ISDN PRI Capabilities and the Asterisk Implementation ... - Asterisk-ES

ISDN PRI Capabilities 

and the Asterisk Implementation 

Mark A. Vince, P.E., AT&T 

Matt Fredrickson, Digium 

© 2008 AT&T Knowledge Ventures. All rights reserved. AT&T 

and the AT&T logo are trademarks of AT&T Knowledge Ventures. 

Astricon 2008 

September 25, 2008 

Phoenix, AZ

Why Are We Talking About ISDN? 

• PSTN has a large user community 

• ISDN PRI is an “incumbent” interface 

• Matured over 20 years 

• Standardized.... well sorta... vendor specific 

• Way to integrate VOIP and “rest of the world” 

• Learn from those who have gone before 


and the AT&T logo are trademarks of AT&T Knowledge Ventures. Page 2

ISDN Application Architecture 

PSTN 

4E, 5E, 

DMS, 

Siemens, 

ISDN 

PRI 

IP 

VOIP 

Router 

Legacy 

PBX 

TDM 

IP 



Areas of Discussion 

• ISDN PRI overview 

Layer 1 – T1 

Layer 2 – Q.921 

Layer 3 – Q.931 

• Asterisk PRI architecture 

Zaptel (Dahdi) 

Libpri 

• Connecting to Asterisk – Real examples 

Avaya PRI 

Cisco PRI 

=>> Asterisk zaptel, zapata settings 



What is ISDN PRI? 

Integrated Services Digital Network 

Common infrastructure for voice, data and video (circa 1980s) 

Feature rich signaling interface 

Two types: BRI and PRI 

Primary Rate Interface 

Provided via a T1 / E1 (1.544 Mbs / 2.048 Mbs) 

Bearer channels (B-channels, H-channels), signaling channel (D-channel) 

Two types: Facility Associated Signaling (FAS) and Non Facility Associated 

Signaling (NFAS) 

Basic Rate Interface – BRI – 2B+D not covered today 

H-channels not covered today 



Primary Rate Interface – PRI – 23B+D / 30B+D 

Facility Associated Signaling (FAS) 

D-Channel 

B-Channel 

B-Channel 


Protocol 

Q.931 

Q.921 

T1 / E1 

B-Channel 

B-Channel 

B-Channel 

• Looks like a “regular” T1 

• All signaling in the D-channel 

• B-channels are true “pipes” 



Primary Rate Interface – PRI – nB+D 

Non Facility Associated Signaling (NFAS) 

With D-Channel Back-UP (DBU) 

Primary 

D-Channel 

#0 

Back-up 

D-Channel 

#1 

D-Channel 

B-Channel 

D-Channel 

Q.931 

Q.921 

#2 

B-Channel 

B-Channel 

T1 / E1 

B-Channel 

#n 



ISDN PRI Layer 1 – Physical Layer 

• T-1, 1.544 Mbps = 24 x 64 kbps + 8 kbps (framing) 

• DS1 Signal Options 

ESF – Extended Super Frame – What type of framing? 

B8ZS - Bipolar 8 Zero code Substitution – How to handle all zeroes? 

• Electrical Consideration 

CSU - Channel Service Unit / DSx level – How long are the wires? 

• Synchronization / Clocking 

Take from telephone network, generally 

Slips significantly impact modems (fax), reduce voice quality 

=>> zaptel.conf 


and the AT&T logo are trademarks of AT&T Knowledge Ventures. Page 8 

span=1,1,0,esf,b8zs 

bchan=1-23 

dchan=24

ISDN PRI Layer 2 – Link Layer - Q.921 

• Exists Only In The D-channel 

• LAP-D – Link Access Procedure - D 

• HDLC – High level Data Link Control Protocol 

• Point-to-Point protocol only 

• Network / User concept 

• Specific Elements 

SABME– Synchronous Balanced Mode Extended / 

UA - User Acknowledge 

RR / RNR – Receiver Ready or Receiver Not Ready 

=>> zapata.conf 

pritimer => t200,1000 

signalling=pri_net / pri_cpe 



ISDN PRI Layer 3 – Protocol Layer - Q.931 

• General Concepts 

Exists only in the D-channel, on top of Q.921 

Messages with Information Elements (IEs) 

