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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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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 PRI Layer 3 Q.931 - Continued
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 PRI Layer 3 Q.931 - Continued
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
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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
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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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Avaya PRI Configuration for Asterisk
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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Avaya PRI Configuration for Asterisk
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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Avaya PRI Configuration for Asterisk
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
Note: Not all settings described
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Avaya PRI Configuration for Asterisk
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
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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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Cisco PRI Configuration for Asterisk
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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Cisco PRI Configuration for Asterisk
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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Cisco PRI Configuration for Asterisk
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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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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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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