Tellabs Hardware Echo Cancellers

Hardwiring a Tellabs modular echo canceller card


Tellabs are a multinational corporation that has been in the business of providing carrier-grade telco equipment (particularly echo cancellation modules) for a number of years. Their gear has enjoyed large-scale deployment at most of the major carriers at some point or other and thus appears frequently on the secondary market, such as eBay. Because they're manufacturing for the carrier market their echo cancellation equipment tends to be very high density - commonly deployed as 16x24 channel T1 cards in a single rack mount chassis. Obviously 384 channels is a bit much for most people and sourcing and mounting a full-size dual power shelf (ie. cage) to the wall for just one card is equally overkill so this page will attempt to guide you through hard-wiring a single card to work without a shelf.


Many have noted that when working with asterisk on distant loops, using a tellabs ec totally corrects the echo issue. It works so well in many circumstances that the zaptel ec can be disabled and there are no audible echo or sound quality issues. Tweaking gains also becomes much much simpler, since you only need to listen for volume and over-attenuation. Cards can be found for less than $50 on ebay, and power supply's less than $30. (Yes, it has to be a 'negative' 48 volt power supply, a telco version, and the tellabs brand is recommended).


Your mileage may vary, you may burn your fingers and/or kill your board. No liability is implied or assumed. Consider yourself warned!

Models of Echo Canceller cards

This is simply based on the cards I have to hand at this time and may be lacking. If you have additional information please do add it.

Tellabs have produced a substantial variety of echo cans over the years - two of the most commonly available are the 253x (mid '80s, part numbers 82253x) and the more modern 257x series (mid to late 90's, pn# 81257x). These cards are pin-compatible and differ mechanically only in the overall thickness of the cards, the 253x being approximatly 2" thick and the newer 257x 1" thick. The 253x is a fixed configuration card, the 257x can accept daughterboards that add additional features such as additional memory (ie. longer tail handling), DSPs for noise reduction and so on.

Known models include:

2531 - 32ms T1 Echo Canceller Module (2" thick, non-expandable)
2532 - 64ms T1 Echo Canceller Module (2" thick, non-expandable)
2571 - 32ms T1 Echo Canceller Module (1" thick, expandable)
2572 - 64ms T1 Echo Canceller Module (1" thick, expandable)
2574G - +32ms Endpath Expansion Daughterboard for 257x cards
2574G - +64ms Endpath Expansion Daughterboard for 257x cards
2581 - 32ms E1 Echo Canceller Module (1" thick, expandable, different pinout!)
2582 - 64ms E1 Echo Canceller Module (1" thick, expandable, different pinout!)

Models of Shelves

There are a quite a few of varieties of shelf, common configurations include the highly desireable 2 slot 253C, the 8 slot 253B wire-wrap cages and the more modern 16 slot Amphenol connector equipped 255A. The 257x cards, being only one inch thick are backwards compatible with the 253A/C cages, but the 2 inch thick 253x are obviously not forward compatible with the 255 series.

The 253 shelf is a very simple unit, providing edge connectors for each card and a small integrated power distribution/serial interface card. Connections to trunks are made via wire-wrap posts on each edge connector.

The newer 255 shelves include a seperate removable 2555 Alarm and Access card and provide trunk interconnection through a bank of 50-pin Amphenol connectors.

All shelves provide inputs for two seperate/redundant 48vdc power sources, discrete fusing for each card as well as relay driven outputs for signalling the presence of an alarm condition within the shelf. Finally a method of connecting a serial terminal (a 'craft port' in telco lingo) is also provided - DB25 of on older units, RJ45 on newer.

The edge-connectors used by Tellabs in their shelves are quite clever - the fingers are designed in such a way that the various send and receive T1 lines directly touch each other when the card is removed from the chassis, thereby maintaining the electrical circuit that forms the T1. In practice this means you can remove and reinsert a card with the T1 in service and only trigger a brief yellow alarm, however this is obviously best avoided.

What is -48vdc power?

Nearly all telephone central offices, where these cards were meant to live, are powered from -48. The shelf, ground, and the positive terminal of the power supply are all connected together. The "hot" side of the supply is negative with respect to ground. Why? Because any leakage current on a telephone line will cause metal to build up on something that is negatively charged with respect to its surroundings (earth) and will cause the metal to dissolve into the earth if the ground is negative. Tiny amounts to be sure, but that's why positive ground was chosen.

