E1 / T1 / TDM

good old ISDN technology
2 Mbits/s (E1) or 1.54 Mbits/s (T1) synchronous, full-duplex
not used much anymore in telephony (everything moves to SIP/IP)
still used quite a bit in 2G/3G cellular networks, even in 2018!

E1/T1/TDM in 2G/3G networks

traditionally all interfaces over E1/T1

Abis (RSL/OML over LAPDm) from BTS to BSC
A (BSSAP/SCCP/MTP) from BSC to MSC
ISUP/MTP for calls between MSCs and from/to PSTN
MAP/TCAP/SCCP/MTP between MSC/VLR, SGSN, HLR, GW-MSC, IW-MSC, …
Gb (BSSGP/NS/FR) betewen PCU and SGSN
Iub (Inverse ATM multiplex) over 4xE1 to RNC

E1/T1/TDM in 2G/3G networks today: Abis

TDM based Abis on decline
- back-haul networks increasingly switch TDM to IP as 4G is co-located with 2G
- but: Lots of BTSs still have physical E1
- Equipment like Ericsson SIU used to convert E1 to IP (proprietary protocols)
- TDM link remains between BTS and SIU-style converter

E1/T1/TDM in 2G/3G networks today: A

TDM based A on the decline
- 3GPP has an official interoperable protocol: AoIP
- adopted by many more modern MSCs
- OsmoBSC supports 3GPP AoIP (yay!)

E1/T1/TDM in 2G/3G networks today: Core

TDM based core network connections still prevalent
- lots of legacy switches (MSCs) and STPs around
- signaling interconnect among MNOs and MVNOs often still TDM
- full MAP+CAP over TCAP/SCCP/MTP stack required

E1/T1/TDM interfacing from Linux / Osmocom

we’ve had E1/T1 based Abis for ages
libosmo-abis supports mISDN + DAHDI drivers
- PCI + PCIe cards readily available
- still extremely expensive (OK in CN, not next to each BTS)
- PCI cards of course require a rather large (ATX, ITX, …) computer

TDMCTRL implements equalizer, elastic buffer, CRC, framing, HDLC, …

Osmocom E1/T1/TDM interfacing use-case

many E1/T1 based BTSs decommissioned around the world
refurbished traders have quantities in stock for very low price
using those BTSs with OsmoBSC + friends is an inexpensive way of
- deploying carrier-grade tier-1 BTS equipment
- with excellent environmental, RF sensitiviy, RF power and high MTBF
- for very low cost
but: The E1/T1 card + associated PC are more expensive than your BTS :(

E1/T1/TDM interfacing wishlist

in 2018, you just want a very small E1/USB or E1/Ethernet adapter
can be used with laptop when on the road, debugging something
can be used with Raspberry Pi, Beaglebone or whatever other mall, inexpensive embedded Linux board
you want to pay a realistic price, not some fantasy price (Digium & co)

Building an E1/T1 adapter

Ok, so let’s build an E1 adapter using existing chips…

existing E1/T1 controllers
- many (including Infineon) already EOL due to the demise of ISDN
- have arcane bus interfaces (parallel Intel/Motorola bus like 1980)
- are ridiculously expensive
- come in very large acane packages

The SD-TDM Plan [tm]

The road to software-defined E1:

Simply use a LIU (Line Interface Unit) + Magnetics
- this converts the HDB3 ternary signal to a serial bit-stream
serialize/deserialize that stream from a microcontroller
do everything else in software, including framing, CRC4, …

Hardware Option A: TI PRU

TI processors like the AM335x on the Beagleboard have PRU cores
- PRU: Programmable Realtime Unit
PRU allows high-speed "real time" bit banging
PRU can serialize/deserialize and provide buffers to ARM core with Linux
E1 adapter could be a beaglebone cape
Beaglebone could actually run entire OsmoBSC + OsmoMGW, too, using 3gPP AoIP over back-haul

Hardware Option B: XMOS

XMOS has a very unusual microcontroller architecture
RISC CPU core @ 500 MHz with programmable serdes
- except USB + Ethenet, no other hard peripherals
- all peripherals (including I2C, SPI, ..) implemented in software!
- could be a simple / small E1/T1 to USB or to Ethernet converter

Hardware Option C: Programmable Logic

Using FPGA or CPLD one can of course synthesize a E1 core
but that’s not really software defined anymore
toolchain trouble (except yosys/arachne/ice40)
just seems like overkill for a slow 2 Mbits/s signal

Further Reading

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