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Network design
As
Figure 80: Committed information rate (burst range)
CIR is guaranteed, whereas traffic beyond the CIR usually is not. This is how Frame Relay is
intended to work. CIR is a committed and reliable rate, whereas burst is a bonus when network
conditions permit it without infringing upon the CIR of any user. For this reason, burst frames
are marked as discard eligible (DE), and are queued or discarded when network congestion
exists. Although experience has shown that customers can achieve significant burst throughput,
it is unreliable and unpredictable, and not suitable for real-time applications like IP Telephony.
Therefore, the objective is to prevent voice traffic from entering the burst range and being
marked DE. One way to accomplish this is to prohibit bursting by shaping the traffic to the CIR
and setting the excess burst size (B
prevents data traffic from using the burst range.
Additional frame relay information
One interesting piece of knowledge is that most IXCs convert the long-haul delivery of Frame
Relay into ATM. That is, the Frame Relay PVC is converted to an ATM PVC at the first Frame
Relay switch after leaving the customer premise. It is not converted back to Frame Relay until
the last Frame Relay switch before entering the customer premise. This is significant because
ATM has built- in Class of Service (CoS). A customer can contract with a carrier to convert the
Frame Relay PVC into a constant bit rate (CBR) ATM PVC. ATM CBR cells are delivered with
lower latency and higher reliability.
Finally, under the best circumstances, Frame Relay is still inherently more susceptible to delay
than ATM or TDM. Therefore, after applying the best possible queuing mechanism, one should
still expect more delay over Frame Relay than is present over ATM or TDM.
VPN
Many definitions exist for Virtual Private Networks (VPNs). VPNs refer to encrypted tunnels that
carry packetized data between remote sites. VPNs can use private lines, or use the Internet
through one or more Internet Service Providers (ISPs). VPNs are implemented in both
dedicated hardware and software, but can also be integrated as an application to existing
hardware and software packages. A common example of an integrated package is a firewall
product that can provide a barrier against unauthorized intrusion, as well as perform the security
features that are needed for a VPN session.
The encryption process can take from less than 1 millisecond (ms) to 1 second or more, at each
end. Obviously, VPNs can represent a significant source of delay, and therefore have a negative
affect on voice performance. Avaya VPN products encrypt traffic with less than 1ms of delay,
and thus are appropriate for IP Telephony. Also, because most VPN traffic runs over the Internet
and there is little control over QoS parameters for traffic crossing the Internet, voice quality may
suffer due to excessive packet loss, delay, and jitter. Users might be able to negotiate a
service-level agreement with the VPN provider to guarantee an acceptable level of service.
Before implementing IP Telephony with a VPN, users should test their VPN network over time to
ensure that it consistently meets the requirements that are specified in the Avaya IP Voice
Quality Network Requirements Document Summary.
306 Avaya Application Solutions IP Telephony Deployment Guide
on page 305 shows, traffic up to the
– determines the burst range) to zero. However, this also
e
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