Retarus Fax-over-IP (FoIP) Connectivity

Retarus offers state-of-the-art inbound fax services using Fax-over-IP (FoIP) connections via SIP trunks. This solution enables businesses to transition from traditional analog fax systems to a fully digital, IP-based infrastructure, aligning with modern communication standards while maintaining essential fax functionality.

By leveraging FoIP connectivity, organizations can:​

• Enhance reliability
  Digital transmission significantly reduces the risk of errors associated with analog lines, ensuring more consistent delivery of critical documents.

• Improve scalability
  Easily adjust capacity to meet changing business needs without hardware limitations or complex infrastructure changes.

• Ensure compliance
  Meet industry-specific regulatory requirements (such as HIPAA, SOX, GDPR) through secure, encrypted, and auditable fax transmissions.

• Integrate seamlessly
  Connect with existing IT environments and VoIP infrastructures.

To use FoIP connectivity for inbound faxing, you must establish one or more SIP trunks directly from your VoIP carrier or network environment to a Retarus data center. For high availability and business continuity, setting up at least two SIP trunks to separate Retarus data centers is strongly recommended.

Retarus offers SIP trunk connections via:

• Public internet (unencrypted or encryted using SIP/TLS and SRTP)

• VPN connections

• Dedicated lines

This flexibility accommodates various network configurations and security requirements.

FoIP connectivity is offered as part of a Retarus fax services project and must be evaluated together with Retarus Technical Consulting and your relevant technical experts.

Connectivity setup

SIP Trunks

A SIP trunk is a SIP signaling association between two endpoints, such as your PBX (Private Branch Exchange) or Session Border Controller (SBC) and a Retarus SBC.

Retarus SIP trunks:

• Do not use or support registration (basic authentication).

• Must be routed within your infrastructure to the Retarus SBCs for incoming calls.

Customer requirements

As default configuration for each SIP trunk, Retarus supports:

• One predefined static customer SIP signaling IP address.

• Up to two predefined customer Real-Time Transport Protocol (RTP) media IP addresses.

You must ensure that all SIP and RTP traffic is routed through these predefined IP addresses. Typically, you can achieve this by using an SBC or an IP-PBX acting as a media relay.

Retarus does not support the STUN, TURN, or ICE protocols.

VPN connectivity

If you are using private IP addresses over VPN, you may need to perform Network Address Translation (NAT) on your side.

Security and encryption

Retarus supports secure communication by encrypting:

• SIP signaling with Transport Layer Security (TLS)

• Media streams with Secure Real-time Transport Protocol (SRTP)

Both SIP and media must be encrypted. Partial encryption is not supported and will lead to call rejections.

Retarus supports fax transmission over G.711 only. T.38 encryption is not supported. When using SIP/TLS and SRTP, fax traffic is transmitted via G.711.

Service availability and performance

High availability

To ensure high availability of Retarus Fax Inbound Services, at least two SIP trunks connecting to two separate Retarus data centers are required. You can choose between the following setup options:

• Active-active setup
  Incoming calls are distributed across both data centers, providing load balancing and redundancy.

• Active-passive setup
  Incoming calls are routed to the primary data center, while the secondary data center is used only in case of a failure.

An active-active setup is recommended for optimal load balancing and continuous availability. To enhance redundancy, you can also configure two separate endpoints.

Concurrent calls

A SIP trunk setup supports a minimum of 10 concurrent calls, controlled by Call Admission Control (CAC) as defined in your commercial agreement. You can order additional capacity in blocks of 10 concurrent calls to accommodate growth.

WAN connection requirements

The following specifications are recommended for your WAN connection:

• 100 kbit/s bandwidth per concurrent call

• Round-trip delay of less than 40 milliseconds for RTP traffic

• Jitter below 20 milliseconds

Call setup and media

Number formatting

For successful fax reception:

• The destination number in the SIP URI must be in the full ITU-T E.164 format with a leading "+" (example: +498955280000)

• The origin number should also be in E.164 format.

RTP media and codecs

For FoIP connections, Retarus supports:

• G.711 a-law and μ-law codecs during call setup (fax pass-through)

• Switching to T.38 (version 0 or 3) through re-INVITE messages, if both sides support it.

You can restrict the allowed codecs during the initial configuration process.

Retarus supports fax transmission using uncompressed codecs such as G.711 to ensure optimal quality and reliability. Compressed codecs like G.729 are not supported.