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.
In this tutorial, we'll describe the E1 and T1 protocols, alarms, and framing formats. We'll also show how alarms are used in conjunction with drop and insert capabilities to promote the delivery of voice and data services over T1 links. Let's start the discussion by looking at E1.
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 signal (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:
Note: The Sx bits are reserved for international use and not discussed here.
- A correct FAS word in frame N.
- Bit 2 = 1 in frame N+1
- A correct FAS word in frame N+2.
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.
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 FAS. 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 exist at the near end of a link, the near end will send a RAI alarm the to 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.
The T1 frame consists of 24 timeslots (Figure 7). 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 super-frame (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 super-frame 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.
Drop and Insert
Drop and insert applications allow voice and data to be sent over the same link. Figure 11 is a SS7 application with E1 links connected to an SS7 Network. In this example, only one data channel is used by the message transfer part 2 (MTP2) application.
Figure 11: SS7 application with E1 links connected to an SS7 network.
The data channel is for call setup/tear-down information. The rest of the channels are for voice. The adapter used is this application will use a TDM switch to direct the data and voice channels to the appropriate destination. For example, TS1 is the time slot used by the MTP2 application above the WAN. The switch will direct TS1 channel to the software. All the other voice channels received on port 0 will be routed to port 2. Figure 11 only shows data going in one direction. The drop and insert application can be bi-directional.
Figure 12 is a example of how the signaling channel is accessed and processed by the SS7 Front End on an E1 network. There are 32 channels: one for framing, one for signaling, and 30 for voice. The voice channels are redirected to the media back-end servers. The media back-end servers are connected to carrier networks, which handle the voice traffic. The advantage of drop and insert is that it saves one trunk because the voice calls are still sent to the carrier network and the data channel can be extracted. The SS7 front-end servers terminates SS7 traffic at the MTP2 layer and sends MTP3 and higher layer signaling messages to a switch.
Figure 12: SS7 front-end server processing signaling and voice channels.
As can be seen in Figures 11 and 12, the voice channels are routed from port 0 to port 2. What happens if frame alignment is lost on link 0 between the SS7 network and the SS7 front-end server? Port 0 at the SS7 front-end server will receive a LFA alarm. This is where the media back end must also be notified that the data that is routed to it is in error. The SS7 front-end server must notify both the SS7 network and the media back-end server that there is a problem. Port 0 on the SS7 front-end server will respond with a RAI to the SS7 network (Figure 13). Port 2 on the SS7 front-end server will send the AIS alarm to the media back end. This notifies the media back-end server that the SS7 front end is still alive, but there is a problem somewhere in the network. Once the LFA goes away and frame alignment is re-established, the RAI and AIS will be disabled.
Figure 13: SS7 front-end server processing E1 alarms in double-frame format.
Figure 14 provides a final example for E1 networks for alarm management when the frame format is E1 multi-frame. When frame alignment is lost between the SS7 front-end server and the SS7 network, an LFMA and LFA alarm are received at the SS7 front-end server. In this case, the SS7 front-end server will enable RAI in timeslot 0 and timeslot 16 and send it back to the SS7 network. AIS is sent to the media back end. This is all 1's in all timeslots. When double-frame synchronization is established, the RAI in timeslot 0 and the AIS in all timeslots will be disabled. RAI in timeslot 16 and AIS in timeslot 16 will continue until multi-frame synchronization has been established.
Figure 14: SS7 front-end server processing alarms in an E1 multi-frame format.
In summary, T1 and E1 networks can support both voice and data over the same line. The drop and insert functions are used to extract the data channel and route the voice calls to the appropriate destination. Alarms are used between the links to notify the destination and the sender of the data that there is a problem on the link.
About the Author
Christopher Murray is a senior software engineer at RadiSys. He has worked on communications protocols and device drivers for the last 10 years. Chris has a Master of Science in Computer Science from the University of HoustonClear Lake and can be reached at firstname.lastname@example.org.