T.38 Fax Over IP: What it is & How it Works

Fax machines have been a staple in offices for decades, allowing documents to be quickly sent and received over telephone lines.

But with the rise of the internet and IP networks, an improved way to send faxes emerged – T.38 fax over IP.

T.38 fax utilizes packet-switched IP networks to transmit fax data, providing many benefits over traditional faxing.

In this comprehensive guide, we will explore what T.38 fax is, its features, how it compares to older fax protocols, the advantages it provides, and how it modernizes an old technology.

Whether you are new to T.38 or looking to better understand this increasingly popular fax standard, this article will provide you with a helpful overview and technical details on how T.38 fax over IP works.

What is T.38 Fax?

T.38 is an ITU-T standard protocol that defines how real-time Group 3 fax can be supported over IP networks. Here are some key points about T.38 fax:

T.38 allows fax transmissions to be carried over Internet Protocol (IP) networks in real-time. This allows faxes to be sent and received using modern VoIP phone systems and networks.
It supports transmission of standard Group 3 faxes, the most commonly used fax mode. Group 3 faxes can encode data at up to 14.4 kbit/s using standard compression techniques.
T.38 was ratified as an ITU standard in 1998. It has been updated several times since to add features and improve performance. The current version is T.38 Amendment 7 published in 2018.
To transport T.38 faxes over IP, the fax data is encapsulated in UDP or TCP packets. Session initiation and control are handled via SIP or H.323 call signaling protocols.
T.38 utilizes redundancy and forward error correction techniques to compensate for packet loss on IP networks. This helps ensure the reliable transport of fax data.
Gateways or media gateways with T.38 support are used to convert between the public switched telephone network (PSTN) and VoIP/IP network for interoperability.

Overall, T.38 provides a standard way to transport traditional faxes over modern IP-based phone systems and networks.

It maintains compatibility with existing fax machines while gaining the advantages of IP packet networks.

How does T.38 Fax Work?

T.38 is an ITU standard protocol that defines how real-time Group 3 fax can be supported over IP networks. It allows the transmission of fax documents over VoIP networks.
It works by converting analog fax signals to digital signals that can be transmitted over packet networks. At the transmitting fax machine, the document is scanned and the analog signal is digitized and packetized.
These fax packets are then transmitted over the IP network to the receiving gateway/fax machine. The receiving gateway converts the packets back to analog fax signals that the receiving fax machine can print out as a regular fax document.
T.38 uses TCP or UDP as the transport protocol and includes mechanisms for error correction, compression, and network signaling to account for packet loss, latency, and other issues associated with VoIP transmission.
It supports standard fax modes like G3, super G3, and V.34. T.38 is designed to work even on low-bandwidth networks.
Overall, T.38 allows the same fax transmission capabilities over IP that are possible over traditional telephone networks. It enables fax to integrate with VoIP and modern communication infrastructures.

T.38 Fax Over IP Features

T.38 fax over IP provides several benefits and advanced features compared to traditional faxing over PSTN lines:

Supports fax transmissions worldwide over IP networks – T.38 is not limited to local PSTN connections which enables global reach.
Better reliability than PSTN faxing – T.38 incorporates redundancy, forward error correction, and retransmission of lost fax packets to improve reliability.
Faster call setup and improved quality – IP networks have less latency and degradation compared to PSTN lines which improves fax transmission speed and quality.
Lower cost – Sending faxes over existing IP infrastructure reduces costs compared to analog phone lines. T.38 is included with most VoIP platforms and IP PBX systems.
Compatibility with G3 fax machines – T.38 maintains backward compatibility with the large installed base of standard fax machines.
Security – T.38 faxes can be encrypted via SIP/TLS or IPsec protocols to provide confidentiality and prevent eavesdropping.
No need for fax boards – Special fax hardware cards are not required since T.38 fax packets traverse the network just like other VoIP calls.
Advanced features – T.38 enables fax store-and-forward, fax broadcasting to multiple recipients, and fax forwarding to email or other destinations.
Centralized management – IP-based fax infrastructure enables administrators to manage fax servers and gateways centrally.

