E1 tutorial
- an overview of the basics of the E1 digital
transmission telecommunications link standard used predominantly in Europe.
E1 is a standard for a digital telecommunications links is
widely used within Europe. It enables the transmission of several (multiplexed)
voice/data channels simultaneously on the same transmission facility. More
specifically it has an overall bandwidth of 2048 kbps and provides 32 channels
each supporting a data rate of 64 kbps. The lines are mainly used to connect
between the PABX (Private Automatic Branch eXchange), and the CO (Central
Office) or main exchange.
The E1 standard defines the physical characteristics of a
transmission path, and as such it corresponds to the physical layer (layer 1) in
the OSI model. Technologies such as ATM and others which form layer 2 are able
to pass over E1 lines, making E1 one of the fundamental technologies used within
telecommunications.
A similar standard to E1, known as T1 has similar
characteristics, but it is widely used in North America. Often equipment used
for these technologies, e.g. test equipment may be used for both, and the
abbreviation E1/T1 may be seen.
E1 beginnings
Development The life of the standards started back in the
early 1960s when Bell Laboratories, where the transistor was invented some years
earlier, developed a voice multiplexing system to enable better use to be made
of the lines that were required, and to provide improved performance of the
analogue techniques that were used. The step of the process converted the signal
into a digital format having a 64 kbps data stream. The next stage is to
assemble twenty four of the data streams into a framed data stream with an
overall data rate of 1.544 Mbps. This structured signal was called DS1, but it
is almost universally referred to as T1.
In Europe, the basic scheme was taken by what was then the
CCIT and developed to fit the European requirements better. This resulted in the
development of the scheme known as E1. This has provision for 30 voice channels
and runs at an overall data rate of 2.048 Mbps. In Europe E1 refers to both the
formatted version and the raw data rate.
E1 Applications and standards
The E-carrier standards form part of the overall Synchronous
Digital Hierarchy (SDH) scheme. This allows where groups of E1 circuits, each
containing 30 circuits, to be combined to produce higher capacity. E1 to E5 are
defined and they are carriers in increasing multiples of the E1 format. However
in reality only E3 is widely used and this can carry 480 circuits and has an
overall capacity of 34.368 Mbps.
Physically E1 is transmitted as 32 timeslots and E3 has 512
timeslots. Unlike Internet data services which are IP based, E-carrier systems
are circuit switched and permanently allocate capacity for a voice call for its
entire duration. This ensures high call quality because the transmission arrives
with the same short delay (Latency) and capacity at all times. Nevertheless it
does not allow the same flexibility and efficiency to be obtained as that of an
IP based system.
In view of the different capacities of E1 and E3 links they
are used for different applications. E1 circuits are widely used to connect to
medium and large companies, to telephone exchanges. They may also be used to
provide links between some exchanges. E3 lines are used where higher capacity is
needed. They are often installed between exchanges, and to provide connectivity
between countries.
E1 description
An E1 link runs over two sets of wires that are normally
coaxial cable and the signal itself comprises a nominal 2.4 volt signal. The
signalling data rate is 2.048 Mbps full duplex and provides the full data rate
in both directions.
For E1, the signal is split into 32 channels each of 8 bits.
These channels have their own time division multiplexed slots. These are
transmitted sequentially and the complete transmission of the 32 slots makes up
a frame. These Time Slots are nominated TS0 to TS31 and they are allocated to
different purposes:
- TS0 is used for synchronisation, alarms and messages
- TS1 - TS 15 used for user data
- TS16 is used for signalling, but it may also carry user data
- TS17 - TS31 are used for carrying user data
Time slot 0 is reserved for framing purposes, and alternately
transmits a fixed pattern. This allows the receiver to lock onto the start of
each frame and match up each channel in turn. The standards allow for a full
Cyclic Redundancy Check to be performed across all bits transmitted in each
frame.
E1 signalling data is carried on TS16 is reserved for
signalling, including control, call setup and teardown. These are accomplished
using standard protocols including Channel Associated Signalling (CAS) where a
set of bits is used to replicate opening and closing the circuit. Tone
signalling may also be used and this is passed through on the voice circuits
themselves. More recent systems use Common Channel Signalling (CCS) such as ISDN
or Signalling System 7 (SS7) which sends short encoded messages containing call
information such as the caller ID.
Several options are specified in the original CEPT standard
for the physical transmission of data. However an option or standard known as
HDB3 (High-Density Bipolar-3 zeros) is used almost
exclusively.
Future
E1 and also T1 are well established for telecommunications
use. However with new technologies such as ADSL, DSL, and the other IP based
systems that are now being widely deployed, these will spell the end of E1 and
T1. Nevertheless they have given good service over many years, and they will
remain in use as a result of this wide deployment for some years to come.
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