Voice Gateways: The Bridge Between Worlds

Traditional Telephony vs Voice over IP

Traditional Telephony:

  • Well known and stable
  • Proprietary PBX/Key-System
  • Central Office
  • Time/Frequency Division Multiplexing (TDM/FDM) T1 with 24 channels – TDM uses time slots to split connections via single connection. FDM is used on DSL connections, 1 single cable using different frequencies for voice and DSL for example.

Voice Over IP:

  • Open ish standards for multi vendor communication
  • Unified Networks and staff
  • Internet Telephony Service Service Providers (ITSP)
  • Toll Bypass
  • Phased Migration Supported

Voice Gateway

  • Entry and exit point of the system
  • Bridge the old and new
  • Provide a line of security
  • Provide VOIP ‘services’

Voice Processing

  • Brain of the network
  • Device Control
  • Dial Plans and Permissions
  • Phone Features and Control

Media Termination Point – Call on hold is cut off essentially and held on the MTP via the VG whilst on hold.

VoIP Design Models




  • Single Site
  • Centralised Multisite
  • Multisite Distributed (With a Gatekeeper – Lord of the WAN! Central control for everyone. Policies BW for example. Can set network into ‘zones’. Can also be centralised ‘phonebook’ when you don’t know where to go. Centralised management.
  • Split Cluster over WAN (Pub and Sub separated by WAN link)

Common Cisco VoIP Hardware

  • Voice Gateways – 2900 Series with a Voice feature IOS – Modules added for voice capabilities. VWIC for Voice T1 for example. PoE switch/FXS/FXO ports etc..
  • ATA
  • Voice Switches – PoE capabilities, QoS support, Voice VLANs etc..
  • CUCM
  • IPT Endpoints