Variants – AT&T (4ESS, 5ESS), ETSI, NI-1, NI-2, QSIG, Nortel (DMS100) 

• Call Control / Signaling Protocol 

switchtype=national 

• Code Sets – Collections of Information Elements (IEs) 

Code Set 0 – Default set for collection of IEs 

Code Set 5 – National specific set for IEs 

Code Set 6 – Network specific set for IEs 

Code Set 7 – User specific set for IEs 

• Locking Shift – Used to move between Code Sets 



ISDN PRI Layer 3 – Q.931 - Continued 

• ISDN Messages for Call Control 

SETUP Message – Here is what I want.... 

• Bearer Capability IE – Voice, data, encoding √ 

• Channel IE – Which channel on the PRI 

• Called Number IE – Number to be called 

• Calling Number IE 

√ 

• II / OLI IE (Information Indicator / Originating Line Identification) √ 

• UUI IE– User-to-User Information – Data associated with the call 

• Lower Layer Compatibility IE – Subrate data multiplexing scheme 

• Display Name IE – PBX user name √*CodeSet 0 only 

√ 

√ 

CONNECT Message– Here is what you got.... 

• Call was accepted 

• Connected Number IE – Who really answered 

√ Asterisk supported 




• ISDN Messages for Call Control - Continued 

Disconnect Message – I’m leaving.... 

• Channel IE – Which channel on the PRI √ 

• UUI IE– User-to-User Information – Data associated with the call 

Release Message – Goodbye.. 

• ISDN Messages - Others 

Facility Message 

• Calling Party Name IE – PSTN Calling Party Name √ 

• Channel IE – Which channel on the PRI 





• ISDN Messages for Maintenance 

RESTART Message - Set to known state (idle) √ 

• Channel – Which channel on the PRI* 

RESTART-ACKNOWLEDGE Message √ 

• Channel – Which channel on the PRI 

resetinterval=3600 

SERVICE Message - Change channel state 


• Change Status – Requested state change (in-service, maintenance, out-of-service) 

SERVICE-ACKNOWLEDGE Message 


• Change Status – Final state (in-service, maintenance, out-of-service) 


* Some implementations do more than one channel at a time 



State must be 

communicated 

to 

application ! 

(chan-zap)

Traditional ISDN Information Element Flow 

PSTN 

4E, 5E, 

DMS, 

Siemens, 


PRI 

Setup => 

Facility => 

Called Party Number 

Calling Party Number 

Display Name 

UUI, II/OLI 

Legacy 

PBX 

Screen 

Pop 

Calling Party Number 

Channel ID Display Name 

Called Party Number UUI 

Bearer Capability Calling Party Name 

Restart =>

ISDN PRI Layer 3 – Message Associated - User 

to User Information 

• IE associated with setup and clearing ISDN messages 

• Typical Applications -> Call Center / Computer Telephony (CTI) 

Provides a “tag” to identify supporting data regarding the call 

Stays with the call 

Can be modified as call is forwarded 

• Size of message depends on vendor implementation 

4ESS – 198 bytes (across all Code Sets) 

5ESS – 64 bytes 

DMS250 – 128 bytes 

DMS100 – 0 bytes 

Avaya Definity – 120 bytes 

• Novel Applications 

Wrong number but pass data to called party 

Pass URL (alphanumeric only) 



ISDN PRI Layer 3 – Temporary Signaling Connections 

• Standard D-channel messages, difference is B-channel 

• Two Types 

Call Associated Temporary Signaling Connection (CA-TSC) 

Non Call Associated Temporary Signaling Connection (NCA-TSC) 

or Call Independent Signaling Connection (CISC) (QSIG) 

• CA-TSC Application -> Distributed Communication System® (DCS) 

Pass station data that might otherwise not be passed 

• NCA-TSC Application 

Carry Message Waiting Indication (MWI) information 

Pass query information on call queue parameters 

• Can be used through some PSTN network switches 

Allows for consolidated voicemail in the enterprise 



Asterisk PRI Architecture 

* 



Architectural Overview 






Architectural Overview – Asterisk Core 

• Asterisk Core 

– Control is in the Dialplan 

– Allows channels and applications to access each other in a 

standard way. Regardless of underlying protocols or bearer 

transmission. 

