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2008 AT&T Knowledge Ventures. All rights reserved. AT&Tand the AT&T logo are trademarks of AT&T Knowledge Ventures.

ISDN PRI Capabilities

and the Asterisk Implementation

Mark A. Vince, P.E., AT&TMatt Fredrickson, Digium

Astricon 2008

September 25, 2008

Phoenix, AZ


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

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

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


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ISDN Application Architecture

PSTN VOIP

IP

ISDNPRI

Router

4E, 5E,DMS,

Siemens,

IPTDMLegacyPBX


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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


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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


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

Facility Associated Signaling (FAS)

B-Channel

B-ChannelB-ChannelB-Channel

B-Channel

D-Channel

Q.921

Q.931

T1 / E1

Looks like a regular T1 All signaling in the D-channel B-channels are true pipes

ISDNProtocol


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Primary Rate Interface PRI nB+D

Non Facility Associated Signaling (NFAS)

Q.921

Q.931

T1 / E1

B-Channel

D-Channel

B-Channel

B-Channel

B-Channel

D-Channel

Primary

D-Channel

Back-up

D-Channel

#0

#2

#1

#n

With D-Channel Back-UP (DBU)


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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 8Zero 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

dchan=24

bchan=1-23

span=1,1,0,esf,b8zs


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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

pritimer => t200,1000=>> zapata.conf

signalling=pri_net / pri_cpe


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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

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

switchtype=national


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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


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ISDN Messages for Call Control - Continued Disconnect Message Im 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

Asterisk supported


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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

SERVICE Message - Change channel state Channel Which channel on the PRI Change Status Requested state change (in-service, maintenance, out-of-service)

SERVICE-ACKNOWLEDGE Message Channel Which channel on the PRI Change Status Final state (in-service, maintenance, out-of-service)

* Some implementations do more than one channel at a timeAsterisk supported

State must be

communicatedto

application !

(chan-zap)

resetinterval=3600


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Traditional ISDN Information Element Flow

PSTN

ISDN

PRI

4E, 5E,

DMS,Siemens,

LegacyPBX

Setup =>

Screen

Pop

Facility =>

Calling Party Name

Restart =>


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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)


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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


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Asterisk PRI Architecture

*


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Architectural Overview


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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.


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Architectural Overview Kernel Side

Zaptel/DAHDI Communicates with Asterisk via chan_zap/chan_dahdi D

oes

n

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

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


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Zaptel/DAHDI Overview


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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


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chan_dahdi/libpri


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chan_zap/libpri

Libpri

Sits on top of a zapteldchan

Handles all Q.921 (layer 2)and Q.931 (layer 3)messaging necessary for D

-channel operation

Provides simple interface tochan_zap for making calls

over D-channel Q.931 Messages translate

into easy to use PRI events

Chan_zap

Utilizes libpri to maintains

ignaling services on D-channel

Directly interfaces with bearers throughzaptel (includes

re

ading and writing audio data)

Provides glue interface betweenlibpri


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Useful CLI Commands

pri debug span x Enables debugging on the given span (Q.931 call level)

pri intensive debug span x Enables enhanced debugging on the given span (Q.931 + Q.921)

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

U f l


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Useful

Configur

a

t

ion Options (chan_dahdi.conf/zapata.conf) inbanddisconnect=[yes,no]

Allows a caller to hear inband disconnect data that the remoteend may play at disconnect, instead of releasing the B channelimmediately

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, or10 secs) waits 10 seconds for the D-channel to come back up

before hanging up calls.


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Mistakes NOT to make

Don't configure a non-NFAS PRI withthe

trunkgroup and spanmap settings (those are only for NFAS)

instead use implicit method of declaration: signalling=pri_net[or pri_cpe] channel=1-23


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HOW TO Enable Supplementary Services

Calling Name over Facility message facilityenable=yes You will see (in pri

debug

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

Two B-Channel Transfer (Works on 5ESS, NI2, DMS100 (RLT) Q.SIG)

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


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What's New?

Zaptel/DAHDI

D

AHDI will be released soon!

Many driverfixes

,updates, and optimizations.

MTP2 channel type (for SS7)

Libpri

Service message support! BRI Support (1.4.4 of

libpri

,1.6.0 and trunk of Asterisk)

signalling

=

b

ri_net, bri_cpe, bri_cpe_ptmp


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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)


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Configuration for /etc/asterisk/zapata.conf

switchtype=national (Set protocol of PRI)

resetinterval = 3600 (Set audit frequency)

signalling = pri_cpe (PRI user side)

channel => 1-23 (Designate B-Channels)


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ISDN Configuration Examples

Configurations

for

Avaya and Cisco PRIWith

Asterisk


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Avaya PRI Configuration for Asterisk

Three Step Process

Add a DS1/E1 board (TN767x or TN464x)

Add a signaling group

Add a trunk group


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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


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Add a signaling group

Command: add signaling-group xx

For FAS:

Associated Signaling? y Primary D-Channel: 01B1124For NFAS:

Associated Signaling? n Primary D-Channel: 01B1124

Secondary D-Channel: 01A0924

Trunk Brd (tables) Interface ID (tables)01B11 0

01A09 1Note: Not all settings described


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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

Add a trunk group

Command: add trunk-group xx



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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



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Cisco PRI Configuration for Asterisk

Three Step Process

Add a DS1/E1 interface

Add a signaling interface

Some additional settings


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Add a DS1/E1 interface

Command: controller T1 x/y

framing esf

linecode b8zs

cablelength short 133

pri-group timeslots 1-24


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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 12no cdp enable


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Additional settings

Command: isdn switch-type primary-4ess

Command: trunk group xx

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

hunt-scheme sequential both down


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and the AT&T logo are trademarks of AT&T Knowledge Ventures.Page 44

Summary

High level overview of ISDN

Identified various Asterisk configuration settings

Detailed description of Asterisk ISDN internal architecture

Covered two real examples


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2008 AT&T Knowledge Ventures. All rights reserved. AT&Tand the AT&T logo are trademarks of AT&T Knowledge Ventures. Page 45

References

Digium zapata.conf Configuration, August 2006http://www.asterisk.org/doxygen/Config_zap.html

AT&T PRI Specification, 1999http://www.att.com/cpetesting/pdf/tr41459_99.pdf

General Information on ISDNhttp://www.protocols.com/pbook/isdn.htm

Rarig, Harry, ISDN Signal Distribution Network, IEEECommunications Magazine, Vol. 32, No. 6, June 1994

Stallings, William, ISDN, An Introduction, MacMillanPublishing Company, ISBN 0024154717, 1989

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