Modem Dial Backup Overview - Avaya G250 Administration

Configuring WAN interfaces

Modem dial backup overview

The modem dial backup feature allows the Avaya G250/G350 Media Gateway to utilize a
modem to provide redundant connectivity between a G250/G350 and IP phones in a small
branch office and their primary Media Gateway Controller (MGC) at the headquarters or a
regional branch office.
Even if the gateway has Standard Local Survivability (SLS - G250 only), or Enhanced Local
Survivability (ELS) using a local S8300 in LSP mode, it is always preferable to continue working
with the primary MGC, since features are lost when the system is fragmented.
Analog modems have limited bandwidth and high latency, and are therefore unfit for carrying
VoIP traffic. However, using Dynamic Call Admission Control (CAC), the G250/G350 can be
configured to report zero bandwidth for bearer traffic to the MGC when the primary WAN link
fails. A matching configuration on the MGC allows it to block new calls, if their bearer is about to
go over the modem dial backup interface, and to alert the user with a busy tone. In this case, the
user is still able to place external calls manually if local PSTN trunks are available. Furthermore,
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
see Administrator's Guide for Avaya Communication Manager, 555-233-506.
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 Chapter 19: Configuring policy
Configuring Weighted Fair VoIP Queuing (WFVQ)
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:
IPSec VPN adds overhead to each packet, further reducing available bandwidth.
Note:
Under ideal conditions, the bandwidth of the analog modem can reach 56kbps for downlink
(53kbps in the US) and 33.6kbps for uplink. However, sub-optimal PSTN quality may degrade
the downlink bandwidth to 33.6kbps or even 28kbps. This may not be enough to carry a single
ISDN-PRI 64kbps 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.
148 Administration for the Avaya G250 and Avaya G350 Media Gateways
Dynamic CAC on page 169. For information about IGAR,
on page 441. For information On WFQ, see
on page 124.