Despite being a communications workhorse for many decades, the traditional TDM PBX has a number of problems. It’s big, mobility is difficult if not impossible, and there is no way to create a geographic disaster recovery configuration. I like to say that if you have a building, you have a PBX. If you have another building, you have another PBX. If the PBX in building A goes down, its users cannot take advantage of the PBX in building B even if building B is across the street.
All of that changed in the new millennium with the creation of the IP-PBX. Systems shrank due to the move from proprietary hardware to off-the-shelf servers and standard rack technology. IP supports mobility beyond the physical length limitations of analog and digital telephone lines. IP technology also allows the PBX to be split into gateways that can be dispersed geographically. This means that you can have a PBX in building A that serves users in building B with an IP-connected remote gateway.
Additionally, if you put a call processor in the remote gateway, it can act as a backup PBX for the users in building A. In the Avaya world this capability is known as Flatten, Consolidate, and Extend (FCE) and is implemented with Enterprise Survivable Servers (ESS) and Local Survivable Processors (LSP).
There are a tremendous number of positive attributes to the ESS and LSP architecture. Every call processor in the remote sites runs the exact same code and database as the main system. This allows for a secondary call processor to take over for the entire system (ESS) or a subset of the system (LSP) and continue functioning as if it was the prime call processor. Replicated translations, user configurations, and call center vectors allow remote sites to provide consistent levels of service and functionality during disasters for all telephone types.
However, there is one disadvantage to this gateway architecture and that is cost. Adding a call processor to the remote sites isn’t cheap. Of course, every Avaya H.248 gateway (e.g. G430 and G450) offers a level of survivability that doesn’t require a call processor. That functionality is called Standard Local Survivability (SLS). SLS provides a bare bones level of survivability for analog, digital and H.323 telephones. Unfortunately, SLS doesn’t support SIP endpoints and probably never will.
Enter the SIP Survivable Gateway (SSG). The SSG offers survivability for SIP telephones and SIP telephones only. On a sunny day those SIP telephones receive their call processing functionality from a core call processor (either the prime or ESS site). However, if the SSG loses connection to the core due to a WAN failure or some other catastrophic problem in the core, the SSG will function as a SIP proxy for its telephones and provide them with basic SIP call features (i.e. SIPPING-19).
SIP Survivable Gateways for an Avaya system come in two flavors. The first are the MediaPack and Mediant gateways from AudioCodes. The MediaPack gateways top out at 25 SIP telephones and the Mediant 1000 MSBG gateway can support up to 600 SIP telephones.
Next are the gateways from Avaya. These are the AG2330, SR2330, and SR4134. The AG2330 and SR2330 are for small to medium sized branches and provide SIP survivability for up to 100 SIP telephones.
The SR4134 is designed for larger sites and supports up to 300 SIP telephones. In addition to SIP telephony, the SR2330 and SR41234 (the SR stands for Secure Router) provide data capability for the remote branch. I like to call this an office-in-a-box whereby you can use one gateway for your branch telephony, data router, VPN, and small switching needs.
It’s important to know that all Secure SIP Gateways support analog and digital trunks. So, if the branch goes into survivability mode it can still communicate to the outside world.
There will always be a need for LSP functionality for some remote branches, but there are also cases where a SIP Survivable Gateway might be all that you need. As you analyze your communications needs and plan your SIP roadmap, don’t neglect these small, inexpensive, and highly functional gateways as part of your remote and branch office strategy.
Tao, Zen, and Tomorrow 
Nice perspective on the trend toward IP-PBXs and the associated benefits. Gateways do indeed open up yet additional opportunities. Note that the AudioCodes Media Pack units are particularly good for organizations with a lot of remote locations, such as retail organizations with stores. They are low-cost, and highly effective in this topology.
Thanks, Patrick. These small gateways are perfect for a small office that needs survivability, but is willing to sacrifice a few features when the connection to the main site is lost.