VoIP Considerations

VoIP Considerations

Voice over IP (VoIP) is a protocol optimized for the transmission of voice through the Internet or other packet switched networks. Voice over IP protocols carry telephony signals as digital audio encapsulated in a data packet stream over IP.

VoIP requires a well-engineered, end-to-end network that provides little latency for data stream transmission. Fine-tuning the network to adequately support VoIP involves overcoming the following issues:

Issue		Description
Delay		Delay (or latency) is the amount of time required for the spoken voice to be carried to the receiver's ear. Delays cause long pauses between speaking and receiving, and might result in callers continually interrupting each other. Callers notice roundtrip delays of 250 milliseconds (ms) or more. International standards call for a delay of 150 ms or less.
Jitter		Jitter is the variation of delay in transmissions. Jitter causes strange sound effects as the delay of packets fluctuates. Acceptable levels of jitter vary by vendor, but should be very low (between .5 and 30 ms). Jitter can be controlled to some extent by packet buffers in VoIP equipment.
Packet loss		Packet loss occurs when packets do not arrive. Packet loss causes drop-outs in the conversation. Because voice traffic is time sensitive, lost packets do not need to be retransmitted. Voice traffic is very sensitive to packet loss. Even a 1% loss of packets can be detected. Ideally, Cisco recommends 0% packet loss, although very low (.1-.5% maximum) might still be acceptable.
Echo		Echo is hearing your own voice in the telephone receiver while you are talking. When timed properly, echo is reassuring to the speaker. If echo exceeds approximately 25 milliseconds, it can be distracting and cause breaks in the conversation. Excessive delay can cause unacceptable echo. VoIP implementations use echo cancellers to regulate the echo.

VoIP is typically implemented using switches with additional configuration required on both switches and routers to ensure delivery of VoIP packets for acceptable quality.

• To minimize the number of switch ports required, VoIP phones connect to the switch port, and a corresponding workstation connects to the VoIP phone. Both voice and data traffic is sent through the same switch port.
• Switches with Power over Ethernet (PoE) capability provide electrical power through the Cat 5 cable. This eliminates the need to have a separate power cable for the phone.
• Switches and routers are configured with Quality of Service (QoS) settings to elevate the priority of voice traffic. This helps control delay and jitter.
• To secure VoIP data, the network should have a VoIP Virtual Private Network (VPN) solution. A VPN is a network that uses encryption to allow IP traffic to travel securely over the TCP/IP network. Without a VoIP VPN solution, it is relatively easy to eavesdrop on VoIP calls and even change their content.