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TDM Technology for Multiservice Access

TDM-PDH Access

The TDM (Time Division Multiplexing) technology has been developed for telegraphy systems at the end of the 19th century and applied to the digital telephony during the second half of the 20th.

The synchronous digital transmission shares the bit data stream in Time Slot (TS) to transport together multiple voice or data channels together. Another codirectional transmission transmits the return signals. From this basic process many transmissions have been developed as TDMA in 2G-GSM, PON… with adaptative Time Slot. We will focus on this page in the technology used by Loop Telecom Equipment; E1, T1, E3, DS3, and SDH/SONET are described in another page.

The E1 standard was defined by the CCITT and adopted by most of the countries except North America and a few Asian countries which also use the T1 standard, defined thereafter.

The E1 operates at 2.048Mbps +/- 50ppm. To note:

  • Electrically defined as G.703 and as unframed signal,
  • G.704 standard frame the 2.048 Mbps within 32 TS of 8bits-byte, numbered from TS0 to TS31,
  • TS0 is used for synchronization, alarm, international carrier use,
  • TS16 is used for data or as CAS voice signalization, CCS ISDN D channel,
  • A single frame is 256 bits wide (125µs), and a TS 8 bits wide (or 64Kbps),
  • According to applications, some use dual frame or Multi-Frame (16),
  • Loop Telecom equipment supports AMI or HDB3 Line Code and 75 or 120ohms impedance.

The T1 North America standard transmits at 1.544 Mbps. To note:

  • The framed T1 or DS1 use D4 or ESF framing format,
  • DS1 includes 24 TS of 64Kbps (or DS0) and one 8kbps for framing and synchronization,
  • A single frame is 193 bits wide and a TS 8 bits wide (or 64Kbps),
  • Multi-frames are different according to framing,
  • Loop Telecom equipment supports AMI or B8ZS Line Code and D4 or ESF framing.

The CCITT E3 standard at 34,368 Mbps is no longer supported by Telco, but some equipment still exist. Loop Telecom can transport this interface through O9400R or O9500R with SDH or with PW over IP/Eth/MPLS.

The North America standard DS3 still a standard service at 44.736 Mbps ± 20ppm. Loop Telecom supports the transport of DS3 and the M13 mapping in 28 T1 or 21 E1 Signals with O9400R or O9500R and soon the G7860A*.

Voice Transport G.711. This main standard makes the conversion from Analog voice to Digital voice for the transport inside 64Kbps TS or DS0 over TDM E1/T1 link. This PCM (Pulse Code Modulation) is sampling the analog signal at 8k per second in 8 Bit code word. The non-linear coding used in E1 is the A-Law and in T1 the µ-Law. These interfaces cover the voice band from 300Hz to 3400Hz. All parameters are selectable in Loop equipment to transport single voice channel of E1/T1 voice.

But the E1/T1 does not define the transport between two multiplexers. The PDH (Plesiochronous Digital Hierarchy) technology has been developed since the 1970s. The E1/T1 interfaces from PDH nodes was quasi synchronous (plesiochronous +/-20ppm) and two E1/T1 nodes could not be directly interconnected. At this time of the technology both E1/T1 mux had to be linked over a high order mux E2/T2, same for E2/T2 mux which was interconnected by E3/T3... However now PDH technology has been changed and PDH nodes can be directly interconnect by E1 to E1, thanks to addition of elastic store buffer that compensates +/- 20ppm clock between mux, and same evolution has been granted on T1 devices. PDH term still use to name the E1/T1 TDM multiplexer, even if they are not anymore “Plesiochronous”. But it is necessary to understand that all interfaces inside the PDH multiplexer use the same clock by construction. To avoid loss of frame in transmission between two nodes, both should be connected to a primary clock or, node 2 must be synchronized by the node 1, node 3 by the node 2… but this has a limit and after 4 or 5 nodes… transmission clock become bad and generate errors. Then, large PDH network must be clocked by SDH infrastructure which is able give the primary clock to PDH.

The TDM transmission SDH/SONET basically makes a high precision synchronous network with a unique high-quality clock, from this bit clock it creates a digital hierarchy, with high-order multiplex when crossing large node and inside the node it generates low-order multiplex with sub-rate transmission (TU) and Virtual Containers (VC) of usable data and control mapping of the various signals of different bit rates.

PDH/TDM multiplexers where initially developed for voice distribution, and for industries they have integrated the support of many analog, serial interfaces to become Multiservice TDM/PDH multiplexers.

Loop Telecom Multiservice TDM/PDH multiplexers supports the access of many interfaces including the Ethernet switch/router for EoPDH from 64Kbps to 2Mbps or 8Mbps, terminal server, dry contacts, interfaces dedicated to customer domain as teleprotection, in addition to 2 or 4 wires, magneto, conference… voice, VoIP; we have also added many synchronous/asynchronous serial interfaces with sub-rate. In each interface, we have implemented different modes, parameters… to be compatible with most of actual and old interfaces, these accepts a selection of electric level, impedance, noise… for analog and old legacy interfaces.

Loop Telecom proposes a range of compact and modular TDM/PDH multiplexers, with cross-connect off all TS (non-blocking). Any E1/T1 can be considered as including WAN or tributary.

The redundant CPUs, power supplies, WAN interfaces with 1+1 protection per port make these Mission Critical Communication devices. The transmission can be protected in Ring infrastructure by ULSR or SNCP protection.

References and Recommended Literature: