The following is an overview of relevant specifications for E1 interfaces.
Wikipedia provides a good overview at https://en.wikipedia.org/wiki/E-carrier
G.702 "Digital Hierarchy Bit Rates"¶
- describes the hierarchy of 64k/2048k/8448k/34368k/139264k and their T1 counterparts
G.703 "Physical/electrical characteristics of hierarchical digital interface"¶
Section 11 describes the "E12" interface at 2048 kbit/s
- Nominal bit rate: 2048 kbit/s
- Bit rate accuracy: /-102.4 bit/s)
- Code: High density bipolar of order 3 (HDB3) (a description of this code can be found in Annex A)
- Overvoltage protection requirements: refer to [ITU-T K.20]
The RJ45 / twisted pair interface uses 120 ohms differential/symmetric signaling with a nominal peak voltage of 3V and a nominal space voltage of 0V
G.704 "Synchronous frame structures used at 1544, 6312, 2048, 8448 and 44 736 kbit/s hierarchical levels"¶
Section 2.3 specifies the basic frame structure at 2048 kbit/s
- the basic frame consists of 256 bits, numbered 1 to 256
- the frame repetition rate is 8000 Hz
- bits 1..8 of the frame have special significance, they contain the frame alignment signal (FAS) and the S-bits
- an optional CRC-4 mechanism can be used in bit 1 of the frame, using so-called CRC-4 multiframes composed of 16 frames
Section 5.1 specifies the interface at 2048 kbit/s carrying 64 kbit/s channels
- each of the 64 kbit/s channels occupise 8 bits per timeslot, numbered 1..8
- 32 timeslots of each 8 bits form the basic frame (256bits) described above
- timeslot 0 (TS0) is used for frame alignment and signaling, as described above
- timeslot 1..31 are free for user data. 16 is typically used for signaling [not custom in GSM A-bis, but in general ISDN]
- timeslot 16 can be used for channel-associated signaling (CAS) [not used in context of GSM A-bis]
G.706 "FRAME ALIGNMENT AND CYCLIC REDUNDANCY CHECK (CRC) PROCEDURES RELATING TO BASIC FRAME STRUCTURES DEFINED IN RECOMMENDATION G.704"¶
Section 4 specifies the Frame alignment and CRC procedures at 2048 kbit/s interface
- frame alignment is assumed lost when three consecutive incorrect frame alignment signals (FAS) are received
- frame alignment is assumed lost when bit 2 in TS0 in frmaes not containing the FAS has been received with an error on three consecutive occasions
The following procedure is recommended:
When a valid frame alignment signal is detected in frame n, a check should be made to ensure that a frame alignment signal does not exist in frame n + 1, and also that a frame alignment signal exists in frame n + 2. Failure to meet one or both of these requirements should cause a new search to be initiated in frame n + 2.
G.711 "PULSE CODE MODULATION (PCM) OF VOICE FREQUENCIES"¶
Specifies the aLaw and uLaw encoding of classic ISDN B-channels.
When character signals are transmitted serially, i.e. consecutively on one physical medium, bit No. 1 (polarity
bit) is transmitted first and No. 8 (the least significant bit) last.
I.412 "ISDN USER-NETWORK INTERFACE STRUCTURES AND ACCESS CAPABILITIES"¶
- B-Channels carry G.711 (voice), X.1 (data) or G.722 (wideband voice)
- D-Channels use layered protocol according to I.440, I.441, I.450 and I.451 (I couldn't find any of these)
I.431 "PRIMARY RATE USER-NETWORK INTERFACE - LAYER 1 SPECIFICATION"¶
Sort-of an umbrella specification referring to G.703 / G.704 and specifying details which options to use
- CRC4 shall be used
- D-Channels shall have 64kBps and according to I.412 (default on TS16)
- B-Channels shall have 64kBps and according to I.412
- Remote Alarm Indication (RAI) and LOS, referring to G.706
In the signaling timeslots (typically 64k slots, sometimes 16k sub-slots), HDLC is used for framing. It delimits the start and end of the octet and frame boundaries within the bitstream of the (sub)slot. See wikipedia for a general introduction into HDLC.
ISO/IEC 13239 specifies HDLC.