Distributed vs. Centralized SIP Trunking: Making an Informed DecisionDistributed vs. Centralized SIP Trunking: Making an Informed Decision

There are pros and cons to each architecture, so make sure you understand the factors at work in your enterprise's network.

Today's enterprises are increasingly moving to SIP Trunking as a way to reduce costs and improve network performance. But how does a business decide which deployment model best suits its needs? What are the pros and cons of each model?

SIP Trunking Offers Big Benefits
To review, there are a variety of business benefits related to SIP Trunking, but the main ones are:

* Significant TCO savings: Most customers can reasonably expect savings of 30-50% over TDM legacy systems.
* Simpler management: Eliminates PBX and network access redundancy, streamlines and centralizes call control, and simplifies network management--all while driving significant economies of scale.
* Unified communications: SIP enables you to roll out a vast array of productivity-enhancing features including unified messaging, "anywhere presence", and real-time web-based collaboration for your entire workforce, from HQ personnel to sales road warriors.

SIP Trunking Models: Pros and Cons
There are two flavors of SIP Trunking architecture--distributed and centralized. With the distributed model, multiple sites (possibly each site) have their own separate SIP trunks connecting to service provider networks. In contrast, with a centralized architecture, SIP traffic is backhauled from remote sites to a centralized datacenter (which may be mirrored for redundancy), and from the datacenter, a single SIP trunk (or redundant trunks) delivers the aggregated SIP traffic to the service provider.

Each model has its own advantages and disadvantages.

Distributed SIP Trunks, Pros:
Because each site has its own dedicated access circuit to the provider's network, there is no single point of failure, and engineering for each site's traffic requirements and call routing requires no consideration of the requirements of other sites. Additionally, geographically dispersed sites won't be subject to potential latency issues that might result from tromboning local calls across the country.

Distributed SIP Trunks, Cons:
The biggest downside is that you'll need to invest in and manage an IP-PBX and Session Border Controller (SBC) at every site, forgoing the savings derived from a centralized configuration. Scaling is another issue--increasing bandwidth requirements at sites and adding new sites is more difficult, time consuming, and expensive than in a centralized model. Additionally, you may not be able to "pool" sessions across the network, meaning that if you have an unexpected spike at one site, you can't "borrow" sessions from another where there is idle capacity.

Centralized SIP, Pros:
Approximately two-thirds of enterprises deploy a centralized model with just two or three connected service locations. This architecture significantly eliminates duplication and increases economies of scale. Instead of tens or hundreds of pairs of PBXs and SBCs, you only have to manage a few...and smaller organizations may elect to have only one. Further, by consolidating traffic over just a few access circuits, you eliminate the wasted bandwidth overhead that comes with having unique access circuits at each site; and with COS (Class of Service) packet prioritization, you can run the consolidated SIP access circuits much hotter (i.e., at higher utilization) without losing calls or voice quality. The bottom line is that you'll spend a lot less time and money managing your network.

Centralized SIP, Cons
If you have only one connected hub site, you may have a single point of failure risk, and there is the potential for traffic latency for distant sites--though these challenges can be overcome by strategically distributing your SIP hub sites. Additionally, E911 may not be available from your SIP provider for remote sites, so confirm they can support 911 at all remote sites; alternatively, you can plan to have a TDM circuit at each remote site just for 911.

SIP Creates Significant Benefits over Legacy Solutions
While most enterprises choose a centralized model, the appropriate solution for your business will depend on the factors listed above. Some companies actually implement a hybrid solution where their main business locations are connected to a central hub site or two, while specialized sites (e.g. call centers) have dedicated SIP infrastructure. But no matter which configuration you choose--distributed, centralized, or hybrid--you'll see significant benefits over legacy solutions.