Enterprise VoIP telephony: SIP, UC integration, fleet

The SIP protocol (Session Initiation Protocol, RFC 3261, IETF 2002) handles signalling for voice sessions over IP, while RTP (RFC 3550) carries the media stream encoded in G.711 (64 kbit/s payload, 80 to 100 kbit/s with IP/UDP/RTP header, ITU-T Recommendation G.711) or G.729 (8 kbit/s payload, 20 to 30 kbit/s with header, ITU-T Recommendation G.729). SIP trunks are sized according to the Erlang B model (ITU-T E.500) with an observed ratio in office environments of 1 channel for 4 to 6 users.

Contrarian Kytom position: G.729 is barely justifiable anymore in 2024. Integrator doxa still recommends G.729 to save bandwidth, but on an internal fibre backbone and a carrier SIP trunk of at least 100 Mb/s, the saving of 60 kbit/s per call is marginal compared with the measurable loss of audio quality (MOS 3.9 for G.729 versus 4.4 for G.711, ITU-T Recommendation P.800). Kytom has provisioned G.711 by default since 2022 and reserves G.729 solely for cases of satellite trunk or degraded link.

The typical Kytom architecture on an 850 m² floor plate is structured in 4 layers:

• Fibre backbone: OM4 multimode up to 150 m at 10 Gb/s, or OS2 single-mode over kilometres at 100 Gb/s (ISO/IEC 11801 standard)
• Main server room on an intermediate level (3rd to 5th floor) connected to the carrier entry point
• 42U floor VDI cabinets with PoE+ 30W switches (IEEE 802.3at) or PoE++ 60W (IEEE 802.3bt) for video stations
• Capillary RJ45 category 6A distribution, 2 outlets per workstation

Reference QoS thresholds require jitter below 30 ms, packet loss under 1% and a round-trip delay under 150 ms (ITU-T Recommendation G.114 for mouth-to-ear delay). The contractual SLA of national SIP carriers stands at a minimum of 99.9%, supplemented by an SBC (Session Border Controller) that filters signalling and media streams.

The Kytom method in 5 phases over 12 weeks, from PABX audit to user acceptance

VoIP telephony integration follows strict sequencing, coordinated with the other technical work packages of the fit-out (partitions, suspended ceilings, furniture). The standard timeline corresponds to a fleet of 60 to 180 stations.

1. Audit of the existing fleet (week 1-2): PABX inventory, residual analogue lines, fax, emergency phones for lifts and gates, current numbering plan, ongoing carrier contracts
2. Category 6A VDI cabling (week 1-3): 2 outlets per station, permanent link certification compliant with the structured cabling standards in force, distribution to 42U floor cabinets
3. SBC configuration and numbering plan (week 4-7): carrier SBC configuration, dedicated voice VLAN, QoS DSCP EF (46), SIP rules, G.711/G.729 codec profiles
4. Progressive cutover (week 8-10): number porting over 10 to 15 working days, deployment in batches of 20 to 50 stations, double run over 5 days
5. User acceptance (week 11-12): tests on 5 to 10% of stations, 1 to 2 hours of training per employee, handover of administration to the 2 to 3 internal referents

Conservatory retrofit UPS units cover the core cabinets for 10 to 30 minutes where emergency telephony or the management video room requires it. Kytom works with 4 UC integrators certified for Teams Phone and Zoom Phone.

When this three-month method is not suited to fleets under 30 stations or multi-site fleets above 800 stations. The 5-phase sequencing makes no sense below 30 stations: the ROI falls beyond 36 months and the PABX audit becomes costly compared with a simple turnkey Teams Phone Standard subscription in 2 to 3 weeks. Beyond 800 multi-site stations, phase 4 of the progressive cutover must be re-divided into geographic waves of 6 to 8 weeks each, otherwise level 1 support is overwhelmed and an unmanageable peak of tickets is generated. Finally, on a site with no regulated emergency telephony constraint (no lift, no public-access building), the dedicated SBC can be replaced by the carrier’s shared SBC, which removes the corresponding budget line from the project.

For the IT department: economic and operational benefits of the VoIP cutover

The VoIP cutover generates benefits along three economic and operational axes: reduction of the monthly telecom bill compared with the PSTN, improved reachability of employees, and radical simplification of fleet management (adding or removing a station in a few minutes via web console, versus several days on a traditional PABX). The ROI is generally achieved over a horizon of two to three years for mid-sized fleets, depending on the complexity of the site and the level of UC integration chosen.

IT department reading: what these gains mean for infrastructure management. The reduction in telecom costs is the most visible gain to the CFO but the least strategic for the IT department. The real business lever is the transformation of MAC (Move Add Change): a station addition that used to mobilise several days and a PABX provider ticket is now carried out in a few minutes via the Teams Admin Center or Zoom Admin Portal web console, by an internal level 1 administrator. In concrete terms, on a fleet of 200 stations with 15% annual mobility, this represents 60 to 150 person-days saved each year, redeployable to value-added projects. Traceability is also strengthened: every MAC action is logged, GDPR-auditable and exportable for SOC 2 or ISO 27001 reviews.

Employee reachability improves noticeably through native UC functions: presence, unified messaging, click-to-call from Outlook, iOS/Android mobile soft-phone with seamless WiFi/4G switching.

Limit of the stated gains. The ROI assumes a PABX fleet at the end of its depreciation period and a PSTN carrier contract still active. On an IP centrex fleet already modernised after 2020, the gain is considerably more limited.