T1/E1

T1 and E1 are digital transmission line standards that carry multiple voice or data channels over a single physical circuit. A T1 line (used in North America and Japan) provides 1.544 Mbps of bandwidth across 24 channels (23 voice + 1 signaling when used as PRI), while an E1 line (used in Europe and most other regions) provides 2.048 Mbps across 32 channels (30 voice + 1 signaling + 1 framing when used as PRI). These are the physical layer standards that underpin PRI telephony connections.

How It Works in VICIdial

T1 and E1 circuits connect to VICIdial servers through dedicated hardware interface cards — typically Sangoma (formerly Digium) T1/E1 cards that plug into PCI or PCIe slots. These cards terminate the physical circuit and present the channels to Asterisk through the DAHDI driver framework. Each channel carries one concurrent phone call using G.711 codec at 64 kbps (the native channel rate for both T1 and E1).

A T1 circuit uses a line coding and framing format called ESF/B8ZS and divides its 1.544 Mbps bandwidth into 24 DS0 (64 kbps) time slots. When configured as PRI, 23 slots carry voice (B-channels) and one carries ISDN signaling (D-channel). An E1 circuit uses HDB3 line coding and divides its 2.048 Mbps into 32 time slots, with 30 for voice, one for signaling, and one for framing synchronization.

Multiple T1 or E1 circuits can be combined for higher capacity. A call center needing 92 concurrent PRI channels on T1 would install four T1 cards (4 x 23 = 92 channels). Asterisk manages these as a trunk group, distributing calls across available channels. Configuration is handled in the DAHDI system configuration files (/etc/dahdi/system.conf) and Asterisk’s channel configuration.

Why It Matters

Understanding T1/E1 technology is relevant for VICIdial administrators who manage legacy infrastructure or operate in environments that still rely on physical carrier circuits. While SIP trunks have replaced T1/E1 connections in most modern deployments, many call centers still have T1/E1 circuits in production — either as primary connections with traditional carriers or as backup circuits for redundancy.

The key planning consideration with T1/E1 is fixed capacity. Unlike SIP trunks that can dynamically scale to hundreds of channels, each T1 provides exactly 23 voice channels and each E1 provides exactly 30. Expanding capacity requires ordering new circuits from the carrier (with provisioning lead times of 1-4 weeks), installing additional hardware cards, and configuring the new channels. This rigidity is the primary reason most VICIdial deployments have migrated to SIP trunking, which offers elastic capacity that matches the dynamic scaling needs of modern call center operations.