So you think you are ready to take the VoIP plunge? This article will address some information you need to know before you start looking. Every VoIP manufacturer in the market recommends an infrastructure audit. It is important to know what they are looking for and equally important to look for some things that they may not address in their audits.
When examining your network, health actually comes second. First, you must determine what you have before you determine how well it works. The difference between data communication and voice traffic is in the real-time transmission of voice. We have all been on calls where echo was a problem or where we felt the need to say "over" at the end of every sentence to give the all-clear for the other party to speak. This was fun when we were kids with walkie-talkies, but it is certainly not acceptable for business. In order to understand the problems that can affect performance, let's define some of the key anomalies in voice transmission that may hamper understanding.
Latency represents a delay caused by the actual transmission. One can think of this in terms of throwing a ball. The latency would be the elapsed time between the moment when the ball was thrown and when it was caught. Long latency is not acceptable in voice-grade transmissions. In voice networks, delay of less than 150ms is considered acceptable. This delay can be noticed if two people are using a VoIP system with high latency while looking directly at each other. The result is like watching an old Godzilla movie. You will hear the speaker slightly after you watch his lips move.
Loss is another unacceptable factor in voice transmissions. In a typical voice transmission, a CODEC (Coder-Decoder) changes the voice waves into digitized packets for transmission. The packets are then formed for transmission on the network. VoIP packets carry very small samplings of the voice conversation, typically 20ms. Some loss will not greatly degrade the clarity of the conversation, but a loss of more than 1 percent will. Loss can occur in congested, overworked, or bursty networks.
Jitter is a hurdle for voice quality. Variations in packet delay cause packets to be received in sporadic patterns or out of order. When VoIP is run on a congested network with bursty traffic, jitter will be harder to compensate for at sending and receiving stations. Voice traffic is buffered at the receiving end to help compensate for jitter. In effect, the conversation is stored until enough of the transmission is in memory to play for the receiver. TCP/IP delivers packets based on several routing algorithms. In order for VoIP to work properly, special attention must be paid not only to the paths that the packets take but also to the capacity and health of the network. Excessive jitter makes a conversation indecipherable. New Layer 3 switches offer relief because they are able to understand prioritization.
Sequence errors occur when the packets arrive out of order at the receiving station. TCP/IP packets include keys to their position. A sending station, whether it be a PC or any other TCP/IP device, breaks the packets into datagrams for transmission. These datagrams are assigned a sequence number. If datagrams travel different paths on the network owing to congestion or hardware failure, or cabling issues cause frequent retransmission, the packets will be received out of order. A high level of sequence errors will cause an audible degradation of the conversation.
Echo is great for the Grand Canyon, when you stand at the rim and bellow that you are the best. In a voice system, however, echo is not acceptable. Echo is return loss or signals that are reflected back to the sender. This is typically caused by high return loss in your cable plant. If the echo occurs outside your building, it is a carrier issue; inside your building, it is your cable plant. Echo cancellers will help correct the problem but are very expensive vis-a-vis fixing your cabling.
These errors can be caused by a variety of culprits on your network. One of the most overlooked areas is the cabling plant. When you review your cabling plant, there are a few things you should examine. The first is whether you have enough structured cabling ports at each work area. The standards call for two fully terminated outlets per work area (category 6 is recommended as a minimum, but category 5e has a significant installed base). You can purchase a phone with a switch in it, but in the long run this can be more expensive than adding the additional cable, which is passive and requires no active maintenance costs -- and will last longer than the phone.
Next, check to see whether your switches are Layer 2 or Layer 3. Both can support VoIP, depending on the functionality. VLANs may help, but in either case, some sort of priority marking and QoS understanding is essential. Do not assume that you have to change out all of your active infrastructure to implement VoIP. You may not need to change any, although if you don't have manageable electronics, I highly recommend that you add the software and perhaps firmware features to add that functionality. Remember, sales are sales, and not all salespeople are responsible with your networks. If implementing VoIP is outside your comfort zone, find a resource that can assist in your decisions. You should not be spending unnecessary money. Also remember that each salesperson is trained on his or her specialty and products. You don't go to the dentist and ask him why your foot hurts. Granted, teeth and feet are both part of your body, but you must remember where people's specialties begin and end.
Finally, your electronics should be checked. If you are running a network with electronics that are not manageable, this will be difficult at best. End users will have a hard time reporting data retransmissions because they are generally invisible to them. If you add voice, however, they will certainly come to light. At a minimum, you want to make sure that you have full duplex operations on each port. Do not assume that all is well. Take the time to check speed and duplex functionality. If you are running half duplex, check your settings; if they are set to full duplex or auto and you have half duplex, a good place to start is to check your cabling. Poor cabling or channels that are too long will cause this condition in some cases.
Now that you understand the issues with voice traffic and the role of your electronics and infrastructure, stay tuned for the next article, which will discuss what you can learn from your manageable electronics and how to determine overall health to avoid these problems.
About the author:
Carrie Higbie, global network applications market manager for The Siemon Company, has been involved in the computing and networking industries for nearly 20 years. Carrie has taught classes for Novell, Microsoft and Cisco certifications, as well as CAD/CAE, networking and programming on a collegiate level, and serves as the "Preparing your network for VoIP" site expert on SearchEnterpriseVoice.com.