Managed Networks

The voice and video traffic that is directed by an IP-Centrex controller travels over the LAN inside a building or campus generally at a nominal bit rate of 100 Mbps to and from the desktop. Increasingly, Ethernet links are being created over virtual local area networks (VLANs) between buildings in an urban area, with an operating range of up to at least 20 miles (30 km). In this way metropolitan area networks have been absorbed into VLANs, so that building boundaries have become invisible. The transport of VoIP packets over WANs is still a challenge and requires careful consideration.

LAN Considerations
Most LANs will need some enhancement to support VoIP, if only to provide the in-line power for telephone sets. The LAN for voice/video over IP must be switched, not shared, and any device used in the LAN should be QoS-aware and/or -capable. Admissions control to the LAN will be done in a gateway.

The voice or video bandwidth requirements depend on the codec used in the endpoints. The two voice encoding standards that are commonly used in IP phones are G.711, which was the original ITU recommendation for pulse code modulation and usually generates 64 Kbps per speaker, and G.729 Annex A, for which 8 Kbps must be reserved for each voice transmission.

The G.723.1 recommendation has been extensively used as a low bit-rate encoding algorithm in VoIP gateways, running at 5.3 or 6.3 Kbps, but it is processing-intensive (at around 30 Mips) and therefore has not been widely implemented in IP telephone sets.

If we add some consideration for packeting overhead and round up the numbers somewhat, for bandwidth estimating purposes the figures of 80 Kbps for G.711 and 20 Kbps for G.729A are reasonable.

It is essential to have coding conversion capability at the transition point from one network to another, to allow for telephones, softphones, or video units with differing codecs. This conversion may take place in a separate subsystem, known as a transcoder, or it might be a DSP card in a slot of a router at the edge of a LAN. A practical budget of $30 to $200 per port (i.e., per user) should be allowed for LAN upgrades. Some reprogramming of the edge router will be needed for voice or video, as no PSTN access was likely when the network handled only data.

E911 service requires that local dial tone be provided to each physical location, which will usually be a discrete LAN, and may then be easily identified. However, if each location is just one domain within a larger VLAN (which spans multiple sites), then identification of a specific location will be much more difficult. For details regarding the challenge of meeting E.911 requirements with IP-Centrex

WAN Requirements
The predominant service for carrying VoIP traffic is the VPN. A VPN delivers multiple service classes and interoperates well with the customer's own IP networks, at lower costs than a customer-managed network. A typical example of the four classes of service in an international VPN appears in Table 1.