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SIP Trunking Explained Clearly

Updated March 22, 2021

We’re the first to admit that VoIP isn’t always the easiest to understand.

Sometimes the technology feels needlessly complex. You just want to plug this into that and have it all work. And, yes, you want engineers to name things so us normals can understand them.

But there is method to the madness, we assure you! VoIP systems are organized this way for a reason.

When you actually know what a specific component is used for, then it’s much easier to understand the why and the wherefore.

Take SIP trunking.

Now there’s an engineer’s term! An acronym and a noun used as a verb. You might be turning away already. You might be thinking: “That’s beyond me. I’m no good at that stuff.” Then, when we say that SIP trunking is a crucial component in a VoIP deployment — which it often is — you might give up.

Don’t despair!

We’re going to explain SIP trunking clearly for you.

Our goal in this blog is to answer the following common questions:

  • What is a SIP trunk?
  • What is the PSTN?
  • What is a PBX?
  • How is VoIP different from traditional telephony?
  • What is SIP?
  • Why is SIP trunking important?
  • What are the advantages of SIP trunking?

We’re going to discuss each topic in layman’s terms, using comparisons to help.

Tree Trunks

What is a SIP trunk?

A SIP trunk is the connection between you and your ITSP.

What does that mean? By the end of this blog, you’ll know exactly what that means. Keep it in mind: the connection between you and your ITSP.

But first, let’s start at the start. We need to talk telephones.

Picture this. You want to call someone over a phone. Except your “phone” is a tin can at the end of the wire and the “someone” is your childhood friend. She’s standing around the corner with a tin can in her hand.

The two “phones” are connected by a wire, right? You have a direct connection between the two phones. And the two of you can talk that way.

You can see the limitations of a direct connection system. What if 20 people with 20 telephones want to be able to talk with each other. You can’t all have direct connections to everyone! That’d be very messy. There’d be cables running every which way.

This is primary challenge of telephony: How do you connect someone to everyone?

The answer that’s been developed over the decades is the Public Switched Telephone Network (PSTN).

Telephone Wires

What is the PSTN?

The PSTN is the world-spanning network of cables, switches, exchanges and all the other technology that compose the telephone network. What the Internet is for computers, the PSTN is for phones.

A good way to picture the PSTN is to imagine sending a letter.

The postal system is the equivalent of the PSTN. You could drive your letter to the recipient’s house yourself. That’s the equivalent of a direct connection phone call. You don’t do that, though, do you? Rather than deliver a letter yourself, you give it to the post office and they deliver it for you. There is a network designed to get your letter to its recipient.

The post office is the equivalent of a telephone exchange or switch, as in “public switched telephone network.” We give our letters to the post office, they read the address, then direct the letter to its recipient. They switch the direction of the letter.

The same thing happens with the PSTN. The letter is the equivalent of a phone call. Rather than have everyone directly connected to everyone else, we send a phone call to the telephone service provider, who sends our call over the PSTN, directing the call to its recipient.

Of course, different cities have different post offices, but somehow it all works, right? Everyone’s agreed to get letters to their recipients. The post offices of different cities are the equivalent of different telephone service providers.

That’s not so bad, is it? The PSTN is the network over which all regular phone calls are sent. The companies that direct the calls are telephone service providers. Makes sense!

Now, let’s complicate things a little bit. Let’s say you own a business. You have 100 employees. It’d be messy to set up and expensive to pay for 100 connections to the PSTN for all your employees. Can you simplify this?

The answer is a private branch exchange (PBX).


What is a PBX?

A private branch exchange manages the phone calls from one location, connecting them to the telephone service provider, who sends the calls over the PSTN.

If you go back to the postal system comparison, a PBX is the equivalent of a local post office. All the letters dropped off in a neighborhood are directed to a large sorting center. It’s much easier for people to go to their small, local post office, rather than forcing everyone go to one large post office.

Similarly, it’s much easier for a company to set up a PBX than set up a line for every employee directly to the PSTN. All the calls that a company makes go first to the PBX, which directs them to the telephone service provider, who then directs them over the PSTN to wherever they’re going.

That makes sense, right? Both the postal network and the telephone network are systems designed to get packages to their recipients.

Now get this: the connection between a PBX and a telephone service provider is called a trunk.

We’re back to SIP trunking, and now you understand half of the term. “Trunking” simply refers to connecting a PBX to a service provider. It’s a big connection — that’s why they call it a trunk. What about the other half?

To explain that, we need to explain the difference between VoIP and traditional telephony.