48vdc supplies are quite common in telecom environments and again frequently appear on the secondary market. These cards all consume well under 0.5 amps so if you're only planning to run one card then you should be able to get away with almost any old 48vdc telecom supply or similar. DO NOT try to use an industrial control supply; these are often negative ground (smoke) and not well filtered (hum).

Enough history! What are the pinouts?

(See below for some example photos)
Pins are numbered from 1 to 56, ascending from the 'bottom' of the card where pin 1 is the bottom right hand pin and pin 2 is the bottom left when facing the rear of the card. Pads 1 and 56 should be silkscreened on the board for reference - there is no index slot in the edge connector, after all who in their right mind would use the card with anything but a Tellabs shelf!?! (:biggrin:)

  1  Ground
  2  -48vdc
  3  Not Connected (N/C)
  4  N/C
  5  Send In Tip
  6  Send Out Tip
  7  N/C
  8  N/C
  9  Send In Ring
 10 Send Out Ring
 11 N/C
 12 N/C
 13 Shield
 14 Shield
 15 N/C
 16 N/C
 17 Shield
 18 Shield (connected to pin 18 on both ajacent boards)
 19 N/C
 20 N/C
 21 Receive In Tip
 22 Receive Out Tip
 23 N/C
 24 N/C
 25 Receive In Ring
 26 Receive Out Ring
 27 N/C
 28 N/C
 29 Minor Alarm #1 Normally Closed
 30 Minor Alarm #2 Normally Closed
 31 Minor Alarm #1 Common
 32 Minor Alarm #2 Common
 33 Minor Alarm #1 Normally Open
 34 Minor Alarm #2 Normally Open
 35 Major Alarm #1 Normally Closed
 36 Major Alarm #2 Normally Closed
 37 Major Alarm #1 Common
 38 Major Alarm #2 Common
 39 Major Alarm #1 Normally Open
 40 Major Alarm #2 Normally Open
 41 N/C
 42 N/C 

The next six pins are used to interconnect the cards to a common access serial bus. One will be TX, the other RX and presumably the Ground is common. I leave it as an exercise to the reader to figure out wiring this:
 43 Jumpered to pin 44 on all slots
 44 Jumpered to pin 43 on all slots
 -- These last four are serial comms --
 45 Jumpered to pin 46 on next slot to the right in shelf
 46 Jumpered to pin 45 on next slot to the left in shelf
 47 Jumpered to pin 48 on next slot to the right in shelf
 48 Jumpered to pin 47 on next slot to the left in shelf

The remaining pins are used to assign a 'slot number' to each slot in the shelf and are wired uniquely for every slot. In addition there is board to board jumpering thats purpose remains a mystery:
 49 {unknown, pinned but N/C in all slots?}
 50 {unknown, pinned but N/C in all slots?}
 51 Select bit 0
 52 Sheft Select A (connected to pin 52 on adjacent boards)
 53 Select bit 1
 54 Shelf Select G (Ground)
 55 Select bit 2
 56 Shelf Select A (connected to pin 56 on both adjacent boards)

Jumpers 51, 53, and 55 select the devices address (slot number). They are by default high, ground them to create the binary value of the devices address.
 51 is bit 0
 53 is bit 1
 55 is bit 2
 its plausable that 49 is bit 3, dunno....
 54 is ground

So, to set the IDs:

 slot 0: Jumper 54 to 51 , 53 , 55
 slot 1: Jumper 54 to 51 , 53 , --
 slot 2: Jumper 54 to 51 ,  -- ,  55
 slot 3: Jumper 54 to 51 ,  --  ,  --
 slot 4: Jumper 54 to  -- ,  53 , 55
 slot 5: Jumper 54 to --,  53, --
 slot 6: Jumper 54 to -- , -- ,  55
 slot 7: No jumpers installed.

Serial comms

The cards support configuration through a serial interface as well as from the front panel. The information accessable includes seperate performance counters for both the line and drop sides of the can and can be very useful for debugging timing slips and other T1 level issues, seeing as Asterisk doesn't presently have any CSU mechansim.

