down an individual ADSL or ADSL2+ connection. The process of splitting datastreams is performed at a special datacentre operated by the bonding provider. At the customer’s premises, the separate datastreams are recombined to form a single data stream.
In the upstream direction, for example, when a file is sent to the Internet from a PC, the original file is split into separate streams by special equipment at the customer's premises to be sent on two (or more) ADSL or ADSL2+ connections. These separate streams are then combined at the datacentre.
Bonded ADSL providers have adopted different technical solutions at customers’ premises. For example, with the Evolving Networks service, the necessary processing is performed by a separate ‘Bonded Internet Gateway’ (shown in the photo below). With the Sharedband bonding service, processing is usually performed by a conventional modem router (which is running customised software developed by Sharedband).
Benefits of bonded ADSL
The benefits of a bonded ADSL solution are:
- substantially increased downlink and upstream throughputs (enhanced further through compression, if this is offered)
- avoidance of dramatic performance degradation from 'flooded' connections
- improved quality of service (if this is offered).
Throughputs are close to the throughputs of individual connections added together
Overall, throughputs are close to the aggregated throughputs of individual connections. So, two bonded ADSL or ADSL2+ connections each providing download and upload throughputs of 3 Mbps and 1 Mbps, respectively, would be able to provide download and upload throughputs of nearly 6 Mbps and 2 Mbps.
An increased number of connections would be able to provide even greater throughputs. According to Evolving Networks, there is no problem with bonding together many ADSL or ADSL2+ connections and Evolving Networks has tested more than 12 bonded ADSL links to form a single connection. However, the vast majority of users of ADSL bonding bond together two, three or four broadband connections.
Due to the way bonding works, there is an overhead of about 10% (according to Fusion Broadband), so the resultant throughputs are about 90% of the total throughputs of the individual ADSL or ADSL2+ connections used. This is because additional packets of information have to be transmitted alongside the data carried to provide the bonding system with the information necessary to manage the individual streams and recombine them effectively. Our own testing of the Evolving Network bonded ADSL service shows that the overhead with its service is only 3-4%.
For users struggling with very low download speeds on a single line (say, 2 Mbps or less), ADSL bonding can transform the Internet experience by allowing services (such as HD video streaming) that were previously not possible with a single ADSL or ADSL2+ connection. For example, a single ADSL connection capable of delivering 2 Mbps downlink throughput would not be able to stream a 3.5 Mbps HD video stream from BBC iPlayer, but a bonded solution using two similar ADSL connections would be able to successfully stream the HD video stream.
Some ADSL bonding providers (such as Evolving Networks in the UK and Fusion Broadband in Australia) offer the ability to use a special feature called compression to significantly improve throughputs of bonded lines even further. Compression is a feature that causes all the traffic sent and received to be compressed where possible. Some types of traffic, such as text-based files and web pages are generally highly compressible while others (such as JPEG) images are not. Fusion Broadband claims that, with compression, two ADSL2+ connections that deliver a total uplink throughput of about 1.6 Mbps are capable of delivering in excess of 8 Mbps with compression. Since different data types and files are compressible by different amounts, the throughput improvements from compression can vary. Fusion Broadband expects that it customers will see about double the throughput on a "cross section of compressible data".
ADSL bonding provides much-valued resilience to businesses dependent on Internet connectivity
With bonded ADSL services, businesses have enhanced protection from losing Internet connectivity, which is business-critical for many organisations. If a single ADSL or ADSL2+ connection goes down, instant failover is built in to ADSL bonding so an Internet connection is maintained on the remaining working link(s). There is no loss of connectivity.
For many businesses, the costs associated with a complete loss of Internet connectivity are likely to be dramatically higher than the costs of a bonded ADSL solution.
Evolving Networks ensures that multiple ISPs are used with its bonded ADSL service to provide maximum redundancy by avoiding all links going down due to a single ISP failure. While Sharedband does not specify which ISPs are used with its service, it recommends a diversity of broadband suppliers to maximise the resilience of its service.
The dramatic performance degradation from flooded ADSL or ADSL2+ connections is avoided
An important issue that can affect standard ADSL and ADSL2+ connections is the substantial drop in performance that occurs when a standard connection is fully loaded, that is when the connection is close to, or at, its maximum achievable speed. This could be caused by a single user or single service, for example, a desktop user uploading a large file or an automated online backup service.
