Frequently Asked Questions (FAQ)

1:

What is oversubscription of Frame Relay ports?

A1:

Because data connections tend to be idle as much as they are used, oversubscription of a Frame Relay port is often used as a strategy for achieving more cost-effective WAN connectivity. Oversubscription is another tool network designers use when designing WANs. Designers can "play the averages" by assigning a total committed information rate (CIR) greater than the port speed.

Oversubscription is attractive for saving money on the port connection and is measured in percentages. For example, four 32 K bit/sec permanent virtual circuits (PVC) a total CIR of 128 K bit/sec attached to a 64 K bit/sec port represents a 200 percent subscription. Two 32 K bit/sec PVCs attached to a 64 K bit/sec port equals 100 percent subscription, which represents no oversubscription.

A network configuration consisting of 24 (DLCIs/PVCs) x 56 Kbps (CIR per PVC) = 1.344 Mbps is well within the T1 bandwidth limitation of 1.344/1.536 Mbps (depending on physical line coding; AMI = 1.344 Mbps; B8ZS = 1.536 Mbps). This configuration is not oversubscribing the interface because there is sufficient bandwidth available to support the traffic requirement (1.344 Mbps _ 1.344/1.536 Mbps).

A network configuration of 50 (DLCIs/PVCs) x 56 Kbps (CIR per PVC) = 2.800 Mbps far exceeds the maximum bandwidth supported by a T1 limitation of 1.344/1.536 Mbps (depending on physical line coding; AMI = 1.344 Mbps and B8ZS = 1.536 Mbps). This configuration is oversubscribing the interface because there is not sufficient bandwidth available to support the traffic requirement (2.800 Mbps _ 1.344/1.536 Mbps).

2:

Can Frame Relay carry voice traffic?

A2:

Yes. Voice over Frame Relay (VoFr) is enabled with the use of a VFRAD. Voice Frame Relay access devices (VFRADs) integrate voice into the data network by connecting the router and the PBX at each site in the corporate network to the Frame Relay network.

The VFRADs' prioritization schemes "tag" different applications according to their sensitivity to delay, assigning a higher priority to voice. The VFRADs let the higher priority voice packets go first, keeping the data packets waiting. This prioritization has no negative impact on the queued data traffic because voice transmissions are relatively short and require very little bandwidth (because of voice compression).

The Frame Relay Forum has issued an implementation agreement regarding Voice over Frame Relay. You can read the implementation agreement (FRF.11.1) or downloaded it at www.frforum.com/5000/5000index.html.

3:

Can a Frame Relay site "talk" to an ATM site?

A3:

Yes. Frame Relay to ATM Service Interworking (FRASI) virtual circuits are available by most network service providers. The Frame Relay Forum has issued several implementation agreements regarding FRASI: FRF.5, FRF.8.1, and FRF.18. You can read and download these implementation agreements at www.frforum.com/5000/5000index.html.



Network Sales and Services Handbook
Network Sales and Services Handbook (Cisco Press Networking Technology)
ISBN: 1587050900
EAN: 2147483647
Year: 2005
Pages: 269

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