E1 trunk: definition and a practical example
Short answer: An E1 trunk is a 2.048 Mbit/s digital circuit between the carrier and the PBX that carries up to 30 simultaneous voice channels, with ISDN or R2 signaling.
In Portuguese: Tronco E1
What is an E1 trunk?
An E1 trunk is a dedicated digital connection between the carrier and the company’s PBX, at a rate of 2.048 Mbit/s. It is divided into 32 time slots of 64 kbit/s: slot 0 handles synchronization, slot 16 carries signaling and the remaining 30 carry one voice channel each. That is why an E1 holds up to 30 simultaneous calls.
There are two kinds of signaling. With ISDN (PRI), slot 16 carries messages such as SETUP, CONNECT and RELEASE, and the reason each call ended comes as a Q.850 cause in the release message, such as 17 (busy) or 34 (no circuit available). With R2, widely used in Brazil, signaling is done with line and register signals, which carry less information about why a call failed.
E1 predates IP telephony but is still present in many companies, often connected to an IP PBX through a gateway. ISDN’s Q.850 causes are the same ones SIP carries in the Reason header, which makes it easier to compare failures across the two trunk types.
In practice
E1 compared with a SIP trunk:
| Characteristic | E1 | SIP trunk |
|---|---|---|
| Medium | Dedicated physical circuit | IP network or internet |
| Capacity | 30 channels per E1 | Set by contract |
| Growth | In blocks of 30 channels | By changing the contract |
| Signaling | ISDN or R2 | SIP |
| Failure reason | Q.850 cause (ISDN) | SIP response and Q.850 cause |
| Weak points | Synchronization, cabling, equipment | Link, NAT, SIP ALG, codecs |
With all 30 channels busy, the next incoming call gets busy, with Q.850 cause 34 on ISDN. On R2, the same situation shows up as a busy signal, without the detailed cause.
How Monitor uses this
Monitor calls the number, so it works the same way whether the number is on an E1 or a SIP trunk. The test does not depend on the trunk type: it shows whether the call completed and why it failed. See SIP trunk monitoring.
Frequently asked questions
E1 or SIP: which should I choose?
E1 has a fixed capacity of 30 channels, a dedicated physical circuit and predictable quality, but it requires installation and grows in blocks of 30 channels. A SIP trunk uses the data network, adds channels by contract and usually costs less, but it depends on the link, the firewall and the NAT configuration. Many companies move to SIP and keep the E1 as a backup.
How many simultaneous calls fit on an E1?
Up to 30. An E1 has 32 time slots of 64 kbit/s: slot 0 is used for synchronization, slot 16 carries signaling and the other 30 carry voice. If a company needs 45 simultaneous calls, it needs two E1s, or one E1 plus a SIP trunk. The 31st call gets a busy signal.