In a shelf the serial communications are wired as a loop passing through all the cards. Based on the female db 25 connector on the is wired on the 253A shelf:

The first card is attached as follows:
 db25F   pin 2   --> module x pin 46 
 db25F   pin 20 --> module x pin 48

each of the next modules are in turn connected as:
 module x pin 45  --> module x+1 pin 46
 module x pin 47  --> module x+1 pin 48

finally the last module goes back to the serial connector:
 module x+n  pin 45 --> db25F pin 3 
 module x+n  pin 45 --> db25F pin 8

of course, you also need to:
 connect ground (pin 1 or 54) to the serial connector (db25F pin 7)
 connect RTS (db25F pin 4) to CTS (db25F pin 5)

... and hardwiring without a shelf?

To connect a card for use outside a shelf (using Slot ID 7) it should be as simple as soldering connections directly to:

  1 Ground 
  2 -48vdc

Drop side (ie. T1 facing the source of the echo, eg. the PSTN):
  5 Send In Tip
  9 Send In Ring
 22 Receive Out Tip
 26 Receive Out Ring

Line side (ie. T1 facing the echo-supressed customer, a zaptel interface):
 21 Receive In Tip
 25 Receive In Ring
  6  Send Out Tip
 10 Send Out Ring

  • Note: It's possible to get the line and drop reversed and still have the system work as expected if you have a 257x that is equipped with 'Send Side Echo Cancellation' (Mode 38, Option 1 if equipped)

... and configuring a powered up can?

The simplist method of configuring a can is to dig out your soldering iron and get the serial interface working (see above). Once physically connected you'll need to configure for 1 start, 1 stop, 7 data, even parity, remote echo and no flow control (aka. 7E1). The default is 9600 baud for each module (this can be changed with front panel option 00). As all interactions with the module must be in UPPER CASE caps-lock is recommended (bet you haven't used that key recently!). To access a particular module type @XXX<cr> in the terminal, where XXX is the slot ID number (eg. @1, @6, @12 etc.) of the module, followed by a cariage return.

If all that terminal stuff is just to wimpy for you then you'll want to use the front panel interface. It's nice and obscure... good for making friends and influencing people.

The front panel configuration interface consists of three buttons (Mode, Chan and Option) and two alphanumeric displays (herein the 'top' and 'bottom' display). Each time the Mode button is pressed the Top display will step to the next entry in the Mode table. Pressing the Chan button will cycle through the available channels when the c, d, E or n channels codes are present in the Top display. Finally, the Option button will cycle through the possible options that can be selected for a particular combination of Mode and Channel...

Italicised entries are default values (*)'ed entries require optional hardware or are slaved to other option states. Not all modes or options are present on all modules, but the basics (framing, coding, per-channel config etc.) should be the same.

