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Ask the Analyst: SIP Trunk Service Basics

In reviewing a number of the questions I have received from business customers on SIP Trunk Services, it occurred to me that a number of these could be grouped into "basic' versus "advanced" categories. As a result, I'm dedicating this article to questions and answers on SIP Trunk Service basics. If you are just beginning to learn about SIP Trunk services, this article can be especially helpful.

I’m speaking at Interop New York later this week on SIP Trunking services. I'm told that this is the first presentation Interop has ever done on this subject, so it will be interesting to see what types of questions the audience will ask.

SIP Trunk Services and the PSTN
Q. On SIP Trunks, can we choose our own long distance provider or do we have to use the SIP Trunk service provider for offnet/PSTN calls?

A. In most optimized SIP Trunk services architectures in which the customer deploys SIP and VOIP CPE, SIP Trunk services are only procured for sites that serve as offnet PSTN conduits. Thus the same provider must be used for SIP Trunk services and PSTN connectivity. These same sites often will have connectivity to SIP-based cloud services like hosted IP IVR and ACD, hosted IP audioconferencing, web and videoconferencing services, etc. For more information, please see my response about flat networks and UC in the next section (last question in this article).

Q. Can SIP Trunks avoid local call usage billing (in California, for example)?

A. I assume the question pertains to offnet calls. This depends on the carrier. Some providers’ SIP Trunk prices offer the option in which one can buy local outbound service in quantity. Typically there is a predetermined limit, which if exceeded will incur a per-call surcharge. But other providers bill for every offnet call, whether local or LD. If your company makes many local offnet calls, it's important to use the RFI and RFP process to understand providers' pricing structures.

Q. Is the day of rating a call as Intrastate vs Interstate in the SIP world going away, to essentially an all Interstate pricing model?

A. Over the long, long term in the US, yes. The FCC declared all VOIP calls as jurisdictionally interstate. Although some state PUCs are unhappy about this, the FCC's decision still stands. But these are calls that are end-end VOIP. Thus at present, if a call transits the PSTN, it is billed jurisdictionally (e.g., intra vs. inter-state). SIP Trunk offnet calls (placed to or from a PSTN connection, or calls that transit the PSTN in part) are more numerous than most IT Directors assume. Here, having facts at your fingertips is essential. A review of most companies' bills shows 75-80%+ of all calls (intra and inter-company) are currently offnet. As companies migrate to SIP Trunks, this percentage will decline. But at the very least, a company can't control other companies' migration to SIP Trunks, and so it must plan for a substantial percentage of calls to be offnet for years.

Q. What gains does SIP Trunking provide if the customer is a single location and a "typical" data customer with a T3 pipe for existing Internet access and multiple PRIs for PSTN access?

A. A single-site company obviously doesn't need to employ CPE-based SIP Trunk signaling between multiple sites, because it doesn’t have those sites (this also assumes no employee telecommutes, etc.,-an increasingly rare phenomenon.). But a single-site company can still use carrier-provided SIP Trunk Services for (1) connectivity to the PSTN for incoming/outgoing calls using compression schemes not available on most PRIs. (One bank I'm familiar with made its whole business case just on this.) (2) connectivity to carrier-based SIP Trunking services like hosted IP IVR/ACD services, IP audio, web and videoconferencing services, and as an alternative to buying, maintaining and managing an IP PBX, using hosted VOIP/UC/IP centrex services. Longer term, SIP signaling will become the de-facto signaling to support real-time communications. So even though a single site business can defer using SIP Trunks, it can’t avoid SIP Trunks forever. Ultimately, SIP signaling cannibalizes legacy protocols like in-band signaling, ISDN, etc.

Q. We have multiple locations across several states and these locations are networked via PRIs to our carrier, who bundles local & LD minutes into our monthly recurring charge (MRC). We only pay for Toll-Free calls. (1) Where are all the savings and benefits, and (2) what about the reliability of a SIP-based infrastructure--both CPE and carrier equipment?

A. In the legacy world, a customer like the one above has a pretty sweet deal-assuming the carrier’s MRC is reasonable. It's important to remember that precious little in this world is truly free, and my experience with calls is if you pay for them on a flat rate basis, you typically pay more than you would if the price were usage-based. That's true regardless of protocol--TDM, PRI or SIP. For multisite companies, most of the savings related to SIP comes from re-architecting--from using a MPLS or Ethernet network for intra-company calls, and for using one or two sites as locations to connect to SIP Trunk services to connect to the PSTN (for offnet incoming and outgoing calls). Benefits include inter-company SIP signaling functionality to support multi-company collaboration, and accessing SIP-based cloud services (see more info on this in the very first Q and A above).