VoIP vs traditional telephony

We don’t want to go into the nitty gritty of telephony here. All you need to know is that there are two world-spanning systems of communications being used here: the PSTN and the Internet. These two systems use different types of cables and other equipment to carry signals.

VoIP is a system of telephony that could exist entirely on the Internet. If everyone (or rather, when everyone) gets an IP phone, then we can discard the PSTN in favor of the Internet.

However, the infrastructure of traditional telephony is still widely in use, and you need to be connected with everyone in the world, so we need to deal with the PSTN. And that means we need to deal with different types of cabling.

Today, you need to have an Internet connection. That’s an unavoidable fact. Because you need an Internet connection, you already need to run cable to your Internet service provider.

Do you also have to run cable to your telephone service provider? No. SIP trunking provides an excellent alternative.

Running analog telephone cables is expensive. An analog cable only carries one signal at a time, and the signal degrades badly over distance. New analog connections to the PSTN are essentially non-existent.

Digital or T1 cables are better. When you connect your company to your telephone service provider using T1 cables, you get 23 channels per cable. These channels are virtual lines, that is, they are logically distinct pathways even though they run over the same physical cable.

That’s a great improvement, but it’s still not ideal. Running direct cable is expensive, and you can’t move without dishing out a lot of money to run new T1.

But do you need to run T1? Can you just use your Internet connection?

The answer is: Yes. You can send your voice traffic over the Internet using SIP trunking.

Instead of sending voice traffic over traditional phone cables, you send it to your Internet telephone service provider (ITSP) over your Internet connection. They connect to the PSTN for you. You can more than halve your cabling infrastructure, because current Ethernet cables can run much, much more data than T1 cables.

Actually, because Internet technology is much more advanced, you can send much more than voice traffic, paving the way for unified communications. But that’s a whole other topic!

You might be wondering, though, what that first half of “SIP trunking” stands for.

Woman Sipping Coffee

What is SIP?

The first thing to understand is that computers communicate using protocols.

Computer protocols are kind of like the equivalent of human languages. Picture yourself talking with someone. You’re speaking French and she’s speaking Finnish. Your conversation isn’t getting very far, is it? But if you both speak Frisian, then you’re good to go.

It’s the same with computers. So, finally: What is SIP?

SIP stands for Session Initiation Protocol. SIP is the lingua franca of VoIP, if you will — the common language that computers, phones and other communications technology speak when they’re talking VoIP. A SIP Phone speaks SIP — that’s all it means.

There are actually quite a few VoIP protocols, but SIP is by far the most common, and it’s only growing in popularity. SIP’s primary advantage beyond its reliability is that SIP is an open standard, meaning it’s not proprietary. You can use SIP devices from different companies, and they’ll work together just fine.

So when we talk about SIP trunking, all we mean is that the traffic is sent using the widely used, very reliable, open SIP standard.

Just as in traditional telephony, all the calls from one location can be managed by a single device. That device is known as an IP PBX, VoIP Phone System or VoIP Server. In other words, it’s a PBX that speaks Internet.

Your IP PBX might send the signal to the ITSP or you might use a hosted VoIP solution, which essentially means that you’re paying for access to a specialist company’s IP PBX. Technically, SIP trunking refers to the first option. The advantages of SIP trunking are many.

Thumbs Up

The advantages of SIP trunking

SIP trunking is less expensive, more flexible and more scalable than traditional telephony.

The system goes like this, in a nutshell: ITSPs have a large connection to the PSTN, much larger than any but the largest companies can afford. They also have a great connection to the Internet. You connect to the ITSP over the Internet, and they connect your call to the PSTN. Because they have such a strong connection to the PSTN, they’re able to offer telephone services for less.

And on your end, you only need to worry about your Internet cabling, totally eliminating traditional telephony infrastructure costs. Because you’re only worrying about the Internet, you’re also able to move or scale your phone system very easily. All those hardwired phone cables are gone.

One nice side effect is that you’re able to purchase direct inward dialing (DID) numbers for less, because you’re not paying for all the maintenance and hardware fees that you used to pay for.

You can install a firewall for security. You can register for Enhanced 911 to make sure your location is known in emergencies. The Internet can obscure where signals are actually coming from, due to its diffuse nature, so registering for E911 is important.

One other thing that’s very important to do is to set your Quality of Service (QoS) settings to privilege voice and other time-sensitive data, so you get a good phone signal.

That’s what SIP trunking is on your end.

And now you know pretty much everything you need to know about SIP trunking!

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