Migrating from TDM PSTN trunks to SIP PSTN: Challenges

Migrating to SIP trunking is a strategic decision for the enterprises.  when adopted in right way it can provide flexibility to address immediate and future collaboration needs. However, SIP adoption comes with its own set of challenges including: 

Installation complexity: Lack of uniform standards and substantial protocol, complex configuration, and behavioral makes move between different SIP Trunking solution a challenge.  Leveraging (Enterprise session border controllers) eSBC can help mitigate these issues.

Need for robust WAN infrastructure: To effectively support SIP trunking, organizations need a robust WAN to prevent latency and jitter issues.

Lack of expertise and support: Organizations need to leverage right level of expertise – either in house or through a provider – to customize SIP trunking solution based on organizational needs.

Current Telephony Deployment Approach if Costly and Has Higher Call Rates

In a typical telephony deployment, a bunch of physical wires are delivered from the PSTN service provider to the enterprise. However, TPSTN circuits require high investment to purchase ISDN, BRIs, PRIs or local PSTN gateways, have fixed channels for voice calls and seldom offer buffer. In addition, inefficient least cost routing, lower flexibility to add more lines and parallel infrastructure in addition to IP WAN network adds to the complexity.

Leveraging Right SIP Trunk Model for Enhanced Flexibility and Cost Optimization

SIP trunk allows enterprises to replace these traditional PSTN lines with PSTN connectivity through SIP trunking service. Some of the models include:

Centralized SIP Trunking

In central SIP trunking, only central office location has the direct SIP connectivity to the IP PSTN where all external calls to or from remote sites go through the central location (see figure.1). Centralized SIP trunking requires higher bandwidth for connectivity to both the IP PSTN and the private WAN to service all remote sites. 

The main benefits of centralized SIP tracking include:

• Simpler management of the SIP trunks and SBC and improved optimization of trunks
• Better volume discounts with centralized traffic
• Single WAN link for single point of failure for PSTN connectivity.

Distributed SIP Trunking

In a distributed SIP trunking environment, each site that has connectivity to the WAN has direct SIP connectivity to the IP PSTN. This connectivity may be over the same physical connection. (see figure 2). Benefits of SIP trunking include: 

• Bandwidth optimization as bandwidth of each site determined by its local need.
• Ability to increase bandwidth
• Reduced latency of real-time communication
• Improved redundancy and survivability.
• No requirement of PSTN gateway
• Easier transition to SIP trunking because it mirrors the TDM architecture.

Hybrid SIP Trunking

Hybrid SIP trunking include sites that have distributed SIP trunk model as well as centralized SIP trunk model. In this model each site has direct SIP connectivity to the IP PSTN, and other sites have a centralized SIP trunk deployment, accessing the IP PSTN through the central location. The hybrid SIP trunk deployment model may have multiple central hubs in different regions, or for specific business functions (see figure 3). The benefits of hybrid SIP trunking include:

• Adaptable bandwidth based on site’s specific requirements
• Adaptable capacity to regional SIP offerings at sites that connect directly to the IP PSTN
• Quality of Service (QoS) and Call Admission Control (CAC) for those parts of the network that are linked to a regional or central hub
• Survivability is built in for sites linked to the IP PSTN
• Centralized management tools to support both provisioning and policy monitoring

Selecting the Right SIP Trunk Model: Factors to Consider

While selecting the model some of the important criteria to be considered include:

• Are there limitations on the bandwidth availability at the central hub?
• How much SIP traffic is expected to originate in how many branch offices overall, and how much in each branch office?
• How much growth is expected in call volume, bandwidth usage, and video traffic?

If the bandwidth is limited and there are many branch offices that use lot of bandwidth, then the centralized model may not be the best choice.

A general guideline to selecting the model is that if remote location consumes more than 50% of the bandwidth in the WAN, then the distributed SIP trunk model should be considered. If consumption is less than 20%, the centralized model may be considered. The hybrid model should be considered for situations where branch office demand is high but that demand varies significantly from site to site.

Another deciding factor is geographic distribution. For example, location of remote sites may mandate local termination for enhancing compliance with local regulations. This will require distributed or hybrid model SIP trunk architecture.

The Manifold Benefits of SIP Trunk

• Elimination of parallel network infrastructure and their cost
• Flexibility to roll out new features and functionalities
• Elimination of costly BRIs (Basic Rate Interfaces) and PRIs (Primary Rate Interfaces) subscriptions
• No investment in PSTN gateways and additional line cards
• Edge devices offer low investment path in adding new lines
• Maximum flexibility in dimensioning and usage of lines
• Flexible termination of calls to preferred providers

Engaging with Right Partners for Successful SIP Trunk Implementation

Its critical for organizations to engage right partners to define a short term and long term SIP PSTN strategy, do PoCs and business case analysis to understand the benefits and value SIP PSTN trunking can deliver.  The partner shall help your organization choose the right SIP carrier based on their presence footprint, failover capabilities, SLAs and commercial viability. Your partner shall also help you create a contingency plan and a robust recovery strategy. Finally ensure that your partner understands your business, business challenges and business objectives.

Sushil Kumar Katre

Lead Consultant – Networks Practice

Network Practice, CIS

Sushil has over 14 years of experience in designing and architecting data and voice network. His expertise is in providing network consultancy, design & architecture and presales for global accounts. He is currently part of Cloud and Infrastructure Services in Wipro responsible for network pre-sales.