Cisco NIM-2FXS/4FXOP
2-Port FXS/FXS-E/DID and 4-Port FXO Network Interface Module
Product Highlights
Bridging analog telephony and modern IP networks remains a critical requirement for branch offices, retail locations, and enterprises that still rely on PSTN trunks or legacy analog endpoints. The Cisco NIM-2FXS/4FXOP is a 2-Port FXS/FXS-E/DID and 4-Port FXO Network Interface Module designed for the Cisco 4000 Series Integrated Services Routers (ISRs), delivering a consolidated analog voice solution in a single NIM slot.
The module provides two FXS ports for connecting analog phones, fax machines, or modems — with support for FXS-E and Direct Inward Dialing (DID) — alongside four FXO ports for terminating PSTN trunk lines, PBX connections, or key system interfaces. An integrated onboard DSP handles all voice processing locally, eliminating the need for a dedicated Packet Voice DSP Module (PVDM) on the router motherboard. This simplifies the bill of materials and reduces deployment complexity.
The NIM-2FXS/4FXOP supports online insertion and removal (OIR) on Cisco 4000 Series ISRs, enabling module replacement with minimal service disruption. With support for industry-standard voice protocols including SIP, SCCP, MGCP, and H.323, plus echo cancellation up to 128 ms, this voice NIM integrates seamlessly into Cisco Unified Communications Manager, CME, or SRST environments. It is compatible with the Cisco ISR 4221, 4321, 4331, 4351, and 4431 and requires Cisco IOS XE 16.5 or later.
Features & Benefits
Combined FXS and FXO in One Module
The NIM-2FXS/4FXOP combines 2 FXS/FXS-E/DID ports and 4 FXO ports into a single-wide NIM, conserving valuable router slot real estate. FXS ports deliver dial tone and ring voltage to analog endpoints, while FXO ports terminate incoming PSTN or PBX trunk lines — giving you both station-side and line-side connectivity from one module.
Integrated Onboard DSP
Unlike legacy VIC and VIC2 cards that required a separate PVDM on the router motherboard, the NIM-2FXS/4FXOP includes a dedicated onboard DSP for all voice processing. This reduces the component count in your router build, simplifies ordering, and ensures voice features like echo cancellation and codec transcoding are always available.
Advanced Voice Quality Features
The module supports echo cancellation with up to 128 ms echo-tail length, Voice Activity Detection (VAD), Comfort Noise Generation (CNG), T.38 fax relay, and modem passthrough. These capabilities ensure clear, reliable voice and fax communication even in challenging network conditions.
Online Insertion and Removal (OIR)
Supported on Cisco 4000 Series ISRs, OIR allows the NIM-2FXS/4FXOP to be inserted or removed without powering off the router. This hot-swap capability reduces planned maintenance windows and accelerates module upgrades or replacements in production environments.
Multi-Protocol Flexibility
The module supports SCCP, SIP, MGCP, and H.323 voice signaling protocols, making it compatible with a wide range of Cisco and third-party call control platforms. Whether you run Cisco Unified Communications Manager, CME, or a SIP-based PBX, this NIM integrates without protocol barriers.
Deployment Scenarios
Branch Office Voice Gateway
Deploy the NIM-2FXS/4FXOP in a Cisco ISR 4000 Series router at a branch office to connect analog phones and fax machines via the FXS ports while terminating local PSTN trunks on the FXO ports. This provides a complete analog voice gateway in a single router slot, ideal for sites transitioning to IP telephony while maintaining legacy analog connectivity.
Survivable Remote Site Telephony (SRST)
In SRST deployments, the NIM-2FXS/4FXOP enables the branch router to maintain PSTN connectivity through its FXO ports even when the WAN link to the central call control server is down. Analog endpoints connected to the FXS ports continue to operate, ensuring business continuity during outages.
Retail and Small Office Unified Communications
Retail stores and small offices that need a handful of analog phone lines and local PSTN access benefit from the combined FXS/FXO port density of this module. It eliminates the need for a standalone analog gateway appliance, consolidating voice and data routing into a single Cisco ISR platform.
Legacy PBX Integration
Organizations migrating from a legacy PBX to an IP-based system can use the FXO ports to trunk into the existing PBX while connecting new analog endpoints or overhead paging systems to the FXS ports. This phased approach reduces migration risk and cost.
General
Interfaces
Voice Features
Software Requirements
Compatibility
Compliance
Compatibility Notes
Platform Compatibility
- Cisco ISR 4221 (requires available NIM slot)
- Cisco ISR 4321 (requires available NIM slot)
- Cisco ISR 4331 (requires available NIM slot)
- Cisco ISR 4351 (requires available NIM slot)
- Cisco ISR 4431 (requires available NIM slot)
Software Requirements
- Cisco IOS XE 16.5 or later is required for the NIM-2FXS/4FXOP (the "P" revision).
- Voice applications require the UC technology package (or equivalent voice-enabled IOS XE image).
Important Notes
- The "P" suffix denotes a hardware component revision; there is no change in functionality from the non-P version (NIM-2FXS/4FXO).
- These NIMs are not supported on Cisco 2900 and 3900 Series ISRs.
- The module draws power from the router's NIM slot — no external power supply is required.
Downloads & Resources
Frequently Asked Questions
| Part Number | Product | Key Difference |
|---|---|---|
| NIM-2FXS/4FXO | 2-Port FXS/4-Port FXO NIM (non-P) | Functionally identical to NIM-2FXS/4FXOP but uses an older hardware revision; supports IOS XE 3.14.3+ instead of requiring XE 16.5+ |
| NIM-4FXO | 4-Port FXO Voice NIM | Provides 4 FXO ports only — no FXS/DID ports. Choose when you need PSTN trunk termination without analog station-side connectivity. |
| NIM-2FXS | 2-Port FXS Voice NIM | Provides 2 FXS/DID ports only — no FXO ports. Ideal when you only need analog endpoint connectivity without PSTN trunk termination. |
| NIM-4FXS | 4-Port FXS Voice NIM | Provides 4 FXS/DID ports with no FXO ports. Best for deployments needing more analog endpoints but no direct PSTN line connections. |
