NEC SL1100 Issue 6.0Networking Manual 6 - 11The common scenario for remote IP deployment is: Implementation of an IP Phone with a public IP address talking with anNEC SL1100 behind NAT. An example would be a telecommuter.Implementation of an IP Phone behind a NAT, which connects to theInternet, terminates in a NEC SL1100 behind a different NAT.When selecting VPN equipment it is important to consider Quality ofService. Generally, VPN hardware is connected to Internet connectionswhich are unreliable and out of the control of the customer. However, it ispossible to set prioritization on some VPN units for voice traffic. This doesnot solve the unreliability of the Internet, but helps to ensure that the datatraffic to and from the LAN do not impair the quality of the voice traffic.(Refer to Section 2 QoS on page 6-1). NEC strongly recommends that any VPN hardware used for VoIP has thefacility to prioritize voice traffic.S ECTION 5 CODEC AND B ANDWIDTHThis section describes CODEC and bandwidth and their application with the NECSL1100 system.5.1 CODECsCODEC (COder/DECoder) uses the technology of encoding and decoding asignal. For VoIP, this specifically refers to the algorithm used to convertanalog speech to digital data for transmission on an IP network.The NEC SL1100 system supports three different CODECs: G.711This is the ITU-T recommendation for coding of speech at 64kbps usingPCM (pulse code modulation). This CODEC is often described asuncompressed as it uses the same sampling rate as Time-DivisionMultiplexing (TDM). G.711 has a MOS1 score of 4.2 but uses a largebandwidth for transmission. This CODEC is not commonly used due tothe bandwidth required, although it can be acceptable in LANenvironment (i.e., IP Phones connected over a 100Mbps LAN). G.722G.722 is an ITU standard CODEC that provides 7kHz wideband audio atdata rates from 48 to 64kbps. This is useful in a fixed network VoiceOver IP applications, where the required bandwidth is typically not1. The Mean Opinion Score (MOS) provides a numerical measure of the quality of human speech at the destination endof the circuit. The scheme uses subjective tests (opinionated scores) that are mathematically averaged to obtain aquantitative indicator of the system performance.
