|
|
|
For VoIP to work well, network links must be properly sized with sufficient bandwidth for voice and data traffic. Each voice call uses between 6.3 Kbps and 80 Kbps, depending on the desired codec, quality, and header compression used. G.729 is one of the most promising standards today, using 24 Kbps of bandwidth uncompressed. Interoffice bandwidth demands can be sized using traditional phone metrics such as average call volume, peak volume, and average call length.
Voice traffic must be given absolute priority through the network, and if links are not properly sized or queuing strategies are not properly implemented, it will become evident both with the quality and timeliness of voice and data traffic.
There are three technologies that work well with VoIP: Asynchronous Transfer Mode (ATM), Frame Relay (FR), and Point-to-Point (PPP) circuits. These technologies all have good throughput, low latency, and low jitter. ATM has the added benefit of enhanced QoS. Frame Relay and PPP links are more economical, but lack some of the traffic-shaping features of ATM.
Of the three technologies, Frame Relay is the most difficult WAN circuit to use with VoIP. Congestion in Frame Relay networks can cause frame loss, which can significantly degrade the quality of VoIP conversations. With Frame Relay, proper sizing of the Committed Information Rate (CIR) is critical. In a Frame Relay network, any traffic exceeding the CIR is marked discard eligible, and will be discarded at the carrier's option, if it experiences congestion in its switches. It is very important that voice packets not be dropped; therefore, CIR should be sized to average traffic usage. Usually, 25% of peak bandwidth is sufficient. Also, Service Level Agreements (SLA) should be established with the carrier, which defines maximum levels of delay and frame loss, and remediation should the agreed-to levels not be met.
The following sections discuss ATM, FR, PPP, and other available network technologies. Each section contains recommendations for implementing that technology when a VoIP device like the G700 Media Gateway is part of the network.
|
|
|