Systems Software/Flash Firmware RevisionOPF.xxShows system software/flash firmware revision
2574 Endpath Expansion (*) 1.0Not installed
1.12574G, 32ms installed
1.22574H, 64ms installed
Subassembly (*) 2.0No subassembly installed
2.525VX1 Subassembly is installed
remaineder ommited for the wiki
Channel configuration (per channel)c.xx00E&M/Idlecode Detection
Note: 1. xx is the channel number 01Forced Bypass (Clear Channel)
2. 00 indicates all channels 02Forced Active (Always Cancel)
3. Options 03,04,05,06 only appear for CAS (Mode 63.0) 03FXO-GS
Forced active is intended for diag only 04FXO-LS
it will cause echo at the beginning of each call 05FXS-GS
Data Tone Disabling Control (per channel)d.xx00Disable
Non Lineaer Processor (NPL) control (per channel)n.xx00Disable
ClearCall enhancement (*)E.xx00Disable
Functional Test ModeLPb0In service
1Facility Loopback
02Clear Channel Transparency
3Metallic Bypass
4Drop Side Payload Loopback
5Line Side Payload Loopback
Maintenance Port Baud Rate001.21200 Baud
you get the idea...
Control Port Baud Rate011.2same idea...
Line Side (send out) LBO101330-133 Feet
not complicated...
Drop Side (Recv out) LBO11see previous...
Framing Format203SF (Standard Super Frame) Format
4MBLT (Ft) Format
5ESF (Extended Super Frame) Format
Module Shelf Address/ID30xxxxxx is 1 - 128
Address mode311Auto
2Shelf 255D
Send Side Echo Cancellation (*)380Disable
FLEXWARE Performance Options(*)391Universal
NLP Doubletalk Operation401Off during doubletalk (as per ITU-T recommendations)
2Always on
Dynamic Signal Transfer Control410Off
Minimum ERL4200db
options 44 through 48 excluded for brevity (need daughterboards)...
Maximum Endpath Delay (*)503232ms
9696ms (with 2574H)
128128ms (with 2574H)
Line Coding600AMI with Bit 7 stuffing
2AMI Transparent
Fault Operation610Bypass on Fault
1AIS on Fault
Framing?CRC/Error Transparency Control620Errors are corrected
1Errors are passed through (regenerated)
Busy/Idle detection method630CAS
1Idlecode detection
2Idlecode Detection with CAS Pass-Through
Channel idlecode pattern6407F
17F or FF
Idlecode Logic650Detection on Send OR Receive Side
1Detection on Send AND Receive Sides
2Detection on Send Side Only
3Detection on Receive Side Only
Signalling Disabler660Off
1ITU-T No. 5
2ITU-T No. 6 or No. 7
32600Hz (SF)
Active Channel Logic for CAS671Detection on Send OR Receive side
2Detection on Send AND Receive Sides
3Detection on Send Side Only
4Detection on Receive Side Only
Data Tone Disabler Detection700G.164 with Hold-Band
1G.165 with Hold-Band
2G.164 with Switched-56K
3G.165 with Switched 56K
4G.164 with Hold-Band/CLEARCALL End-of-call
5G.165 with Hold-Band/CLEARCALL End-of-call
Data Tone Disabled Release Mode Control710Hold-Band
options 76 through 85 ommited for brevity (need daughterboards)...
Set System Defaults880At least one system default parameter is not set at its default setting
1All system default settings are currently active (set op 1 to reset module)
Front Panel Lockout890Off (optioning allowed)
1On (View Only mode) (don't set this... :-)

Comments on some specific options:

Modes 40 and 41: Non Linear Processor Opearation
Due to the quantizing noise present in the ulaw encoded PCM used on T1 facilities, some residual echo may remain after the echo estimators have removed the majority of the echo present in the Send path. The NLP removes the residual echo.

Mode 40: NLP Double Talk Operation
The NLP can be optioned to remain active during doubletalk or to deactivate when doubletalk is detected. Leaving the NLP on during doubletalk reduces the likelihood of the far end caller hearing residual echo, and provides better performance when the ERL is close to the minimum ERL selected, but may increase distortion of near end speech. Conversely deactivating the NLP during doubletalk can reduce distortion of the near end signals transmitted to the far end, but may allow some residual echo to be untreated. Note that neither option affects the operation of the NLP during single talk; the NLP is always active during singletalk unless it has been turned off from the front panel or the serial configuration port.

Mode 41: Dynamic Signal Transfer Control
A fundemental drawback to traditional center clipper NLPs has been their tendency to block low level signals from the endpath circuits. The activation of the NLP by far end speech (at the Recv In port) often resulted in audible noise modulation (noise pumping) of idle channel noise and squelching of ambient room noises. DST non linear processing eliminates this problem by providing background signal transparency to low level signals from the endpath curcuits.

Also see Ciscos Echo Analysis for Voice over IP document for a more thorough treatment of processing methods, implications and calculations.

North Americans using Asterisk with PRI/ISDN might want to set:

Mode c.24, option 01 - Don't try and echo cancel the D channel
Mode 20, option 5 - ESF Format
Mode 60, option 1 - B8ZS
Mode 63, option 1 - Idlecode detection (ignoring CAS bits)

There is a button marked ACO (Alarm Cut Out) that disables any external alarm indications (ie. the relay contacts) when momentarially depressed. Push for one second for a lamp test; three seconds for an alarm history code and an alarm history LED display (ie. the most recent alarm encountered). When the ACO button is released the alarm history is cleared. When there has been an alarm that has since cleared itself the green ACO LED will flash to indicate a stored alarm condition.

Applications and equipment arrangements

The Drop side of the echo cancel module is also known as the 'endpath'. This is the side of the echo canceller that is receiving a portion of the audio leaving via the 'Receive Out' line back as echo on the 'Send In' line. The 'endpath' is measured in ms and the net maximum delay of the endpath defines the minimum 'length' of echo canceller required (ie. a 64ms can).