Overall, T.38 provides a broad set of capabilities to securely transport faxes globally over modern IP networks while integrating with existing infrastructure.

T.38 vs G3 Fax (G.711)

T.38 fax over IP differs in a few key ways compared to traditional G3 fax carried over PSTN lines using G.711 encoding:

G.711 applies analog-to-digital encoding of the audio signal at 64 kbit/s. T.38 digitally encapsulates the fax data into packets directly.
G.711 has no procedures for error correction. T.38 incorporates redundancy and forward error correction for reliable transmission over IP networks.
G.711 uses circuit-switching which maintains a constant open line. T.38 is packet-based which dynamically transports packets as needed.
G.711 has limited ability to adapt to network degradations. T.38 negotiates the fax session in real time to find the best codec and settings.
G.711 utilizes telephone infrastructure. T.38 leverages modern IP and VoIP networking.
G.711 has a simple call setup via PSTN signaling. T.38 uses SIP or H.323 which enables more advanced call handling.
G.711 cannot easily cross between PSTN and VoIP networks. T.38 gateways seamlessly bridge between PSTN and IP faxing.
G.711 does not support security features. T.38 can encrypt fax transmissions over IP networks.

While T.38 builds upon the G.711 G3 fax standard, it modernizes faxing to reliably work over global IP networks. T.38 is the recommended approach for faxing with VoIP systems.

The Benefits of Fax over IP

Migrating from traditional PSTN-based faxing to T.38 fax over IP networks provides organizations with several meaningful benefits:

Cost Savings

Reduces separate analog phone line costs for fax machines – Faxes can traverse the same IP network infrastructure already in place for phones, computers, etc.
Leverages VoIP systems companies already have deployed – No need for additional hardware investments.
Consolidates fax and voice traffic into unified solution – Simplifies infrastructure using converged services.
Centralizes fax servers reduce maintenance costs – Eliminates tedious fax machine management.

Reliability Improvements

Built-in redundancy and error correction – Provides reliable faxing even on imperfect IP networks.
Retries and failure recovery procedures – Ensures faxes eventually get through despite interruptions.
Higher uptime than PSTN lines – Not susceptible to physical line outages.
Accommodates packet loss and latency – Still functions well on congested or imperfect networks.

Enhanced Features

Increased fax speed – Leverages high-speed IP networks.
Better fax quality – Avoid signal degradations of PSTN lines.
Advanced features like fax broadcasting, forwarding, and retries.
Centralized server provides high availability and load balancing.
Interoperability with email and other systems via API integration.
Extensive monitoring, reporting, and troubleshooting capabilities.

Overall, T.38 fax over IP provides substantial cost savings, performance gains, and new capabilities compared to traditional PSTN faxing.

Frequently Asked Questions (FAQ)

Ques 1: Is T.38 compatible with existing fax machines?

Ans: Yes, T.38 is designed to be fully interoperable with existing G3 fax machines and fax protocols. It adapts fax transmission to work over IP networks.

Ques 2: Can T.38 transmit faxes to both IP and PSTN destinations?

Ans: Yes, T.38 supports transmitting faxes to traditional PSTN numbers via gateways as well as directly to other endpoints with T.38 support.

Hybrid environments with both VoIP and PSTN are supported.

Ques 3: What call control protocols can be used with T.38?

Ans: SIP and H.323 are the primary protocols used to establish T.38 fax sessions over IP networks. SDP is used for capabilities exchange and negotiation.

Ques 4: Does T.38 support faxing over the public Internet?

Ans: Yes, T.38 can utilize the public Internet but quality levels over the public Internet may not always be ideal for faxing. Private IP networks or VPNs provide better quality assurance.