Retronetworking » Retro-GSM

h1. TCSM2 Exchange Terminal modules

A working Nokia TCSM2 system requires a minimum of 1 and a maximum of 4 Exchange Terminal modules, either ET2E (2xE1) or ET2A (2xT1). The choice of ET module determines whether the TCSM2 system will operate in E1-based ETSI environment or T1-based ANSI environment. (More precisely, the selection between the two fundamental environments is done with a global configuration command on the TRCO - but in order to get a working system, that selection needs to match physically present ET hardware.)

h2. Principle of operation

The Line Interface (LI) between [[TCSM2_TRCO_card|TRCO]] and each ET module is a bidirectional synchronous serial pipe running at 4096 kbit/s, i.e., twice the bit rate of an E1 circuit. There is an 8 MHz clock generated by TRCO and carried to the [[TCSM2_ET1TC_chassis|ET1TC backplane]] by the [[TCSM2_interchassis_cable|interchassis cable]]; the latter backplane then delivers this clock to all ET module slots. (Because the clock rate is 8.192 MHz but the LI bit rate is 4096 kbit/s, there are two clock cycles per bit interval.) There is also an 8 kHz frame sync signal, similarly distributed from TRCO to all connected ET modules. For each individual ET module slot (up to 4 of which can be connected to one TRCO), there is one dedicated differential pair carrying LI bits from TRCO to the ET module, and another dedicated differential pair carrying LI bits in the other direction. This 4096 kbit/s proprietary interface carries all of the traffic for both E1 (or T1) interfaces served by the ET module, although it is unclear from the documentation if the bit streams from the two E1s are interleaved per bit or per 8-bit timeslot.

Whichever interleaving scheme is used, the part of 4096 kbit/s LI that would otherwise correspond to E1 timeslot 0 (only on E1 trunk 0 or perhaps on both E1 trunks, not clear) is repurposed for a private HDLC channel between TRCO and the local processor on the ET module.

h3. Firmware situation

The set of firmware images maintained in flash on TRCO includes those intended for various ET module versions, as analyzed below. However, unlike the situation on the [[TCSM2_transcoder_cards|TR16-S card]] where the "slave" module has no flash memory at all (only presumed DSP boot ROMs), each ET module does need to have its own flash or EPROM (or perhaps OTP ROM like on TRCO) for booting and initial operation. There are two possible working models then:

* One possibility is that the local non-volatile memory on the ET module is an immutable ROM (or EPROM), and the image downloaded from TRCO on each boot then runs entirely in RAM. This possibility _seems_ to be the case with earlier generations of ET2E and ET2A, although we will only know for sure once our C08763 (ET2E-C) module arrives from shields-e.

* The other possibility is that the local non-volatile memory on the ET module is flash, and firmware updates that come in from TRCO are written into this flash. It is possible that Nokia might have switched to this model with ET2E-T and ET2A-T.

h3. Relation to other Nokia platforms

It appears that the same ET2E modules (and perhaps ET2A) were also used in Nokia's BSC of the same era, and maybe in an MSC too. It is an open question if there are any firmware differences, or perhaps differences in jumper or other hardware configuration, between ET2E used in TCSM2 and the same ET2E used in BSC or MSC or other platforms.

h2. ET2A for T1 circuits

I (@falconia) do not currently have an ET2A to play with: unlike ET2E, ET2A does not show up in the online inventory system of the surplus vendor I currently work with, the one willing to sell to ultra-small customers who don't have compliance managers. I might ask them about it at a later time, though.

Compared to E1 modules examined below, ET2A has one distinct advantage: each of the two T1 interfaces is brought out on a standard RJ-48C jack. (Most people call it RJ-45, but RJ-48C is the official designation for this type of jack _when it is used for a T1 interface per FCC specifications_.) However, this advantage is effectively nullified by the problem that [[e1-t1-adapter:icE1usb]] supports only E1, and there is currently no solution for T1 that offers the same level of sanity. (Sanity as in something other than aftermarket Digium cards where the quality of driver code is atrocious, and it is not even clear if a non-working system is a driver problem or defective hardware.)

h2. ET2E for E1 circuits

Even though RJ-48C is an official standard only for T1, many people have adopted the same convenient 8-position modular jack for E1 circuits, including the designers of the already mentioned [[e1-t1-adapter:icE1usb]]. But unfortunately, not Nokia: AFAICT Nokia never made a version of ET2E for TCSM2 that brings out the same convenient jacks. Instead ET2E for 2xE1 was made in two versions in terms of physical interfaces:

h3. ET2E with a DIN 41612 half-C connector

The electrical interface for the two E1 circuits on this ET2E version is the same as it would have been if they had provided RJ-48C jacks, the same 120 ohm electrical interface one would connect directly to an icE1usb. But the connector is a difficult one: it is DIN 41612 of size "1/2 C", i.e., 3 rows with 16 pins in each row. It is the same E1 interface connector as used by the same manufacturer (Nokia) on the [[retronetworking:Nokia_EKSOS_Node_Control_Unit|EKSOS N20 NCU]]. Official mating connectors that are meant to be mounted on cables (free-hanging inline) do exist, but only with a cost-prohibitive MOQ. I tried using the same approach that was used by @laforge on the EKSOS - use a female connector that is officially meant for PCB rather than cable mounting - but I ran into an unexpected problem: the connector does not fit unless I destructively break off some plastic tabs from the connector housing on the ET2E module, which I am quite reluctant to do.

h3. ET2E with coaxial E1 interfaces

The other version of ET2E brings out each E1 interface on a pair of SMB coaxial connectors - it is the 75 ohm unbalanced E1 interface. I don't have this ET2E-C module in my hands yet, but it is on order from the supplier. My current plan is to use it with external adapters that convert the 75 ohm coaxial E1 interface to the 120 ohm version on RJ-48C.

h2. Different generations of ET2E

Aside from the coaxial vs 120 ohm balanced dichotomy, it appears that ET2E was made in 3 different evolutionary generations:

h3. First generation: ET2E and ET2E-C

In the first generation, the 120 ohm balanced version (with the weird connector) was called simply ET2E, whereas the coaxial interface counterpart is ET2E-C. We currently have an ET2E-C module on order from shields-e; when it arrives, we'll be able to tell how it differs from the newer ET2E-T.

Looking at the low-resolution photos of this module on the seller's website, I see a socketed IC in a DIP package that looks like an old-fashioned EPROM; among ET firmware images on TRCO, the image that supposedly supports classic ET2E modules is named @ET2RAMQA.PAC@ - note the "RAM" in the name. Hence I speculate that these ET2E modules use the "sane" model of immutable on-board boot ROM plus RAM-loaded operational code.

h3. Second generation: ET2E-S and ET2E-SC

All we know is the name - we don't know if this version is closer to the original or to ET2E-T.

h3. Third generation: ET2E-T and ET2E-TC

The only module we have on hand currently is ET2E-T, aka C105507. It appears to be a significantly redesigned implementation of ET module functionality compared to the original - but we will (hopefully) get a clearer picture when our ET2E-C arrives and we can compare the two. So far the only aspect we can tell for sure is that ET2E-T has no socketed non-volatile memory ICs of any kind, but there is a flash memory chip soldered directly on the board. The corresponding ET firmware image is @EA2TCSM2.PAC@, said to be common between ET2E-T and ET2A-T, but it is not clear at all whether it is a run-from-RAM image or if it gets written into on-module flash.