How to Prepare Your Network for VoIP


Switching to VoIP is a project that you should take into serious consideration. Voice over IP has many benefits, including reduced maintenance costs, integration of voice, data and video in one infrastructure as well as cost-effective and flexible solutions, amongst many other advantages.

However VoIP deployment cannot and should not take place without proper planning and careful network preparation to accommodate VoIP services. First off, you will need to spend some time doing your own investigation. Various VoIP providers offer different service profiles under different network deployments. Some of them offer VoIP on “closed networks”, not through the internet, by utilizing separate virtual connections on customer CPEs while others provide VoIP through internet connections. Also each one of them offers different Service Level Agreements (SLAs) and performance guarantees. Therefore, before even beginning your network preparation, take some time to check these things first.

Today we’ll take a closer look at what aspects should come under careful consideration and what you’ll need to examine so that you can properly prepare your infrastructure for VoIP. What you need to remember is that Voice service is sensitive to delay, delay variation (jitter) and packet loss. The most important aspects that influence the outcome of these parameters are:

  • Bandwidth requirements
  • Security
  • Voice and data traffic separation
  • Quality of Service
  • Network resilience and high-availability

You really need to understand each one of the above aspects, so take this opportunity and learn about them in this article.

How to Estimate Your Bandwidth Requirements for VoIP

A proper estimation of bandwidth consumption is very important and necessary for proper planning of needed connection trunks to accommodate VoIP traffic. Your bandwidth calculations should be based on the VoIP codec used. If more than one codec is used then you should consider the “worst-case” codec during the busy-hour where the number of concurrent voice calls is about one quarter of all the users in the network.

Let’s take a look at this example:

  • Worst-case codec in terms of bandwidth consumption: G.711
  • Packetization interval: 20 ms
  • Total VoIP users: 100

Using G.711 with a packetization interval of 20 ms, results in bandwidth utilization of about 90 kb/s per voice conversation. To calculate the bandwidth consumption under the above circumstances, you should multiply one quarter of the users (¼ *100) with the 90 kb/s, which results in bandwidth requirement of 2.25 Mb/s during the busy hour.

There are techniques that can further minimize the amount of bandwidth utilization. Voice activity detection (VAD) for example, is known to conserve about 30% of bandwidth by not transmitting packets during silence periods. In our example, if VAD is used the estimated bandwidth calculation would be 1.57 Mb/s.

Security with VoIP

VoIP is susceptible to the same vulnerabilities as the IP data Network. You can read more about network threats and vulnerabilities in my articles on preventing network attacks and dealing with DoS attacks.

Two suggestions I have for you, regarding VoIP security are:

  1. Allow specific IP addresses and transport layer ports for both Voice signaling and media with the use of access lists and restrict as much as you can the usable addresses and ports on the network.
  2. Use a dedicated internal firewall to monitor the traffic flow and secure your network from application level sophisticated threats

Voice and Data Traffic Separation in VoIP

You should consider separating your data traffic from VoIP traffic. To accomplish this task, you will need to apply dedicated layer 2 VLANs to each traffic category. This way you can achieve traffic classification and you can easily apply different QoS profiles to each traffic category. Besides, layer 2 tagging of packets, Voice packets can also be marked at the network layer. Nowadays, most IP phones support Differentiated-Services bit-marking. Therefore, traffic categorization can be achieved at the very beginning of the voice packet generation. This way, service quality can be maintained end-to-end. Of course, your VoIP provider if not trusting your QoS marking, should at least apply similar traffic categorization and eventually prioritization to your VoIP traffic.

Quality of Service with VoIP

If your network is used to carry both data and voice packets, then you should definitely consider Quality of Service (QoS). For protecting your Voice streams and to prevent data traffic from overwhelming your voice conversations, policing and traffic shaping should be applied. Of course in small networks layer 2 tagging of packets is usually enough to provide highly acceptable level of quality. It is your VoIP provider’s responsibility to apply sophisticated Quality of Service methods to offer better service to you. You can learn more about QoS in my article QoS Using the DiffServ Model.

Network Resilience and High-Availability with VoIP

VoIP service cannot tolerate any kind of interruption. You must make sure that you have adequate uplink physical links to carry all your VoIP traffic even in case you lose one of those links. It’s even better if you have an alternative way to carry your traffic in case of losing your primary path. Hardware malfunction should also be considered. More than one VoIP components should be used where appropriate either in load-sharing mode or even in active-standby mode. Moreover, configuration settings on your network devices should also be upgraded to adhere with VoIP service requirements. For example Spanning Tree should be removed or upgraded to Rapid Spanning tree because of Spanning Tree’s 60 seconds of inactivity during connectivity changes.

An Uninterrupted Power Supply (UPS) unit should also be considered. You do not want your Ethernet switches and VoIP devices to go offline in case of power failure. My suggestion is to use Inline power over Ethernet switches. These switches are capable of powering up all attached devices by delivering power over the unused Ethernet wires. The switches can then be attached on a UPS, the later one being able to provide uninterrupted service for a reasonable amount of time until power failure is corrected.

Are You Ready for VoIP?

Well, these are my recommendations and advices for preparing your network for VoIP. Don’t hesitate to ask anything that you’re not sure about. Your VoIP provider should be happy to assist you on your VoIP journey and help to guide you and provide adequate solutions to fit your needs.

Don’t take anything for granted; be open-minded and willing to try new things and remember that we are talking about a technology that is supposed to be cheaper and more promising. I am certain that you will enjoy every moment of your VoIP journey.

I will end this article with one final piece of advice: don’t spend much on new, sophisticated VoIP terminals because there is no rush in doing so. You can reuse your old terminal equipment with the use of an analogue terminal adapter to accommodate those devices.

Good luck and let me know if you have any questions!

 in Cisco


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