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News Details
E1/2M Communication Transmission Knowledge Hotspots
Release time:
2026-03-09
E1/2M transmission was, for more than a decade, the mainstream data‑communication access method. Major carriers’ private‑network access typically relied on E1/2M links. Over time, as fiber‑optic transmission became widespread, private‑network bandwidth requirements outgrew the 2 Mbit/s limit. High‑bandwidth private‑network transport is a hallmark of fiber optics; consequently, 2 Mbit/s private‑network access has gradually faded from its former dominant position in today’s network infrastructure.
Although 2M private‑network access has largely faded from mainstream network applications, it is still frequently encountered in certain specialized scenarios. Here, we’ll revisit the underlying principles and practical applications of E1/2M transmission.
1. Understanding the E1/2M transmission interface
E1 is a telecommunications standard for data rates (a European standard, distinct from the one used in China). It specifies a transmission rate of 2048 kbps. For ease of recall, it is commonly referred to as a “2 Mbit/s interface”; thus, the terms E1 interface and 2 Mbit/s interface convey the same meaning.
T1 is a telecommunications standard (used in the Americas and Japan) with a transmission rate of 1,544 kbps. (T1 interfaces are virtually unused in our country, so they will not be discussed here.)
2. E1/2M transmission interface rate
The data rate of an E1 transmission interface is 2048 kbps. This is calculated as follows: an E1 frame is divided into 32 time slots (denoted as TS), with each slot carrying 8 bits, resulting in a total of 256 bits per E1 frame. Since sampling occurs at 8 kHz, the E1 interface’s throughput is 8 kHz × 256 = 2048 kbps. The data rate for each time slot is 8 bits × 8 kHz = 64 kbps.
3. E1/2M Frame Structure
The E1 interface is divided into three modes.
1) Framing
In framed communication transmission, the 0th timeslot is used to transmit frame synchronization data, while the remaining 31 timeslots can be used to carry user data.
2) Frame Reconstruction
Frame assembly is similar to framing, with the only difference being that control signaling is transmitted in timeslot 16, while traffic data is carried in timeslots 1–15 and 17–31.
3) Non-frame (non-framed)
In non‑frame‑based communication transmission, all 32 time slots are used to carry user data.
4. Principles of Access for E1/2M Transmission Equipment
The 2048 kbit/s (E1) data signal is fed into the transmission equipment, encoded in HDB3 format. After single‑to‑dual conversion, it is transformed into a unipolar code. A dedicated integrated circuit extracts the tributary clock, decodes the signal, and, following bit‑rate adjustment, re‑inserts it into the optical‑signal‑driving bit stream.
The received optical signal enters the photodetector, where it is processed by an equalization and amplification stage and a regeneration circuit to generate an NRZ signal. This signal is then fed into a dedicated integrated circuit for clock recovery and decoding. Following smoothing via a code‑rate recovery circuit and an internal digital phase‑locked loop, it is restored to a 2.048 Mb/s signal and output through an output driver circuit as an HDB3 signal that meets the interface requirements.
5. Common Alarm Indicators on E1/2M Transmission Equipment
1) LOS (Loss of Signal): Signal Loss Alarm
E1 signal loss alarm typically indicates that the E1 interface is not receiving an E1 line signal (the physical link is down). This issue usually originates at the RX side of the E1 interface; please verify whether the E1 line is interrupted or if the interface is loose.
2) LOF (Loss of Frame Alignment): Frame Loss-of-Arrangement Alarm
Frame loss-of-synchronization alarm: This alarm is triggered when a frame loss of synchronization is detected. Typically, it indicates that the synchronization signal has been lost during operation. Please check the E1 line for bit errors and verify whether the terminal equipment is generating any alarms.
3) AIS (Alarm Indication Signal): Alarm Indication Signal (commonly referred to as an all-1 alarm)
A full‑1 alarm generally occurs when a signal is received but contains no useful information. When an AIS alarm is triggered, check whether the line or the remote equipment has lost power. For non‑frame E1 bridge devices, no action is taken in response to an AIS alarm; these devices detect the absence of a payload stream. If there is no payload stream on an E1 channel, the line is considered idle, which can cause an AIS alarm on the associated transmission equipment. This is a normal condition and does not affect line transmission—simply disable the AIS alarm.
The above is a brief overview of the E1 interface, and we hope it has been helpful to you!
Guangzhou Guangqiao Communication Equipment Co., Ltd. specializes in the research, development, and manufacturing of communication equipment. Its main product lines include industrial switches, industrial Ethernet switches, industrial PoE switches, fiber optic transceivers, fiber optic switches, multi‑port transceivers, PoE switches, telephone optical terminals, video optical terminals, protocol converters (bridges), PCM multiplexing equipment, PDH optical terminals, E1 protection switching devices, serial‑port optical terminals, wireless bridges, SFP optical modules, and more. Contact numbers: 400‑8866‑640, 020‑82319301, 020‑82319302. Quality first, competitive pricing.
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