Sonus In Session

The ABCs of SBCs... or What the Heck is an SBC, Anyway?

  

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By Dave Connolly, Marketing Communications at Sonus Networks

Unlike my Sonus colleagues who have worked in the telco space for years and eat alphabet soup (SBCs, SIP, IPv6, MGWs, P-CSCF, etc.) for breakfast, lunch and dinner, I come from a non-technical world where the only time you’re likely to hear the words “G.711” spoken is on Bingo night.  (OK, so I don’t actually play bingo since I’m not 108 years old, but you get the point.)  So, for the non-engineers in the audience, I thought I would explain what an SBC is and why they are becoming so important.  To better understand the role of a Session Border Controller, let’s start with the term itself: session, border, controller.

A session refers to a communications exchange between two endpoints over a network.  A phone call between two parties is a common example of a session.  A smartphone user browsing web pages that are hosted on a remote server is another type of session.  Also, when two or more people exchange instant messages or video images during a conference, that exchange represents a session.  In the world of SBCs,  sessions use an IP-based signaling protocol called SIP (Session Initiation Protocol).  The advantage of SIP is that it allows endpoints to send and receive voice, video and data all using the same signaling language.  So, you can add video to a voice call or include a web link in your videoconference during the same session, which is ideal for applications like Unified Communications.

These SIP sessions traverse one or more networks, which can be a service provider network, an interconnect carrier network or an enterprise network.  The SBC sits at the border of each network to control the amount and type of data that can enter during these sessions.  In this sense the SBC is part firewall, protecting the network from malicious traffic, and part traffic cop, policing how much traffic can enter the network in order to prevent overloads.

Finally, the SBC is a controller, which means it controls not only whether traffic can enter the network, but where it should be sent in the network (referred to as session routing).  It also controls what type of modifications should be made to the traffic (e.g., downgrading an HD voice call to a different voice codec if the endpoint doesn’t support HD voice).  From this, you can see that an SBC is really a network border element that controls SIP sessions.

Who Needs an SBC (and Why)?

Carriers and enterprises need SBCs for a multitude of reasons including:

• Network security
• Call admission control
• Signaling interworking
• Media transcoding
• SIP trunking

Network Security

Session Border Controllers are a fairly recent phenomenon, having grown out of the need to secure networks from Voice over IP (VoIP) traffic.  Before VoIP, voice traffic was carried exclusively over proprietary, private digital networks, which made all voice traffic inherently secure.  With the emergence of VoIP, however, those private networks are now connected to public IP networks like the Internet, which makes voice calls vulnerable to the same malicious attacks as other IP-based communications.  SBCs protect the private network by acting as a back-to-back user agent (which hides the network’s IP address from the outside) , blocking Denial of Service attacks and other things.  SBCs also provide secure communications to and from the network through media and signaling encryption.

Call Admission Control

This is important, because carriers need to balance the flow of traffic into their networks to meet their Service Level Agreements, and enterprises need to balance traffic to meet a high level of customer service through their call center.  SBCs provide the necessary call admission control through certain policies (e.g., use Trunk B when Trunk A is overloaded) and specific call routes (e.g., send calls along the route with the lowest latency).  This, in turn, ensures that VoIP networks have the same levels of quality and responsiveness that end users experience on the traditional PSTN.

Signaling Interworking

Communications networks are often complex systems that have dozens of different devices like routers, gateways, media servers and (of course) SBCs, many of which require some type of signaling mediation in order to talk with one another.  “But isn’t SIP supposed to take care of that?” you ask.  Well, yes and no.  You see, even SIP-based devices require interoperability – for more on that, read the white paper, “SIP-to-SIP: The Interop Dilemma (And How You can Fix It)”.  And that’s not even considering the mediation required for IPv4 devices to talk to IPv6 devices.  SBCs provide the interworking for all of these different devices and protocols, in a sense enabling the machine communications that drive human communications.

Media Transcoding

As if different signaling languages weren’t enough, phones and other endpoints (tablets, laptops) sometimes use different voice codecs (that is, the voice technology that codes and de-codes your voice for digital communications).  This requires a process called transcoding.  A first-generation SBC may have handed the call off to a separate transcoder for translation, but increasingly the SBC itself performs this transcoding as it receives the call.  Transcoding not only ensures that communications sound and look better, it can also reduce the amount of space that each conversation takes in the network.

SIP Trunking

No, it’s not how elephants drink.  SIP trunking is how enterprises can pay peanuts—well, a lot less anyway—for the same voice services they’re getting today by delivering them over more efficient SIP-based trunks.  SIP trunks are created when two SBCs connect to one another in two different networks, such as an enterprise SBC and a service provider SBC.  For large enterprises, the telephony savings alone can equal millions of dollars a year, and the voice quality can be equal or better than toll services.

If your brain is feeling full at the moment, just remember the three S’s—SIP sessions, security, savings (okay, technically that’s four S’s)—and you won’t forget the important role that SBCs play in today’s communications networks.  And for the visually inclined, I recommend this video on The Evolution of SBCs.