The modem dial backup feature allows Branch Gateway to utilize a modem to provide redundant connectivity between Branch Gateway 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 Branch Gateway has Standard Local Survivability (SLS), or Enhanced Local Survivability (ELS) using a local S8300 in Survivable Remote Server 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), Branch Gateway 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, CM 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 IGAR, see Administrator Guide for Avaya Aura® Communication Manager.

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 Branch Gateway and the Remote Access Server (RAS).

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.

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.

Modem dial backup uses Branch Gateway 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.

Using Branch Gateway backup interface functionality, you can designate the Dialer interface as the backup for the main WAN link. However, this method is not always available, since an 'up' WAN link status does not ensure connectivity, and the main WAN link may not even be directly connected to Branch Gateway.

The workaround is to use Branch Gateway object tracking feature to verify connectivity to the primary MGC using Respond Time Reports (RTRs) and object trackers. Configure object tracking to change the state of the Loopback interface accordingly, and configure the Dialer interface as a backup to the Loopback interface.

Modem dial backup uses a modem connected directly to Branch Gateway USB or Console port. The modem can also be used to access Branch Gateway CLI from a remote location. The modem cannot do both at the same time.

Finally, IP routing must be configured so that traffic to and from the site uses the Dialer interface when the primary interface is down. The Dialer interface can work both with static and dynamic routing (OSPF and RIP). Note that the latter mandates the use of unnumbered IP interfaces.