606 Chapter 16: Voice Techniques
of an SVC versus the prior establishment of PVCs is clearly more advantageous from
three aspects:
·
SVCs are more efficient users of bandwidth.
·
QoS for connections do not need to be constant, as with PVCs.
·
The ability to switch calls within the network can lead to the elimination of the Tandem
PBX and potentially the edge PBX.
VoATM Addressing
ATM standards support both a private and public addressing scheme. Both schemes are 20 bytes
in length.
VoATM Routing
ATM uses a private network-to-network interface (PNNI), a hierarchical link-state routing
protocol that is scalable for global usage. In addition to determining reachability and routing
within an ATM network, it is also capable of call setup. As soon as the connection is established,
voice traffic flows between end stations as if a leased line exists between the two. This
specification spells out routing in private networks.
VoATM and Delay
ATM has several mechanisms for controlling delay and delay variation. ATM's QoS capabilities
allow the specific request of constant bit-rate traffic with bandwidth and delay variation
guarantees. The use of VC queues allows each traffic stream to be treated uniquely. In the case
of voice traffic, priority can be given to its transmission. The use of small fixed-size cells
reduces queuing delay and the delay variation associated with variable-sized packets.
Voice over Frame Relay
VoFR is a popular choice for implementing toll bypass. Because standards for VoFR have been
in place for several years, vendor interoperability and functionality have been better tested than
the emerging VoIP standards.
Here are some of the benefits of using VoFR:
·
Frame Relay costs for tie-line functionality are relatively low when compared to the costs
of other WAN circuits.
·
Frame Relay is readily available.
·
Frame Relay can be oversubscribed.
87200333.book Page 606 Wednesday, August 22, 2001 1:41 PM