Presales engineering translates a customer call scenario into interfaces, capacity, licenses and operational requirements. A product comparison should begin with the traffic model, not the catalogue.
This document reflects presales and engineering-selection work. It does not claim that every reviewed design was subsequently deployed by the author.
- Required analog, E1/T1 and SIP interfaces.
- Concurrent calls and busy-hour traffic.
- FXO/FXS port behavior and connected device types.
- Codec and transcoding requirements.
- SIP, SIP-I and interworking requirements.
- Survivability, redundancy and failover expectations.
- Routing, number normalization and security boundaries.
- Monitoring, support and future growth.
Keep different capacity dimensions separate:
| Dimension | Typical input |
|---|---|
| Analog access | Required FXS/FXO ports and simultaneous use |
| TDM | E1 circuits, channel allocation and signaling |
| SIP | Concurrent sessions and call attempts during peak periods |
| Media | Codecs, transcoding, announcements and conferencing |
| Availability | N+1, active/standby or site-level redundancy |
A gateway with enough physical ports may still lack the required session, media or licensed capacity.
- FXS serves analog endpoints and requires electrical compatibility checks.
- FXO connects to analog exchange lines and requires supervision behavior to be validated.
- E1 adds framing, clocking, signaling and channel-allocation requirements.
- SIP adds IP reachability, interoperability, routing, security and media-path requirements.
Terms such as master/slave and active/standby are not interchangeable guarantees. For each proposal, define:
- which component detects failure;
- what configuration or state is synchronized;
- what happens to active calls;
- how new calls select the surviving path;
- how recovery and failback are tested.
Separate physical capacity from enabled software capacity. Confirm which functions, channels and protocols require licenses and how future expansion is activated.
Compare a consolidated modular design with a distributed gateway and analog-access design. Include failure domains, cabling, rack space, management, port density and growth—not only the initial bill of materials.
Convert every requirement into one of four outcomes: supported, supported with a condition, requires clarification or not supported. Record the evidence and avoid assuming compatibility from a similar product name.
Size for concurrent sessions, interfaces, media processing, routing complexity and growth margin. Subscriber count alone is not a voice-capacity calculation.
Validate electrical and signaling behavior: loop parameters, ringing, caller ID, DTMF, polarity requirements and any fax or modem use. “Analog” does not guarantee interoperability in every deployment.
- Customer call-flow diagram.
- Assumptions and open questions.
- Capacity and interface calculation.
- Proposed topology and bill of materials.
- Licensing matrix.
- Compatibility and integration risks.
- Acceptance-test criteria.
A good presales design is auditable: another engineer should be able to trace every selected component and license back to a stated requirement.