Once a connection becomes flooded, a single service can have a dramatic impact on other users and services using a standard ADSL or ADSL2+ connection. Furthermore, downlink-dominated services such as Web browsing can be severely impacted by uplink-dominated services (e.g. file uploading). So, a single file upload could dramatically slow down web browsing for all other users. This is because, while browsing is downlink dominated, acknowledgement requests have to be sent on the uplink. When these uplink messages are queued due to the file uploading, Web browsing speeds are substantially reduced.
According to Fusion Broadband, flooding causes the latency of an ADSL service to increase "dramatically", with a flooded link often displaying latency at five to ten times the normal level. ADSL bonding systems are able to detect when packet queuing events start to occur and can regulate the speed of the specific 'problem' service(s) to keep latency under control. Fusion Broadband says that "a 100% flooded ADSL link will suffer a dramatic increase in latency, where a 100% flooded bonded connection will see a much smaller level of latency movement."
Some bonding providers implement advanced quality of service mechanisms to prioritise certain applications
The ability of a bonding provider to control the connections between the customer's premises (essentially the Ethernet port that the customer's LAN is connected to) and the aggregation datacentre potentially allows them to improve performance by implementing advanced quality of service management. A number of bonding providers, including Evolving Networks in the UK and Fusion Broadband in Australia, offer advanced quality of service options as part of their bonded ADSL offerings.
As shown in the table below, different applications require different amounts of bandwidth and have different sensitivities to delay (latency) and jitter. The relative use and importance for each application will inevitable differ from organisation to organisation. For example, one particular organisation may have widely adopted VoIP and/or video conferencing, while another organisation may perform regular uploading or downloading of large (e.g. video) files.
|Application||Bandwidth required||Requirement for low latency (delay)||Sensitivity to jitter|
|VoIP||Low (depends on number of users)||High||High|
Table: Technical requirements for different types of services
Bonding providers offering quality of service management are able to adjust the performance and priority of different services to best meet the unique requirements of individual businesses, for example:
- by prioritising VoIP and/or video conferencing so that these services work well even when there are high demands on the network (for example, from web browsing and email and file uploads/downloads)
- by reducing the priority of certain 'background' applications (such as downloading/uploading of emails or large files) so that they do not slow down web browsing.
The combined impact of increased throughputs, compression, avoidance of 'flooding' issues and enhanced quality of service can have a dramatic impact on actual and perceived performance.
Bonded ADSL has advantages over load balancing
As discussed in our ADSL loading balancing page, organisations can make use of multiple ADSL or ADSL2+ connections without using ADSL bonding technology. A number of manufacturers now offer so-called load balancing routers, which offer two or more WAN inputs and are relatively affordable. The Cisco RV320 load balancing router (which costs around £150) - shown below - is designed for small businesses and incorporates two WAN inputs, allowing two ADSL or ADSL2+ connections to be used.
Load balancing has a number of disadvantages compared with ADSL bonding.
Firstly, load balancing does not transform two streams into an aggregated stream so cannot transform two 2 Mbps connections into an aggregated 4 Mbps HD video stream, for example, like ADSL bonding can provide. However, a load balancing router can be effective at sharing two or more connections among many users, balancing the traffic between the available broadband connections. So, for example, if there are two ADSL connections, each with a downlink throughput of 2 Mbps, and four users each trying to download a file, the load balancing router would allow each user to download the file at about 1 Mbps. As with ADSL bonding, the total throughput available across all users from a load balancing router is about equal to the combined throughputs of the individual ADSL connections. Unlike load balancing, ADSL bonding allows individual users to access a single data stream at the full throughput and, is therefore, the most flexible.
Secondly, there can be issues with the separate IP addresses used for the individual broadband connections with a load balancing router. With ADSL bonding, a single IP address is provided so that the aggregated Internet connections looks, and performs, like a single broadband connection. With a load balancing router, individual connections retain their IP addresses, and we have found that some applications (such as Skype) can become confused by the presence of multiple IP addresses and stop working.
Thirdly, while load balancing routers generally provide some quality of service mechanisms to enhance performance (particularly when the system is fully loaded), these are not as effective as the quality of service mechanisms possible with ADSL bonding, where greater end-to-end control can be achieved. Furthermore, many small- and medium-sized enterprises lack the IT skills to implement effective quality of service mechanisms locally in their networks, whereas some bonding providers can offer enhanced quality of service as part of their offerings.
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