Components of SIP-based Network Architecture

WAN technologies have migrated from analog environments to digital environments in the past few decades.  With this transition, the cloud is now positioned to better support SIP-based network infrastructures.

Kevin McMahon

April 02, 2015

WAN technologies have migrated from analog environments to digital environments in the past few decades. With this transition, the cloud is now positioned to better support SIP-based network infrastructures.  Alongside the evolution of the WAN, three new classes of networking devices have been developed:

  • The Softswitch
  • The IP-PBX
  • The Session Border Controller

The functions and capabilities of these devices will be explored in light of their support for SIP-based networking environments.

The Impact of the Internet and SIP on the WAN

In the mid-1990s, the Internet Engineering Task Force (IETF) began its development of the Session Initiation Protocol (SIP).  SIP was to the voice world what the Hypertext Transfer Protocol (HTTP) was to the World Wide Web – another application that could be run over an Internet Protocol (IP)-based network – and one that would consume additional WAN bandwidth.

The New Architecture: SIP-based Switching and SIP Trunks

With SIP becoming an industry standard, and with the exponential growth of bandwidth consumption within most organizations, a need arose for a technology solution that could meet all requirements.  This solution was a SIP-based trunk that allowed remote locations to connect directly without all of the protocol conversions.  The evolution of this technology is further proof that the world is moving in the direction of SIP.

Benefits of SIP Trunking

There are many benefits to implementing SIP Trunking.  It can save an enterprise money, since SIP trunks can be purchased in the exact increments needed.  Furthermore, as offices may grow or downsize, SIP trunks can be scaled to any specific capacity – up or down.  

Another benefit is that the SIP trunk can be adjusted for changes in bandwidth requirements, such as for seasonal call volume increases.  Moreover, SIP is a necessary platform for running Unified Communications applications like presence. Finally, SIP trunk lines provide an increased level of reliability and business continuity.  For instance, SIP trunk lines can be programmed to reroute to backup sites if the need arises.

Softswitches – The New Standard in Switching

The Public Switched Telephone Network (PSTN) and Internet can be likened to a freeway – there are specific paths that each call takes from source to destination and a great deal of traffic along the way.  On this journey, there are three basic functions that are required: routing, transmission, and billing.  

While the PSTN has understood and utilized these functions for years, they needed to be adapted for Internet VoIP technologies in order for them to be fully operational on an end-to-end basis.  For the PSTN, these functions are primarily accomplished through the use of switches that are strategically placed around the world.  

The switches are made up of two key elements:

  • Switching fabric - makes the physical connection 
  • Switching logic – provides call routing and control functions, custom calling features, and interfaces to other systems like billing

Switches have gone through an evolutionary process, starting out in the form of telephone operators, then relay-based mechanical switches, and later developing into fully-electronic switches.  A new generation of switches, known as softswitches, are solving many of today’s network migration challenges, providing a path to the future of integrated voice, video, and data applications.  

Softswitches help in the following ways:

  • Replace most of the hardware switching with computer software-based functions
  • Separate the physical switching function from the control logic function, enabling devices in a variety of locations to perform these operations
  • Provide mechanisms to bridge the connectivity gap between existing circuit-switched (PSTN) and evolving packet-switched (VoIP) services

Softswitch Architecture

Softswitches are made up of two major elements: a call agent and a media gateway.  The call agent provides most of the network intelligence, handling the call routing, network signaling, billing, and related functions.  The media gateway delivers the physical connection for an end-to-end path, incorporating a variety of LAN and WAN interfaces like Ethernet, IP, T1/T3, ATM, etc.  The call agent controls the media gateways across the globe, and the type of transmission formats such as analog, LAN, WAN, and so on, that are involved.

Softswitch functions can be divided into four architectural areas:

  1. Transport – carries the call signaling and media (voice, data, video) signals
  2. Call Control and Signaling – controls the devices in the VoIP network and establishes and disconnects media sessions
  3. Service – controls services and applications like applications servers
  4. Management – deals with service provisioning, support, and billing

In order to connect the logical and physical sides of the softswitch architecture, communication must take place between the media gateway controller and media gateway.  This function is accomplished when the media gateway controller sends commands to the media gateway using one of two protocols – either the Media Gateway Control Protocol (MGCP) or the Media Gateway Controller (MEGACO) protocol.


Another key networking device for SIP-based environments is the Private Branch Exchange (PBX).  The PBX has been the workhorse of premises-based communications for several decades.  It acts as a communication circuit between each end station or an end station line and outgoing trunk circuit.  Furthermore, it calls for a limited number of trunk circuits to the central office that are shared amongst the end stations.  The PBX facilitates this trunk sharing.

PBXs are installed at customer sites to provide switching for end-user stations.  This equipment connects end-user stations to one another either onsite or to offsite stations on the company’s private network.  They can also connect end-user stations to other destinations such as cell phones, which are accessible through the PSTN.

PBX technology has gone through the following evolutionary stages: from mechanical switching systems, to analog systems, to digital switches supporting both data and voice, to the current stage of switching technology containing an infrastructure that supports IP technologies.

The PBX is made up of various end-user features.  Smaller switching systems called Key Telephone Systems (KTS) provide basic features like caller ID, call hold, call transfer, voicemail, etc.  These systems are designed for small organizations with 5 to 50 end stations.  Larger PBX systems supporting tens of thousands of end stations provide additional features, including an auto-attendant, conference calling, call detail recording, call center capabilities like call queuing or statistics, and can integrate with other systems like Microsoft Outlook.

Session Border Controllers (SBC)

A final SIP-based WAN network access product, the Session Border Controller (SBCs), provides the following capabilities:

  • Network-to-network security
  • Resolves IP addressing differences between networks
  • Keeps track of key call information like calling and called parties and the call duration
  • Allows VoIP calls to traverse Network Address Translation (NAT) devices and security firewalls
  • Provides admission control
  • Monitors Quality of Service (QoS) and Service Level Agreement (SLA) issues
  • Provides protocol translation between different networking formats

SBCs function as a demarcation point between two networks – an inner network and an outer network.  The architecture consists of two components: a signaling component and a media component, which both logically and physically connect the two networks.  The signaling piece deals with service requests like connects, disconnects, authentication, and authorization.  The media piece deals with the transport of information including QoS and SLA monitoring.

To keep up with the evolution of the WAN and corporate bandwidth demands, advanced networking technologies like Softswitches, IP-PBXs, and Session Border Controllers have developed, providing more capabilities and better functionality for today’s changing networks.  Moreover, it has been estimated that well over half of all companies will be implementing SIP trunking by the end of 2015.  As a result of this increase in adoption comes a need to delve further into the many concepts surrounding SIP Trunking.

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Author Name
Kevin McMahon
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