The E carrier system has been created by the European Conference of Postal and Telecommunications
Administrations (CEPT) as a digital telecommunications carrier scheme for carrying multiple links. The Ecarrier system enables the transmission of several (multiplexed) voice/data channels simultaneously on
the same transmission facility. Of the various levels of the E-carrier system, the E1 and E3 levels are the
only ones that are used.
More specifically E1 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
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 basics
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.
What is T1 and E1?
T1 is a digital carrier signal that transmits the DS – 1 signal. It has a data rate of about 1.544 megabits /
second. It contains twenty four digital channels and hence requires a device that has digital connection.
This digital connection is called as the CSU / DSU – Customer Switching Unit or Digital Switching Unit.
The scalability of the T1 is up to 200 and above users. It also provides some services similar to the
internet provider. Most of the computer uses a T1 connection. This technology makes your modem to
have higher speeds and it is an affordable technology.
E1 is similar to the T1. T1 is the North American term whereas the E1 is the European term for the
transmission (digital). The data rate of E1 is about 2 mega bits per second. It has 32 channels at the
speed of 64 Kbps. It is important to know that 2 channels among the 32 are already reserved. One
channel is used for signaling while the other channel is used for controlling. The difference between T1
and E1 lies in the number of channels here. The speed remains the same. There may be inter –
connection between the E1 and T1 lines. This is interconnected because it is used for international
purpose.
Differences in the physical delivery:
Data Rate: The main difference is the data rate. T1 has a data rate of 1.544 mbps and E1 has a data rate
of 2.048 mbps.
Copper Delivery: In the T1 signal there is a copper delivery among 4 wires. It is grouped into two pairs.
One pair is the RX (1+2) and another is TX (4+5). The RX is the data that is from the network and the TX is
to the network. In the E1, there are two types of physical delivery; balanced physical delivery and
unbalance physical delivery. The unbalance physical delivery has 4 copper wires. It is similar to that of
T1. Whereas in the balance physical delivery there is a coax connector which has one cable for RX and
one cable for TX.
Services: T1 has a specific type of service. It has repeaters for every six thousand feet, a pulse or
waveform shape and a jitter. The E1 has 32 timeslots. This can be said as DS. Each DS is about 8 bits
wide.
Differences in the Framing Format:
Framing: In T1, there are two types of framing formats. One is D4 (twelve bits group) – used in aligning
the equipment which is used for framing and another is ESF (twenty four bits group) – used in aligning
the frames as well as in the maintenance of the channel which is facilitated by the data link. In E1, there
are two framing formats. One is a called the double frame – it uses the DS0 and another is the
multiframe which is the independent form.
E1 and T1 lines are used to carry data and voice between carrier links. Historically, the T1 circuit has
been used in North America and the E1 circuit in Europe. T1 and E1 lines can support both voice and
data on a single link. This gives substantial cost savings to the carriers by using existing data networks
and routing the voice to other destinations.
When a link of T1 or E1 call setup/teardown data arrives at the data center, the data channel is
extracted and voice is sent to a media backend for the voice call to be processed. This function is called
drop and insert. Each T1 and E1 line has a protocol to notify the other end of any errors that occur. The
method of notification is called alarms.
Alarms are used in T1 or E1 to notify a node of a problem on a link. Alarms classify the nature of the
problem. If a problem arises in drop and insert mode on a link, both links must also be notified.
E1 Protocol Basics
The E1 frame is composed of 32 timeslots (Figure 1). Timeslots are also called DS0s. Each timeslot is 8
bits. Therefore, the E1 frame will be (32 timeslots * 8 bits) = 256 bits. Each timeslot has a data rate of
64,000 bits/second. There will be 64,000 bits/second/8 bits = 8000 frames a second. The E1 frame will
arrive every 1 second/8000 frames/sec = 125 microseconds. The line rate will be (32 channels * 8
bits/channel)/ frame * 8000 frames/second = 2048000 bits/second.
Figure 1: Diagram of the E1 frame.
Timeslot 0 is used for frame synchronization and alarms. Timeslot 16 is used for signaling, alarms, or
data. Timeslot 1 to 15 and 17 to 31 are used for carrying data.
An alarm is a response to an error on the E1 line or framing. Three of the conditions that cause alarms
are loss of frame alignment (LFA), loss of multi-frame alignment (LFMA), and loss of signal (LOS).
