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SIP Trunking Explained

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If you know very little about telecommunications but have some basic understanding of the Voice over IP technology – you will have no trouble grasping the concept of SIP trunking. Understanding why DLS chooses not to offer this service may take some explaining.

Understanding Trunking

The word “Trunk” has more than one meaning but in communications it came to represent a concept of a part that can be divided into branches and vice versa. More specifically, the term “trunk” is used to describe a transmission channel between two switching systems. Such transmission channel could be comprised of one or more communications circuits. When you think about public switched telephone network (PSTN), you imagine many different telephone switches all connected to each ot

her using trunks. Each trunk would typically consist of multiple trunk lines. Trunk sizes vary depending on how many trunk lines are in it.

Since the 1970s telecom services largely relied on the technology called TDM (Time Division Multiplexing). This technology allowed delivery of fixed number of voice channels per digital circuit. For example: T-1 circuit would contain 24 channels, PRI – 23 channels, etc.  Each channel could be used for voice or data. This technology represented a significant leap from the analog switches because it allowed delivery of multiple channels over a 4-wire connection eliminating costly requirement for individual copper pairs to be run from the central office switch. A business could purchase a digital circuit(s) and pool some or all of its channels into trunk. These channels would then be referred to as “trunk lines”.

From the prospective of a PBX phone system, however, “inside trunk lines” are channels connecting endpoint devices to the main body of the phone system. One of the primary purposes of PBX system was to share a limited number of “outside trunk lines” among greater number of “inside trunk lines” providing cost savings by not requiring each user to have their own line.

Sounds confusing? Well, maybe it is a little. The important part is to remember that “trunk” is made up of “trunk lines”. To add to confusion, many telecom professionals and PBX installers began to use the words “trunk” and “trunk line” interchangeably even though this was technically inaccurate.

Why SIP?

With the introduction of VoIP technology, many protocols such as H.323, MGCP, Skinny and others were designed to simulate trunk lines over the Internet protocol. Each trunk line would require a fixed amount of bandwidth. SIP on the other hand did not simulate “trunk lines”. Instead it offered a simplified option of establishing and managing a “session” between network applications. Simply put SIP establishes and manages a session that exists only while Internet applications need it to communicate. These applications could be voice, video, chat, SMS or any other.

How SIP Trunking Works

SIP trunking offered by ITSPs (Internet Telephony Service Providers) connects customer’s PBX phone systems to the PSTN network. This service is also referred to as “Origination and Termination”. Calls originating from and terminating on the public switched telephone network are delivered with the help of Session Initiation Protocol (SIP) via service providers “soft switch”.

The beauty of VoIP of course is that all this happens over the Internet and the soft switch can be located anywhere.

Who Offers SIP Trunking?

SIP trunks are offered by most incumbent telephony carriers or Internet telephony service providers who specialize in wholesale connectivity or offer connectivity to premise-based PBX.

Some VoIP providers which offer hosted PBX services also choose to offer SIP trunking to customers that prefer to operate their own premise-based PBX systems although this service puts them in direct competition with their own hosted product.

Different SIP Trunking Pricing Plans

Various ITSPs package their SIP trunking services in accordance with their philosophy. Some charge based on the fixed number of concurrent sessions (“trunk lines”), others charge squarely based on the number of minutes used by the sessions. Most telephone companies, large or small, nowadays offer SIP trunking. There are two categories of SIP trunking offerings targeted to different types of users.

Wholesale SIP Trunking is a service offered by ITSPs to ITSPs. It’s pricing is typically tailored to high voice call volume. Purchaser is usually required to meet usage volume commitment in order to qualify to certain pricing tiers.

Retail SIP Trunking is targeted towards businesses that wish to utilize premise-based PBX system. These premise-based systems are either older legacy systems that rely on VoIP gateways to connect them with the ITSPs or more modern VoIP systems.

Some of the early IP PBX systems allowed connecting VoIP phones using but still required TDM or analog trunk lines to connect to outside world. Even now many premise-based IP PBXes continue to support conventional and analog and digital trunk lines requiring specialized telephony hardware.

Retail SIP trunks are priced in a variety of ways. Some plans offer unlimited or near unlimited sessions while others only allow a set number.

Per-minute usage charges vary from carrier to carrier. Call quality may also vary depending on audio codecs supported by your PBX and ITSPs soft switch.

If you have resources to maintain and support your own PBX or a full featured Unified Communications system, you may want to consider purchasing different trunks from multiple providers and utilize least cost routing (LCR) if your PBX call routing engine allows it. While one provider offers great domestic per-minute rates, another one may offer lower international calling rates. Programming and maintaining LCR within your PBX is not a trivial task and must be done by an experienced professional. Also, VoIP termination rates have a tendency to change often and keeping track of them requires certain level of commitment.

Due to openness and differences in the implementations of the SIP protocol, use of SIP trunking typically requires a thorough interoperability testing to ensure VoIP service compatibility with the customers PBX. Such testing is commonplace but there are times when compatibility issues arise even after interoperability tests were performed. Once again, an experienced VoIP engineer can help resolve these issues in most cases.

Premise-based PBX systems may also have licensing limitations preventing their owners from taking full advantage of unlimited SIP trunking. Developers of the commercially available PBX software saw this as a way to license their products based on the number of sessions they could open to the phone company.

How Hosted PBX Changes The Game

As an ISP entering VoIP market without any preconceived notions about how telecommunication services should function, DLS had focused its development on a hosted PBX service. Our vision of the hosted VoIP service encompassed assuming a role of a one-stop Internet Telephony and Unified Communications Service Provider. As in most cases with the Hosted PBX, we provide voice, video  and other communication services directly to endpoint devices eliminating the need for customers to purchase SIP trunking altogether. Our Hosted PBX service offers unlimited inbound and outbound sessions. “Unlimited sessions” probably sounds too much like a marketing term (and it is), but for all practical purposes, the ceiling of concurrent number of calls is set high enough to be deemed unlimited even by the most demanding call center operator.

Many ITSPs offer usage-based and unlimited calling pricing plans. Unlimited calling pricing model favors high usage clients leaving ITSP exposed to fraudulent uses and phreaking, but that is the risk many VoIP service providers are willing to take on.

To every rule, however, there are exceptions. Not all hosted PBX providers offer unlimited number of sessions per connected device. Some ITSPs are limited by their softs switch software licensing costs, which may depend on the total number of concurrent VoIP sessions offered.

Why Don’t We Offer SIP Trunking?

Since 2004 DLS has focused on delivering rich and flexible Unified Communications experience rather than using VoIP technology to sell usage-based trunk lines and minutes as telephone carriers did for many years.

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