The E1 link or circuit is probably the most commonly used format within the E carrier system. It is used for connecting a variety of elements within a network - typically small exchanges, mobile base stations and the like will use E1 circuits.
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 there 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 line 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 latency or delay and it has the same capacity at all times. However circuit switched lines do not provide the same flexibility and efficiency that is offered by packet switched data systems.
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 & E1 frame format
An E1 link or E1 line 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.
To provide signal structure, there is a frame that has been devices. The E1 frame format has been devised to provide a frame of 32 time slots of octets, i.e. 8 bits each which are numbered 0 to 31, or as more often seen, TS0 to TS31. Obviously TS stands for Time Slot. The E1 frame repetition rate is 8000 Hz.
The E1 frame Time Slots are nominated TS0 to TS31 and they are allocated to different purposes:
- TS0: This E1 frame time slot is used for synchronisation, alarms and messages. It 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.
- TS1 - TS15: These time slots are used for user data
- TS16: E1 signalling data is carried on TS16. This includes 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. Data may also be carried on this time slot.
- TS17 - TS31: These E1 frame times slots are used for carrying user data
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.
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