Tellabs present the most common equipment arrangement as being a 'long-distance' arrangement, thus:

                     ---------         ------           ------         ---------
                 -->|         |------>|Si  So|---...-->|Ri  Ro|------>|         |>--
                /   |CO Switch| drop  | echo |  line   | echo | drop  |CO Switch|   \
 Phone<->Hybrid<    |         | side  | can  |  side   | can  | side  |         |    >Hybrid<->Phone
                \--<|         |<------|Ro  Ri|<--...---|So  Si|<------|         |<--/
                     ---------         ------           ------         --------- 

 \_____________Near end__________________/\___Long Haul__/\__________________Far end________________/

In this application, the Near end module provides echo-free service to the Far end subscriber by removing the echo of their own voice that is present at its Send-in (Si) port. Likewise, the Far end module provides echo-free service to the Near-end subscriber by removing the echo of their voice that is present at its Send-in (Si) port. Put differently, the Long Haul facility is always protected from carrying remote-end echos.

For an average Asterisk installation the equipment arrangement might look something like:

                           ------           ---------
                 -------->|Si  So|-------->|         |
                /   drop  | echo |  line   |Asterisk |
           PSTN<    side  | can  |  side   |         |<->VoIP network with echo-free handsets
                \<--------|Ro  Ri|<--------|         |
                           ------           --------- 

If you were doing a PBX private-network using Asterisk, you might have something along the lines of:

                           ------           ---------            ---------           ------
                 -------->|Si  So|-------->|         |          |         |-------->|Ri  Ro|------>>--
                /   drop  | echo |  line   |Asterisk |   IAX2   |Asterisk |  line   | echo | drop     \
           PSTN<    side  | can  |  side   |         |<-------->|         |  side   | can  | side      >Remote PBX
                \<--------|Ro  Ri|<--------|         |          |         |<--------|So  Si|<------<--/
                           ------           ---------            ---------           ------ 

Obviously this provides the ultimate level of echo protection facility as it should be able to handle acoustic coupling in the PBX handsets as well as reverberation from the room when Executives use their speakerphones...

    • Note - Echo cancellers rely on the echoed signal not demonstrating significant phase-jitter. Ie. if the 'tail length' of the endpath were to be continuously varying, even by a few milliseconds, then the echo canceller will have a much harder time removing the echo as it will be constantly tracking back and forth. It follows therefore that you must not try to place the drop sides of the echo canceller facing the zaptel interface. At this time zaptel hardware is notorious for not delivering outbound audio in a highly predictable fashion and this will cause poor performance. Furthermore, the endpath of a VoIP haul may be very, very long in comparison to the PSTN or PBX sides - ie. 10ms cpu time near end, 5ms codec time near end, 20ms on the network, 5ms codec time far end, 10ms cpu time far end... throw in another 10ms to allow for network jitter and you're getting pretty close to the limit of a 64ms can already. Compare this to the delay for national fiber optic based PSTN circuits of 3 ms + (0.005 ms/km * distance in km). More background in Ciscos Echo Analysis for Voice over IP.

258x E1 echo cancellers

The 258x E1 echo cancellers use a different connector, need a different power-supply and have a different pinout. To power the card, you need a +5V power supply, not a -48V one. They use a 96 pin DIN 41612 connector with 3 rows each with 32 pins. The rows are marked A,B and C on the PCB, the pins are marked 1 to 32. As I have no documentation about the real pinout and no shelf available, the following information was guessed with a multimeter.

The pinout for these cards is:

C 1 - GND
C10 - +5V
C11 - Receive Tip - Send-In
C13 - Receive Ring - Send-In
A15 - Transmit Tip - Receive-Out
A17 - Transmit Ring - Receive-Out
C15 - Receive Tip - Receive-In
C17 - Receive Ring - Receive-In
A11 - Transmit Tip - Send-Out
A13 - Transmit Ring - Send-Out
C22 - ACO
C27 - Serial Receive - TTL!
A27 - Serial Transmit - TTL!

Drop side is: C11, C13, A15 and A17
Line side is: C15, C17, A11 and A13
The card will have ID 50 after these connections.

To configure the card via the serial terminal (which is the only possible option), you need to connect a level-converter (for example using a max232) to ports C27 and A27 of the card. After this the card will be recognized at ID 50, so type @50<CR> in a terminal set to 9600,7,e,1 to get its attention.