(2) Reliability is a fair question, and many providers offer dual homed access to diverse SIP Trunk nodes as a way to attain high availability on the services side of the connection. Depending on the provider, this may be offered at no additional charge if the homing begins at the provider’s access node (typically one SIP Trunk service node homes to many access nodes). Typically, providers charge additional fees if a physically diverse second SIP Trunk is also provisioned. While 5Es and 4Es offer high reliability, they are not foolproof. So, some carriers offer similar forms of business continuity for companies that want very high assured availability to the PSTN. On the CPE side of the access connection, although IP PBXs weren't as reliable as TDM PBXs for a number of years, that is no longer the case. And again, various IP PBX or gateway vendors provide different methods to assure high availability--some offer a more "baked in" approach than others. But here's the bottom line: carriers are investing as little as they can in legacy PSTN services and networks. For instance, some are asking state PUCs to approve longer and longer PSTN time to repair intervals, and others have asked the FCC what it will take for them to de-commission their PSTN toll networks. All traditional carriers are making plans and investments to grow their SIP-based networks to migrate customers to them. So using SIP Trunks isn’t a matter of "if", but "when".

WAN and Access-Related Questions
Q. Can companies purchase SIP bandwidth as a stream (e.g., X Kbs or Mbs) instead of concurrent calls so we can use more aggressive compression schemes to place more calls on the SIP trunk?

A. I know of no major enterprise-class services provider who supports this. All require the purchase of concurrent call paths, typically engineered for peak calling periods (e.g., busy hour). In addition, since the SIP Trunk interfaces to the provider's node, the only compression schemes that can be used are those the provider has authorized. This can vary by make/model of SIP certified CPE employed at the customer site. Some companies may elect to use different compression schemes on the intra-company calls they place on their MPLS networks, but if the call is transiting the SIP Trunk for offnet purposes (both on an incoming and outgoing call basis), it must use one of the SIP Trunk provider's approved codecs--the most common today are G.711, G.729 a and b.

Q. Can SIP Trunks be used to interconnect to legacy networks like Frame Relay?

A. To my knowledge, no major business-class SIP Trunk service provider formally supports this. The entire point of moving to SIP is to be based on IP services. It's possible some customers may interconnect their FR networks to SIP Trunk services via interconnections of multiple CPE devices (for instance, from FRAD to router to SBC or IP PBX, and from there connect to a provider's SIP Trunk service), but call quality can certainly suffer. Also, most providers don't formally support Voice over FR, and those that do are in the process of discontinuing the service (at the very least, they are making such plans). In these circumstances it's probably not financially prudent to make investments for the future based on an assumption about continued carrier support for legacy network infrastructure.

Q. Will interconnecting SIP Trunks to the public Internet result in uneven call quality?

A. In a word, yes.

The weak link here is the public internet (and the potential number of providers, service quality, and hops involved in an end-end call), not the SIP Trunk service. Some businesses are willing to make the tradeoff of cost and quality, many others seem oblivious of the uneven call quality, although the far end of the call usually is extremely aware of it. Typically companies are interested in using the public internet for SOHO/branch office connections, or for offnet calling (to other businesses). My general advice is to use SIP Trunks for offnet PSTN connectivity (when calling to/from other companies), and to look at dedicated access that combines both SIP Trunks and data services for smaller sites.

Q. What about connecting very small sites like locations with less than 5-10 people?

A. I know some companies' sites are so small that the only access currently available on a widespread basis is 2G mobile service, POTS, cable or DSL. When looking at the calling patterns of these sites, I think many companies will discover that the majority of calls to and from the site(s) in question are not intra-company, but inter-company (e.g., offnet-where one end is PSTN). If your provider doesn’t offer a quality small-site SIP solution (such as a small-site focused hosted VOIP/UC service, or SIP Trunking service over certain types of cable modem service, as some MSOs do today), then the most reliable thing to do for now is to retain POTS or look at wireless (2G) services. It's perfectly understandable that businesses want to swing the same type of hammer across all their sites, but many haven’t been able to do this for decades--they've used PRI at large and medium sized locations and POTS at most small ones, and on the data side, a combination of higher-speed and higher quality access connections for large and medium sites, and DSL or cable modem for smaller sites). Eventually more providers will offer small-site focused solutions that cannibalize POTS connectivity, but that could take some time.

Q. What about interconnecting SIP Trunks to one provider’s public Internet service?

A. If the provider offers the same QoS capabilities as the SIP Trunk service and the customer's interconnecting CPE, this can work in environments where the entire call is on the same provider’s network (or the provider has interconnection agreements and SLAs that support end-end QoS on a multi-provider basis). Depending on the provider, if the call is inter-company, the call may be billed as either on-net or offnet. But such QOS-based Internet services are the exception, not the norm today.