The LFA condition, also called an out-of-frame (OOF) condition, and LFMA condition occur when there
are errors in the incoming framing pattern. The number of bit errors that provokes the condition
depends on the framing format. The LOS condition occurs when no pulses have been detected on the
line for between 100 to 250 bit times. This is the highest state alarm where nothing is detected on the
line. The LOS may occur when a cable is not plugged in or the far end equipment, which is the source of
the signal, is out of service.
The alarm indication signals (AIS) and remote alarm indication (RAI) alarms are responses to the LOS,
LFA, and LFMA conditions. The RAI alarm is transmitted on LFA, LFMA, or LOS. RAI will be transmitted
back to the far end that is transmitting frames in error. The AIS condition is a response to error
conditions also. The AIS response is an unframed all 1's pattern on the line to the remote host. It is used
to tell the far end it is still alive.
AIS is the blue alarm, RAI is the yellow alarm. A red alarm that can occur after a LFA has existed for 2.5
seconds. It is cleared after the LFA has been clear for at least one second.
E1 Double Frame
There are two E1 frame formats, the double frame and the multi-frame. The synchronization methods
are different in the two frame formats. Synchronization can be achieved after receipt of three E1 frames
in double frame format. The synchronization information is carried in timeslot 0. This is called the frame
alignment signal (FAS).
The FAS is a pattern "0011011" that specifies the alignment of a frame. The FAS is in timeslot 0 in
alternate frames (figure 2). Bits 2 through 8 are the FAS. The other frame's (N+1) bit 2 is set to 1. Frame
alignment is reached if there is:
1. A correct FAS word in frame N.
2. Bit 2 = 1 in frame N+1
3. A correct FAS word in frame N+2.
Note: The Six bits are reserved for international use and not discussed here.
Figure 2: Frame N for an E1 double frame
Figure 3: Frame N+1 for an E1 double frame.
What happens if synchronization is not achieved or has been achieved and lost? This condition is called
LFA. If three in four alignment words are in error, an LFA is declared. This is if bit 2 in Frame N+1 is set to
0. The near end must respond to the far end that there is an alignment problem. This is done with the
RAI alarm. The A bit (bit 3) in all N+1 frames is used for sending the RAI alarm to the far-end equipment.
E1 Multi-frame
In multi-frame format, the synchronization for multi-frame requires 16 consecutive good frames. The
multi-frame structure also has two extra features. It provides channel associated signaling (CAS) and a
cyclic redundancy check (CRC).
CAS is sent in timeslot 16 of each frame. It is CAS information that can denote on-hook and off-hook
conditions of telephone calls. Figure 4 shows how CAS information is sent.
Figure 4: diagram illustrating how CAS information is sent.
In frame 1, the information for channels 1 and 16 is sent. In frame 2, the information for frames 2 and
17 is sent. Only 4 bits are used to denote on-hook and off-hook conditions. Of the four bits, not all are
always used. Refer to figure 9 for the definitions of the ABCD bits for on hook/off hook conditions.
Notice that timeslot 16 of frame 0 does not send this information.
The extra feature to multi-frame is the addition of a CRC. This resides in timeslot 0 (Figure 5). The Cx bits
are for the four-bit CRC which resides in bit 1 of frames 0, 2, 4, 6, 8, 10, 12, and 14. The E and S bits are
for international use and will not be discussed here.
Figure 5: Format of timeslot 0 of an E1 multi-frame.
The FAS pattern for multi-frame is also "001011". This is in bit 1 of frames 1, 3, 5, 7, 9, and 11. Notice
now that the FAS is 1 bit in each of the frames vertically. This pattern is called a multi-frame alignment,
when all 16 frames are correct. By reviewing Figure 5, notice that there is a double frame FAS horizontal
in frames 0, 2, 4, 6, 8, 10, 12, and 14 which is 0011011. Frame alignment can still be achieved with this
FA. This will align as double frame with the 3 steps listed above. Double frame alignment is achieved
before multi-frame alignment.
If synchronization is not achieved or has been achieved and then lost a LMFA condition will be declared.
This denotes that the FAS was not received correctly in 16 frames. If double frame alignment has been
lost, LFA will also be declared. The LMFA and LFA conditions are handled differently. When the LMFA
condition exists at the near end of a link, the near end will send a RAI alarm to the far end pf the link.
The RAI is transmitted by setting the Y bit to 1 in timeslot 16 Figure 6. The LFA alarm will be handled as it
is in double frame by setting the A bit in bit 3 of every N+1 frame.
