The 4 Horseman of the Virtual PBX Apocalypse Part 4: Bandwidth

In this multiple part series, I’m exploring the four evils that are the curse of high-quality Virtual PBX: Bandwidth, JitterPacket loss and Latency.

In this final installment, we talk about Bandwidth.

What is Bandwidth?

Bandwidth is often interpreted to be the speed that data passes through a given circuit. It’s commonly measured in Kilo (thousand), Mega (Million) or Giga (Billion) bits per second. 

If your internet provider says that they provide 3 megabits per second, this means that the line has a theoretical transport capacity 3 million bits per second.

Does this mean that data will pass fast? Usually not. Many other limitations can cause average data transport speeds to be less than the theoretical limit. The key factor for you, the IT Manager, to keep in mind is that bandwidth is a scarce commodity. While it’s been getting cheaper in recent years, it still costs money and the more bandwidth you need, the more you’re going to pay. 

Why is Bandwidth Important to My Virtual VoIP PBX?

As an IT Manager, it’s important to know your company’s bandwidth requirements, especially when you add voice traffic to what may be an already taxed data network.  

On an office’s local area network (LAN), adding voice traffic is usually not much of an issue because modern LANs can often transport up to 1 Gigabits per second or more. This is great. Ethernet is fast, but it only works for short distances. Hence its use in LANs. 

But your internet connection (WAN) is a different story. It needs to travel great distances. Depending on the size and data requirements of your company, your WAN’s bandwidth speeds are always much, much slower. 

This matters if you’re thinking about a Virtual PBX solution because the voice traffic relies on this WAN connection to make it back to the server that manages all your voice calls.

Here are some typical bandwidth speeds for WAN connections. 

How to Measure Your Company’s Bandwidth Requirements

So, to get the most out of Virtual PBX solution, it’s important to have a good idea of the bandwidth requirements of your voice calls.

Here’s how to do it in 4 simple steps:

Step 1:  Know Your Busy Hour Call Volume

This is straightforward. How many concurrent phone calls does your company make during its busiest time of the day? What is your peak call volume? 10 concurrent conversations, 20 concurrent conversations? Whatever it is, I recommend you overestimate this number to be on the safe side. 

Step 2:  Know Your Codec and its Packets Per Second Capability

Codecs are the technology used to compress (CO) and decompress (DEC) your voice signal across the network. There are two primary codecs used these days for voice traffic – G.711 and G.729. G.711 does not compress the data per say, while G.729 compresses the data to about a 1/8th of its size. This compression is obviously more efficient use of available bandwidth, but it comes at a cost of lower voice quality.

Since bandwidth is more precious on WAN connections, the more efficient G.729 standard is often used. Using G.729, your typical packets per second is about 50 bits per second. 

Step 3:  Know Your Total Packet Size Over the WAN

Your VoIP data has three primary packet sections- Layer 2 Header, the IP/UDP/RTP Header and the Payload (the actual data). For typical VoIP data packets using the G.729 codec, each packet will consist of the following:

  • Layer 2 Header – 6 bytes
  • IP/UDP/RTP Header – 40 bytes
  • Payload – 20 bytes
  • Total – 66 Bytes

Step 4:  Calculate Your Required Bandwidth

Now that you know all the elements, calculating the bandwidth for one phone call is reduced to arithmetic: 

Packets Per Second x Packet Size Bytes/Second x 8 bits/second x Busy Hour Call Volume

So for a company with 10 concurrent phone calls during its busy hour, the required bandwidth would be:

50 Packets/second x 66 Bytes/second x 8 bits/second x 10 = 264,400 bits per second (264 Kbps) of required bandwidth. 

Keep in mind that we’re talking about voice conversations that require full duplex i.e. bandwidth in both directions. This is so people can hear and talk at the same time in a more natural conversation. This effectively doubles the bandwidth needs to 528 Kbps for a small office of 10 concurrent phone calls during the busy hour. 

Talk to Your Provider

If you’re considering Virtual PBX as a possible solution for your small business, knowing your bandwidth requirements is great to have at the tip of your tongue. Besides being just good to know, it also a great way to know if your provider is under or overselling you on bandwidth. 

In our experience, getting the proper bandwidth on the WAN connection is the most important factor to a good Virtual PBX experience. It’s worth the extra time and effort to know your bandwidth requirements so that your transition over to Virtual PBX is smooth, painless and virtually unnoticeable by your users. 


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About the Author

Randy Kremlacek

President | Head Chef

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Randy Kremlacek is CEO and Head Chef of TeleDynamic Communications, a Digium Select Partner and four time Digium Pinnacle Award winner. TeleDynamic Communication, Inc. provides premise-based PBX, Virtual PBX, and SIP Trunking services.  READ MORE

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