A manual for the 258x and other tellabs echo cancellers can be found at

Done! You speak Tellabs and can consider yourself trained in the dark art of hardware echo cancellation configuration. Your users are now nicely echo-supressed so go spend some time getting the zaptel echo canceller to work properly. Someone. Please.


The following is a PDF file on the 258x (E1 Echo can)

The following is a PDF file on the 2572 (T1 Echo can)

This is a zoom-in of a Tellabs, front of the card with wiremap:

The attachment id given is not valid.

This is a zoom-in of a Tellabs, back of the card with wiremap:

The attachment id given is not valid.

Wiremap of the 586b jacks:

The attachment id given is not valid.

This a picture of the front of the Tellabs 253C shelf, as you can see there are 2 slots but only one is used. The open gap is either because I didn't get anything to fill it up with, or because it's meant to work with the older 2" ones.
The attachment id given is not valid.

This is a picture of the back of the Tellabs 253C shelf, at the far left it has 2 sets of connectors, the top one is for external alarms, and the bottom for power and ground. To the right of it you can see 2 DB15 connectors for the Drop side and Line side of the T1 for slot 2 (called System 2). Underneath it explains the pinouts (if conecting to a RJ45, then you would do DB15 pin 1-9 to RJ45 pin 1-2, and DB15 pin 3-11 to RJ45 pin 4-5). To the right of it there is room for a knockout in the frame, I got no clue what can go in there.
Next, to the right are 2 sets of DIP switches, the top one is meant for assigining which shelf is controled from the RS-232 (since it can be daisy chained), the bottom one tells the system that this shelf is the last one in a chain.
Next, to the right are 2 RS-232s the top one marked as Out and the bottom as In, they are meant for connecting an external sourec that will control the Echo cancelling on a per channel basis, from the 2572 manual:

The Maintenance SCP (available only on the 255D and 257D Shelves) can be accessed at
a DB25 female connector on the rear of the shelf or via an RJ-11 jack on the front panel of
the 2555, 2555A, or 2555S Module. A straight-through RS-232-D cable is required when
accessing the rear mounted DB25 connector. When accessing the RJ-11 jack, a standard
modular telephone cable can be used.
The Control SCP can also be accessed at a DB25 female connector on the rear of the shelf.
Although the maintenance menus can be accessed from the Control SCP, this SCP is
intended for applications where the switching system or an adjunct processor issues ASCII
commands to enable and disable echo cancellation.

For proper communication to all 257X Modules installed on an SCP link, each module must
be assigned a unique SCP address. SCP addresses are automatically assigned by position
within the shelf. A shelf is assigned a range of SCP addresses via DIP switches or wire-wrap
pins, depending upon the shelf used. See the appropriate shelf technical manual for

The above gives you a clue as to what these RS-232s are meant and what the DIP switches do.
Next, to the right is another knokout, again I got no clue what it's meant for.
Next, to the right are the 2 DB15s for the Drop and Line of the T1s of System 1 (Slot 1).
The attachment id given is not valid.

Example photos of a 2572 EC

Visit for some example photos of a soldered on non-serial 2572 Echo Can. (01/2006)

One good testing tip is to loop the two sides before installation. You should be able to get an AIS Loop. Then you can go straightthrough to your channel bank, and cross-over to your t1 asterisk card.

You may need to adjust the signalling settings per channel, as I needed to since I am mixing fxs and fxo. Asterisk was giving me odd ring indications until I did this. I beleive I had to adjust option 63 in order to access the per-channel settings.

Be sure to check options 20, 60 and 63 for your setup...

If you need help, visit www d0t adcomcorp d0t com for contact info. GW 01/2006

Example photos of a 2582 E1 EC

Photos of a connected E1 2582 canceller: (01/2006)

These connections were soldered temporarily to pins plugged directly into the connector, until I got hold of the matching DIN connector which is now used instead. Photos of this will be coming soon, too.
The 4 wires going to the front of the picture are the serial tx and rx lines and 5v power for the level converter.

The fault LED is lit because the unit can't detect two working power-supplies, but works fine anyway.
Created by: kb1_kanobe, Last modification: Sun 12 of Feb, 2006 (12:22 UTC) by lytledd
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