Figure 6: Format of timeslot 16 of an E1 multi-frame.
The AIS is sent as all 1's in the frame. It is declared when there are less than three zeros in a 250-ms
period. All timeslots will be filled with 1's. This is sent in double frame and multi-frame when the LFA
occurs. When LMFA condition occurs, AIS will be sent only in timeslot 16.
T1 Basics
The T1 frame consists of 24 timeslots . Each timeslot is 8 bits. The first bit of each
frame is used for synchronization. The T1 frame is 24 timeslots * 8 bits = 192 bits
+ 1 synchronization bit = 193 bits. Each timeslot has a data rate of 64,000
bits/second. There are 64,000(bits/second)/8 bits which is 8000 frames a second.
A T1 frame will arrive every 1 second/8000 frames/sec = 125 microseconds. The
data rate is (24 channels * 8 bits/channel)/1 frame * 8000 frames/second =
1536000 bits/second. The total line rate is (24 channels * 8 bits/channel + 1
synchronization bit)/1 frame* 8000 frames/second = 1544000 bits/second.
Figure 7: Diagram depicting the T1 frame.
The same error conditions and alarms provided in E1 exist in T1. They are LFA, LMFA, LOS, AIS (or blue),
and RAI (or yellow). The RED alarm can also occur after a LFA has existed for 2.5 seconds. It is cleared
after the LFA has been clear for at least one second.
Bit 1 of each frame is used for frame synchronization. Timeslots 1 to 24 are used for data. Two T1
framing formats will be described. The formats are the D4 frame super-frame and the extended superframe (ESF).
T1 Super Frame
The D4 super-frame is made up of 12 individual T1 frames (Figure 8). There are
two types of framing bits, the terminal framing bits (Ft) and the signaling framing
bits (Fs).
Figure 8: Diagram of a typical D4 super frame.
The Ft framing bits identify the framing boundary. The Fs framing bits identify the super-frame
boundaries. The Ft framing bits and the Fs framing bits form a 12-bit framing pattern "100011011100".
The Ft and Fs bits can be combined for synchronization or they can be independent. When the Ft and Fs
bits are combined for synchronization, two errors within 4/5/6 framing bits will give a LFA condition for
Ft and a LFMA condition for Fs. The 4/5/6 number of framing bits is configurable. When Ft and Fs are
independent, two errors within 4/5/6 Ft bits will give a LFA and LFMA condition. Two errors within 4/5/6
Fs framing bits will only give a LFMA condition.
In T1, the LFA and LFMA condition are handled the same. When framing errors occur, the RAI will be
sent to the far-end equipment. The RAI is sent by setting bit 2 in every timeslot of each frame.
D4 uses two signaling bits in the 6th and 12th frames (see Figure 8 above). These are called A and B bits.
The A and B bits are used in CAS also called robbed bit signaling. The A and B bits replace the last bit in
the timeslot of frame 6 and 12. This only occurs when the timeslots are used for voice. The robbed bit
does not affect the quality of the voice. The value of these bits determines the state of the telephone
channel.
There are actually four signaling bits used in CAS designated as the A, B, C, and D bits (Figure 9). All four
are used in the ESF format (see Figure 10 below). When there is no call on a channel, it is in the on-hook
state. When there is an active call on a channel, then it is in the Off Hook state.
Figure 9: ABCD signaling bits in a T1 frame.
T1 Extended Super Frame
ESF consists of 24 individual T1 frames (Figure 10). The framing bits have been doubled from superframe 12 to 24. The 24 framing bits have three different functions: FAS, data link (DL), and the CRC. The
FAS is used for framing and synchronization in frames 4, 8, 12, 16, 20, and 24. The DL bits are used for
sending performance information and alarms in frames 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21 and 23. The
CRC is used to monitor the transmission quality of the ESF. The CRC6 bits are transmitted and received in
frames 2, 6, 10, 14, 18, and 21 giving a 6-bit CRC.
Figure 10: Diagram illustrating the ESF frame.
Synchronization is achieved with the FAS pattern "001011". Synchronization is lost when there are 2
errors in a configurable 4/5/6 framing bits. When synchronization is lost a RAI is sent with the pattern
"1111111100000000" in the DL bits. This pattern continues over multiple extended super frames.
The AIS alarm is sent as all 1's in the D4 and ESF frame formats. It is declared when there are less than 3
zeros in 12 or 24 frames. All timeslots will be filled with 1's.