Local Access Facilities

With an MPLS-based WAN providing the public network links for IP-Centrex, the customer is essentially concerned about the bandwidth to and from the network. The typical IP-VPN service provider defines a committed access rate (CAR), which is comparable to the more familiar committed information rate that was defined for frame relay services. For VoIP the CAR should be a minimum of 20 Kbps for each active voice conversation over a symmetrical, digital bit stream, and with an Ethernet interface at the customer's premises.

For a single user (e.g., at home) with a PC, a standard, analog dial-up connection and a 56-Kbps modem are adequate to handle voice conversations through the IP-Centrex service. Where video communications are used this minimum bit rate requirement rises to 200 Kbps. At sites with multiple users (i.e., anywhere other than the smallest home office) the local access is now generally described as a broadband link, or a "fat pipe." This is in contrast to legacy Centrex, which in most cases is delivered by each phone line using one of the copper wire pairs (which are utilized as "thin" pipes in this case, as a bandwidth of only 4 kHz or 64 Kbps is required).

Broadband used to be defined, over the past decade, as bit rates above 45 Mbps (DS-3) in North America, or 34 Mbps (E-3) in other countries. But common usage now seems to define broadband as referring to any bit rates higher than 128 Kbps. An informal survey of documents from 14 countries showed that the lower threshold considered for broadband ranged from 200 Kbps to 30 Mbps.

We can assume that today's multimedia applications demand a symmetrical 1 Mbps per active individual user. Table 3.3 shows the desirable available access bit rate for different types of premises.