• Channel Drivers 

– Provide protocol independent interface for Asterisk to control 

– Read, Write, Dial, Hangup 

• Applications 

– Can use the Asterisk core to read and write audio data and 

initiate signalling over channels. 






Architectural Overview – Kernel Side 

• Zaptel/DAHDI 

– Communicates with Asterisk via chan_zap/chan_dahdi 

– D 

oes 

n 

(‏signalling ot actually do any of the ISDN signaling (only RBS bit 

– Provides services for higher layers that actually do the signalling 

• Zaptel/DAHDI Hardware Drivers 

– Provide “clear channel timeslots” to which zaptel communicates 

– Interrupt driven Pseudo-TDM bus 



Zaptel/DAHDI Overview 



Zaptel Services 

• Provides Services to Userland 

– D-channel 

– B-channel 

– Separates channels 

• Provides Services for those Channels 

– Echo cancelation 

– Conferencing 

– HDLC bit stuffing, unstuffing 

– CRC checking/generation 

– Time Slot Interchange 



chan_dahdi/libpri 



chan_zap/libpri 

Libpri 

• Sits on top of a zaptel 

“dchan” 

• Handles all Q.921 (layer 2) 

and Q.931 (layer 3) 

messaging necessary for D 

-channel operation 

• Provides simple interface to 

chan_zap for making calls 

over D-channel 

• Q.931 Messages translate 

into easy to use PRI events 



Chan_zap 

• Utilizes libpri to maintain 

s 

i 

g 

naling services on D-channel 

• Directly interfaces with bearers through 

zaptel (includes 

r 

e 

a 

(‏data ding and writing audio 

• Provides glue interface between 

libpr 

i

Useful CLI Commands 

• pri debug span x 

(‏level – Enables debugging on the given span (Q.931 call 

• pri intensive debug span x 

(‏Q.921‎ – Enables enhanced debugging on the given span (Q.931 + 

• pri show span x 

– Sho 

ws 

s 

ome useful information about span (up, down, timer values, etc). 

• pri [set,unset] debug file [filename] 

– Outputs all PRI debug enabled by the 

pri 

d 

ebug commands to the given filename (or disables it with unset). 



• pri show spans

Useful 

Configur 

a 

t 

• inbanddisconnect=[yes,no] 

(‏chan_dahdi.conf/zapata.conf‏)‏ ion Options 

– Allows a caller to hear inband disconnect data that the remote 

end may play at disconnect, instead of releasing the B channel 

immediately 

• pritimer=t309,10000 

– Used in cases with periodic instability. Instead of hanging up all 

calls when the D-channel drops, a guard timer (10,000 ms, or 

10 secs) waits 10 seconds for the D-channel to come back up 

before hanging up calls. 



Mistakes NOT to make 

• Don't configure a non-NFAS PRI with 

t 

h 

e 

(‏NFAS trunkgroup and spanmap settings (those are only for 

– instead use implicit method of declaration: 

• signalling=pri_net[or pri_cpe] 

• channel=1-23 



HOW TO – Enable Supplementary Services 

• Calling Name over Facility message 

– facilityenable=yes 

– You will see (in pri 

debug 

s 

p 

an x) a facility message sent containing caller name information. 

• Two B 

- 

C 

h 

a 

(‏Q.SIG nnel Transfer – (Works on 5ESS, NI2, DMS100 (RLT) 

– facilityenable=yes 

– transfer=yes 



– Don't do anything in the Dial() application which would prohibit native bridging (any of the options that require inband 

audio

What's New? 

Zaptel/DAHDI 

Libpri 

• 

D 

AHDI will be released soon! 

• Many driver 

fixe 

s 

, 

updates, and optimizations. 

(‏SS7‎ • MTP2 channel type (for 

• Service message support! 