Q. Is it necessary to have the same provider for data services/MPLS and SIP Trunking services?

A. It depends on your architecture. If the only connection between the two is through your CPE, then you can employ different providers. The CPE must support the CoS options available from both the MPLS and SIP Trunk service providers. But many customers will want to use the same access connection for both SIP Trunks and MPLS and/or Internet services, especially at medium-small locations. In such instances, it is usually necessary to use the same provider for all. The only exception I'm aware of--some providers’ SIP Trunk services use Internet connections to reach multiple carriers. While this is possible, such an architecture will have difficulty providing a high-level of call quality assurance (see question # 2 above).

Q. Can a company use one broadband service provider to interconnect to another SIP Trunk service provider?

A. In most cases, providers don't support this. Please see my answer to the preceding question.

Q. Can SIP Trunking from one provider be a node on another carrier's MPLS network?

A. In the US, very few carriers directly interconnect their MPLS services with each other, so this type of arrangement is still considered the exception.

Q. How can a branch site make an outbound call if their WAN connection (that supports intra-company calls) is down?

A. If that is the only connection, it can’t. There are a number of potential solutions. One low tech approach-- employers may reimburse employees that use their personal cell phones in such circumstances. But if you are interested in wireline/SIP connectivity, this is one of the reasons companies employ both Internet and MPLS on SIP Trunks--they use the Internet connection for backup connectivity in the event MPLS fails (for both voice and data applications). The call quality may not be great, but it may be good enough on an exceptional basis for a number of intra-company applications. For sites with a great deal of intra-company connectivity (eg., offnet calls), the company should consider other scenarios. These include using a backup MPLS connection to the data center (preferably from a secondary provider), or another SIP Trunk that has a diverse path to a second carrier SIP node (same carrier as primary SIP Trunk/MPLS provider), or to a PRI for access to the PSTN, etc.

Q. Is it necessary to use dedicated access for SIP trunks?

A. Yes. All service providers offer T1, T3s, OC-X as the physical layer (to support SIP Trunk services). Some will use Ethernet at layer 2. Some support their own variation of business-class DSL or cable modem access. But there is no IP equivalent of POTS switched access for real-time applications like VoiP.

Q. What about using a SIP Trunk service provider who doesn’t have a local access/services footprint in all the locations we need?

A. Even large carriers with substantial ILEC footprints don’t have ubiquitous dedicated access-- and so they wholesale physical access from local carriers in their out-of-region markets. The same is true of smaller carriers. But like PRI, the SIP Trunk service provider supplies the signaling and transport services to the customer site(s), which interconnect to the (certified) CPE. Some providers who make extensive use of other carriers' local access make special arrangements for attaining higher reliability (typically through diverse routing). This can be more expensive, but is important for customers whose applications require high availability. Often companies prefer to use the same provider for both local access and SIP Trunk services--this is perfectly understandable from both an economic and carrier management perspective. But in and of itself it doesn't guarantee higher service availability, better time to restore in the event of a problem, etc.

Q. Can a company provide its own access (customer-provided access--CPA) to the carrier’s SIP Trunk service node?

A. Depending on if and what the provider supports, yes. Most commonly this is accomplished through OC-X connectivity. CPA interconnection usually is not a question of SIP Trunking, but of the carrier's physical layer access interconnection capabilities and policies.

Q. Should we use a flat network to achieve UC?

A. Compared to the hierarchical structure employed in many companies' FR networks, yes. For the sake of call quality, there are two main architectural scenarios:

1. When using MPLS or Ethernet services with CoS and SIP signaling for intra-company calls, it’s advisable that all locations directly connect to a data center that houses the necessary VOIP/UC/SIP CPE. The data center accesses the SIP Trunking service for all those sites (for offnet calls, for cloud-based carrier SIP services). For survivability, it’s prudent to employ backup/alternate data center(s) that also have SIP Trunking services. See earlier discussion on business continuity.

2. Some businesses employ hosted VOIP/UC services now, and I think many more will in the coming years. In most deployments, each site directly accesses the hosted VOIP/UC service--they do not transit a data center. Today, some carriers provide SIP Trunks as part of these services (some have yet to do so, which perturbs me). The carrier moves both on-net and offnet calls through its infrastructure--on-net typically calls transit the provider's managed VOIP service and are included in the basic hosted VOIP/UC fee, offnet calls connect to the carrier's PSTN network.

Some readers will be interested in additional SIP Trunking information. These earlier No Jitter postings can be of help:

Status of SIP Trunk Services and Evaluation Criteria: Pierce Pierce Pierce

Prior Ask the Analyst Question and Answer Postings Pierce Pierce