Gateway Protocols

Designing and Deploying VoIP over ISDN


Voice over IP (VoIP) over Integrated Service Digital Network (ISDN) is sometimes a desirable combination, especially in enterprise networks using IP Telephony. The features required to provide necessary quality of service (QoS) for VoIP, Low Latency Queuing (LLQ), Class Based Weighted Fair Queuing (CBWFQ), and Link Fragmentation and Interleaving (LFI), are supported for ISDN and the combination works. However, there are significant design considerations to take into account. This document discusses the caveats and limitations involved in using these VoIP related QoS features with ISDN, and provides some tested sample configurations.



Cisco recommends that you have knowledge of these topics:

  • ISDN

  • Point-to-Point Protocol (PPP)

  • Multilink PPP (MLPPP)

  • LFI

  • LLQ


  • Compressed Real Time Protocol (cRTP)

This document does not provide technology training on these subjects but rather an explanation of how these technologies work together in a VoIP network. See the Related Information