How Does SD-WAN Work?

This is the second post in our series about SD-WAN, the first was What is SD-WAN, which described what an SD-WAN is and highlighted some business applications.  This second post is a high-level description of how an SD-WAN works and a little bit about what makes it such an exciting development in Business Networking.

How Does an SD-WAN Work?

The technology behind SD-WAN is not new – what is new is putting it all together in a package.  It’s based on a technology called SDN (Software Defined Networking) that corporate network admins have been using to manage their Local Area Networks.  Centrally managing a WAN is not new – vendors already offer various load balancing and failover technologies and are always happy to sell multiple links to a site. But those technologies, combined with SD-WAN’s ability to add dynamic load balancing to share network bandwidth across connections is a package that is better than the sum of its parts.

Dynamic Load Balancing

Dynamic load balancing is more responsive than traditional failover or load balancing.  In a traditional failover setup, a router uses a primary connection until it senses that the connection is down, where it will switch all traffic to the backup connection.  In this scenario, the backup connection is idle unless the primary connection is down.  Also, most traditional failover methods take several seconds to switch to the backup connection.

Traditional load balancing works in a similar way. It will manage two or more connections by using an algorithm to set up connections across them.  Unlike SD-WAN, all connections must be the same type.  If one of the connections were to go down, the load balancing would suspend activity on all circuits, recalculate the traffic balance across the remaining connections and reassign traffic.  This would handle failover, but it would interrupt all network traffic, including voice calls, and new connections would have to be established.

An SD-WAN uses intelligent load balancing to manage two or more connections of any type at a single site.   It does this by continuously measuring one-way packet loss, latency, jitter and available bandwidth on each path and sending traffic over the connection that is performing the best, or even by using both connections at once for different traffic. Since a complete failure on one circuit counts as an “issue”, you get failover for free.  There are two different methods that SD-WAN vendors use to perform load balancing, session-based and packet-based.

More traditional session-based load balancing makes the determination of which connection to use at the start of each internet session.  If problems are detected on the connection, or if the connection goes down, the SD-WAN will terminate the current session and restart one on another connection.  The unfortunate aspect of this type of load balancing, described earlier, is that in a failover situation it can take a few seconds to restart sessions on the working connections.  This is usually fine for most types of internet traffic, but for VoIP calls it will cause calls in progress to drop and they must be reestablished.

Packet based load balancing, on the other hand, determines which connection to use constantly and can switch back and forth between connections within a session.  If problems are encountered on a connection, traffic can be re-routed to another connection seamlessly, and it can fail over in a few milliseconds.  This means that near-instantaneous failover can be accomplished without interrupting the session, or the voice call.

WAN Management

Managing your wide area network through software has many distinct advantages. In the past, making changes to the network configurations of your branch offices would have taken a large planning effort and also would have had to be applied by an on-site technician.  Rolling out a new hosted voice system, for example, would have required setting up bandwidth allocations at each location, usually requiring a technician to be sent on-site to make the changes.  Using an SD-WAN, changes can be made from a central location using a graphical user interface.

QoS (or Quality of Service) is a technology used to prioritize network traffic that is more sensitive to delay.  For example, email traffic can experience a 300 millisecond delay and no one will notice.  A 300 ms delay on voice traffic, however, will result in choppy audio and dropped calls.  QoS is not supported for internet traffic generally because internet routers are too busy routing enormous volumes of traffic to worry about things like prioritization.  Some versions of SD-WAN can actually provide rudimentary QoS because they manage traffic at both ends of the connection.  An SD-WAN can detect delay on a connection and route around it, or prioritize voice traffic at both ends of the connection.

What Does This Mean for Corporate Wide Area Networks?

Flexible – MPLS networks can take weeks to set up whereas commodity broadband connections can be set up in a few days to a week.  Even faster in the case of LTE and fixed wireless.  This means that branch offices can be set up quickly and integrated instantly into the the corporate WAN.  Also, SD-WANs can tie together connections from multiple vendors, so there’s no more waiting for a single vendor to provision “off-net” connections to complete your WAN. Moving sites is now as simple as moving the SD-WAN router and implementing it on a new set of connections at the new site.

Less Expensive – A broadband link can be one tenth the cost of an MPLS connection and tying a few of them together in an SD-WAN gives you the same reliability for a lot less money.  For large organizations, this will result in significant cost savings on their corporate networks.  For smaller organizations, it means that highly available managed networks are now within the reach of more modest budgets.

More Reliable – Most connections do suffer from random bouts of delay or jitter just in the course of a normal day and most commodity internet connections are best-effort – not covered by any service level agreement.  In addition, even the most reliable network connection can be interrupted by an errant backhoe.  An SD-WAN ties multiple connections together into one so that your traffic is always using the best connection every second of every day.  Because they can handle different types of connections, an SD-WAN can even be used to incorporate wireless connections to improve resistance to backhoes.

Better Management – Very few people have the training and experience to manage traditional corporate WANs, and even then each router vendor has their own protocols and management software.  SD-WANs vastly simplify the management of corporate networks because they reduce network options to the bare minimum and automate the rest. Their graphical user interfaces and common protocols make SD-WANs manageable even by relatively inexperienced personnel.



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