• BRI Support (1.4.4 of 

libpr 

i 

, 

(‏Asterisk 1.6.0 and trunk of 

– signallin 

g 

= 

b 

bri_cpe_ptmp ri_net, bri_cpe, 



Configuration for /etc/zaptel.conf 

span=1,1,0,esf,b8zs (Set up T1 span) 

bchan=1-23 

(Assign B-Channels) 

dchan=24 

(Assign D-Channels) 



Configuration for /etc/asterisk/zapata.conf 

switchtype=national 

resetinterval = 3600 

signalling = pri_cpe 

channel => 1-23 

(Set protocol of PRI) 

(Set audit frequency) 

(PRI user side) 

(Designate B-Channels) 



ISDN Configuration Examples 

Configurations 

for 

Avaya and Cisco PRI 

With 

Asterisk 



Avaya PRI Configuration for Asterisk 

Three Step Process 

• Add a DS1/E1 board (TN767x or TN464x) 

• Add a signaling group 

• Add a trunk group 




Add a DS1/E1 Board 

Command: add ds1 xxYzzzz 

• Name: Asterisk Access 

• Line Coding: b8zs 

• Line Compensation: 1 

• Framing Mode: esf 

• Signaling Mode: isdn-pri 

• Connect: pbx 

• Interface: network 

• Country Protocol: 1 

• Protocol Version: a 

• Near-end CSU Type: other 



Note: Not all settings described


Add a signaling group 

Command: add signaling-group xx 

For FAS: 

• Associated Signaling? y 

• Primary D-Channel: 01B1124 

For NFAS: 

• Associated Signaling? n 

• Primary D-Channel: 01B1124 

• Secondary D-Channel: 01A0924 

• Trunk Brd (tables) Interface ID (tables) 

01B11 0 

01A09 1 

Note: Not all settings described 




Add a trunk group 

Command: add trunk-group xx 

• Group Type: isdn 

• Group Name: Asterisk PSTN Access 

• Direction: two-way 

• Outgoing Display? Y 

• Service Type: tie 

• Codeset to Send Display: 0 

• Codeset to Send National IEs: 6 

• Trunk Hunt: ascend 

Note: Not all settings described 




Add a trunk group, continued 

• Maintenance Tests? Y 

• Send Name: y 

• Send Calling Number: y 

• Numbering Format: public 

• Outgoing Channel ID Encoding: preferred 

• UUI IE Treatment: service-provider 

• Replace Restricted Numbers? n 

• Replace Unavailable Numbers? n 

• Send Connected Number: y 

• Send UCID? n 

• Send Codeset 6/7 LAI IE? n 

• US NI Delayed Calling Name Update? n 



Note: Not all settings described

Cisco PRI Configuration for Asterisk 

Three Step Process 

• Add a DS1/E1 interface 

• Add a signaling interface 

• Some additional settings 




Add a DS1/E1 interface 

Command: controller T1 x/y 

framing esf 

linecode b8zs 

cablelength short 133 

pri-group timeslots 1-24 




Add a signaling interface 

Command: interface Serialx/0:23 

description "D-Channel for Asterisk" 

no ip address 

encapsulation hdlc 

no logging event link-status 

no snmp trap link-status 

isdn switch-type primary-ni 

isdn incoming-voice voice 

isdn map address ^1 plan isdn type national 

isdn map address ^011 plan isdn type international 

isdn disconnect-cause busy 

isdn outgoing display-ie 

trunk-group 12 

no cdp enable 




Additional settings 

Command: isdn switch-type primary-4ess 

Command: trunk group xx 

description "Trunk Group 12 & 23 - 4ESS PRI" 

hunt-scheme sequential both down 



Summary 

• High level overview of ISDN 

• Identified various Asterisk configuration settings 

• Detailed description of Asterisk ISDN internal architecture 

• Covered two real examples 



References 

• Digium zapata.conf Configuration, August 2006 

http://www.asterisk.org/doxygen/Config_zap.html 

• AT&T PRI Specification, 1999 

http://www.att.com/cpetesting/pdf/tr41459_99.pdf 

• General Information on ISDN 

http://www.protocols.com/pbook/isdn.htm 

• Rarig, Harry, ISDN Signal Distribution Network, IEEE 

Communications Magazine, Vol. 32, No. 6, June 1994 

• Stallings, William, ISDN, An Introduction, MacMillan 

Publishing Company, ISBN 0024154717, 1989

ISDN PRI Capabilities and the Asterisk Implementation ... - Asterisk-ES

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