V.151 Gateway

VOCAL V.151 Gateway is a standards based implementation that incorporates reliable and efficient transport of analog modulated signals generated by PSTN text telephone across IP network. This solution supports text over IP (ToIP) operation in Voice Band Data (VBD) mode and in the Text Relay (TR) mode. Possible connection scenarios may include PSTN Text Telephone at one end and another PSTN Text Telephone or an IP Text Telephone on the other end.

VOCAL’s v.151 implementation can be combined with other VOCAL v.15x series of recommendations like ITU-T V.150.1 for Modem-over-IP (MoIP) or ITU-T V.152 for VBD. Although for each V.15x recommendation, there is a requirement that the support for it has to negotiated with the remote gateway specifically.


Payload Type Indication – This default method for switching between transport modes using RTP payload types are supported in the gateways.
SSE – Signaling State Events are Supported, it is a mechanism by which the media states are synchronized between the media gateways.
SPRT – Simple Packet Relay Transport for text relay is supported along with different data types (I_OCTET, I_OCTET_CS, I_RAW_OCTET) to carry the data from one gateway to another.
SDP – Different variations of the Session description based on the requirement available, e.g., port sharing where you can send the SPRT and the RTP over the same port.

Interoperability – Our gateways use a number of modulations and character encodings with many other different vendors.
Scalability – A large number of channels on a single gateway can be supported, starting from 10s to 100s based on the gateway.
Flexibility – VOCAL’s V.151 Gateway runs in the form of hardware independent library for a number of the target environment platform (any standard Linux or Windows distribution).


  • Digital control messages replacing analog tone events, for passing the protocol over hops.
  • Supported data modulations include V.23, V.21, Bell 103 modem standards as well as standard fax modulations.
  • Eliminate extra trunking because of convergence of data, voice, and fax.
  • Interconnection of multiple communications networks, such as wireless, wired PSTN, and IP networks, is an important capability for the defense system.
  • Telemetry application transport over IP-based networks can give reliable and economic solutions.

ITU V.151 identifies different modes for transmitting text over IP networks.

TEXT Relay – the audio signal from the local test telephone is demodulated at the on-ramp gateway and transmitted over the network through the SPRT protocol and re-modulated at the off-ramp gateway for transmission to the local text telephone. The gateways maintain state for the call session.

Voiceband (VBD) – The signal is encoded by a speech codec such as G.711 and transmitted as data packets over the network. VBD mode may be used over reliable networks with predictable throughput and negligible packet loss.

Flexible Implementation:

  • Based on decades of experience engineering modems and VoIP telecommunication systems, VOCAL is uniquely capable of creating telephone systems that run purely in software, or on your custom hardware system, and interfacing with your customized interfaces and specialized applications. Whether your text telephones are on one or both ends of the network, VOCAL has a safe and reliable solution.
  • VOCAL’s systems are standards based and fully compatible, so replacing an existing v.151 gateway will happen seamlessly. Variations include communications with gateways or standalone endpoints that send v.151 messages directly on the IP link.
V.151 Network Overview