Avaya Media Gateway G250, Administration Manual

Page: 292 / 840

Configuring WAN interfaces
292 Administration for the Avaya G250 and Avaya G350 Media Gateways
Avaya Communication Manager 3.0 Inter-Gateway Alternate Routing (IGAR) may be
configured to become active in such a case and to use the PSTN for transporting the voice
bearer transparently between the sites, transparently to the user. For information about
Dynamic CAC in the G250/G350, see
Dynamic CAC on page 317. For information about IGAR,
see Administrator Guide for Avaya Communication Manager , 03-300509.
Modem dial backup is a generic data dial backup feature that can carry not only signalling but
every type of IP traffic. However, the low bandwidth of an analog modem would be likely to
cause congestion. The administrator must therefore ensure that VoIP signaling has priority over
the Dialer interface. This can be performed using access control lists (ACL), QoS lists, and
Weighted Fair Queuing (WFQ) priority schemes. The administrator should apply these tools in
both the G250/G350 and the Remote Access Server (RAS). For information on ACL and QoS
lists, see
Configuring policy on page 637. For information on WFQ, see Weighted Fair VoIP
Queuing (WFVQ) on page 254.
You can configure modem dial backup to dial to an enterprise-owned RAS or to the Internet via
an Internet Service Provider (ISP). Most ISPs mandate the use of the internal IPSec VPN
gateway process to encrypt the traffic as it goes over the Internet.
Note:
Note:
IPSec VPN adds overhead to each packet, further reducing available bandwidth.
Under ideal conditions, the bandwidth of the analog modem can reach 56 kbps for downlink
(53
kbps in the US) and 33.6 kbps for uplink. However, sub-optimal PSTN quality may
degrade the downlink bandwidth to 33.6 kbps, or even 28 kbps. This may not be enough to
carry a single ISDN-PRI 64 kbps D-Channel for signalling over H.248 to and from the MGC,
even without considering the need to support IP phones and/or analog or DCP trunks.
VoIP signaling consumes bandwidth when setting up and tearing down calls. However,
calculations, testing, and field experience show that an analog modem can easily support a
small branch office when the expected Busy Hour Call Completion (BHCC) is limited.
Note:
Note:
The low bandwidth and high Round-Trip-Time (RTT) of analog modems
(~100 ms) may lead to acceptable changes in Post-Dial-Delay (PDD) and
offhook-to-dialtone delays.
Modem dial backup uses the G250/G350’s backup interface functionality to activate the Dialer
interface for modem dial backup when the primary interface fails and to deactivate the Dialer
interface when the primary interface is up again. Currently, modem dial backup does not
support such features as Dial On Demand Routing (DDR), callbacks, or RAS. Modem dial
backup cannot receive backup calls. For more information about backup interfaces, see
Backup
interfaces on page 289.
Note:
Note:
You can only backup one interface with modem dialer backup.